The variety of possible autopilot / flight director system (AFDS) modes means that the 757 can be operated in a very flexible way. To avoid any possibility of error these modes must be thoroughly understood. The following information is intended as an amplification of information already contained in your company technical manual.

The first point to be made is that the AFDS mode control panel switchlights, when pressed and illuminated, merely indicate mode requests. Modes which are armed or engaged can only be confirmed by reference to the ADI mode annunciations.

To allow autothrottle engagement, the A/T switch must be at ARM. The autothrottle can be engaged either by manual selection of the EPR or SPEED switchlights, or automatically through selection of certain pitch modes.

When engaged manually two autothrottle modes are available, EPR and SPD.

When EPR mode is engaged the autothrottle will open the thrust levers to the maximum limit currently selected on the Thrust Mode Select Panel. EPR mode cannot be engaged on the ground unless TO is indicated on the Thrust Mode Select Panel. At 80 knots on take-off the autothrottle enters an unpowered condition known as Throttle Hold (THR HOLD annunciated) to guard against auto throttle runaway. It remains in this mode untila different thrust limit (other than TO) is selected.

When SPD mode is engaged, the autothrottle will attempt to control aircraft speed to the value selected in the speed window. The maximum power available in the SPD mode is determined by the current selection on the Thrust Mode Select Panel.

Automatic engagement of the autothrottle is discussed under the related pitch modes.

Seven pilot selected roll modes are available:

 > Heading Hold (HDG HOLD)
 > Heading Select (HDG SEL)
 > Lateral Navigation (LNAV)
 > Localiser Capture (LOC)
 > Approach (APP)
 > Take-off (TO)
 > Go-around (GA)
 
 There are two "sub-modes", attitude hold (ATT) and Rollout mode (ROLLOUT).

On pushing the Heading Hold button the AFDS commands the wings to roll level, and then maintains the resultant heading. HDG HOLD appears on each ADI. It does not maintain the heading one is turning though on selection, as the resultant heading after the wings roll level will usually differ slightly.

If an autopilot is engaged after take-off with TO still annunciated, the autopilot will engage in ATT mode (or HDG HOLD mode if bank is less than 5 degrees).

A "sub-mode", attitude hold (ATT) is automatically engaged if the bank angle exceeds 5 degrees when the F/D or autopilot is first engaged (i.e. no previous mode selected). ATT mode holds the existing bank angle (maximum 30 degrees) until another mode is selected.

Rotating the Heading Selector sets a heading in the Heading Window, and appropriately positions the selected Heading Marker on both HSIs.

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 is engaged, the AFDS will turn the aircraft the shortest way round to the selected heading. Once the heading select mode is engaged and the direction of turn is established the heading cursor can be taken past the tail of the aircraft and the current direction of turn will continue.

Pushing the LNAV button arms or engages the AFDS in LNAV mode. This mode permits the FMC to control the roll channel and so follow the active route. If the aircraft is more than 2.5nm from the active route, LNAV appears white (armed) on each ADI. Previously engaged roll modes remain active. If the aircraft is not on a heading which will intercept the active leg of the FMC route, the FMC light illuminates, and the FMC MSG lights illuminate and "NOT ON INTERCEPT HEADING" appears in the scratch pad of both CDUs.

If the aircraft is at or within 2.5nm or turning radius from the active route, LNAV appears green (engaged) on each ADI. The AFDS then follows the active route.

LNAV mode is disengaged by selecting HDG HOLD or HDG SEL mode, or when localiser caputure occurs.

Pushing the LOC switch arms or engages the AFDS in LOC mode. LOC appears white (armed) on each ADI prior to localiser capture. The AFDS is armed to capture and track inbound on the ILS front course. The capture point will vary based on range and intercept angle.

LOC capture will not occur if the intercept angle exceeds 120 degrees.

Initial roll modes such as LNAV, HDG SEL or HDG HOLD will remain engaged until LOC capture. G/S capture will not occur in this mode.

A second push of the LOC button prior to LOC capture will cancel the arming.

Note that LOC 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 many traditional aircraft.

