Subject: Installation of Terrain Awareness and

 Warning System (TAWS) Approved

 for Part 23 Airplanes

Date: 6/14/00

Initiated By: ACE-100

AC No: 23-18

Change:

1. PURPOSE............................................................................................................................................................... 3

2. RELATED REGULATIONS AND DOCUMENTS............................................................................................ 3

A. AIRWORTHINESS REGULATIONS........................................................................................................................... 3

B. OPERATING REGULATIONS................................................................................................................................... 4

C. AC, ORDERS, TSO, AND RELATED DOCUMENTS................................................................................................. 4

D. INDUSTRY DOCUMENTS. ...................................................................................................................................... 6

E. OTHER DOCUMENT............................................................................................................................................... 6

3. DEFINITIONS. ...................................................................................................................................................... 6

4. BACKGROUND. ................................................................................................................................................... 9

A. REGULATIONS...................................................................................................................................................... 9

B. TSO-C151A....................................................................................................................................................... 10

C. NEED FOR GUIDANCE......................................................................................................................................... 11

5. DISCUSSION. ...................................................................................................................................................... 11

A. REGULATORY BASIS. ......................................................................................................................................... 11

6. SYSTEM DESCRIPTION................................................................................................................................... 12

A. CLASS A TAWS EQUIPMENT AS DEFINED IN TSO-C151A. ................................................................................ 12

B. CLASS B TAWS EQUIPMENT, AS DEFINED IN TSO-C151A. ............................................................................... 12

7. AIRWORTHINESS CONSIDERATIONS. ....................................................................................................... 14

A. INITIAL INSTALLATION. ...................................................................................................................................... 14

B. FOLLOW-ON INSTALLATIONS.............................................................................................................................. 14

C. PROJECT SPECIFIC CERTIFICATION PLAN............................................................................................................ 15

D. SYSTEM SAFETY ASSESSMENT........................................................................................................................... 16

E. SOFTWARE. ........................................................................................................................................................ 17

F. POSITION SOURCES............................................................................................................................................. 17

G. TERRAIN DATABASE.......................................................................................................................................... 19

H. DISPLAY PRESENTATION.................................................................................................................................... 19

I. ALERTS. .............................................................................................................................................................. 22
AC 23-18 6/14/00

2

J. ALERT PRIORITIZATION....................................................................................................................................... 22

K. SYSTEM INHIBIT................................................................................................................................................. 25

L. FLIGHT DATA RECORDER................................................................................................................................... 25

M. SYSTEMS EVALUATIONS.................................................................................................................................... 26

N. GROUND TEST REQUIREMENTS. ........................................................................................................................ 26

O. FLIGHT-TEST REQUIREMENTS............................................................................................................................ 26

P. AFM OR AFMS. ................................................................................................................................................ 30
6/14/00 AC-23-18

3

1. PURPOSE. This advisory circular (AC) establishes an acceptable means, but not the only

means, of obtaining Federal Aviation Administration's (FAA) airworthiness approval for the

installation of a TAWS that has been approved under Technical Standard Order (TSO)-C151a,

Terrain Awareness and Warning System, in a Part 23 airplane. The FAA’s TSO process is a

means for obtaining FAA design and performance approval for an appliance, system, or product;

however, the TSO does not provide installation approval. This AC serves to provide guidance

for designing an acceptable installation for a TAWS that complies with TSO-C151a. The

guidance provided is specific to installations of these systems on airplanes certificated under

Title 14 of the Code of Federal Regulations (14 CFR) Part 23 [commonly referred to as Part 23

of the Federal Aviation Regulations (FAR)]. It describes the airworthiness considerations for

such installations as they apply to the unique features of the TAWS and the interface of the

TAWS with other systems on the airplane.

Equipment that does not meet the minimum performance standards specified in TSO-C151a shall not be

identified as TAWS equipment. Applicants that are not required to install TAWS and choose to install

the system may deviate from the guidelines in this AC as necessary, provided the level of safety for the

airplane’s existing certification basis is not degraded.

Like all advisory material, this AC is not mandatory and does not constitute a regulation. It is issued for

guidance purposes and to outline a method of compliance with applicable airworthiness requirements.

Because the method of compliance presented in this AC is not mandatory, the terms "shall" and "must"

used in this AC apply only to an applicant who chooses to follow this particular method and prescribed

standards of TSO-C151a.

2. RELATED REGULATIONS AND DOCUMENTS.

a. Airworthiness Regulations. These acceptable means of compliance refer to the

applicable sections of 14 CFR Part 23. The corresponding paragraphs of the former Civil Air

Regulations (CAR) are shown in parenthesis for airplanes certificated under CAR.

§ 23.771 (3.381) Pilot compartment.

§ 23.773 (3.382) Pilot compartment view.

§ 23.777 (3.3841) Cockpit controls.

§ 23.1301 (3.651 Equipment: Function and installation.

and 3.652)

§ 23.1303 (3.655(a)) Flight and navigation instruments.

§ 23.1309 Equipment, systems, and installations.

§ 23.1311 Electronic display instrument systems.

§ 23.1321 (3.661 Arrangement and visibility.

and 3.662)

§ 23.1322 Warning, caution, and advisory lights.

§ 23.1331 (3.668) Instruments using a power source.

§ 23.1351 (3.681, Electrical Systems and Equipment: General.

3.682, and 3.685)

§ 23.1353 (3.683) Storage battery design and installation.
AC 23-18 6/14/00

4

§ 23.1357 (3.690, Circuit protective devices.

3.691, and 3.692)

§ 23.1359 Electrical system fire protection.

§ 23.1361 (3.688 and 3.689) Master switch arrangement.

§ 23.1365 (3.693) Electric cables and equipment.

§ 23.1367 (3.694 Switches.

and 3.695)

§ 23.1381 (3.696 Instrument lights.

and 3.697)

§ 23.1383 (3.698 and 3.699) Taxi and landing lights.

§ 23.1431 (3.721) Electronic equipment.

§ 23.1459 Flight recorders.

§ 23.1501 (3.735 Operating Limitations and Information: General.

and 3.737)

§ 23.1529 Instructions for Continued Airworthiness.

§ 23.1541 (3.755) Markings and Placards: General.

§ 23.1543 (3.756) Instrument markings: General.

§ 23.1545 (3.757) Airspeed indicator.

§ 23.1559 (3.770) Operating limitations placard.

§ 23.1581 (3.777) Airplane Flight Manual and Approved Manual Material:

General.

§ 23.1583 (3.761 Operating limitations.

and 3.778)

b. Operating Regulations. More specifics on these regulations are provided in paragraph 5.

§ 91.223 Terrain awareness and warning system

§ 121.354 Terrain awareness and warning system

§ 121.360 Ground proximity warning—glide slope deviation alerting

system

§ 135.153 Ground proximity warning system

§ 135.154 Terrain awareness and warning system

c. AC, Orders, TSO, and Related Documents. Later revisions are acceptable. The

following publications may be obtained free of charge from the U.S. Department of Transportation,

Subsequent Distribution Office, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD

20785:

AC 20-115B Radio Technical Commission for Aeronautics (RTCA),

Inc., RTCA/DO-178B

AC 20-136 Protection of Aircraft Electrical/Electronic Systems Against the

Indirect Effects of Lightning

AC 21-16D RTCA Document DO-160D
6/14/00 AC 23-18

5

AC 23-15 Small Airplane Certification Compliance Program

AC 23.1311-1A Installation of Electronic Displays in Part 23 Airplanes

AC 23.1309-1C Equipment, Systems, and Installations in Part 23 Airplanes

AC 20-112 Airworthiness and Operational Approval of Airborne Systems

to be Used in Lieu of Ground Proximity Warning System

AC 20-130A Airworthiness Approval of Navigation or Flight Management

Systems Integrating Multiple Navigation Sensors

AC 20-138 Airworthiness Approval of Global Positioning System (GPS)

Navigation Equipment for Use as a VFR and IFR Supplemental

Navigation System

AC 90-45A Approval of Area Navigation Systems for Use in the U.S. National

Airspace System

FAA Order 8110.4B Type Certification

FAA Order 8260.3B United States Standard for Terminal Instrument Procedures

(TERPS)

TSO-C10b Altimeter, Pressure Actuated, Sensitive Type

TSO-C67 Airborne Radar Altimeter Equipment

TSO-C92c Airborne Ground Proximity Warning Equipment

TSO-C106 Air Data Computer

TSO-C113 Airborne Multipurpose Electronic Displays

TSO-C115b Airborne Area Navigation Equipment Using Multi-Sensor Inputs

TSO-C117a Airborne Windshear Warning and Escape Guidance Systems for

Transport Airplanes

TSO-C129a Airborne Supplemental Navigation Equipment Using the Global

Positioning System (GPS)
AC 23-18 6/14/00

6

TSO-C145 Airborne Navigation Sensors Using the Global Positioning System

(GPS) Augmented by the Wide Area Augmentation System

(WAAS)

TSO-C151a Terrain Awareness and Warning System

The FAA technical reports listed below can be obtained from the National Technical Information

Service in Springfield, Virginia 22161.

