Yuan Liu

N&R Engineering, Parma Heights, Ohio

Jonathan S. Litt

Glenn Research Center, Cleveland, Ohio

T. Shane Sowers

Vantage Partners, Brook Park, Ohio

A. Karl Owen

Consultant, North Royalton, Ohio

Ten-Huei Guo

Glenn Research Center, Cleveland, Ohio

Application and Evaluation of Control Modes for

Risk-Based Engine Performance Enhancements

NASA/TM—2015-218452

January 2015

AIAA–2014–3637
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Yuan Liu

N&R Engineering, Parma Heights, Ohio

Jonathan S. Litt

Glenn Research Center, Cleveland, Ohio

T. Shane Sowers

Vantage Partners, Brook Park, Ohio

A. Karl Owen

Consultant, North Royalton, Ohio

Ten-Huei Guo

Glenn Research Center, Cleveland, Ohio

Application and Evaluation of Control Modes for

Risk-Based Engine Performance Enhancements

NASA/TM—2015-218452

January 2015

AIAA–2014–3637

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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NASA/TM—2015-218452 1 

Application and Evaluation of Control Modes for 

Risk-Based Engine Performance Enhancements 

Yuan Liu 

N&R Engineering 

Parma Heights, Ohio 44130 

Jonathan S. Litt 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

T. Shane Sowers 

Vantage Partners, LLC 

Brook Park, Ohio 44142 

A. Karl Owen 

Consultant 

North Royalton, Ohio 44133 

Ten-Huei Guo 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The engine control system for civil transport aircraft imposes operational limits on the propulsion 

system to ensure compliance with safety standards. However, during certain emergency situations, 

aircraft survivability may benefit from engine performance beyond its normal limits despite the increased 

risk of failure. Accordingly, control modes were developed to improve the maximum thrust output and 

responsiveness of a generic high-bypass turbofan engine. The algorithms were designed such that the 

enhanced performance would always constitute an elevation in failure risk to a consistent predefined 

likelihood. This paper presents an application of these risk-based control modes to a combined 

engine/aircraft model. Through computer and piloted simulation tests, the aim is to present a notional 

implementation of these modes, evaluate their effects on a generic airframe, and demonstrate their 

usefulness during emergency flight situations. Results show that minimal control effort is required to 

compensate for the changes in flight dynamics due to control mode activation. The benefits gained from 

enhanced engine performance for various runway incursion scenarios are investigated. Finally, the control 

modes are shown to protect against potential instabilities during propulsion-only flight where all aircraft 

control surfaces are inoperable. 

Nomenclature 

Alt altitude 

C-MAPSS40k Commercial Modular Aero-Propulsion System Simulation 40k 

EOL end-of-life 

EPR engine pressure ratio 

FAA Federal Aviation Administration 
NASA/TM—2015-218452 2 

FADEC full authority digital engine control 

FPA flight path angle 

FR faster response 

Hdg heading 

HPC high-pressure compressor 

HPT high-pressure turbine 

NASA National Aeronautics and Space Administration 

OT overthrust 

PCA propulsion controlled aircraft 

PIO pilot-induced oscillation 

PLA power lever angle 

Ps30 combustor static pressure 

T48 high-pressure turbine exit total temperature 

TO takeoff 

TCM Transport Class Model 

VBV variable bleed valve 

VSV variable stator vane 

Ȗ flight path angle 

ij roll angle 

1.0 Introduction 

The performance of propulsion systems used on modern civil transport aircraft is generally limited by 

mandatory adherence to safety standards set by regulatory bodies such as the Federal Aviation 

Administration (FAA). Limits placed on operating parameters such as spool speeds and gas temperatures 

serve to minimize the probability of engine failure but also represent a ceiling on performance capabilities 

such as thrust production and responsiveness. However, this prioritization may not be optimal during 

certain emergency scenarios. Namely, maximizing the chance of survival may require accepting a higher 

risk of engine failure (Ref. 1). 

Numerous studies have been conducted into the possibility, benefits, and risks of pushing engine 

performance beyond maximum design limits. Recent examples of such efforts include applying sliding 

mode controllers (Ref. 2) and L1 adaptive control theory (Ref. 3) to improve dynamic engine response. 

Researchers at the National Aeronautics and Space Administration (NASA) have developed control 

modes that extract greater thrust output (overthrust mode) and improve thrust responsiveness to throttle 

commands (faster response mode) with an emphasis on minimizing changes to the conventional engine 

control architecture (Refs. 4 and 5). Studies have also outlined the benefits of enhancing maximum engine 

performance, particularly with respect to directional stability and control for a damaged aircraft (Refs. 6 

to 9). Additional scenarios such as runway incursions and propulsion-only flight have also been evaluated 

via piloted simulations (Ref. 10). Investigations have also been conducted into characterizing the engine 

failure risk that accompanies the usage of these enhanced performance modes. Statistical methods were 

used to quantify the likelihood of disk and blade failure and compressor instabilities due to overthrust and 

faster response, respectively (Refs. 11 and 12). A recent study utilized these statistical models to redesign 

the NASA-developed overthrust and faster response control modes such that the enhanced performance 

delivered is based on the elevation of failure risk to a consistent predefined level (Ref. 13). This paper 

documents the continuation and application of that effort. 

