AC 25-23

DATE: 5/22/00

Airworthiness Criteria for the

Installation Approval of a

Terrain Awareness and Warning System (TAWS)

for Part 25 Airplanes

5/22/00 AC 25-23

Page i (and ii)

CONTENTS

PARAGRAPH PAGE

1. PURPOSE.................................................................................................................................. 1

2. RELATED REGULATIONS AND DOCUMENTS. ............................................................. 1

3. DEFINITIONS.......................................................................................................................... 4

4. BACKGROUND....................................................................................................................... 7

5. DISCUSSION............................................................................................................................ 8

6. SYSTEM DESCRIPTION. ...................................................................................................... 9

7. AIRWORTHINESS CONSIDERATIONS FOR CERTIFICATION PROGRAMS....... 10

8. PROJECT SPECIFIC CERTIFICATION PLAN (PSCP)................................................. 11

9. SYSTEM SAFETY ASSESSMENT...................................................................................... 12

10. SOFTWARE. ........................................................................................................................ 13

11. POSITION SOURCE. .......................................................................................................... 13

12. TERRAIN DATABASE. ...................................................................................................... 15

13. DISPLAY PRESENTATION .............................................................................................. 15

14. ALERTS. ............................................................................................................................... 19

15. ALERT PRIORITIZATION. .............................................................................................. 20

16. SYSTEM INHIBIT............................................................................................................... 21

17. FLIGHT DATA RECORDER............................................................................................. 22

18. SYSTEMS EVALUATIONS WITH SIMULATORS. ...................................................... 22

19. GROUND TEST CONSIDERATIONS. ............................................................................. 22

20. FLIGHT TEST CONSIDERATIONS. ............................................................................... 23

21. AIRPLANE FLIGHT MANUAL (AFM)/AIRPLANE FLIGHT MANUAL

 SUPPLEMENT (AFMS)...................................................................................................... 28

5/22/00 AC 25-23

Page 1

1. PURPOSE. 

a. This Advisory Circular (AC) describes an acceptable means for obtaining FAA

airworthiness approval for the installation of a Terrain Awareness and Warning System

(TAWS) that has been approved under Technical Standard Order (TSO)-C151a, Terrain

Awareness and Warning System, or later revisions. 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 is compliant with

TSO-C151a. The guidance provided is specific to installations of these systems on

transport category airplanes certificated under 14 CFR part 25 [commonly referred to as

part 25 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.

b. The guidance provided in this document is directed to airplane and avionics

manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation

Administration transport airplane type certification engineers and their designees.

c. Like all advisory circular material, this AC is not, in itself, mandatory, and does

not constitute a regulation. It is issued to describe an acceptable means, but not the only

means, for demonstrating compliance with the requirements for transport category airplanes.

Terms such as “shall” and “must” are used only in the sense of ensuring applicability of

this particular method of compliance when the acceptable method of compliance

described in this document is used. While these guidelines are not mandatory, they are

derived from extensive Federal Aviation Administration and industry experience in

determining compliance with the pertinent regulations.

d. This advisory circular does not change, create any additional, authorize changes in,

or permit deviations from, regulatory requirements.

2. RELATED REGULATIONS AND DOCUMENTS.

a. Regulations. 14 CFR parts:

§ 25.1301 Function and installation

§ 25.1303 Flight and navigation instruments

§ 25.1309 Equipment, systems, and installations

§ 25.1321 Arrangement and visibility

§ 25.1322 Warning, caution, and advisory lights

§ 25.1333 Instrument systems
AC 25-23 5/22/00

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§ 25.1351 Electrical systems and equipment -- General

§ 25.1353 Electrical equipment and installations

§ 25.1357 Circuit protective devices

§ 25.1381 Instrument lights

§ 25.1383 Landing lights

§ 25.1431 Electronic equipment

§ 25.1459 Flight recorders

§ 25.1529 Instructions for Continued Airworthiness

§ 25.1541 Markings and placards -- General

§ 25.1581 Airplane flight manual -- General

§ 25.1585 Operating procedures

§ 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

b. FAA Advisory Circulars (AC), Orders, and Technical Standard Orders

(TSO). The AC’s and TSO’s listed below can be obtained from the U.S. Department of

Transportation, General Services Section, M-443.2, Washington, D.C. 20590.

• AC 20-112, Airworthiness and Operational Approval of Airborne Systems

to be Used in Lieu of Ground Proximity Warning System, February 19,

1981.

• AC 20-115B, Radio Technical Commission for Aeronautics, Inc.

Document RTCA/DO-178B, January 11, 1993

• AC 20-130A, Airworthiness Approval of Navigation or Flight

Management Systems Integrating Multiple Navigation Sensors, June 14,

1995.

