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
CONTENTS
PARAGRAPH PAGE
1. PURPOSE1
2. RELATED REGULATIONS AND DOCUMENTS1
3. DEFINITIONS4
4. BACKGROUND7
5. DISCUSSION8
6. SYSTEM DESCRIPTION9
7. AIRWORTHINESS CONSIDERATIONS FOR CERTIFICATION PROGRAMS10
8. PROJECT SPECIFIC CERTIFICATION PLAN (PSCP)11
9. SYSTEM SAFETY ASSESSMENT12
10. SOFTWARE13
11. POSITION SOURCE13
12. TERRAIN DATABASE15
13. DISPLAY PRESENTATION15
14. ALERTS19
15. ALERT PRIORITIZATION20
16. SYSTEM INHIBIT21
17. FLIGHT DATA RECORDER22
18. SYSTEMS EVALUATIONS WITH SIMULATORS22
19. GROUND TEST CONSIDERATIONS22
20. FLIGHT TEST CONSIDERATIONS23
21. AIRPLANE FLIGHT MANUAL (AFM)/AIRPLANE FLIGHT MANUAL
SUPPLEMENT (AFMS)28
Page i (and ii)
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 Page 1
§ 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 Page 2
• 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 DO- 160D
• 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 Page 3
• 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.
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, Page 5
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.
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.
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 Page 8
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.
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.