Related to LOC (and APP) is the ILS course selector at the rear of the centre control stand. The importance of selecting the correct ILS QDM must be stressed. Mis-setting of the course selector seriously compromises the ability of the autopilot / flight director to correcly establish on the localiser. The results can be extremely confusing with the ADI and MAP displays showing one thing and the autopilot / flight director attempting to acheive something else! If the AFDS demands seem inconsistent with intercepting the localiser beam, check the ILS course selector.

Approach mode combines localiser capture with glideslope capture to permit ILS approach.

A "sub-mode" of Approach, Rollout provides runway centreline control. It engages at 5ft RA and replaces LOC annunciation. The centreline of the localiser is maintained using autopilot control of the rudder and nosewheel steering. Rollout mode remains until the aircraft stops or the autopilots are disengaged.

TO and GA are track hold modes and available only during initial climb or go-around. Note that by delaying the selection of a roll mode after take-off, the aircraft will maintain the runway centreline. This can be very useful on occasion.

Seven pilot selected pitch modes are available:

 > Take-off (TO)
 > Altitude Hold (ALT HOLD)
 > Vertical Speed (VS)
 > Approach (GS annunciated)
 > Vertical Navigation (VNAV)
 > Fight level change (SPD annunciated)
 > Go-around (GA)

There are two "sub-modes", Altitude Capture (ALT CAP) and Flare mode (FLARE).

TO mode can only be selected on the ground (automatically by tuning on both F/D's). Pitch commands indicate 8 degrees nose-up until rotation then the demand increases to hold a speed of V2+15 on initial climb-out, or if an engine fails the existing airspeed with a minimum of V2 and a maximum of V2+15. TO mode is a F/D mode only. If an autopilot is engaged prior to reaching Aa, the autopilot will engage in VS, not TO. Note that the take-off mode is not a take-off pitch director. The aircraft must be rotated at the correct rate to the initial rotation attitude, then the Flight Director commands can be followed to hold the appropriate speed.

On pushing the Alitude Hold switch, ALT HOLD appears on both ADIs and the AFDS maintains the altitude at which the switch is pushed. Altitude Capture is a sub-mode of Altitude 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.

Aircraft speed is not necessarily being controlled. This possible lack of speed control can have undesireable results if the autopilot is engaged and the pitch attitude is such that there is a rapid speed trend. If the speed trend is unacceptable, there is no recourse but to disengage the autopilot. It is for this reason that while the autopilot is certified to be engaged above 500ft after take-off, the aircraft should be stabilised on the desired speed before selecting an autopilot to CMD.

The Vertical Speed Selector, which rotates up and down, sets the desired vertical speed in the Verical Speed Window. Pusing the Vertical Speed Switch selects AFDS Vertical Speed mode, and V/S appears on each ADI. This causes the existing verical speed to be displayed in the Verical Speed Window. VS can be engaged without selecting a new altitude; when a selected altitude is reached, automatic level-off occurs. The AFDS pitch will maintain the vertical speed diplayed in the Vertical Speed Window. If the AFDS is engaged in V/S from FL CH or VNAV, the A/T automatically engages in SPD mode if armed.

VS is a pitch lock and requires caution when used. Although the autothrottle will attempt to control airspeed within the power available, when very high rates of climb or descent are selected this is obviously not possible. Secondly, unlike VNAV and FL CH, when in VS mode it is possible to fly away from the altitude selected in the ALT select window, depriving the crew of ALT CAP protection and in some cirumstances, of the white ALT ALERT lights.

Pushing the APP swtich arms or engages AFDS in APP mode. LOC and G/S appear white (armed) on each ADI prior to localiser and glideslope capture. LOC and G/S appear green (engaged) on each ADI when capture occurs respectively. Either localiser or glideslope can be captured first.

APP mode allows automatic engagement of other autopilot channels on an ILS LAND 3 approach. Single, dual or triple autopilot operation is permitted.

G/S capture can occur only when the intercept track angle is within 80 degrees of the LOC front course.

Approach mode is used only where a full ILS localiser and glideslope is available. A single autopilot can operate only one stabiliser trim motor. Both trim motors are desireable on approach but particularly during an automatic go-around; otherwise pitch trim occurs at half speed. Boeing therefore designed the autopilot such that with an autopilot engaged selecting APP also armed the other two autopilots for automatic engagement at 1,500ft RA.