DOT/FAA/CT-96/1 Human Factors Design Guide

DOT/FAA/AAR-95/3 FAA Aircraft Certification Human Factors and Operations

Checklist for Standalone GPS Receivers

d. Industry documents. Obtain RTCA, Inc. documents listed below from RTCA, Inc.,

1140 Connecticut Avenue NW, Suite 1020, Washington, D.C. 20036-4001.

RTCA/DO-160D, Environmental Conditions and Test Procedures for Airborne

Equipment

RTCA/DO-161A, Minimum Performance Standards - Airborne Ground Proximity Warning

Equipment

RTCA/DO-178B, Software Considerations in Airborne Systems and Equipment

Certification

RTCA/DO-200, Preparation, Verification and Distribution of User-Selectable Navigation

Data Bases

RTCA/DO-200A, Standards for Processing Aeronautical Data

RTCA/DO-208, Minimum Operational Performance Standards for Airborne Supplemental

Navigation Equipment Using Global Positioning System (GPS)

e. Other document.

The FAA and Industry Guide to Product Certification, January 25, 1999. Available on internet:

http://av-info.faa.gov/dst/CPIGUIDE.pdf

3. DEFINITIONS.

a. Alert: A visual, aural, or tactile stimulus presented to either attract attention or convey

information regarding system status or condition, or both.
6/14/00 AC 23-18

7

b. Aural Alert: A discrete sound, tone, or verbal statement used to annunciate a condition,

situation, or event.

c. Caution Alert: An alert requiring immediate flightcrew awareness. Subsequent corrective action

normally will be necessary.

d. Class A TAWS Equipment: A class of equipment that is defined in TSO-C151a. As a

minimum, it will provide alerts for the following circumstances:

• Reduced required terrain clearance.

• Imminent terrain impact.

• Premature descent.

• Excessive rates of descent.

• Excessive closure rate to terrain.

• Negative climb rate or altitude loss after take-off.

• Flight into terrain when not in landing configuration.

• Excessive downward deviation from an ILS glideslope.

• Descent of the airplane to 500 feet above the terrain or nearest runway elevation

(voice callout “Five Hundred”) during a non-precision approach.

Class A TAWS installations must provide a terrain awareness display that shows either the

surrounding terrain or obstacles relative to the airplane, or both. See paragraph 5 to determine

which operating rules and aircraft configurations require Class A TAWS equipment.

e. Class B TAWS Equipment: A class of equipment that is defined in TSO C151a. As a

minimum, it will provide alerts for the following circumstances:

• Reduced required terrain clearance.

• Imminent terrain impact.

• Premature descent.

• Excessive rates of descent.

• Negative climb rate or altitude loss after take-off.

• Descent of the airplane to 500 feet above the terrain or nearest runway elevation

(voice callout “Five Hundred”) during a non-precision approach.

Class B TAWS installation may provide a terrain awareness display that shows either the

surrounding terrain or obstacles relative to the airplane, or both. See paragraph 5 to determine

which operating rules and aircraft configurations require Class B TAWS equipment.

f. Controlled Flight Into Terrain (CFIT): An accident or incident in which the airplane, under the

flightcrew’s control, is inadvertently flown into terrain, obstacles, or water without either sufficient or

timely flightcrew awareness to prevent the event, or both.

g. Corrected Altitude: A computed aircraft altitude designed to help ensure optimal operation of

the TAWS function through all phases of flight and atmospheric conditions. Corrected altitude can be
AC 23-18 6/14/00

8

computed using pressure altitude, GPS altitude, radio altitude, terrain and runway elevation, etc. data to

reduce or eliminate errors potentially induced in corrected barometric altitude by temperature extremes,

non-standard altitude conditions, and altimeter miss-sets.

h. Failure: The inability of the equipment or any sub-part of that equipment to perform within

previously specified limits.

i. False Alert: A warning or caution that occurs when the design terrain warning or caution

threshold of the system is not exceeded.

j. Forward Looking Terrain Avoidance (FLTA): The FLTA function looks ahead of the airplane

along and below the airplane’s lateral and vertical flight path and provides suitable alerts if a potential

CFIT threat exists.

k. Hazard: A state or set of conditions that, together with other conditions in the environment,

could lead to an accident.

l. Hazardously Misleading Information (HMI): An incorrect depiction of the terrain threat

relative to the airplane during an alert condition (excluding source data).

m. Imminent Terrain Impact: An alert when the airplane is currently below the elevation of a

terrain cell along the airplane’s lateral projected flight path and it is based upon the vertical projected

flight path. The equipment predicts that the terrain clearance will be less than the value given in the

RTC column of TSO-C151a, Table 3.1.

n. Nuisance Alert: An inappropriate alert, occurring during normal safe procedures, that occurs as

a result of a design performance limitation of TAWS.

o. Obstacle: A structure that may be in the flight path of the airplane. The obstacle database

generally contains the catalog of obstacles of 100 feet or greater in height located within North America

and portions of the Caribbean. Caution and warning visual and audio alerts are provided when a conflict

with the obstacle is detected.

p. Premature Descent Alert (PDA): A function of the TAWS that uses the airplane’s current

position and flight path information, as determined from a suitable navigation source and airport

database, to determine if the airplane is hazardously below the normal (typically 3 degree) approach path

for the nearest runway as defined by the alerting algorithm.

q. Required Terrain Clearance (RTC): The minimum requirements for obstacle or terrain

clearance as defined by United States Standard for Terminal Instrument Procedures (TERPS), FAA

Order 8260.3B, and the Aeronautical Information Manual (AIM). The TAWS required RTC by phase of

flight is shown in TSO-C151a, Table 3.1.
6/14/00 AC 23-18

9

r. Terrain Awareness and Warning System (TAWS): A system that provides the flightcrew with

sufficient information and alerting to detect a potentially hazardous terrain situation and so the

flightcrew may take effective action to prevent a CFIT event.

s. Terrain Awareness Display: A display of the surrounding terrain or obstacle(s) relative to the

airplane.

t. Terrain Database: Terrain or obstacle information stored within a TAWS.

u. Time-shared Display: A display that shows terrain information, plus additional

information from other systems [for example, an Electric Flight Instrument System/Navigation

Display/Multi-Function Display (EFIS/ND/MFD)].

v. Visual Alert: The use of projected or displayed information to present a condition, situation, or

event to the flightcrew.

w. Warning Alert: An alert for a detected terrain threat that requires immediate flightcrew action.

4. BACKGROUND.

a. Regulations. Beginning in the early 1970’s, a number of studies looked at the occurrence

of “controlled flight into terrain” (CFIT) accidents, where a properly functioning airplane under

the control of a fully qualified and certificated crew is flown into terrain (or water or obstacles)

with no apparent awareness on the part of the crew.

Findings from these studies indicated that many such accidents could have been avoided if a warning

device called a ground proximity warning system (GPWS) had been used. As a result of these studies

and recommendations from the National Transportation Safety Board (NTSB), in 1974 the FAA required

all Part 121 certificate holders (that is, those operating large turbine-powered airplanes) and some Part

135 certificate holders (that is, those operating large turbojet airplanes) to install TSO approved GPWS

equipment (§§ 121.360 and 135.153) (39 FR 44439, December 18, 1974).

