64. Vertical Modes. This paragraph discusses modes that are implemented in many FGSs that are used primarily for pitch control of the airplane. The criteria identified reflect operational experience gained from the use of these modes.
a. Target altitude selection. To avoid unconstrained climbs or descents for any altitude transitions when using applicable vertical modes, the altitude select controller should be set to a new target altitude before the vertical mode can be selected. If the design allows the vertical mode to be selected before setting the target altitude, then consideration should be given to the potential vulnerability of unconstrained climb or descent leading to an altitude violation or controlled flight into terrain (CFIT). Consideration should also be given to appropriate annunciation of the deviation from previously selected altitude and/or subsequent required pilot action to reset the selected altitude.
b. Vertical speed. In the Vertical Speed mode, the FGS should smoothly acquire and maintain a selected vertical speed. Consideration should be given to the situation where the selected value is outside the performance capability of the airplane. Another situation to consider is use of Vertical Speed mode without autothrust. These situations could potentially lead to a low-speed or high-speed condition and corresponding pilot awareness vulnerabilities. For discussion of acceptable means of compliance for these situations, see Chapter 5, paragraph 57 of this AC.
c. Flightpath angle. In the Flightpath Angle mode, the FGS should smoothly acquire and maintain the selected flightpath angle. Consideration should be given to the situation where the selected value is outside the performance capability of the airplane. The use of Flightpath Angle mode without autothrust—potentially leading to a low-speed or high-speed condition and corresponding pilot awareness vulnerabilities—should also be considered. Acceptable means of compliance have included a reversion to an envelope protection mode or a timely annunciation of the situation.
d. Indicated airspeed/Mach hold (speed on elevator). In the Airspeed/Mach Hold mode, the FGS should maintain the airspeed or Mach at the time of engagement.
e. Indicated airspeed/Mach select (speed on elevator). In the Airspeed/Mach Select mode, the FGS should acquire and maintain a selected airspeed or Mach. The selected airspeed or Mach may be either pre-selected or synchronized to the airspeed or Mach at the time of engagement.
f. Flight level (FL) change (speed on elevator). In the FL Change mode, the FGS should change altitude in a coordinated way with thrust control on the airplane. The autopilot/FD will typically maintain speed control through the elevator. The autothrust function, if engaged, should control the thrust to the appropriate value for climb or descent.
g. Altitude capture.
(1) Mode transition. The Altitude Capture mode should command the FGS to transition from a vertical mode to smoothly capture and maintain the selected target altitude with consideration of the rates of climb and descent experienced in service.
(2) Guidelines for altitude capture mode. In-service experience has shown that certain implementations have the potential to cause pilot confusion that may lead to altitude violations. Accordingly, the following are guidelines for the Altitude Capture mode.
(a) Automatic arming of mode. The Altitude Capture mode should be automatically armed to ensure capture of the selected altitude. Annunciation of the armed status is not required if the Altitude Capture mode is armed at all times. If the FGS is in the Altitude Capture mode, it should be annunciated.
(b) Engagement from any vertical mode. The Altitude Capture mode should engage from any vertical mode if the computed flightpath will intercept the selected altitude and the altitude capture criteria are satisfied, except as specified during an approach (for example, when the glidepath for Approach mode is active).
(c) Changing climb/descent command references. Changes in the climb/descent command references (for example, the vertical speed command reference), with the exception of those made by the flightcrew using the altitude select controller, should not prevent capture of the target altitude.
(d) Capturing selected altitude. The Altitude Capture mode should smoothly capture the selected altitude, using an acceptable acceleration limit and pitch attitude with consideration for occupant comfort.
(e) Minimizing acceleration overshoot. The acceleration limit may, under certain conditions, result in an overshoot. To minimize the altitude overshoot, the normal acceleration limit may be increased, consistent with occupant safety.
(f) Selecting other vertical modes. Pilot selection of other vertical modes at the time of altitude capture should not prevent or adversely affect the level off at the target altitude. One means of compliance is to inhibit transition to other pilot-selectable vertical modes (except Altitude Hold, Go-Around, and Approach modes) during altitude capture, unless the target altitude is changed. If glidepath capture criteria are satisfied during altitude capture, then the FGS should transition to glidepath capture.
