or deviations from the flight path should be conducted at the upper limit.
Appropriate airplane loading to produce the critical results should be used.
33.7.2.4 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 is included in the AFM. All maneuvers should be accomplished smoothly, accurately, and without sustained nuisance oscillation.
33.7.2.5 In addition to performance of intended function of each mode examined, the FGS should cause no sustained nuisance oscillations, undue control activity, or sudden large attitude changes, especially when configuration or power/thrust changes are taking place.
33.7.2.6 When use of the FGS is permitted with any of its functions inoperative
(e.g., autothrust, yaw damper, etc.), the system should be evaluated with these functions both operative and inoperative.
33.7.3 Procedures: FGS Protection Features.
33.7.3.1 Section 25.1329(h) requires that, when the flight guidance system is in use, a means must be provided to avoid excursions beyond an acceptable margin from the speed range of the normal flight envelope. The FGS itself may contain protection features to aid the flightcrew in assuring that the boundaries of the flight envelope or operational limits are not exceeded.
The means to alert the flightcrew to a boundary or for the system to intervene may vary, but certain operational scenarios can be used to assess the performance of the system in providing the protection function. The procedures in the following paragraphs can be used to evaluate the protection features as required by § 25.1329(h), regardless of whether they are included in the FGS itself or invoked by other means.
33.7.3.2 Low Speed Protection.
Low speed protection is intended to prevent loss of speed leading to an unsafe condition. If the FGS remains in the existing mode, a suitable alert should be provided to annunciate the low speed condition. In this case, note the pilot response to the alert and the recovery actions taken to maintain the desired vertical path and to accelerate back to the desired speed. The following scenarios should be considered when evaluating the low speed protection means that must be present when the FGS is in use:
33.7.3.2.1 High Altitude Cruise Evaluation.
1. At high altitude at normal cruise speed, engage the FGS into an Altitude Hold mode and a heading or lateral navigation (LNAV) mode.
33-7
2. Engage the autothrust into a speed mode.
3. Manually reduce one engine to idle power or thrust.
4. As the airspeed decreases, observe the FGS behavior in maintaining altitude and heading/course.
5. When the low speed protection feature becomes active, note the airspeed and the associated aural and visual alerts including possible mode change annunciations for acceptable operation.
33.7.3.2.2 Altitude Capture Evaluation at Low Altitude.
1. At a reasonably low altitude (e.g., approximately 3000 feet above MSL where terrain permits) and at 250 knots, engage the FGS into Altitude Hold and a heading or LNAV mode.
2. Engage the autothrust into a speed mode.
3. Set the altitude pre-selector to 5000 feet above the current altitude.
4. Make a flight level change to the selected altitude feet with a 250 knots climb at maximum climb power or thrust.
5. When the FGS first enters the Altitude Capture mode, reduce thrust/power on one engine to idle.
6. As the airspeed decreases, observe the airplane trajectory and behavior.
7. When the low speed protection condition becomes active, note the airspeed and the associated aural and visual alerts including possible mode change annunciations for acceptable operation.
33.7.3.2.3 High Vertical Speed Evaluation.
1. Engage the FGS in the Vertical Speed mode with a very high rate of climb.
2. Set the thrust/power to a value that will cause the airplane to decelerate at approximately 1 knot per second.
3. As the airspeed decreases, observe the airplane trajectory and behavior.
4. When the low speed protection condition becomes active, note the airspeed and the associated aural and visual alerts including possible mode change annunciations for acceptable operation.
33.7.3.2.4 Approach Evaluation.
1. Conduct an instrument approach with vertical path reference.
2. Couple the FGS to the localizer and glideslope (or LNAV/VNAV, etc.).
33-8
3. Cross the final approach fix/outer marker at a reasonably high speed at idle thrust/power until low speed protection activates.
4. As the airspeed decreases, observe the airplane trajectory and behavior.
5. When the low speed protection becomes active, note the airspeed and the associated aural and visual alerts including possible mode change annunciation for acceptable operation.
6. Note the pilot response to the alert and the recovery actions taken to recover to the desired vertical path and the re-capture to that path and the acceleration back to the desired approach speed.
