235 1 Alerting to an impending capture of a pre-selected altitude; and
2 Alerting to an uncommanded departure from an assigned (selected) altitude.
(c) Low and high rates of vertical speed should be included in the evaluation with captures from above and below the pre-selected altitude.
(d) Determine adequacy of the alert light location and its visibility under various lighting conditions, and adequacy of the aural warning.
(e) A flight simulator may be used to perform appropriate tests.
(11) Ground Proximity Warning System (GPWS) .
(a) Initial approvals of new GPWS should include flight evaluation with an instrumented airplane. The instrumented flight test of a computer type may be demonstrated on one airplane and the results made applicable to any other airplane with only a functional test being required.
(b) Initial approval of new individual TSO approved (TSO’d) sensors, which have an equivalent function and interface as previously approved sensors, and are interfaced with a previously approved computer, may be approved by ground test. Applicable sensors (systems) are the air data computer (ADC), the radio altimeter (R/A), and the airplane’s ILS equipment.
(c) Subsequent installations of identical computers and interfacing sensors in other airplane types will require a ground test and an uninstrumented functional flight test to determine proper functioning, freedom from interference, adequate sound and light levels, inhibit switch details, etc.
(d) The functional flight test should include construction of a profile to selectively actuate each mode that the applicant elects to certify (not all modes are required by the FAA).
Refer to the equipment supplier’s system description to aid in constructing the flight profile.
(e) Evaluate the GPWS warning light and other annunciators for visibility in day and night lighting conditions.
(f) Evaluate the sound level of the aural GPWS warning at V MO with headsets on and with and without simultaneous radio communications.
(g) Verify that the self-test function and the flap override switch perform their intended functions. The gear override switch should be verified as well.
(h) Simulate failures of the ADC, R/A’s and ILS systems (open circuit breakers) and verify that the GPWS detects the faults and illuminates the GPWS fail annunciator.
(i) Verify that conducting a radio altimeter self-test in flight will not induce a false GPWS failure warning.
(j) Verify that the GPWS does not adversely interfere or interact with other airplane systems.
(k) Verify that the GPWS is free of inadvertent activation during electrical bus or generator switching.
(l) Verify that the caution & warning priorities are assessed and that they include systems such as windshear. Example, windshear has priority over the traffic collision avoidance system (TCAS).
(12) Enhanced Ground Proximity Warning System (EGPWS). (RESERVED.)
(13) Master Caution/Master Warning System (MCS/MWS) .
(a) This system provides inputs to master warning (MW) and master caution (MC) lights located on the glareshield in front of each pilot and to the CAWS. (See paragraph 170e(8)). It contains the sensors, switches and logic required to assess the requirement for the warning and caution annunciations.
(b) The system may contain logic to assess priority of the various warnings in order to sequence simultaneous actuation. If so, ground, flight, or bench tests may have to be conducted to verify this logic.
(c) Some cautions may be inhibited during various phases of flight, particularly during the final stages of a Category III automatic landing operation (below alert height). If so, ground or flight tests may have to be structured to verify the inhibit logic. Also, certain warnings may be inhibited during the high speed stages of a takeoff roll. If this feature is incorporated, it too should be carefully substantiated.
(d) Simulate failure of selected sensor inputs to verify that the MCS/MWS computer detects the faults and illuminates the fail annunciator (if installed).
(e) Verify that there is no unacceptable mutual interference between the MCS/MWS and other systems and equipment.
(14) Flight Warning Computer (FWC) .
(a) The FWC is a key subset of the overall warning, caution, and alerting system on the more modern airplanes. It is a microprocessor-based system that works in conjunction with, or totally replaces, the CAWS and MC/MW systems.
(b) The FWC incorporates sufficient computer capacity to perform many more functions than the MCS/MWS and, in addition to outputting to the visual warning displays and
237 aural warning speakers, outputs hundreds of alerts/messages to other display media, most often an electronic display (multi-function CRT on the instrument panel or the CDU portion of the FMS).
