August 2011

NASA/TP-2011-217167

Energy Navigation: Simulation Evaluation and 

Benefit Analysis

David H. Williams, Rosa M. Oseguera-Lohr, and Elliot T. Lewis

Langley Research Center, Hampton, Virginia
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August 2011

NASA/TP-2011-217167

Energy Navigation: Simulation Evaluation and 

Benefit Analysis

David H. Williams, Rosa M. Oseguera-Lohr, and Elliot T. Lewis

Langley Research Center, Hampton, Virginia
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not 

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration. 
i

Contents

Abbreviations and Symbols......................................................................................................................................... iii

Introduction ...................................................................................................................................................................1

Background....................................................................................................................................................................2

RNAV Arrival Procedures.........................................................................................................................................2

Flight Management Systems and RNAV Procedures................................................................................................3

Traffic Control and RNAV Procedures.....................................................................................................................3

Energy Navigation Concept.......................................................................................................................................4

Low Noise Guidance Flight Deck Tool.................................................................................................................4

Expanded Energy Guidance Tool..........................................................................................................................4

ENAV Software.....................................................................................................................................................4

Related studies...........................................................................................................................................................5

Experiment Descriptions ...............................................................................................................................................6

Objectives and Approach...........................................................................................................................................6

Piloted Simulation Experiment..............................................................................................................................6

Benefits Analysis Batch Study ..............................................................................................................................7

Simulation Facilities..................................................................................................................................................7

Piloted Flight Simulator.........................................................................................................................................7

Air Traffic Operations Laboratory.........................................................................................................................8

RNAV Procedures and Charts...................................................................................................................................8

Louisville Arrival ..................................................................................................................................................8

Atlanta Arrivals .....................................................................................................................................................9

ATC Environment .....................................................................................................................................................9

Piloted Simulation Study .......................................................................................................................................9

Benefits Analysis Batch Study ............................................................................................................................10

Experiment Design ..................................................................................................................................................10

Piloted Simulation Test Conditions.....................................................................................................................10

Batch Simulation Test Conditions.......................................................................................................................11

Piloted Simulation Crew Procedures...................................................................................................................12

Batch Simulation Pilot Modeling ........................................................................................................................12

Piloted Simulation Test Matrix................................................................................................................................13

Subject Pilots.......................................................................................................................................................14

Batch Simulation Test Matrix..................................................................................................................................14

Data collection.........................................................................................................................................................15

Objective data ......................................................................................................................................................15

Subjective data.....................................................................................................................................................15

Results and Discussion ................................................................................................................................................16

Piloted Simulation Study .........................................................................................................................................16

Vertical Profile ....................................................................................................................................................16

Noise....................................................................................................................................................................16

Fuel Use...............................................................................................................................................................17

Workload .............................................................................................................................................................18

Other Subjective Data..........................................................................................................................................18

Post-Test Questionnaire Results..........................................................................................................................19

Benefits Analysis Batch Study ................................................................................................................................21

Validation of Batch Simulation ...........................................................................................................................21

Fuel......................................................................................................................................................................22

Noise....................................................................................................................................................................24

Final Stabilization................................................................................................................................................24

Concluding Remarks ...................................................................................................................................................25

References ...................................................................................................................................................................27

Appendix A ENAV Software Description..................................................................................................................28

Environment ............................................................................................................................................................28
ii

Pilot interface...........................................................................................................................................................28

Algorithm Inputs......................................................................................................................................................28

Lateral Route Definition ......................................................................................................................................28

Vertical Segment Definition ................................................................................................................................28

Weather Definition ..............................................................................................................................................29

Aircraft State Data ...............................................................................................................................................29

Aircraft Mode Control Data.................................................................................................................................29

Aircraft Planning Data.........................................................................................................................................30

ENAV Altitude and Speed Limits.......................................................................................................................30

Algorithm Outputs...................................................................................................................................................30

Guidance Output..................................................................................................................................................30

Trajectory Output ................................................................................................................................................30

Appendix B Flight Manual Bulletin ...........................................................................................................................32

Appendix C Questionnaires........................................................................................................................................41
iii

Abbreviations and Symbols

ADRS Air Data Radar Simulation

AGL Above Ground Level

ASDO Airspace Super Density Operations

ASTOR Aircraft Simulator for Traffic Operations Research

ATC Air Traffic Control

ATL Atlanta Hartsfield-Jackson International Airport

ATOL Air Traffic Operations Laboratory

ATOS Air Traffic Operations Simulation

CAS Calibrated Airspeed

CDA Continuous Descent Arrival

CDU Control Display Unit

EFIS Electronic Flight Information System

ENAV Energy Navigation

FAF Final Approach Fix

FMB Flight Manual Bulletin

FMC Flight Management Computer

FMS Flight Management System

IAF Initial Approach Fix

ILS Instrument Landing System

INM Integrated Noise Model

JPDO Joint Planning and Development Office

LNAV Lateral Navigation

LNFP Low Noise Flight Procedures

LNG Low Noise Guidance
iv

MCP Mode Control Panel

MSL Mean Sea Level

NAS National Airspace System

ND Navigation Display

NextGen Next Generation Air Transportation System

nm nautical miles

NWS National Weather Service

OPD Optimized Profile Descent

PFD Primary Flight Display

PM Pilot Model

QAT Quiet Aircraft Technology project

RFD Research Flight Deck

RNAV Area Navigation

RPFMS Research Prototype Flight Management System

RUC Rapid Update Cycle

SDF Louisville-Standiford International Airport

SEL Sound Exposure Level

STAR Standard Terminal Arrival Route

TBO Trajectory Based Operations

VNAV Vertical Navigation

Vref Reference Speed
1

Abstract

This paper presents results from two simulation studies investigating

the use of advanced flight-deck-based energy navigation (ENAV) and 

conventional transport-category vertical navigation (VNAV) for 

conducting a descent through a busy terminal area, using Continuous 

Descent Arrival (CDA) procedures. This research was part of the Low 

Noise Flight Procedures (LNFP) element within the Quiet Aircraft 

Technology (QAT) Project, and the subsequent Airspace Super Density 

Operations (ASDO) research focus area of the Airspace Project. A 

piloted simulation study addressed development of flight guidance, and 

supporting pilot and Air Traffic Control (ATC) procedures for high 

density terminal operations. The procedures and charts were designed to 

be easy to understand, and to make it easy for the crew to make changes 

via the Flight Management Computer Control-Display Unit (FMC-CDU) 

to accommodate changes from ATC. The test runs were intended to 

represent situations typical of what exists in many of today’s terminal 

areas, including interruptions to the descent in the form of clearances 

issued by ATC. A subsequent non-piloted simulation study provided 

extended benefits analysis for situations not tested in the piloted study.

The results showed that the pilots were able to conduct the descents 

with both forms of guidance, even when interrupted by ATC with 

instructions that took the aircraft off of the programmed path, laterally, 

vertically or both. The uninterrupted descent resulted in the most optimal 

profile, which translated to the lowest perceived noise levels and lowest 

amount of fuel burned. These benefits were reduced in the VNAV cases 

as the interruptions from ATC increased, but remained more consistent 

in the ENAV cases. The workload associated with conducting the 

descents were rated comparably for the two forms of guidance. Pilots 

were able to conduct the descents with a minimum amount of time 

allowed for studying the instructions and charts they were given, and 

expressed a strong preference for displaying the ENAV energy guidance 

on the Primary Flight Display. The extended benefits analysis identified 

the final approach region as the primary location for ENAV benefits, 

with limited improvements seen in the enroute descent region.

Introduction

The noise generated by aircraft during 

departure and arrival flight operations continues 

to be a significant problem at most major airports 

in the United States. Complaints from the 

communities surrounding these airports often 

result in restrictions to the number and type of 

operations that can be conducted in the 

surrounding areas. They also result in significant 

delays to construction of new runways and 

extension of existing runways. These restrictions 

in turn limit the capacity of the airport and can 

result in economic hardship for the airport, 

airlines, and communities served by the airport.

Improvements to the design of jet engines over 

the past several decades have reduced jet engine 
2

noise and greatly reduced the noise footprint of 

individual aircraft. However, the increasing 

number of flights and the expansion of population 

in the vicinity of airports have prompted renewed 

interest in methods for noise abatement. 

Procedural solutions to the noise problem, which 

involve changing the way pilots operate their 

aircraft to minimize the perceived noise on the 

ground below, have been investigated for a 

number of years, and several promising 

techniques have been developed. The primary 

advantage of procedural solutions is that benefits 

can be achieved without making design changes 

to the aircraft engines or airframe. The major 

challenges involved with the use of operational 

noise abatement procedures include development 

of acceptable pilot procedures, development of 

flight guidance techniques, and development of 

acceptable procedures for Air Traffic Control 

(ATC).

This paper presents results of two studies that 

were conducted as part of NASA’s Quiet Aircraft 

Technology (QAT) and NextGen Airspace 

Projects. The primary goal of the QAT Project 

was to identify technology which can be applied 

to aircraft and to flight operations that will reduce 

the community noise generated by aircraft. The 

objective was to reduce noise by 10 dB, with 

flight operations contributing 2 dB to the total 

noise reduction. The element within the QAT 

Project that addressed the operational issues was

called Low Noise Flight Procedures (LNFP), and 

included the development of flight guidance, and 

supporting pilot and ATC procedures for low 

noise operations. The NASA NextGen Airspace 

Project conducts fundamental research supporting 

development of the Joint Planning and 

Development Office (JPDO) Next Generation Air 

Transportation System (NextGen). A key 

research focus area within the Airspace Project 

involves Airspace Super Density Operations 

(ASDO) at the major airport terminals. The 

efficiency and noise characteristics of arrivals and 

departures within the airport terminal airspace are 

primary concerns of ASDO.

The studies described in this paper involved 

the development and testing of operational 

procedures for conducting continuous descent

arrivals using a flight deck tool that computes and 

displays energy cues for pilots to use to fly the 

most optimal descent for the given conditions. 

Descents conducted with the use of this energy 

guidance are compared with those conducted 

using conventional aircraft guidance.

Background

RNAV Arrival Procedures

The Continuous Descent Arrival (CDA) has 

been identified as a beneficial method for 

operationally reducing community noise near 

airports. As the name implies, a CDA optimally 

consists of an uninterrupted descent through the 

terminal area for an arriving aircraft, without any 

level altitude segments. The CDA is designed to 

minimize level flight at low altitudes, which 

produces more noise than descending segments, 

due to the higher thrust setting required to 

maintain level flight. Also, the CDA design keeps 

the aircraft higher throughout most of the descent 

through the terminal area, which allows for 

increased noise attenuation (see Figure 1). The 

higher altitude, lower thrust characteristics of 

CDAs also contribute to potentially significant 

fuel savings.

Considerable research and operational testing 

of CDA procedures has been conducted over the 

past decade [1-3]. Several airports, such as 

Heathrow in London and Schiphol in Amsterdam,

have operational CDA procedures that are used 

mainly during night-time low traffic-density 

operations. Area navigation (RNAV) arrival 

procedures with CDA characteristics (called 

Optimized Profile Descents, or OPD) are being 

designed and tested at several airports in the 

United States. The FAA NextGen 

Implementation Plan includes RNAV with OPD 

as a key element of Trajectory Based Operations 

(TBO) in the future National Airspace System

(NAS).

There are major obstacles, however, that limit 

the ability to use CDAs on a regular basis, 

especially during high traffic-density periods.
3

One is the lack of flight guidance to properly 

manage thrust and drag when interrupted by ATC 

for spacing with other aircraft while flying a nearidle CDA. Another obstacle is the lack of 

operational techniques that can integrate current 

ATC procedures with CDA and OPD procedures. 

This is particularly important during high traffic 

density operations, when controllers rely on the 

ability to issue speed and routing changes for 

maintaining aircraft separation and spacing. 

These studies address both the flight guidance 

issues as well as procedures needed to conduct 

CDA descents. The following two sections 

expand on the issues of Flight Management 

Systems and Air Traffic Control, as they relate to 

the CDA procedures.

Flight Management Systems and RNAV

Procedures

Since the early 1980s, standard avionics on 

commercial transport aircraft have included 

systems for managing various aspects of the flight 

trajectory. A modern Flight Management System 

(FMS) allows pilots to plan the trajectory for an 

entire flight, and includes Vertical Navigation 

(VNAV) functions that can compute a 

performance-based vertical trajectory for the 

aircraft. The VNAV function also provides flight 

guidance to follow the computed trajectory, and 

thus could be used to follow a CDA trajectory. 

Reference 4 includes a report on a study where 

CDA procedures were demonstrated, using 

commercial FMS VNAV functions to conduct the 

descent. Limitations in both the basic 

functionality of VNAV as well as in pilot 

understanding of VNAV guidance, however, have

prevented widespread adoption of VNAV-based 

CDA procedures for operational use. Three main 

limitations associated with use of existing VNAV 

for CDA procedures are:

• The lack of a standard pre-defined lateral 

path that is continuous from top of descent 

to the runway, and can be used as the basis 

for a CDA trajectory,

• The lack of flexibility in operation of the 

VNAV function, that does not allow pilots 

to easily make speed changes during the 

descent, while maintaining the CDA 

vertical trajectory, and

• The lack of consistent operation between 

different versions of VNAV as 

implemented for different aircraft.

These VNAV limitations could make it 

difficult, if not impossible, for aircraft to conduct 

CDAs in busy terminal areas, where controllers 

typically rely on tactical procedures to space 

traffic. The first item above can be addressed by 

the design of RNAV arrival procedures. Many 

new RNAV arrivals are being designed to have 

ending locations that coincide with the Initial 

Approach Fixes (IAF) of the final approach 

procedures. This allows a continuous lateral path 

on which a CDA can be overlaid. The second 

item can be addressed through pilot/controller 

techniques, such as those developed for this study, 

and expanded pilot training using existing VNAV 

functionality. The third item requires adoption of 

new performance standards for FMS VNAV to 

ensure consistent interoperability between 

different systems. Enhanced guidance modes, 

such as the Energy Navigation (ENAV) concept 

described in this paper, can assist in alleviating 

the limitations of VNAV and provide consistent 

performance with minimal pilot training. 

Traffic Control and RNAV Procedures

CDA procedures using VNAV can be readily 

implemented in the terminal area for single 

aircraft operations. However, multiple aircraft 

following CDA procedures for landing at the 

same or parallel runways present a significant 

challenge for ATC. Typically, terminal area air 

traffic controllers (approach controllers) will 

utilize vectoring techniques in order to sequence 

arriving aircraft for landing, and to provide 

adequate lateral and vertical separation between 

aircraft. This requires controllers to make tactical 

changes to the aircraft heading and airspeed, in 

addition to using staggered altitude profiles, to

facilitate a safe and orderly flow of traffic to the 

runways. The fixed lateral routing needed for 
4

CDA descents is seldom used for busy terminal 

areas. On arrival segments that have defined 

lateral routing, controllers typically use speed 

control to achieve and maintain desired spacing 

intervals between aircraft. Aircraft flying 

continuous descent procedures have higher energy 

than those flying current-day procedures, mostly 

because of their higher altitude throughout the 

arrival. It may not be possible for an aircraft to 

dissipate this higher energy if the flying distance 

is shortened, thus making it difficult to comply 

with vectors that shorten the aircraft’s flight path.

To effectively utilize FMS-based CDA 

procedures while also maintaining separation in a 

high traffic density terminal area, controllers must 

be able to specify changes in the lateral path, 

airspeed, and altitude of all aircraft. To be usable, 

CDA procedures must be flexible enough to 

accommodate these tactical clearances. 

Procedures for this experiment were developed to 

take these considerations into account.

Energy Navigation Concept

The ENAV concept combines dynamic, 

energy-based reference trajectory generation with 

real-time flight guidance and energy-based auto 

throttle commands. Originally developed at 

NASA Langley as a low-altitude terminal area 

tool during the QAT Project in the early 2000s, 

the concept has been expanded to include full 

arrival operations in support of the NASA 

NextGen Airspace Systems Program.

Low Noise Guidance Flight Deck Tool

The original Low Noise Guidance (LNG)

concept was developed specifically to reduce 

community noise and engine emissions during 

terminal arrival situations. A prototype LNG 

flight deck tool was developed to construct high 

energy, low thrust arrival trajectories under the 

dynamic conditions of high density terminal 

arrival situations [4]. Depiction of energy state 

relative to the reference trajectory and display of 

the location of events needed to manage the 

energy of the aircraft (flap and landing gear 

deployment) were key elements of LNG. 

The primary challenge in developing LNG 

was the dynamic nature of real-world terminal 

arrival situations and the need for continuous 

adjustment to the reference trajectory based on 

speed, altitude and lateral path changes from ATC 

as well as fluctuating atmospheric conditions 

encountered during flight. To accommodate these 

dynamic conditions, the state of the aircraft 

relative to the reference trajectory, the current 

atmospheric winds and temperatures, as well as 

the pilot selected target speed and altitude limits 

were continuously monitored by the LNG 

algorithm. A new reference trajectory was 

generated when any of these parameters exceeded 

tolerances from the expected or modeled 

conditions. Normal flight plan changes, such as 

cost index, routing, cruise altitude and desired 

speed schedules, were handled by standard 

VNAV with the resulting trajectory becoming the 

new basic trajectory for LNG.

Expanded Energy Guidance Tool

Following completion of the piloted 

simulation described in this report, the LNG tool 

was expanded to support the extended cruise to 

landing operations envisioned for the NextGen 

ASDO concepts. Additional logic was added to 

the LNG tool in order to handle the optimized 

descents from cruise altitude as well as arbitrary 

altitude and airspeed crossing constraints at 

waypoints along the arrival. During this time 

period, NASA was also supporting government, 

industry and academia efforts at developing and 

testing CDA operations and dedicated RNAV 

arrival procedures with Optimized Profile 

Descents [3]. Lessons learned in these tests were 

incorporated in the new ENAV algorithm and 

research Flight Management System used for

NASA testing.

ENAV Software

The original LNG and extended ENAV 

software were designed to operate as complete 

and stand-alone flight guidance modules. A 

single procedure call to the software provided all 

inputs required by the guidance and returned all 

necessary guidance signals needed for both flight 
5

displays and the airplane auto-flight systems. 

Details of the ENAV software are contained in 

Appendix A.

Related studies

The experiments documented in this report 

make use of the LNG and ENAV flight deck 

tools. It is useful to recall a previous study, which 

examined the use of an early version of this 

energy-based guidance to maintain the ideal CDA 

profile. This study provided researchers with an 

initial look at the benefit of and pilot acceptability 

issues associated with ENAV for conducting 

CDA operations. 

The study [4] paired individual subject pilots 

(acting as the flying pilot) with a researcher 

(acting as the non-flying pilot). Because this 

pairing did not truly represent an airline crew, a 

full set of crew procedures was not developed or 

evaluated. However, a charted procedure was 

developed and used, and the pilots were given a 

set of crew procedures that described how the 

LNG algorithm could be used to conduct the 

CDA descent.

The LNG algorithm was designed and tested 

as a VNAV sub-mode, in a simulated subsonic jet 

transport aircraft with advanced all-electronic 

displays. The subject pilots in this experiment 

were able to use the LNG guidance to effectively 

conduct low-noise arrivals, with no major 

problems conducting the descent profiles as 

outlined in the procedures they were given. This 

included the test runs where they were required to 

make route and speed changes. Compared to the 

baseline runs, noise under the flight path was 

reduced by at least 2 decibels at distances from 3 

nm out to 17.5 nm from the runway, with peak 

reductions of 8.5 decibels at about 10.5 nm. Fuel 

consumption was also reduced by about 17% for 

the LNG conditions compared to baseline runs for 

the same flight distance. 

A standard CDA procedure, in which the 

pilots used charted altitude crossing conditions 

with extended glideslope on final approach (a 

CDA using conventional guidance), also proved 

effective in reducing noise and fuel consumption. 

Without the benefit of continuous VNAV 

guidance, however, the pilots were not able to 

consistently achieve continuous descents. The 

level-altitude segments prior to glideslope 

intercept resulted in additional required thrust, 

and subsequently reduced the potential noise 

benefit. Peak noise reductions of 6.5 decibels and 

fuel savings of about 8% were achieved with the 

standard CDA procedure compared to the 

baseline runs.