In 1978, the FAA extended the GPWS requirement to Part 135 certificate holders operating smaller

airplanes: turbojet-powered airplanes with 10 or more passenger seats. These operators were required to

install TSO-approved GPWS equipment or alternative ground proximity advisory systems that provide

routine altitude callouts whether or not there is any imminent danger (§ 135.153) (43 FR 28176, June 29,

1978). This requirement was considered necessary because of the complexity, size, speed, and flight

performance characteristics of these airplanes. The GPWS equipment was considered essential in

helping the pilots of these airplanes to regain altitude quickly and avoid what could have been a CFIT

accident.

Installation of GPWS’s or alternative FAA-approved advisory systems was not required on turbopropeller powered (turboprop) airplanes operated under Part 135 because, at that time, the general

consensus was that the performance characteristics of turboprop airplanes made them less susceptible to

CFIT accidents. For example, it was thought that turboprop airplanes had a greater ability to respond
AC 23-18 6/14/00

10

quickly in situations where altitude control was inadvertently neglected, as compared to turbojet

airplanes. However, later studies, including investigations by the NTSB, analyzed CFIT accidents

involving turboprop airplanes and found that many of these accidents could have been avoided if GPWS

equipment had been used.

Some of these studies also compared the effectiveness of the alternative ground proximity advisory

system to the GPWS. GPWS was found to be superior in that it would warn only when necessary,

provide maximum warning time with minimal unwanted alarms, and use command-type warnings.

Based on these reports and NTSB recommendations, in 1992 the FAA amended § 135.153 to require

GPWS equipment on all turbine-powered airplanes with 10 or more passenger seats (57 FR 9944,

March 20, 1992).

After these rules were issued, advances in terrain mapping technology permitted the development of a

new type of ground proximity warning system that provides greater situational awareness for

flightcrews. The FAA has approved certain installations of this type of equipment, known as the

enhanced ground proximity warning system (EGPWS). However, in the proposed final rule, the FAA is

using the broader term “terrain awareness and warning system” (TAWS) because the FAA expects that a

variety of systems may be developed in the near future that would meet the improved standards

contained in the proposed final rule.

The TAWS improves on existing GPWS systems by providing the flightcrew much earlier aural and

visual warning of impending terrain, forward looking capability, and continued operation in the landing

configuration. These improvements provide more time for the flightcrew to make smoother and gradual

corrective action.

In 1998, the FAA issued Notice No. 98-11, Terrain Awareness and Warning System (63 FR 45628,

August 26, 1998), proposing that all turbine-powered U.S.-registered airplanes type certificated to have

six or more passenger seats (exclusive of pilot and copilot seating), be equipped with an FAA-approved

terrain awareness and warning system. On March 23, 2000, the FAA issued Amendments 91-263, 121-

273, and 135-75 (65 FR 16736, March 29, 2000) with an effective date of March 29, 2001. These

amendments amended the operating rules to require that all U.S. registered turbine-powered airplanes

with six or more passenger seats (exclusive of pilot and copilot seating) be equipped with an FAA

approved TAWS. See paragraph 5 for more details.

b. TSO-C151a. On November 29, 1999, the FAA issued TSO-C151a and canceled

TSO-C151. The FAA took this action to clarify and correct certain paragraphs and tables that

appeared in TSO-C151. The FAA has determined that the changes to TSO-C151a are of a

clarifying and corrective nature, and that these changes do not alter the original intent of the

airworthiness requirements of the paragraphs or tables being changed. TSO-C151a prescribes

the minimum design standards that a TAWS must meet to be identified with the TSO-C151a

marking. The TAWS described in the TSO is representative of the “next generation” GPWS. In

addition to the standards and test procedures required for the FLTA functions, the TAWS

incorporates the standards and test procedures for basic GPWS equipment (as defined in
6/14/00 AC 23-18

11

TSO-C92c and in RTCA DO-161A). For further in-depth information concerning the TAWS’

capabilities, refer to TSO-C151a.

c. Need for Guidance. The FAA’s TSO process is a means of obtaining FAA design and

performance approval for an appliance, system, or product. However, the TSO does not provide

procedures for installation approval or procedures for design or implementation of an installation. With

future rulemaking and heightened interest by manufacturers and operators to equip Part 23 airplanes with

TAWS systems that are compliant with TSO-C151a, the FAA has recognized the need to establish

guidance material for the design and test requirements for the installation of such systems. This AC has

been developed as the means for providing such guidance for Part 23 airplanes. This AC describes the

airworthiness considerations for designing a TAWS installation. The airworthiness considerations

discussed apply only to the interface of the TAWS with other aircraft systems.

5. DISCUSSION.

a. Regulatory Basis. New requirements under 14 CFR Parts 91, 121, and 135 require the

installation of the TAWS. Specifically:

(1) § 91.223 states that no person may operate a turbine-powered U.S.-registered airplane

configured with six or more passenger seats, excluding any pilot seat, unless that airplane is equipped

with an approved terrain awareness and warning system that meets the requirements of Class B

equipment of TSO-C151. This section does not apply to: (a) Parachuting operations when conducted

entirely within a 50 nautical mile radius of the airport from which such local flight operations began, (b)

Fire fighting operations, and (c) Flight operations when incident to the aerial application of chemicals

and other substances.

(2) § 121.354 states that no person may operate a turbine-powered airplane unless that airplane

is equipped with an approved terrain awareness and warning system, including a terrain awareness

display, that meets the requirements for Class A equipment of TSO-C151.

(3) § 135.154 states that no person may operate a turbine-powered U.S.-registered airplane

configured with six to nine passenger seats, excluding any pilot seat, unless that airplane is equipped

with an approved terrain awareness and warning system that meets the requirements of Class B

equipment of TSO-C151. It also states that no person may operate a turbine-powered U.S.-registered

airplane configured with 10 or more passenger seats, excluding any pilot seat, unless that airplane is

equipped with a terrain awareness and warning system that meets the provisions of Class A equipment

of TSO-C151.

(4) Part 121 and 135 regulations also mandate that GPWS be replaced with a TAWS approved

under TSO-C151.

(5) For the compliance schedule, §§ 121.360 and 135.153 have an expiration date of

March 29, 2005, for the use of current GPWS systems. U.S. registered airplanes manufactured

after March 29, 2002, must be equipped with TAWS. Turbine-powered airplanes manufactured

on or before March 29, 2002, must be equipped with TAWS by March 29, 2005.
AC 23-18 6/14/00

12

6. SYSTEM DESCRIPTION. TAWS is intended to provide flightcrews with aural and visual

alert aids aimed at preventing a CFIT accident through increased terrain awareness.

a. Class A TAWS equipment as defined in TSO-C151a.

(1) Class A TAWS equipment provides three principal alerting functions. These

are:

(a) FLTA function, which includes:

• Reduced required terrain clearance.

• Imminent terrain impact.

(b) PDA function.

(c) Basic GPWS functions, as defined in TSO-C151a and RTCA DO-161A, which

include:

• Excessive rates of descent.

• Excessive closure rate to terrain.

• Negative climb rate or altitude loss after take-off.

• Flight into terrain when not in landing configuration.

• Excessive downward deviation from and ILS glideslope.

• Descent of the airplane to 500 feet above the terrain or nearest runway

elevation (voice callout “Five Hundred”).

(2) The Class A TAWS system will require a display. The terrain display and terrainthreat alerting are made possible by the TAWS’ acceptance of a variety of input parameters.

Color displays are not required; monochromatic displays have been allowed and will continue to

be allowed. These parameters are used in conjunction with a terrain and airport database(s) that

reside within the TAWS computer. The Class A TAWS places an airplane symbol on a digital

terrain map and applies terrain display algorithms. Terrain mapping information may be

provided on either a weather radar (WXR) display, Electronic Flight Instrument System (EFIS)

display, or other compatible display screens. Aircraft position information is provided by either

the Flight Management Computer (FMC), Global Positioning System (GPS), or other source of

positional information that meets the requirements specified in paragraph 7.f. of this AC.