 
NASA/TM—2015-218452 3 

The objective of this paper is to present and evaluate the application of the risk-based enhanced 

performance control modes. A brief description of the control modes is provided in Section 2.0. The control 

modes were implemented in a combined engine/aircraft digital simulation and evaluated using both an 

autopilot flight control system and realistic flight simulator hardware operated by an experienced pilot. The 

engine and aircraft models are used extensively in research and well-documented (Refs. 14 and 15). 

Section 3.0 provides a concise overview of the computer models and the flight simulator system. Section 4.0 

presents the experimental methodology and results. Using the engine/aircraft model and flight simulator 

system, the control modes were tested in various emergency situations to determine their effectiveness. The 

scenarios tested represent a range of severity. First, the effects of simply activating the control modes during 

steady, level flight were studied. Next, the control modes were evaluated for a runway incursion situation. 

Runway incursions cause a sudden reduction in available takeoff distance (Ref. 16), which additional thrust 

may be able to overcome. Finally, the control modes were applied to propulsion-only flight control. There 

have been several instances where airframe malfunction or damage caused the failure of all flight control 

surfaces and the crew resorted to engine thrust modulation to fly the aircraft (Refs. 17 to 19). These 

incidents prompted extensive research by NASA into propulsion-controlled aircraft (PCA) flight, which 

identified slow thrust response as a hindrance (Refs. 20 and 21). Simulation results demonstrate the benefits 

provided by the control modes for a propulsion-controlled aircraft. 

2.0 Control Modes 

Two types of engine performance enhancements are considered and evaluated in this paper; both 

involve modifying the control system to improve the thrust output characteristics of the engine. 

Overthrust (OT) refers to pushing the engine beyond its designed maximum thrust output capability. 

Faster response (FR) involves speeding up the dynamics of the thrust output, especially for throttle 

transients from low power settings (e.g., flight idle). This section briefly describes the control modes that 

provide these performance enhancements. The interested reader is referred to the dedicated publication 

for a more complete treatment (Ref. 13). 

Exceeding the maximum design performance limits naturally increases the likelihood of mishap within 

the engine. Therefore, a basic premise for the control modes that were developed is that their design 

should be based on models of component failure probability. With such a model, the risk of failure can be 

quantified based on operating conditions whether the engine operates within or beyond its normal limits. 

Conversely, the extent by which engine performance may be increased can be quantified by defining a 

maximum allowable probability of failure. This definition also acts as the objective for the control mode 

design. The concept is best elucidated through examples; the following subsections describe the 

architecture of the OT and FR control modes. 

2.1 Overthrust Mode 

Increasing thrust output requires hotter gas temperatures within the engine and faster spool speeds. 

Thus, the design of the OT control mode was based on the risk of compressor/turbine disk failure and 

turbine blade failure. Under normal operation, these failure probabilities vary greatly based on factors 

such as engine power, health, and ambient conditions, but must remain below 10–5 per flight hour per 

regulations defined by the FAA (Ref. 22). The design objective of the OT control mode was to increase 

maximum thrust output such that it corresponded to a consistent risk of failure of 10–3 per hour. 

A statistical model was used to relate the probability of failure to operating parameters such as spool 

speeds and gas temperatures (Ref. 11). The control mode algorithms were designed based on a simplified 

version of this risk model: disk failure risk was calculated as a function of core speed, and blade failure 

risk a function of core speed and high-pressure turbine (HPT) exit total temperature (T48). As such, the 

10–3 failure rate limit can be converted to a single core speed limit for disk failure and a two-variable 

temperature/speed threshold for blade failure. Thus, when activated, the OT mode disables the default 

speed and temperature protection logic within the engine control system. During maximum overthrust, the 
NASA/TM—2015-218452 4 

control mode uses feedback logic to push the operating point of the engine to the aforementioned disk and 

blade failure thresholds, stopping at whichever limit is reached first. Reproduced from Reference 13, 

Figure 1 compares the probability of blade failure with overthrust against that with maximum baseline 

thrust for 180 different flight and engine health conditions. Figure 2 plots the HPT exit temperatures and 

core speeds of the same 180 overthrust cases with a threshold representing the 10–3 blade failure risk. In 

general, the OT mode consistently increases the failure risk to the desired elevated failure rate limit by 

driving the engine to the temperature/speed threshold. The hollow points in both figures represent 

conditions where a combustor static pressure (Ps30) limit, which was not bypassed, was reached before 

the risk threshold. Increases in the net thrust output for the 180 cases ranged from 10 to 25 percent over 

the maximum design level. 

Figure 1.—Overthrust operation at various flight conditions. 

OT control mode increases maximum thrust output until a 

predefined failure probability unless maximum combustor 

pressure limit was reached. 

Figure 2.—Values of HPT exit temperature and core speed during 

overthrust operation at various flight conditions. Maximum 

combustor pressure limit prevents operating on the desired risk 

threshold for certain conditions. 

