L-1011 Autoflight: Altitude Alert (22-46)

The altitude alert lights are located on the Captain's and First Officer's instrument panel. When the aircraft is within 750 ft. of the selected altitude the steady amber ALTITUDE light will illuminate and an audio C-Cord alert will sound.

When the aircraft is within 25 ft. of the selected altitude, the altitude light will go out. The altitude light will remain extinguished as long as the aircraft remains within 250 ft. of the selected altitude.

If the aircraft leaves a selected altitude without resetting he altitude warning, you will get a flashing altitude light and audio alert when the aircraft departs more than 250ft from the assigned altitude.

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.

Original Flight Hardware: Altitude Select Panel

The Altitude Select panel is part of the auto pilot system on the Lockheed L-1011. The panel is located next to the NAV Mode Panel on the first officer side of the autopilot. There are a number of selector elements on the panel:

TEST .... Altitude Alert System Test Switch
COMPUTATION CHANNEL SELECTOR ... has a setting of NORM and STBY for the altitude computation system.
ALT PRESELECT SYSTEM ARMING SWITCH ... pressing momentary switch the first time arms APFDS for automatic capture of selected altitude.
ALTITUDE SELECTOR CONTROL KNOB ... provides selection of the target altitude.



Panel Location