Automatic Flight - Comments

The following comments are drawn from the
British Airways Short-haul Flight Training Study Guide

1.   The variety of possible Autopilot Flight Director System (AFDS) modes enables the 757 to be operated in a very flexible way. To avoid any possibility of error these modes must be thoroughly understood. The following notes are intended as amplification of information in the Operations Manual.

2.   The first point to be made is that mode arming or engagement can only be confirmed by reference to the Flight Mode Annunciation (FMA) panel at the top of the EADI. The Mode Control Panel (MCP) switch lights, when pressed and illuminated, merely indicate mode requests.

Autothrottle Modes

3.   To allow Autothrottle engagement, the Autothrottle switch must be at ARM. The Autothrottle can be engaged either by manual selection of the EPR or SPD switch lights, or automatically through selection of certain pitch modes.

4.   MANUAL. When engaged manually only two Autothrottle modes are available, SPD and EPR.

5.   AUTOMATIC. Automatic engagement of the Autothrottle will be discussed under the related pitch modes.

Roll Modes

6.   Seven pilot selected roll modes are available:

There are two sub-modes, Attitude Hold (ATT) and ROLLOUT.

Pitch Modes

7.   Seven pilot selected pitch modes are available:

There are two pitch sub-modes: ALT CAP and FLARE.

Single and Dual Function Pitch Modes

8.   There are two kinds of pitch modes:

9.   Moving from a dual function mode to a single function mode will transfer an engaged Autothrottle with it. And because a speed would already have been nominated, the Autothrottle mode that now shows engaged is SPD (with one rare exception covered later).

10.   But the rules governing the transfer of a designated speed are different for FLCH and for VNAV:

11.   Because of this, whenever ALT CAP occurs it is always good practice to check the speed that appears in the MCP speed window.

12.   Some additional points about interrelationship between modes can now be made, particularly between Autothrottle and pitch modes.

Interactions/inhibitions Associated With TO Thrust

13.   Autothrottle Speed Inhibition and Altitude Capture. There are two inhibitions associated with having TO selected on the TMSP:

  1. Autothrottle SPD mode cannot be engaged when TO is annunciated on EICAS. Inhibition of the SPD mode on take off prevents unpleasant complications if mistakenly SPD were to be selected instead of EPR at start of takeoff. So when EPR is engaged for takeoff (THR HOLD annunciated at 80 kts) the Autothrottle cannot transfer to Autothrottle SPD until CLB or other thrust limit is selected.

  2. While VNAV can be armed, it cannot engage while TO is still annunciated on EICAS.

14.   These two inhibitions can have implications in the procedures after acceleration altitude. The normal procedure after takeoff is straightforward at acceleration altitude select CLB thrust, VNAV, FLAPS 5 (or 1). But certain confusing events can occur if a low level off altitude is acquired after takeoff. If ALT CAP occurs while still in the TO pitch mode (flight director only) or VS (autopilot basic mode), the Autothrottle remains in the EPR mode. Then with EPR power set and the aircraft flying level, acceleration will be very rapid. This impending situation can be recognized however, when at acceleration altitude, VNAV may arm but refuse to engage, while ALT CAP shows in the EADI pitch mode annunciation. In this case to control aircraft speed check CLB (usually CLB2) thrust has been selected, then select the command airspeed bug to VREF+80 (or any desired speed) and manually engage Autothrottle SPD. (With TO still selected on the TMSP, Autothrottle SPD is inhibited and cannot be engaged.) For a recap refer to AFDS PROCEDURES AT acceleration altitude.

15.   Autothrottle Speed and Flight Level Change. FLCH is the pitch speed lock. It is important to remember this fact. Consequently you cannot select FLCH and Autothrottle SPD at the same time. If FLCH has been engaged and Autothrottle SPD is subsequently selected, FLCH disengages and the pitch mode reverts to the basic mode, VS.

16.   If ALT CAP takes place when in FLCH, Autothrottle SPD is automatically engaged (i.e. Autothrottle changes from EPR mode to SPD). This is logical - on reaching the selected altitude pitch demand changes from speed lock to ALT HOLD, hence the Autothrottle reverts to SPD hold. The speed that now appears in the MCP speed window is the actual aircraft speed at the moment of ALT CAP, not the previously selected speed. This may or may not be the same figure. Also beware the SPD mode inhibition if still in T/O thrust - in this case Autothrottle will remain in EPR.