1.EͲ07

1.EͲ06

1.EͲ05

1.EͲ04

1.EͲ03

1.EͲ02

1.EͲ01

1.E+00

0 20 40 60 80 100 120 140 160 180

Failure Probability

Flight Condition

Baseline Risk

Elevated Risk

Baseline, Max. Power

Overthrust, Ps30 Active

Overthrust

1900

1950

2000

2050

2100

2150

2200

11500 11700 11900 12100 12300 12500

T48,

R

Core Speed, RPM

Risk Threshold

Overthrust, Ps30 Active

Overthrust
NASA/TM—2015-218452 5 

2.2 Faster Response Mode 

When considering improvements in transient thrust response, the primary risk is usually high-pressure 

compressor (HPC) stall. Thus, the design of the FR control mode required a method to determine the 

probability of HPC stall during engine operation. Details of this stall risk calculation are available in 

Reference 13. Essentially, the process involved attaching an uncertainty element due to engine-to-engine 

variation to the stall margin output values from any deterministic compressor or engine simulation. This 

uncertainty was modeled using a statistical compressor stability stack-up, letting the deterministic 

simulation represent the mean of an engine fleet (Ref. 23). The result was a function that related the mean 

stall margin to the probability of stall stemming from engine-to-engine variation. This relationship 

(Figure 3, reproduced from Ref. 13) was used to guide the faster response mode design. Namely, the 

control mode objective was to shorten the thrust response time of the engine up to a certain allowable 

probability of stall. This limit was notionally set to 10–3, which corresponds to a mean stall margin of 

approximately 2.3 percent. 

The control mode design process employed several previously developed techniques to achieve faster 

thrust response (Refs. 4 and 5). First, the bandwidth of the primary fuel controller was increased to 

improve thrust response for relatively small throttle movements. Next, the customer bleed and HPC stator 

vane control schedules were modified to increase spool speeds at idle engine power. Finally, the 

acceleration control schedule was modified to allow the HPC to attain lower stall margins during large 

transients. Specifically, a new schedule was developed such that the minimum transient HPC stall margin 

would be approximately 2.3 percent for a wide range of operating conditions (Ref. 13). Figure 4 compares 

the minimum HPC stall margin attained during large throttle transients at 540 different flight and engine 

health conditions. Power transients using the baseline engine resulted in a large scatter of minimum stall 

margin values. With the faster response mode active, the minimum stall margin values—and hence, the 

probability of stall—were collapsed to a relatively consistent level for all operating conditions. 

Reductions in the time required for the idle-to-full-power transient generally ranged from 5 to 20 percent. 

Figure 3.—Relationship to determine probability of stall due to 

engine-to-engine variation when using a deterministic 

compressor/engine model. 

1.EͲ12

1.EͲ10

1.EͲ08

1.EͲ06

1.EͲ04

1.EͲ02

1.E+00

0.5 1.5 2.5 3.5 4.5 5.5

Stall Probability

Mean Stall Margin, %
NASA/TM—2015-218452 6 

Figure 4.—Minimum HPC stall margin attained during large 

throttle transients at various operating conditions. 

3.0 Simulations and Test Bed 

This section provides brief descriptions of the aircraft model, engine model, and the flight simulation 

test bed used to evaluate the performance enhancing control modes. 

3.1 Engine Simulation 

The control modes were applied to an engine simulation called the Commercial Modular 

Aero-Propulsion System Simulation 40k (C-MAPSS40k) (Ref. 14). Developed by NASA Glenn Research 

Center, C-MAPSS40k is a nonlinear dynamic model of a generic, high-bypass, dual-spool turbofan 

engine with a design thrust capability of approximately 40,000 lb. The overall engine simulation, which is 

written in a combination of MATLAB/Simulink (The Mathworks, Inc.) and C, is a modular 

interconnection of models representing major engine components such as the inlet, fan, compressors, 

combustor, turbines, and nozzles. The control inputs to the engine—fuel flow rate, variable stator vane 

(VSV) setting, and variable bleed valve (VBV) setting—are calculated by a comprehensive gainscheduled feedback control system representative of the Full Authority Digital Engine Control (FADEC) 

systems used on modern transport aircraft (Ref. 24). The control system utilizes sensed engine 

information—engine simulation output parameters processed by models that emulate sensor noise and 

dynamics. VSV and VBV commands are scheduled on spool speeds and inlet conditions. Fuel flow rate is 

calculated by gain-scheduled feedback control on engine pressure ratio (EPR). The fuel flow command is 

passed through a suite of limit protection controllers to prevent compressor stall and excursions beyond 

allowable speed, temperature, and pressure thresholds. The control commands are processed by actuator 

models that ensure the engine receives realistic input values. 

3.2 Aircraft Simulation 

A modified version of the Transport Class Model (TCM) (Ref. 15) was selected as the airframe 

simulation. Developed for controls research by NASA Langley Research Center, the TCM is a dynamic 

simulation of a modern, mid-sized, narrow-body, twin-engine, commercial transport aircraft implemented 

in the MATLAB/Simulink environment. The aircraft aerodynamic characteristics evolved from windtunnel testing of a 5.5 percent sub-scale model of a transport aircraft. The hypothetical, full-scale airframe 

that the TCM represents weighs 185,000 lb and has a wing area of 1,951 ft2

 and wing span of 125 ft. The 

simulation also contains models of the control surface actuators to capture realistic response times and 

nonlinearities. The default propulsion systems within the TCM were replaced by two copies of the 

C-MAPSS40k turbofan engine model. The control system for each C-MAPSS40k engine was augmented 

with the overthrust and faster response control modes. 

0

2

4

6

8

10

12

14

0 100 200 300 400 500

Minimum Stall Margin,

%

Flight Condition

Baseline

Fast Response
NASA/TM—2015-218452 7 

The TCM version used for this work also includes an autopilot-like flight control system that allows 

the user to easily execute any desired flight profile. The control system calculates the control surface 

deflections (e.g., elevator, aileron, etc.) and engine throttle positions required to meet air speed, altitude, 

and heading demands. The air speed controller (“auto-throttle”) is a feedback control system that 

automatically adjusts the engine throttle settings. The altitude controller consists of several nested 

feedback loops: the altitude feedback loop calls a flight path angle controller, which itself calls a pitch 

angle controller that calculates elevator deflection. Likewise, heading control is accomplished by calling a 

roll angle controller that calculates aileron deflection. These flight control algorithms are leveraged to 

demonstrate the benefits of enhanced engine performance. 

