Automatic Flight - AFDS

Amendment: Boeing Flight Operations Technical Bulletin 757-74 (AFDS Mode Control Panel Faults) included.

Introduction

The Autoflight System (AFS) consists of the:

• Autopilot Flight Director System (AFDS)

• Autothrottle system (A/T)

The Mode Control Panel (MCP) and Flight Management Computer (FMC) control the AFDS and, with the Thrust Mode Select Panel (TMSP), the Autothrottle system to perform climb, cruise and descent.

Autopilot Flight Director System

The AFDS consists of three Flight Control Computers (FCCs) and the MCP.

The MCP provides control of the autopilot, flight director, Altitude Alert, and Autothrottle systems. The MCP selects and activates AFDS modes, and establishes altitudes, speeds, and climb/descent profiles.

Flight Control Computers

The three FCCs, left, centre, and right, control separate hydraulically powered autopilot control servos to operate flight controls. The autopilot controls ailerons and elevators. Rudder commands are added only during a multiple autopilot approach, landing and rollout. Nose wheel steering is also added during rollout from an automatic landing. During an ILS approach with all three autopilots engaged, separate electrical sources power the three FCCs.

Boeing On certain aircraft, the Captains altimeter settings are followed irrespective of autopilot selection. For further information see the Air Data section.

Autopilot engagement requires at least two operable FCCs. Commands from the two FCCs are compared to prevent one FCC from commanding an autopilot hardover. The left AC bus normally powers the left and centre FCCs except during Autoland operations after Bus Isolation occurs. When the electrical busses are isolated for an automatic Landing, the centre FCC receives power from the Hot Battery Bus and Standby Inverter. If Left AC Bus power is lost inflight, the left and centre FCCs will be unpowered. With only the right FCC powered, no autopilots can be engaged; however, the right flight director will operate.

The FCCs also provide inputs for AFDS operating mode displays and Flight Director commands on the EADI.

MCP Switches

Mode Control Panel
[click on an area for more information]

MCP switches select automatic flight control and flight director modes. A light in the lower half of the switch illuminates to indicate the mode is armed or active. Respective roll and pitch flight mode annunciations on the FMA will also display to indicate mode engagement. Autothrottle modes are discussed later in this section.

Most modes activate with a single push. These modes include:

• Flight level change (FLCH).
• Heading hold (HDG HOLD).
• Heading select (HDG SEL).

• Vertical speed (V/S).
• Altitude hold (ALT HOLD).

Other modes arm or activate with a single push. These modes are:

• Lateral navigation (LNAV).
• Vertical navigation (VNAV).
• Localiser only (LOC).

• ILS Approach (APP).
• As installed - Back course localiser (B/CRS).
• As installed - Control wheel steering (CWS).

All modes deactivate by disengaging the Autopilot and turning both Flight Directors off. After localiser and glideslope capture, the localiser and glideslope modes can only be deactivated by disengaging the autopilot and turning both flight directors off or by selecting GA mode. When armed, VNAV, LNAV, LOC, B/CRS and APP modes can be disarmed by pushing the mode switch a second time.

Desired target values can be selected on the MCP for:

• Airspeed.
• Mach.
• Heading.
• Vertical speed.
• Altitude.

All parameters except vertical speed can be preselected before autopilot and/or flight director engagement.

Autopilot Engagement

Autopilot engagement requires at least two FCCs and pushing one of the MCP autopilot engage switches.

Autopilot Disengagement

Normal autopilot disengagement is through either control wheel autopilot disengage switch.

As installed
The autopilots can also be disengaged by the MCP autopilot disengage bar.

The A/P DISC light illuminates and the EICAS warning message AUTOPILOT DISC displays when the autopilot has been manually or automatically disconnected.

AFDS Failures

During single autopilot operation, failures affecting the active mode annunciate on the FMA. If the failure affects only the active mode:

• The autopilot remains engaged in an attitude-stabilising mode.
• An amber line is drawn through the mode annunciation.
• The AUTO PILOT light illuminates.
• The EICAS caution message AUTOPILOT displays.

If unwanted operation is noticed or when an autopilot failure is annunciated the autopilot should be disconnected and the airplane flown manually.

Failures affecting all autopilot modes result in an autopilot disengagement accompanied by an aural warning. Depending on the system failure, it may be possible to re-engage an autopilot by pushing the autopilot engage switch.

Flight Director Display

Flight director steering indications normally display any time the related F/D switch is ON.

The steering indications are also displayed when the related flight director switch is OFF and a go-around switch is pushed, if airspeed is greater than 80 knots and the flaps are not retracted. In this case, the flight director display can be removed by cycling the respective flight director switch on and then off.

A flight director failure in either pitch or roll causes the respective steering bars to disappear. If both axes become unusable, both command bars disappear.

AFDS Flight Mode Annunciations

Flight mode annunciations are displayed on the EADI. Mode annunciations from left to right are:

Autothrottle

Pitch

Roll

AFDS status

Active modes display at the top of the flight mode annunciation boxes in large green letters. Armed modes (except for TO on the ground and GA in flight) display in smaller white letters at the bottom of the fight mode annunciator boxes. When a mode changes, the new active mode is highlighted with a green rectangle around it for several seconds.

Autothrottle Modes

FMA Autothrottle annunciations are:

• EPR - Autothrottle controlling to the selected EPR reference thrust.

• SPD - Autothrottle controlling thrust to maintain speed selected in IAS/MACH or, if VNAV engaged, the speed as programmed by the FMC.

• IDLE - Autothrottle is reducing or has reduced thrust to idle. It may engage in a VNAV descent. It will, after FLARE is engaged.

• THR HOLD - Thrust levers remain in existing position or where manually placed. The Autothrottle servos are de-energised.