(c) The added computer capacity also enables incorporating rather complex inhibit algorithms which, in turn, dictates a requirement for structuring a very complex ground test to verify operation of the inhibits. Review of the system description document will probably be required before any ground, flight, or bench tests can be defined.
(d) The flight evaluation should incorporate the tests described under CAWS and MC/MW systems.
(e) Verify that the self-test feature functions properly.
(f) Simulate failure of selected sensor inputs to verify that the FWC detects the faults and illuminates the FWC fail annunciator.
(g) Demonstrate freedom from nuisance warnings by observing and logging nuisance events as they occur throughout the flight demonstration program. Assess the status at the end of the program by examining the log.
(15) Stall Warning Computer (SWC) .
(a) The SWC should be evaluated in conjunction with the stall tests described in Chapter 2, Section 6 of this AC. Adequacy of the various warnings (stick shaker, warning light, aural (and voice) warning) should be assessed, in addition to the speed schedules.
(b) Demonstrate freedom from nuisance warnings during the takeoff, landing, and maneuvering flight tests described in Chapter 2 of this AC. Further testing may be required to show freedom from nuisance warnings by the test requirements in Chapter 8 of this AC if special airworthiness approvals are sought.
(16) Takeoff Warning System (TOW) .
(a) The TOW system is usually a subset of the CAWS or MCS/MWS or FWC and is generally inhibited at nose wheel liftoff during the takeoff maneuver. It is usually armed when the squat switch indicates that the airplane is on the ground and one or more power levers are advanced beyond a prescribed position or engine thrust setting. The system alarms if the flaps/slats/stabilizer/spoiler/brakes are not set correctly for takeoff.
(b) Determine that each event is annunciated properly by conducting a static ground test.
(c) Flight evaluation during takeoff roll should be a qualitative assessment of the clarity, tone, and volume of each warning. Some systems incorporate a TEST switch, which facilitates activation during takeoff. If this feature is not available, it may suffice to check the
238 audio level of a warning during touch and go landings (e.g., the stabilizer warning while accelerating to VR speed).
(d) Simulate failure of selected sensor inputs to verify that the TOW system detects the faults and illuminates the fail annunciator (if installed).
(e) Verify that the system’s arming status is not affected by electrical power transients, which occur when switching from ground or APU power, during engine start, following bus priority checks, etc.
(f) Verify that there is no unaccepta ble mutual interference between the TOW system and other airplane systems and equipment.
(17) Instrument Comparator System .
(a) If an instrument comparator system is installed and activated, ensure the system monitors validity of and compares (side 1 vs. side 2) pitch and roll attitude, heading, radio altitude, localizer, and glideslope deviation information used for display in the cockpit instruments.
(b) When these parameters exceed a preset threshold or trip level, or if an invalid condition is detected in any of the displayed information, an appropriate annunciator to that parameter will be illuminated.
(c) A ground test on the airplane should be conducted to demonstrate performance of intended functions and validate the detector threshold levels. If annunciation is suppressed under certain conditions, these conditions should also be checked.
(d) A flight evaluation should also be conducted to determine adequacy of the annunciator light locations and visibility under various lighting conditions and that the annunciators are properly identified.
(e) During flight, verify that the system is free from nuisance warnings and is compatible with other electronic systems.
(18) Reactive Windshear Warning System . A complete and comprehensive demonstration program (simulator and flight test) is contained in AC 25-12, “Airworthiness Criteria for the Approval of Airborne Windshear Warning Systems In Transport Category Airplanes,” dated November 2, 1987.
(19) Traffic Alert and Collision Avoidance System . A complete and comprehensive demonstration program (laboratory/ground/flight test) is contained in AC 20-131A, “Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders,” dated March 29, 1993.
f. Procedures-Recording Systems .
(1) Cockpit Voice Recorder (CVR) .
(a) Demonstrate operation of the CVR self-test function and bulk erase feature.
