Safety Investigation Report 2018:1 Factual Information/1.6/1.6.8 Aircraft Systems/2
SAFETY INVESTIGATION REPORT MH370 (9M-MRO)
2) Autopilot Flight Director System
The autopilot is engaged by operation of either of two A/P pushbutton switches on the Mode Control Panel (MCP) located on the glareshield panel (Figure 1.6C [below]). Once engaged the autopilot can control the aircraft in various modes selected on the MCP. Normal autopilot disengagement is through either control wheel autopilot disengage switch. The autopilot can disengage if the flight crew override an autopilot command through the use of the control column, control wheel or rudder pedals (when the yaw axis is engaged for approach).
Figure 1.6C - Autopilot Mode Control Panel
The autopilot can also be disengaged by pulling down on the A/P Disengage Bar on the MCP. The autopilot will also disengage automatically for failures of systems on which it relies upon for specific operations. The Autopilot Flight Director System (AFDS) consists of three Autopilot Flight Director Computers (AFDCs), one MCP, and six backdrive actuators (one each for the Captain’s and First Officer’s control column, control wheel, and rudder pedals). The left and right 28V DC buses power the left and right AFDCs, respectively and the MCP while the 28V DC battery bus powers the centre AFDC.
Emergency power from the Ram Air Turbine (RAT) generator does not power these busses and as a result the autopilot will not function with RAT electrical power.
- a) Take-off Mode
The Take-off (TO/GA) mode controls roll and pitch during take-off. Also, the Thrust Management Computing Function (TMCF) controls thrust during take-off. Turning a flight director on while the aircraft is on the ground, or activating either TO/GA switch while on the ground, will engage Take-off mode.
- b) Roll Modes
The following AFDS roll modes are available during climb, cruise and descent (Figure 1.6D [below]):
- i) Lateral Navigation
Pushing the Lateral Navigation (LNAV) switch arms or disarms the LNAV mode. The commands come from the active Flight Management Computing Function (FMCF) when there is a valid navigation data base and an active flight plan.
- ii) Heading Hold/Track Hold
Pushing the Heading Hold (HDG HOLD)/Track hold (TRK HOLD) switch selects Heading or Track hold. In this mode, the aircraft holds either heading (HDG) or track (TRK). If the HDG/TRK display on the MCP shows TRK, the aircraft holds track. If the HDG/TRK display on the MCP shows HDG, the aircraft holds heading.
Figure 1.6D - Lateral Mode Switches and Indicators
- iii) Heading Select/Track Select
Pushing the Heading Select (HDG SEL)/Track Select (TRK SEL) switch (inner) selects Heading Select or Track Select modes. In this mode, the aircraft turns to the heading or track that shows in the heading/track window. Pushing the Heading/Track (HDG/TRK) Reference switch alternately changes the heading/track reference between heading and track. Rotating the Heading/Track selector (middle) sets the heading or track in the heading/track window. If the HDG/TRK display shows HDG, the aircraft goes to and holds the heading that shows in the heading/track window. If the HDG/TRK display shows TRK, the aircraft goes to and holds the track that shows in the heading/track window. Rotating the Bank Limit selector (outer) sets the bank limit when in the Heading Select or Track Select modes. In the AUTO position, the limit varies between 15 - 25°, depending on True Airspeed. When the other detented positions are selected, the value is the maximum, regardless of airspeed.
- iv) Roll Attitude Hold
The Roll Attitude Hold mode is used to hold the roll attitude that exists at the time the flight director is first turned on, or the autopilot is first engaged. The Roll Attitude Hold mode is activated, and ATT annunciated, if the bank angle is greater than 5 degrees when either:
- A flight director is turned on with the autopilot not engaged; or
- The autopilot is initially engaged with no flight director on.
- i) Lateral Navigation
- c) Pitch Modes
The following AFDS pitch modes are available during climb, cruise and descent (Figure 1.6E [below]):
- i) Vertical Navigation
Pushing the vertical navigation (VNAV) switch arms or disarms the VNAV mode. In this mode, the AFDS uses vertical steering commands provided by the Flight Management Computer Function (FMCF). The FMCF vertical steering commands come from the active FMCF based on the navigation data and the active flight plan.
- ii) Vertical Speed/Flight Path Angle
Pushing the Vertical Speed/Flight Path Angle (V/S-FPA) switch selects the V/S or FPA mode. Rotating the V/S-FPA selector Up or Down sets the vertical speed or flight path angle in the vertical speed/flight path angle window. Pushing the V/S-FPA Reference switch alternately changes vertical speed/flight path angle window references between vertical speed and flight path angle. The vertical speed or flight path angle command is an elevator command. The pilot uses this mode to change flight levels. The pilot must set the engine thrust necessary to hold the vertical speed or flight path angle command. When the V/S/FPA display shows V/S, the aircraft goes to and holds the vertical speed that shows on the vertical speed/flight path angle window.