• FLCH - Autothrottle controlling to a maximum of the selected mode reference thrust during climb, and to a minimum thrust during descent.

• GA - Autothrottle controlling to a maximum reference thrust to maintain a climb rate of at least 2000 fpm. If both flight directors and the autopilot are off, Autothrottle controls to go-around reference thrust subject to flap and VMO limit speeds.

Roll Modes

Roll annunciations are:

LNAV Arm LNAV by pushing the LNAV switch. The light illuminates and LNAV annunciates on the FMA roll mode annunciator in white characters below the current roll mode. LNAV (armed) - LNAV is armed to activate when parameters are met. LNAV (active) - LNAV activates when in position to turn onto the active route leg. In flight, selection causes immediate activation if within 2½ nm of the active leg. HDG HDG SEL (active) - airplane turns to or maintains the heading selected in the MCP heading window. HDG HOLD (active) - AFDS holds present heading. When turning, AFDS holds the heading reached after rolling wings level. ATT ATT (active) - when the autopilot is first engaged or the flight director is first turned on in flight, AFDS holds a bank angle between 5 and 30 degrees and will not roll to wings level. When the bank angle is less than 5 degrees, AFDS returns to wings level HDG HOLD. When the bank angle is greater than 30 degrees, AFDS returns to 30 degrees of bank. LOC LOC (armed). AFDS captures the localiser when within range and within 120 degrees of the localiser course. LOC (active). AFDS follows the localiser course. B/CRS (As Installed) B/CRS (armed). AFDS is armed to capture and track inbound on back course of localiser after localiser capture. B/CRS (active). AFDS tracks inbound on back course of localiser. TO On the ground, TO annunciates by selecting either F/D switch ON when both flight directors are OFF TO roll and pitch guidance become active at lift-off. GA In flight, go-around arms with flaps out of up or at glideslope capture. There is no flight mode annunciation for go-around armed in flight; although the reference thrust limit changes to GA. Go-around is activated in flight by pushing a GA switch. The roll steering indication provides guidance to maintain the ground track present at mode engagement. Pushing either GA switch: Automatic arming occurs with glideslope capture or extension of wing flaps. Engages AFDS and Autothrottle in GA mode if previously armed. Provides windshear guidance during GA mode if previously armed. If Flight Director is OFF, activates Flight Director in GA mode. Cancels all thrust de-rates if selected after take-off. ROLLOUT ROLLOUT (armed). Displays below 1500 feet Radio Altitude and activates below 5 feet. ROLLOUT (active). After touchdown, AFDS uses rudder and nose wheel steering to steer the airplane on the localiser centreline.

Pitch Modes

Pitch annunciations are:

TO On the ground, TO annunciates by selecting either F/D switch ON when both flight directors are OFF. The flight director pitch bar indicates an initial pitch of approximately eight degrees up. After takeoff, the AFDS commands a pitch attitude to maintain: Pitch command greater of V2 + 15 or lift-off speed + 15. If current airspeed remains above the target speed for 5 seconds, target airspeed resets to current airspeed, to a maximum of V2 + 25 knots. IAS/MACH window speed if IAS/MACH window speed is changed to a speed greater than the target speed. Note: AFDS uses the speed set in the IAS/MACH window prior to takeoff for V2. GA After the reference thrust limit changes from takeoff to climb, GA arms whenever: flaps are out of up or at glideslope capture When a go-around is initiated the commanded speed is the MCP IAS/MACH window or current airspeed, whichever is higher. If the airspeed increases and remains above the initial target airspeed for five seconds, target airspeed resets to current airspeed to a maximum of the IAS/MACH window speed plus 25 knots. If airspeed at initiation of go-around is greater than IAS/MACH window plus 25 knots, that speed is maintained. GA displays as the reference thrust limit on the primary EICAS engine display. VNAV Arm VNAV by pushing the VNAV switch. The light illuminates and VNAV annunciates on the FMA pitch mode annunciator in white characters below the current pitch mode. VNAV provides pitch commands to maintain the FMC computed airspeed/path: VNAV SPD (active) - the AFDS maintains the FMC speed displayed on the EADI and/or the CDU CLIMB or DESCENT pages. During speed intervention, use the MCP IAS/MACH selector to manually set the speed. VNAV PTH (active) - the AFDS maintains FMC altitude or descent path with pitch commands. For a non-entered headwind, thrust may increase to maintain the VNAV descent path. If the MCP altitude window is set to the current cruise altitude as the airplane approaches the top of descent, the CDU scratchpad message RESET MCP ALT displays. When a VNAV descent is initiated before the top of descent (T/D) and the airplane descent path subsequently intercepts the VNAV descent path, the pitch annunciation changes from VNAV SPD to VNAV PTH. V/S Pushing the V/S switch opens the vertical speed window and displays the current vertical speed. It also opens the IAS/MACH window (if blanked). Pitch commands maintain the rate of climb or descent selected in the V/S window. SPD Pushing the SPD switch opens the IAS/MACH window (if blanked). Pitch commands maintain IAS/MACH window airspeed or Mach. ALT CAP A transition manoeuvre entered automatically from a V/S, FLCH, or VNAV climb or descent to selected MCP altitude. Engages but does not annunciate during VNAV transition. ALT HOLD Altitude Hold mode is activated by: Pushing the MCP altitude HOLD switch or Capturing the selected altitude from a V/S or FLCH climb or descent G/S Autopilot flight director system follows the ILS glideslope. FLARE FLARE (armed) - during Autoland, FLARE displays below 1500 feet Radio Altitude. FLARE (active) - during Autoland, flare activates at 50 feet Radio Altitude. FLARE deactivates at touchdown and the nose wheel smoothly lowers to the runway.

ASA