A subsequent study [5] addressed the issue of 

conducting CDAs using only conventional 

guidance technology (VNAV). Although the 

CDA profile cannot be maintained exactly using 

only conventional VNAV, the profile can be 

approximated, and could still provide a significant 

noise advantage over current-day procedures. In 

recent years there has been much interest in the 

development of flexible CDA procedures that 

could be used in terminal areas with moderately 

heavy traffic levels. The degree to which a CDA 

profile can be maintained is dependent partially 

on how well the charted procedures are designed, 

and partially on how much ATC lets the aircraft 

stay on the profile. The charted and crew 

procedures used for this experiment were 

developed with these factors in mind.

Results from the experiment indicated that, 

with appropriate charts and crew procedures, 

CDA descents could be flown with VNAV 

guidance and provide fuel savings and noise 

reduction benefits over current-day operations, 

even when interrupted by ATC with instructions 

that took the aircraft off of the VNAV path, 

laterally, vertically or both. The uninterrupted 

descent resulted in the most optimal profile, 

which translated to the lowest perceived noise 

levels and lowest amount of fuel burned. The

potential fuel savings could be significant, on the 

order of 10% to 20% less fuel used in the descent 

segment. These benefits were reduced as the 

interruptions from ATC increased and took the 

aircraft off its optimal descent path, but all the 

CDA runs were quieter and more fuel efficient 

than those with current-day (non-CDA) 

procedures. The workload and pilot acceptability 
6

associated with conducting the descents were 

rated comparable to current-day procedures.

At the conclusion of the test runs, pilots were 

introduced to the concept of energy-based 

guidance for conducting CDA descents. Reactions 

were very positive from the subject pilots, with all 

of them indicating this type of guidance could 

make CDA descents much easier to conduct, with 

the only cause for concern being to ensure that the 

aircraft speed was above the flap extension speed 

prior to reaching the flap extension point that is 

generated and displayed on the ND by the lownoise algorithm.

The experiments described in this document 

continue the work from these two previous 

studies, incorporating updated versions of charts, 

training, and crew procedures, along with the 

latest modifications to the LNG and ENAV 

algorithms. The studies assessed the effectiveness 

and potential benefits of both conventional 

VNAV and advanced energy guidance 

technologies for conducting CDA procedures.

Experiment Descriptions

This report presents the results of two separate 

but related experiments conducted by the same 

research team. The first was a piloted simulation 

experiment conducted in 2005 as part of the 

NASA QAT Project. The second was a nonpiloted batch simulation of the extended ENAV 

concept conducted in 2010 as part of the NASA 

NextGen Airspace Systems Program. Although 

separated by several years, the studies are linked 

and focused on different aspects of the same 

concept.

Objectives and Approach

The piloted simulation experiment described 

in this report was the culmination of a series of 

experiments evaluating low noise flight guidance 

and procedures, focused primarily on qualitative 

pilot assessment (references 4 and 5). The batch 

simulation experiment was designed to provide a 

more quantitative assessment of the ENAV 

guidance concept. The following sections detail 

the specific objectives and approach taken for 

each experiment.

Piloted Simulation Experiment

The main objective of the piloted simulation

experiment was to assess the effectiveness of 

using LNG, in comparison with conventional 

VNAV, for conducting CDA procedures. 

Secondary experiment objectives were to identify 

strengths, weaknesses, and potential sources of 

confusion in using the LNG tool and CDA 

procedures.

The first objective, assessing the effectiveness 

of LNG and VNAV for conducting CDAs, 

required both objective and subjective data to 

determine whether or not the pilots could conduct 

a descent using the CDA procedures with 

acceptable levels of perceived workload. The 

effectiveness of the guidance would be 

determined by reduction of perceived noise levels 

and fuel use. Subjective data from subject pilots 

would give supporting data on workload and 

acceptability of the guidance and CDA 

procedures.

The second objective, identification of 

strengths, weaknesses, and potential sources of 

confusion, is important to the development of 

clear and concise CDA and LNG procedures, 

since the CDA differs from current-day 

procedures in subtle but important aspects. The 

primary source of data for determining this was 

from pilot comments and observations of how the 

crews conducted the descent.

To achieve these experiment objectives, a 

series of four simulator test runs was developed, 

based on a scenario that consisted of a descent 

and approach into a moderately busy terminal 

area, Louisville-Standiford International Airport 

(SDF). Each of the test runs used for this study 

had slight variations that represented situations 

typical of what an aircraft might encounter in that 

environment. The subject pilots were given 

instructions and information on how to conduct 

the CDA descent. Following practice time in the 

simulator, the pilots completed the test runs, 
7

during which objective and subjective data were 

recorded.

Benefits Analysis Batch Study

The primary objective of the batch study was

to further evaluate the benefits and efficiency 

gains of ENAV compared to conventional VNAV 

during high-density terminal arrival operations. 

These benefits were measured in terms of fuel 

burn and community noise levels. Secondary 

objectives were to evaluate operational 

enhancements afforded by ENAV, including 

reduced dispersion of final approach stabilization 

altitude, elimination of high-energy landing 

conditions, and potential reduction in missedapproach conditions.

To accomplish the batch study, results of the 

piloted simulation were evaluated and a model of 

pilot behavior was developed. Details of the pilot 

model development are included in the 

Experiment Design section of this report. This 

pilot model allowed operation of the simulated 

airplane without the need for pilot test subjects. 

Test conditions were then designed to exercise 

ENAV and VNAV under a variety of arrival 

conditions. The Louisville scenarios developed 

for the piloted simulation tests were used to 

validate the behavior of the airplane simulation 

model, pilot model and FMS logic used by the 

batch airplane simulation. Additional Louisville 

scenarios were then tested to explore wind 

conditions and ATC speed changes not included 

in the piloted simulation. Finally, more extensive 

arrival situations were tested using a simulated 

Atlanta Hartsfield-Jackson International airport 

(ATL) terminal environment. The Atlanta 

scenarios involved full arrivals from high-altitude 

cruise to landing using an RNAV/OPD arrival 

developed for a companion study conducted at

NASA Ames to explore Controller Managed 

Spacing research topics. The RNAV procedure 

represents a state of the art current-day (circa 

2008) procedure designed to take maximum 

advantage of FMS capabilities while observing 

the airspace restrictions of the high-density 

Atlanta environment. The batch study was 

conducted several years after the conclusion of 

the piloted simulation study and utilized an 

enhanced ENAV algorithm designed to 

accommodate full descent from cruise altitude 

with arbitrary altitude and speed constraints at 

waypoints on the arrival.

Simulation Facilities

The two studies utilized different research 

facilities to accomplish the research objectives. 

The piloted simulation experiment was conducted 

in a high-fidelity cockpit simulator in order to

properly assess flight crew workload and 

procedures. The batch simulation experiment was 

conducted using multiple simultaneous airplane 

simulations without the need for a full flight deck 

environment.

Piloted Flight Simulator

The facility used for the piloted simulation 

study was the NASA Langley Research Center 

Research Flight Deck (RFD) simulator (Figure 2). 

The RFD simulator cockpit is an engineering cab 

designed to represent the conventional flight deck 

of an advanced current-day commercial airplane. 

The simulation model used in the RFD consisted 

of full six degree of freedom equations of motion 

with high-fidelity aerodynamic and propulsion 

models of a current twin-engine jet transport 

aircraft. The cab is populated with flight 

instrumentation, including the overhead 

subsystems panels, center-aisle throttle quadrant, 

and full Electronic Flight Instrumentation System 

(EFIS) displays. The Primary Flight Display 

(PFD), Navigation Display (ND), and Engine 

Indicating and Crew Alerting System (EICAS) 

displays were representative of current display 

formats used in modern Boeing-style aircraft. 

The flight management computer was a 

Honeywell FMC-PIP used in Boeing 757/767 

airplanes. The Mode Control Panel (MCP) was 

also from the Boeing 757 airplane. The cockpit 

contains a “Panorama” visual out-the-window 

display system. This system provides a 200 

degree by 40 degree visual out-the-window 

display to add realism to piloted experiments.

During these simulation tests, cockpit 
8

modifications included a non-standard control 

panel for the ND, and format modifications to the 

baseline ND and PFD. The non-standard ND 

control panel was located on the aisle stand just 

aft of the throttles. A description of the ND and 

PFD is contained in the Flight Manual Bulletin in 

Appendix B.

Air Traffic Operations Laboratory

The benefits analysis batch study utilized the 

NASA Air Traffic Operations Laboratory 

(ATOL) running multiple airplane simulations 

communicating via data-link with scripted ATC 

commands. Pilot modeling was used to operate 

the airplane simulations and human intervention 

was not required during the experiment data runs. 

A description of the basic design and capabilities 

of this simulation is provided in [6].

Aircraft Model. The aircraft model used in 

the batch study was the Aircraft Simulator for 

Traffic Operations Research (ASTOR). This is a 

medium-fidelity simulation of a modern jet 

transport aircraft utilizing the same core Six 

Degree-Of-Freedom simulation model as used in 

the piloted simulation cab described previously. 

The aerodynamic and engine models, however, 

were simplified using trimmed drag polars and 

tables of steady-state engine parameters similar to 

those used by flight management systems for 

trajectory predictions.

Flight Management System. The Research 

Prototype Flight Management System (RPFMS) 

used by the ASTORs was designed and developed 

by NASA to provide a research platform for 

studying FMS-related air traffic management 

topics. The core trajectory prediction and 

guidance algorithms evolved from the flight 

guidance software of the NASA Transport 

Systems Research Vehicle [7] and a PC-based 

simulation [8]. The trajectory prediction and 

VNAV guidance logic has been modified to 

replicate the behavior of the commercial FMS in 

the piloted simulation cab during the descent 

flight phase.

Pilot Modeling. The Pilot Model (PM) used 

by the ASTORs was designed to follow rules to 

support batch mode activities without human 

interactions. The PM has a class structure which 

consists of Sensors, Rules, and Actions objects. 

These objects are developed to sense flight 

conditions, recognize alerts and advisories,

generate needed actions, and execute these 

actions, respectively, to maintain normal flight 

operations. The specific PM rules and actions 

used in this study are described in the Experiment 

Design section of this report.

RNAV Procedures and Charts

Charted RNAV arrival procedures were 

developed for each of the simulation studies 

described in this report.

Louisville Arrival

The charted arrival used for the piloted 

simulation study was called the SILENT CDA 

RNAV Arrival (Figure 3), and was based on the 

then existing CHERI Two Standard Terminal 

Arrival (STAR) into Louisville. The CHERI Two 

Arrival brings aircraft arriving from the West on a 

direct course to the IIU VOR, which is a radio 

navigational aid located southeast of the airport. 

Normally, aircraft landing to the South are 

vectored towards the airport, onto a right 

downwind or base leg. The SILENT CDA RNAV 

STAR included transitions for arrivals from the 

South (New Hope, EWO), West (CHERI), and 

North (DANNY).

Crossing restrictions of 13,000 ft and 240 

knots were applied at CHERI and DANNY for 

handoff from Enroute Center airspace. The next 

hard altitude restriction was applied at CHRCL, 

the Final Approach Fix (FAF) for runway 17R. A 

speed restriction of 190 knots was applied at the 

SILNT waypoint to force aircraft to slow and 

deploy initial approach flaps in preparation for 

intercept of the final approach leg at SPYRS. An 

altitude restriction of at or above 6,000 feet at 

SILNT was used to avoid low altitude segments 

prior to final approach. SPYRS, BLGRS and 

CHRCL were also waypoints on the Instrument 

Landing System (ILS) 17R Approach, and the 
9

FMS would link the Arrival and Approach into a 

continuous path when both were selected by the 

flight crew.

Atlanta Arrivals

The benefits analysis batch study extended the 

evaluation of ENAV to the more complex, high 

density terminal environments being tested in the 

NASA NextGen ASDO research focus area. A 

primary terminal of interest in ASDO is the 

Atlanta Metroplex. A recent simulation study of 

Controller Managed Spacing concepts developed 

candidate RNAV arrival procedures into Atlanta. 

These arrival procedures provided East and West 

arrivals merging to a common runway with 

altitude and speed crossing constraints at 

waypoints that are consistent with current-day 

Atlanta airspace design. The crossing constraints 

permitted an idle descent from cruise into the 

terminal area, followed by partial power 

continuous descent segments to final approach. 

These arrivals proved acceptable to air traffic 

controllers during human-in-the-loop testing, and 

represent the current state of the art in RNAV 

procedure design. Figure 4 illustrates these 

RNAV arrivals. The altitude and speed crossing 

restrictions for these arrivals are shown in Table 

1.

The crossing restrictions at NOFIV from the 

West and BYRDS from the East provided the 

bottom of descent anchors for the idle descents 

from cruise. The routes merged at HAVAD, 

indicated by the shaded section of the table, and 

followed a fairly rigid vertical profile to final 

approach. A custom waypoint, named ELLLE, 

was inserted on final approach to allow a 90 

degree turn from base to final. All other 

waypoints were taken from existing arrival and 

approach procedures at Atlanta.

The initial condition points, IC_SFO and 

IC_ORF, provided a common route distance of 

150 nm for the West and East arrivals. These 

points represented the initial cruise locations of 

the aircraft flying the arrivals and were not part of 

the RNAV Arrival procedures. The aircraft 

would initialize at these locations and proceed 

direct to the first waypoint on the Arrival 

(CALCO or ODF).

ATC Environment

The two experiments required different levels 

of ATC simulation. The piloted study required 

live interaction with an air traffic controller to 

provide a realistic environment and proper 

communication workload. The non-piloted batch 

simulation needed a way of mimicking ATC 

speed clearances without human interaction. The 

following sections describe the methods used to 

accomplish these ATC requirements.

Piloted Simulation Study

A PC-based Air Traffic Control simulation 

was used to help provide realism to the subject 

pilots’ experience in the simulator. The simulation 

used was called the Multi-Aircraft Control 

System (MACS), a program developed in-house 

at NASA Ames Research Center for internal 

NASA use. An air traffic controller operating the 

MACS controller station watched the ground 

tracks of all the simulated air traffic (including the 

subject crew’s aircraft) on a PC display. All other 

air traffic (excepting the subject crew’s aircraft) 

were pre-recorded and played back during the 

runs. An example of the controller display is 

shown in figure 5.

Communications between the other aircraft 

and the controller were scripted and recorded on a 

desktop computer as audio files that could be 

played back at the appropriate times by the 

controller. The recorded communications were all 

natural voice using standard terminology. The 

controller communications were recorded by the 

controllers participating in the study. 

Communications from other aircraft were 

recorded by a variety of other people with piloting 

backgrounds. The controller also had scripts for 

the communications with the subject pilots, but 

had some flexibility for real-time requests or 

questions from the pilots. The subject pilots were 

able to hear all the pre-recorded communications, 

but talked directly with the controller, enabling 

him to issue clearances and vectors to them in 
10

real-time and adjust as needed for each particular 

situation. Position data for the other aircraft were 

also transmitted to the simulator, so the subject 

pilots could see them as traffic targets on their 

ND. 

Benefits Analysis Batch Study

The benefits analysis batch study was 

designed to run without human pilots, and there 

was no need to include an ATC simulation. There 

was, however, the need to provide speed changes 

to the aircraft at specific locations along the 

arrival in order to mimic ATC speed clearances. 

This was accomplished by using the data link 

capability in ATOL along with custom pilot 

model rules. A series of free text data link 

messages would be sent to the aircraft with a 

distance to FAF and a commanded calibrated 

airspeed. The pilot model software would store 

these clearances, and then open the MCP speed 

window and dial the appropriate speed at the 

designated distance to FAF. This provided a 

repeatable way of mimicking a controllerinstructed speed change at a specific location on 

the arrival. The pilot model would then close the 

MCP speed window just prior to the FAF to allow 

the airplane to decelerate and deploy flaps and 

gear for landing using the programmed FMS 

schedule.

Experiment Design

This section describes the test conditions and 

primary test variables for each experiment.

Piloted Simulation Test Conditions

Four different CDA test conditions were 

developed for this experiment: a nominal 

condition, where the CDA was flown without 

interruptions from ATC and three variations with 

different interruptions to the nominal path. Two 

versions of each of the four scenarios were 

developed: one arriving at the waypoint DANNY 

(north of the airport), and the other arriving at the 

waypoint CHERI (west of the airport). The four 

test conditions are described in this section, and 

are referred to with the following shorthand 

notation: a) Nominal for the uninterrupted CDA; 

b) Slow for the condition where the descent was 

interrupted with speed reductions from ATC; c) 

Vector for the condition where the descent was 

interrupted with an off-route vector in addition to 

the speed reductions; and d) Stress for the descent 

that was shortened significantly towards the end 

of the run. Examples of lateral paths for the four 

different run conditions are shown in Figure 6.

Nominal. The simplest CDA test run was an 

uninterrupted descent, flown with no changes to 

the charted procedure. After being cleared for the 

CDA, no further instructions from ATC (other 

than frequency changes) were issued until the 

approach clearance was issued on the base leg. 

The resulting descent would be near-optimal 

CDA. When flown properly, the vertical profile 

for this run had no level segments.

Slow. The second CDA test run began the 

same as a Nominal CDA descent, but was 

interrupted by ATC with speed changes that 

required the crew to come off of the optimal 

descent profile to some extent. A speed reduction 

from 240 knots to 210 knots was issued just after 

passing the CHERI or DANNY waypoint. A 

second speed change to 180 knots was issued just 

prior to the SILNT waypoint. Since the 

uninterrupted CDA was designed with faster 

speeds at near idle thrust, it was expected that this 

test run would result in a descent that was less 

optimal.

Vector. The Vector test run began the same as 

the Nominal and Slow CDA runs, but included 

more interruptions from ATC. In this run, the 

aircraft was taken off the CDA with a vector to 

the West just after the speed reduction to 210 

knots as shown by Vector Routes in Figure 6. 

The resulting lateral path was not as direct as in 

the Nominal or Slow runs. The vertical trajectory 

did not require any level segments if properly 

flown, but had two segments that were very 

shallow. It was expected that this test run would 

result in a descent that was less optimal than 

either the Nominal or Slow test runs. 

Stress. The fourth CDA test run began the 
11

same as the Nominal CDA test run, but was 

interrupted by ATC on the base leg with a 

heading vector that shortened the path distance (as 

shown in figure 6). It was expected that this test 

run would result in a high energy state and force 

timely pilot action to recover the vertical profile. 

The intent of this condition was to stress both the 

vertical guidance as well as pilot workload.

All piloted simulation runs were conducted 

using a nominal, vertically-varying wind field 

representative of typical winds in the Louisville 

area (Table 3). The same winds were used in all 

piloted simulation runs.

Batch Simulation Test Conditions

The fuel and noise benefits observed in the 

piloted simulation study were obtained using a 

single aircraft type, flown at the same weight, 

using the same atmospheric conditions along the 

same routes. The same speed restrictions and 

controller vectors were also issued to the flight 

crews at common locations on the route. These 

parameters were held constant in order to isolate 

the effects of flight crew performance. The batch 

study, however, exercised some of these 

previously controlled variables as independent 

variables with pilot performance and actions held 

constant.

The batch simulation was designed as an 

observational experiment rather than a 

randomized, Monte-Carlo style test. The values 

for the independent variables were chosen to 

cover the full operational range expected to be 

encountered in order to excite and/or uncover 

deficiencies or strengths in the ENAV guidance. 

A randomized design, while permitting more 

rigorous statistical analysis, would not ensure the 

full range of conditions would be exercised. 

Aircraft Weight. The landing weight of an 

aircraft has a direct and proportional affect on the 

flap deployment and final approach speeds for the 

same aircraft type. The heavier the aircraft, the 

higher the speeds will be for flap deployment and 

for landing. A speed restriction at a waypoint or 

an ATC-instructed speed reduction may require 

different flap settings depending on the current 

aircraft weight. This change in flap setting will 

result in different drag and deceleration 

characteristics and different thrust requirements 

along a common vertical path. The fuel usage of 

a particular aircraft will therefore be dependent on 

weight. The Louisville scenarios used 17 weights 

providing a one knot difference in approach speed 

between each weight covering the entire weight 

range of the airplane. For the Atlanta scenarios, 

four weights were chosen that evenly covered the 

approach speed variation of the airplane.