Figure 1.6E - Vertical Mode Switches and Indicators
- iii) Flight Level Change
Pushing the Flight Level Change (FLCH) switch selects the FLCH mode. In this mode, the AFDS will control to the speed target in the IAS/MACH window, providing climb and descent guidance and control. FLCH mode may be used with autothrottles, or with manual throttle control. When the IAS/MACH display shows IAS, the elevator command holds the speed that shows on the IAS/MACH window. When the IAS/MACH display shows MACH, the elevator command holds the MACH that shows on the IAS/MACH window. Rotating the IAS/MACH selector sets the speed in the IAS/MACH window. Pushing the IAS/MACH Reference switch alternately changes the IAS/MACH window between IAS and MACH. The Thrust Management Computing Function (TMCF) supplies the engine thrust commands.
- iv) Altitude Hold
Pushing the Altitude Hold (ALT) switch selects the Altitude hold mode. In this mode, the aircraft holds the barometric altitude present when the pilot pushes the altitude HOLD switch. Altitude Capture and Hold can also be engaged from a climb or descent as the aircraft approaches the altitude that is selected and displayed in the altitude window.
- i) Vertical Navigation
- d) Landing Modes
The following AFDS functions are available for landing:
- i) Localizer
The Localizer (LOC) mode captures and holds the aircraft to a localizer flight path. - ii) Glideslope
The Glideslope (G/S) mode captures and holds the aircraft to a vertical descent flight path. - iii) Flare
The flare (FLARE) mode controls the aircraft to a smooth touchdown at a point past the glideslope antenna. This is a computed command and is not part of the glideslope mode. - iv) Runway Alignment
In crosswind conditions, the runway alignment mode supplies roll and yaw control to decrease the aircraft crab angle for touchdown. The runway alignment mode also includes roll and yaw control for an engine failure in approach during autoland. - v) Rollout
After touchdown, the rollout (ROLLOUT) mode controls the aircraft to the runway centre line. Aircraft deviation from the localizer centre line supplies rudder and nose wheel steering signals. - vi) Go-Around
The go-around (TO/GA) mode controls roll and pitch after an aborted approach. Also, the TMCF controls thrust during go- around.
Pushing the LOC switch arms or disarms the localizer as roll mode. Pushing the Approach (APP) switch arms or disarms the localizer as roll mode and G/S as pitch mode (Figure 1.6F [below]).
Figure 1.6F - Approach Mode Switches
- i) Localizer
- e) Autothrottle (Thrust Management Computing Function)
The autothrottle (A/T) commands the thrust levers to achieve an engine thrust setting, or a selected airspeed. The A/T is armed by raising one or both A/T Arm switches, and is engaged by a pushbutton switch on the MCP (Figure 1.6G [below]).
During normal flight operations, the flight crew uses the Thrust Management Computing Function (TMCF) to perform several routine or normal operations and tasks. These operations or tasks relate to autothrottle modes. The A/T modes operate in these flight phases:
Figure 1.6G - Autothrottle Switches
- Take-off (TO)
- Climb (CLB)
- Cruise (CRZ)
- Descent (DES)
- Approach (APP)
- Go-around (GA)
Autothrottle thrust mode annunciations relate to pitch mode annunciations on the Primary Flight Display (PFD).
- f) Autothrottle Modes
- i) Take-off
In take-off (TO), the autothrottle controls thrust to the TO thrust limit. The autothrottle mode annunciation on the PFD is thrust reference (THR REF). At a threshold air speed, the autothrottle mode annunciation on the PFD changes to HOLD.
- ii) Climb
These are the three autothrottle mode selections in climb (CLB):
These are the autothrottle mode annunciations for these modes:
- THR REF when VNAV engages
- THR when FLCH engages
- SPD or THR REF when autothrottle mode engages.
The autothrottle speed mode only engages when VNAV, FLCH, and TO/GA are not active, and the aircraft is in the air.
- iii) Cruise
In cruise, the pitch mode could be VNAV PTH, VNAV ALT or MCP ALT; the corresponding A/T mode is SPD.
- iv) Descent
These are the three autothrottle modes in descent (DES):
- VNAV
- FLCH
- Autothrottle speed mode
These are the autothrottle mode annunciations in descent:
- v) Approach
SPD is normal mode in approach with glideslope active or in a manual approach (APP).
- Go-Around
A go-around (GA) mode request causes the autothrottle mode to change to THR. A second GA request causes the autothrottle mode to change to THR REF. The TO/GA switch must be pushed to request GA. - Flare Retard
Flare retard occurs when a specified altitude threshold has been achieved when in SPD mode, or during an Autoland approach with a command from the autopilot flight director system (AFDS). The autothrottle mode changes to IDLE during a flare retard.
- Go-Around
- vi) Autothrottle Disconnect
The autothrottle disconnects when there is a manual autothrottle disconnect or when there is thrust reverser application. This occurs after initial touchdown during rollout. The autothrottle will disconnect automatically for certain system faults.