3.3 Flight Simulator 

A commercially available, self-contained flight simulator was used to obtain piloted evaluations of the 

control modes. The flight simulator system (Figure 5)—the Modular Flight Deck developed by Precision 

Flight Controls, Inc.—is a realistic two-seater cockpit that includes pilot and co-pilot yoke and pedal 

controls, throttle quadrant, three interior computer screens to simulate instrumentation, and five exterior 

high-definition screens for visualization. By default, this flight simulation system utilizes the X-Plane 

software (Ref. 25) for both flight dynamics and graphical rendering. For this application, however, the 

combined TCM/C-MAPSS40k model is used to drive the physics of the flight simulator, with X-Plane 

handling visualization only. 

Figure 5.—Interior view of flight simulation test bed for piloted evaluations. 
NASA/TM—2015-218452 8 

4.0 Application and Evaluation 

In order to evaluate the effects and benefits of the control modes, several simulation experiments were 

conducted using the TCM/C-MAPSS40k model and flight simulator system. The experiments fall into three 

categories: control mode activation, runway incursion, and propulsion-only flight. All three scenarios were 

simulated using the aforementioned TCM flight control system without human pilot input. This was to 

ensure consistency across all test cases for proper assessment of the control modes. In addition, piloted 

evaluations using the flight simulator hardware were performed for the propulsion-only scenario. 

4.1 Control Mode Activation 

A single switch was used to simultaneously activate both enhanced performance control modes. In the 

flight simulator, this switch is located on a custom overhead panel in the cockpit. The signal from this 

switch is linked to a flag variable within the engine control system that triggers both control modes. For the 

non-piloted computer simulation cases, this flag variable was manipulated directly. Once online, the faster 

response control mode increases thrust responsiveness to throttle movement for both small and large 

transients. The OT mode, though active, requests overthrust only when throttle position is near the top 

portion of its range of motion. The assumption is that if OT mode is active but the throttle position is not 

fully pushed forward, the pilot does not require OT and/or prefers OT to be readily available on standby. 

The above implementation also ensures that activating but not yet utilizing the control modes causes 

minimal disturbance to the flight parameters. In other words, pilot effort required to return the aircraft to 

its state prior to mode activation should be small to none. To examine this scenario, the TCM flight 

control system was used to execute a standard descent trajectory. The aircraft was commanded to descend 

wings-level from 5,000 ft at –3° flight path angle and 135 kts. At 2,000 ft, the overthrust and faster 

response modes were activated but the descent profile remained unchanged. These conditions represent a 

situation where enhanced engine performance could potentially be desirable. The aircraft is in a lowenergy state (altitude, speed, power) and any emergency resulting in an aborted landing would likely 

require a rapid increase in engine power. 

Figure 6 and Figure 7 present the time histories of several engine and aircraft parameters of interest, 

respectively, for a 200-sec window during the descent. The flight control system calculated the elevator 

and throttle inputs required to maintain a steady descent. The small oscillations present in the engine 

parameters (thrust and core speed) result from a numerical instability of the C-MAPSS40k engine 

simulation at low power settings. As shown by the time traces of altitude and air speed, the frequency and 

magnitude of these oscillations are not significant enough to affect the aircraft flight dynamics. The 

performance enhancing control modes were activated at 2,000 ft altitude, indicated by the red line on the 

throttle plot. Since the engine was operating at low-power settings, the OT mode had no effect on engine 

performance. The faster response mode commanded an offset to the scheduled HPC VSV position. As 

previously mentioned, this action increases spool speeds without appreciably impacting the thrust output 

beyond a brief transitional oscillation. As a result, the effect of mode activation on the aircraft flight 

dynamics is nearly imperceptible. 
NASA/TM—2015-218452 9 

Figure 6.—Effects of control mode activation on engine parameters. 

Figure 7.—Effects of control mode activation on flight parameters. 

150 200 250 300 350

12

13

14

15

Throttle, %

Mode Active

150 200 250 300 350

5200

5400

5600

5800

Thrust, lbf

150 200 250 300 350

9500

9600

9700

9800

9900

Core Speed, rpm

150 200 250 300 350

-6

-4

-2

0

2

dVSV, deg

Time, s

150 200 250 300 350

0

2000

4000

Altitude, ft

150 200 250 300 350

134

135

136

Air Speed, kt

150 200 250 300 350

-3.4

-3.2

-3

Elevator, deg

Time, s
NASA/TM—2015-218452 10 

4.2 Runway Incursion 

Runway incursion simulations were conducted to evaluate the effectiveness of the control modes during 

an emergency situation. It was expected that the extra thrust delivered by the OT mode would result in 

shorter takeoff (TO) distances. A total of 24 TO cases were simulated. Figure 8 summarizes the key 

characteristics common to all TO simulations. For each case, the aircraft was positioned stationary at x0 with 

TO flap positions, and engine throttles were increased from 0 to 90 percent. To simulate visual detection of 

an incursion, 100 percent throttle and 20° pitch up were commanded at some distance down the runway (xP). 

