m. Systems Evaluations. Aircraft simulators, mock-ups, system integration tools, etc., independently or in conjunction with computer generated test scenarios developed by the TAWS manufacturer, may be used to evaluate specific installations of TAWS equipment. If these tools are used to evaluate a particular TAWS installation, they should adequately represent the target aircraft configuration. The level of fidelity required will depend on the type of credit being sought. Some of the characteristics of a TAWS installation and flight deck integration that may be evaluated are displays, alert prioritization, mode transitions, pop-up displays, auto-ranging, self test, operational workload issues, accessibility and usability of the TAWS controls, and systems failure modes.
n. Ground Test Requirements. A ground test should be conducted for each TAWS installation. The level of testing required will be determined by the scope of the installation (whether an initial installation of the model versus a follow-on installation). In addition to system evaluations noted above, ground test should evaluate the location of TAWS controls, displays, and visual alerts, aural alerts, self-test functions, identified failure modes, all discrete inputs and sensors interfaced with the TAWS equipment, electro-magnetic interference (EMI)/electro-magnetic compatibility (EMC) testing, and electrical transient effects. The visibility and characteristics of any provided display should be evaluated under all anticipated cockpit lighting conditions. Ground tests conducted using computer generated test equipment accepted by the TAWS manufacturer may be used in place of flight-testing for basic Class B TAWS equipment. Similar testing may be used to minimize the extent of flight-testing required for Class A TAWS equipment.
o. Flight-Test Requirements. The level of flight-test required to validate a particular TAWS installation will be based on the type of airplane, airplane system architecture (digital, hybrid digital-analog discrete inputs, or analog discrete inputs), and credit given for previously certified installations, system evaluation, and ground testing. Airplanes incorporating digital or hybrid digitalanalog discrete data interface architectures may require flight-testing to ensure proper data transfer among various systems. The actual requirement for a flight-test needs to be evaluated for each installation. If ground testing is not adequate to verify proper function of all interfaced equipment, additional flight-testing for TAWS functions may be necessary.
(1) Flight-Test Matrix. Table 4 shows TAWS functions that should have a flight-test evaluation for the following examples. These examples are intended to assist in determining the extent of flight-testing for some potential or likely TAWS configurations:
TABLE 4. Flight-Test Matrix TAWS Functions Example Example Example Example Example Example FLTA PDA X or Basic GPWS Terrain Display TAWS Horizontal Position Source
(a) Example 1. This is the first time the manufacturer's equipment has been installed in any airplane for the purpose of receiving initial TC or STC approval. If such is the case, then a complete and thorough ground and flight-test program should be conducted to verify the adequacy of the installation.
(b) Example 2. This involves a follow-on installation of a previously approved TAWS in which a required sensor input has not been previously approved for the specific manufacturer's equipment. For example, if the sensor that provides barometric altitude (or equivalent) to the TAWS equipment has not been previously approved, the flight-test evaluation should focus on the TAWS functions affected by barometric altitude such as FLTA and PDA.
(c) Example 3. This involves a follow-on installation of a previously approved TAWS in which the terrain display has not been previously approved. In this case, the focus of the flight-test evaluation should be on display related issues and tests specified in paragraph
7.0. (6), below, of this AC.
(d) Example 4. This involves a follow-on installation of a previously approved TAWS in which the horizontal position source sensor input has not been previously approved for the specific manufacturer's equipment. In this case, the focus of the flight-test evaluation should be on the adequacy of the horizontal position source and the display of the terrain as determined by that horizontal position source. Verification of the function and proper interface installation may require additional flight-testing for the FLTA, PDA, and basic GP WS.
(e) Example 5. This involves a follow-on installation of a previously approved TAWS in which the vertical position source to the TAWS equipment has not been previously approved. In this case, the focus of the flight-test evaluation should be on the TAWS functions affected by the vertical position source such as one of the basic GPWS modes, PLTA, or PDA functions. Only one test is required to assure that the vertical position source input is properly installed to the TAWS.
(f) Example 6. This involves a follow-on installation of a particular manufacturer's TAWS in an airplane that was previously approved with the same manufacturer's basic GPWS equipment (per TSO-C92c). The TAWS installation uses the same interfaces used for the previous GPWS installation plus those additional interfaces associated with the TAWS installation. If such is the case, then basic GPWS testing may not be necessary. Flight-tests can be limited to the interfaces, features and functions (including basic GPWS) added with the TAWS installation. For example, interfaces that enable additional GPWS functions or features, such as position based alerting desensitization, altitude callouts, bank angle limits, etc., would be evaluated along with TAWS functions.
(2) FLTA Flight-Test Requirements. Flight-testing to verify the proper operation of the FLTA function can be conducted in an area where the terrain or obstacle elevation for the test runs is known within approximately 300 feet. This test may have a potential hazard. Test runs are recommended to be level flight at approximately 500 feet above the terrain/obstacle of interest. The test runs should verify that all alerts (caution and warnings) are given at an appropriate point in the test run, that all pop-up, auto-range, or other display features are working correctly, and that the display depicts the terrain accurately.
