247 provided the total head sensing installation is designed to provide true readings over the angleof-attack range involved.
b. Procedures . None.
179. Pitot Heat Indication Systems - § 25.1326. [Reserved]
180. Magnetic Direction Indicator - § 25.1327. [Reserved]
181. Automatic Pilot System - § 25.1329.
a. Explanation .
(1) On most modern airplane installations, the autopilot, automatic thrust control (if provided), and flight director are integrated and are identified as a flight guidance system (FGS).
Although a HUD system, if installed, is generally a separate system, evaluation of its modes of operation would be identical to a flight director system. For these reasons, the systems flight test evaluation criteria provided here also apply to HUDs and head up guidance systems (HGS).
(2) The system pertains to equipment or combinations of equipment that provide the means for airplane control and guidance, whether by means of pilot supervised automatic systems or through guidance to the pilot for manual control. This includes sensors, computers, power supplies, servomotors/actuators, and associated wiring.
(3) The system should include any indications and controllers necessary for the pilot to manage and supervise the system.
(4) Additional guidance material related to FGS operation, testing, and approval is contained in the following FAA Advisory Circulars:
(a) AC 25-15, “Approval of Flight Management Systems in Transport Category Airplanes,” dated November 20, 1989.
(b) AC 25.1329-1B, “Approval of Flight Guidance Systems,” dated July 17, 2006.
(c) AC 91-16, “Category II Operations - General Aviation Airplanes,” dated August 7, 1967.
(d) AC 120-28D, “Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout,” dated July 13, 1999.
(e) AC 120-29A, “Criteria for Approval of Category I and Category II Weather Minima for Approach,” dated August 12, 2002.
b. Procedures .
(1) General .
(a) The evaluation of an FGS should primarily concentrate on system intended function and safe operation. In certain areas, degree of performance will require specific criteria commensurate with the type of operation. Criteria and requirements for flight should consider the applicant’s engineering analysis. The combination of analysis and flight evaluation will form the whole of the certification requirement and, as such, should be in harmony and provide full evaluation. The flight evaluations supplement the analysis as required in both the representative operating conditions of the airplane and for critical operating conditions. The critical conditions are determined from analysis of the total FGS/airplane modes of operation and configuration conditions.
(b) The requirement for demonstrating safe operation normally should include induced failures during flight. The requirement for failure demonstrations is also an outgrowth of the analysis submitted by the applicant and is a result of the particular design being evaluated
(e.g., consideration should be made for multiple channel systems specifically designed to be selfadaptive to failure conditions).
(c) The automatic pilot system should be installed and adjusted so that the system tolerances established during certification tests can be maintained in normal operation. This may be assured by conducting flight tests at the extremes of the tolerances. Those tests conducted to determine that the automatic pilot system will adequately control the airplane should establish the lower limit, and those tests to determine that the automatic pilot will not impose dangerous loads or deviations from the flight path should be conducted at the upper limit. Appropriate airplane loading to produce the critical results should be used.
(d) Means should be provided to indicate to the flightcrew the current mode of operation and any modes armed by the pilot. Selector switch position is not acceptable as a means of indication.
(e) The system should be demonstrated to perform its intended function in all configurations in which it may be used throughout all appropriate maneuvers and environmental conditions, including turbulence, unless an appropriate operating limitation or statement is included in the AFM. All maneuvers should be accomplished smoothly, accurately, and without sustained nuisance oscillation.
(f) In addition to performance of in tended function of each mode examined, the automatic pilot system should cause no sustained nuisance oscillations, undue control activity, or sudden large attitude changes, especially when configuration or power changes are taking place.
(g) When automatic functions are provided that may be used with these systems
(e.g., automatic throttle control or yaw damper, etc.), and use of the system is permitted with any of these functions inoperative, the system should comply with these flight test requirements with these additional functions operative and inoperative.
(h) In addition to the quick release controls of § 25.1329(d), in order to show compliance with § 25.1309, an alternative means of disengagement for the automatic pilot should be provided. It should be readily accessible in flight.
(i) It should be possible to disengage the automatic pilot system at any time without unacceptable out-of-trim forces.
(j) The controls, indicators, and warnings should be designed to minimize crew errors. Mode and system malfunction indications should be presented in a manner compatible with the procedures and assigned-tasks of the flightcrew. The indications should be grouped in a logical and consistent manner, and be visible under all lighting conditions.