(g) Changing target altitude. The FGS must be designed to minimize flightcrew confusion concerning the FGS operation when the target altitude is changed during altitude capture. See § 25.1329(i). It should be suitably annunciated and appropriate for the phase of flight.
(h) Barometric pressure adjustment. Adjusting the datum pressure at any time during altitude capture should not result in loss of the capture mode. The transition to the pressure altitude should be accomplished smoothly.
(i) Maintaining reference airspeed. If the autothrust function is active during altitude capture, the autopilot and autothrust functions should be designed such that the FGS maintains the reference airspeed during the level-off maneuver. For example, if the autopilot changes from a speed mode to an altitude capture or control mode, then the autothrust should transition to a speed mode to maintain the reference airspeed.
h. Altitude hold.
(1) Entering mode. The Altitude Hold mode may be entered either by flightcrew selection or by transition from another vertical mode.
(a) Pilot selection.
1 Level flight. When initiated by pilot action in level flight, the Altitude Hold mode should provide guidance or control to maintain altitude at the time the mode is selected.
2 Climbing or descending. When initiated by pilot action when the airplane is either climbing or descending, the FGS should immediately initiate a pitch change to arrest the climb or descent and maintain the altitude when level flight (for example, less than 200 feet per minute) is reached. The intensity of the leveling maneuver should be consistent with occupant comfort and safety.
(b) Automatic transition. When initiated by an automatic transition from Altitude Capture, the Altitude Hold mode should provide guidance or control to the selected altitude.
(2) Mode transition annunciation. Automatic transition into the Altitude Hold mode from another vertical mode should be clearly annunciated for flightcrew awareness.
(3) Barometric pressure adjustment. Any airplane response due to an adjustment of the datum pressure should be smooth.
i. VNAV.
(1) FMS path.
(a) Acquire vertical path. In the VNAV mode, the FGS should acquire and maintain the vertical flightpath commanded by a flight management function (that is, FMS or equivalent). If the airplane is not on the desired FMS path when the VNAV mode is selected, the FGS VNAV mode should go into an armed state or provide guidance to smoothly acquire the FMS path. The flightcrew should establish the airplane on a flight profile to intercept the desired FMS path. The FGS should transition from the armed state to an engaged state at a point where the FGS can smoothly acquire and track the FMS path.
(b) Deviation from vertical path. If the aircraft is flying a vertical path (for example, VNAV path), then the deviation from that path should be displayed in the primary field of view, such as the PFD, navigation display (ND), or other acceptable display.
(2) Climb or descent.
(a) Autothrust function. When VNAV is selected for climb or descent, the autothrust function (if installed and engaged) should maintain the appropriate thrust setting.
When leveling after a VNAV climb or descent, the autothrust function should maintain the target speed.
(b) Preclude VNAV climb. The FGS should preclude a VNAV climb, unless the MSP altitude window is set to an altitude above the current altitude.
(c) Preclude VNAV descent. The FGS should preclude a VNAV descent, unless the MSP altitude window is set to an altitude below the current altitude, except when on a final approach to a runway.
(d) Preclude MSP altitude fly-through. The FGS should not allow the VNAV climb or descent to pass through a MSP altitude, except when on a final approach to a runway.
Note: See paragraph 67, Special Considerations for VNAV Approach Operations Related to Selecting a Target Altitude, of this AC.
65. Multiaxis Modes. This paragraph discusses modes that are implemented in many FGSs that are used in an integrated manner for pitch, lateral/directional control, and thrust management of the airplane. The criteria identified reflect operational experience gained from the use of these modes.
a. Takeoff mode.
(1) Vertical guidance. In the Takeoff mode, the vertical element of the FGS should provide vertical guidance to acquire and maintain a safe climb out speed after initial rotation for takeoff.
(2) Lateral guidance. In the Takeoff mode, the lateral element of the FGS, if implemented, should maintain runway heading/track or wings level after liftoff. A separate lateral mode annunciation should be provided.
(3) Rotation guidance. If rotation guidance is provided, the use of the guidance should not result in a tail strike. It should be consistent with takeoff methods necessary to meet takeoff performance requirements up to 35 feet above ground level (AGL). If no rotation guidance is provided, the pitch command bars may be displayed during takeoff roll but should not be considered as providing rotation guidance, unless it is part of the intended function.
(4) Autothrust. The autothrust function should increase and maintain engine thrust to the selected thrust limits (for example, full takeoff thrust, derate).