Wind Conditions. Another major 

independent variable in the benefits study was 

atmospheric winds aloft. The nominal vertical 

wind profile shown in Table 2 was used as the 

baseline winds for the Louisville scenarios. Two 

additional wind profiles were tested, using double 

or half the wind magnitudes specified at the 

altitudes of Table 2. The wind directions 

remained the same for all three wind profiles.

The wind conditions for the Atlanta scenarios 

used a different approach. The historic data for 

winds aloft for every day over a two year period 

were collected from archived NOAA data from 

2008 and 2009. The wind profiles were extracted 

from the Rapid Update Cycle (RUC) 20 km grid 

data for the location corresponding to the 

Peachtree National Weather Service (NWS) site 

(location shown in Figure 4). This location is 

used by the NWS to launch Radiosonde weather 

balloons on a twice daily basis. The extracted 

RUC data was found to be a good match to the 

balloon data and provided a complete altitude 

profile for use in the simulation. A profile of 

wind speed and direction versus pressure altitude 

was created using each pressure point in the RUC 

data for the time corresponding to the morning 

balloon launch. The profiles were then analyzed 

to determine the general characteristics of the 

wind, and the suitability of the profile for use in 

the experiment. The following were the 

characteristics deemed significant:

• Easterly winds at 1,000 ft pressure altitude 

(headwind component near touchdown).
12

• Easterly winds at 3,000 ft pressure altitude 

(headwind component near final approach 

fix).

• Northerly winds at 5,000 ft pressure altitude 

(crosswind component near final approach 

intercept).

The wind profiles were grouped according to 

these characteristics using the criteria in Table 3.

Individual wind profiles for each of the 

available 679 days were created for testing in the 

batch simulation. In addition, average profiles 

were created for the All, Headwind, Tailwind, 

Southerly Crosswind and Northerly Crosswind

data shown in Table 3. Plots of the resulting 

averaged profiles are shown in Figure 7. 

The final independent variable in the benefits

analysis batch study was speed changes issued to 

the aircraft. Tactical speed changes, typically 

required for spacing and separation from other 

aircraft, interrupt the descent of an aircraft and 

force the FMS guidance to adapt to the new 

speed. Flight crews were instructed to respond to 

tactical speed interruptions using the speed 

window on the aircraft MCP. Conventional 

VNAV will revert to a mode called “Speed 

Intervention” in order to honor the speed entered 

by the flight crew in the MCP. Depending on 

where during the descent this speed is entered, the 

VNAV system will use different control strategies 

to manage the speed. The system used in the 

NASA simulator, a commercial FMS unit, would 

normally use pitch control to fly the speed and 

require pilot intervention with thrust and/or drag 

to follow the aircraft vertical path. Once the 

aircraft was on the approach portion of the route 

(inside the initial approach fix), the VNAV 

system reverted to flying the vertical path with 

pitch and would use thrust to manage the speed. 

The location of the tactical speed interruption can 

thus have an effect on the efficiency of the flight. 

The magnitude of the speed change can also 

affect the need for early (or late) deployment of 

aircraft flaps. Both the magnitude and location of 

tactical speed changes were varied in the benefits 

analysis batch study.

Piloted Simulation Crew Procedures 

Procedures for the crew to use as a guideline 

in conducting the CDA descent were included in a 

Flight Manual Bulletin (FMB). The FMB 

described the RNAV CDA procedure and 

included instructions for pilots on how to manage 

their speed, altitude, and lateral route while 

conducting the CDA (see Appendix B). A 

subsection of the FMB provided specific 

instructions on how to use LNG to conduct the 

CDA procedure. For all test runs, the pilots were 

instructed to maintain their normal operating 

procedures as much as possible, except for the 

instructions in the FMB that were specific to 

conducting the CDA and using LNG.

The pilots were instructed to remain in VNAV 

and Lateral Navigation (LNAV) as much as 

possible. This included the times when ATC 

issued changes in lateral route or speed. The 

speed changes were to be made using the SPEED 

INTERVENE function, thereby allowing the 

aircraft to remain in VNAV guidance. Off-route 

vectors could be executed using Heading mode, 

but LNAV was to be engaged as soon as possible 

after being vectored back on course. Pilots were 

cautioned that using SPEED INTERVENE would 

affect how well the aircraft maintained the 

vertical path, and they needed to be aware of their 

position relative to the vertical path. This meant 

that pilots needed to maintain awareness of the 

aircraft flight modes as much as possible.

Batch Simulation Pilot Modeling

The batch simulation used pilot modeling 

based on rules developed from the crew 

procedures of the piloted simulation and observed 

behavior of the flight crews from that study. 

Table 4 summarizes the pilot actions that were 

modeled in the batch simulations.

Flap extension was based on current flap 

position, current airspeed, target airspeed and 

distance to the Final Approach Fix (FAF), as 

shown in table 5. The pilot model would always 

extend flaps to the next setting when the airspeed 

approached a prescribed minimum tolerance value 
13

above the minimum speed allowed for the current 

flap setting. For current flaps of 0, 1 and 20, this 

minimum tolerance value was 5 knots. For flaps 

15 and 25, the pilots tended to deploy flaps 

earlier, and the tolerances were set to 12 and 14 

knots, respectively. The need for additional 

deceleration on final approach required pilot 

model criteria for flap deployment at 0, 1 and 5 

degrees based on distance to FAF and speed 

above the target speed.

Deployment of landing gear was based on 

criteria developed for the flight tests of CDA 

procedures conducted at Louisville, as described 

in reference 3. Normal criteria, based on 

intercepting the ILS glideslope, were not 

applicable to descents that followed the glideslope 

for a considerable distance. Instead, the distance 

to FAF was used as a guide for deployment of 

landing gear. The value found acceptable during 

CDA flight trials was deployment of gear at one 

nautical mile before the FAF. This criterion was 

used for the pilot model in the batch simulation. 

The pilots in the piloted simulation study were 

told to deploy gear prior to the FAF without a 

numerical value provided.

Piloted Simulation Test Matrix

Since workload and acceptability results were 

primary metrics for the piloted simulation, it was 

important that the crews be paired from the same 

company and aircraft type, to maintain continuity 

in their normal operating procedures. It was 

recognized that this constraint would limit the 

time that the subject pilots from each crew could 

both be expected to be available to participate in 

the test. It was also considered necessary to 

complete the test within a period of approximately 

nine hours, to minimize any negative effects due 

to fatigue from an excessively long day. To meet 

these constraints, the test was structured such that 

each crew could complete their run matrix in one 

9-hour day. This made it difficult to completely 

counterbalance the test matrix by the control 

variables (run condition, pilot flying, type of 

guidance used), as this would have produced too 

many runs for the intended time frame of one 9-

hour day per crew. 

Another experiment design concern that arose 

was that it was unrealistic to expect the pilots to 

be able to switch between guidance modes (LNG 

vs. VNAV) from one run to the next, without any 

adverse effects on their performance. This backand-forth switching could potentially happen from 

one run to the next if the conditions were either 

counterbalanced or randomized. However, some 

level of counterbalancing was considered 

necessary. To address the guidance switching 

issue, it was decided that the run conditions for 

each crew needed to be grouped by type of 

guidance used (that is, all the LNG runs together, 

and all the VNAV runs together); every crew 

would complete two sets of runs, one each with 

the two types of guidance.

If each pilot was allowed to fly one run as 

Flying Pilot (FP) and one as Non-Flying Pilot 

(NFP), the number of runs required was sixteen (4 

conditions times 2 guidance modes times 2 

pilots). This was more than it was felt could be 

accomplished in one 9-hour day. Since normally 

either pilot would fly any given arrival, the two 

pilots were considered equivalent from the 

standpoint of performance (noise, fuel, trajectory 

flown) results. This allowed the number of runs 

per crew to be divided in half, resulting in eight 

runs, which could easily be completed in one day, 

However, if both pilots flew some of the 

conditions as FP and some as NFP they could 

both provide ratings on workload and 

acceptability, which could all be combined for 

analysis, essentially doubling the pool of 

subjective data obtained. 

A final issue of concern was the fact that 

conducting eight runs based on the same initial 

conditions might soon result in boredom, since 

they would know what to expect at the start of 

each run. To help mitigate boredom that might 

result from conducting the same approach so 

many times in a row, the arrival transitions were 

designed to initiate from two different directions, 

which were considered equivalent in terms of the 

CDA condition that was being investigated. 

The resulting combinations of guidance, 

arrival transition, and flying pilot, and numbering 
14

scheme used to identify each of the conditions is 

shown in Table 6. For example, conditions 1 and 

2 are both VNAV Nominal runs, but one is flown 

by the Captain and one by the First Officer, and 

one initiates on the DANNY Transition and one 

on the CHERI Transition. These two conditions 

are considered equivalent, and both represent the 

VNAV Nominal condition.

Each crew flew half of conditions 1-12, and 

either 13a and 14a, or 13b and 14b, for a total of 

eight test runs. The runs were selected such that 

for every crew, approximately half the runs were 

flown by each pilot. Because the guidance 

conditions were grouped together, this variable 

was balanced from one crew to the next, so that 

half the crews flew the VNAV group first, 

followed by the LNG group, and the other half 

flew the LNG group first, followed by the VNAV 

group. Within each of these groups, the four runs 

were randomized. Table 7 shows the order in 

which the test runs were conducted.

Subject Pilots

Twenty-four subject pilots were used for this 

study and were recruited as crews of two (one 

Captain and one First Officer), both from the 

same airline. All the subject pilots were required 

to be currently flying Boeing-style EFIS-equipped 

aircraft, such as B747-400 or B777, to minimize 

the amount of training time needed for this study. 

The resulting eight crews were each assigned a 

day to participate in the study. A single day was 

required for training and completion of the test 

matrix of runs.

Generally, the subject pilots recruited for this 

experiment had a great deal of experience. The 

subject crews represented four different airlines. 

The mean total flying hours for the pilots was 

over 10,000 hours, with a low of 3,300 hours and 

a high of over 20,000 hours. Most of the subject 

pilots had greater than 1,000 hours in either the 

B747-400 or B777 type aircraft. Also, when they 

participated in the study most had more than 100 

flying hours in the previous 90 days (only four 

indicated they had fewer than 100 flying hours in 

the previous 90 days).

Batch Simulation Test Matrix

The benefits analysis batch simulation focused 

on the effects of aircraft weight, atmospheric 

wind, and simulated ATC-required speed 

variations during the arrival. The Nominal and 

Slow CDA procedures from the piloted simulation 

study were replicated in the batch simulation in 

order to compare and validate the batch 

simulation with the piloted simulation. The same 

routes were then flown with variations in aircraft 

weight, various wind conditions, and changes in 

the location and magnitude of ATC-directed 

speed changes. The Atlanta routes were flown 

using the same pilot procedures as developed for 

the Louisville scenarios, except using the Atlanta 

RNAV arrivals.

The test matrix for the Louisville scenarios is 

shown in Table 8. Each guidance mode (ENAV 

and VNAV) was flown on each route using 3 

wind conditions, 7 combinations of speed 

schedules, and 17 different weights for a total of 

1428 individual runs. The shaded values in the 

table indicate the test conditions used in the 

piloted simulation. The magnitude of the nominal 

winds was halved and doubled to examine the 

effect of winds on the guidance. The ATC speeds 

represented the range of speed variations that 

could be expected from ATC-directed speed 

changes along the route. Nominal speed was the 

uninterrupted case where the airplane flew 

charted speeds as programmed in the FMS. The 

remaining speeds were the ATC-directed speed 

changes issued at the same locations on the routes 

as used in the piloted simulation. Each speed 

combination represented an operationally realistic 

situation, covering the speed range available for 

jet transport operations. Finally, 17 weights were 

chosen to provide a complete range of final 

approach reference speeds (Vref) for the simulated 

airplane type. Each weight value was chosen to 

provide a one knot change in Vref.

The test matrix for the Atlanta scenarios was 

designed to exercise ENAV in a wide variety of 

realistic atmospheric wind conditions with aircraft 

weights and ATC speed conditions spanning the 

full capability of the aircraft. The test matrix was 
15

divided into two main sections. The first section 

was designed to explore the impact of recorded 

wind variations on the guidance for uninterrupted 

arrivals using charted speeds and altitude 

restrictions as programmed for the RNAV arrivals 

in the FMS. For these scenarios, the archived 

winds from the Peachtree weather station location 

for every day in 2008 and 2009 were used. The 

only days not used were those with tailwinds on 

final approach of greater than 10 knots. The 

resulting matrix consisted of 679 individual wind 

scenarios. For each wind scenario, 16 aircraft 

were flown, distributed between guidance mode, 

arrival route, and weights, as shown in Table 9. 

This resulted in a total of 10,864 individual runs. 

The second section of the Atlanta evaluation 

was designed to explore the impact of speed 

interruptions during the arrival, as might occur 

from ATC speed changes for traffic spacing. 

Table 10 shows the test matrix variables used for 

this evaluation.

Three nominal locations were chosen where 

speed changes would typically be issued by ATC. 

These locations were further modified by shifting 

them early or late by 2 nm, resulting in three sets 

of speed change locations. Six combinations of 

speed change values were then defined that would 

bracket the available speed capability of the 

aircraft. The winds used in these scenarios were 

the averages for the headwind, tailwind, southerly 

crosswind, and northerly crosswind, as described 

in the section on wind conditions. The resulting 

test matrix, including two guidance modes along 

two arrival routes, was 1152 runs.

Data collection 

The effectiveness of ENAV and VNAV in the 

piloted simulation experiment was determined by 

whether or not its use resulted in a decrease in 

noise and fuel use. Acceptability and workload 

associated with the guidance and CDA procedures 

were assessed from questionnaire data, as another 

measure of effectiveness. Identification of 

strengths, weaknesses, and potential sources of 

confusion was determined partially from results 

of how well the pilots were able to follow the 

procedure, and partially from the pilots’ 

questionnaires. 

Objective data

The main parameters used for determining the 

effectiveness of the guidance and CDA 

procedures from the recorded simulator data were 

altitude, airspeed, latitude/longitude, throttle 

activity, and fuel use. Many of these parameters 

were used for computing noise exposure levels 

using software programs developed external to 

NASA. Other parameters of interest were the 

operating modes (VNAV PATH, VNAV SPD, 

LNAV), and use of flaps, speed brakes, and 

landing gear.

Subjective data

Data such as workload and acceptability 

ratings are considered subjective, because they 

represent the pilots’ opinions, rather than direct 

measurements of physical (or simulated) 

quantities. Subjective data collected for the 

piloted simulation experiment included workload 

ratings and structured questionnaires, which 

solicited ratings of acceptability, clarity, and ease 

of use, and open-ended questions and comments.

The workload ratings were obtained by means 

of a Bedford scale [5]. This is a 10-point scale 

adapted from the Modified Cooper-Harper scale 

commonly used in flight testing for rating aircraft 

handling qualities.

Some of the questions asked the pilots to 

provide a numbered rating used a 7-point scale, 

with the mid-point representing the neutral point. 

Some of the questions required a “Yes” or “No” 

answer, and others asked for comments or other 

explanations. Most of the questions on the final 

questionnaire were also given to the pilots in a 

pre-test questionnaire before conducting the test 

runs in the simulator. When the pilots were given 

the pre-test questionnaire, they were not told that 

they would be seeing the questions again after the 

test runs were completed, thus enabling 

researchers to obtain “before and after” responses 

to many of the questions. 
16

Prior to beginning the initial briefing on the 

study, the pilots were given a test of their 

knowledge of certain VNAV modes. The nine 

questions on the test were intended to provide the 

researchers with some indication of the pilots’ 

level of understanding of VNAV modes that they 

had developed from their previous flying and 

simulator experiences. The pilots were not told in 

advance that they would be given this test, nor 

were the results discussed with them. 

Questionnaires are included in Appendix C.

Results and Discussion

Two separate but related simulation 

experiments were conducted in 2005 and then 

2010 in order to evaluate the effectiveness, pilot 

workload, and potential benefits of an energy 

management flight guidance concept. The 

analysis of the results from these experiments is 

divided into two major sections. The first section 

presents results from the piloted simulation 

experiment. The second presents the results of 

the non-piloted batch experiment.

Piloted Simulation Study

The two main indicators of the effectiveness of 

the flight guidance are whether it resulted in a 

reduced level of perceived noise level under the 

flight path, and whether it resulted in reduced fuel 

use. The vertical guidance mode (VNAV or LNG) 

affected the vertical profile flown by the aircraft 

and, in turn, the noise and fuel usage. The 

following sections examine the effect of guidance 

mode on the vertical profile followed by the 

effects on noise and fuel.

For data analysis and reporting purposes, the 

run conditions are referred to as Nominal

(uninterrupted CDA), Slow (runs where the 

descent speed was reduced), Vector (runs where 

the aircraft was temporarily taken off the LNAV 

path), and Stress (runs where the lateral path was 

substantially shortened just prior to turning base 

leg). This terminology is used for the remainder 

of this report.

Vertical Profile

The CDA design of the RNAV arrivals was 

intended to provide a continuously descending 

flight path requiring low thrust levels. The 

guidance mode affects the actual trajectory flown. 

Figures 8 through 11 show the altitude and 

airspeed profiles for each of the test conditions. 

As expected, there was little difference in the 

profiles for the Nominal condition. The Slow and 

Vector runs, however, exhibit an overall higher 

altitude profile for LNG while retaining the same 

speed profile. This higher profile, coupled with 

dynamic gear and flap deployment, provides the 

noise and fuel benefits for LNG.

Noise

Perceived noise level was determined using 

the FAA Integrated Noise Model (INM) program 

with proprietary noise data tables from the Boeing 

Company, as described in [4]. Aircraft trajectories 

and parameters such as thrust level from the 

simulation were provided as inputs to the INM 

program, along with environmental factors 

(airport elevation, atmospheric pressure and 

temperature). The INM program then computed 

the Sound Exposure Level (SEL) at location 

points defined every 250 feet along the flight 

path, which were further reduced to obtain the 

average noise level for the range (along-track) 

between 3 nm and 15 nm. This is the region 

where there is the greatest need for reduction in 

noise levels. This also coincides with the data 

obtained in [5] for the identical simulated aircraft 

following typical baseline vectored routes into 

Louisville. Table 11 shows these results in 

tabular form for the different run conditions

compared to the baseline data from reference [5].

The reduction in noise using VNAV compared 

to the baseline data was substantial in all cases. 

The data from the VNAV runs for the case of a 

near-optimal CDA descent (as represented by the 

Nominal condition), produced a reduction in the 

perceived noise levels of 4 decibels compared to 

baseline data. LNG runs resulted in an additional 

1 decibel beyond what could be accomplished 

with VNAV only. The noise reduction obtained 
17

by using LNG remained relatively constant for the 

Slow and Vector conditions, while the benefit 

from VNAV guidance was reduced. This is 

indicative of the fact that the use of LNG allows 

the crew to maintain a more optimal descent when

interrupted with additional speed or altitude 

constraints. The majority of the noise benefit, 

however, comes from the CDA design and not the 

guidance.

Fuel Use

Fuel use was compared among the four 

different run conditions, and between VNAV and 

LNG runs. Since all runs for the same test 

conditions were repeated from the same initial 

conditions for both VNAV and LNG, a direct 

comparison of fuel usage could be made. 

Table 12 shows a comparison of the fuel use 

between VNAV and LNG runs, as well as the 

percent reduction these numbers represent for the 

LNG conditions. Generally speaking, for any 

given condition (Nominal, etc.), the percent 

reduction in fuel use by using LNG versus VNAV 

is less than 10 percent. In the Nominal conditions 

the fuel difference is especially small. This result 

was expected, as it represents a near-optimal 

CDA (the aircraft was able to conduct an 

uninterrupted CDA descent), and there would not 

be a significant advantage to using LNG over 

VNAV. In the Slow and Stress conditions, the 

required speed changes (and path changes in the 

Stress condition) caused more fluctuations in the 

speed and altitude when VNAV was used, 

whereas LNG was able to re-compute a new 

optimal trajectory that accommodated the 

changes. The limited number of data points and 

lack of variation in aircraft type, weight, winds or 

ATC speed changes prevent any definitive 

conclusions on fuel benefits afforded by LNG. 