The control laws for glideslope capture assume the aircraft is flying level in VNAV or ALT HOLD. In this case pitch demand will maintain level flight until the centre 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 deflection), the flight control computer will intepret this as a glide path deviation and may demand a sharp dive or pull-up to correct. This should be avoided by using LOC to establish on the localiser beam and not arming APP until in ALT CAP or ALT HOLD or actually on the glideslope. However, do not hesitate to arm APP early if LOC and GS capture appear to be about to coincide with an ALT CAP.

VNAV combines the features of both ALT HOLD (and A/T SPD) and FL CH modes under the control of the FMC, so VNAV can climb, cruise or descend the aircraft. Note ADI pitch annunciation shows VNAV continuously while in this mode but the A/T annunciation above shows:

 > EPR in the climb
 > SPD in the cruise
 > IDLE on commencing descent, then THR HOLD

This is the pitch speed lock, enabling the pilot to climb or descend at a selected speed. Additionally FL CH commands automatic engagement of autothrottle in the EPR mode (provided the A/T ARM switch is at ARM). After comparing present altitude with the value selected in the ALT window, either MAX EPR thrust (as selected in the Thrust Mode Select Panel) or IDLE thrust is applied by the autothrottle system. Note that in FL CH the ADI pitch mode annunciation shows SPD while the A/T mode annunciation above it shows FL CH in a climb and FL CH followed by THR HOLD in a descent.

GA mode works in a similar way to FL CH. In GA, after the initial rotation a pitch demand is given to climb at the higher of achieved airspeed or the commanded bug speed (i.e. approach speed). Autothrottle is automatically engaged in GA thrust, however once the rate of climb exceeds 2,000fpm the throttles retard to limit the climb rate to this figure. Note that if the commanded airspeed is selected to a higher figure (e.g. VRef+80), pitch demand will seek to increase speed to the new datum. This feature is used in the GA procedure after Aa to accelerate the aircraft.

Effectively, there are two kinds of pitch mode. Each of the seven pitch modes serves either a single or dual function.

Single function modes are:

 > VS
 > ALT HOLD
 > TO
 > GS (and FLARE)

These are often referred to as "Standard" modes. They can accept an autothrottle mode being engaged as well, but they will not themselves demand the autothrottle.

Dual function modes are:

 > VNAV
 > FL CH
 > GA

These are often referred to as "Smart" modes. They have a permanent command to engage autothrottle. In VNAV the commanded airspeed is obtained from the FMC. In FL CH and GA, the commanded airspeed is that shown in the MCP window. In all modes except VNAV CRZ the pitch mode is controlling airspeed and the autothrottle will be in EPR.

Moving from a "Smart" mode to a "Standard" mode will transfer an engaged autothrottle with it. Because a speed has already been nominated the autothrottle mode that now shows engaged is SPD. There is one rare exception which is disussed in the following section on Altitude Capture.

Autothrottle SPD mode cannot be engaged when TO is selected on the Thrust Mode Select Panel. Inhibition of the speed mode on take-off prevents unpleasant complications in case SPD were to be selected instead of EPR at the start of the take-off. When EPR is engaged for take-off (THR HOLD annunciated at 80 knots) the autothrottle cannot transfer to autothrottle SPD until CLB or another thrust limit is selected. As will be seen below, this has implications in the procedures after Aa.

The normal procedure after take-off is straightforward - At Aa select VS, Flap 5 and Climb Thrust. If ALT CAP occurs while still in the TO pitch mode or VS, the autothrottle remains in EPR mode. Remember these pitch modes are standard or single function modes and send no signals to the autothrottle system. With EPR power set and the aircraft flying level, acceleration would be very rapid. This impending situation can be recognised when, at Aa, VS refuses to engage while ALT CAP shows in the ADI pitch mode annunciation. In this case to control aircraft speed it is necessary to select the command airspeed bug to VRef + 80 (or any desired speed) and engage autothrottle SPD, but first complete the initial Aa procedure (Flaps 5, climb thrust). Otherwise with TO still selected on the Thrust Mode Select Panel, autothrottle SPD is inhibited and cannot be engaged.