• AC 20-136, Protection of Aircraft Electrical/Electronic System Against the

Indirect Effects of Lightning, March 5, 1990
5/22/00 AC 25-23

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

Navigation Equipment for Use as a VFR and IFR Supplemental

Navigation System, June 25, 1994.

• AC 21-16D, Radio Technical Commission for Aeronautics Document DO160D

• AC 25-4, Inertial Navigation Systems (INS), February 18, 1966.

• AC 25-10, Guidance for Installation of Miscellaneous Nonrequired

Electrical Equipment, March 6, 1987.

• AC 25-11, Transport Category Airplane Electronic Display Systems, July

16, 1987.

• AC 25-12, Airworthiness Criteria for Approval of Airborne Windshear

Warning Systems in Transport Category Airplanes, November 2, 1987.

• AC 25-16, Electrical Fault and Fire Prevention and Protection, April 5,

1991.

• AC 25.1309-1A, Systems Design and Analysis, June 21, 1988.

• AC 25.1581-1, Airplane Flight Manual, July 14, 1997.

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

National Airspace System, February 21, 1975.

• FAA Order 8110.4A, Type Certification Process

• 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-C115, Airborne Navigation Equipment Using Multi-Sensor Inputs

• TSO-C117a, Airborne Windshear Warning and Escape Guidance Systems

for Transport Airplanes
AC 25-23 5/22/00

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• TSO-C129a, Airborne Supplemental Navigation Equipment Using the

Global Positioning System (GPS)

• 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

c. Industry documents.

(1) The Radio Technical Commission for Aeronautics (RTCA) documents listed

below are available from RTCA, Inc., 1140 Connecticut Avenue N.W., Suite 1020,

Washington, D.C. 20036-4001.

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

Equipment

• DO-161A, Minimum Performance Standards - Airborne Ground

Proximity Warning Equipment

• DO-178B, Software Consideration in Airborne Systems and Equipment

Certification

• DO-200, Preparation, Verification and Distribution of User-Selectable

Navigation Databases

• DO-200A, Standards for Processing Aeronautical Data

• DO-208, Minimum Operational Performance Standards for Airborne

Supplemental Navigation Equipment Using Global Positioning System

(2) The Society of Automotive Engineers (SAE) documents listed below are

available from SAE, 400 Commonwealth Drive, Warrendale, Pennsylvania 15096-0001.

• Society of Automotive Engineers (SAE) Aerospace Recommended

Practice (ARP) 4761, Guidelines and Methods for Conducting the Safety

Assessment Process on Civil Airborne Systems and Equipment

• Society of Automotive Engineers (SAE) Aerospace Recommended

Practice (ARP) 4754, Certification Considerations for Highly Integrated

or Complex Aircraft Systems

3. DEFINITIONS.

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

convey information regarding system status or condition, or both

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

condition, situation, or event.
5/22/00 AC 25-23

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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.

(1) 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.

(2) This class of TAWS equipment also must provide a terrain awareness display

of the surrounding terrain and/or obstacles relative to the airplane. See TSO C151a,

Table 11-1, 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.

(1) 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.

(2) This class of TAWS equipment does not require a terrain awareness display

of the surrounding terrain and/or obstacles relative to the airplane. See TSO C151a,
AC 25-23 5/22/00

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Table 11-1, 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. Failure: The inability of the equipment or any sub-part of that equipment to

perform within previously specified limits

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

caution threshold of the system is not exceeded.

i. Forward Looking Terrain Avoidance (FLTA): A TAWS functional

requirement to provide look-ahead terrain and obstacle protection along and below the

airplane’s lateral and vertical flight path.

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

environment, could lead to an accident.

k. Hazardously Misleading Information (HMI): An incorrect depiction of the

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

l. Imminent Terrain Impact: An alert that will notify the flightcrew when the flight

path of the airplane is projected to impact terrain.

m. Nuisance Alert: An alert that is caused by improper setting of the terrain alerting

threshold.

n. Obstacle: A human made structure that is in the flight path of the aircraft.

o. Premature Descent Alert (PDA): A warning system’s ability to detect when the

aircraft is hazardously below the normal (approximately 3 degrees) approach path for the

nearest runway, and to provide a timely alert.

p. Required Terrain Clearance (RTC): The minimum requirements for obstacle

terrain clearance as defined by United States Standard for Terminal Instrument

Procedures (TERPS) FAA Order 8260.3B and the Aeronautical Information Manual

(AIM).

q. Terrain Awareness and Warning System (TAWS): A generic term used to

describe an alerting system that provides the flightcrew with sufficient information and

time to detect a potentially hazardous terrain situation and avoid CFIT.
5/22/00 AC 25-23

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

relative to the airplane.

