Cause of crash in Buffalo (Colgan 3407) now evident: update.

Posted on 26. Feb, 2009 by in Featured

The NTSB has detailed concern with the autopilot mode that the aircraft was in. Why is that important? I’m not familiar with the Q-400 flight director and autopilot so I will cover systems in general.

There are various modes of a flight director and autopilot combination. The two used the most are VSI and IAS. In VSI mode you simply dial in the rate of descent or climb in a digital window. If for example, the pilot sets -700 the aircraft will descend at 700 feet per minute. The IAS (Indicated Air Speed) mode is based on airspeed, by setting 160 in the window the aircraft will descend at 160 knots. The IAS mode protects you from stall by holding an acceptable speed; however it is not accurate in controlling descent. You have to manipulate the throttles to get the desired rate of descent. If you pull power the nose will lower to maintain indicated airspeed thus increasing the rate of descent. Adding power does the opposite.

A very important part of the equation; most small aircraft do not have auto throttles. It is reported that the Q-400’s at Colgan did not have auto throttles installed as standard equipment. Auto throttles maintain the selected airspeed, up to the capability of the engines. Think of it as cruise control. Without auto throttles the crew must maintain the airspeed via manual manipulation of the throttles.

If an autopilot is set to a specific rate of descent and the throttles are not set to maintain a desired speed, the aircraft will either decelerate or accelerate. Thus, if the power was not set correctly or monitored during the approach, the aircraft could slow to a dangerous speed. It appears the NTSB is looking very close at the mode the autopilot was in, as a causal factor.

A reminder from a past post; the autopilot will continue to trim into a stall. Making it extremly pitch sensitive.

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