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.
Does this mean that if the ALT HOLD switch is pressed, the aircraft will hold the current altitude, not the altitude in the altitude select window?
ReplyDeleteAnd that if the aircraft is holding a captured altitude, changing the altitude in the altitude select window will have no effect until an alternative pitch mode such as V/S, IAS or MACH is selected?
That is correct George. The Altitude Select Arm is used for "capture" and will cause the aircraft to level out once the selected altitude is reached. However, simply dialing an altitude into the Alt Select window and arming it will not cause the aircraft to pitch up or down. The pilot would have to pitch the aircraft and/or use the V/S mode to "fly" the aircraft towards the selected altitude. Once the selected altitude is reached the ALT ARM light goes out and the auto flight system switches from V/S mode to ALT HOLD mode. Later versions of the APFDS included a "VNAV" mode that allowed for pitch instructions to be sent to the autopilot form the FMS ... however, the -1 model did not have that.
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