The distance at which the aircraft cleared an altitude of 50 ft (xC) was then recorded. This procedure was 

executed for six different values of xP with the baseline engine controller with no enhanced performance 

modes. To ascertain the benefits of using the enhanced performance control modes, these runs were 

repeated with the additional step of activating the control modes at xP (full throttle would, therefore, 

correspond to maximum overthrust). Additionally, this suite of baseline and enhanced performance 

simulations were carried out using both new (50-hr) and end-of-life (EOL) engines, by appropriately setting 

the deterioration level option within the C-MAPSS40k engine model, for the total of 24 TO runs. 

In order to examine in detail the relevant engine and flight parameters during a representative TO 

incursion run, we consider the case where xP equals 1,500 ft. In this case, an incursion was detected when 

the aircraft was 1,500 ft into the TO ground roll, at which point the throttles were increased from 90 to 

100 percent and the flight control system issued the appropriate elevator command to attempt to pitch the 

vehicle up to 20°. For the enhanced performance cases, mode activation occurred at this time as well. The 

variations in altitude, air speed, net thrust (both engines), exhaust gas temperature (EGT), and highpressure compressor stall margin throughout the TO run are shown in Figure 9. For all cases, the aircraft 

remains on the ground for at least 500 ft beyond xP before it gains enough air speed to take off. The 

distance required to take off and climb to 50 ft (xC) increases with declining engine health. In this case, 

this relationship is due to a maximum limit of 1,500 R imposed on the EGT of the baseline engines in 

order to comply with FAA failure risk requirements. As an engine ages, producing consistent thrust levels 

results in a higher EGT due to decreasing component efficiencies. As Figure 9 shows, the EGT of the 

baseline EOL engine was already near the 1,500 R limit at 90 percent throttle, before the incursion. 

Therefore, unlike the new engine, increasing throttle to 100 percent does not have a significant effect on 

thrust output. Usage of the OT mode, however, shortened xC by approximately 150 ft regardless of the 

engine health condition. In fact, xC for an EOL engine with OT is slightly shorter than that of a new 

baseline engine. None of the incursion cases posed a risk of compressor stall since the transient from 

90 percent to full power, though fast, was relatively small. 

Figure 8.—Runway incursion scenario. Aircraft is stationary at x0. Maximum engine power and 

20q pitch up command occurs at xP. Aircraft clears 50 ft altitude at xC. 
NASA/TM—2015-218452 11 

Figure 9.—Aircraft and engine parameters during an example 

incursion simulation (xP equals 1,500 ft) for baseline and 

enhanced performance cases. 

 

1500 2000 2500 3000

0

50

100

Altitude, ft

Incursion

Baseline, new

Baseline, EOL

Enhanced, new

Enhanced, EOL

1500 2000 2500 3000

80

100

120

140

Air Speed, kts

1500 2000 2500 3000

6

7

8

x 104

Thrust, lbf

1500 2000 2500 3000

1400

1500

1600

EGT, R

1500 2000 2500 3000

15

20

Stall Margin, %

Distance, ft
NASA/TM—2015-218452 12 

The improvement provided by the OT mode is possible because the conventional speed and 

temperature limits within the engine control system are bypassed. Instead, recall that maximum thrust 

output is determined by a risk threshold that is a function of core speed and HPT exit temperature (T48). 

These values, along with the risk boundary, are plotted in Figure 10 for the TO incursion simulation 

shown in Figure 9. The portion of each trajectory from the low-speed, low-temperature region of the 

graph (lower-left) up until the X marker (incursion detection) represents the initial engine spool-up from 

0 to 90 percent throttle. Hence, the baseline and enhanced cases are indistinguishable since the modes had 

not been activated yet. The remainder of the trajectory from X onwards represents the transient from 

90 to 100 percent throttle. For the baseline new engine, core speed and T48 increased slightly but 

remained well short of the risk boundary (as they should, per the normal allowance on the risk of failure). 

The trajectory of the baseline EOL engine beyond the 90 percent throttle point (i.e., X marker) is 

imperceptible due to the EGT limit. OT mode activation, however, signals the acceptance of an elevated 

failure risk. Thus, for both new and EOL cases, increasing throttle to maximum pushes core speed and 

T48 to the risk boundary. 

The results of all 24 incursion simulations are summarized in Figure 11 and Figure 12. Figure 11 

presents the relationship between the aircraft speed at xP (where full throttle and pitch up was initially 

commanded) and the distance traveled from xP to xC (where the aircraft cleared 50 ft altitude) for all runs. 

With the control modes, the aircraft equipped with EOL engines was able to generally outperform that 

with new (baseline) engines. The difference in xC between the baseline and enhanced performance cases 

is plotted in Figure 12 for both new and EOL engines. The benefits diminish as mode activation occurs at 

higher speeds. The aircraft configuration, and thus the minimum air speed required to take off, was 

identical for all runs. Hence, these results suggest the benefits provided by increased engine performance 

in the incursion scenario stem primarily from a faster ground acceleration to the minimum TO speed 

(with faster climb to 50 ft as a secondary factor). 

Figure 10.—Engine risk parameters relative to overthrust risk threshold during 

incursion simulation. 

 

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3

x 104

1200

1400

1600

1800

2000

2200

2400

Core Speed, RPM

T48, R

Risk Boundary

Baseline, new

Baseline, EOL

Enhanced, new

Enhanced, EOL

Incursion
NASA/TM—2015-218452 13 

Figure 11.—Effects of engine performance enhancements on the additional 

distance beyond xP needed to clear 50 ft. 