(5) Operation and performance. The FGS design should address all engine and engine-inoperative conditions consistent with the following takeoff system performance characteristics after liftoff.
(a) Transitions between flight phases. Takeoff system operation should be continuous and smooth through transition from the runway portion of the takeoff to the airborne portion and reconfiguration for en route climb. The pilot should be able to continue the use of the same primary display(s) for the airborne portion as for the runway portion. Changes in guidance modes and display formats should be automatic.
(b) Pitch attitude and climb speed/normal operation. The vertical axis guidance of the takeoff system during normal operation should result in the appropriate pitch attitude and climb speed for the airplane considering the following factors:
1 Normal rate rotation of the airplane to the commanded pitch attitude at VR (takeoff rotation speed) - 10 knots for all engines operative and VR - 5 knots for engine out should not result in a tail-strike.
2 The system should provide commands that lead the airplane to smoothly acquire a pitch attitude that results in capture and tracking of the all-engine takeoff climb speed, V2 (takeoff safety speed) + X. X is the all-engine speed additive from the AFM (normally 10 knots or higher). If pitch limited conditions are encountered, a higher climb airspeed may be used to achieve the required takeoff path without exceeding the pitch limit.
(c) Engine-out operation. For engine-out operation, the system should provide commands that lead the airplane to smoothly acquire a pitch attitude that results in capture and tracking of the following reference speeds:
1 V2, for engine failure at or below V2. This speed should be attained by the time the airplane has reached 35 feet altitude.
2 Airspeed at engine failure, for failures between V2 and V2 + X.
3 V2 + X, for failures at or above V2 + X. Alternatively, the airspeed at engine failure may be used, provided it has been shown that the minimum takeoff climb gradient can still be achieved at that speed.
(d) Lateral commands during takeoff mode. If implemented, the lateral element of the Takeoff mode should maintain runway heading/track or wings level after liftoff and a separate lateral mode annunciation should be provided.
b. Go-around mode. Characteristics of the Go-Around mode and resulting flightpath should be consistent with a manually flown go-around.
(1) Vertical elements. The vertical element of the FGS Go-Around mode should initially rotate the airplane or provide guidance to rotate the airplane to arrest the rate of descent.
(2) Speed. The FGS should acquire and maintain a safe speed during climbout and airplane configuration changes. Typically, a safe speed for go-around climb is V2, but a different speed may be found safe for wind shear recoveries. See AC 25-12.
(3) Autothrust.
(a) Thrust during climb. The autothrust function, if installed, should increase thrust and either maintain thrust to specific thrust limits or maintain thrust for an adequate, safe climb. The autothrust function should not exceed thrust limits (for example, full go-around thrust or derated go-around thrust limits). Furthermore, the autothrust function should not reduce
52 thrust for wind below the minimum value required for an adequate, safe climb or reduce the thrust lever position below a point that would cause a warning system to activate.
(b) Pitch attitude during go-around. The initial go-around maneuver may require a significant change in pitch attitude. It is acceptable to reduce thrust to lower the pitch attitude for comfort of the occupants when a safe climb gradient has been established. It should be possible for the pilot to re-select the full thrust value, if needed.
(4) Engagement. The Go-Around mode should engage when go-around is selected by the pilot, even if the MSP selected altitude is at or below the go-around initiation point. The airplane should climb until another vertical mode is selected or the MSP altitude is adjusted to an altitude above the present aircraft altitude.
(5) All engine and engine-out capability. The FGS design of the Go-Around mode should address all engine and engine-out operation. The design should consider an engine failure resulting in a go-around and the engine failure occurring during an all engine go-around.
c. Approach mode.
(1) Final approach path. In the Approach mode, the FGS should capture and track a final approach lateral and vertical path, if applicable, from a NAV or landing system, for example, ILS, GLS, RNP, area navigation (RNAV). See the current editions of AC 120-28 and AC 120-29.
(2) Mode annunciations. The FGS should annunciate all operationally relevant approach modes. Modes that are armed and waiting for capture criteria to be satisfied should be indicated in addition to the active precapture mode. A positive indication of the capture of the previously armed mode should be provided.
(3) Submodes. The FGS may have submodes that become active without additional crew selection. An assessment of the significance of these submode transitions to the flightcrew should be made. If assessed to be significant (for example, flare), positive annunciation of the transition should be provided.
