L-1011 Autoflight: VOR Arm, Capture and Track (22-29)

The usual heading guidance prior to initial VOR capture is provided by the Heading Select mode. Notice during pre-capture guidance that a pitch mode is engaged.

The capture of a VOR radial is similar to all other radio beam captures in that it is done in three steps: (1) arm, (2) capture and (3) track; with only the first two steps being annunciated. When VOR capture occurs, VOR ARM changes to VOR on both AFCS MODE annunciators. During capture, a maximum bank angle of 28º may be obtained at any true airspeedl it is not limited as a function of true airspeed as it is in the Heading Select mode. Track status is achieved when the bank command becomes 5º or less. Subsequent bank commands will not exceed 10º during VOR track status. There is no annunciation change to indicated track status.

L-1011 Autoflight: Altitude Capture (22-27)

In the example above (sorry for the poor image quality) our aircraft is using vertical speed as pre-capture guidance for altitude select. Notice at the bottom of the climb that VS was selected and is engaged and the aircraft is climbing at 800 feet per minute. The selected altitude is 5,000 feet, ALT SELECT has been selected by dialing in the new altitude and the computers have been armed.

At capture, the VS light would extinguish. Because of the different closure rates the capture point changes. At a vertical speed of one thousand feet per minute, the capture point would be approximately two hundred and sixteen feet, or approximately 10 seconds from the selected altitude. A smooth, asymptotic capture is possible because the computer senses the altitude error and the rate of closure on the selected altitude, and initiates the required pitch maneuver sufficiently in advance to perform the capture maneuver without imposing an excessive change in the flight load factor (g).

At eight feet from the selected altitude, track is established and the ALT light will illuminate and the ALT ARM message on the AFCS modes annunciations will change to ALT.

An automatic capture can also be made in CWS, provided that a flight director is on (the system cannot be armed in CWS alone), but the capture will not be asymptotic unless flight director guidance is followed. In a "hands off" FD/CWS capture, the capture maneuvering is not inititated until the the selected altitude is passed through, so that there will be some overshoot. On reaching the selected altitude, CWS/FD is the same as WMD or CMD/FD in that the APFDS automatically engages the ALT Hold mode.

The Altitude Select knob performs two services simultaneously. Not only does it select an altitude to be captured, but it also selects the reference altitude for the altitude alert system which will be discussed later.

The desired altitude can be selected and the mode armed for capture at any time, even before takeoff. It will remain armed until the altitude selected is being traced, or until it is disarmed by a second actuation of the ARM switch, by selection of TURB, by selection of a difference altitude, or by switching off both flight directors and both autopilots. Interim altitudes can be captured, held, and released by appropriate actuation of the ALT switch without disarming the ALT Select mode. When this id done, there will be simultaneous annunciation of the Altitude Hold and Altitude Select modes.

L-1011 Autoflight: Altitude Select Mode (22-26)

Altitude select mode provides the capability of selecting a desired altitude to be captured. It can be used for either autopilot control or flight director guidance and is compatible with any roll axis mode.

Engagement of altitude select is accomplished by entering the desired altitude in the altitude select window using the altitude select knob. Below 10,000 feet a twenty-foot change in altitude will automatically arm the altitude select mode. Above 10,000 feet, a 100-foot altitude change is required. Any altitude between 0 and 59,000 feet may be selected.

Verification of mode engagement is accomplished by observing the ALT ARM message on both AFCS MODES annunciators.

The mode may be manually released by disengaging both flight directors and disconnecting the autopilot from the command (CMD) configuration. It will be automatically released if a required interlock function is lost or, if after initiation of the altitude capture maneuver, a superseding pitch axis mode is selected.

If an autopilot is not engaged in CMD, and the ALT Select Flight Director Guidance Commands are followed, the altitude capture maneuver will be asymptotic, with subsequent automatic engagement of the ALT Hold mode for flight director guidance only.

L-1011 Autoflight: Altitude Hold Overshoot (22-25)

Altitude hold is a command mode which provides automatic control of barometric altitude. When altitude hold is engaged, the aircraft will maintain the selected altitude with one autopilot engaged in command or control wheel steering, and provides flight director guidance to maintain altitude when in manual control with a flight director engaged.

Altitude hold maintains the selected altitude by means of pitch control and therefore is not compatible with any other pitch mode. Altitude hold is compatible with the autothrottle system and with all heading and navigation modes until glideslope capture occurs.

The barometric pressure reference for the altitude hold mode is the instantaneous pressure existing at the time the ALT switch is pressed or the mode is engaged. When the aircraft is climbing or descending at the time of manual engagement, an overshoot will occur but the airplane will return to and capture the engagement altitude whether in command or control wheel steering. Any flight director guidance mode can be engaged while in control wheel steering.