(k) The means provided to comply with § 25.1329(h) should also give an appropriate indication when there is an inadvertent change or disengagement of a mode.
(2) Takeoff Mode .
(a) AC 25-15, “Approval of Flight Management Systems in Transport Category Airplanes,” contains some basic criteria and airspeed tolerance criteria that should be included in the flight evaluation of systems containing takeoff guidance. Additional testing considerations, which should be considered for inclusion in the flight test plan, are presented in paragraphs (b) through (s), below.
(b) The roll axis control law will probably be a heading hold or roll attitudehold/wings-level mode. The performance of the roll axis control function is considered to be obvious during the normal course of flight testing and, therefore, is not addressed with specific test requirements herein. The pitch axis control law discussed below represents a system providing computed command guidance (flight director, HGS, and automatic pilot after liftoff) representing the reference speed to be flown during second segment climb-out.
(c) For a normal all-engines-operating (AEO) takeoff, the reference speed should be the normal AEO initial takeoff climb speed selected by the applicant (typically V2+10 knots).
In the event that an engine failure occurs during the takeoff maneuver, at an airspeed greater than V1, the reference speed should be:
1 V 2 if an engine failure occurs prior to V2.
2 Existing speed if engine failure occurs between V 2 and the AEO initial climb speed.
3 The AEO initial takeoff climb speed for all other conditions, if appropriate.
(d) Evaluate takeoff performance with different flap settings and different loading conditions (gross weight and center of gravity) to cover the various flight conditions. Use normal rotation and cleanup procedures unless noted otherwise.
(e) Evaluate abnormal/simulated failure conditions including:
1 Late rotation.
2 Early rotation.
3 Simulated engine failure at V 1.
4 Simulated engine failure at V 2+20 knots.
5 Simulated engine failure at V 2+5 knots.
(f) Evaluate the ability of the autopilot to command the correct speed for the conditions in the testing described in paragraphs (c), (d), and (e) above.
(g) If a variable takeoff thrust derate feature is provided, perform takeoff demonstrations using up to the maximum derated thrust level for which approval is to be granted.
(h) If an ATTCS is included on the ai rplane, evaluate its performance of intended function in conjunction with the above.
(i) Conduct takeoffs with and without, as applicable, automatic throttles, yaw damper, automatic brake system, control wheel steering, and any other automated equipment configurations, and combinations thereof, for which the applicant is seeking certification approval. Include all sensor and instrument switching combinations.
(j) Evaluate Slow/Fast display behavior, if applicable, in conjunction with these takeoff tests.
(k) Evaluate the autopilot behavior with both series and parallel rudder (if available) configurations, particularly in conjunction with simulated engine failure tests.
(l) Evaluate “fade-out” of parallel rudder (with simulated engine failure) during autopilot takeoff mode transition to automatic altitude captures (if this feature is available).
(m) Evaluate pitch axis takeoff behavior during simultaneous heading select turns during climb-out.
(n) Evaluate mode transition from ta keoff to other approved modes using flight director, HGS, and automatic pilot guidance (i.e., from pitch axis takeoff mode to IAS Hold or Vertical Speed or Altitude Hold or vertical navigation with autopilot operation (VNAV); from roll axis takeoff mode to Heading Select or VOR or lateral navigation with autopilot operation (LNAV), etc.).
(o) Evaluate all reversion modes appropriate to the design. Most speed command takeoff modes will be fail-passive (or “fail-soft”) and will probably contain reversion modes.
Consult the system description, failure modes and effects analysis (FMEA), and the responsible systems engineer for definition of the appropriate evaluation. This may be accomplished in an appropriate simulator environment.
(p) Evaluate adequacy of mode annunciations and controls and displays associated with takeoff mode operation.
(q) In addition to evaluating speed performance to the criteria shown in Table 2 of AC 25-15, “Approval of Flight Management Systems in Transport Category Airplanes,” evaluate command bar flyability, dynamics of the airplane while following the commanded guidance, initial pitch target presented during and immediately following rotation, etc.
(r) Evaluate takeoff pitch attitude limit, if applicable, for compatibility and command harmony.
(s) Quantitative time history data should be derived with an on-board data system to facilitate post-flight evaluation of performance and to provide type inspection report (TIR) records.