(4) Mode engagement sequence. Glideslope capture mode engagement may occur prior to localizer capture. However, it is the flightcrew’s responsibility to ensure proper safe obstacle/terrain clearance when following vertical guidance when the airplane is not established on the final lateral path.
Note: Additional guidance and criteria are contained in AC 120-29 and AC 120-28.
66. Autothrust Modes. This paragraph discusses modes that are implemented in many FGSs that are used primarily for controlling the engines on the airplane. The criteria identified reflect operational experience gained from the use of these modes.
a. Thrust mode. In the Thrust mode, the FGS should command the autothrust function to achieve a selected target thrust value.
b. Speed mode. In the Speed mode, the FGS should command the autothrust function to acquire and maintain the selected target speed value, assuming that the selected speed is within the speed range of the normal flight envelope. The autothrust system may fly a higher airspeed than the selected target speed during an approach when operating in winds or turbulent conditions.
c. Retard mode. If a Retard mode is implemented in the FGS, it should work in the same manner for both automatic and manual landings when the autothrust function is engaged.
67. Special Considerations for VNAV Approach Operations Related to Selecting a Target Altitude.
a. Approach operations. The FGS vertical modes should allow the pilot to set the target altitude to a missed approach value prior to capturing the final approach segment. This should be possible for capturing from both above and below the final approach segment.
b. VNAV path operations. It should be possible to define a descent path to the final approach fix and another path from the final approach fix to the runway with the target altitude set for the missed approach altitude. Appropriate targets and descent points should be identified by the FMS.
68. Control Wheel Steering (Control Steering through the Autopilot).
a. General. In the control wheel steering (CWS) mode, the FGS allows the flightcrew to maneuver the airplane manually through the autopilot control path. CWS is considered an FGS mode, as it is a specific function of the FGS. However, during CWS operation, the pilot is in control of the aircraft, rather than the FGS. Operationally, CWS is identical to the pilot flying the airplane during manual flight. In both cases, the pilot is in control of the flightpath and speed of the airplane. The only difference is the mechanization of how the actual flight control surfaces are moved. No automatic FGS commands are involved during CWS operation, with the possible exception of automatic aircraft trim function(s). Therefore, paragraphs in this AC, such as those that discuss speed protection and performance objectives, should be applied only to those autopilot modes with which the FGS is in control of the flightpath of the airplane. Such paragraphs in this AC should not be applied to CWS, unless such features are specifically provided while the FGS is in the CWS mode.
b. Design. If provided, a CWS mode should meet the following:
(1) Control forces. It should be possible for the pilot to maneuver the airplane using the normal flight controls with the CWS mode engaged and to achieve the maximum available control surface deflection without using forces so high that the controllability requirements of § 25.143(d) are not met. Excessive discontinuities in control force that might adversely affect the flightpath should not be encountered.
(2) Bank and pitch attitudes. The maximum bank and pitch attitudes that can be achieved during CWS operations should be limited to those necessary for the normal operation of the airplane.
54 Note 1: Normal operational limits are typically 35 degrees in roll and +20 degrees to -10 degrees in pitch.
Note 2: Limits that can be physically overcome with forceful control commands by the pilot are sometimes referred to as “soft limits.”
(3) Control behavior. It should be possible to perform all normal maneuvers smoothly and accurately without nuisance oscillation. It should also be possible to counter all normal changes of trim due to change of configuration or power, within the range of flight conditions in which CWS may be used.
(4) Stall and stall recovery. The stall and stall recovery characteristics of the airplane should remain acceptable. It should be assumed that recovery from such a stall is made with CWS in use, unless automatic disengagement of the autopilot is provided.
(5) Adjustments to trim. In showing compliance with § 25.143(f), consideration should be given to adjustments to trim that may be made by the autopilot in the course of maneuvers that can reasonably be expected. Some alleviation may be acceptable in the case of unusually prolonged maneuvers, provided that the reduced control forces would not be potentially hazardous.
(6) Takeoff and landing. If the use of CWS for takeoff and landing is to be permitted, it should be shown that:
(a) Sufficient control, in both amplitude and rate, is available without encountering force discontinuities.
(b) Reasonable mishandling is not hazardous (for example, engaging the autopilot while the elevators or ailerons are held in an out-of-trim position).