L-1011 Autoflight: Vertical Speed Engagement (22-24)

In this example, the aircraft is climbing at 1,000 feet per minute. The vertical speed window indicates 1,000 feet per minute and the thumb-wheel has no effect. When vertical speed is selected by pressing the "VS" switch, the VS light will illuminate indicating mode selection. Remember, verification of mode selection con only be made by checking for the VS message displayed in both AFCS MODES annunciators. The vertical speed thumb-wheel is now active and is adjusted to a new vertical speed of 800 feet of climb per minute. The aircraft will automatically pitch over and maintain a vertical speed of 800 feet per minute.

L-1011 Autoflight: Vertical Speed Limits (22-23)

Maximum vertical speed climb command is 4,000 feet per minute and maximum descent command is 8,000 feet per minute.

If the existing climb or descent is in excess of the maximum vertical speed command values upon engagement, the flight director (if engaged) will command a return to the maximum command value. If the autopilot is in command, there will be an automatic, smooth return to the maximum command value. Either situation is highly unlikely to happen in normal operation.

L-1011 Autoflight:: Engage-Lever CMD (22-22)

An autopilot may be engaged in command at any time after takeoff, however, only one autopilot may be in command at a time unless the approach/land (A/L) mode is selected.

When an engage lever is placed to command and no modes are selected, the Aircraft will respond exactly as if control wheel steering (CWS) had been engaged. Command and control wheel steering are the same until a pitch or navigation mode is selected.

When only a pitch command mode is engaged, roll will still be control wheel steering. This is also true with only a roll mode engaged. Remember, in order to have pitch and roll operate in command, both a pitch and roll mode must be engaged.

Automatic pitch trim is provided during operation in all of the pitch axis modes when either autopilot is engaged in the basic (CWS) or command (CMD) configurations. The automatic trim system acts to relieve any load on the autopilot servers to prevent transients when the autopilot is either manually or automatically disengaged. There are two automatic pitch trim systems and at least one must be operative in order to engage either autopilot. A manually controlled electric pitch trim system is also provided; however; this function is disabled when either autopilot channel is engaged, except when a fore or aft force is applied to the control wheel during CWS operation. Manual trim inputs can then be used to reduce the force, as desired.

L-1011 Autoflight: Turbulence Mode (22-20)

Turbulence mode provides pilot-assisted, constant attitude flight through turbulent air. The autopilot control authority is reduced in order to soften its response to attitude changes caused by the turbulence.

Turbulence is an autopilot mode only and cannot be selected unless an autopilot is engaged. The TURB mode may be selected any time either autopilot is in CWS or CMD and A/L TRACK or GO AROUND is not established. The TURB light will illuminated indicating mode selection. Verification of mode engagement is accomplished by checking for TURB legend on in both AFCS MODES annunciators. When turbulence is selected while the autopilot engage lever is in the CMD position, the solenoid held lever drops from CMD to CWS. This action essentially cancels all other modes.

No other mode can be selected while the turbulence mode is engaged. The turbulence mode can only be released by pressing the TURB switch a second time or by disengaging the autopilot. Either of these actions then, is a prerequisite to resuming normal autopilot-controlled flight after a turbulence encounter.

If a flight director is also on when turbulence is selected, its command bars will be biased out of view but no FD warning flag will appear. When TURB is canceled, the command bars will return if the FD switch was left on but it will have a new reference. The new reference is the attitude existing at the time of TURB cancellation since all previous modes were canceled when TURB was selected.

L-1011 Autoflight: Proportional Pitch Trim (22-19)

Proportional pitch trim is active during control wheel steering operation but only while pressure is being applied to the control wheel. When pressure is released, the auto trim will operate. It is not necessary to use the trim but it more closely approximates the feel of the manual airplane.

Proportional pitch trim is also active on the ground when control wheel steering is selected and a pressure of four pounds is applied to the control wheel while trimming.

L-1011 Autoflight: Control Wheel Steering Heading Hold (22-18)

If the bank angle is less than 3 degrees at the time CWS is engaged, the wings will roll level and the existing heading would be maintained.

L-1011 Autoflight: CWS Pitch and Bank Limits (22-17)

When an autopilot is engaged in CWS, the airplane will maintain the existing pitch attitude if it is less than 18 degrees and the existing bank angle if it is less than 35 degrees. If pitch or roll are more than 18 degrees respectively, the autopilot will return the airplane to these limits and then hold it there.

Therefore, pitch and bank limits can be manually exceeded through the control wheel. When control wheel pressure is released, the aircraft will automatically return to the maximum limits.

