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.