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

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

information from other systems [e.g., an Electric Flight Instrument System/Navigation

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

u. Visual Alert: The use of projected or displayed information to present a

condition, situation, or event to the flightcrew.

v. Warning Alert: An alert for a detected terrain threat that requires immediate

flightcrew action.

4. BACKGROUND.

a. General. Controlled flight into terrain (CFIT) has been a principal contributor to

commercial jet airplane hull losses and fatalities. Before 1975, the worldwide

commercial jet airplane fleet averaged approximately eight CFIT accidents per year.

When the original Ground Proximity Warning System (GPWS) was first introduced on

aircraft in the mid-1970’s, it dramatically reduced accident rates. The GPWS is a

computer-based system that, among other things, provides the flightcrew with adequate

warning (both aural and visual) of inadvertent contact of the airplane with the terrain or

other obstacles, taking into account such items as crew recognition and reaction times.

b. Regulatory Background. In 1974, the FAA issued Amendments 121-114 and

135-12 (39 FR 44440, December 24, 1974). Those amendments required that all part 121

certificate holders (i.e., those operating large turbine-powered airplanes) and certain part

135 certificate holders (i.e., those operating large turbojet-powered airplanes) install

GPWS equipment approved under TSO-C92c (Airborne Ground Proximity Warning

Equipment) in their airplanes. (In 1978, the FAA extended this same requirement to part

135 certificate holders operating smaller turbojet-powered airplanes.) That TSO

prescribes the minimum performance standards that GPWS equipment must meet to be in

compliance with the applicable regulation.

c. New Systems. The use of GPWS has not entirely eliminated the problem of CFIT,

however, and CFIT accidents continue to occur. Recent advances in terrain mapping

technology have led to the development of a new type of GPWS that provides greater

situational awareness to flightcrews. It is designed to help reduce the CFIT accident rate

even further by providing additional information and warning to the flightcrew in

situations where the airplane may be inadvertently approaching significant rising terrain

or a man-made obstruction. There is an increased interest in certifying these systems on

transport airplanes due to recent and continuing accidents attributed to CFIT.
AC 25-23 5/22/00

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d. TSO-C151a. The FAA has issued Technical Standard Order (TSO)-C151a,

Terrain Awareness and Warning System, which prescribes the minimum design standards

that a Terrain Awareness and Warning System (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

Forward Looking Terrain Avoidance (FLTA) functions, the TAWS incorporates the

standards and test procedures for Basic GPWS equipment (as defined in TSO-C92c and

in RTCA document DO-161A, Minimum Performance Standards - Airborne Ground

Proximity Warning Equipment). For further in-depth information concerning the TAWS’

capabilities, refer to TSO-C151a.

e. Need for Guidance.

(1) 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 heightened interest by manufacturers and operators to equip

transport category 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 Advisory Circular (AC) has been

developed as the means for providing such guidance.

(2) 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 on transport category airplanes.

5. DISCUSSION. The following sections of 14 CFR parts 91, 121, and 135 require the

installation of the TAWS:

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

airplane configured with 6 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-C151a.

b. § 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-C151a.

c. § 135.154 states that no person may operate a turbine-powered U.S.-registered

airplane configured with 6 to 9 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-C151a. It also states that no person may
5/22/00 AC 25-23

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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-C151a.

d. The parts 121 and 135 regulations also mandate that GPWS be replaced with a

TAWS approved under TSO-C151a.

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) Forward-Looking Terrain-Avoidance (FLTA) function, which includes:

• Reduced required terrain clearance.

• Imminent terrain impact.

(b) Premature Descent Alert (PDA) function.

(c) Basic Ground Proximity Warning System (GPWS) functions, as defined

in TSO-C151a and RTCA Document 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

terrain-threat alerting are made possible by the TAWS’ acceptance of a variety of input

parameters. 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 provisions specified in paragraph 11. (Position Source) of this AC.
AC 25-23 5/22/00

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

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

(a) FLTA function, which includes:

• Reduced required terrain clearance

• Imminent terrain impact

(b) Premature Descent Alert (PDA) function

(c) Basic GPWS functions, as defined in TSO-C151a and RTCA Document

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 provisions of paragraph 13. (Display Presentation) of

this AC.

(3) The Class B TAWS equipment will be required to interface with an approved

GPS for horizontal position information.

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

7. AIRWORTHINESS CONSIDERATIONS FOR CERTIFICATION PROGRAMS

a. The scope of the applicant’s program should be directed toward airworthiness

approval through the Type Certification (TC) or Supplemental Type Certification (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 must 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 must

be coordinated with the responsible FAA Directorate, as described in FAA Order 8110-

4A, Type Certification.

b. The remainder of this AC describes in detail specific airworthiness considerations

that applicants should take into account as part of the certification process.
5/22/00 AC 25-23

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8. PROJECT SPECIFIC CERTIFICATION PLAN (PSCP). 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. A crucial part of this process is the

development of a “Project Specific Certification Plan (PSCP).” The applicant should

develop a comprehensive PSCP that includes the following:

a. Project Description: A summary of the project.

b. System Description:

(1) 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.