L-1011 Autoflight: Control Wheel Steering (22-16)

The two basic functions of control wheel steering are attitude hold and heading hold.

Control wheel steering allows the airplane to be hand flown with normal airplane feel. The airplane will remain where the pilot puts it as long as it is within the control wheel steering limits. That is, if you move the controls with a control pressure greater than four pounds, the airplane responds. When you release pressure on the controls, the airplane stays where you put it.

L-1011 Autoflight: DUAL A/L NOT AVAIL LIGHT (22-15)

The DUAL A/L NOT AVAIL light (A/L .. Autoland), located on the Pilot's caution and warning panel, serves as a cruise monitor that forewarns of the unavailability of dual A/L whenever it detects a failure that may preclude achievement of dual A/L track status. Its activation is not dependent upon prior selection of A/L. Its purpose is to allow ample time to refile if the destination forecast would require dual A/L.

This warning can be reset and will remain out until the landing roll, when it will be recalled automatically as the airspeed passes through 90 knots. The recall serves as a reminder for postflight logbook entry and as an advisory message for maintenance personnel and for the next flight crew, if not corrected.

L-1011 Autoflight: Autopilot Warnings (22-14)

The AFCS WARNING annunciators show changes in autopilot status resulting from failures detected by the autopilot monitors. The example above shows an alert created be the autopilot state transition from "engaged" to "disconnected".

L-1011 Autoflight: AFCS Warning (22-13)

An AFCS WARNING annunciator is located at the top of each Pilot's instrument panel. It gives error and warning information regarding the autoflight system.

L-1011 Autoflight: Verification of Command Mode Engagement (22-12)

Each time a command mode is selected, both autopilot computers will respond by displaying the selected mode on the respective AFCS MODES annunciator. Each autopilot computer has an AFCS MODES annunciator located on the main instrument panel. In the example above, autopilot A is in the command position and vertical speed has been selected as a pitch command mode. The "VS" light on the Pitch Panel indicates mode selection, verification of both autopilot computers operating in the "VS" pitch mode is accomplished by observing the "VS" message in both AFCS MODES annunciators.

"VS" is also displayed int he PITCH Panel window (digital autopilot system only) along with the actual vertical with the actual vertical speed at the time of engagement.

L-1011 Autoflight: AFCS Mode Annunciators (22-11)

Verification of mode engagement is accomplished by observing the message on both AFCS MODES annunciators. An AFCS MODES annunciator is located on the top of each pilot's instrument panel.

L-1011 Autoflight: Control Wheel Steering (22-9)

There are two basic modes of operation that an autopilot can be engaged in, control wheel steering (CWS) or command (CMD mode. In control wheel steering, the autopilot receives its pitch and roll inputs from the control wheel. The aircraft will remain where you put it. If you move the control wheel with a force of four pounds or more, the aircraft will respond. When you release the controls, the aircraft stays where you put it. Remember, only one autopilot can be engaged at a time.

When an engage lever is moved to CWS, it is held in that position electrically after the autopilot/flight director computer determines that the system is working properly. Anytime the signals are not valid, the lever will automatically drop to the OFF position.

L-1011 Autoflight: Flight Director Command Bars (22-8)

As you already know, each flight director switch connects its respective autopilot/flight director computer to command bars on the same side. Autopilot A to the Captain's command bars, autopilot B to the First Officer's command bars. Either or both flight directors may be selected simultaneously and are independent of autopilot engagement. With the flight director switch off, the command bars are biased out of view as shown on the First Officer's side. Placing the flight director switch to ON will bring the command bars into view as shown on the Captain's side.

In the pictured example:

Autopilot/flight director computer A will now provide steering information to the Captain since both autopilot autopilot levers are off, but the FD for the Captain's side is on. With Autopilot lever A on, autopilot A would fly the aircraft in pitch and roll and the flight director command bars would indicate to the Captain what the autopilot is doing.

L-1011 Autoflight: Engage Levers (22-7)

Autopilot A has an FD switch to engage the Captain's flight director (APFD Engage Panel) and a lever to engage one side of the dual pitch and roll autopilot servos. Both the Captain's flight director and one side of the autopilot servos can be driven by autopilot/flight director computer A. The First Officer's side does the same thing with autopilot B.

Either autopilot is capable of flying the aircraft in pitch and roll. Yaw is automatic in pitch and roll and will be discussed in a later blog posting.

Both flight directors may be on at the same time, each using individual autopilot computers. Only one autopilot may be engaged at a time. The second autopilot is electro-mechanically locked in the OFF position. A normal configuration is to have one autopilot engaged and both flight directors on. The only exception for both autopilots to be engaged is when the approach/land mode is engaged.