Flight upsets - what they are

Flight upsets, as defined for a large commercial transport aircraft, might be considered unchallenging if they occurred in a military fighter or an aerobatics capable light aircraft. As a guide, the definition accepted by the US Federal Aviation Administration is any situation in which the attitude exceeds:

25° nose up; 45° bank; 10° nose down; a combination of the above.

In normal airline flying, take-off and early climb rarely exceeds 17° nose up, turns are usually about 25° bank and descent - depending on aircraft configuration - may not even involve a nose-down attitude but, if a rapid descent is required, it will rarely require more than 5° nose down.

The reason these deviations from normal airline flying attitudes are considered an "upset" is the fact that a large transport aircraft - compared with military fast-jets or aerobatic types - has a relatively slow rate of response to control inputs, a low power/weight ratio, high inertia and low g-loading tolerances.

Unless a pilot reacts quickly and correctly to the situation the aircraft can exceed its flight envelope. A steep nose-down attitude - especially combined with high power - can cause the aircraft to accelerate through its never-exceed speed; a high nose-up attitude - especially combined with low power - can cause rapid deceleration to less than stalling speed; a high bank angle can lead to many problems, the most obvious of which is the tendency for the nose to drop and the aircraft to enter a spiral descent. A high bank angle can also tempt a pilot who has been taken by surprise to pull excessive positive g to prevent nose-drop.

An airline pilot would not normally choose an attitude in excess of the definitive upset thresholds, so the implication is that something has happened to put the aircraft in that situation - whether it was a pilot's lack of attention to a developing condition, an environmental factor like turbulence, windshear or icing, or something abnormal about the aircraft in terms of systems, structure, controls or engines.

Unless it is daylight and there is a clear natural horizon, a pilot who is taken by surprise on discovering an extreme or unusual attitude or speed can immediately develop disorientation or vertigo and a crucial part of upset training is teaching pilots the disciplines of using instruments to identify what is happening and ignoring physical sensory perceptions in favour of the indications provided by the serviceable flight performance and power instruments.

Source: Flight International