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

flight engineer panels.

(3) Information regarding software aspects of certification and any applicationspecific 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.

c. Project Schedule:

(1) A detailed project schedule that identifies all major milestones and schedules

for any required deliverables (i.e., test plans).

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

authority validation requirements.

d. Certification Basis and Means of Compliance:

(1) 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.

(2) 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 (i.e., STC, TSO, letter of approval, etc.) should be

included in this section.
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e. Communication and Coordination:

(1) If applicable, identification of all designated engineering representatives

(DER), designated airworthiness representatives (DAR), designated manufacturing

inspection representatives (DMIR), or designated alteration station (DAS) specialists

working on the program.

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

stipulations, coordination, and limitations that are placed upon those delegations.

f. Human Factors Plan: This section should address human factors issues and

provide human factors support for decisions regarding the flightcrew interface issues

resulting from the TAWS installation.

g. Testing Plan: This section should contain the requirements for the planning,

preparation, and conduct of FAA required testing, including any delegations.

h. Conformity Plan: This section should describe the activities associated with

conformity of parts, simulators, and aircraft installations.

i. Continued Airworthiness Plan: This section should provide the instructions for

continued airworthiness for the TAWS installation.

j. Compliance Documentation: This section should describe the procedures for

submittal and processing of compliance documentation.

9. SYSTEM SAFETY ASSESSMENT.

a. The applicant should perform a system safety assessment (SSA) to establish the

hazards associated with the proposed installation. The applicant should develop the SSA

in accordance with the guidance provided in AC 25.1309-1A (System Design and

Analysis) and Society of Automotive Engineers (SAE) Aerospace Recommended Practice

(ARP) 4761 (Guidelines and Methods for Conducting the Safety Assessment Process on

Civil Airborne Systems and Equipment).

b. The TSO-C151a requires a probability of 10-5 for unannunciated failure,

hazardously misleading information (HMI), and false alerts at the box level. Therefore,

the box as installed must meet the following criteria:

(1) The probability of a failure that would lead to the loss of all functions as they

are described in paragraph 6. (System Description) of this AC shall be less than or equal

to 10-3 per flight hour.
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(2) The probability of a false caution and/or warning alert due to undetected or

latent failures shall be less than or equal to 10-4 per flight hour.

(3) The probability of an unannunciated failure of the system to provide the

required alerting functions due to undetected or latent failures shall be less than or equal

to 10-4 per flight hour.

(4) The probability of the system to provide HMI to the TAWS display due to

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

(5) Failure of the installed TAWS shall not degrade the integrity of any essential

or critical system installed in the airplane with which the TAWS interfaces.

10. SOFTWARE. The applicant should provide evidence that the TAWS software

meets the requirements of TSO-C151a and that it meets the appropriate software levels

for any added feature (s).

11. POSITION SOURCE. TAWS uses the estimated position of the airplane with

reference to the terrain/obstacle to determine when an alert should be annunciated. The

applicant should provide evidence that the TAWS position source (horizontal and

vertical) is suitable for each phase of flight (i.e., enroute, remote/oceanic, terminal, and

approach, as provided by TSO-C151a, Appendix 1, Section 10) for which approval is

sought.

a. Horizontal Position Source.

(1) TAWS that interface with previously approved navigation systems:

(a) Class A equipment may use approved Area Navigation (RNAV), which

may include GPS.

(b) Class B equipment is required to interface with an approved GPS for

horizontal position.

(c) Previously approved RNAV and GPS systems that are used for

navigation are considered suitable for TAWS horizontal position inputs.

(d) The RNAV system must meet the requirements of TSO-115, or be

installed in accordance with AC 90-45A for RNAV systems.

(e) GPS must meet the requirements of TSO-C129a, or be installed in

accordance with AC 20-130A and/or AC 20-138.
AC 25-23 5/22/00

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(f) If a Wide Area Augmentation System (WAAS) is used as a position

source, it should meet TSO C-145.

(g) TAWS functions that require this navigation source for operations should

be automatically disabled when the source has degraded to a point where its accuracy can

no longer support the TAWS functions (i.e., IRU, dead reckoning). An annunciation of

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

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

longer available, and the appropriate crew action should be noted in the AFM. A means

for testing the position source for accuracy and reliability is unnecessary for these

systems.