These limitations are addressed in the batch 

simulation benefits analysis section of this report.

Since this experiment did not include a 

baseline descent (i.e., a descent with current-day 

procedures), the results from reference [5] were 

used to illustrate the reduction in fuel use due to 

the CDA procedures as well as the vertical 

guidance mode. This previous study is very 

comparable to the current study in that it used 

procedures very similar to those used in the 

current study, and was conducted in the same 

simulator, under similar conditions and pilot 

training. The RNAV STAR used in that study 

was different than the one used in this study; 

however, the baseline procedures were 

representative of controller vectoring and could 

be used for comparison with the RNAV 

procedures in this study. As was done in the prior 

study, the fuel data were examined along a 

common path distance. This allowed comparison 

of the fuel benefits afforded by the CDA vertical 

profile used in this study against the fuel used for 

non-CDA vertical descent procedures from the 

prior study. 

Table 13 shows the percent reduction in fuel 

use from the reference 5 baseline condition, 

separately for VNAV and LNG runs. Each 

condition was adjusted to a common range of 82.6 

nm, the minimum distance needed to include the 

Top Of Descent for each simulation run. These 

fuel numbers thus represent the fuel difference 

due to the vertical profile flown for each test 

condition. In Figure 12, the fuel numbers are 

presented normalized by the path distance (fuel 

used per nm), as another way to illustrate the 

differences among the different conditions and 

between VNAV and LNG runs.

These results showed that use of LNG to 

conduct the CDAs resulted in a greater reduction 

of fuel used over that seen with VNAV use. As 

with the noise level, the differences were not as 

great for the Nominal condition, which 

represented the closest to an optimal CDA 

descent. As the aircraft was taken off the optimal 

descent, however, the differences are more 

obvious. In the Slow, Vector, and Stress

conditions, the percent fuel reduction was about 

twice as much when LNG was used versus when 

VNAV was used. These differences illustrate the 

advantage that can be gained by using LNG, 

which continually updates the aircraft’s descent, 

taking into account any required changes to the 

trajectory, to re-compute a new optimal descent 

trajectory.
18

The fuel data presented in Table 13 and Figure 

12 do not include the fuel benefit afforded by 

shortening path distance by using the RNAV 

CDA STAR. This benefit can be quite 

substantial, and is discussed in more detail in 

reference [5].

Workload 

Perceived workload ratings were collected 

from the subject pilots at several points during 

each run. These ratings were based on the 

Bedford scale (Figure 13), which asks pilots to 

rate their spare capacity (i.e., their ability to 

complete other tasks after having completed the 

primary task of flying the descent). The pilots 

were asked to provide a rating at three points 

during the run (Figure 14): A) during the descent 

from 18,000 ft, B) on base leg (two miles after 

crossing the waypoint SILNT), and C) on final 

(two miles after crossing the waypoint BLGRS). 

During the Stress conditions, the lateral path did 

not take the aircraft over SILNT, but rather cut 

short the path and went direct to BLGRS. Because 

of this shortcut, the second workload rating (after 

SILNT) was skipped for these runs.

Results of the pilot ratings are summarized in 

Table 14, and shown graphically in Figure 15. 

Overall, the ratings were low on the scale (below 

3.5), indicating that the pilots felt the workload 

levels were at a very acceptable level. According 

to the scale, a rating of 3 indicates “enough spare 

capacity for all desirable additional tasks”. Each 

pair of data sets (VNAV vs. LNG at each 

condition and evaluation point) was compared 

with a Mann-Whitney Rank Sum test, which 

showed no statistical differences between each of 

the two data sets. From these results we can 

conclude that, for this group of pilots, CDA 

descents could be conducted with either LNG or 

VNAV without any significant differences in 

perceived workload for the flight crew. Also, in 

both cases the workload required to conduct 

CDAs was considered to be at an acceptably low 

level when conducted according to the 

instructions provided.

Other Subjective Data

VNAV Test. Prior to starting the briefing at 

the beginning of the day, pilots were given a pretest on VNAV operations. The questions on the 

VNAV test were designed to assess the pilots’ 

level of understanding of the basic operation of 

VNAV in a descent and presented situations that 

could be encountered during a CDA procedure. 

These results were of interest only as a means of 

assessing the understanding of VNAV for the 

pilots as a group, and not as a means of grading 

any individual pilot’s knowledge. The results of 

the test were of no consequence to the pilots’ 

subsequent qualification for participation in the 

study. In an operational environment, the 

incorrect interpretation of the mode of operation 

of the autoflight system could result in a lessthan-optimal descent profile being flown.

Most of the 24 subject pilots correctly 

answered all the questions on the VNAV test. Out 

of the eight questions on the test, only three of 

them had some incorrect responses. The pilots 

were not given advance knowledge of this test, 

but they were told that they should have a good 

understanding of the operating modes of the 

autoflight system. Because of this requirement, it 

was anticipated that the pilots who volunteered to 

participate in this study would be wellexperienced and knowledgeable on VNAV and 

would do well on this test.

The first question had only one incorrect 

answer. To stay on an FMC-programmed vertical 

path, in which autoflight mode must the aircraft 

be? The correct answer was VNAV Path, but one 

pilot answered VNAV Speed.

Three pilots answered the second question 

incorrectly. The question was: “While in a VNAV 

descent with the MCP Speed window closed, what 

speed will the autoflight system attempt to 

maintain?” The correct answer was “the FMCprogrammed speed on the VNAV Descent page”. 

The three pilots who answered this question 

incorrectly gave the same answer, which was that 

the system would attempt to maintain the ECON, 

or most efficient speed.
19

The other question with incorrect answers 

was: “With the aircraft descending in VNAV, if 

the MCP speed window is opened (speed 

intervene) to lower the speed below the FMCprogrammed descent speed, the vertical path 

flown by the aircraft will initially be:” The correct 

answer was “higher than the FMC-programmed 

path”. Four pilots gave incorrect answers to this 

question. Two of these pilots gave “lower than the 

FMC-programmed path” as the answer, and the 

other two gave “the same as the FMCprogrammed path” as the answer.

A sample of the VNAV test given to the pilots 

is included in Appendix C.

Post-Run Questionnaires. After each test run 

was completed, the subject pilots were asked to 

rate (using a 7-point scale) how difficult, how 

clear, and how acceptable were the procedures for 

maintaining the aircraft vertical path, speed, and 

lateral path. This resulted in 9 ratings for each test 

run for each pilot. The results were compiled and 

plotted as means and standard deviations, grouped 

by whether they were VNAV or LNG runs. 

Results are shown in Figure 16 for the VNAV 

Nominal, Slow, Vector, and Stress conditions, and 

in Figure 17 for the LNG Nominal, Slow, Vector, 

and Stress conditions. 

Post-Test Questionnaire Results

The Post-Test Questionnaire was given to the 

subject pilots immediately after completion of the 

final test run of the day. This questionnaire was 

intended to give the pilots the opportunity to 

evaluate the entire process, including the training, 

materials, and the cockpit procedures. They also 

were asked to indicate preferences regarding the 

appearance of the energy indicator, such as on 

which display (PFD or ND) they preferred to see 

it, and which elements of the LNG tool were most 

useful. The questions from the VNAV Test and 

the Pre-Test Questionnaire were repeated in this 

questionnaire, to assess whether there were any 

differences in opinion after having actually flown 

the test runs.

Arrival Chart Information. When asked 

whether they would have liked to see any other 

information on the chart, most (83%) answered 

“no”; the remaining responses suggested 

terrain/obstruction information, and additional 

waypoint speed/altitude constraints. All pilots 

answered “no” to the question “Do you have any 

suggestions for improvements to the chart?” Most 

(92%) said they had enough time to study the 

chart, with only 2 (8%) indicating they did not 

have enough time. 

Procedures. The pilots were asked to rate the 

procedures for managing path, speed, and altitude, 

as outlined in the FMB. For all three questions, a 

seven-point scale was used, ranging from Very 

Difficult (corresponding to a rating of 1), to Very 

Easy (corresponding to a rating of 7), with the 

midpoint being Average (a rating of 4). Their 

responses to all three questions were very similar. 

The results indicated that they felt the procedures 

for managing altitude between the waypoints 

CHERI and BLGRS were easy (Mean = 6.0, 

Standard Deviation = 1.0). The procedures were 

rated similarly easy for managing speed (M = 5.9, 

SD = 1.0), and for managing path (M = 6.1, SD = 

1.0). 

When asked whether there was any 

information missing that would help make the 

procedure easier, the majority of the pilots (71%) 

said there was nothing missing, but the remaining 

7 pilots had suggestions that ranged from 

reducing the “clutter,” to adding more graphics to 

show the mode changes to be expected. Other 

suggestions included additional prompts, such as 

for gear and other reminders of some of the 

altitude/speed interactions due to the different 

modes.

When asked whether they had suggestions for 

improvements to the FMB, only two answered 

“yes”, one suggesting keeping standard 

terminology, and the other said that the chart 

seemed to follow the standard format, with all the 

necessary information to safely fly the descent 

except for terrain information.

The pilots were asked if they were 
20

uncomfortable with the 700-ft stabilization 

altitude. Seventeen of them (71%) answered that 

they were not uncomfortable with it. When asked 

what altitude they would prefer, the answers 

varied from 500 ft (in VMC) up to 1,000 ft, with 

the most common answer being 1,000 ft. Some 

indicated this altitude for IMC, with a lower one 

(500 ft) for VMC. Many also answered that they 

preferred 700-800 ft.

Most pilots indicated that they had enough 

time to study the FMB, with only three (13%) 

indicating that they would have liked more time.

When asked to rate their level of 

understanding of the procedures, the mean rating 

was 6.4 (SD=0.6), indicating that most pilots felt 

they understood the BLGRS2 RNAV Arrival 

procedure very well. A seven-point scale was 

used for this question, with the lowest rating of 1 

indicating “I do not understand it at all”, the 

midpoint rating of 4 indicating “I understand it, 

but still have some questions,” and the highest 

rating of 7 indicating “I understand it 

completely”.

The final question on the procedures asked the 

pilots if they could think of any situations where a 

pilot who is not as familiar with VNAV might 

become confused regarding what the altitude or 

speed of the aircraft should be while conducting 

this type of descent procedure. Pilots cited 

possible confusion after an ATC intervention in 

speed, altitude or heading, then trying to resume 

the FMS path. Specifically some pilots mentioned 

knowing where to resume the charted speed after 

having been issued another speed by ATC. 

Another issue that was mentioned was possible 

confusion due to some pilots setting intermediate 

altitudes in the MCP rather than the final altitude 

for the descent. Some pilots’ comments were 

more general, saying that for pilots unfamiliar 

with VNAV some parts of the procedure could 

get confusing, but with practice they would be 

able to do it better. 

LNG Tool. The final set of questions was 

specifically about the LNG tool and consisted 

mostly of multiple-choice answers. The first 

question asked the pilots how much they used the 

LNG tool display during the LNG runs. Half of 

the pilots responded with “during most of the time 

it was displayed (about 75% of the time)”, and 

most of the rest (46%) answered “during the 

entire time it was displayed”. One pilot responded 

that he used it “about half the time it was 

displayed” (see Figure 18).

The second question asked the pilots which 

part of the LNG tool they found most useful. As 

shown in Figure 19, half the pilots answered that 

they found the energy indicator diamond the most 

useful. Eight percent found the limits on the 

energy indicator most useful, and 29% found the 

event markers on the ND (29%) the most useful. 

The pilots that indicated “other” responses (17%) 

indicated that they liked the Flight Path Vector on 

the Vertical Situation Display on the ND, which 

technically was not really part of the LNG tool.

When asked whether there was any part of the

LNG tool that they found particularly useful, the 

answers varied, but basically covered a range of 

features: the energy diamond (particularly on the 

PFD), the event markers (mentioned several 

times), the vertical situation display (not really 

part of the LNG tool), and the box around the 

energy diamond, which was a prompt to close the 

MCP speed window (see Figure 19).

When asked whether there was any part of the 

LNG tool that they found particularly confusing, 

most (83%) answered “no”, and most of the 

remaining answers referred to the small (and 

difficult to read) energy diamond on the ND. One 

pilot commented that the energy indicator display 

needed an explanation of what “high” and “low” 

actually meant.

When asked if there was any part of the LNG 

tool that they found particularly intuitive (i.e., 

easy to use), the answers were fairly evenly 

divided among the different components – energy 

diamond on the PFD, event markers, and the 

flight path vector. Other components mentioned 

were the limit indicators and the trend arrows on 

the energy display.
21

The pilots were asked which energy indicator 

display (PFD, ND, or both equally) they used the 

most. Two-thirds answered that they used the 

PFD display most, followed by both equally 

(21%), and 3 (12%) that indicated they used the 

ND display most (Figure 19). 

When asked where they would prefer to have 

the energy indicator displayed (PFD, ND, or 

both), 75% indicated that they would prefer to 

have it displayed on the PFD, 21% on both, and 

only 1 pilot indicated that he would prefer to have 

it on the ND (see Figure 20).

The pilots were asked whether they would be 

comfortable having the energy indicator displayed 

only on the PFD, only on the ND, or it should be 

displayed in both places. The vast majority (86%) 

answered “yes” to having it displayed only on the 

PFD, with only 3 pilots answering “no” (see 

Figure 21). For the ND, 58% answered that they 

would not be comfortable having the energy 

indicator displayed only on the ND, and the rest 

indicated that they would. The option of having it 

displayed in both places received almost equal 

votes for yes (9 votes, 53%) and no (8 votes, 

47%). When asked “For most pilots, where do 

you think it would be most appropriate to have the 

LNG energy indicator,” the responses were: PFD 

67%, ND 8% and both 25% (see Figure 22).

VNAV Operations. The final two questions 

asked pilots to estimate how often, in their normal 

day-to-day operations, they used VNAV below 

11,000 ft and below 6,000 ft. The most common 

response (see Figure 23) was “sometimes”, with 

almost half the pilots (46%) responding that they 

used it below 11,000 ft, and 42% indicating that 

they used it below 6,000 ft. The remainder were 

evenly split for the below 11,000 ft question, 

between “every flight” and “often”. For the below 

6,000 ft question, the remaining responses were 

split between “often” and “never”. These results 

support the presupposed notion that it is very 

common for pilots to use VNAV below 11,000 ft, 

but not as much so below 6,000 ft.

Benefits Analysis Batch Study

The potential benefits of ENAV guidance were

examined in batch simulation using the test matrix 

of runs previously described for both the Louiville 

scenarios as well as full cruise to landing 

scenarios in the Atlanta terminal area. The 

Louisville scenarios replicated the conditions 

flown in the piloted simulation study with 

additional weight, wind and speed restrictions 

being included. The Atlanta scenarios explored 

more restrictive terminal altitude constraints with 

full cruise to landing operations. The following 

sections present the results of these tests in terms 

of fuel, noise and airplane stabilization metrics. 

Also described is a comparison of the batch and 

piloted simulation performance and behavior.

Validation of Batch Simulation

The batch study was conducted using a

different aircraft simulation and FMS than was 

used in the piloted simulation. A series of test 

runs were conducted to compare the performance 

of the batch simulation with that of the piloted 

simulation. The Nominal and Slow test conditions 

from the piloted simulation were replicated using 

the batch simulation. The resulting altitude and 

airspeed profiles are compared in Figures 24 and 

25 for both VNAV and ENAV guidance.

The VNAV Nominal profiles compared quite 

well between the piloted and batch simulations. 

The batch simulation VNAV altitude profile was 

a bit lower as the aircraft approached the 6,000' 

altitude constraint due to different logic in 

handling the vertical altitude constraints. The 

FMS in the piloted simulation used a simple fixed 

flight-path angle between altitude constraints 

while the research FMS built a shallow 

deceleration segment for the speed constraint at 

6,000 feet. One of the piloted simulation profiles 

departed the Nominal altitude profile when a high 

speed condition resulted in reversion to VNAV 

SPEED mode as the airplane approached 10,000 

feet altitude. In this case, the pilot applied speed 

brake and allowed the aircraft to descend below 

the programmed path and stabilized the speed on 

the next crossing constraint of 190 knots. The 
22

batch simulation VNAV logic included this mode, 

however, these simulation runs did not excite this 

behavior. Only one of the piloted simulation runs 

encountered this situation.

The VNAV Slow profiles also compared well, 

with the batch simulation altitude tracking the low

end of the piloted simulation profiles. This was 

due to the lack of a manual thrust mode in the 

batch simulation pilot model. During the piloted 

simulation runs, the pilots were able to manually 

apply thrust when the aircraft was in VNAV 

SPEED mode with autothrottle set to HOLD. In 

this particular scenario, the aircraft would 

normally descend below the programmed altitude 

while the autopilot applied pitch control to hold 

the selected airspeed. VNAV would stop the 

descent using thrust to prevent descending below 

the next waypoint altitude crossing constraint. 

This is the modeled behavior of the batch 

simulation. Several of the pilots in the piloted 

simulation monitored the vertical deviation during 

these VNAV SPEED descents and manually 

applied thrust to prevent descending too far below 

the programmed path. The significance and 

implications of this behavior are discussed in the 

fuel and noise benefits results sections.

The ENAV profiles were also slightly 

different between the batch and piloted 

simulations. Coding changes to the ENAV logic, 

as well as improved performance modeling 

compared to LNG, resulted in slightly different 

trajectories.

The major difference in behavior between the 

piloted and batch simulations was the timing of 

flap and gear deployment. Pilot modeling for the 

batch simulation based flap deployment on 

current speed, target speed, and distance to FAF 

as shown in Table 5. A comparison of flap 

deployment between the piloted simulation and 

the batch simulation for identical Louisville 

scenarios is shown in Figures 26 and 27. The 

speed at which flaps were deployed agreed well 

between the 2 simulations for the flaps 1, 5, 25 

and 30 settings. Flaps 15 showed a larger 

difference, primarily because of the target 

airspeed criterion used by the pilot model in the 

batch simulation. All batch simulation flap 

deployment speeds, however, were well within 

one standard deviation of the observed 

deployment speeds in the piloted simulation. The 

distance to the runway at which flap deployment 

occurred, however, was significantly different for 

the flap 1, 5 and 15 settings, as shown in Figure 

27. The pilot model deployed flaps at least 5 nm

earlier than the average distance for the pilots in 

the piloted simulation. The flaps 1 and 5 

conditions occurred at speeds that were very close 

to the speeds being held constant for extended 

distances along the arrival. The pilot model 

would deploy flaps as soon as the speed reached 

the criteria specified in Table 5. The pilots, 

however, realizing they were holding an airspeed 

that was slightly greater than the minimum speed 

for the current flap setting, would not deploy flaps 

until the target speed dropped below the minimum 

speed. Several pilots commented they would 

have deployed flaps earlier if they were actually 

flying and there was any turbulence causing 

fluctuations in airspeed. This real-world 

consideration was the primary reason for using 

the more conservative flap-deployment criteria in 

the batch simulation. The ENAV criteria for flap 

deployment were also modified to include the 

additional margins above minimum speed for the 

batch simulation runs. The flap deployment 

criteria were thus consistent for the VNAV and 

ENAV comparisons in the batch simulation.

Fuel

Fuel benefits were determined by comparing 

the fuel usage resulting from flying the scenarios 

using ENAV compared to the same scenarios 

flown using conventional VNAV.

Louisville Scenarios. The piloted simulation 

provided an indication of the fuel benefits 

afforded by using LNG guidance for the 

Louisville scenario. Table 15 presents the fuel 

use results for the same scenario with an 

expanded set of airplane weights, atmospheric 

winds, and ATC speed restrictions using the latest 

ENAV guidance.