(b) During ground operations prior to and after engine start; during takeoff, climbout, cruise at VMO / MMO, and during landing approach. Except where specifically noted, obtain the following CVR recordings:
1 Flightcrew conversations using the area microphone.
2 Flightcrew conversations using the oxygen mask microphones and the boom-microphone/handset, and the hand-held microphones in flight and during ground operations.
3 Radio transmissions by each crew member during cruise.
4 Audio signals identifying navigation aids (through cockpit speakers and on radio channels) during landing approach.
5 PA announcement during cruise.
6 Selective (based on analysis) aural warning signals (see CAWS) during appropriate flight phases.
(c) Use CVR circuit breaker to limit CVR recording to available capacity.
(d) Verify that all controls, displays, and annunciators consistent with normal and abnormal crew procedures, are satisfactorily identified, accessible, operable, and visible.
(e) EMC. Verify by observation that no adverse effects are present in the required flight systems.
(f) Following the flight, test the CVR audio recording in an appropriate laboratory, and evaluate for clarity of the recorded messages. Preferably the evaluators should not have advance information concerning the flightcrew conversations.
(g) Verify that the hot mike feature at each position allows for recording regardless of the position of the interphone transmit key switch (§ 25.1457(c)(5)).
(2) Digital Flight Data Recorder (DFDR) .
(a) Demonstrate operation of the DFDR se lf-test feature and, prior to collection of test data, set the trip number and date on the code switches of the Trip and Data Encoder, if so equipped.
(b) The certification data will be derived automatically during any appropriate flight that encompasses operation throughout the full flight envelope. Hand recorded data of heading (magnetic or true), barometric altitude, pitch attitude, indicated airspeed, and time (UTC) should be collected at several stabilized flight conditions for subsequent data correlation purposes.
(c) Following the flight, the data should be retrieved for correlation and accuracy verification.
(d) Demonstrate performance of intende d function of the DFDR for all parameters specified in the operating rules. (Refer to appendix B of part 121 for the relevant data ranges, accuracies, and recording intervals.) Accuracy will be verified at stabilized points by correlating against, and comparing with, an approved instrumentation system or manual notes.
(e) Electromagnetic Compatibility (EMC). Verify by observation that no adverse effects are present in the required flight systems.
g. Engine Interfacing Systems . [Reserved]
h. Stability Augmentation Systems . [Reserved]
i. All Weather Operation (Reduced Visibility) Systems . [Reserved]
171. Flight and Navigation Instruments - § 25.1303.
a. Explanation .
(1) Section 25.1303(b)(1) requires an airspeed indicator to be visible at each pilot station. Additionally, if airspeed limitations vary with altitude, airspeed indicators must have a maximum allowable airspeed indicator showing the variation of VMO with altitude. In practice, this maximum allowable airspeed indicator requirement is rarely complied with in a literal sense.
The variation in VMO is not normally depicted literally as a function of altitude, but rather as a radial marker on the airspeed indicator whose position varies as a function of altitude. This presentation of the maximum allowable airspeed limit, as a function of altitude, is approved based on a substantiation of equivalent safety with the literal interpretation of the rule.
(2) Production tolerances for speed warning devices at V MO/MMO are required to be taken into account in accordance with § 25.1303(c).
(3) Section 21.127(b)(2) requires each production flight test to include “An operational check of each part or system operated by the crew while in flight to establish that during flight, instrument readings are within normal range.” Nowhere in these requirements is there any inference that the finite performance or quantitative limits, defined during type certification, need to be determined for each production airplane.