(2) TAWS equipment with internal GPS position source: Class A and Class B

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 DO-208 or equivalent.

When this alarm limit is activated, the GPS-computed position is considered unsuitable

for the TAWS and the functions that require GPS for operations should be automatically

disabled and an annunciation provided to the flightcrew. If the installation does not

provide this automatic feature, then an indication should be provided to the flightcrew

that the function is no longer available, and the appropriate crew action should be noted

in the AFM. This type of TAWS equipment will not be approved for navigation unless it

meets the provisions of paragraph 11.a.(1) of this AC. 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.

b. Vertical Position Source. The following sources are considered suitable for use

in establishing vertical position. They may be used as a sole source or in combination.

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

provides the appropriate vertical information for the TAWS functions.

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

altitude source that meets the requirements of TSO-C10b, or later versions, is considered

acceptable as a source of vertical position information.

(2) 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. Class B equipment does not require a radio

altitude input.

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

computer that meets the accuracy requirements of TSO-C106, or later versions, is

considered acceptable as a source of vertical position information.
5/22/00 AC 25-23

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

combination with other sources of vertical position. If GPS is used as the only means for

the determination of vertical position then it must meet the requirements of TSO-C145, or

later versions.

12. 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:

a. 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 the airplane

owner/operator about updates, or the airplane owner/operator can learn about updates.

The procedure must contain sufficient information to enable the owner/operator 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.

b. 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

13. DISPLAY PRESENTATION. The terrain display for TAWS is primarily a

strategic planning tool that the flightcrew will use to identify potential flight vertical of

horizontal errors and correct these errors before an alert occurs. It is not intended for

tactical navigation during alert conditions. When an alert occurs, the flightcrew should

perform the appropriate procedures before referencing the terrain display for information.

NOTE: A terrain display is not mandatory for Class B equipment. (For further

definition, see paragraph 6.b. of this AC and TSO-C151a.)

a. Terrain Display.

(1) 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 systems. 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 the

customer/user. Regardless of what format is used, the display presentation must enhance

the flightcrew’s terrain situational awareness.
AC 25-23 5/22/00

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(2) The terrain display system is an output from the TAWS; therefore:

(a) the possibility of failure of that system should be no greater than 10-3 per

average flight hour, and

(b) 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.

(3) 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.

(4) The applicant should consider the selection of a terrain display where the

display is used for multiple functions. In these cases, a means should be provided to

select and de-select the display of terrain. However, care must be exercised in the

selection of such a multifunction implementation, to ensure that the display sharing is

appropriate for the specific functions. The use of the TAWS situation awareness display

should not unacceptably detract from the usability of existing functions. Since the TAWS

display is not to be used for navigation, the use of the display should not impair the ability

of the pilot to perform required navigation functions. An example of such an impairment

would be an installation that forces the pilot to chose between the TAWS display and the

needed navigation information in situations where both are likely to be used

simultaneously and continuously (e.g., instrument approach in the vicinity of hazardous

terrain). If the timesharing of the display between TAWS and other functions is deemed

acceptable, the design must facilitate simple switching between the functions, with

minimal time delays, so that both functions are sufficiently accessible in realistic flight

scenarios.

(5) The terrain display should be installed in a location that is acceptable for

monitoring by the pilot as a planning tool to identify potential flight path conflicts. In

general, the terrain display should be in a location similar to other situation awareness

displays, such as electronic maps and weather radar.

b. Terrain Display Presentation. TAWS equipment shall be designed to interface

with a terrain display, either color or monochromatic. If there are multiple terrain views

available, at least one terrain display shall be capable of providing all of the following

terrain-related information:

(1) 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.

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

pilot may estimate the distance to the terrain of interest.
5/22/00 AC 25-23

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(3) 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.

(4) Variations in terrain elevation depicted relative to the airplane’s elevation

(above and below) should be visually distinct. Terrain that is more than 2000 feet below

the airplane’s elevation need not be depicted.

(5) Terrain that generates alerts should be displayed in a manner to distinguish it

from non-hazardous terrain, consistent with the caution and warning alert level.

(6) 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.

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

display must be viewable by one of the pilots.

(8) An inhibited failed and/or inoperative TAWS should be indicated to the

flightcrew in a manner consistent with the flight deck design philosophy.

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

the terrain alerting function of the TAWS.

(10) Terrain mapping should allow the flightcrew to determine relative terrain

elevation.

(11) The terrain display should be designed so that the flightcrew can readily

determine if terrain is a threat to the airplane.

(12) The colors and/or textures used for threat terrain should be intuitive and

indicate the immediacy of the threat.

(13) If the terrain is presented on a shared display, the terrain mode and terrain

information should be easily distinguishable from weather and other features.

(14) 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.