The Nominal speed condition showed a 5.9 
23

percent reduction in fuel using ENAV for the 

expanded weight and wind conditions in the batch 

simulation compared to a 2.7 percent fuel 

reduction using LNG in the piloted simulation

(Table 12). This 3 percent improvement in the 

ENAV batch results was attributed to the 

conservative pilot modeling in the batch 

simulation, with early flap deployment and no 

throttle adjustments to correct low path 

conditions. This conservative pilot modeling, 

however, is felt to be more representative of 

average airline pilot performance, especially those 

pilots with limited experience using VNAV in the 

terminal environment. Training and experience 

with VNAV is necessary to improve performance 

and achieve the benefits observed by the 

motivated and experienced pilots who participated 

in the piloted simulation. ENAV, however, was 

designed to assist even inexperienced pilots 

achieve consistent performance.

Similar fuel savings comparisons are seen for 

the 210/180/170 speed condition in Table 15 and 

the comparable Slow condition in Table 12. 

Again, ENAV exhibited about 3 percent more 

fuel savings than observed in the piloted 

simulation using LNG. Fuel savings in the batch 

simulation, however, were seen to decrease as the 

ATC-required speed conditions increased. As the 

aircraft is held at higher speeds closer to the 

runway, there is little the ENAV guidance can do 

to optimize flap and gear deployment. 

The average fuel savings using ENAV for all 

speed and wind conditions in the Louisville 

scenarios was about 78 lbs or 7.5% greater than 

the fuel used with VNAV. The impact of half or 

double the wind magnitude was found to have 

little effect on this result, as seen in Table 15b. 

Pilot training on the use of VNAV, manual 

throttle correction of below-path conditions and 

delayed flap extension could reduce this benefit 

by perhaps 2 or 3 percent.

Atlanta Scenarios. The extended arrival 

scenarios, based on Atlanta airspace, evaluated 

the use of ENAV under highly constrained 

terminal arrival operations as envisioned for

NextGen. These scenarios included opposite 

direction arrivals starting at cruise altitudes of 

37,000 feet, arriving from the West, and 36,000 

feet, arriving from the East. The terminal routing 

included altitude and speed crossing restrictions 

designed to allow continuous descents while 

maintaining separation from other arrival and 

departure traffic. These restrictions, deemed 

essential for high density traffic operations, limit 

the ability of airborne optimization to improve 

individual aircraft performance. Within the 

constrained regions of the arrival, the airborne 

optimization is limited to guidance for managing 

the energy state of the aircraft through flap and 

gear configuration changes. In addition, highenergy situations requiring additional drag 

through speed brake can be identified. Vertical 

optimization of altitude, however, is no longer 

viable.

Results of the fuel used during the Atlanta 

scenario arrivals are presented in Table 16. A fuel 

reduction using ENAV of about 49 lbs, or 3.5 % 

of the total fuel used in the scenario was observed 

for the wind-condition tests. This fuel reduction 

was essentially the same for the East and West 

routes, although the percentage was slightly 

different due to different total fuel used in the 

different directions. From the start of the 

simulation to waypoint ELLLE, the first waypoint 

on final approach, the fuel reduction was about 17 

lbs. The remaining 32 lbs of fuel savings

occurred on final approach, between ELLLE and 

the runway, a distance of only 13 nautical miles. 

This result indicates the majority of fuel savings 

for these scenarios is attributed to optimization of 

flap and landing gear deployment on final 

approach. The enroute and initial descent 

segments in the terminal area account for only 

about one third of the fuel savings. This indicates 

that the dynamic recalculation of the descent 

trajectory during descent by ENAV did not result 

in much fuel savings for these scenarios.

The effect of ATC speed restrictions was 

found to be minor, with a slight increase in 

ENAV fuel savings for the early location speed 

changes and least benefit for the late changes. 
24

Noise

The perceived noise levels under the flight 

path for the batch simulation arrivals were 

computed using the noise tables provided by 

Boeing, as described in Reference 3. The 

instantaneous peak noise level (LMAX) values 

were computed from these tables for each 

recorded trajectory point along the arrival and 

then averaged over the distance from 15 nm to 3 

nm from the runway to obtain a single noise 

metric for each flight. This metric was consistent 

with the sound exposure level (SEL) noise metric 

obtained for the piloted simulation study. The use 

of instantaneous LMAX values allowed 

computation of noise metrics directly from the 

airplane state, configuration and engine data 

without requiring post-processing using the 

modified FAA INM program. The relative 

difference in noise levels between two trajectories 

using the averaged peak noise was found to be the 

same as the difference in averaged SEL noise.

Louisville Scenario. The noise metrics for 

the Louisville batch simulation runs are presented 

in Table 17. The overall noise reduction achieved 

using ENAV was about 2 decibels. The largest 

reductions were seen for the slower speed 

conditions, with less reduction occurring with 

faster speeds. The wind conditions also showed 

small differences in noise reduction, with about a 

one decibel difference between the double and 

half magnitude wind fields. 

As with the fuel savings, the batch noise 

benefits for ENAV were slightly greater than the 

noise benefits of LNG from the piloted simulation 

results shown in Table 11. The overall lower 

altitudes of the VNAV trajectories in the batch 

simulation, as well as the earlier flap deployment, 

produced about one decibel of additional noise 

compared to the piloted simulation. Pilot training 

and vigilance would be required to avoid this 

additional noise penalty.

Atlanta Scenarios. The noise metrics for the 

Atlanta batch simulation runs are presented in 

Table 18. As with the Louisville scenarios, the 

overall noise reduction achieved with ENAV was 

about 2 decibels. The East and West routes 

merged at about 17 nm from the runway, so there 

was no difference in the region covered by the 

noise metric. The only noticeable difference in 

noise levels occurred for the late speed 

conditions, where the benefit of ENAV was 

reduced. This is consistent with fuel savings 

results and again illustrates the penalty of 

constraining the aircraft to the point where there

is insufficient range remaining to optimize flap 

and gear deployment.

Final Stabilization

The final metric considered in the batch 

experiment was the altitude at which the airplane 

achieved final-approach stabilization. For the 

purposes of this experiment, the airplane was 

considered stabilized on the approach when the 

following criteria were achieved:

• Airspeed within 10 knots of final approach 

speed,

• Landing gear down and flaps fully extended 

to landing configuration,

• Flight path angle less than 0 degrees and 

greater than -3.5 degrees, and

• Throttle 5% above idle.

The target stabilization altitude for the arrivals 

in the batch study was 1,000 feet above ground 

level (AGL). Pilot procedures for achieving 

proper stabilization are quite varied and difficult 

to realistically model in non-piloted simulations. 

Observations from the piloted simulation runs, 

review of airline procedures, and discussions with 

the pilots resulted in the modeled procedures used 

in the batch experiment. The primary means for 

achieving the 1,000 foot stabilization was gear 

extension by one nautical mile before the Final 

Approach Fix. Target speed selection of final 

approach speed occurs after crossing the FAF. 

Flap deployment occurs as the speed approaches 

the minimum for the current flap setting. Speed 

margins above the minimum were measured in 

the piloted simulation and used to trigger flap 
25

deployment in the batch simulation. The ENAV 

guidance provided cues, both visually on the 

Navigation display as well as audio and visual 

Engine Indicating and Crew Alerting System 

(EICAS) alerts, for all gear and flap deployment. 

The pilot model in this study responded instantly 

to both the distance to FAF and speed margin 

cues for VNAV and the gear and flap deployment 

cues for ENAV.

Louisville Scenarios. The stabilization 

altitudes for the Louisville runs are presented in 

Table 19. The results for all simulation runs 

showed a mean stabilization altitude for the 

ENAV guidance of 1,029 feet with a standard 

deviation of 55 feet. This compared to the mean 

altitude of 1,172 feet and standard deviation of 73 

feet for VNAV. The 143-foot lower stabilization 

altitude resulted in about a half of a nautical mile 

shorter distance flown in landing configuration 

and was a primary factor in the lower fuel use of 

the ENAV runs. The correspondingly later gear 

and flap deployment for ENAV contributed to the 

lower noise levels. 

The deviation in stabilization altitude was not 

significantly lower for ENAV, primarily due to 

the various ATC speed conditions included in the 

scenarios. The ENAV variation under nominal 

speed conditions was a remarkably low 4 feet, 

nearly within the system repeatability margin. 

The speed-constrained conditions, however, had 

standard deviations from 15 to 94 feet. This 

variation in the standard deviation indicates that 

the ENAV logic for flap deployment under offnominal speed conditions was not operating as 

effectively as hoped. The VNAV conditions, that 

based gear and flap deployment on simple 

distance and speed margins, exhibited less 

variation in the standard deviation of stabilization 

altitude.

Atlanta Scenarios. The stabilization altitudes 

for the Atlanta runs are presented in Table 20. 

The mean stabilization altitudes were a bit lower 

for these scenarios compared to the Louisville 

scenarios due to the approximately .5 nm shorter 

distance from the FAF to the runway. The 

VNAV cases, which based gear extension on 

distance to FAF, exhibited mean stabilization 

altitudes about 120 feet lower than the Louisville 

scenarios. The ENAV cases, with guidance cues 

based on deceleration predictions to the runway, 

were only about 25 feet lower. The overall result 

was a smaller benefit from ENAV in terms of 

distance flown in the landing configuration and a 

correspondingly lower fuel benefit.

The large sample of wind conditions, many 

including large fluctuations of wind on final 

approach, produced larger variations in the 

measured stabilization altitudes for both ENAV 

and VNAV. The overall standard deviation in 

altitude for ENAV was 63 feet, up from 55 feet in 

the Louisville scenarios. The VNAV scenarios 

exhibited a more significant increase in standard 

deviation to 121 feet from 73 feet in the 

Louisville runs. Again, ENAV exhibited large 

variations in some of the speed conditions shown 

in Table 20b, in particular the higher speed 

conditions. In general ENAV handled the large 

variations in wind and speeds better than VNAV, 

but certainly not to a significant degree. 

Additional work is needed to refine the ENAV 

logic for handling flap deployment under offnominal speed conditions.

Concluding Remarks

Two simulation studies investigated the use of 

an Energy Navigation (ENAV) flight-deck tool 

for conducting a descent through a busy terminal 

area using Continuous Descent Arrival (CDA) 

procedures. A piloted simulation study evaluated 

crew procedures, display concepts, and workload 

while providing an initial assessment of the 

effectiveness and benefits of the energy guidance. 

The crew procedures and RNAV chart were 

designed to be easy to understand, and to make it 

easy for the crew to make changes via the Flight 

Management Computer Control-Display Unit 

(FMC-CDU) and the auto-flight system Mode 

Control Panel (MCP) to accommodate changes 

from ATC. The test runs were intended to 

represent situations typical of what exists in many 

of today’s terminal areas, including interruptions 

to the descent in the form of clearances issued by 

ATC. The batch simulation further evaluated the 
26

benefits and efficiency gains of ENAV during

high-density terminal arrival operations. These 

benefits were measured in terms of fuel burn,

community noise levels and final approach 

stabilization altitude.

The results of the piloted simulation showed 

pilots were able to conduct the descents, even 

when interrupted by ATC with instructions that 

took the aircraft off of the VNAV path, laterally, 

vertically or both. The uninterrupted (Nominal) 

descent resulted in the greatest noise and fuel 

benefits for both LNG and VNAV descents. The 

perceived noise level was reduced by 

approximately 1 decibel for the LNG runs 

compared to the VNAV runs for the uninterrupted 

descents, and about 2 decibels for the interrupted 

descents. Relative to the baseline conditions from 

a prior study [5], the noise reductions of VNAV 

and LNG were about 4 to 5 decibels, respectively. 

The potential fuel savings in the descent segment 

were similar for the uninterrupted VNAV and 

LNG runs (13% and 16%, respectively compared 

to baseline). These benefits were reduced as the 

interruptions from ATC increased and took the 

aircraft off its optimal descent path; however, in 

these cases the savings with LNG were 

approximately twice those of VNAV. 

The workload associated with conducting the 

CDAs was rated comparable to workload with 

current-day non-CDA procedures for both the 

LNG and VNAV descents. Pilots were able to 

conduct the descents with a moderate amount of 

time allowed for studying the instructions and 

charts they were given. Minor improvements to 

the charts and procedures were identified that can 

clarify the altitude and speed constraints and 

clearances, and reduce the potential for confusion.

Pilots expressed a strong preference for 

locating the Energy Indicator on the PFD rather 

than the ND, partially due to its function as a tool 

for managing speed in the descent, and partially 

due to the greater space available for display.

The results of the batch simulation showed the 

fuel and noise benefits observed in the piloted 

study were representative of benefits achieved 

under a larger variety of atmospheric wind 

conditions and ATC speed interruptions. The 

average fuel savings using ENAV for all speed 

and wind conditions in the Louisville scenarios 

was about 78 lbs or 7.5% less than the fuel used 

with VNAV. The impact of half or double the 

wind magnitude was found to have little effect on 

this result. The Atlanta scenarios, which included 

a large variety of wind conditions, more 

restrictive altitude constraints and a shorter 

distance from the Final Approach Fix (FAF) to 

the runway, had an average fuel savings of 49 lbs 

or 3.5% using ENAV. The effect of ATC speed 

restrictions was found to be minor, with a slight 

increase in ENAV fuel savings for the early 

location speed changes, and least benefit for the 

late changes.

The majority of the fuel savings with ENAV 

was found to occur on the final approach segment 

of the arrival. Nearly two thirds of the total fuel 

savings occurred during the last 13 nautical miles 

of the arrival, indicating that the primary fuel 

efficiency of ENAV is resulting from delayed flap 

and landing gear deployment. The dynamic 

recalculation of the descent trajectory in response 

to unplanned winds and/or ATC speed changes 

did not account for a significant fuel savings.

The overall noise reduction achieved using 

ENAV was about 2 decibels greater than the noise 

reduction achieved for VNAV for the batch 

simulation scenarios. Late ATC speed restrictions 

were found to reduce the benefits of ENAV since 

little time or distance remained to optimize flap 

and gear deployment.

Final stabilization height was reduced using 

ENAV with a smaller dispersion in stabilization 

altitudes. The distance from FAF to the runway 

affected the VNAV stabilization height since gear 

deployment was based on the FAF location. In 

general ENAV handled large variations in wind 

and speeds well and showed more consistent 

variations in stabilization height. ENAV logic for 

flap deployment under off-nominal speed 

conditions prior to the FAF, however, did not 

operate as effectively as hoped and did not 

provide much benefit over conventional flap 
27

deployment logic. 

These two simulation studies demonstrated 

pilot acceptability of terminal RNAV arrival 

procedures incorporating near-idle continuous 

descent with both conventional VNAV as well as 

enhanced ENAV flight guidance. The arrival 

procedures tested in the studies are representative 

of future NextGen trajectory-based operations and 

are consistent with the latest RNAV arrival 

designs being implemented by the FAA. The 

addition of ENAV guidance to the flight deck has 

the potential to improve the efficiency and 

environmental benefits of these RNAV arrival 

procedures while reducing the flight crew 

workload and training requirements needed to 

routinely fly these procedures. Results from these 

studies indicate the majority of the efficiency and 

environmental benefits with ENAV occur on final 

approach and further refinement of ENAV flap 

and gear deployment logic is needed to maximize 

these benefits.

References

[1] Erkelens, L.J.J.: Research on Noise Abatement 

Procedures, NLR TP 98066, February, 1998.

[2] Clarke, J.P.; Brown, J.; Elmer, K.; Hunting, C.; 

McGregor, D.; Shivashankara, B.; Tong, K., Warren, 

A..; and Wat, J.: Continuous Descent Approach 

Flight Demonstration Test at Louisville 

International Airport, ICAT-2003-1, March 2003.

[3] Clarke, J.P.; et al.: Development, Design, and 

Flight Test Evaluation of a Continuous Descent 

Approach Procedure for Nighttime Operation at 

Louisville International Airport, Cambridge, MA: 

PARTNER-COE-2005-002, 2006.

[4] Williams, D.H.; Oseguera-Lohr, R.M.; and Lewis,

E.T.: Design and Testing of a Low Noise Flight 

Guidance Concept, NASA/TM 2004-213516, Dec. 

2004.

[5] Oseguera-Lohr, R.M.; Williams D.H.; and Lewis,

E.T.: Crew Procedures for Continuous Descent 

Arrivals Using Conventional Guidance, NASA/TM 

2007-214538, Feb. 2007.

[6] Peters, M. E.; Ballin, M. G.; and Sakosky, J. S.: A 

Multi-Operator Simulation for Investigation of 

Distributed Air Traffic Management Concepts,

AIAA Guidance, Navigation, and Control 

Conference & Exhibit, Monterey, California, AIAA 

2002-4596, August 5-8, 2002.

[7] Williams, D. H.; and Green, S. M.: Airborne 4D 

Flight Management in a Time-Based ATC 

Environment, NASA TM 4249, March 1991.

[8] Palmer, M.T.; Abbott, T.A.; and Williams, D.H.: 

Development of Workstation-Based Flight 

Management Simulation Capabilities within NASA 

Langley’s Flight Dynamics and Control Division,

Ninth International Symposium on Aviation

Psychology, Columbus, OH, April,1997.

[9] Roscoe, A. H.; and Ellis, G.A.: A Subjective Rating 

Scale for Assessing Pilot Workload in Flight: A 

Decade of Practical Use, RAE TR-90019, March, 

1990.

[10] Forsyth, D.W.; Gulding, J.; and DiPardo, J.: 

Review of Integrated Noise Model (INM) Equations 

and Processes, NASA CR-2003-212414, May, 

2003.
28

Appendix A

ENAV Software Description

Environment

The ENAV software operates as a software 

module within a simulation or flight environment. 

ENAV is designed to operate in the final cruise, 

descent and approach phases of flight. The 

software expects inputs consistent with these 

operations. Aircraft state conditions must be 

within the operational envelope of the airplane, 

and the airplane should be within approximately 

200 nm of the landing runway.

Pilot interface

ENAV is intended to function as a custom 

FMS LNAV and VNAV mode. As such, there is 

no special or unique pilot interface. When ENAV 

is active, the standard LNAV and VNAV 

guidance are presented to the pilot. The only new 

indications to the pilots will be an energy error 

indication on the Primary Flight Display (PFD) 

and/or Navigation Display (ND), and ENAVspecific events on the ND. In addition, vertical 

trajectory information is available for optional 

display on a Vertical Situation Display (VSD). 

The ENAV events can also be annunciated to the 

pilots via the standard aural and visual alerting 

system if desired.

Algorithm Inputs

Inputs to the ENAV software consist of lateral 

route definition, vertical segment definition, 

weather definition, and aircraft state data. These 

parameters must be passed in a manner that 

prevents the ENAV software from altering their 

contents in the calling program. The ENAV 

process determines the computations required 

based on the input data. 

Lateral Route Definition

This is an ASCII string that defines the lateral 

routing for the ENAV trajectory. Two route 

strings may be sent to ENAV: one for the active 

route and another for the “mod” or provisional 

route. The active route is required for ENAV 

active guidance. The provisional route is 

optional.

The strings are a series of latitude-longitude 

waypoints separated by a period in the sequence 

starting from the first, being at or behind the 

airplane location, and the last being the runway

threshold.

The string may be set to NULL if a call to the 

ENAV procedure does not contain new route 

data. There must be a minimum of two waypoints 

for a valid input of route data. The maximum 

number of waypoints is 60. The maximum length 

of the route string is defined in the interface 

header file.

The ENAV routine automatically determines if 

the input active route string is sufficiently 

different from the current active route to warrant a 

new route calculation. In order to force a new 

route calculation, the calling program must add 

the string “NEW.” to the beginning of the active 

route string. 

Vertical Segment Definition

The vertical segment definition is passed to the 

ENAV software as a data structure of string 

parameters that adhere to the naming convention 

described in the Vertical Path section of 

Trajectory Definition. The structure defines the 

number of vertical segments and provides ASCII 

strings for each required parameter for each 

segment. A maximum of 16 vertical segments 

may be used.

The first record contains headers for each 

parameter of the segment definition (these 

correspond to each parameter in the data 

structure). Subsequent records contain the 

segment definition for each segment, starting 

from the runway working backwards to the 

airplane. The first record must be the final pretouchdown conditions at which the airplane is 

stabilized for landing (i.e., stabilization altitude). 
29

The final record must be a level altitude segment 

ending at the airplane range. 