(c) Runaway rates and control forces are such that the pilot can readily overpower the autopilot with no significant deviation in flightpath.
(d) Any lag in aircraft response induced by the CWS mode is acceptable for the intended maneuver.
(7) Automatic reversion to CWS mode. The autopilot, when engaged, should not automatically revert to the CWS mode by applying an input to the control column or wheel, unless the autopilot is in a capture mode (for example, Altitude Capture or Localizer Capture).
When the force is released, the autopilot should return to the previously engaged capture mode or to the Track/Heading Hold mode.
c. Terminology and application of advisory material. The term “CWS” is currently used by industry to refer to several different types of systems. Several other types of systems that are similar in nature to CWS, but functionally different from it, are described below.
Paragraph 68.b, Design, of this AC applies to systems as described in paragraph 68.a, General, of this AC. It does not apply to those systems described in paragraphs 68.c(1) and 68.c(2) below.
55 These descriptions are intended to be generic. Implementation may vary from airplane to airplane.
(1) Touch control steering (TCS). A TCS system is available on many business and commuter aircraft. With a TCS system, a pilot is able to physically disengage the autopilot servos from the flight control system, usually by pushing and holding a button on the control wheel, without causing the autopilot system itself to disengage or lose its currently selected modes. The pilot may then maneuver the airplane as desired using the aircraft’s flight control system (that is, the autopilot servos are not part of the control loop). The pilot is then able to reconnect the autopilot servos to the flight control system by releasing the TCS button. Using the new orientation of the aircraft as a basis, the autopilot will then reassume control of the airplane using the same mode selections as were present before the selection of TCS. This type of system on some aircraft is also sometimes referred to as CWS.
(2) Supervisory override. A function available on some aircraft is referred to as a “supervisory override” of an engaged autopilot. With this function, a pilot is able to physically overpower an engaged autopilot servo by applying force to the flight deck controls. With a supervisory override, the autopilot does not automatically disengage due to the pilot input. This allows the pilot to position the airplane as desired using the flight deck controls without first disengaging the autopilot. When the pilot releases the controls, the autopilot reassumes control of the airplane using the same mode selections as were present before the supervisory override.
69. Special Considerations for the Integration of Fly-by-Wire Flight Control Systems and FGS.
a. Speed protection. Speed protection features may be implemented in the fly-by-wire flight control system. However, if speed protection is also implemented within the FGS, it should be compatible with the envelope protection features of the fly-by-wire flight control system. The FGS speed protection (normal flight envelope) should operate to or within the limits of the flight control system (limit flight envelope).
b. System degradation. Information should be provided to the flightcrew about the impact on the FGS following degradation of the fly-by-wire flight control systems.
70—74 [RESERVED]
56 CHAPTER 7. FLIGHT GUIDANCE SYSTEM INTEGRATION
75. General. Throughout the preceding chapters of the document, FGSs and functions have been considered as being separate and distinct from other systems and functions on the aircraft. It is recognized that in complex aircraft designs, the flight guidance functions are closely integrated with other avionics functions and that the physical integration of these systems may have a bearing on how airplane level safety is assessed. The following paragraphs provide guidance on the likely FGS system integration issues found in more complex aircraft system designs and the interfaces that should be considered within the bounds of demonstrating the intended function, performance, and safety of the FGS.
76. System Integration Issues.
a. Common and cascading failure modes. Integration of other aircraft systems with the FGS has the potential of reducing the independence of failure effects and partitioning between functions. This is particularly the case where hardware and software resources are shared by different systems and functions (for example, aircraft data network and Integrated Modular Avionics (IMA) architectures). In addition to considering the reliability and integrity aspects of the FGS as a separate system, it may be necessary to address the effects of FGS failures with respect to fault propagation, detection, and isolation within other systems. The overall effect on the aircraft of a combination of individual system failure conditions occurring as a result of a common or cascade failure, may be more severe than the individual system effect.
For example, failure conditions classified under § 25.1309 as “minor” or “major” by themselves may have hazardous effects at the aircraft level when considered in combination. With regard to isolation of failures and particularly combination of failures, the ability of the alerting system to provide clear and unambiguous information to the flightcrew becomes of significant importance.