NOTE: The terrain selected should be at least 15 NM from the nearest airport to conduct the test as described. If this is not practical, the fly-over altitude will have to be lowered to 300 feet or less above either the terrain or obstacle, or both, in order to generate a TAWS alert.
(3) PDA Flight-Test Requirements. Flight-testing to verify the proper operation of the PDA function can be conducted in any airport area within 10 NM of the nearest runway. The airplane should be configured for landing at approximately 1500 feet AGL along the final approach segment of the runway at approximately 10 NM from the runway. At the 10 NM point, a normal three degree flight path angle descent can be initiated and maintained until the PDA alert occurs.
NOTE: The runway selected for this test should be relatively free from either terrain or obstacles to preclude activation of the FLTA function. If not feasible, then increase the barometric altitude by 1 inch of mercury (hg) to allow radio altitude inputs to trigger a PDA alert. Approximately level terrain along the final approach segment will exercise the PDA function. This test may also exercise the 500 foot voice callout. The adequacy of the PDA aural alert should be verified during this test. This test will also verify the adequacy of the airport data base, the navigation source input and either the barometric or radio altitude inputs to TAWS, or both.
(4) Basic GPWS Flight-Test Requirements. Flight-testing to verify the proper operation of basic GPWS functions can be conducted in any area where the terrain elevation is known to the flightcrew. The following information is intended to provide guidance for conducting flight-tests to exercise and verify the proper operation of each GPWS function. The 27
need to conduct flight-testing for follow-on TAWS installations will depend upon the nature of the new or modified sensors and their impact on that particular GPWS function.
(a) Excessive Rates of Descent. This test can be conducted at any location, but descents toward near level terrain are recommended for best results and ease of correlation with RICA DO-161 envelopes. For Class A equipment, exercising this test verifies the proper inputs to the TAWS of barometric altitude (and the corresponding computation of barometric altitude rate) and radio altitude. For Class B equipment, exercising this test verifies the proper inputs to the TAWS of barometric altitude, the height above terrain as determined from the GPS position and the corresponding terrain elevation from the terrain data base. Only one test run is required to determine proper installation.
(b) Excessive Closure Rate To Terrain. This test must be conducted in an area of known rising terrain. It is recommended that one level test run at an altitude between 500-1000 feet above the terrain elevation be conducted. For Class A equipment only, this test will verify the proper input to the TAWS of the radio altitude.
(e) Negative Climb Rate or Altitude Loss After Takeoff. This test is conducted immediately after takeoff before climbing above 700 AGL, or above runway elevation. For Class A equipment, exercising this test verifies the proper inputs to the TAWS of barometric altitude, barometric altitude rate and radio altitude. For Class B equipment, exercising this test verifies the proper input of barometric altitude and height above terrain as determined from the GPS position and the corresponding terrain elevation from the terrain data base.
(d) Flight Into Terrain When Not In Landing Configuration. This test should be conducted while on a visual approach to a runway and include evaluation of the flap override function. For Class A equipment only, exercising this test verifies the proper input to the TAWS of barometric altitude, barometric altitude rate and radio altitude as well as the gear and flap sensor inputs to TAWS.
(e) Excessive Downward Deviation From an ILS Glideslope. This test should be conducted during an ILS approach and include evaluation of the glideslope cancel function. For Class A equipment only, this test will verify the proper installation of the ILS Glideslope input to TAWS.
(f) Voice Callout "Five Hundred." This test is conducted during an approach to a runway. For Class A and B equipment, this test will verify the proper input to the TAWS of barometric altitude, radio altitude, and height above terrain as determined by either radio altitude or by access to the terrain data base.
(5) Terrain Display Flight-Test Requirements. Flight-testing to verify the proper operation of the terrain display should be conducted while verifying all the other required TAWS functions. Emphasis should be placed on verifying compliance with the requirements specified in paragraph 7.h. (2) of this AC during normal airplane maneuvering during all phases of flight.
Pop-up and auto-ranging features should be evaluated if applicable. It is recommended to 28
perform sustained turns to evaluate symbol stability, flicker, jitter, display update rate, color cohesiveness, readability, the use of color to depict relative elevation data, caution and warning alert area depictions, map masking and overall suitability of the display.
(6) TAWS Horizontal Position Source Flight-Test Requirements. For TAWS equipment interfaced with an approved GPS (WAAS) navigation system, no additional horizontal position source testing is necessary. (For TAWS equipment incorporating an internal GPS (WAAS) position source, a suitable ground or flight-test to validate positional accuracy is necessary.) A low level overflight of a known ground reference point, reference AC 20-130A or AC 20-138, is acceptable. In addition, a 360 degree turn at 30 degrees of bank is necessary to show continuity of position data. Note: Additional test equipment will likely be required to observe the GPS data.
