33.3.1.1.7 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 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.
33.3.1.1.8 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. Further, an increase in the takeoff distance or the accelerate-stop distance as a result of airspeed lag should not exceed 100 feet. 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 feet of increased distance.
33.3.1.1.9 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. Further, 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 feet 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.
33.3.1.2 Configuration.
Airspeed calibration tests should be conducted in the following configurations:
Weight—between maximum takeoff and maximum landing.
CG position—optional.
Takeoff configuration(s)—ground roll.
33-2
Wing flaps and landing gear—all combinations of positions used to show compliance with the takeoff, climb, and landing requirements of part 25.
Power or thrust—as required.
33.3.2 Procedures.
33.3.2.1 Any one or any desired combination of the procedures in paragraphs
33.3.2.2 or 33.3.2.3 of this AC 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 exceed 1.23 VSR. The highest speed should not exceed VMO/MMO. 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.
33.3.2.2 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:
Time of day.
Airplane’s indicated airspeed.
Reference indicated airspeed.
Pressure altitude.
Ambient air temperature.
Wing flap position.
Landing gear position.
33.3.2.3 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.
33.3.2.4 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. Further, if airspeed lag causes an 33-3
increase of more than 100 feet 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:
33.3.2.4.1 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.
33.3.2.4.2 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.
33.3.2.4.3 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 lag-corrected 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.
33.4 Static Pressure Systems—§ 25.1325(d) and (e).
33.4.1 Explanation.
33.4.1.1 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.
33.4.1.2 The theoretical relationship between airspeed error and altimeter error may be used to derive an altimeter calibration from the airspeed 33-4
calibration, or vice versa, if both use the same static vent, provided any error in total pressure over the angle-of-attack range is taken into account.
33.4.2 Procedures.
None.
33.5 Pitot Heat Indication Systems—§ 25.1326. [Reserved]
33.6 Magnetic Direction Indicator—§ 25.1327. [Reserved]
33.7 Flight Guidance System—§ 25.1329.
33.7.1 Explanation.
33.7.1.1 On most modern airplane installations, the autopilot and flight director are integrated into a single computer, use common control laws, and are identified as a flight guidance system (FGS). The FGS is primarily intended to assist the flightcrew in the basic control and tactical guidance of the airplane. The system may also provide workload relief to the pilots and provide a means to fly a flight path more accurately to support specific operational requirements, such as reduced vertical separation minimum (RVSM) or required navigation performance (RNP). To perform these functions, the FGS typically includes an integrated autopilot, autothrust, and flight director. The FGS functions also include flight deck alerting, status, mode annunciations (instrument displays), and associated information displayed to the flightcrew for situation awareness.
Also included are those functions necessary to provide guidance and control with an approach and landing system, such as an instrument landing system (ILS), microwave landing system (MLS), global navigation satellite system (GNSS), or a GNSS landing system (GLS).
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.
Note: For applicants considering type certification of a HUD system, we recommend that the flight technical error be characterized during certification testing. This data will be required if the applicant chooses to seek operational credit for advanced performance-based navigation applications.
33.7.1.2 For the purposes of this AC, the FGS includes all the equipment necessary to accomplish the FGS function, including the sensors, computers, power supplies, servo-motors/actuators, and associated wiring. It also includes 33-5
any indications and controllers necessary for the pilot to manage and supervise the system.
33.7.1.3 A flight test program should be established that confirms the performance of the FGS for the modes of operation and the operational capabilities supported by its design. The operational implications of certain failures and failure conditions may require flight evaluation. Also, the pilot interface with FGS controls and displays in the cockpit will need to be assessed. Some aspects of the FGS design may be validated by laboratory test and/or simulator evaluation.
33.7.1.4 The scope of the flight demonstration program will depend on the operational capability being provided, including any new and novel features. Early coordination with the FAA is recommended to reduce certification risks associated with the flight demonstration program. The intent of the flight demonstration program is to confirm that the operation of the FGS is consistent with its use for the intended flight operations of the airplane type and configuration. The modes of the FGS should be demonstrated in representative airplane configurations and under a representative range of flight conditions.
33.7.1.5 Additional guidance material related to FGS operation, testing, and approval is contained in the following ACs:
AC 25-15.
AC 25.1329-1C.
AC 120-28D.
AC 120-29A.
33.7.2 Procedures: General.
33.7.2.1 The evaluation of an FGS should primarily concentrate on the intended function(s) and safe operation of the system. Flight test evaluations supplement analysis, laboratory testing, and simulator testing for showing compliance with the applicable part 25 requirements.
33.7.2.2 The requirement for demonstrating safe operation should include evaluation of FGS failure conditions that a system safety assessment identifies as needing to be validated by flight test.
33.7.2.3 The FGS should be installed and adjusted so that the system tolerances established during certification tests can be maintained in normal operation. This may be addressed by conducting flight tests at the extremes of the tolerances. Those tests conducted to determine that the FGS will adequately control the airplane should establish the lower limit, and those tests to determine that the FGS will not impose dangerous loads 33-6
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