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.
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.
At 80 kts on takeoff the Autothrottle enters an unpowered condition known as Throttle Hold (THR HOLD) to guard against Autothrottle runaway. It remains in this mode until a thrust limit other than TO is selected.
Note 1: If ALT CAP occurs while THR HOLD is annunciated, the Autothrottle will remain in EPR mode at takeoff thrust.
Note 2: VNAV can be armed but will not engage while TO thrust is annunciated on EICAS.
When SPD is engaged, the Autothrottle will attempt to control aircraft speed at the value selected in the MCP speed window. The maximum power available in the SPD mode is determined by the current selection on the TMSP (ie GA, CLB, CLB1, CLB2, CONT or CRZ).
5. AUTOMATIC. Automatic engagement of the Autothrottle will be discussed under the related pitch modes.
6. Seven pilot selected roll modes are available:
There are two sub-modes, Attitude Hold (ATT) and ROLLOUT.
Heading Hold commands the wing to roll level, then maintains the resulting heading.
If an autopilot is engaged after takeoff with TO annunciated, the autopilot will engage in HDG HOLD if bank angle is less than 5°.
If the bank angle exceeds 5° when the autopilot is first engaged, Attitude hold (a sub-mode) is automatically engaged. ATT mode holds the existing bank angle (maximum 30°) until another mode is selected.
The same rule applies when the flight director or autopilot is selected on in flight with no FCS modes already annunciated.
In Heading Select mode the AFDS will remember the turn direction only when HDG SEL is already engaged. If the Heading Selector is turned first and then HDG SEL engaged, the AFDS will turn the aircraft the shorter way around.
Once HDG SEL is engaged and the direction of turn established, the heading cursor can be taken past the tail of the aircraft and the current direction of turn will continue.
Localizer permits capture of an ILS localizer beam only. It does not enable capture of a VOR radial - there is no facility for the AFDS to be directly coupled to a VOR radial as in most previous aircraft.
Related to the LOC (and APP) is the ILS course selector at the rear of the center control stand. The importance of selecting the correct ILS QDM must be stressed. The ILS deviation pointers on the EADI are versed to give back course indications when the aircraft is tracking more than 90° to the ILS QDM selected. This course selector controls the beam bar course on the ILS display of the EHSI. It does not affect the ILS localizer deviation display on the EADI unless the course selector is grossly miss-set when BACK COURSE indications may still remain on an intercept heading. Nor does it affect the MAP display. However, miss-selection of the course selector seriously compromises the ability of the autopilot/flight director to correctly establish on the localizer. The results can be extremely confusing with the EADI and MAP displays showing one thing and the autopilot / flight director attempting to achieve something else! So if AFDS demand seems inconsistent with intercepting the localizer beam - check the ILS course selector.
A sub-mode of APP, Rollout provides runway centerline control. It engages at 5 ft RA and replaces the LOC annunciation. The localizer centerline is maintained using autopilot control of the rudder and nosewheel steering. Rollout mode remains engaged until the autopilots are disengaged.
Note: The drift applied is that calculated on initial engagement; drift is not corrected as the aircraft climbs.
7. Seven pilot selected pitch modes are available:
There are two pitch sub-modes: ALT CAP and FLARE.
TO mode can only be selected on the ground (automatically by switching both FDs ON). Pitch commands are fixed at 8° until rotation, when the demand increases to hold a speed of V2+15 on initial climb out, or, if an engine fails, existing airspeed with a minimum of V2 and maximum V2+15. TO mode is a FD mode only. If an autopilot is engaged prior to selecting another mode, the autopilot will engage in VS, not TO.
Note: The Takeoff mode is not a takeoff pitch rate director. The aircraft must be rotated at the correct rate to the initial rotation attitude. Then follow the flight director to hold the appropriate speed.
In Altitude Hold mode, the AFDS commands the altitude existing when the switch is pushed. Pushing the switch a second time re-datum's again to the new existing altitude. This feature can be useful to prevent a "bunt" if the aircraft has a high vertical speed when the ALT HOLD button is first pushed.
Altitude capture is a sub-mode of ALT HOLD. The performance of the aircraft is such that ALT CAP can be annunciated several thousand feet before the selected altitude. Once ALT CAP is annunciated the Flight Control Computers will provide demands to level the aircraft at the altitude selected in the ALTITUDE window at the moment of ALT CAP, even if this figure is subsequently changed. ALT CAP announces that the selected altitude is in the process of being acquired, that the previous pitch mode has been disengaged, and that the pitch attitude is being controlled to achieve a smooth altitude capture.
Note: Aircraft speed is not necessarily being controlled. This possible lack of speed control can have undesirable results if the autopilot is engaged and the pitch attitude is such that there is a rapid speed trend. It is for this reason that while the autopilot is certificated to be engaged above 500 feet after takeoff, the aircraft should be stabilized on the desired speed before selecting an autopilot to CMD. If the speed trend is unacceptable, two options are available:
A note of caution when using Vertical Speed. This is a pitch lock and although the Autothrottle will attempt to control airspeed within the power range available, when very high rates of climb or descent are selected this is obviously not possible. Secondly, unlike VNAV and FLCH, when in VS it is possible to fly away from the altitude selected in the Altitude window, depriving the crew of ALT CAP protection and in some circumstances, of the white ALT ALERT lights.