17.   Autothrottle Re-Engagement. VNAV, FLCH and GA have a permanent command to engage autothrottle. In these dual function modes, if the Autothrottle is manually disengaged, re-engagement can be achieved by switching the Autothrottle ARM switch to OFF, then back to ARM. This rearms the Autothrottle circuit and the dual function mode circuitry will recapture the autothrottle.

18.   If Autothrottle is manually disengaged while in a single function pitch mode (e.g. VS, ALT HOLD or APP) it will be necessary to reselect the SPD or EPR button as required to reengage the autothrottle. The ARM switch does not need to be rearmed.

19.   Autothrottle Idle and Throttle Hold. We have already identified the circumstances when the Autothrottle is automatically engaged, ie with the Autothrottle ARM switch at ARM, whenever FLCH, VNAV or GA is engaged. In these automatic modes, the maximum thrust available is determined by the thrust limit selected (and announced on EICAS).

20.   When descending in FLCH or VNAV, IDLE is annunciated on the EADI. Provision is made for pilot adjustment of the rate of descent by a sub-mode (THR HOLD). If the throttles are adjusted while IDLE is annunciated, then power is removed from the Autothrottle and the throttles will remain where manually positioned. THR HOLD is annunciated to advise of this dormant state. This condition remains until the aircraft levels off, either in VNAV or by ALT CAP when Autothrottle SPD mode is automatically re-engaged.

21.   In a VNAV or FLCH descent, the dormant Autothrottle can be “woken up” if required by cycling the Autothrottle ARM switch OFF then back to ARM again. IDLE will again be annunciated and the throttles will close to the appropriate idle limit.

22.   THR HOLD is also annunciated if a CRZ DES is being made in VNAV. In a CRZ DES the pitch mode initially sets up a target rate of descent of 1000 fpm. while the Autothrottle controls speed to the previous cruise value. Once the rate of descent is approximately 1000 fpm, the pitch mode changes to a speed hold mode and the Autothrottle becomes dormant; THR HOLD is annunciated to invite the pilot to make any further throttle adjustments required.

Limitations to Engagement of LNAV and VNAV

23.   LNAV and VNAV put the AFDS under the control of the ACTIVE pages of the FMC, i.e. ACTIVE ROUTE and ACTIVE CLB, CRZ or DES pages. Notice the emphasis on ACTIVE. If the Route has not been activated or if any essential performance information is omitted during FMC initialization, then LNAV and VNAV will arm but cannot engage.

24.   VNAV cannot engage if TO thrust is still annunciated. The AFDS MCP VNAV switch light will illuminate if pressed, to indicate a mode request, but VNAV will not engage, and VNAV will only annunciate as armed in the EADI.

Note: It is for this reason that the procedure on takeoff when reaching acceleration altitude is select CLB power, then VNAV.

25.   If, when climbing or descending in VNAV, altitude intervention occurs (i.e. the selected altitude in the MCP is more restrictive than the altitude entered in the FMC) the AFDS automatically disengages VNAV and returns pitch and speed control back to the autopilot/flight director MCP. The pitch mode reverts to ALT CAP, then ALT HOLD, while the Autothrottle changes from FMC computed EPR or IDLE to Autothrottle SPD mode, holding existing speed.

Note: The FMC speed is not transferred to the Autothrottle speed mode, what appears in the MCP window is the actual aircraft speed at the moment of ALT CAP, and it is this speed that the Autothrottle will now hold.

26.   If the altitude entered in the FMC is more restrictive than the altitude selected on the MCP, the resulting level off can be very subtle. There is no white altimeter altitude alert light and VNAV remains annunciated in the EADI as the rate of climb or descent reduces to zero. There are two clues to the VNAV altitude capture however the Autothrottle mode annunciation on the EADI changes from EPR or IDLE to SPD, while if the aircraft was climbing the thrust limit annunciated on EICAS changes from CLB to CRZ.

Summary

[British Airways Short-haul Flight Training Study Guide]

EICAS Messages