Figure 12.—Improvement in distance required to clear 50 ft due to engine 

performance enhancements. 

 

800

1300

1800

2300

2800

3300

20 40 60 80 100 120 140

Additional distance to clear 50', ft

Speed at incursion detection, kts

Baseline, new

Baseline, EOL

Enhanced, new

Enhanced, EOL

0

50

100

150

200

250

300

350

20 40 60 80 100 120 140

Improvement, ft

Speed at incursion detection, kts

New

EOL
NASA/TM—2015-218452 14 

The data also act as a starting point for determining which incursion situations warrant the use of 

enhanced engine performance for the hypothetical TCM aircraft. For example, if an incursion is detected 

when the aircraft has accelerated to 60 kts during its TO run, although the TO distance can be shorted by 

more than 250 ft by activating the control modes, the aircraft must still travel at least an additional 2,200 

ft before clearing the 50-ft threshold. Instead, it may be possible to stop the aircraft within that distance by 

applying full brakes and thrust reversers. On the other hand, the stopping distance may be unacceptably 

long if an incursion occurs at higher speeds. In such cases, any improvement provided by the control 

modes may be desirable in order to prevent a collision. Availability of enhanced engine performance 

modes represents an additional degree of freedom that can be integrated into the safety guidelines for such 

emergency situations. Since the control modes are designed based on risk of engine failure, they can be 

more readily incorporated into this guideline development.

4.3 Propulsion-Only Flight 

Although the runway incursion scenario involved activating both emergency control modes, the extra 

thrust provided by the OT mode is primarily responsible for shortening TO distance. To highlight the 

faster response mode, this subsection examines the benefits of using the control modes with a propulsioncontrolled aircraft (PCA). For these simulation cases, the TCM aircraft was flown straight and level at 

5,000 ft and 200 kts when all flight control surfaces (elevator, aileron, rudder, spoilers, and flaps) were 

failed (i.e., frozen in the position at time of failure). The two engines represented the only controllable 

“actuators” that can affect the flight trajectory. 

A feedback control system (Figure 13) was developed to facilitate flying the aircraft with engines 

only. This control system—a hypothetical emergency system that is not currently implemented in any 

civil transport in service—is based on, but highly simplified from, the extensively researched and tested 

PCA control system developed by NASA (Ref. 26). The control system attempts to maintain desired 

values of flight path angle (Ȗ) and roll angle (ij) through throttle modulation. The flight path angle 

controller consists of a proportional-integral feedback control structure that calculates throttle commands 

for both engines. The roll controller, also proportional-integral, calculates a differential value that is 

added to the left and subtracted from the right engine throttle setting. The gains for both controllers are 

fixed (i.e., no scheduling) and not optimal in any mathematically rigorous sense. Nevertheless, this 

implementation was sufficient for demonstrating the benefits of enhanced engine performance. 

Figure 13.—Propulsion-only flight control system calculates collective and differential throttle 

settings to maintain flight path angle (FPA) and roll. 
NASA/TM—2015-218452 15 

Figure 14.—Command profiles for altitude (Alt) and heading (Hdg) 

variation during propulsion-only flight. Each profile is executed 

separately, with the other command constant (e.g., altitude 

command profile with heading constant at 270q). 

Two scenarios, both involving flight path maneuvers using only the engines, were simulated for the 

non-piloted evaluation. The command profiles are shown in Figure 14. In the first scenario, the aircraft 

had to climb from 5,000 to 6,000 ft before eventually descending to 3,000 ft while maintaining a fixed 

heading. In the second, the aircraft, while maintaining an altitude of 5,000 ft, had to turn left from a 

heading of 270° to 180°, and then turn right to 300°. Each of the two examples was simulated with the 

following three configurations: 

x Nominal aircraft (i.e., no control surface failure, no PCA). 

x Control surfaces failed at 60-sec mark; PCA using baseline engines. 

x Control surfaces failed at 60-sec mark; PCA with enhanced performance modes active. 

As previously mentioned, the TCM contains a simple, autopilot-like flight control system. This system 

was used to fly all test cases. For the nominal cases, the command profiles shown in Figure 14 were 

directly entered into the flight control system as inputs. Additionally, the auto-throttle was set to maintain 

200 kts air speed. For the control surface failure cases, the altitude and heading portions of the TCM flight 

control system, which calculate flight path angle and roll commands, respectively, were redirected to the 

propulsion-only control system shown in Figure 13. It is important to note that without elevator authority, 

it is not possible to control altitude and air speed independently. Thus, air speed deviations were expected 

for the propulsion-only cases. 

Engine health was also varied for each of the two scenarios described above. However, maneuvering 

the aircraft via throttle modulation emphasizes the responsiveness rather than magnitude of the engine 

thrust output. For these scenarios, the results for EOL and new engines were found to be quite similar 

qualitatively and quantitatively. Therefore, this paper presents the results for new engines only. 