(4) Section 25.1303(c) requires that turbine-powered airplanes be equipped with a speed warning device that will provide aural warning whenever the speed exceeds VMO plus 6 knots or MMO + 0.01 M. The regulations specify that the upper limit of the production tolerances permitted for the warning device must be at a speed not greater than the prescribed warning speed.
b. Procedures . The applicant should substantiate, by appropriate ground and/or flight tests with possible production instrument error corrections, that the system operates within the boundaries established by the regulation. Understanding that other procedures may be acceptable, this could be accomplished in accordance with the following:
(1) If the maximum allowable airspeed varies as a function of altitude, and is indicated by means other than a maximum airspeed versus altitude presentation in the airspeed indicator, it should be substantiated that the chosen means of maximum allowable airspeed indication provides an accurate presentation of the relationship between VMO and altitude. The indicated VMO should not exceed the actual VMO for any altitude by more than six knots. This is particularly important for airplanes that have a step function in VMO at an altitude of 10,000 feet due to bird strike considerations below that altitude.
(2) During production flights, the speed warning device must provide warning at a speed equal to or less than VMO + 6 knots or MMO + 0.01 M, minus the sum of the values stipulated in (a) and (b) below:
(a) The possible error of the airspeed indicator or Mach meter used as the reference.
(b) Three knots or 0.005 M, if the airspeed indicator or Mach meter used as the reference has a static or pitot pressure source different from the pressure sources of the warning devices.
(3) The speed warning device must be shown to comply with the requirements of § 25.1303(c), as explained in paragraph (a) and (b) below:
(a) During ground test using calibrated reference instruments, the installed speed warning device must provide warning at a speed equal to or less than VMO + 6 knots or MMO + 0.01M. The procedure for this test should be processed in accordance with § 21.127(b)(5) or § 21.143(a)(3).
(b) For each type design, a test should be conducted to show satisfactory correlation between the operation of the warning device during flight and the data obtained during the ground tests.
172. Powerplant Instruments - § 25.1305. [Reserved]
173. Miscellaneous Equipment - § 25.1307. [Reserved]
174. Equipment, Systems, and Installations - § 25.1309.
a. Explanation . The following procedures outline and paraphrase the appropriate provisions of § 25.1309. Further definition and explanation, if required, may be found in part 25 and in AC 25.1309-1A, “System Design and Analysis,” dated June 21, 1988.
b. Procedures .
(1) Evaluate functioning of required installed equipment to verify that performance is as intended under any foreseeable operating and environmental conditions.
(2) Evaluate failure conditions, as appropriate, to determine their impact on the capability of the airplane or the ability of the crew to operate it.
(3) Review, as appropriate, any design an alyses, proposals, studies, or tests that correlate probabilities of failure condition occurrence with the effects of those failure conditions, to determine that they are properly categorized for the appropriate criticality level.
(4) Verify that adequate warnings are provided of unsafe conditions, and that these warnings enable the flightcrew to take appropriate corrective action with a minimum of error.
(5) For probable operating combinations of required electrical installations, verify that power is provided for probable durations:
(a) Under normal conditions;
(b) After failure of any one generator, converter, or storage device;
(c) After failure of one engine on a two-engine airplane; and
(d) After failure of two engines on airplanes with three or more engines.
(6) For probable operating combinations of required electrical installations that must be provided with an alternate source of power in accordance with § 25.1331(a), verify that power is provided for probable durations after failure of any one power system.
Section 2. Instruments: Installation
175. Arrangement and Visibility - § 25.1321. [Reserved]
176. Warning, Caution, and Advisory Lights - § 25.1322. [Reserved]
177. Airspeed Indicating System - § 25.1323.
a. Explanation .
(1) Methods . Unless a calibrated reference system is provided, the airspeed system should be calibrated throughout as wide a range as necessary to cover the intended flight tests.
The procedures of this section are for the purpose of showing compliance with § 25.1323(b) and are not intended to cover the speed range of the flight tests. If an alternate airspeed indicating system is provided, it should be calibrated. The airspeed indicating system should be calibrated in accordance with the following methods:
(a) The tests should be conducted in stabilized flight at airspeeds throughout the speed range for the airplane configurations to be tested. The airplane’s airspeed system should be calibrated against a reference airspeed system or a groundspeed course.