Flight level change (FL CH) is the autopilot speed lock. It is important to remember this fact as consequently you cannot select FL CH and auto throttle SPD at the same time. If FL CH has been selected and autothrottle SPD is engaged FL CH disengages and the pitch mode reverts to V/S.

If ALT CAP takes place when in FL CH, autothrottle SPD is automatically engaged (i.e. autothrottle annunciation from FL CH (climb) or THR HOLD (descent) becomes SPD). This is logical, on reaching the selected altitude pitch demand changes from speed lock to altitude hold, hence autothrottle reverts to SPD hold ("Smart" mode to "Standard" mode). The speed that now appears in the MCP SPD window is the actual aircraft speed at the moment ALT CAP occurs, not the previously selected speed. This may or may not be the same figure. Also beware of the SPD mode inhibition while in TO thrust, in this case the autothrottle will remain in EPR.

VNAV, FL CH and GA have a permanent command to engage autothrottle. In these "Smart" 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 re-arms the autothrottle circuit and the "Smart" mode circuitry will re-capture the autothrottle.

If autothrottle is manually disengaged while in a "Standard" pitch mode (e.g. VS, ALT HOLD, APP) it will be necessary to re-select the SPD or EPR button as required to re-engage the autothrottle. The ARM switch does not need to be cycled.

The circumstances in which the autothrottle is automatically engaged have already been discussed (i.e. with the autothrottle ARM switch at ARM, whenever FL CH, VNAV or GA are engaged). In these automatic modes, the maximum thrust available is determined by the thrust limit selected (and annunciated on EICAS). When descending in VNAV, IDLE is annunciated on the ADI. Provision is made for pilot adjustment of the rate of descent by a sub-mode (THR HOLD). If the throttles are manually adjusted when IDLE is annunciated, then power is removed from the autothrottle and it will remain in the same position. THR HOLD is annunciated to advise of this dormant state.

When descending in FL CH, FL CH is initially annunciated, this then changes to THR HOLD once the throttles are closed.

In a VNAV or FL CH descent, the dormant autothrottle can be "woken up" if required by cycling the autothrottle ARM switch. IDLE will again be annunciated and the throttles close.

LNAV and VNAV put the AFDS under the control of the "ACTIVE" pages of the FMC. That is to say the "Active" route and "Active" climb, cruise or descent page. This differentiates "Active" pages from others which provide information but are not currently being used to control the aircraft.

VNAV cannot be engaged if TO thrust is still annunciated (that is because VNAV could not operate in CRZ if autothrottle SPD were still to be inhibited). The AFDS mode control panel switchlight will illuminate if pressed to indicate a mode request, but VNAV will not engage, nor will VNAV be annunciated on the ADI.

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 mode control panel. 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 that the FMC speed is not transferred to the autothrottle speed mode, what appears in the MCP speed window is the actual aircraft speed at the moment of ALT CAP, and this is the speed the autothrottle will subsequently hold.

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

These are the "Standard" pitch modes:

 > VS
 > ALT HOLD
 > TO
 > GS (and FLARE)

These are the "Smart" pitch modes:

 > VNAV
 > FL CH
 > GA

If the first pitch mode to be selected is a "Standard" mode, autothrottle will not engage automatically. Pilot input is required to select autothrottle in EPR or SPD as required.

Engaging a "Smart" mode picks up autothrottle EPR or SPD, whichever the "Smart" mode requires. The actual autothrottle mode is then annunciated on the ADI.

Transfer from a "Smart" to a "Standard" pitch mode takes autothrottle into SPD mode (unless autothrottle SPD mode is inhibited by TO thrust selection in which case the autothrottle remains at EPR).

With TO selected in the Thrust Mode Select Panel, autothrottle SPD cannot be engaged, nor can VNAV.

The flexibility of the AFDS modes often gives the pilot more than one option to achieve a desired end. In order to enhance crew co-operation and to avoid any possibility of confusion at critical times certain standard procedures are followed at Aa after take-off or after go-around.