The guidelines used for the piloted evaluation of the propulsion-only scenario were similar to the 

framework described above with the exception of some modifications to facilitate testing. First, the 

nominal aircraft case was omitted. Piloted evaluation was only conducted on the cases with failed 

surfaces. Second, when the flight control surfaces failed, the pitch and roll trim controls in the cockpit 

0 100 200 300 400 500 600

2000

4000

6000

Alt Cmd, ft

Scenario 1: Climb/Descend

0 100 200 300 400 500 600

150

200

250

300

350

Hdg Cmd, deg

Time, s

Scenario 2: Turn
NASA/TM—2015-218452 16 

were remapped to the flight path angle and roll commands, respectively, for the PCA control system. The 

pilot controlled the crippled aircraft with these two trim controls. Third, the surfaces were failed 

immediately upon start of the simulation, fixed to their neutral positions. The pilot was then instructed to 

allow the PCA system to trim the aircraft to straight and level flight before commencing with the 

maneuverability tests. Fourth, the pilot was not instructed to follow a fixed flight trajectory for either of 

the two maneuvers. The goal was simply to achieve each altitude or heading waypoint in a controlled 

manner before progressing to the next. In other words, there was no time requirement to transition from 

one altitude/heading to another. Therefore, the piloted tests can only be evaluated in terms of whether the 

maneuver was safely performed, not how well the baseline and enhanced performance flight trajectories 

match. Finally, the tests were repeated several times, but the pilot was not informed whether or not 

enhanced engine performance was active during each run; he was simply instructed to fly each maneuver 

as well as he could. 

4.3.1 Climb/Descend Maneuver 

The non-piloted simulation results of the altitude maneuver case are shown in Figure 15. As expected, 

with the flight control surfaces failed, the aircraft is unable to maintain air speed, which decreases as the 

vehicle climbs and increases as it descends. Nonetheless, usage of the enhanced performance modes in 

conjunction with the propulsion-only control system (“PCA, Enhanced”) results in an altitude trajectory 

that is nearly identical to that of the nominal aircraft. On the other hand, the propulsion-only aircraft with 

baseline engines (“PCA, Baseline”) could only successfully perform the ascent portion of the commanded 

profile. On the descent portion, the aircraft was unable to return to level flight and entered into unstable 

oscillations of altitude and air speed. 

More detailed investigation determined that a combination of the flight control system and the slow 

dynamic thrust response of the engines at low power settings resulted in an instability akin to pilotinduced oscillations (PIO). Figure 16 shows the throttle command and thrust response for both 

propulsion-only cases (thrust is normalized as a percentage of 40,000 lb so that a time response 

comparison with throttle can be made). Without elevator control, low engine power was required to 

maintain the negative flight path angle during the descent (between 300 and 400 sec). 

Figure 15.—Climb/descend maneuver: comparing nominal aircraft 

against propulsion-only flight control without and with engine 

performance enhancing control modes. 

100 200 300 400 500 600

2000

4000

6000

Altitude, ft

Nominal

PCA, Baseline

PCA, Enhanced

100 200 300 400 500 600

150

200

250

Air Speed, kts

Time, s
NASA/TM—2015-218452 17 

Figure 16.—Climb/descend maneuver: throttle command and thrust 

response (normalized to 40,000 lb) for propulsion-only cases. 

Figure 17.—Climb/descend maneuver: piloted evaluations. 

Returning to level flight required a sharp increase in power just after the 400-sec mark. Unfortunately, the 

altitude controller was designed to expect changes in flight path angle that are faster than what the 

baseline engines could provide, resulting in overly aggressive corrective commands. Both the slow thrust 

response and the aggressive throttle commands are evident for the baseline case shown in Figure 16. 

Instability does not occur during the ascent portion because the transition from climb to level flight 

requires decreasing power, which the engines can adequately respond to with or without enhanced 

performance. 

Figure 17 shows the results of the piloted simulation of the climb/descend maneuver. The test was 

conducted a total of five times, twice with enhanced engine performance active. The altitude trajectories 

are relatively similar to those obtained using the autopilot control system (Figure 15). There are, however, 

small differences in air speed from the non-piloted case; recall that for the piloted simulations, the control 

surfaces were fixed to their neutral positions, whereas for the autopilot case, the aircraft was trimmed to 

100 200 300 400 500 600

0

20

40

Baseline

100 200 300 400 500 600

0

20

40

Enhanced

Time, s

Throttle, %

Thrust, %

0 100 200 300 400 500

2000

4000

6000

Altitude, ft

0 100 200 300 400 500

150

200

250

300

Air Speed, kts

Time, s

PCA, Baseline (3 runs)

PCA, Enhanced (2 runs)
NASA/TM—2015-218452 18 

200 kts before failure occurred. For two of the three baseline engine cases, the pilot was unable to prevent 

the aforementioned altitude/speed oscillations. For the unstable cases, the pilot was aware of the control 

surface inoperability and therefore did not command erroneous, overcompensating inputs like the 

autopilot. Hence, the instability was at least partially caused by an incompatibility between the slow 

response time of the baseline engines and the simplistic design of the PCA control system. Nevertheless, 

the oscillations did not occur when using the enhanced performance modes, whether the aircraft was 

autonomously or pilot controlled. Thus, enhanced engine performance provides a noticeable buffer 

against such instabilities, regardless of the nature of their cause. This type of protection could be crucial 

during a situation where the flight crew would have limited time to acclimate to the unfamiliar handling 

qualities of the damaged aircraft. 

4.3.2 Turning Maneuver 

Figure 18 summarizes the aircraft parameters of interest for the turning maneuver simulation runs. 

With the enhanced performance modes active, the aircraft with total control surface failure was able to 

reproduce the heading trajectory of a nominal aircraft. However, the rolling maneuvers required to enter 

and exit the turns are not ideal. The difficulty lies in maintaining a constant altitude while changing the 

roll angle of the aircraft. With the control surfaces frozen, rolling the aircraft was accomplished through 

Figure 18.—Turning maneuver: comparing nominal aircraft 

against propulsion-only flight control without and with engine 

performance enhancing control modes. 