(3) Climb, Cruise, Descent, and Holding Modes .
(a) The acceptability of the autopilot system performance may be based on subjective judgment, taking into account the experience acquired from similar equipment and the general behavior of the airplane. The acceptable performance may vary according to airplane type and model.
(b) Examination of the following modes are considered appropriate for inclusion in this section:
Pitch Attitude Hold Roll Attitude Hold Vertical Speed Hold/Select Heading Hold/Select Altitude Hold/Select VOR Navigation IAS Hold/Select Lateral Navigation Mach Hold/Select Backcourse Level Change Non-Precision Approach Vertical Navigation VOR Turbulence (Penetration) Localizer (only) Area Navigation
Evaluation/approval of these modes generally will not require derivation of quantitative flight data.
(c) Special characteristics associated with some specific modes worthy of note are:
1 Operation of the system should not result in performance for which the pilot would be cited during a check ride (i.e., exceeding a speed target of 250 knots by more than 5 knots, if appropriate, during operations below 10,000 feet altitude, or overshooting a target altitude by more than 100 feet during capture of the pre-selected altitude).
2 Where the automatic pilot system has the ability to acquire and maintain a pre-selected altitude, it should be shown in particular that:
(aa) With the autopilot engaged, if the pilot fails to advance the throttles following an altitude acquisition from the descent, ensure that the airplane exhibits no hazardous characteristics, if recovery action is taken within a reasonable period after the onset of stall warning, or other reliable warning. The autopilot should disconnect at stall warning unless proven safe beyond this point.
NOTE: Compliance with this provision need not be demonstrated if adequate means are provided to prevent such an error.
(bb) Resetting the datum pressure or the selected altitude at any time during altitude capture should not result in hazardous maneuvers.
3 The altitude hold mode should be evaluated:
(aa) During turning flight.
(bb) During accelerating and decelerating flight.
(cc) With engagement of the mode from climbing and descending flight conditions.
4 Thoroughly explore transition of VOR navigation from cruise/holding to the approach mode. Include appropriate procedures in the AFM if special procedures or limitations are deemed to be necessary.
(4) Go-Around Mode .
(a) AC 25-15 also contains some basic criteria and airspeed tolerance criteria for go-around systems that should be included in the flight evaluation. Also, AC 120-28D, “Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout,” contains additional go-around criteria pertinent to this flight evaluation, if the airplane is to be approved for CAT III operation.
(b) As with the takeoff mode, the go-around roll axis control law will probably be a heading-hold or roll attitude-hold/wings-level mode and will not be addressed herein. The
253 pitch axis control law may be an angle-of-attack control law system, but most modern systems will have control laws for speed control. The speed should be compatible with that used for a manually controlled go-around without guidance.
(c) Under no circumstances should the system command a go-around speed less than 1.13 VSR1 for the go-around flap setting (which equates to approximately 1.3 VSR0 + 3 knots for the landing flap setting) or other firmly established minimum control speed (e.g., VMCL) established for that particular airplane. In making this evaluation, consideration should be given to the autopilot roll axis control law and its effect on minimum control speeds; if the autopilot employs a wings-level control law, the minimum control speeds will be considerably increased over the pilot flying values that are determined with some degree of bank into the operating engine(s).
(d) Evaluate go-around performance with normal conditions for each landing flap setting through a full range of gross weight and center of gravity conditions. Use normal cleanup procedures to the appropriate approach climb flap setting.
(e) Evaluate go-around performance with simulated engine failure conditions occurring:
1 Just prior to go-around initiation;
2 Just after go-around initiation; and
3 During climb-out, both before and after initiation of altitude capture mode.
(f) If engine-out approach/landing approval is sought by the applicant, then evaluate go-around performance initiated from an engine-out approach.
(g) It should be demonstrated that the autopilot will command the correct speed to accomplish a go-around for the conditions in the testing described in paragraphs (d), (e), and
(f), above.
(h) Flight path control following an engine failure during go-around should not require exceptional piloting skill or alertness.
(i) Conduct go-arounds with and without, as applicable, automatic throttles, yaw damper, automatic ground spoilers, control wheel steering, and any other equipment configurations, and combinations thereof, for which the applicant is seeking certification credit.
Include all sensor and instrument switching combinations.
(j) Evaluate Slow/Fast display behavior, if applicable, in conjunction with these go-around tests.