(b) A reference airspeed system should consist of either of the following:
1 An airspeed impact pressure and static pressure measurement device (or devices) that are free from error due to airplane angular changes relative to the direction of the free stream or due to slipstream variation resulting from changes in airplane configuration or power. In addition, the device or devices should have a known calibration error when located in the free stream; or
2 Any other acceptable airspeed calibration method (e.g., the altimeter method of airspeed calibration).
(c) When establishing the airplane’s true airspeed by means of the groundspeed course, flight between the two reference points should be made at constant airspeed in two successive runs in opposite directions to eliminate the effect of wind. The runs should be made only in stable wind. The time to make the runs should be obtained by means of some calibrated device. The speed runs should not be made nearer the ground surface than a wingspan’s length.
(d) If an alternate system is provided, it may be calibrated against either the reference system or the airplane’s system.
(e) An acceptable means of compliance when demonstrating a perceptible speed change between 1.23 VSR to stall warning speed (§ 23.1323(d)) is for the rate of change of IAS with CAS to be not less than 0.75.
(f) An acceptable means of compliance when demonstrating a perceptible speed change between VMO to VMO + 2/3 (VDF - VMO) (§ 23.1323(e)) is for the rate of change of IAS with CAS to be not less than 0.50.
(g) Airspeed Lag . With the advent of electronic instruments in the cockpit, the pneumatic signals from the pitot and static sources are processed and digitized in the Air Data Computer (ADC) and then filtered and transported to the cockpit display. As a result of the data processing and filtering, the associated time lag, and, consequently, airspeed lag at the cockpit display, can be an important consideration in the airspeed indicating system calibration during ground acceleration. As stated in § 25.1323(b), the calibration for an accelerated takeoff ground
244 run must determine the system error, which is the relation between indicated and calibrated airspeeds. The system error is the sum of the pneumatic lag in the pressure lines, airspeed lag due to time lags in processing the data, and static source, position error.
1 Airspeed lag should be measured during ground acceleration tests or determined by analysis. Increments should be developed for a range of airplane gross weights considering airspeed lag at V1 and the associated increase in accelerate-stop and takeoff distances due to lag. The error due to lag in the airspeed indicating system during ground acceleration should not be greater than 3.0 knots throughout the takeoff operating envelope of the airplane. Furthermore, an increase in the takeoff distance or the accelerate-stop distance as a result of airspeed lag should not exceed 100 ft. The 3 knots limitation is intended to establish the maximum acceptable systematic error. Even though the lag may be within the 3 knots limit, an airspeed correction may be required to stay within the 100 ft. of increased distance.
2 Corrections may be applied directly in the ADC or they may be introduced via the ground airspeed calibration provided in the AFM. If corrections are applied directly in the ADC, it is possible to display calibrated airspeed in the cockpit. Furthermore, if acceleration data are input, the airspeed error can be computed and accounted for in real time, assuming the time lag is known. The alternative would be to use an airspeed lag increment derived from calibration tests that would represent a range of conditions within the takeoff envelope. After correction, an increase in distance due to lag should be less than 100 ft throughout the takeoff envelope, whether applied in the ADC or AFM. Consideration should be given to short field, lighter weight takeoffs (higher acceleration), as well as maximum weight and higher V1 speeds, in deriving the increment.
(2) Configuration . Airspeed calibration tests should be conducted in the following configurations:
(a) Weight - between maximum takeoff and maximum landing.
(b) C. G. position - optional.
(c) Takeoff configuration(s) - ground roll.
(d) Wing flaps and landing gear - all combinations of positions used to show compliance with the takeoff, climb, and landing requirements of part 25.
(e) Thrust - as required.
b. Procedures .
(1) Any one or any desired combination of the procedures in subparagraphs (2) through (4) of this paragraph may be used for calibrating the airspeed indicating system. The airspeed should be measured or determined simultaneously from the airplane’s system and the reference system during stabilized runs for at least five speeds spaced throughout the speed range, the lowest not to be less than 1.23 VSR. The highest speed should not exceed VMO/MMO.