(15) If additional terrain views are provided, they must not present information

that is inconsistent or incompatible with the features described above. Each view should

be consistent with the other as far as color, scale, and textural information. When

transitioning between views, the range scale should remain the same for each view. Each

view will be carefully evaluated to ensure that it does not interfere with or reduce the

effectiveness of the TAWS as a whole, and that it meets the other relevant portions of this
AC 25-23 5/22/00

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Advisory Circular. Since such displays may include functionality that goes beyond the

scope of this AC, they also will be evaluated based on other relevant regulations,

guidance material, or industry standards, as appropriate.

c. Pop-Up Mode-Switching Functionality.

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

incorporate the following considerations in its design:

(a) The pop-up functionality should automatically display TAWS-related

information when a TAWS caution alert occurs.

(b) The pop-up functionality should be implemented consistently for

weather, Predictive Windshear System (PWS), and traffic alerts.

(c) The display system should be designed so that it is very evident that an

automatic pop-up has occurred.

(d) The terrain display mode should be annunciated on the display. If this is

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

(e) Manually switching back to the original mode of operation should require

minimal effort.

(f) 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.

(2) Pop-Up Inhibit Feature. For dual displays, pop-up functionality can be

inhibited if the terrain map is on at least one display when a terrain alert occurs.

However, if the terrain map is not on a display when a terrain alert occurs, the terrain

map, with the alerts, should be automatically displayed on both the left and right displays.

(3) 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:
5/22/00 AC 25-23

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TABLE 1.

Prioritization of Pop-Up Displays Where TAWS and

Predictive Windshear System (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

(4) 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.

d. 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 threat on the display.

Switching back to a manually-selected range should require minimal effort.

14. ALERTS.

a. In addition to being complaint with the requirements of § 25.1322, the TAWS

alerts should be clear, concise, and unambiguous.

b. The Human Factors Plan specified in the PSCP (see paragraph 8.f.) should be used

to establish that the visual and aural alerts are consistent with the alerting philosophy of

the airplane flight deck in which the TAWS equipment is installed. This is particularly

important with retrofit installations, which may include the re-use of previously-installed

alerting annunciations. The plan should consider that the visual alerts are:

• located in the pilots primary field of view, as defined by the human factors

plan

• consistent with their associated voice, or aural call out

• consistent with the colors specified in § 25.1322, both in the discrete and

textural display formats.

15. ALERT PRIORITIZATION.
AC 25-23 5/22/00

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a. 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 alert prioritization scheme such that:

(1) Only one alert is given at any one time, and

(2) 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 alerts for multiple alerting systems. If such is the

case, a prioritization scheme is not required. However, if

simultaneous alerts can be given, then the aural words must be

understandable and the associated visual alerts must not be confusing

to the flightcrew.

b. Implementing this prioritization scheme within the TAWS equipment is

acceptable. Table 2, below, displays an example of the recommended alert prioritization:
5/22/00 AC 25-23

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TABLE 2.

Recommended Alert Prioritization between

the TAWS and Other Systems Installed

Priority Description

Highest 1 Reactive Windshear Warning

Class A/B 2 Sink Rate Pull-Up Warning (Excessive Rates of Descent)

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

Class A/B 4 Terrain Awareness Pull-Up Warning (FLTA)

5 Predictive Windshear Warning

6 Minimums (Voice Callouts)

Class A/B 7 Terrain Awareness Caution (FLTA)

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

Configuration)

Class A/B 9 PDA (“Too Low Terrain”) Caution

Class A/B 10 Altitude Callouts (Voice callouts)

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

Configuration)

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

Configuration)

Class A/B 13 Sink Rate (Excessive Rates of Descent)

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

Class A 15 Glideslope (Excessive Downward Deviation From an ILS Glideslope)

16 PWS Caution

17 Approaching Minimums (Voice Callouts)

18 Bank Angle (Voice Callouts)

19 Reactive Windshear Caution

20 TCAS RA (“Climb,” “Descend,” etc.)

 Lowest 21 TCAS TA (“Traffic, Traffic”)

16. SYSTEM INHIBIT.

a. A means for one of the flightcrew to inhibit the following TAWS functions must

be provided:

• FLTA

• PDA
AC 25-23 5/22/00

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• Basic GPWS - Flight into terrain when not in the landing configuration

• Basic GPWS – Excessive downward deviation from the ILS glideslope

b. Care should be taken to ensure that instinctive, inadvertent, or habitual reflexive

action by the flightcrew does not inhibit the these functions. (Provisions for a guarded

switch could be a possible means of compliance.) Appropriate annunciation of the

inhibited functions must be provided to the flightcrew. Flightcrew procedures for

disabling various TAWS functions should be identified in the Airplane Flight Manual

(AFM). For Class A equipment, inhibiting the FLTA and PDA functionality should not

affect the Basic GPWS functions.