(1) Pressure Altitude Variations in Cold Weather Flight-Test Requirements. The TAWS may be designed to account for the effects of cold weather on barometric altitude while determining vertical position. Flight-testing may be required for the initial installation approval only, unless a suitable ground verification procedure is conducted. Testing requirements will depend on the design of the cold weather compensation.
(8) Added Features Flight-Test Requirements. Flight-testing may be required to verify the proper operation of added features such as Windshear Detection, Bank Angle, Altitude Call Outs "Approaching Minimums," or other features not required by TSO-C151a.
p. AFM or AFMS. An AFM or AFMS containing the limitations, restricted areas of operation, and operational considerations for either Normal or Abnormal Procedures should be provided for each installation. The applicant should make an evaluation to determine if there are any limitations of the system and, if so, how they will affect aircraft operations. Any limitations affecting operations shall be included in the AFM or AFMS. As a minimum, the applicant should provide instructions in the Limitations Section of the AFM or AFMS that include the following:
(1) Limitations. The following instructions should be included in the Limitations Section of all AFM or AFMS:
(a) Navigation must not be predicated upon the use of the TAWS.
NOTE: The terrain display is intended to serve as a situational awareness tool only. It may not provide either the accuracy or fidelity, or both, on which to solely base decisions and plan maneuvers to avoid terrain or obstacles.
(b) To avoid giving unwanted alerts, the TAWS must be inhibited when landing at an airport that is not included in the airport database.
(c) The use of the terrain awareness warning and terrain display functions is prohibited during QFE (atmospheric pressure at airport elevation) operations.
NOTE: This limitation may not apply to systems that use other sources of altitude measurement to determine the airplanes vertical position.
(d) The ABC Model XXX TAWS Pilot's Guide, P/N <insert part number>, dated <insert date> (or later appropriate revision) should be immediately available to the flightcrew.
The software status stated in the pilot's guide should match that displayed on the equipment.
(e) Pilots are authorized to deviate from their current ATC clearance to the extent necessary to comply with TAWS warnings.
(1) Terrain database. The area of intended operation should be identified in the AFM or AFMS (that is, North America, Europe, etc.)
(2) Restricted Areas of Operation. Known areas or other factors within the approved areas of operation that adversely affect navigation performance to the extent that the TAWS will be potentially unreliable or misleading, should be identified in the AFM or AFMS. Areas of operation where the terrain data base resolution or accuracy leads to nuisance alerts should also be identified.
This situation may occur if:
• The data are not properly analyzed,
• There is not enough data to map the terrain adequately,
• The resolution of the data is not adequate for the type of approved position source, or
• There are changes in the area (volcanoes, earthquakes, construction) that have not been noted.
NOTE: For Class A TAWS equipment and where operational procedures require a route structural analysis (RSA), airports or runways that are susceptible to nuisance alerts should be identified. This analysis can be accomplished by computer simulation. Engine-out procedures also should be considered when developing the RSA.
(3) Operational Considerations for either Normal or Abnormal Procedures, or both. In addition to the GPWS operational procedures, consider the following:
(a) Terrain Awareness Caution. When a terrain awareness CAUTION occurs take positive corrective action until the alert ceases. Stop descending or initiate either a climb or a turn, or both, as necessary, based on analysis of all available instruments and information.
(b) Terrain Awareness Warning. If a terrain awareness WARNING occurs, immediately initiate and continue a climb that will provide maximum terrain clearance, or any similar approved vertical terrain escape maneuver, until all alerts cease. Only vertical maneuvers are recommended, unless either operating in visual meteorological conditions (VMC), or the 30
pilot determines, based on all available information, that turning in addition to the vertical escape maneuver is the safest course of action, or both.
(c) NOTE: Cold Weather Operations. For those systems that do not provide extreme cold temperature compensation of barometric altitude (corrected altitude), the AFM, AFMS, or Supplemental AFM should include the following note: Operations at extreme cold temperatures, for example, -30 degrees Celsius, will result in a significant reduction in terrain clearance provided by TAWS alerts." To reduce or eliminate errors potentially induced in barometric altitude temperature extremes, the design should include features such as corrected altitude or special operational procedures. For operations in cold weather, either the system should be able to account for variations in extreme cold weather temperatures, for example, -30 degrees Celsius, or additional flightcrew procedures should be considered to address pressure altitude limits for vertical position determination.
(d) GPWS Mode 4. A procedure must be established for the operation of the inhibition of Mode 4 warnings based on flaps being in other than the landing configuration. The GPWS Mode 4 function may be deactivated to prevent nuisance warnings when following approved procedures that specify landing with flaps not in normal landing position or landing gear up.
(e) GPWS Mode 5. The GPWS should provide a capability to inhibit or cancel the below-glideslope alert for an approach.
S/ Michael K. Dahl Acting Manager, Small Airplane Directorate Aircraft Certification Service 31