• 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 Page 23
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.
TABLE 3.
Flight Test Matrix TAWS Example Example Example Example Example Example FUNCTIONS 1 2 3 4 5 6
F L T A 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 X X * 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.
(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 Page 26
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:
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:
(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.
Donald L. Riggin Acting Manager, Transport Airplane Directorate Aircraft Certification Service Page 30