APP (GS annunciated)
Approach mode is used only where a full ILS (Localizer and Glideslope) is available. The control laws for glide path capture assume the aircraft is flying level in VNAV or ALT HOLD. In this case pitch demand will maintain level flight until the center of the glide path is reached. If, however, the aircraft is still descending while approaching the glide path from either above or below, when it enters the glide path capture range (1/3 dot), the Flight Control Computers will interpret this as a glide path deviation and may demand a sharp dive or pull up to correct. This could be avoided by using LOC to establish on the localizer beam and not arming APP until in ALT CAP or ALT HOLD or actually on the glide path. However, do not hesitate to arm APP early, if LOC and GS capture appear to be about to coincide with and ALT CAP
Flight Level Change is a pitch speed lock, enabling the pilot to climb or descend at a selected speed. This is why in FLCH the EADI pitch mode annunciation shows SPD. Additionally FLCH commands automatic engagement of Autothrottle in the EPR mode (provided the Autothrottle ARM switch is at ARM). After comparing present aircraft altitude with the value selected in the ALT window, either maximum EPR thrust (as selected in the TMSP) or IDLE thrust is applied by the Autothrottle system. If the difference between existing altitude and selected altitude is small, the Autothrottle may apply less than full EPR or IDLE power. The Autothrottle mode annunciation in the EADI will show FLCH (in a climb) or IDLE (in a descent).
Note: EADI pitch annunciation shows VNAV SPD during climb and VNAV PTH during cruise or descent. The Autothrottle annunciation in VNAV shows EPR in the climb, SPD in the cruise and IDLE on commencing descent.
In VNAV descent, the AFDS attempts to follow a descent path calculated by the FMC prior to top of descent and several assumptions are made about idle thrust, a wind that decreases with altitude, and linear increase in temperature and pressure. If the aircraft speed increases beyond a certain limit during descent, the pitch mode will change from VNAV PTH to VNAV SPD and the aircraft will leave the VNAV path. Pilot action can be taken to increase drag and regain the path, whereupon the pitch mode will automatically recapture VNAV PTH. Or the pilot can limit his action to monitoring the divergence from the path as frequently after a period of time the path is regained without any pilot action being required.
GoAround mode works in a similar way to FLCH. In GA, after the initial rotation a pitch demand is given to climb out at the higher of achieved airspeed or the command bug speed (i.e. approach speed). Autothrottle is automatically engaged in GA thrust, however once the rate of climb reaches 2000 ft/min. the throttles retard to limit the climb to this figure.
Note: If the command airspeed is selected to a higher figure (e.g. VREF+80), pitch demand will seek to increase speed to the new datum by reducing pitch attitude. This feature is used in the GA procedure after acceleration altitude to accelerate the aircraft.
8. There are two kinds of pitch modes:
Dual Function Pitch Modes. Of the seven pitch modes, three (VNAV, FLCH and GA) have a permanent command to engage Autothrottle. They are, therefore, dual function modes. In VNAV, the command airspeed is obtained from the FMC. In FLCH and GA the commanded airspeed is that shown in the MCP speed window; during climb and descent the pitch mode is controlling airspeed and Autothrottle will be in EPR (or one of its sub-modes IDLE or THR HOLD).
Single Function Pitch Modes. VS, ALT HOLD, TO, GS (and FLARE) are single function modes. An Autothrottle mode can be separately engaged by the pilot as well but these single function pitch modes will not themselves demand or automatically engage the Autothrottle.
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:
Coming out of FLCH, where the climb or descent speed is displayed in the MCP speed window, that speed is retained when ALT CAP occurs or alternatively VS or ALT HOLD ie selected.
Coming out of VNAV, the existing aircraft speed is always transferred, not the VNAV target speed.
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.
13. Autothrottle Speed Inhibition and Altitude Capture. There are two inhibitions associated with having TO selected on the TMSP:
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.
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.
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.
Single Function Modes: VS, ALT HOLD, APP and TO
Dual Function Modes: VNAV, FLCH and GA
If the first pitch mode to be selected is a single function mode Autothrottle will not engage automatically (Pilot input is required to select Autothrottle in EPR or SPD as required).
Engaging a dual function mode picks up Autothrottle EPR or SPD, whichever that mode requires (The actual Autothrottle mode is then annunciated on the EADI).
Transfer from a dual function mode to a single function pitch mode takes Autothrottle into SPD mode (unless Autothrottle SPD is inhibited by TO thrust selection in which case Autothrottle remains EPR).
With TO selected in the TMSP, Autothrottle SPD cannot be engaged, nor can VNAV.
[British Airways Short-haul Flight Training Study Guide]