The ENAV software contains a default vertical 

segment definition. The ENAV software uses this 

default vertical segment definition if the input 

vertical segment definition data indicates zero 

segments. The calling program explicitly sets the 

number of segments to zero to enable using the 

default vertical segments.

The software modifies the vertical segment 

definition to honor all waypoint constraints 

contained in the route definition input string. This

modification only occurs with the default vertical 

segment definition.

Weather Definition

This is an ASCII string with the vertical 

weather profile. The string is a series of weather 

records containing pressure altitude in 100s of 

feet, wind direction in degrees, wind speed in 

knots and temperature deviation from standard 

day in degrees C. The weather record entries are 

separated by a period in the sequence starting 

from the lowest altitude to the highest. Altimeter 

setting, in inches of mercury, may optionally be 

input as the first weather record

There is a minimum of one vertical weather 

point and/or one altimeter setting for a valid input 

of weather data. The maximum length of the 

weather string is defined in the interface header 

file. If a valid weather string is not input by the 

calling program, standard day, no-wind conditions 

will be used. To specifically force standard day, 

no-wind conditions, the weather string must be set 

to “STANDARD_DAY”.

The ENAV routine will test the input weather 

string to determine if the weather data has 

changed sufficiently to require a new trajectory 

calculation. Repeatedly sending the same weather 

string will not result in new trajectories being 

generated. NULL weather strings are ignored, 

and the previous weather input will be used on 

any trajectory calculation. If a valid weather 

string has never been sent, ENAV will default to 

standard day with no wind.

Aircraft State Data

The following aircraft parameters are required 

on each call to the ENAV procedure:

• Time (current time of day), sec

• Latitude, deg

• Longitude, deg

• Barometric altitude (altimeter), feet

• Vertical speed, fpm

• Calibrated airspeed, knots

• True airspeed, knots

• Ground speed, knots

• True track angle, deg

• Wind speed, knots

• Wind direction, deg

• Static air temperature, deg C

• Flap selection, deg

• Engine power setting (EPR for NASA 

757)

• Landing gear handle (0=up or 

off,1=down)

• Weight, lbs

Aircraft Mode Control Data

The following MCP parameters are required 

on each call to the ENAV procedure:

• Selected altitude, feet

• Selected CAS, knots

• Selected Mach

• Speed window status

0 = closed,

1 = open,

2 = closed with externally defined speed 

target

• LNAV active guidance mode

(0 = no, 1 = yes)

• VNAV active guidance mode

(0 = no, 1 = yes)

The MCP data are used in the automatic 

updating feature of ENAV.
30

Aircraft Planning Data

The following data are needed from the 

aircraft FMS for vertical trajectory planning:

• Final approach reference speed (Vref)

• Flag indicating a Mach/CAS descent

• Descent calibrated airspeed

• Descent Mach

ENAV Altitude and Speed Limits

The default operating altitude and speed for 

ENAV is below 12,500 feet and less than or equal 

to 250 knots. These operating limits may 

optionally be extended to higher altitudes and 

speeds using the MaxAltitude and MaxCAS input 

parameters. These limits should not be extended 

beyond 41,000 feet altitude and 350 knots CAS.

Algorithm Outputs

The ENAV software is designed for real-time 

flight guidance and primarily provides dynamic 

outputs on return from each call to the ENAV 

procedure. The software also provides output of 

the active reference flight trajectory in both full 

engineering units and an ASCII string suitable for 

input to ACARS-capable FMS via a data link 

formatted flight plan modification.

Guidance Output

The following parameters are output on each 

return from the ENAV procedure:

• Target barometric altitude, feet

• Target vertical speed, ft/min

• Command vertical speed, ft/min (for 

autopilot pitch control)

• MCP vertical speed, ft/min (for manual 

pitch control)

• Target calibrated airspeed, knots (for 

autothrottle speed control)

• Reference calibrated airspeed, knots

• Descent calibrated airspeed, knots

• Target mach

• Flag indicating Mach or CAS target

• Reference ground speed, knots

• Energy error, feet

• Energy error rate, ft/sec

• Maximum energy error, feet

• Minimum energy error, feet

• Distance to go to the runway threshold, 

feet

• Elapsed along-track range from start of 

trajectory, feet

• Estimated time to go to the runway, sec

• Target true track angle, deg

• Cross track error, feet

• Nominal bank angle, deg

• Bank angle command, deg

• Flag indicating drag required

• Throttle mode, integer code

0 = ENAV_THROTTLE_SPD

1 = ENAV_THROTTLE_IDLE

2 = ENAV_THROTTLE_DORMANT

When guidance computations are not valid, 

these output parameters are set to a unique value 

indicating they are invalid. The value for invalid 

data is defined in the interface header file.

Additionally, an event string is output that 

defines the upcoming ENAV events as well as 

error codes. The contents of the event string 

include:

• Name of the event

• Location (latitude and longitude)

The ENAV events are also output as a data 

structure containing the event name, latitude, 

longitude, and along-path distance to go to the 

event.

Trajectory Output

The calling program maintains a data structure 

that matches the trajectory definition contained in 

the ENAV interface header file. A pointer to this 

structure is passed to ENAV in the calling 

statement. The ENAV software will populate this 

data structure whenever a new trajectory is 

computed. The number of trajectory points in the 

trajectory is explicitly set to zero by the ENAV 

software if there is no active ENAV trajectory.
31

In addition to the trajectory structure 

maintained by the calling program, a trajectory 

change point (TCP) structure is provided in the 

outputs. The contents of this structure are:

• Name

• Latitude, deg

• Longitude, deg

• Elapsed range, feet

• Altitude, feet

• CAS, knots

• Groundspeed, knots

• ETA, sec
32

Appendix B

Flight Manual Bulletin
33

INSTRUCTIONS TO SUBJECT PILOTS

____________________________________________________________________________

FLIGHT MANUAL BULLETIN

____________________________________________________________________________

Revision 4.5, 7/29/2005

CONTINUOUS DESCENT APPROACH (CDA) 

BACKGROUND AND SUMMARY

The Continuous Descent Approach (CDA) is a beneficial method for operationally reducing 

community noise near airports, in addition to lowering fuel usage during the descent. The CDA is 

designed to minimize level altitude segments during approach, especially at lower altitudes. This 

keeps the aircraft higher and at reduced thrust settings during most of the descent in the terminal 

arrival area. Typically, the altitude crossing restriction at the Initial Approach Fix (IAF) of the 

Instrument Landing System (ILS) approach is raised so that glideslope intercept occurs 

approximately at the turn to final approach. A CDA can be flown accurately with a Flight 

Management System (FMS)-equipped aircraft; however the way in which the vertical profile is flown

can vary substantially across pilots and aircraft types. If the vertical profile is not properly managed, 

the aircraft could reach the IAF altitude early, requiring an undesired level segment to the IAF. 

Properly managing the aircraft’s total energy (speed and altitude) throughout the descent can 

minimize this problem. To best comply with altitude and speed restrictions, while remaining close to 

the optimal descent trajectory, the CDA should be flown with vertical guidance in the VNAV (Vertical 

Navigation) mode. To conduct an effective CDA descent with VNAV, pilots should have a good 

understanding of how this mode transitions among its sub-modes and interacts with other autoflight 

systems to meet speed and altitude constraints.

CDA procedures (RNAV arrivals) were developed for Louisville International Airport (SDF), using 

routing typical of what is used by Air Traffic Control (ATC) Approach Control for vectoring 

operations. ATC will issue the clearance to descend on the CDA RNAV arrival. While on the arrival, 

the crew is responsible for complying with charted speed and altitude crossing restrictions. ATC may 

issue speeds and/or altitudes that are different from those shown on the chart. The crew must 

comply with any new instructions issued by ATC, until instructed to resume the RNAV arrival. The 

crossing conditions at the end of the CDA allow for a smooth transition to the ILS approach, however 

ATC may clear the aircraft for a different type of approach.

This bulletin describes clearances and crew procedures for conducting the CDA. Additional 

instructions are in the Appendix for aircraft equipped with Low Noise Guidance (LNG), a flight deck 

tool that displays aircraft energy error and provides guidance for a noise-optimal descent. 

EXAMPLES OF NOMINAL CDA CLEARANCES

Descent clearance for the Silent RNAV Arrival: 

• "Descend via the Silent RNAV Arrival";

 “Proceed direct ABCDE, then descend via the Silent RNAV Arrival”

This clearance may be issued at any time, and allows the crew to descend on the SILNT RNAV 

Arrival until the fix CHRCL. Typically it is issued before the initial fix on the arrival (CHERI, DANNY, 

or KWIET). However, this descent clearance may also be issued in conjunction with a “direct to” one 

of the other waypoints on the arrival. This allows the aircraft to continue descending on the 

remainder of the RNAV arrival, via the indicated waypoint. The crew must comply with all charted
34

altitude and speed restrictions from (and including) that waypoint on, unless otherwise instructed by 

ATC.

Pilots shall advise ATC upon beginning the descent (e.g., “...leaving 18000 feet...”). Pilots shall 

advise ATC upon initial contact that they are descending on the SILNT RNAV Arrival (“...descending 

through XXXXX feet on the SILNT RNAV Arrival”).

Deviations from the CDA RNAV Arrival:

At any time after initiating the descent, ATC may issue speed, heading, or altitude changes that are 

different from those on the RNAV Arrival. Subsequent instructions may be issued to continue the 

descent on the Arrival. Examples of instructions taking the aircraft off the Arrival are listed below 

(these may be issued individually or grouped):

• "Turn left (right), heading XXX." 

This instruction takes the aircraft off of the RNAV Arrival. ATC-issued instructions must be complied 

with until the crew is cleared to again descend via the Arrival, at which point the charted restrictions 

once again apply.

• "Descend and maintain YYYY." 

This instruction requires the crew to descend and maintain the ATC-issued altitude until instructed 

otherwise by ATC. If the crew is subsequently cleared to continue descending on the Arrival, the 

crew must revert to the charted altitude profile, beginning with the “to” waypoint, where the arrival is 

re-joined. If there is no altitude restriction at this waypoint, the last ATC-issued altitude is used. If 

ATC did not issue a new altitude clearance, then the next charted altitude restriction is used.

• "Reduce speed to XXX knots.”

This instruction requires the crew to maintain the issued speed until instructed otherwise by ATC. If 

the crew is subsequently cleared to continue descending on the Arrival, the crew must revert to the 

charted speed profile; this begins with the first waypoint (the “to” waypoint) on the arrival, where it is 

re-joined. If there is no speed restriction at this waypoint, the last ATC-issued speed is used.

A normal approach clearance is issued prior to the IAF waypoint for the approach procedure.

CREW PROCEDURES FOR CONDUCTING THE CDA

While descending on the RNAV Arrival, keeping the aircraft in full LNAV/VNAV mode allows the 

crew to comply with charted altitude and speed constraints, and to maintain a profile close to the 

optimal CDA profile. If ATC issues instructions that take the aircraft off of the RNAV Arrival, the crew 

may use other autoflight functions to comply with the verbal instructions, but should remain in VNAV 

as much as possible throughout the descent to achieve the most efficient profile. Pilots must remain 

cognizant of their aircraft’s autoflight modes at all times.

Lateral Route Management

Depending on traffic conditions, ATC may alter the RNAV route by issuing instructions that take the 

aircraft off the charted route, with subsequent instructions to rejoin the procedure. For timely 

compliance, the crew may use the HDG (heading select) function on the Mode Control Panel (MCP) 

to turn towards the cleared waypoint, while remaining in VNAV. To continue on the RNAV route, the 

crew must modify the FMC route to go “direct-to” the indicated waypoint. After the new route is 

executed, the crew must then re-engage LNAV and VNAV (if not already engaged) to continue.
35

Altitude Management

With the aircraft in VNAV, the descent will be managed according to the VNAV vertical profile. 

However, it is crucial that pilots monitor throughout the descent the amount of deviation from the 

programmed VNAV path using the vertical path deviation indicator on the Navigation Display (ND). If 

the aircraft’s path deviates too much from the programmed path, it might not be possible to 

recapture it until the aircraft is on final approach. This could result in VNAV exhibiting unexpected 

behaviors. If the aircraft path were excessively low, an extended level segment might result, 

producing excessive noise. If the aircraft path were excessively high, it might not be possible to 

intercept the glide slope and continue the approach.

If the pilot feels that VNAV is not managing the vertical path in an acceptable manner, speedbrakes 

and throttle should be used to augment VNAV guidance, or another pitch mode may be selected to 

better comply with the vertical profile.

Speed Management

With the aircraft in VNAV, speed is controlled by the autoflight guidance to comply with the speed 

profile programmed in the FMC. If ATC issues a speed change, the crew must use the VNAV speed 

intervene function to comply with the ATC instruction in a timely manner, by pushing the MCP SPD 

knob to open the MCP speed window and dialing in the new speed. Descending in this mode will 

likely cause the aircraft to deviate from the programmed vertical profile, increasing the possibility that 

the descent could be completed too early, and the aircraft would level off prior to intercepting the 

glide slope. This makes the descent less optimal (CDA descents are designed to avoid low-altitude 

level segments.) To minimize the possibility of having a level segment, the aircraft’s descent rate 

may be adjusted (using thrust while in THR HOLD and VNAV SPD) to obtain a better profile and 

meet the next waypoint constraint.

If the crew is then cleared to resume the RNAV Arrival while still in VNAV-speed-intervene, they may 

either continue in this mode (managing all subsequent charted speed reductions by dialing them in 

the MCP speed window), or return to full VNAV (by de-selecting the MCP speed knob).

If the crew chooses to return to full VNAV, the SPD knob must not be pushed until after crossing the 

next speed-constrained waypoint on the RNAV arrival. If the SPD knob is pushed prior to that, the 

aircraft may revert to the old VNAV descent speed, unless the new one is programmed into the 

FMC. If the old descent speed is much higher than the current aircraft speed, this might cause the 

autoflight guidance to command abrupt throttle changes to meet the next speed constraint.

It is crucial to monitor the vertical path deviation indicator whenever the speed intervene function is 

used, to ensure that the aircraft remains close to the vertical path. The crew may use another pitch 

mode to ensure compliance with restrictions (such as the 250-kt speed requirement below 10,000 

feet), if they feel that remaining in VNAV will not do so. Speedbrakes may be used to add drag.

Flap and Gear Deployment

To maximize noise reduction, the landing gear extension should be delayed until the Final Approach 

Fix. If speed is nominal, landing gear should be extended at the FAF. If speed is more than 10 kts 

high approaching the FAF, gear may be extended early to enable the aircraft to achieve a stabilized 

approach by 1000 feet AGL.

Flap deployment should follow the normal speed schedule. As speed is reduced throughout the 

descent, the corresponding flaps should be deployed.
36
37
38

winds (on the CDU Descent Forecast Page) and actual winds, an energy error will be displayed

even if air speed and altitude are correct. 

With the aircraft tracking the desired vertical path with VNAV engaged, an energy deviation will 

consist predominantly of a speed error. It is preferable to dissipate energy through the use of 

spoilers on the initial part of the descent, to fine-tune the energy without the use of throttle. If further 

energy dissipation is required, flaps may be deployed (or a notch added) before arriving at the 

DECEL or FLAP points, as appropriate. If energy gets too low, gradually increase thrust to achieve 

a satisfactory trend on the energy indicator. 

Since the auto throttle will normally manage thrust to maintain (or achieve) the VNAV speed target, it 

may not be possible to correct an energy error while coupled in VNAV and auto throttle without 

changing the VNAV target speed. Higher-than-predicted headwinds and/or intentional low airspeeds 

typically result in a low energy condition. Conversely, lower-than-predicted headwinds or intentional 

high airspeeds will result in a high energy condition. Too-low energy while in auto throttle is

generally not a problem and corrective action (such as increasing the target speed using VNAV 

speed intervene) is not necessary. Too-high energy, however, should be corrected using additional 

drag (if throttle is at idle) and/or reducing the aircraft speed.

General rule for using LNG: If energy is too high, reduce thrust or add drag. If energy is too low, 

increase thrust or reduce drag.

Lateral Path Management

The low noise guidance algorithm requires a continuous lateral path to the runway in order to 

calculate the CDA vertical trajectory and the geographical locations of the guidance events. 

Once calculated, the guidance events are valid only while the aircraft remains on or near the 

programmed lateral path (in LNAV). If the aircraft deviates significantly from the lateral path, e.g. 

when being radar vectored, the vertical path is no longer valid and will be removed from the Vertical 

Situation Display. LNG vertical guidance and energy error display will remain active and provide 

approximate guidance to rejoin the FMS path in an efficient manner. Large deviations from the 

lateral path should be corrected by executing a DIRECT TO a waypoint on the lateral path to ensure

coupled lateral and vertical guidance.

Speed and Flap Schedule

If Air Traffic Control (ATC) issues a new speed to the LNG aircraft, the crew must use the speedintervene function (pushing the speed knob on the Mode Control Panel) to expedite the speed 

change while remaining in VNAV. After LNG re-calculates a new path, the speed knob should be 

pushed again to exit the speed-intervene function. Recalculation of the path usually takes about one 

second, and is evident by a slight change in the location of TOD and other ND events. Exiting from 

speed-intervene enables the aircraft to once again closely follow the LNG vertical path guidance, 

resulting in a more optimal descent profile. If the crew does not exit the speed-intervene function, a 

box will appear around the energy diamond as a prompt to close the MCP window. It is important to 

close the MCP window when this prompt appears, because failing to do so will over-ride any

downstream speed reductions computed by LNG, and affect the aircraft’s energy level. This feature 

is only active with flaps up.

Deployment of flaps and landing gear will take place at a slightly later point than during a non-CDA 

approach, primarily due to the higher-energy approach profile. The CDA profile while using VNAV 

LNG is designed for a constant speed descent between the TOD and DECEL points. If needed, 

VNAV will then shallow the descent between the DECEL and FLAP points to decelerate the aircraft
39

to VFE. To reduce screen clutter, the DECEL point is not shown if the distance to the FLAP point is 

less than 1/2 nm. Flaps may be extended when the aircraft speed is in the appropriate range.

After the FLAP-5 point, additional flaps should be extended as necessary as the aircraft speed 

naturally decays, or as needed to manage energy. Pilots should keep the MCP speed window open 

after the FLAP-5 point, and adjust the speed as would normally be done for this part of the descent. 

Speeds issued by ATC must be adhered to as would normally be the case, and the MCP speed 

window should be open prior to glideslope capture, to ensure that the throttle does not make any 

rapid adjustments. After flaps have been extended, the “MCP window open” box around the energy 

diamond will not appear.

Gear Extension

The location of the GEAR event point is prescribed in order to reduce aircraft noise close to the 

airport. If the aircraft energy is good when nearing the GEAR point, the landing gear should be 

lowered crossing the GEAR point. If energy is slightly high, the gear may be extended earlier than 

the GEAR point to help dissipate excess energy. If energy is low, airspeed should be maintained 

with throttle, at the flaps 5 or flaps 15 minimum maneuver speed until the gear point is reached.
40

CDA PROCEDURE WITH LNG

The following table contains a summary of procedures for conducting a continuous descent 

approach in VNAV, with and without the LNG function.

CDA Approach without LNG CDA Approach with LNG

Must have active route in FMC to enable 

guidance (VNAV) to fly CDA automatically.

same

Prior to TOD and in VNAV, dial MCP altitude 

window to next altitude constraint.

same

Comply with charted restrictions until FAF. same

VNAV guidance follows vertical path in FMC; 

autothrottle adjusts to follow VNAV speeds. same

If new speed is issued, use speed intervene to 

expedite; remain in speed intervene until back 

on Arrival; closely watch vertical path deviation.

Compliance with charted altitude crossing 

restrictions is crucial.

If new speed is issued, use speed intervene to 

expedite; exit speed-intervene after new 

descent path is computed; Ensure energy

remains in green range

Aircraft will automatically decelerate to comply 

with charted crossing constraints. same

Extend flaps according to current speed. Extend flaps at specified LNG event locations 

and/or according to current speed.