(3) The Class A TAWS for operations under Part 135 (turbine-powered U.S.-registered

airplane configured with 10 or more passenger seats, excluding any pilot seat) will be required to

incorporate either an integral (internal) GPS or interface with an independent approved GPS for

horizontal position information.

b. Class B TAWS equipment, as defined in TSO-C151a.

(1) Class B TAWS equipment provides three principal alerting functions. These

are:
6/14/00 AC 23-18

13

(a) FLTA function, which includes:

• Reduced required terrain clearance

• Imminent terrain impact

(b) PDA function

(c) Basic GPWS functions, as defined in TSO-C151a and RTCA DO-161A, which

include:

• Excessive rates of descent

• Negative climb rate or altitude loss after take-off

• Descent of the airplane to 500 feet above the terrain or nearest runway

elevation (voice callout “Five Hundred”).

(2) The Class B TAWS will not require a display. If a display is installed with a Class

B TAWS, it should meet the requirements of paragraph 7.h. of this AC.

(3) The Class B TAWS will be required to incorporate either an integral (internal) GPS

or interface with an independent approved GPS for horizontal position information that meets the

requirements specified in paragraph 7.f. of this AC.

(4) The Class B TAWS will not require an interface with a radio altimeter.

c. Note: The FAA is considering developing minimum operational performance standards

intended for piston-powered and turbine-powered airplanes when configured with less than six

passenger seats, excluding any pilot seats. At this time, the FAA is not considering a

requirement for these installations. If standards are developed, they may be identified as Class C

TAWS requirements. It is envisioned that Class C TAWS would only be an option that the

operator may install for additional terrain situational awareness.
AC 23-18 6/14/00

14

7. AIRWORTHINESS CONSIDERATIONS.

a. Initial Installation. For initial approval of a particular TAWS equipment installation, the

scope of the applicant’s program should be directed toward airworthiness approval through the Type

Certificate (TC) or Supplemental Type Certificate (STC) process. The guidance provided in this AC

also is appropriate for applicants who choose to exercise their Designated Alteration Station (DAS)

authorization for STC approval. As part of the amended TC or STC program, the applicant should

identify if the changes to the type certificated airplane constitute a significant change, but not one so

extensive as to require a new TC in accordance with § 21.19 (Changes Requiring a New Type

Certificate). If the design change is considered significant, the certification program should be

coordinated with the responsible FAA Directorate, as described in FAA Order 8110.4B, Type

Certification.

b. Follow-on Installations. For TAWS Class A and B equipment that has already obtained

initial installation approval via the TC or STC process, approval may be obtained using either the STC,

amended TC, or FAA Form 337 (Major Repair and Alteration) process.

(1) For TAWS Class B installations involving only simple altitude (vertical position source)

and GPS data inputs, which utilize a format specified as acceptable in the TAWS equipment installation

instructions, approval for return to service can be accomplished using FAA Form 337. The TAWS

equipment manufacturer should provide a listing of acceptable altitude (vertical position source) and

GPS equipment. Flight-testing should not be required for these installations.

(2) TAWS Class A and certain Class B installations that includes additional features beyond

the minimum TAWS Class B equipment requirements (that is, terrain display, radio altitude or landing

gear, flap, glideslope sensor inputs, etc.) will require a more extensive installation evaluation. Some

flight-testing will likely be required for these installations as under Flight-Test Requirements.

(a) The installer should provide satisfactory evidence of similarity of the proposed TAWS

installation to previously approved installations incorporating the same interfaces (sensors, displays,

etc.). In any similarity evaluation, all changes to the system or its installation should be assessed for their

effect on a system to meet the certification requirements.

(b) When using the STC or amended TC process, all required data pertaining to

the installation should be submitted to the Aircraft Certification Office (ACO). These data

should include the manufacturer’s operating and installation instructions, fault analysis for the

installation, installation details, structural substantiation, and system wiring diagrams. The ACO

will define necessary ground and flight-tests.

(c) When using the FAA Form 337 process, all required data pertaining to the

installation should be submitted to the Flight Standards District Office (FSDO). These data

should include the manufacturer’s operating and installation instructions, fault analysis for the

installation, installation details, structural substantiation, system wiring diagrams, and results of

required ground and flight-test evaluations. Limitations associated with the equipment should be
6/14/00 AC 23-18

15

copied from the installation instructions to the Airplane Flight Manual (AFM), Airplane Flight

Manual Supplement (AFMS), or Supplemental AFM. In situations where additional FAA

evaluation is necessary, required flight evaluations will be conducted by the cognizant ACO or,

when authorized, by a Flight-Test Pilot Designated Engineering Representative (DER) in

accordance with the procedures used by the ACO. These evaluations should be conducted to

verify that the design and installation performs its intended function under the expected operating

conditions, that there are no adverse interactions between the TAWS and existing aircraft

systems, and that prior approvals of present aircraft equipment have not been compromised. The

approval for return to service must be signed by one of the entities noted in 14 CFR Part 43 (that

is, repair station, manufacturer, holder of an inspection authorization).

c. Project Specific Certification Plan. On January 25, 1999, the FAA, in coordination with

the Aerospace Industries Association (AIA) and the General Aviation Manufacturers Association

(GAMA), introduced "The FAA and Industry Guide to Product Certification." This aid communicates

the design and production certification process for aircraft. It describes how to plan, manage, and

document an effective and efficient product certification process. The applicant should use this guide to

develop their Project Specific Certification Plan (PSCP). The applicant should develop a comprehensive

PSCP that includes the following:

(1) Project Description. A summary of the project.

(2) System Description.

(a) A comprehensive system description that includes a brief summary of the product as it

relates to existing flight deck displays, sensors, added sensors, switches, annunciator lights, control

panels, electrical components, interior arrangement, other interfaces, product part number, etc.

(b) A layout and description of any changes to the flight instrument panels.

(c) Information regarding software aspects of certification and any application-specific

integrated circuits (ASIC) approved under TSO-C151a should be referenced and made available upon

request. Additional documentation may be required for added features that were not approved under

TSO-C151a.

(3) Project Schedule.

(a) A detailed project schedule that identifies all major milestones and schedules for any

required deliverables (that is, test plans).

(b) Schedules for operational and maintenance aspects, as well as foreign authority

validation requirements.
AC 23-18 6/14/00

16

(4) Certification Basis and Means of Compliance.

(a) A certification matrix that identifies the applicable regulations, AC’s, current policies,

certification basis, and the procedures or methods that will be used to comply with those regulations.

(b) Any testing or analyses applicable to the project that have been previously approved by

the FAA under an STC, TC, TSO, or Parts Manufacturer Approval (PMA) held by the applicant. The

approval date, letter reference number, and references as to how the specific approval was granted (that

is, STC, TSO, letter of approval, etc.) should be included in this section.

(5) Communication and Coordination.

(a) If applicable, identification of all Designated Engineering Representatives (DER)

Designated Airworthiness Representatives (DAR), Designated Manufacturing Inspection Representative

(DMIR) or DAS specialists working on the program.

(b) Identification of all delegated functions, which should include any stipulations,

coordination, and limitations that are placed upon those delegations.

(6) Testing Plan. This section should contain the requirements for the planning, preparation,

and adequacy of conducting the FAA required testing, including any delegations. The applicant should

address human factors issues and provide human factors support for decisions regarding the flightcrew

interface issues resulting from the TAWS installation.

(7) Conformity Plan. This section should describe the activities associated with

conformity of parts, simulators, and aircraft installations.

(8) Continued Airworthiness Plan. This section should provide the instructions for

continued airworthiness for the TAWS installation in accordance with § 23.1529.

(9) Compliance Documentation. This section should describe the procedures for submittal and

processing of compliance documentation.

d. System Safety Assessment.

(1) A system safety assessment (SSA) should be performed that establishes the hazards

associated with the proposed installation. The SSA should be developed in accordance with

AC 23.1309-1C.

