(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.
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 14
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.
(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 flighterew 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 (105) for unannunciated failure, HMI, and false alerts at the TAWS computer level. The box, as installed, should meet the following criteria:
(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 103 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 104 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+ 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 to104 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.
1. 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 TSO- C151a (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.
(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.
(ill) 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 18
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 103 per average flight hour, and the possibility of misleading information on the display due to undetected or latent failures should be no greater than 104 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.
(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.
(1) 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.
() 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.
(I) 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 20
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.
(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 22
(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) 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) 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") 23
TABLE 3. Recommended voice prioritization between the Class B TAWS and other systems installed Priority Description for Class B TAWS Highest 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 Altitude Callouts (Voice callouts) 9 Sink Rate (Excessive Rates of Descent) 10 Don't Sink (Negative Climb Rate or Altitude Loss After Take-off) 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.
I. 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.