33.7.3.3 High Speed Protection.
High speed protection, which may either be included in the FGS or provided by other means, is intended to prevent a gain in airspeed leading to an unsafe condition. If the FGS remains in the existing mode with reversion to high speed protection, a suitable alert should be provided to annunciate the high speed condition. In this case, note the pilot response to the alert and the recovery actions taken to maintain the desired vertical path and to decelerate back to the desired speed. The following scenarios should be considered when evaluating the means used to provide protection from high speed excursions while the FGS is in use, as required by § 25.1329(h):
33.7.3.3.1 High Altitude Level Flight Evaluation with Autothrust Function.
1. Select autothrust off, if an automatic wake-up function is provided;
otherwise, select autothrust on.
2. Engage the FGS in Altitude Hold.
3. Select a power or thrust level that will result in an acceleration that would cause, without intervention (either automatic or manual), a speed/Mach beyond VMO/MMO.
4. As the airspeed increases, observe the behavior of the high-speed protection condition and any autothrust reactivation and thrust/power reduction, as applicable.
5. Assess the performance of the FGS to control the airspeed to VMO/MMO, or other appropriate speed.
33.7.3.3.2 High Altitude Level Flight Evaluation Without Autothrust Function.
1. Set a power/thrust level that will result in acceleration that would cause, without intervention (either automatic or manual), a speed/Mach beyond VMO/MMO.
2. As the airspeed increases, observe the basic airplane overspeed warning activate.
33-9
3. Observe activation and effectiveness of the high speed protection means and note any FGS indications and behavior.
4. Maintain the existing power or thrust level and observe the airplane depart the selected altitude.
5. After sufficient time has elapsed to verify and record FGS behavior, reduce the thrust/power as necessary to cause the airplane to begin a descent.
6. Observe the FGS behavior during the descent and subsequent altitude capture at the original selected altitude.
33.7.3.3.3 High Altitude Descending Flight Evaluation with Autothrust Function.
1. Select autothrust off (with automatic wake-up function) with power or thrust set to maintain airspeed 10 percent below VMO/MMO with the FGS engaged in Altitude Hold.
2. Select Vertical Speed mode with a vertical speed that will result in an acceleration that would cause, without intervention (either automatic or manual), a speed/Mach beyond VMO/MMO.
3. As the airspeed increases observe the autothrust function reactivate and reduce power or thrust towards idle.
4. Observe the activation and effectiveness of the high speed protection means and note any FGS indications and behavior.
33.7.4 Procedures: FGS Takeoff Mode.
33.7.4.1 AC 25-15 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 33.7.4.2 through 33.7.4.19 below.
33.7.4.2 There are no test conditions specifically identified for evaluating the roll axis control function. The performance of this function should be evaluated during the normal course of flight testing. The test conditions for the pitch axis control function identified below is for a system providing computed command guidance (flight director, Head Up Guidance System (HGS), and automatic pilot after liftoff) to achieve and maintain a reference speed to be flown during second segment climb-out.
33.7.4.3 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). If an engine failure occurs during the takeoff maneuver, at an airspeed greater than VEF, the reference speed should be:
V2, if an engine failure occurs prior to V2.
33-10
The existing speed if engine failure occurs between V2 and the AEO initial climb speed.
The AEO initial takeoff climb speed for all other conditions, if appropriate.
33.7.4.4 Evaluate takeoff performance with different flap settings and different loading conditions (gross weight and CG) to cover the various flight conditions. Use normal rotation and cleanup procedures unless noted otherwise.
33.7.4.5 Evaluate abnormal/simulated failure conditions including:
Late rotation.
Early rotation.
Simulated engine failure at VEF.
Simulated engine failure at V2+20 knots.
Simulated engine failure at V2+5 knots.
33.7.4.6 Evaluate the ability of the FGS to command the correct speed for the conditions in the testing described in paragraphs 33.7.4.3, 33.7.4.4, and
33.7.4.5 above.
33.7.4.7 If a variable takeoff power or thrust derate or reduced thrust (power) feature is provided, perform takeoff demonstrations to cover the range of derated/reduced power or thrust levels for which approval is requested.