(2) The FAA expects an SSA for the installation to establish, by quantitative or qualitative

analysis, the probability of system failure, false alerts, and unannunciated failures. The presentation of

hazardously misleading information (HMI) on the terrain display, or the unannunciated loss of the terrain

warning functions as a result of TAWS computer failure, is considered a major failure condition by

TSO-C151a. TSO-C151a requires major failure condition (10-5) for unannunciated failure, HMI, and

false alerts at the TAWS computer level. The box, as installed, should meet the following criteria:
6/14/00 AC 23-18

17

(a) The probability of failure conditions of the installed system that would lead to the loss of

all the functions, as described in paragraph 6.a. and 6.b. above, should be less than or equal to 10-3 per

flight hour.

(b) The probability of a failure condition for either a false caution or warning alert, or both,

should be less than or equal to 10-4 per flight hour due to undetected or latent failures.

(c) The probability of a failure condition for an unannunciated failure of the system to

provide the required alerting functions should be less than or equal to 10-4 per flight hour due to

undetected or latent failures.

(d) The probability of a failure condition for the system to provide hazardously misleading

information (HMI) to the TAWS display should be less than or equal to10-4 per flight hour due to

undetected or latent failures.

(e) Failure of the installed TAWS should not degrade the integrity of any installed system

with the TAWS interfaces that could have either hazardous or catastrophic failure conditions as defined

by AC 23.1309-1C.

e. Software. The applicant should submit data that the TAWS software meets the

requirements of TSO-C151a and that it meets the appropriate software levels for any added feature (s).

Software implementing the TAWS functions defined in TSO-C151a should be developed at least to

Level C, as defined in RTCA DO-178B, or an acceptable alternative approved by the FAA.

f. Position Sources. TAWS uses the estimated position of the airplane with reference to

either the terrain or obstacle to determine when an alert should be annunciated. The applicant

should provide evidence that the TAWS horizontal and vertical position sources are suitable for

each phase of flight (that is, enroute, terminal, approach, and departure) as provided by TSOC151a (Appendix 1, Section 10) for which approval is sought.

(1) Horizontal Position Source.

(a) TAWS that interface with previously approved navigation systems.

Previously approved Area Navigation (RNAV) and GPS systems that are used for navigation are

generally considered suitable for TAWS horizontal position inputs. An RNAV system must

meet the requirements of TSO-C115 or be installed in accordance with AC 90-45A. A GPS must

meet the requirements of TSO-C129a or be installed in accordance with AC 20-130A or AC

20-138. If a Wide Area Augmentation System (WAAS) is used as a position source it must meet

the requirements TSO-C145. A means for testing the position source for accuracy and reliability

is unnecessary for these systems.

(i) Class A TAWS equipment installed in airplanes operated under Part 121 may

use approved RNAV, which may include GPS.
AC 23-18 6/14/00

18

(ii) Class A TAWS for operations under Part 135 (turbine-powered U.S.-

registered airplane configured with 10 or more passenger seats, excluding any pilot seat) are

required to interface with an independent approved GPS for horizontal position information.

(iii) The Class B TAWS equipment is required to interface with an independent

approved GPS for horizontal position information.

(b) TAWS equipment with internal GPS position source. Class A and Class B

TAWS equipment that use a GPS internal to the TAWS for horizontal position information must

use a GPS that is capable of detecting a positional error that exceeds the appropriate alarm limit

for the existing phase of flight, in accordance with TSO-C129a or equivalent. When this alarm

limit is activated, the GPS-computed position is considered unsuitable for the TAWS. The

applicant must be able to demonstrate that the TAWS, as installed on the aircraft, provides the

appropriate position information (latitude/longitude) for the TAWS functions.

(c) Degraded Horizontal Position Source. TAWS functions should be

automatically disabled when the horizontal position source has degraded to a point where its

accuracy can no longer support these functions (that is, IRU, dead reckoning). Annunciation of

the loss of the TAWS function is required. If the installation does not provide this automatic

feature, then an indication should be provided to the flightcrew that the TAWS function is no

longer available and the appropriate crew action noted in the AFM or AFMS.

(2) Vertical Position Source. The following sources are considered suitable for use in

establishing vertical position. The applicant is required to demonstrate that the TAWS, as

installed in the aircraft, provides the appropriate vertical information for the TAWS functions.

(a) Barometric Altitude. Vertical position information provided by a barometric

altitude source that meets the accuracy specified in TSO-C10b, or later versions, is considered

acceptable as a source of vertical position information.

(b) Radio Altimeter. Vertical position information provided by a radio altimeter

that meets the accuracy specified in TSO-C67, or later versions, is considered acceptable as a

source of vertical position information for height above ground only. Radio altitude by itself is

not adequate as the source of vertical position information for all phases of flight. Class B

equipment does not require a radio altitude input.

(c) Air Data Computers. Vertical position information provided by an air data

computer that meets the accuracy specified in TSO-C106, or later versions, is considered

acceptable as a source of vertical position information.

(d) GPS. Vertical position information provided by GPS may be used in

combination with other sources of vertical position (that is, barometric altitude). In this

implementation, the GPS must meet the accuracy and integrity requirements of TSO-C129a. If

GPS data is used as the only means for the determination of vertical position, then it must meet
6/14/00 AC 23-18

19

the accuracy and integrity requirements of TSO-C145, or later versions. Either of these

implementations is considered acceptable as a source of vertical position information.

g. Terrain Database. The terrain database for TAWS has been validated via the

TSO-C151a authorization process, so there is no need to revalidate the database during the installation

process. However, there are two areas of concern that must be addressed during the installation process:

(1) Updates. The installed system must be capable of accepting updated terrain databases (and

obstacle databases as well, if included). The TAWS manufacturers must have a procedure in place

whereby the manufacturer can inform either the airplane owner or operator, or both, about updates or

how they can learn about updates. The procedure must contain sufficient information to enable them to

make a logical safety decision as to whether it is appropriate to purchase and install the update. This

information should be contained in the instructions for continued airworthiness.

(2) Valid Regional Data. The instructions for continued airworthiness should identify the

procedures for determining the status of the terrain database. Operators should use this information to

determine if the current terrain database is appropriate for the area of intended operation. The area of

intended operation should be identified in the AFM or AFMS.

h. Display Presentation.

(1) Terrain Display.

(a) The TAWS equipment approved under TSO-C151a must be capable of providing terrain

and alerting data to display hardware. The display hardware may be stand-alone or interfaced with

existing equipment, such as a weather radar (WXR), Navigation displays, or other compatible display

system. The actual display presentation format that the flightcrew sees will depend on the onboard

display hardware, the options made available by the TSO-C151a manufacturer, and the features desired

by either the customer or the user. Regardless of whatever format is used, the display presentation

should enhance the flightcrew’s terrain situational awareness.

(b) The terrain display system is an output from the TAWS; therefore, the possibility of

failure of that system should be no greater than 10-3 per average flight hour, and the possibility of

misleading information on the display due to undetected or latent failures should be no greater than 10-4

per average flight hour. For a Class A TAWS, the software development assurance level should be at

least to Level C as defined in RTCA DO-178B or an acceptable alternative approved by the FAA. For

Class B TAWS, the software development assurance level should be at least to Level D providing the

required alerts and visual annunciations are independent of the terrain display(s). If the required alerts

and visual annunciations are integrated on the displays, the software development assurance should be at

least Level C. NOTE: A terrain display is not mandatory for Class B equipment.

(c) During the development of the TAWS equipment, the applicant should use a

representative sample of pilots to participate in the design and evaluation process for the

proposed presentation format.
AC 23-18 6/14/00

20

(d) The applicant should consider the selection of terrain display where the display is

utilized for multiple functions. In these cases, a means should be provided to select and de-select

the display of terrain.

(2) Terrain Display Presentation. TAWS equipment shall be designed to interface with a

terrain display, either color or monochromatic. The terrain display should be capable of providing the

following terrain-related information:

(a) The terrain should be depicted relative to the airplane’s position such that the pilot may

estimate the relative bearing to the terrain of interest.

(b) The terrain should be depicted relative to the airplane’s position such that the pilot may

estimate the distance to the terrain of interest.

(c) The terrain depicted should be oriented to either the heading or track of the airplane. In

addition, a north-up orientation may be added as a selectable format. A north-up orientation should only

be used for strategic (flight planning) purposes.