(k) Evaluate the autopilot behavior with both series and parallel rudder (if available) configurations, particularly in conjunction with simulated engine failure tests.
(l) Evaluate “fade-out” of parallel rudder (with simulated engine failure) during autopilot go-around mode transition to automatic altitude captures (if this feature is available).
(m) Verify that when automatic go-around is engaged, subsequent momentary ground contact will not cause its disengagement or a mode reversion out of go-around.
(n) Structure the test program such that altitude loss during the go-around maneuver can be determined as a function of height above the ground at go-around initiation.
Altitude loss during the maneuver is directly proportional to the rate of descent on initiation of go-around.
(o) Go-around altitude loss is defined as the difference in altitude between the selection of the go-around mode and the minimum height above the ground achieved during the maneuver.
(p) If the airplane is to be approved for Category III operations, the go-around altitude loss information should be included in the AFM (see appendix 6 of AC 120-28D). The preferred format for presentation of this information in the AFM is as a cross-plot of altitude loss (ordinate of the crossplot) versus altitude of mode initiation (abscissa of the crossplot) as shown below.
Figure 181-1 Altitude Loss versus Altitude Go-Around Mode
EXAMPLE ALTITUDE LOSS DURING GO-AROUND (FT) go-around initiated at 50 feet is The expected altitude loss during between 38 feet and 45 feet.
50 40 These data are derived from tests to 30 determine altitude losses during go arounds.
They were conducted with the autopilot 20 engaged, with all engines functional and with simulated engine failures, and at all 10 approved flap settings. There is essentially no difference in results, regardless of aircraft configuration.
20 40 60 80 100
ALTITUDE AT GO-AROUND INITIATION (FT)
(q) Evaluate the go-around maneuver under conditions with automatic spoilers armed and go-around initiated at a low altitude, which results in wheel contact with the runway surface. Verify that momentary deployment of the ground spoilers under these conditions is acceptable.
(r) In addition to evaluation of speed performance to the criteria shown in Table 2 of AC 25-15, "Approval of Flight Management Systems in Transport Category Airplanes," evaluate command bar flyability, the dynamics of the airplane while following the commanded guidance, etc. The control action and flight path during the initial rotation should not be significantly different from those of a manually controlled go-around without command guidance.
(s) The go-around from any point on the approach to touchdown should not require exceptional piloting skill, alertness, or strength, and should ensure that the airplane remains above the obstacle limitation surface specified in AC 120-29A, "Criteria for Approval of Category I and Category II Weather Minima for Approach."
(t) Evaluate pitch axis go-around mode behavior with simultaneous heading select turns during climb-out.
(u) Evaluate mode transitions from go-around to other approved modes using flight director, HGS, and automatic pilot guidance (i.e., from pitch axis go-around mode to IAS Hold or Vertical Speed or Altitude Hold or VNAV; from roll axis go-around mode to Heading Select or VOR or LNAV; etc.).
(v) Evaluate all reversion modes appropriate to the design. Most speed command go-around modes will be fail-passive (or “fail-soft”) and will probably contain reversion modes.
Consult the system description, the FMEA, and the responsible system engineer for definition of the appropriate evaluation. It may be possible to perform this testing in a simulator.
(w) Evaluate adequacy of mode annunciations and controls and displays associated with the go-around maneuver.
(x) Evaluate the go-around pitch attitude limit (if applicable) for compatibility and command harmony.
(y) “Fixed Pitch Go-Around.” Missed approaches using the calibrated attitude markings on the Attitude Director Indicator (ADI) should be demonstrated to be an acceptable alternative technique for low visibility operations (CAT II & CAT III), in the event that automatic or flight director go-around modes are not available. A sufficient sample size should be collected to establish the transitional altitude loss. The target attitude for go-around (13 to 17 degrees is usually the target) should be established by the applicant. Use normal cleanup procedures. Simulate various failure conditions that would force the pilot to revert to this configuration. (e.g., simultaneous loss of autoland and go-around computation).
(z) Quantitative time-history data should be derived with an on-board data system to facilitate post-flight evaluation of performance and to provide TIR records.
(5) Instrument Landing System (ILS) Approach Mode .