100 200 300 400 500

150

200

250

300

350

Heading, deg

100 200 300 400 500

-20

0

20

Roll, deg

100 200 300 400 500

4600

4800

5000

5200

5400

Altitude, ft

Nominal

PCA, Baseline

PCA, Enhanced

100 200 300 400 500

160

180

200

220

240

Air Speed, kts

Time, s
NASA/TM—2015-218452 19 

Figure 19.—Turning maneuver: throttle command and thrust response (normalized to 40,000 lb) for 

propulsion-only cases. 

differential thrust. However, changes to the roll angle rotate the lift vector (e.g., increasing roll decreases 

the vertical component of the lift vector, causing the aircraft to descend). Therefore, in order to maintain 

altitude, changes in the total thrust were required, which interfered with the differential thrust commands. 

For these turning simulations, since the total and differential thrust portions of the propulsion-only flight 

control system were not coupled, some altitude change while rolling was unavoidable and the roll 

response of the aircraft had a tendency to overshoot. It is important to note that maintaining a steady nonzero roll angle does not require differential thrust; therefore, the control system was able to maintain a 

steady turn at constant altitude. The conflicting differential and total thrust commands occurred when roll 

angle was dynamic. 

Without enhanced engine performance, however, the aircraft became unstable for reasons similar to 

those in the climb/descent case. Figure 19 shows the throttle commands and thrust responses for the 

baseline and enhanced engines, differentiating between the left and right engines. Again, 

overcompensation in the commands due to the relatively slow response of the baseline engines is evident. 

In this case, the instability begins approximately 250 sec into the run. Just prior to this point, differential 

thrust was applied to return the aircraft to level flight and exit out of the first turn. Total thrust was 

decreased to correct the corresponding increase in altitude. However, similar to the descent example, 

engine thrust could not be increased quickly enough to return the aircraft to a stable altitude. 

Figure 20 presents the results of the piloted evaluation for this scenario. The pilot performed the 

maneuver six times, half of which utilized enhanced performance control modes. In this case, instability 

did not occur for any of the runs. Moreover, unlike the autopilot case, the control modes do not appear to 

offer any benefits since all six flight trajectories are relatively similar. This apparent discrepancy is due to 

the more conservative flying style of the human pilot—a result of knowing the impaired nature of the 

aircraft—as compared to the automated counterpart. Comparing the roll time histories in Figure 18 and 

Figure 20, the pilot rolled into and out of each turn more slowly than the autopilot. Moreover, the pilot 

prioritized turning over maintaining altitude. The autopilot with PCA controls treated both the heading 

maneuver and altitude hold commands equally. Therefore, although the autopilot exerted significantly 

tighter control over altitude for the duration of the maneuver with the control modes active, it could not 

maintain stable flight using the baseline engines. On the other hand, the piloted maneuvers were within 

the capabilities of the baseline engines and did not require the extra protection provided by the enhanced 

performance modes. 

100 200 300 400 500

0

20

40

Baseline

Left Engine

Throttle, %

Thrust, %

100 200 300 400 500

0

20

40

Enhanced

Time, s

100 200 300 400 500

0

20

40

Right Engine

100 200 300 400 500

0

20

40

Time, s
NASA/TM—2015-218452 20 

Figure 20.—Turning maneuver: piloted evaluations. 

5.0 Summary and Conclusions 

This paper evaluates the effects and benefits of previously developed engine performance enhancing 

control modes on aircraft operation, particularly during emergency scenarios. The control modes, faster 

response and overthrust, provide improved thrust capability in terms of dynamic responsiveness and 

maximum output, respectively. Increased performance is dependent upon the allowed elevation of engine 

failure risk. A notional implementation of the control modes is presented using a combined engine/aircraft 

simulation. Computer simulations and piloted evaluations were conducted to identify the potential 

benefits the control modes provide to an aircraft during an emergency. The following summarizes the 

conclusions that can be drawn from the results presented in this paper: 

x The control modes were implemented such that activation does not significantly affect the aircraft 

flight dynamics. Therefore, minimal pilot effort is required to maintain the flight trajectory prior to 

mode activation. 

x For runway incursions, the benefits of enhanced engine performance—overthrust in particular—

diminish with later detection of the incursion event. Enhanced engine performance represents an 

additional consideration in the trade-off study that must be conducted to determine the guidelines for 

runway incursions. 

0 50 100 150 200 250 300 350

150

200

250

300

350

Heading, deg

0 50 100 150 200 250 300 350

-20

0

20

Roll, deg

0 50 100 150 200 250 300 350

4600

4800

5000

5200

5400

Altitude, ft

0 50 100 150 200 250 300 350

150

200

250

300

Air Speed, kts

Time, s

PCA, Baseline (3 runs)

PCA, Enhanced (3 runs)
NASA/TM—2015-218452 21 

x For the emergency scenario of propulsion-only flight where the aircraft experiences a total loss of 

control surface authority, the enhanced performance modes provide some protection against 

instabilities similar to pilot-induced oscillations. Computer simulation of various flight maneuvers 

showed that using the control modes prevented these instabilities where the baseline engines were 

unsuccessful. However, piloted evaluations showed that the baseline engines could suffice in some 

instances if the inputs were more conservative and maneuverability requirements more lax. 

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