Expect normal approach clearance. same

At glideslope capture, autothrottle reverts to 

SPD mode, pilot controls speed through MCP. same

If aircraft trajectory is in acceptable range, 

extend landing gear prior to FAF.

If energy is in green range, extend landing gear 

at GEAR point on ND
41

Appendix C

Questionnaires
42

VNAV Questionnaire

1) To stay on an FMC-programmed vertical path, in which autoflight mode must the aircraft be?

a. FLCH (Flight Level Change)

b. VNAV SPD (Vertical Navigation Speed)

c. VSPD (Vertical Speed)

d. VNAV PTH (Vertical Navigation Path)

2) While in a VNAV descent with the Mode Control Panel (MCP) Speed window closed, what 

speed will the autoflight system attempt to maintain?

_______________________________________________________________________

3) While in a VNAV descent with the MCP speed window open, what speed will the autoflight 

system attempt to maintain?

_______________________________________________________________________

4) In VNAV PTH mode, the autoflight system will sacrifice _______________ in order to 

maintain ______________.

5) In VNAV SPD mode, the autoflight system will sacrifice _______________ in order to 

maintain ______________.

6) While in a descent in VNAV PTH mode, a speed is assigned (verbally) by ATC. What action 

would you normally take in order to comply with the speed assignment, and in what mode would 

the aircraft then be? 

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

7) With the aircraft descending on a VNAV path and thrust mode in throttle hold, what speed 

will the autoflight system attempt to maintain?

8) With the aircraft descending in VNAV, if the MCP speed window is opened (speed 

intervene) to lower the speed below the FMC-programmed descent speed, the vertical path flown 

by the aircraft will initially be:

a) higher than the FMC-programmed path

b) lower than the FMC-programmed path

c) the same as the FMC-programmed path
43

Post-Run Questionnaire
44

Use the scale below to answer the following question.

1 2 3 4 5 6 7 

|___________|___________|___________|___________|___________|___________|

 Very Average Very

 Difficult Easy

Very difficult: required much harder mental effort to complete this task than what I normally 

experience during this phase of flight for most of my commercial operations today.

Average: required about the same amount of mental effort to accomplish this task as what I 

normally experience during this phase of flight for most of my commercial operations today

Very Easy: required much less mental and/or physical effort to accomplish this task than what I 

normally experience during this phase of flight for most of my commercial operations today.

Rate how difficult it was during this test run to manage:

run number 1 2 3 4 5 6 7 8 9 10 11 12

vertical path

speed

lateral path

Comments:

1_____________________________________________________________________________

______________________________________________________________________________

2_____________________________________________________________________________

______________________________________________________________________________

3_____________________________________________________________________________

______________________________________________________________________________

4_____________________________________________________________________________

______________________________________________________________________________

5_____________________________________________________________________________

______________________________________________________________________________

6_____________________________________________________________________________

______________________________________________________________________________

7_____________________________________________________________________________

______________________________________________________________________________

8_____________________________________________________________________________

______________________________________________________________________________

9_____________________________________________________________________________

______________________________________________________________________________

10____________________________________________________________________________

______________________________________________________________________________

11____________________________________________________________________________

______________________________________________________________________________

12____________________________________________________________________________

______________________________________________________________________________
45

Use the scale below to answer the following question.

1 2 3 4 5 6 7 

|___________|___________|___________|___________|___________|___________|

 Very Average Very

 Unclear Clear

Very Unclear: I did not understand the procedures at all.

Average: I understood the procedures, but had to think carefully about what I needed to do.

Very clear: I understood the procedures completely, and knew exactly what I needed to do.

Rate how clear it was during this test run to manage:

run number 1 2 3 4 5 6 7 8 9 10 11 12

vertical path

speed

lateral path

Comments:

1_____________________________________________________________________________

______________________________________________________________________________

2_____________________________________________________________________________

______________________________________________________________________________

3_____________________________________________________________________________

______________________________________________________________________________

4_____________________________________________________________________________

______________________________________________________________________________

5_____________________________________________________________________________

______________________________________________________________________________

6_____________________________________________________________________________

______________________________________________________________________________

7_____________________________________________________________________________

______________________________________________________________________________

8_____________________________________________________________________________

______________________________________________________________________________

9_____________________________________________________________________________

______________________________________________________________________________

10____________________________________________________________________________

______________________________________________________________________________

11____________________________________________________________________________

______________________________________________________________________________

12____________________________________________________________________________

______________________________________________________________________________
46

Use the scale below to answer the following question.

1 2 3 4 5 6 7 

|___________|___________|___________|___________|___________|___________|

 Very Average Very

Unacceptable Acceptable

Very Unacceptable: I did not like the procedures at all, and would not use them in normal 

operations.

Average: I liked the procedures and would use them in normal operations, but would like to see 

some improvements.

Very acceptable: I liked the procedures very much, and would use them without any 

improvements.

Rate how acceptable it was during this test run to manage:

run number 1 2 3 4 5 6 7 8 9 10 11 12

vertical path

speed

lateral path

Comments:

1_____________________________________________________________________________

______________________________________________________________________________

2_____________________________________________________________________________

______________________________________________________________________________

3_____________________________________________________________________________

______________________________________________________________________________

4_____________________________________________________________________________

______________________________________________________________________________

5_____________________________________________________________________________

______________________________________________________________________________

6_____________________________________________________________________________

______________________________________________________________________________

7_____________________________________________________________________________

______________________________________________________________________________

8_____________________________________________________________________________

______________________________________________________________________________

9_____________________________________________________________________________

______________________________________________________________________________

10____________________________________________________________________________

______________________________________________________________________________

11____________________________________________________________________________

______________________________________________________________________________

12____________________________________________________________________________

______________________________________________________________________________
47

Post-Test Questionnaire

1) To stay on an FMC-programmed vertical path, in which autoflight mode must the aircraft 

be?

a. FLCH (Flight Level Change)

b. VNAV SPD (Vertical Navigation Speed)

c. VSPD (Vertical Speed)

d. VNAV PTH (Vertical Navigation Path)

2) While in a VNAV descent with the Mode Control Panel (MCP) Speed window closed, 

what speed will the autoflight system attempt to maintain?

_______________________________________________________________________

3) While in a VNAV descent with the MCP speed window open, what speed will the 

autoflight system attempt to maintain?

_______________________________________________________________________

4) In VNAV PTH mode, the autoflight system will sacrifice _______________ in order to 

maintain ______________.

5) In VNAV SPD mode, the autoflight system will sacrifice _______________ in order to 

maintain ______________.

6) While in a descent in VNAV PTH mode, a speed is assigned (verbally) by ATC. What 

action would you normally take in order to comply with the speed assignment, and in what 

mode would the aircraft then be? 

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

_________

7) With the aircraft descending on a VNAV path and thrust mode in throttle hold, what 

speed will the autoflight system attempt to maintain? 

______________________________________

8) With the aircraft descending in VNAV, if the MCP speed window is opened (speed 

intervene) to lower the speed below the FMC-programmed descent speed, the vertical path 

flown by the aircraft will initially be:

a) higher than the FMC-programmed path

b) lower than the FMC-programmed path

c) the same as the FMC-programmed path

 
48

Chart

9) Is there any other information you would like to see on chart? Y N

a. If no, please explain

10) Do you have any suggestions for improvements to the chart?

11) Did you have enough time to study the chart? Y N

Flight Manual Bulletin

After studying the crew procedures in the Flight Manual Bulletin for conducting the CDA 

procedure:

12) I think the procedures for managing altitude between the waypoints CHERI and BLGRS are:

 1 2 3 4 5 6 7

 |___________|___________|___________|___________|___________|___________|

Very Average Very

Difficult Easy 

Comments:

13) I think the procedures for managing speed between the waypoints CHERI and BLGRS are:

 1 2 3 4 5 6 7

 |___________|___________|___________|___________|___________|___________|

Very Average Very

Difficult Easy 

Comments:
49

14) I think the procedures for managing the lateral path between the waypoints CHERI and 

BLGRS are:

 1 2 3 4 5 6 7

 |___________|___________|___________|___________|___________|___________|

Very Average Very

Difficult Easy 

Comments:

15) Was there any information missing from the FMB that you think would be helpful for 

understanding how to conduct the CDA? Y N

If yes, please explain

16) Do you have any suggestions for improvements to the chart?

17) Were you uncomfortable with the 700-ft stabilization altitude? Y N

(comment):____________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

18) What altitude would you prefer for a required stabilization altitude?____________________
50

Flight Manual Bulletin

19) Did you have enough time to study the FMB? Y N

20) Please rate your level of understanding of the BLGRS2 RNAV Arrival procedure:

 1 2 3 4 5 6 7

 |___________|___________|___________|___________|___________|___________|

I do not I understand I understand

understand it, but still have it completely

it at all some questions 

 

21) Can you think of any situations where a pilot who is not as familiar with VNAV might 

become confused regarding what the altitude or speed of the aircraft should be while conducting 

this type of descent procedure?

LNG tool

22) How much did you use the LNG tool display during the LNG runs?

a. During the entire time it was displayed

b. During most of the time it was displayed (about 75% of the time)

c. About half the time it was displayed

d. Not very much (about 25% of the time

e. Not at all

23) Which part of the LNG tool did you find most useful?

a. Energy indicator diamond

b. Limits on energy indicator

c. Event markers on ND (any one in particular? ______________________)

d. Other ______________________________________________________

24) Was there any part of the LNG tool that you found particularly useful? 
51

25) Was there any part of the LNG tool that you found particularly confusing?

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

26) Was there any part of the LNG tool that you found particularly intuitive (i.e., easy to use)?

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

27) Which LNG energy indicator display did you use the most?

a. PFD

b. ND

c. both equally

28) Where would you prefer to have the LNG energy indicator?

d. PFD

e. ND

f. Both

29) Would you be comfortable with the LNG displayed only on:

a. The PFD Y N

b. The ND Y N

c. It should be in displayed in both places Y N

30) For most pilots, where do you think it would be most appropriate to have the LNG energy 

indicator?

g. PFD

h. ND

i. Both

31) In normal day-to-day operations, how often do you use VNAV:

a) below 11000 ft altitude 

 ____every flight ____often ____sometimes ____never

b) below 6000 ft altitude 

 ____every flight ____often ____sometimes ____never
52

Table 1. Waypoint Crossing Restrictions for Atlanta Arrivals

West Arrival, SFO to ATL East Arrival, ORF to ATL

Waypoint Altitude Speed Waypoint Altitude Speed

IC SFO Cruise (FL370) Cruise (.8 Mach)

CALCO ----- --

VIKNN ----- -- IC ORF Cruise (FL360) Cruise (.8 Mach)

HERKO ----- -- ODF ----- --

RPTOR ----- -- FLCON ----- --

NOTRE ----- -- DIRTY ----- --

NOFIV 11000 230 BYRDS 10000 230

FANEW ----- --- COSEL ----- ---

HAVAD 6000 210 HAVAD 6000 210

ELLLE 5400 180 ELLLE 5400 180

BALLI ----- --- BALLI ----- ---

AJAAY 2700 170 AJAAY 2700 170

R-26R 1076 --- R-26R 1076 ---

Table 2. Winds Used During Piloted Simulation

Altitude 

(ft, MSL)

Wind Speed 

(Knots)

Wind Direction 

(True deg)

0 2.4 208

2461 5.7 242

4921 9.2 262

9843 16.0 273

13801 20.7 274

18289 25.9 273

23574 31.8 272

30065 38.9 270

33999 42.3 270

Table 3. Wind criteria for Atlanta scenarios

Condition Number of Days Criteria

All 679 Easterly winds at 1000 ft greater than -10.

Headwind 239 a) Easterly winds at 1000 ft greater than 0.

b) Easterly winds at 3000 ft greater than 0.

c) Absolute value of Northerly winds at 5000 ft less than 10.

Tailwind 117 a) Easterly winds at 1000 ft greater than -10 and less than 0.

b) Easterly winds at 3000 ft less than 0.

c) Absolute value of Northerly winds at 5000 ft less than 10.

Southerly Crosswind 149 a) Northerly winds at 5000 ft greater than 10.

b) Easterly winds at 1000 ft greater than -10.

Northerly Crosswind 126 a) Northerly winds at 5000 ft less than -10.

b) Easterly winds at 1000 ft greater than -10.

Headwind to Tailwind on final 31 a) Easterly winds at 1000 ft greater than -10 and less than 0.

b) Easterly winds at 3000 ft greater than 0.

Tailwind to Headwind on final 17 a) Easterly winds at 1000 ft greater than 0.

b) Easterly winds at 3000 ft less than 0.
53

Table 4. Pilot Model Actions

Action VNAV ENAV

Flap Extension Rules based on airspeed, target 

airspeed and distance to FAF 

EICAS message

Gear Extension Distance from FAF EICAS message

Speed brake Rules based on vertical deviation 

and FMC drag message

Above energy limit

MCP speed selection MCP speed changes sent via data 

link

Same as VNAV

Table 5. Flap Extension Criteria for Pilot Model

Current Flap 

Position

Criteria for selecting next flap setting

Current Speed Target Speed Distance

0 or 1 Less than Minimum Flap 

Speed + 5 knots

any any

0 or 1 Less than Maximum Flap 

Speed - 5 knots

Less than current 

speed - 10 knots

Within 5 nm

of FAF

5 Less than Minimum Flap 

Speed + 12 knots

any any

5 Less than Maximum Flap 

Speed - 5 knots

Less than current 

speed - 10 knots

Within 5 nmi 

of FAF

20 Less than Minimum Flap 

Speed + 5 knots

any any

25 Less than Minimum Flap 

Speed + 14 knots

any any
54

Table 6. Test Conditions for Piloted Simulation

Condition 

Number

Guidance Condition 

Name

Arrival

Transition

Flying

Pilot

1 VNAV Nominal DANNY Capt

2 VNAV Nominal CHERI 1st Off.

3 VNAV Slow CHERI Capt

4 VNAV Slow DANNY 1st Off.

5 VNAV Vector CHERI Capt

6 VNAV Vector DANNY 1st Off.

7 LNG Nominal DANNY Capt

8 LNG Nominal CHERI 1st Off.

9 LNG Slow CHERI Capt

10 LNG Slow DANNY 1st Off.

11 LNG Vector CHERI Capt

12 LNG Vector DANNY 1st Off.

13a VNAV Stress CHERI 1st Off.

14a LNG Stress CHERI 1st Off.

13b VNAV Stress CHERI Capt

14b LNG Stress CHERI Capt
55

Table 7. Run ordering for the Piloted Simulation Experiment

1st run 2nd run 3rd run 4th run 5th run 6th run 7th run 8th run

crew 1 1 5 13 4 7 11 10 14

crew 2 12 14 9 8 13 2 6 3

crew 3 14 12 9 8 13 6 2 3

crew 4 5 13 1 4 10 11 14 7

crew 5 5 1 4 13 7 11 14 10

crew 6 14 9 12 8 6 2 3 13

crew 7 8 14 9 12 13 3 6 2

crew 8 13 5 1 4 7 10 11 14

crew 9 4 13 5 1 10 7 14 11

crew 10 12 14 8 9 2 6 13 3

crew 11 9 8 12 14 3 6 2 13

crew 12 5 13 4 1 7 11 14 10

Table 8.- Test matrix for Louisville scenarios

Guidance (2) Route (2) Winds (3) ATC Speeds (7) Weight / Vref (17)

ENAV CHERI Half Nominal nominal 160000 / 117

VNAV DANNY Nominal 190 / 170 / 160 163000 / 118

Double Nominal 200 / 180 / 160 165000 / 119

210 / 180 / 170 168000 / 120

220 / 190 / 170 170000 / 121

230 / 200 / 170 173000 / 122

240 / 200 / 180 175000 / 123

178000 / 124

180000 / 125

183000 / 126

185000 / 127

188000 / 128

190000 / 129

193000 / 130

195000 / 131

198000 / 132

200000 / 133

Table 9.- Test matrix for wind evaluation scenarios in Atlanta

Wind (679) Guidance (2) Route (2) Weight / Vref (4)

ENAV East Arrival 160000 / 117

VNAV West Arrival 175000 / 123

185000 / 127

200000 / 133
56

Table 10.- Test matrix for speed interruption scenarios in Atlanta

Guidance (2) Route (2) Winds (4) Speed location (3) Speed schedule (6) Weight / Vref (4)

ENAV East arrival Headwind early 210 / 190 / 170 160000 / 117

VNAV West arrival Tailwind nominal 220 / 200 / 170 175000 / 123

Southerly late 220 / 200 / 180 185000 / 127

Northerly 230 / 210 / 180 200000 / 133

230 / 210 / 190

240 / 220 / 190

Table 11. – Average noise reduction (3 to 15 nm from runway), in dB SEL

Condition VNAV LNG

Nominal 4.0 5.0

Slow 3.4 5.3

Vector 3.8 5.0

All 3.7 5.1

Table 12. – Average fuel used per run, and percent reduction for LNG runs

Condition Distance,

nm

VNAV Fuel LNG Fuel

Percent

reduction 

Average, lbs with LNG

Standard 

Deviation, 

lbs

Average, 

lbs

Standard 

Deviation, 

lbs

Nominal 83.7 1172.1 50.6 1140.0 63.6 2.7

Slow 83.9 1234.5 45.7 1131.0 38.4 8.4

Vector 88.4 1362.6 29.3 1242.3 75.3 4.5

Stress 83.2 1194.3 40.9 1087.3 52.6 9.0

All 84.8 1242.3 85.0 1165.0 100.2 6.2

Table 13. – Fuel reduction for VNAV and LNG runs compared to reference 5 Baseline

Fuel, 

VNAV

Fuel, 

LNG

Fuel,

Baseline -

VNAV

Fuel, 

Baseline -

LNG

Percent 

reduction,

VNAV

Percent

reduction,

LNG

Nominal 1150.2 1116.7 160.1 193.6 12.2 14.8

Slow 1209.2 1106.6 101.1 203.7 7.7 15.5

Vector 1245.7 1181.9 64.6 128.4 4.9 9.8

Stress 1183.8 1076.9 126.5 233.4 9.7 17.8

Common distance of 82.6 nm.