17. FLIGHT DATA RECORDER. For those applications that require crash-survivable

flight data recording in accordance with the requirements of § 25.1459(e) (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 a

FLTA and/or PDA-inhibit.

18. SYSTEMS EVALUATIONS WITH SIMULATORS. Simulators may be used as

a tool to evaluate specific installations of TAWS. The level of simulation 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 via simulation 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.

19. GROUND TEST CONSIDERATIONS.

a. A ground test should be conducted for each TAWS installation. The level of

testing required will be determined by the scope of the installation (First of a Model vs.

Follow-on). Some items to consider for ground test should include:

• an acceptable location of TAWS controls, displays, and annunciators;
5/22/00 AC 25-23

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• exercise of self-test functions;

• evaluation of identified failure modes;

• evaluation of all discretes and TAWS interfaces;

• electro-magnetic interference (EMI)/electro-magnetic compatibility (EMC)

testing; and

• electrical transient testing.

b. Considerations can also be made for evaluating display characteristics if it can be

shown that all of the performance aspects of the display that are available during flight

can be evaluated on the ground.

20. FLIGHT TEST CONSIDERATIONS.

a. The level of flight test required to validate a particular TAWS installation will be

based on the type of airplane, airplane system architecture, and credit given for previously

certified installations, simulation and ground testing. The actual requirement for a flight

test needs to be evaluated for each installation. First time/model installations and new

sensor inputs will require flight test. Follow on installations that introduce changes in

flight deck configurations may require flight testing if they can not be evaluated as

specified in paragraph 18. of this AC. The evaluation of new sensor models may require

flight test, unless it can be shown through a sensitivity analysis that the new sensor’s

dynamic characteristics during flight are compatible with the current sensor parameters,

and will not effect the performance of the TAWS.

The following examples are intended to assist in determining the flight test guidelines for

some potential or likely TAWS configurations. Table 3, which follows, provides a

summary of these examples.

(1) Example 1. This is the first time the vendor’s equipment has been installed

in any airplane for the purpose of receiving a first time TC/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.

(2) 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 vendor’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.

(3) 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
AC 25-23 5/22/00

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focus of the flight test evaluation should be on display related issues and tests specified in

paragraph 20.e. (Terrain Display Flight Test Considerations) of this AC.

(4) 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 vendor’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. In addition, since the

navigation system provides track and ground speed information to TAWS, which affect

the alerting logic, an FLTA functional flight test evaluation test is warranted. Some basic

GPWS installations use the horizontal position information to desensitize some of the

TSO C92c modes. Verification of the function and proper interface installation may

require additional flight testing for the basic GPWS.

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

TAWS in which the radio altitude 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 radio altitude such as one of the Basic GPWS modes or the PDA

function. Only one test is required to assure that the radio altimeter input is properly

installed to the TAWS. This example does not apply to Class B TAWS.

(6) Example 6. This involves an initial installation of a vendor’s TAWS in an

airplane that was previously approved with Basic GPWS equipment (per TSO- C-92c)

from the same vendor, Same basic GPWS algorithms, and the same sensors that are used

for the GPWS installation are used for the TAWS installation. If such is the case, then

Basic GPWS testing is not required to be accomplished for the TAWS installation.

However, there are exceptions; the following situation is an example:

Exception: A basic GPWS has a TSO C92a that is installed in an

aircraft. Subsequently, the TAWS (TSO C151a) is to be installed.

The sensors used for the basic GPWS (TSO C92c) used for the

TAWS are the same, except that some of the algorithms were changed

and some features were added. Ultimately, an evaluation must be

made to check the differences between the basic GPWS algorithms

that were certified and the basic GPWS algorithms used for the

TAWS to determine if any flight testing is necessary.
5/22/00 AC 25-23

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TABLE 3.

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

Terrain Display X X X X

Horizontal Position

Source

X XX

* Class A equipment

b. FLTA Flight Test Considerations.

(1) 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 to

be within approximately 300 feet.

(2) Test runs are recommended to be level flight at approximately 500 feet above

the terrain/obstacle of interest. The test runs should verify that:

(a) all alerts (caution and warnings) are given at an appropriate point in the

test run,

(b) all pop-up, auto-range, or other display features are working correctly,

and

(c) 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 the terrain/obstacle in order to generate a TAWS

alert.

c. PDA Flight Test Considerations.

(1) 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.
AC 25-23 5/22/00

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(2) 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.

(3) 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

terrain/obstacles to preclude activation of the FLTA function. If not

feasible, then increase the barometric altitude by 1 in. 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 also will verify

the adequacy of the airport database, the navigation source input, and

the barometric and/or radio altitude inputs to TAWS.

d. Basic GPWS Flight Test Considerations. 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 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.