(See also Chapter 8, Safety Assessment, of this AC.)
b. Risk of error. Complex and highly integrated avionics systems present greater risk for development error. With nontraditional HMIs, there is also the potential for operational flightcrew errors. Moreover, integration of systems may result in a greater likelihood of undesirable and unintended effects.
c. System safety analysis. When credit is taken for shared resources or partitioning schemes, these should be justified and documented within the system safety analysis. When considering the functional failures of the system, where such partitioning schemes cannot be shown to provide the necessary isolation, possible combination failure modes should be taken into account. An example of this type of failure would be multiaxis active failures, where the control algorithms for more than one axis are hosted on a single processing element. Further, the functional integration of control functions such as control surface trimming, yaw channel, and stability augmentation, while not strictly FGS, should be considered.
77. Functional Interfaces.
a. Interfacing sensors. In its simplest form, the FGS may be considered as interfacing with sensors that provide the necessary inputs to enable computation of its various functions.
Typically, these sensors will include air and inertial data, engine control, and NAV sensors such
57 as ILS, Very high frequency Omnidirectional Range (VOR), and distance measuring equipment (DME). In the case of engine control, a feedback loop may also be provided. The FGS may also be considered as providing inner loop closure to outer loop commands. The most common interface is with the FMS, which provides targets for LNAV and VNAV in the form of steering orders.
b. Potential inconsistencies between systems. In demonstrating the intended function and performance of both the FGS and systems providing outer loop commands, the applicant needs to address potential inconsistencies between limits provided by the two different systems.
One example of this is possible conflicts between FMS steering commands with basic FGS pitch and bank angle limits. Failure to address these points can result in discontinuities, mode switching, and reversions, leading to erroneous NAV and other possible safety issues (for example, buffet margin at high altitude). Similar issues arise in the inner loop, across the functional interface between FGS and flight controls. In fly-by-wire aircraft, the loss of synchronization between the two can result in mode anomalies and autopilot disengagement.
c. Function and performance. The applicant should demonstrate the intended function and performance of the FGS across all possible functional interfaces. The alerting system should also be assessed to ensure that accurate and adequate information is provided to the flightcrew when dealing with failures across functional interfaces.
78—82 [RESERVED]
58 CHAPTER 8. SAFETY ASSESSMENT
83. General.
a. Basic safety requirements. Section 25.1309 defines the basic safety requirements for airworthiness approval of airplane systems and the current edition of AC 25.1309-1, System Design and Analysis, provides an acceptable means of demonstrating compliance with this rule.
This chapter provides additional guidance and interpretive material for the application of § 25.1309 to the approval of FGS.
b. Safety assessment. A safety assessment should be performed to identify the failure conditions, classify their hazard level according to the guidance of AC 25.1309-1, and establish that the failure conditions occur with a probability corresponding to the hazard classification or are mitigated as intended. The safety assessment should include the rationale and coverage of the FGS protection and monitoring philosophies employed. The safety assessment should include an appropriate evaluation of each of the identified FGS failure conditions and an analysis of the exposure to common mode/cause or cascade failure in accordance with AC 25.1309-1.
Additionally, the safety assessment should include justification and description of any functional partitioning schemes employed to reduce the effect/likelihood of failures of integrated components or functions.
c. Validation of safety assessment. There may be situations where the severity of the effect of a failure condition identified in the safety analysis needs to be validated. Laboratory, simulator, or flight test may accomplish the validation of the assessment. The test facility for each validation test should be chosen based on specific criteria, such as the difficulty of setting up the test conditions, potential hazard to an airplane and the test crew, and the validity of that validation test in that specific facility.
d. Coordination with the FAA. It is recommended that the safety analysis plan and the functional hazard assessment be coordinated with the FAA early in the certification program.
84. FGS Failure Conditions. One of the initial steps in establishing compliance with § 25.1309 for a system is to identify the failure conditions that are associated with that system.
The failure conditions are typically characterized by an undesired change in the intended function of the system. The failure condition should identify the functionality affected and the effect on the airplane and/or its occupants, specify any considerations relating to phase of flight, and identify any flightcrew action or other means of mitigation that are relevant.
a. Functionality. The primary functions of a FGS may include the following:
(1) Automatic control of the airplane’s flightpath utilizing the airplane’s aerodynamic control surfaces.
(2) Guidance provided to the flightcrew to achieve a particular desired flightpath or maneuver via information presented on a HUD or HDD system.
(3) Control of the thrust applied to the airplane.