(d) Variations in terrain elevation depicted relative to the airplane’s elevation (above and

below) should be visually distinct. Terrain that is more than 2,000 feet below the airplane’s elevation

need not be depicted.

(e) Terrain that generates alerts should be displayed in a manner to distinguish it from nonhazardous terrain, consistent with the caution and warning alert level.

(f) The terrain display presentation should be clear, unambiguous, and readily usable by the

flightcrew during day and night operations under all ambient lighting conditions expected in service.

(g) The terrain display should be viewable in direct and reflected sunlight, and at least one

display should be viewable by the minimum required flightcrew.

(h) An inhibited, failed, or inoperative TAWS should be indicated to the flightcrew in a

manner consistent with the flight deck design philosophy.

(i) The terrain display presentation should complement and be compatible with the terrain

alerting function of the TAWS.

(j) Terrain mapping should allow the flightcrew to determine relative terrain elevation.

(k) The terrain display should be designed so that the flightcrew can readily determine if

terrain is a threat to the airplane.

(l) Either the colors or textures, or both, used for threat terrain should be intuitive and

indicate the immediacy of the threat. For warnings and cautions, § 23.1322 provides specific

requirements for the assignment of red and amber for visual annunciations. Red should be used
6/14/00 AC 23-18

21

as the warning annunciation for emergency operational conditions when immediate flightcrew

recognition is required, and immediate correction or compensatory action may be required.

Amber should be used for the cautionary annunciation for abnormal operational conditions when

immediate flightcrew awareness is required and subsequent flightcrew action may be required.

White, or another unique color, should be used for advisory annunciations of operational

conditions that require flightcrew awareness and when action may be required. Green should be

used for indication of safe operating conditions. For color terrain displays, an acceptable method

for color allotment is as follows: (1) red for terrain or obstacle threat warning area when the

airplane is well below the threat, (2) yellow or amber for terrain or obstacle threat caution area

when the airplane is about the same altitude as the threat, (3) green for terrain obstacle area when

the airplane is well above the threat, and (4) black for no significant terrain or obstacle. For

monochromatic displays, the intensity of the data, such as the use of dot density or brightness,

shows the relationship between terrain and the airplane altitude.

(m) If the terrain is presented on a shared display, the pilot may select the information to be

displayed at any given time. Terrain mode and terrain information should be easily distinguishable from

weather and other features.

(n) For color displays, the selected colors should complement the discreet visual and aural

alerts that are presented to the flightcrew. Accordingly, any colors that are used for the threat terrain

display should match the colors used for the discreet visual alerts.

(3) Pop-Up Mode-Switching Functionality.

(a) General Considerations. If implemented, an automatic pop-up feature should

incorporate the following considerations in its design:

• The pop-up functionality should automatically display in a heading up or track up

orientation.

• The pop-up functionality should automatically display TAWS-related information

when a TAWS warning or caution alert occurs.

• The display system should be designed so that it is very evident that an automatic

pop-up has occurred.

• The terrain display mode should be annunciated on the display. If this annunciation is

not feasible then a mode annunciation light should be installed near the terrain

display.

• Manually switching back to the original mode of operation should require minimal

effort.

• Automatic switching back to the original mode of operation after the caution

or warning ceases should not be allowed unless it is part of the aircraft design

philosophy.
AC 23-18 6/14/00

22

(b) Pop-Up Inhibit Feature. When dual terrain displays are provided in conjunction with

pop-up functionality, the terrain map and alerts should automatically be presented on at least one of the

displays.

(c) Prioritizing Pop-Up Displays Between Systems. In installations where the TAWS and

the Predictive Windshear System (PWS) share the same display, and automatic pop-up functionality is

employed, the display priorities indicated in Table 1, below, are recommended:

TABLE 1. Prioritization of pop-up displays where the TAWS and the

PWS share the same display

Priority Description

Highest 1 Terrain Awareness Warning

2 Predictive Windshear Warning

3 Terrain Awareness Caution

4 Predictive Windshear Caution

5 Standard Terrain Display

Lowest 6 Weather Radar

If the system provides alerting for obstacle threats, the prioritization for warnings and cautions should be

the same as those for terrain. The priorities are listed in the table above.

(4) Auto-Range Switching Mode. An auto-ranging function during alerts is not required.

However, if provided, an auto-ranging display should be designed so that it is very evident to the

flightcrew that the range has been automatically selected. The range selected for auto-ranging should

provide a usable depiction of the terrain or obstacle threat on the display. Switching back to a manuallyselected range should require minimal effort.

i. Alerts. Alerts should be clear, concise, and unambiguous. The alerting system should be:

(1) Consistent with the alerting philosophy of the airplane flight deck in which the TAWS

equipment is installed; and

(2) Within the flightcrew’s primary field of view

(3) Generation of aural and visual alerts should not be dependent upon display of terrain data.

j. Alert Prioritization.

(1) Installations of TAWS on aircraft also equipped with a Reactive Windshear System (RWS),

Predictive Windshear System (PWS), and Traffic Alert and Collision Avoidance System (TCAS) should

include an aural prioritization scheme such that:

(a) Only one aural alert is given at any one time, and
6/14/00 AC 23-18

23

(b) Aural alerts for situations requiring immediate action by the flightcrew have priority in

situations where conditions for multiple alerts may occur. Note: In older aircraft the system architecture

may preclude the prioritization of aural alerts for multiple alerting systems. If such is the case, an aural

prioritization scheme is not required. However, if simultaneous alerts can be given, then the alerts must

be understandable and distinguishable.

(2) Implementing this voice prioritization scheme within the TAWS equipment is acceptable.

An example of the recommended voice prioritization is shown in Table 2 for Class A TAWS, and Table

3 for Class B TAWS, if provided, but it is not required.

TABLE 2. Recommended voice prioritization between the Class A TAWS and

other systems installed

Priority Description For Class A TAWS

Highest 1 Reactive Windshear System Warning

2 Sink Rate Pull-Up Warning (Excessive Rates of Descent)

3 Terrain Closure Pull-Up Warning (Excessive Closure Rates)

4 Terrain Awareness Pull-Up Warning (FLTA)

5 Predictive Windshear System Warning

6 Minimums (Voice Callouts)

7 Terrain Awareness Caution (FLTA)

8 Too Low Terrain (Flight Into Terrain When Not in Landing

Configuration)

9 PDA (“Too Low Terrain”) Caution

10 Altitude Callouts (Voice callouts)

11 Too Low Gear (Flight Into Terrain When Not in Landing

Configuration)

12 Too Low Flaps (Flight Into Terrain When Not in Landing

Configuration)

13 Sink Rate (Excessive Rates of Descent)

14 Don’t Sink (Negative Climb Rate or Altitude Loss After Take-off)

15 Glideslope (Excessive Downward Deviation From an ILS Glideslope)

16 Predictive Windshear System Caution

17 Approaching Minimums (Voice Callouts)

18 Bank Angle (Voice Callouts)

19 Reactive Windshear System Caution

20 TCAS RA (“Climb,” “Descend,” etc.)

 Lowest 21 TCAS TA (“Traffic, Traffic”)
AC 23-18 6/14/00

24

TABLE 3. Recommended voice prioritization between the Class B TAWS and

other systems installed

Priority Description for Class B TAWS

Highest 1 Reactive Windshear System Warning

2 Sink Rate Pull-Up Warning (Excessive Rates of Descent)

3 Terrain Awareness Pull-Up Warning (FLTA)

4 Predictive Windshear System Warning

5 Minimums (Voice Callouts)

6 Terrain Awareness Caution (FLTA)

7 PDA (“Too Low Terrain”) Caution

8 Altitude Callouts (Voice callouts)

9 Sink Rate (Excessive Rates of Descent)

10 Don’t Sink (Negative Climb Rate or Altitude Loss After Take-off)

11 Predictive Windshear System Caution

12 Approaching Minimums (Voice Callouts)

13 Bank Angle (Voice Callouts)

14 Reactive Windshear System Caution

15 TCAS RA (“Climb,” “Descend,” etc.)

 Lowest 16 TCAS TA (“Traffic, Traffic”)

k. System Inhibit.