245 The speed spread between the test speeds should be limited to 10 knots from 1.23 VSR to 1.5 VSR or VFE, and 20 knots from 1.5 VSR to VMO.
(2) Speed course : The airspeed, power, and altitude should be stabilized before entering the speed course. Constant airspeed should be maintained during each run. The runs should be made in both directions on reciprocal headings for each speed over the speed course.
The following data should be recorded:
(a) Time of day at beginning of run.
(b) Time to make run.
(c) Pressure altitude.
(d) Ambient air temperature.
(e) Airspeed at several intervals during run.
(f) Wing flap position.
(g) Landing gear position.
(h) Course distance.
(3) Reference airspeed system : Stabilized runs at the test speeds listed in this paragraph should be made. The airspeed from the airplane’s airspeed system and the reference airspeed system should be read simultaneously. The following data should be recorded:
(a) Time of day.
(b) Airplane’s indicated airspeed.
(c) Reference indicated airspeed.
(d) Pressure altitude.
(e) Ambient air temperature.
(f) Wing flap position.
(g) Landing gear position.
(4) Other acceptable airspeed calibration methods . Stabilized flight runs at the test speeds should be made, and the necessary data recorded, to establish the airplane’s airspeed system error and the configuration of the airplane. Calibration methods may also include airspeed boom, static trailing cone, and radar range.
(5) The procedures presented in this paragraph pertain to the calibration of the airspeed indicating system during takeoff ground acceleration. In particular, airplanes with electronic instruments in the cockpit must account for the airspeed lag at the cockpit display associated with data processing and filtering as required by § 25.1323(g). The airspeed indicating system should not have a lag in excess of 3 knots at the V1 speed during any takeoff condition.
Furthermore, if airspeed lag causes an increase of more than 100 ft. in takeoff or accelerate-stop distances, a lag correction should be applied to the airspeed indicating system. Airspeed lag should be determined by one of the following methods:
(a) Conduct ground acceleration tests for a range of airplane gross weights to calibrate IAS at the cockpit display against the reference CAS. Determine airspeed lag from the calibration data by comparing the cockpit displayed airspeed with the reference calibration speed for a given gross weight and V1 speed.
(b) Determine airspeed lag by analysis using a computer program suitable for AFM development. Compute takeoffs for a range of gross weights to determine the acceleration at V1. Calculate airspeed lag at V1 for a corresponding acceleration and a known time lag due to data processing and filtering. The analysis should also consider other sources of airspeed lag as appropriate, such as the pneumatic lag in the pressure lines for the pitot and static sources.
(6) Having established the calibration data, one acceptable method of adjusting for airspeed lag is to apply corrections directly in the ADC data processing to result in a lagcorrected airspeed at the cockpit display. Another would be to include an airspeed lag correction in the takeoff ground speed calibration of IAS vs. CAS in the AFM. A single airspeed lag increment can be developed as the correction for the range of gross weights and corresponding accelerations at V1. This increment, when applied to the calibration, should result in no more than a 100 ft. increase in takeoff or accelerate-stop distances due to airspeed lag for any takeoff condition. A more accurate correction would result from presenting airspeed lag as a function of airplane acceleration based on the calibration data. If acceleration data are available in the ADC, a real time correction for lag during the takeoff can be applied in the data processing.
178. Static Pressure Systems - § 25.1325(d) and (e).
a. Explanation .
(1) If the altimeter installation is of the pressure type, its operation will be affected by any error that exists in the measurement of the static air pressure. Since the accuracy of the altimeter is of utmost importance, the static air vent system should be calibrated. If separate or alternate vent systems are employed for the altimeter and airspeed indicator, separate calibrations are required. Where the altimeter, rate of climb indicator, and airspeed indicators are vented to the same static systems, the altimeter calibration may be made in conjunction with the airspeed calibrations.
(2) The theoretical relationship between airspeed error and altimeter error may be used to derive an altimeter calibration from the airspeed calibration if both use the same static vent,