(1) 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 DO-161 envelopes. For Class A equipment, exercising this test verifies

the proper installation of barometric altitude (and the corresponding computation of

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

verifies the proper installation of barometric altitude, the height above terrain as

determined from the GPS position and the corresponding terrain elevation from the

terrain database. Only one test run is required to determine proper installation.

(2) 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 installation of the radio altimeter.

(3) 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 installation of barometric

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

test verifies the proper installation of barometric altitude and height above terrain as
5/22/00 AC 25-23

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determined from the GPS position and the corresponding terrain elevation from the terrain

database.

(4) Flight into Terrain when Not in Landing Configuration. This test should be

conducted while on a visual approach to a runway. For Class A equipment only,

exercising this test verifies the proper installation of barometric altitude, barometric

altitude rate and radio altitude as well as the gear and flap sensor inputs to TAWS.

(5) Excessive Downward Deviation from an ILS Glideslope. This test should be

conducted during an ILS approach. For Class A equipment only, this test will verify the

proper installation of the ILS Glideslope input to TAWS.

(6) Voice Callout “Five Hundred Feet.” This test is conducted during an

approach to a runway. For Class A and B equipment, this test will verify the proper

installation of barometric altitude, radio altitude, and height above terrain as determined by

either radio altitude or by access to the terrain database.

e. Terrain Display Flight Test Considerations.

(1) Flight testing to verify the proper operation of the Terrain Display should be

conducted while verifying all the other required TAWS functions.

(2) Emphasis should be placed on verifying compliance with the provisions

specified in paragraph 13.a. (Terrain Display) of this AC during normal airplane

maneuvering during all phases of flight. Pop-up and auto-ranging features should be

evaluated, if applicable.

(3) The FAA recommends that the applicant 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.

f. Added Features Flight Test Considerations. Flight testing may be required to

verify the proper operation of added features such as:
AC 25-23 5/22/00

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 windshear detection,

 bank angle,

 altitude call outs “Approaching Minimums,” or

 other features not required by TSO-C151a.

g. Pressure Altitude Variations in Cold Weather. 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, unless a suitable verification procedure can be

conducted. This will depend on the design of the cold weather compensation.

21. AIRPLANE FLIGHT MANUAL (AFM)/AIRPLANE FLIGHT MANUAL

SUPPLEMENT (AFMS). 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 must be included in the AFM/AFMS. As a

minimum, the applicant should provide instructions in the Limitations Section of the

AFM/AFMS that include the following:

a. Limitations. The following instructions should be included in the Limitations

section of all AFM/AFMS:

(1) 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 the accuracy and/or fidelity

on which to solely base decisions and plan maneuvers to avoid

terrain or obstacles.

(2) To avoid giving unwanted alerts, the TAWS must be inhibited when landing

at an airport that is not included in the airport database.

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

prohibited during QFE (atmospheric pressure at airport elevation) operations.

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

sources of altitude measurement to determine the airplanes vertical

position.

b. Restricted Areas of Operation.

(1) Areas of operation or other factors that adversely affect navigation

performance to the extent that the TAWS will be potentially unreliable or misleading

should be identified in the AFM/AFMS. Areas of operation where the terrain database

resolution or accuracy leads to nuisance alerts also should be identified. This situation

may occur if:
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(a) the data are not properly analyzed;

(b) there are not enough data to map the terrain adequately;

(c) the resolution of the data is not adequate for the type of approved position

source; or

(d) there are changes in the area (volcanoes, earthquakes, construction) that

have not been noted.

(2) A route structural analysis (RSA) may be needed to identify those airports or

runways that are susceptible to nuisance alerts. This analysis can be accomplished by

computer simulation. Engine-out procedures also should be considered when developing

the RSA.

c. Operational Considerations for Normal/Abnormal Procedures. In addition to

the GPWS operational procedures, consider the following:

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

verify the airplane flight path and correct it, if required. If in doubt, perform a climb until

the CAUTION alert ceases.

(2) 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 operating in visual meteorological conditions

(VMC), and/or the pilot determines, based on all available information, that turning in

addition to the vertical escape maneuver is the safest course of action.

(3) Cold Weather. For operations in cold weather, either the system should be

able to account for variations in cold weather at temperatures at or below 0° Celsius, or

additional flightcrew procedures should be considered to address pressure altitude limits

for vertical position determination. Limitations may be needed to address insufficient

determination of the airplane’s vertical position in cold weather temperatures.

(4) 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, as

required by § 121.360.
AC 25-23 5/22/00

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Donald L. Riggin

Acting Manager, Transport Airplane Directorate

Aircraft Certification Service