33.7.4.8 If an ATTCS is included on the airplane, evaluate its performance of intended function in conjunction with the above.
33.7.4.9 Conduct takeoffs with and without, as applicable, autothrust, yaw damper, autobrakes, 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.
33.7.4.10 Evaluate Slow/Fast display behavior, if applicable, in conjunction with these takeoff tests.
33.7.4.11 Evaluate the FGS behavior with both series and parallel rudder (if available) configurations, particularly in conjunction with simulated engine failure tests.
33.7.4.12 Evaluate “fade-out” of parallel rudder (with simulated engine failure) during FGS takeoff mode transition to automatic altitude captures (if this feature is available).
33-11
33.7.4.13 Evaluate pitch axis takeoff behavior during simultaneous heading select turns during climb-out.
33.7.4.14 Evaluate mode transition from takeoff to other approved modes using flight director, HGS, and automatic pilot guidance (e.g., from pitch axis takeoff mode to Indicated Airspeed (IAS) Hold, Vertical Speed, Altitude Hold, or Vertical Navigation (VNAV) mode; from roll axis takeoff mode to Heading Select, radial tracking, or Lateral Navigation (LNAV) mode, etc.).
33.7.4.15 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.
33.7.4.16 Evaluate adequacy of mode annunciations and controls and displays associated with takeoff mode operation.
33.7.4.17 In addition to evaluating speed performance to the criteria shown in Table 2 of AC 25-15 evaluate command bar flyability, dynamics of the airplane while following the commanded guidance, initial pitch target presented during and immediately following rotation, etc.
33.7.4.18 Evaluate takeoff pitch attitude limit, if applicable, for compatibility and command harmony.
33.7.4.19 Quantitative time history data should be recorded with an onboard data system to facilitate post-flight evaluation of performance and to provide type inspection report (TIR) records.
33.7.5 Procedures: FGS Climb, Cruise, Descent, and Holding Modes.
33.7.5.1 The acceptability of the FGS 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.
33.7.5.2 Examination of the following modes are considered appropriate for inclusion in this section:
Altitude Hold/Select.
Area Navigation.
Backcourse.
Heading Hold/Select.
33-12
IAS Hold/Select.
Lateral Navigation.
Level Change.
Localizer (only).
Mach Hold/Select.
Non-Precision Approach.
Pitch Attitude Hold.
Roll Attitude Hold.
Turbulence.
Vertical Navigation.
Vertical Speed Hold/Select.
VOR.
VOR Navigation.
33.7.5.3 Evaluation/approval of the modes in the paragraph above generally will not require derivation of quantitative flight data.
33.7.5.4 Special characteristics associated with some specific modes worthy of note are:
33.7.5.4.1 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).
33.7.5.4.2 Where the FGS has the ability to acquire and maintain a pre-selected altitude, it should be shown in particular that:
1. With the autopilot function of the FGS engaged without autothrust, if the pilot fails to advance the throttles following an altitude acquisition from a descent, ensure that the FGS speed protection function activates to avoid an unsafe speed excursion.
2. Resetting the datum pressure or the selected altitude at any time during altitude capture should not result in hazardous maneuvers.
33.7.5.4.3 The altitude hold mode should be evaluated:
1. During turning flight.
2. During accelerating and decelerating flight.
33-13
3. With engagement of the mode from climbing and descending flight conditions.
33.7.5.4.4 Thoroughly explore transition of VOR navigation from cruise/holding to the approach mode. Include appropriate procedures or limitations in the AFM if special procedures or limitations are deemed to be necessary.
33.7.6 Procedures: FGS Go-Around Mode.
33.7.6.1 AC 25-15 contains some basic criteria and airspeed tolerance criteria for go-around systems that should be included in the flight evaluation. Also, AC 120-28D contains additional go-around criteria pertinent to this flight evaluation if the airplane is to be approved for CAT III operation.
33.7.6.2 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. The 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.
33.7.6.3 The system should not command a go-around speed less than 1.13 VSR1 for the go-around flap setting or the 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 may be considerably higher than the AFM values that are determined with some degree of bank into the operating engine(s).
33.7.6.4 Evaluate go-around performance with normal conditions for each landing flap setting through a full range of gross weight and CG conditions. Use normal cleanup procedures to the appropriate go-around flap setting.
