Quote:
Originally Posted by Randall Stevens
To expand on my last question, if the altitude is changed, what determines how quickly the autopilot will reach that altitude? Is it just a standard rate of ascent/descent? Is that how they estimated that the altitude of 100' was selected by the co-pilot?
To change altitude, the pilot first selects the altitude on the Mode Control Panel. The airplane will not being a climb or descent until the pilot selects the type of climb or descent, either VNAV, IAS, or VS (nomenclature may vary from one type of aircraft to another, but the including flight logic is similar).
VNAV (Vertical NAVigation) will cause the plane to descend according to a programmed vertical profile in the flight management system. Unlikely that this mode was used as it will respect altitude limits on the flight plan and wouldn't achieve the rapid descent we saw in this case.
IAS mode will maintain the selected speed of the plane. To make the plane descend, power is reduced and the plane noses over to maintain the selected speed. Setting a high speed and pulling the power to idle will achieve a dramatic decent rate.
VS (Vertical Speed) lets the pilot set a desired climb or descent rate, e.g. 3000 fpm. This is not used except for small altitude changes because it provides no speed protection. So if I select a 4000 fpm climb, the plane will keep bringing the nose up in an effort to maintain this climb rate and the airspeed will bleed off until the plane stalls (the Airbus won't let the plane get to this point; it will lower the nose). If the pilot selects a 4000 fpm decent, the airplane will likely overspeed if the power isn't brought back to idle thrust.
When using IAS or VS, the first step (selecting an altitude) isn't actually required, i.e. the plane will attempt to satisfy the selected mode and fly away from the selected altitude.