Mean fuel for Baseline (reference 5) runs = 1310.3 lbs.
57

Table 14. Pilot Workload Ratings (out of 10 possible)

(a) in Descent

VNAV 

Mean 

VNAV

StdDev

LNG 

Mean

LNG

StdDev

Nominal 1.9 0.9 1.5 0.7

Slow 1.8 0.7 1.7 0.7

Vector 2.0 0.9 2.2 1.0

Stress 1.8 0.9 1.8 1.2

All 1.9 0.1 1.8 0.3

(b) after SILNT

VNAV 

Mean

VNAV 

StdDev

LNG 

Mean

LNG 

StdDev

Nominal 2.2 1.0 1.9 0.9

Slow 2.0 0.9 2.4 1.0

Vector 2.5 1.1 2.4 1.0

Stress -- -- -- --

All 2.2 0.2 2.2 0.3

(c) after BLGRS

VNAV 

Mean 

VNAV 

StdDev 

LNG 

Mean

LNG 

StdDev

Nominal 2.1 1.0 2.1 0.8

Slow 2.1 1.1 1.9 0.6

Vector 2.3 1.3 2.1 0.8

Stress 2.5 0.9 2.7 1.5

All 2.2 0.1 2.0 0.1
58

Table 15. Fuel usage at runway for Louisville batch simulation

(a) Speed conditions

Speed

Condition

VNAV ENAV ENAV - VNAV

number Mean

(lbs)

Stdev

(lbs)

number Mean

(lbs)

Stdev

(lbs)

Absolute

(lbs)

Percent

nominal 102 968.7 58.3 102 911.7 49.1 -57.0 -5.9

190 / 170 / 160 102 1170.2 72.0 101 1034.5 66.8 -135.7 -11.6

200 / 180 / 160 102 1120.9 68.3 102 993.2 53.8 -127.8 -11.4

210 / 180 / 170 102 1072.2 71.4 102 950.9 65.4 -121.3 -11.3

220 / 190 / 170 102 1011.5 59.1 102 943.3 51.7 -68.2 -6.7

230 / 200 / 170 102 979.8 53.7 102 948.9 53.7 -30.8 -3.1

240 / 200 / 180 102 957.0 60.5 102 955.6 50.2 -1.4 -0.1

All 714 1040.0 99.4 713 962.5 66.9 -77.6 -7.5

(b) Wind conditions

Wind

Condition

VNAV ENAV ENAV - VNAV

number Mean

(lbs)

Stdev

(lbs)

number Mean

(lbs)

Stdev

(lbs)

Absolute

(lbs)

Percent

nominal 238 1047.8 88.3 237 971.7 46.6 -76.1 -7.3

double 238 974.3 83.7 238 900.6 36.9 -73.7 -7.6

half 238 1098.0 84.5 238 1015.2 56.6 -82.9 -7.5

All 714 1040.0 99.4 713 962.5 66.9 -77.6 -7.5
59

Table 16. Fuel usage for Atlanta batch simulation

(a) Wind conditions

Runway

All Guidance VNAV ENAV ENAV - VNAV

Arrival Number Mean

(lbs)

Stdev

(lbs)

Number Mean

(lbs)

Stdev

(lbs)

Number Mean

(lbs)

Stdev Absolute

(lbs)

Percent

East 5384 1586.0 226.9 2692 1610.6 230.6 2692 1561.4 220.5 -49.2 -3.1

West 5384 1176.7 104.0 2692 1201.0 99.9 2692 1152.4 102.4 -48.6 -4.0

All 10768 1381.4 270.3 5384 1405.8 271.2 5384 1356.9 267.2 -48.9 -3.5

Final Approach (Waypoint ELLLE)

All Guidance VNAV ENAV ENAV - VNAV

Arrival Number Mean

(lbs)

Stdev

(lbs)

Number Mean

(lbs)

Stdev

(lbs)

Number Mean

(lbs)

Stdev

(lbs)

Absolute

(lbs)

Percent

East 5384 1302.6 199.7 2692 1311.4 199.7 2692 1293.9 199.4 -17.6 -1.3

West 5384 892.2 116.4 2692 899.9 116.5 2692 884.4 115.8 -15.5 -1.7

All 10768 1097.4 262.4 5384 1105.7 262.8 5384 1089.2 261.7 -16.5 -1.5

(b) ATC speed conditions

ATC VNAV ENAV ENAV - VNAV

Speed Location Number Mean

(lbs)

Stdev

(lbs)

Number Mean

(lbs)

Stdev

(lbs)

Absolute

(lbs)

Percent

Nominal N/A 32 1379.9 212.2 32 1330.1 212.6 -49.8 -3.6

210_190_170 Early 32 1485.7 228.2 32 1428.2 217.8 -57.5 -3.9

220_200_170 Early 32 1446.5 227.9 32 1388.8 211.2 -57.7 -4.0

220_200_180 Early 32 1433.6 226.2 32 1364.6 210.0 -69.0 -4.8

230_210_180 Early 32 1406.4 235.3 32 1343.0 214.6 -63.3 -4.5

210_190_170 Nominal 32 1458.9 212.0 32 1413.0 217.1 -46.0 -3.1

220_200_170 Nominal 32 1422.3 211.9 32 1377.8 209.4 -44.5 -3.1

220_200_180 Nominal 32 1408.9 210.6 32 1353.7 207.8 -55.2 -3.9

230_210_180 Nominal 32 1378.4 210.3 32 1338.1 214.0 -40.3 -2.9

210_190_170 Late 32 1445.7 210.8 32 1407.8 218.5 -37.9 -2.6

220_200_170 Late 32 1405.5 210.6 32 1392.9 211.7 -12.5 -0.9

220_200_180 Late 32 1403.4 210.0 32 1366.6 213.0 -36.8 -2.6

230_210_180 Late 32 1356.9 213.0 32 1376.4 216.2 19.5 1.4

All All 416 1417.9 216.6 416 1375.5 212.3 -42.4 -3.0
60

Table 17. Noise levels for Louisville batch simulation

(a) Speed conditions

Speed

Condition

VNAV ENAV ENAV - VNAV

number Mean

(dB)

Stdev

(dB)

number Mean

(dB)

Stdev

(dB)

Mean

(dB)

nominal 102 61.9 0.2 102 60.3 0.4 -1.6

190 / 170 / 160 102 64.6 0.4 102 61.0 1.2 -3.6

200 / 180 / 160 102 63.7 1.5 102 60.1 0.1 -3.6

210 / 180 / 170 102 63.7 1.3 102 59.8 0.5 -3.9

220 / 190 / 170 102 62.7 0.9 102 60.8 0.2 -1.9

230 / 200 / 170 102 62.9 0.4 102 61.5 0.5 -1.4

240 / 200 / 180 102 61.9 1.2 102 61.5 0.6 -0.4

All 714 63.0 1.3 714 60.7 0.9 -2.3

(b) Wind conditions

Wind

Condition

VNAV ENAV ENAV - VNAV

number Mean

(dB)

Stdev

(dB)

number Mean

(dB)

Stdev

(dB)

Mean

(dB)

nominal 238 63.1 1.3 238 60.7 0.8 -2.4

double 238 62.7 1.4 238 60.9 0.9 -1.9

half 238 63.3 1.2 238 60.6 0.9 -2.8

All 714 63.0 1.3 714 60.7 0.9 -2.3
61

Table 18. Noise levels for Atlanta batch simulation

(a) Wind conditions

Runway

All Guidance VNAV ENAV ENAV - VNAV

Arrival Number Mean

(dB)

Stdev

(dB)

Number Mean

(dB)

Stdev

(dB)

Number Mean

(dB)

Stdev

(dB)

Mean

(dB)

East 5384 60.3 1.5 2692 61.5 1.2 2692 59.2 0.6 -2.3

West 5384 60.4 1.6 2692 61.6 1.3 2692 59.2 0.6 -2.4

All 10768 60.4 1.5 5384 61.5 1.3 5384 59.2 0.6 -2.4

(b) ATC speed conditions

ATC VNAV ENAV ENAV - VNAV

Speed Location Number Mean

(dB)

Stdev

(dB)

Number Mean

(dB)

Stdev

(dB)

Mean

(dB)

Nominal N/A 32 60.9 1.2 32 58.5 0.3 -2.4

210_190_170 Early 32 63.0 0.3 32 60.3 2.6 -2.7

220_200_170 Early 32 62.2 1.4 32 60.1 2.6 -2.2

220_200_180 Early 32 62.1 2.0 32 58.9 0.3 -3.3

230_210_180 Early 32 61.3 2.1 32 58.8 0.3 -2.5

210_190_170 Nominal 32 62.0 1.8 32 59.8 2.4 -2.2

220_200_170 Nominal 32 61.9 1.5 32 59.7 2.3 -2.2

220_200_180 Nominal 32 61.4 1.8 32 58.7 0.2 -2.8

230_210_180 Nominal 32 61.4 1.7 32 58.6 0.2 -2.8

210_190_170 Late 32 62.5 0.6 32 61.5 0.8 -0.9

220_200_170 Late 32 63.1 0.3 32 62.2 0.8 -0.8

220_200_180 Late 32 63.1 0.3 32 61.8 0.6 -1.3

230_210_180 Late 32 61.9 0.7 32 62.4 0.3 0.5

All All 416 62.1 1.5 416 60.1 2.0 -2.0
62

Table 19. Stabilization altitude for Louisville batch simulation

(a) Speed conditions

Speed

Condition

VNAV ENAV

number Mean

(ft, AGL)

Stdev

(ft, AGL)

number Mean

(ft, AGL)

Stdev

(ft, AGL)

nominal 102 1170.4 71.7 102 1008.2 3.5

190 / 170 / 160 102 1181.7 83.1 101 1055.4 93.5

200 / 180 / 160 102 1182.0 81.1 102 1041.4 51.7

210 / 180 / 170 102 1172.6 73.2 102 1011.5 14.7

220 / 190 / 170 102 1170.5 70.1 102 1030.6 53.9

230 / 200 / 170 102 1170.6 71.3 102 1042.2 65.3

240 / 200 / 180 102 1157.4 57.7 102 1012.1 17.1

All 714 1172.2 73.1 713 1028.7 54.7

(b) Wind conditions

Wind

Condition

VNAV ENAV

number Mean

(ft, AGL)

Stdev

(ft, AGL)

number Mean

(ft, AGL)

Stdev

(ft, AGL)

nominal 238 1166.7 79.6 237 1032.2 47.0

double 238 1175.4 78.6 238 1026.7 69.7

half 238 1174.4 59.2 238 1027.3 43.6

All 714 1172.2 73.1 713 1028.7 54.7
63

Table 20. Stabilization altitude for Atlanta batch simulation

(a) Wind conditions

All Guidance VNAV ENAV

Arrival Number Mean

(ft, AGL)

Stdev

(ft, AGL)

Number Mean

(ft, AGL)

Stdev

(ft, AGL)

Number Mean

(ft, AGL)

Stdev

(ft, AGL)

East 5384 1021.8 101.0 2692 1050.6 121.4 2692 993.0 63.3

West 5384 1022.1 100.3 2692 1050.6 120.4 2692 993.6 63.0

All 10768 1021.9 100.6 5384 1050.6 120.9 5384 993.3 63.2

(b) ATC speed conditions

ATC VNAV ENAV

Speed Location Number Mean

(ft, AGL)

Stdev

(ft, AGL)

Number Mean

(ft, AGL)

Stdev

(ft, AGL)

Nominal N/A 32 1053.5 103.8 32 989.8 58.6

210_190_170 Early 32 1055.7 102.1 32 1024.8 24.4

220_200_170 Early 32 1057.5 100.2 32 1024.9 25.7

220_200_180 Early 32 1027.5 100.1 32 928.7 109.8

230_210_180 Early 32 1031.8 99.7 32 932.0 109.3

210_190_170 Nominal 32 1059.6 98.4 32 1025.1 25.7

220_200_170 Nominal 32 1058.4 98.8 32 1024.8 25.9

220_200_180 Nominal 32 1032.1 104.2 32 927.5 110.7

230_210_180 Nominal 32 1029.8 102.8 32 922.7 111.0

210_190_170 Late 32 1042.0 110.5 32 1021.4 26.0

220_200_170 Late 32 1000.4 131.2 32 1018.7 25.8

220_200_180 Late 32 991.4 126.5 32 965.9 78.2

230_210_180 Late 32 776.3 235.2 32 965.4 83.9

All All 416 1016.6 140.2 416 982.4 83.1
64

Figure 1. Illustration of vertical profiles for a CDA and current-day approach profile.

Figure 2. NASA LaRC Research Flight Deck Simulator
65

Figure 3. SILENT CDA RNAV STAR used for this experiment.
66

Figure 4. Atlanta RNAV arrival routes.

Figure 5. MACS Controller display showing traffic.

ELLLE

HAVAD

COSEL

BYRDS

DIRTY

FLCON

ODF

NOFIV FANEW

NOTRE

RPTOR

HERKO

VIKNN

CALCO

R-26R AJAAY BALLI

IC SFO

IC ORF

PEACHTREE
67

Figure 6. Lateral paths of piloted test runs.

37.5

38

38.5

39

39.5

-87.5 -87 -86.5 -86 -85.5

Latitude, deg

Longitude, deg

Nominal and Slow Routes

DANNY

SILNT SPYRS

BLGRS

CHRCL

RUNWAY CHERI

Vector Routes

Stress

Route

Initial Condition 1

Initial Condition 2
68

(a) Wind speed.

(b) Wind direction.

Figure 7. Wind profiles for Atlanta wind models.

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 10 20 30 40 50 60 70 80 90 100

Altitude, feet

Wind speed, knots

All Data

Headwind

Tailwind

South Crosswind

North Crosswind

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 30 60 90 120 150 180 210 240 270 300 330 360

Altitude, feet

Wind direction, degrees (from)

All Data

Headwind

Tailwind

South Crosswind

North Crosswind
69

a) Altitude profile.

b) Calibrated airspeed profile.

Figure 8. Vertical profiles for Nominal test condition.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Altitude, feet

Distance to runway, nmi

VNAV

LNG

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

VNAV

LNG
70

a) Altitude profile.

b) Calibrated airspeed profile.

Figure 9. Vertical profiles for Slow test condition.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Altitude, feet

Distance to runway, nmi

VNAV

LNG

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

VNAV

LNG
71

a) Altitude profile.

b) Calibrated airspeed profile.

Figure 10. Vertical profiles for Vector test condition.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Pressure altitude, feet

Distance to runway, nmi

VNAV

LNG

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

VNAV

LNG
72

a) Altitude profile.

b) Calibrated airspeed profile.

Figure 11. Vertical profiles for Stress test condition.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Pressure altitude, feet

Distance to runway, nmi

VNAV

LNG

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

VNAV

LNG
73

Figure 12. Fuel used in descent.

0.0

5.0

10.0

15.0

20.0

25.0

Nominal Slow Vector Stress

Run Type

Fuel used per nm path distance

VNAV descent

LNG descent

Baseline fuel/nm
74

Figure 13. Bedford Workload Rating Scale

Was it

possible to

complete the

task?

No Task abandoned. Pilot unable

to apply sufficient effort.

Was

workload

satisfactory

without

reduction?

Was

workload

tolerable for

this task?

Yes

Yes

Yes

No

Workload insignificant.

Workload low.

Enough spare capacity for all

desirable additional tasks.

Insufficient spare capacity for

easy attention to additional tasks.

Reduced spare capacity;

additional tasks cannot be given

the desired amount of attention.

Little spare capacity; level

effort allows little attention

to additional tasks.

Very little spare capacity, but

maintenance of effort in the

primary tasks not in question.

Very high workload with almost

no spare capacity. Difficulty in

maintaining level of effort.

Extremely high workload. No

spare capacity. Serious doubts as to

ability to maintain level of effort.

1

4

5

6

7

8

9

10

Start

No

2

3
75

Figure 14. Locations of workload ratings

37.5

38

38.5

39

39.5

-87.5 -87 -86.5 -86 -85.5

Latitude, deg

Longitude, deg

DANNY

SILNT SPYRS

BLGRS

CHRCL

RUNWAY CHERI

Initial Condition 1

Initial Condition 2

Workload locations

A

A

A

A

B

C
76

(a) Nominal Conditions

(b) Slow Conditions

(c) Vector Conditions

(d) Stress Conditions

Figure 15. Workload ratings.

Point C

Point B

Point A

Pilot Ratings

1 2 3 4 5 6 7 8 9 10

VNAV runs

LNG runs

Nominal Conditions

Point C

Point B

Point A

Pilot Ratings

1 2 3 4 5 6 7 8 9 10

VNAV runs

LNG runs

Slow Conditions

Pilot Ratings

1 2 3 4 5 6 7 8 9 10

VNAV runs

LNG runs

Point C

Point B

Point A

Vector Conditions

Pilot Ratings

1 2 3 4 5 6 7

VNAV runs

LNG runs

Stress Conditions Point C

Point A
77

(a) Nominal conditions

(b) Slow conditions

Figure 16. Post-run pilot ratings for VNAV conditions.

VNAV Nominal

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage

VNAV Slow

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage
78

(c) Vector conditions

(d) Stress conditions

Figure 16. Concluded.

VNAV Vector

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage

VNAV Stress

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage
79

(a) Nominal conditions

(b) Slow conditions

Figure 17. Post-run pilot ratings for LNG.

LNG Nominal

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage

LNG Slow

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage
80

(c) Vector conditions

(d) Stress conditions

Figure 17. Concluded.

LNG Vector

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage

LNG Stress

Pilot Rating

0 1 2 3 4 5 6 7

Vertical Path

Speed

Lateral Path how acceptable

how clear

how difficult to manage
81

Figure 18. Results from post-test questionnaire, question number 22.

Figure 19. Results from post-test questionnaire, questions number 23 and 27.

Figure 20. Results from post-test questionnaire, question number 28.

How much did you use the LNG tool display during the LNG runs?

entire time most of the time about half not very much not at all

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

46% 50%

4% 0% 0%

Which part of the LNG tool

did you find most useful?

diamond limits events other

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

50%

8%

29%

17%

Which LNG energy indicator display

did you use the most?

PFD ND both equally

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

67%

12%

21%

Where would you prefer to have the LNG energy indicator?

PFD ND both

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

75%

4%

21%
82

Figure 21. Results from post-test questionnaire, question number 29.

Figure 22. Results from post-test questionnaire, question number 30.

Figure 23. Results from post-test questionnaire, question number 31.

Would you be comfortable with the LNG displayed:

N Y N Y N Y

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

only on the PFD only on the ND should be in both places

14%

86%

58%

42% 53% 47%

For most pilots, where do you think it would be most appropriate to have the LNG energy indicator?

PFD ND both

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

67%

8%

25%

In normal day-to-day operations, how often

do you use VNAV below 11000 feet

every flight often sometimes never

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

In normal day-to-day operations, how often

do you use VNAV below 6000 feet

every flight often sometimes never

number of responses

0

2

4

6

8

10

12

14

16

18

20

22

24

29% 25%

46%

0% 0%

29% 29%

42%
83

(a) VNAV altitude profiles for Nominal runs..

(b) VNAV calibrated airspeed profiles for Nominal runs.

Figure 24. Comparison of piloted and batch trajectories for Nominal runs.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Altitude, feet

Distance to runway, nmi

Piloted Simulation

Batch Simulation

0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

Piloted Simulation

Batch Simulation
84

(a) ENAV altitude profiles for Nominal runs.

(b) ENAV calibrated airspeed profiles for Nominal runs..

Figure 24. Concluded.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Altitude, feet

Distance to runway, nmi

Piloted Simulation

Batch Simulation

0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

Piloted Simulation

Batch Simulation
85

(a) VNAV altitude profiles for Slow runs.

(b) VNAV calibrated airspeed profiles for Slow runs.

Figure 25. Comparison of piloted and batch trajectories for Slow runs.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Altitude, feet

Distance to runway, nmi

Piloted Simulation

Batch Simulation

0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

Piloted Simulation

Batch Simulation
86

(a) ENAV altitude profiles for Slow runs.

(b) ENAV calibrated airspeed profiles for Slow runs.

Figure 25. Concluded.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10 20 30 40 50 60 70 80 90

Altitude, feet

Distance to runway, nmi

Piloted Simulation

Batch Simulation

0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90

Calibrated airspeed, knots

Distance to runway, nmi

Piloted Simulation

Batch Simulation
87

Figure 26. Comparison of flap extension speeds for piloted and batch simulations.

Figure 27. Flap extension distance to runway for piloted and batch simulations.

0 5 10 15 20 25 30 35

130

140

150

160

170

180

190

200

210

220

230

Flap setting, degrees

Calibrated airspeed, knots

Piloted simulation

Batch simulation

Minimum limit

0 5 10 15 20 25 30 35

0

5

10

15

20

25

30

35

Flap setting, degrees

Distance to runway, nmi

Piloted simulation

Batch simulation
REPORT DOCUMENTATION PAGE Form Approved

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 4. TITLE AND SUBTITLE

Energy Navigation: Simulation Evaluation and Benefit Analysis 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Williams, David H.; Oseguera-Lohr, Rosa M.; Lewis, Elliot T.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

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14. ABSTRACT

This paper presents results from two simulation studies investigating the use of advanced flight-deck-based energy navigation 

(ENAV) and conventional transport-category vertical navigation (VNAV) for conducting a descent through a busy terminal 

area, using Continuous Descent Arrival (CDA) procedures. This research was part of the Low Noise Flight Procedures 

(LNFP) element within the Quiet Aircraft Technology (QAT) Project, and the subsequent Airspace Super Density Operations 

(ASDO) research focus area of the Airspace Project. A piloted simulation study addressed development of flight guidance, and 

supporting pilot and Air Traffic Control (ATC) procedures for high density terminal operations. The procedures and charts 

were designed to be easy to understand, and to make it easy for the crew to make changes via the Flight Management 

Computer Control-Display Unit (FMC-CDU) to accommodate changes from ATC. 

15. SUBJECT TERMS

Air traffic control; Descent; Flight management systems; Navigation; Simulation

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3. DATES COVERED (From - To)

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

411931.02.61.07.01 

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/T3-2011-217167

16. SECURITY CLASSIFICATION OF:

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 08 - 2011