33.7.6.5 Evaluate go-around performance with simulated engine failure conditions occurring:
Just prior to go-around initiation, Just after go-around initiation, and During climb-out, both before and after initiation of altitude capture mode.
33.7.6.6 If engine-out approach/landing approval is sought by the applicant, then evaluate go-around performance initiated from an engine-out approach.
33-14
33.7.6.7 It should be demonstrated that the FGS will command the correct speed to accomplish a go-around for the conditions in the testing described in paragraphs 33.7.6.4, 33.7.6.5, and 33.7.6.6 above.
33.7.6.8 Flight path control following an engine failure during go-around should not require exceptional piloting skill or alertness.
33.7.6.9 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.
33.7.6.10 Evaluate Slow/Fast display behavior, if applicable, in conjunction with these go-around tests.
33.7.6.11 Evaluate the autopilot behavior with both series and parallel rudder (if available) configurations, particularly in conjunction with simulated engine failure tests.
33.7.6.12 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).
33.7.6.13 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.
33.7.6.14 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 when the go-around is initiated.
33.7.6.15 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.
33.7.6.16 Go-around altitude loss information should be included in the AFM, especially if the airplane is to be approved for low visibility (Category I, II, or III) approaches. An example of how such information has been presented in the AFM is shown in figure 33-1 below.
33-15
Figure 33-1. Altitude Loss versus Altitude at Go-Around Mode Initiation
33.7.6.17 Evaluate the go-around maneuver under conditions with automatic spoilers armed and go-around initiated at an altitude low enough to result in wheel contact with the runway surface. Verify that momentary deployment of the ground spoilers under these conditions is acceptable.
33.7.6.18 In addition to evaluating speed performance to the criteria shown in Table 2 of AC 25-15 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.
33.7.6.19 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.
33.7.6.20 Evaluate pitch axis go-around mode behavior with simultaneous heading select turns during climb-out.
33.7.6.21 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, Vertical Speed, Altitude Hold, or VNAV;
from roll axis go-around mode to Heading Select VOR, or LNAV, etc.).
33-16
33.7.6.22 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.
33.7.6.23 Evaluate adequacy of mode annunciations and controls and displays associated with the go-around maneuver.
33.7.6.24 Evaluate the go-around pitch attitude limit (if applicable) for compatibility and command harmony.
33.7.6.25 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° 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).
33.7.6.26 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.
33.7.7 Procedures: FGS Instrument Landing System (ILS) Approach Mode.
33.7.7.1 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. Further guidance is provided below:
33.7.7.2 For airworthiness approval to Category I (CAT I) minimums:
33.7.7.2.1 Conduct a series of approaches (usually 4 or more) on Type I rated ILS beams to a radio altitude of 160 feet. (Twenty percent below the CAT I decision height of 200 feet).
33.7.7.2.2 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.
33-17
33.7.7.2.3 At least three Type I beams should be included in the evaluation, one of which should exhibit very noisy localizer and glideslope characteristics.
33.7.7.2.4 Failure modes/conditions described in paragraph 33.7.8.6 of this AC, appropriate to ILS approach modes, should be conducted.
33.7.7.2.5 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 without exceptional piloting skill or strength.
33.7.7.3 For airworthiness approval to Category II (CAT II) minimums for each candidate system (automatic pilot, flight director, and HGS):
33.7.7.3.1 Conduct a series of approaches on Type II rated ILS beams to a radio altitude of 100 feet.
33.7.7.3.2 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.
33.7.7.3.3 At least three Type II beams should be included in the evaluation.
33.7.7.3.4 When relatively minor changes are made to the approach control laws or when a different display (i.e., flight director instrument) is used 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).
33.7.7.3.5 The approaches should be made in conditions chosen to show that the performance is satisfactory within the permitted extremes of weight, CG, wind speed, localizer capture angles, glideslope captures from below and above (if appropriate) the beam, captures at various ranges from runway threshold, etc.
33.7.7.4 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.
33.7.7.5 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.
33.7.7.6 It is possible that many of the above tests can be performed in a simulator.
33-18