(1) Inhibiting the FLTA and PDA functionality should not impact the basic GPWS functions.

Appropriate annunciation of the inhibited functions should be provided to the flightcrew. Flightcrew

procedures for disabling various TAWS functions should be identified in the AFM or AFMS. System

inhibit should include a means such that it can not be operated instinctively, inadvertently, or by habitual

reflexive action. Provisions for a guarded switch could be a possible means of compliance.

(2) System Self-Test. Self-test features should be inhibited in flight unless provisions are made

to ensure the self-test cycle does not interfere with normal operations.

l. Flight Data Recorder. For those applications that require crash-survivable flight data

recording in accordance with the requirements of § 23.1459 (Flight recorders), a means should be

provided to record the FLTA alerts in the same manner as is currently done for the basic GPWS. It is

not necessary to distinguish between basic GPWS and the new FLTA and PDA alerts. A means also

should be provided to record either a FLTA or PDA-inhibit, or both.
6/14/00 AC 23-18

25

m. Systems Evaluations. Aircraft simulators, mock-ups, system integration tools, etc.,

independently or in conjunction with computer generated test scenarios developed by the TAWS

manufacturer, may be used to evaluate specific installations of TAWS equipment. If these tools are used

to evaluate a particular TAWS installation, they should adequately represent the target aircraft

configuration. The level of fidelity required will depend on the type of credit being sought. Some of the

characteristics of a TAWS installation and flight deck integration that may be evaluated are displays,

alert prioritization, mode transitions, pop-up displays, auto-ranging, self test, operational workload

issues, accessibility and usability of the TAWS controls, and systems failure modes.

n. Ground Test Requirements. A ground test should be conducted for each TAWS

installation. The level of testing required will be determined by the scope of the installation (whether an

initial installation of the model versus a follow-on installation). In addition to system evaluations noted

above, ground test should evaluate the location of TAWS controls, displays, and visual alerts, aural

alerts, self-test functions, identified failure modes, all discrete inputs and sensors interfaced with the

TAWS equipment, electro-magnetic interference (EMI)/electro-magnetic compatibility (EMC) testing,

and electrical transient effects. The visibility and characteristics of any provided display should be

evaluated under all anticipated cockpit lighting conditions. Ground tests conducted using computer

generated test equipment accepted by the TAWS manufacturer may be used in place of flight-testing for

basic Class B TAWS equipment. Similar testing may be used to minimize the extent of flight-testing

required for Class A TAWS equipment.

o. Flight-Test Requirements. The level of flight-test required to validate a particular

TAWS installation will be based on the type of airplane, airplane system architecture (digital, hybrid

digital-analog discrete inputs, or analog discrete inputs), and credit given for previously certified

installations, system evaluation, and ground testing. Airplanes incorporating digital or hybrid digitalanalog discrete data interface architectures may require flight-testing to ensure proper data transfer

among various systems. The actual requirement for a flight-test needs to be evaluated for each

installation. If ground testing is not adequate to verify proper function of all interfaced equipment,

additional flight-testing for TAWS functions may be necessary.
AC 23-18 6/14/00

26

(1) Flight-Test Matrix. Table 4 shows TAWS functions that should have a flight-test

evaluation for the following examples. These examples are intended to assist in determining the

extent of flight-testing for some potential or likely TAWS configurations:

TABLE 4. Flight-Test Matrix

TAWS Functions Example

1

Example

2

Example

3

Example

4

Example

5

Example

6

FLTA X X X X X

PDA X X X or X

Basic GPWS X X X X

Terrain Display X X X X

TAWS Horizontal

Position Source

X XX

(a) Example 1. This is the first time the manufacturer’s equipment has been

installed in any airplane for the purpose of receiving initial TC or STC approval. If such is the

case, then a complete and thorough ground and flight-test program should be conducted to verify

the adequacy of the installation.

(b) Example 2. This involves a follow-on installation of a previously approved

TAWS in which a required sensor input has not been previously approved for the specific

manufacturer’s equipment. For example, if the sensor that provides barometric altitude (or

equivalent) to the TAWS equipment has not been previously approved, the flight-test evaluation

should focus on the TAWS functions affected by barometric altitude such as FLTA and PDA.

(c) Example 3. This involves a follow-on installation of a previously approved

TAWS in which the terrain display has not been previously approved. In this case, the focus of

the flight-test evaluation should be on display related issues and tests specified in paragraph

7.o.(6), below, of this AC.

(d) Example 4. This involves a follow-on installation of a previously approved

TAWS in which the horizontal position source sensor input has not been previously approved for

the specific manufacturer’s equipment. In this case, the focus of the flight-test evaluation should

be on the adequacy of the horizontal position source and the display of the terrain as determined

by that horizontal position source. Verification of the function and proper interface installation

may require additional flight-testing for the FLTA, PDA, and basic GPWS.

(e) Example 5. This involves a follow-on installation of a previously approved

TAWS in which the vertical position source to the TAWS equipment has not been previously

approved. In this case, the focus of the flight-test evaluation should be on the TAWS functions

affected by the vertical position source such as one of the basic GPWS modes, FLTA, or PDA

functions. Only one test is required to assure that the vertical position source input is properly

installed to the TAWS.
6/14/00 AC 23-18

27

(f) Example 6. This involves a follow-on installation of a particular manufacturer’s

TAWS in an airplane that was previously approved with the same manufacturer’s basic GPWS

equipment (per TSO-C92c). The TAWS installation uses the same interfaces used for the

previous GPWS installation plus those additional interfaces associated with the TAWS

installation. If such is the case, then basic GPWS testing may not be necessary. Flight-tests can

be limited to the interfaces, features and functions (including basic GPWS) added with the

TAWS installation. For example, interfaces that enable additional GPWS functions or features,

such as position based alerting desensitization, altitude callouts, bank angle limits, etc., would be

evaluated along with TAWS functions.

(2) FLTA Flight-Test Requirements. Flight-testing to verify the proper operation of

the FLTA function can be conducted in an area where the terrain or obstacle elevation for the test

runs is known within approximately 300 feet. This test may have a potential hazard. Test runs

are recommended to be level flight at approximately 500 feet above the terrain/obstacle of

interest. The test runs should verify that all alerts (caution and warnings) are given at an

appropriate point in the test run, that all pop-up, auto-range, or other display features are working

correctly, and that the display depicts the terrain accurately.

NOTE: The terrain selected should be at least 15 NM from the nearest airport to

conduct the test as described. If this is not practical, the fly-over altitude will have to

be lowered to 300 feet or less above either the terrain or obstacle, or both, in order to

generate a TAWS alert.

(3) PDA Flight-Test Requirements. Flight-testing to verify the proper operation of the

PDA function can be conducted in any airport area within 10 NM of the nearest runway. The

airplane should be configured for landing at approximately 1500 feet AGL along the final

approach segment of the runway at approximately 10 NM from the runway. At the 10 NM point,

a normal three degree flight path angle descent can be initiated and maintained until the PDA

alert occurs.

NOTE: The runway selected for this test should be relatively free from either terrain

or obstacles to preclude activation of the FLTA function. If not feasible, then increase

the barometric altitude by 1 inch of mercury (hg) to allow radio altitude inputs to

trigger a PDA alert. Approximately level terrain along the final approach segment

will exercise the PDA function. This test may also exercise the 500 foot voice

callout. The adequacy of the PDA aural alert should be verified during this test. This

test will also verify the adequacy of the airport data base, the navigation source input

and either the barometric or radio altitude inputs to TAWS, or both.

(4) Basic GPWS Flight-Test Requirements. Flight-testing to verify the proper

operation of basic GPWS functions can be conducted in any area where the terrain elevation is

known to the flightcrew. The following information is intended to provide guidance for

conducting flight-tests to exercise and verify the proper operation of each GPWS function. The
AC 23-18 6/14/00

28

need to conduct flight-testing for follow-on TAWS installations will depend upon the nature of

the new or modified sensors and their impact on that particular GPWS function.