(a) A complete and comprehensive description of flight test evaluation for automatic pilot and flight director system ILS approach mode approval, including HUDs, is contained in AC 120-29A, “Criteria for Approval of Category I and Category II Weather Minima for Approach,” and in AC 91-16, “Category II Operations - General Aviation Airplanes.”
Further guidance is provided below:
(b) For airworthiness approval to Category I (CAT I) minimums:
1 Conduct a series of approaches (usually 4 or more) on CAT I rated ILS beams to a radio altitude of 160 ft. (20 percent below the CAT I decision height of 200 ft).
2 Conduct the approaches with and without automatic throttles, with and without yaw damper, alternate flap setting and so forth, for all configurations and combinations of equipment for which the applicant seeks approval.
257 3 At least three CAT I beams should be included in the evaluation, one of which should exhibit very noisy localizer and glideslope characteristics.
4 Failure modes/conditions described in paragraph 181b(7)(f) of this AC, appropriate to ILS approach modes, should be conducted.
5 The definition of a successful approach is one that positions the airplane at the decision height (DH), such that the airplane can be safely landed.
(c) For airworthiness approval to Category II (CAT II) minimums for each candidate system (automatic pilot, flight director and HUD):
1 Conduct a series of approaches on CAT II rated ILS beams to a radio altitude of 100 feet.
2 For initial system approval, approximately 20 approaches (total for each affected system) are required to examine each configuration/combinations of equipment for which the applicant seeks credit.
3 At least three CAT II beams should be included in the evaluation.
4 When relatively minor changes are made to the approach control laws or when a different display (i.e., flight director instrument) is incorporated with the system, approximately 9 approaches (total for each affected system) have been found to be sufficient
(i.e., 3 each on 3 different ILS beams).
5 The approaches should be made in conditions chosen to show that the performance is satisfactory within the permitted extremes of weight, center of gravity, wind speed, localizer capture angles, glideslope captures from below and above (if appropriate) the beam, captures at various ranges from runway threshold, etc.
(d) If approval is sought for ILS approaches initiated with one engine inoperative, and with the airplane trimmed at the point of glide path intercept, the automatic flight control system should be capable of conducting the approach without further manual trimming. For airplanes with three or more engines, the loss of a second critical engine should not cause a rate of lateral deviation from the ILS course greater than 3° per second (averaged over a 5-second period), or produce hazardous attitudes.
(e) Unless it is shown that failure of the automatic pilot system to disengage during the approach, when the pilot operates the quick release control on the control wheel, is improbable, then it should be demonstrated that the pilot can control the airplane manually without operating any of the other disengagement controls.
(f) It is possible that many of the above tests can be performed in a simulator environment.
(6) Control Wheel Steering (CWS) .
(a) It should be possible for the pilot to overpower the automatic pilot system, and achieve the maximum available control surface deflection, without using forces that exceed the pilot control force limits specified in § 25.143(d).
(b) The maximum bank and pitch attitudes that could be achieved without overpowering the automatic pilot system should be limited to those necessary for the normal operation of the airplane. Typically ±35° in roll and +20° to -10° in pitch.
(c) It should be possible to carry out all normal maneuvers and to counter normal changes of trim due to change of configuration etc., within the range of flight conditions in which control wheel steering may be used, without encountering excessive discontinuities in control force that might adversely affect the flight path.
(d) The stall and stall recovery characteristics of the airplane should remain acceptable with control wheel steering in use.
(e) In showing compliance with § 25.143(f), account should be taken of such adjustment to trim as may be carried out by the automatic pilot system in the course of maneuvers that can reasonably be expected. Some alleviation may be acceptable in the case of unusually prolonged maneuvers, provided the reduced control forces would not be hazardous.
(f) If the use of this mode for takeoff and landing is to be permitted, it should be shown that:
1 Sufficient control, both in amplitude and rate, is available without encountering force discontinuities;
2 Reasonable mishandling is not hazardous (e.g., engaging the autopilot while the elevators or ailerons are held in an out-of-trim position); and
3 Runaway rates and control forces are such that the pilot can readily overpower the automatic pilot with no significant error in flight path.
(7) Failure Modes/Malfunction Tests .
(a) General .
1 Failure conditions of the system should be simulated in such a manner as to represent the overall effect, and worse case effect, of each failure condition about all axes.
2 Investigations should include the effects of any failure, and any combination of failures, not shown to be extremely improbable.