(a) Excessive Rates of Descent. This test can be conducted at any location, but

descents toward near level terrain are recommended for best results and ease of correlation with

RTCA DO-161 envelopes. For Class A equipment, exercising this test verifies the proper inputs

to the TAWS of barometric altitude (and the corresponding computation of barometric altitude

rate) and radio altitude. For Class B equipment, exercising this test verifies the proper inputs to

the TAWS of barometric altitude, the height above terrain as determined from the GPS position

and the corresponding terrain elevation from the terrain data base. Only one test run is required

to determine proper installation.

(b) Excessive Closure Rate To Terrain. This test must be conducted in an area of

known rising terrain. It is recommended that one level test run at an altitude between 500-1000

feet above the terrain elevation be conducted. For Class A equipment only, this test will verify

the proper input to the TAWS of the radio altitude.

(c) Negative Climb Rate or Altitude Loss After Takeoff. This test is conducted

immediately after takeoff before climbing above 700 AGL or above runway elevation. For Class

A equipment, exercising this test verifies the proper inputs to the TAWS of barometric altitude,

barometric altitude rate and radio altitude. For Class B equipment, exercising this test verifies

the proper input of barometric altitude and height above terrain as determined from the GPS

position and the corresponding terrain elevation from the terrain data base.

(d) Flight Into Terrain When Not In Landing Configuration. This test should be

conducted while on a visual approach to a runway and include evaluation of the flap override

function. For Class A equipment only, exercising this test verifies the proper input to the TAWS

of barometric altitude, barometric altitude rate and radio altitude as well as the gear and flap

sensor inputs to TAWS.

(e) Excessive Downward Deviation From an ILS Glideslope. This test should be

conducted during an ILS approach and include evaluation of the glideslope cancel function. For

Class A equipment only, this test will verify the proper installation of the ILS Glideslope input to

TAWS.

(f) Voice Callout “Five Hundred.” This test is conducted during an approach to a

runway. For Class A and B equipment, this test will verify the proper input to the TAWS of

barometric altitude, radio altitude, and height above terrain as determined by either radio altitude

or by access to the terrain data base.

(5) Terrain Display Flight-Test Requirements. Flight-testing to verify the proper

operation of the terrain display should be conducted while verifying all the other required TAWS

functions. Emphasis should be placed on verifying compliance with the requirements specified

in paragraph 7.h.(2) of this AC during normal airplane maneuvering during all phases of flight.

Pop-up and auto-ranging features should be evaluated if applicable. It is recommended to
6/14/00 AC 23-18

29

perform sustained turns to evaluate symbol stability, flicker, jitter, display update rate, color

cohesiveness, readability, the use of color to depict relative elevation data, caution and warning

alert area depictions, map masking and overall suitability of the display.

(6) TAWS Horizontal Position Source Flight-Test Requirements. For TAWS

equipment interfaced with an approved GPS (WAAS) navigation system, no additional

horizontal position source testing is necessary. (For TAWS equipment incorporating an internal

GPS (WAAS) position source, a suitable ground or flight-test to validate positional accuracy is

necessary.) A low level overflight of a known ground reference point, reference AC 20-130A or

AC 20-138, is acceptable. In addition, a 360 degree turn at 30 degrees of bank is necessary to

show continuity of position data. Note: Additional test equipment will likely be required to

observe the GPS data.

(7) Pressure Altitude Variations in Cold Weather Flight-Test Requirements. The

TAWS may be designed to account for the effects of cold weather on barometric altitude while

determining vertical position. Flight-testing may be required for the initial installation approval

only, unless a suitable ground verification procedure is conducted. Testing requirements will

depend on the design of the cold weather compensation.

(8) Added Features Flight-Test Requirements. Flight-testing may be required to

verify the proper operation of added features such as Windshear Detection, Bank Angle, Altitude

Call Outs “Approaching Minimums,” or other features not required by TSO-C151a.

p. AFM or AFMS. An AFM or AFMS containing the limitations, restricted areas of

operation, and operational considerations for either Normal or Abnormal Procedures should be

provided for each installation. The applicant should make an evaluation to determine if there are

any limitations of the system and, if so, how they will affect aircraft operations. Any limitations

affecting operations shall be included in the AFM or AFMS. As a minimum, the applicant

should provide instructions in the Limitations Section of the AFM or AFMS that include the

following:

(1) Limitations. The following instructions should be included in the Limitations

Section of all AFM or AFMS:

(a) Navigation must not be predicated upon the use of the TAWS.

NOTE: The terrain display is intended to serve as a situational awareness tool

only. It may not provide either the accuracy or fidelity, or both, on which to solely

base decisions and plan maneuvers to avoid terrain or obstacles.

(b) To avoid giving unwanted alerts, the TAWS must be inhibited when landing at an

airport that is not included in the airport database.

(c) The use of the terrain awareness warning and terrain display functions is

prohibited during QFE (atmospheric pressure at airport elevation) operations.
AC 23-18 6/14/00

30

NOTE: This limitation may not apply to systems that use other sources of

altitude measurement to determine the airplanes vertical position.

(d) The ABC Model XXX TAWS Pilot’s Guide, P/N <insert part number>, dated

<insert date> (or later appropriate revision) should be immediately available to the flightcrew.

The software status stated in the pilot’s guide should match that displayed on the equipment.

(e) Pilots are authorized to deviate from their current ATC clearance to the extent

necessary to comply with TAWS warnings.

(f) Terrain database. The area of intended operation should be identified in the AFM

or AFMS (that is, North America, Europe, etc.)

(2) Restricted Areas of Operation. Known areas or other factors within the approved

areas of operation that adversely affect navigation performance to the extent that the TAWS will

be potentially unreliable or misleading, should be identified in the AFM or AFMS. Areas of

operation where the terrain data base resolution or accuracy leads to nuisance alerts should also

be identified.

This situation may occur if:

• The data are not properly analyzed,

• There is not enough data to map the terrain adequately,

• The resolution of the data is not adequate for the type of approved position source,

or

• There are changes in the area (volcanoes, earthquakes, construction) that have not

been noted.

NOTE: For Class A TAWS equipment and where operational procedures require a

route structural analysis (RSA), airports or runways that are susceptible to nuisance alerts should

be identified. This analysis can be accomplished by computer simulation. Engine-out

procedures also should be considered when developing the RSA.

(3) Operational Considerations for either Normal or Abnormal Procedures, or

both. In addition to the GPWS operational procedures, consider the following:

(a) Terrain Awareness Caution. When a terrain awareness CAUTION occurs take

positive corrective action until the alert ceases. Stop descending or initiate either a climb or a

turn, or both, as necessary, based on analysis of all available instruments and information.

(b) Terrain Awareness Warning. If a terrain awareness WARNING occurs,

immediately initiate and continue a climb that will provide maximum terrain clearance, or any

similar approved vertical terrain escape maneuver, until all alerts cease. Only vertical maneuvers

are recommended, unless either operating in visual meteorological conditions (VMC), or the
6/14/00 AC 23-18

31

pilot determines, based on all available information, that turning in addition to the vertical escape

maneuver is the safest course of action, or both.

(c) NOTE: Cold Weather Operations. For those systems that do not provide

extreme cold temperature compensation of barometric altitude (corrected altitude), the AFM,

AFMS, or Supplemental AFM should include the following note: “Operations at extreme cold

temperatures, for example, -30 degrees Celsius, will result in a significant reduction in terrain

clearance provided by TAWS alerts.” To reduce or eliminate errors potentially induced in

barometric altitude temperature extremes, the design should include features such as corrected

altitude or special operational procedures. For operations in cold weather, either the system

should be able to account for variations in extreme cold weather temperatures, for example, –30

degrees Celsius, or additional flightcrew procedures should be considered to address pressure

altitude limits for vertical position determination.

(d) GPWS Mode 4. A procedure must be established for the operation of the

inhibition of Mode 4 warnings based on flaps being in other than the landing configuration. The

GPWS Mode 4 function may be deactivated to prevent nuisance warnings when following

approved procedures that specify landing with flaps not in normal landing position or landing

gear up.

(e) GPWS Mode 5. The GPWS should provide a capability to inhibit or cancel the

below-glideslope alert for an approach.

S/

Michael K. Dahl

Acting Manager, Small Airplane Directorate

Aircraft Certification Service