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reverse thrust. In cases where the squat switch did not properly sense ground contact the
pilot still could manually deploy speed brakes and over-ride the inhibitions against reverse
thrust.
Subsequent designs of ground sensing systems began to limit pilot authority. The thought
was, if we can prevent the pilot from activating a system at the wrong time, pilot error can be
prevented. Actually, pilot error may just be supplanted by design error. Preventing a pilot
from over-riding a system, supposes that the designer has evaluated all possible contingencies,
and that the designerÕs judgement over the situation, is superior to the pilotÕs.
On-ground sensing designs that limit pilot authority can have disastrous consequences. An
example of this can be seen in an early A-320 design. In a particular case that was not
envisioned by the designer, the aircraft in this example (an Airbus A-320) landed on a wet
runway, during very adverse conditions, and commenced to hydroplane. There was
insufficient weight on the wheels to trigger the aircraft on-ground logic (weight on both wheels
was required). Reverse thrust was locked out. Because the aircraft had not transitioned to the
ground mode, no deceleration systems were available. The aircraft eventually slowed enough
to place sufficient weight on the landing gear olio that allowed the deceleration devices to
become available. Unfortunately, valuable time was lost and the aircraft was not able to stop
on the available runway.
The most compelling argument for manual over-ride is the fact that all possible
contingencies can never be totally predicted.
This system has now been modified to allow a slight extension of the speed brakes when only
one wheel senses weight to more firmly place the aircraft weight on the wheels allowing full
activation of the ground sensing system. This should prevent a reoccurrence of this type of
accident. Although there is still no manual over-ride of the system, the report issued
regarding this accident, called for additional pilot authority through the following
recommendation2:
Possibility should be analyzed to introduce the emergency use of ground spoilers and
thrust reversers independently of meeting the criteria imposed by aircraft logics.
3.1 Recommendation
To maintain pilot authority over a malfunctioning on-ground sensing system, a method to
over-ride the system by the pilot must be provided
4.0 STALL PROTECTION
2 Report on the Accident to Airbus A320-211 Aircraft in Warsaw on 14 September 1993. Page 45.
Revision 14.0 6
Stall protection is another example of a protection system that could have disastrous results if
full pilot authority is not maintained. This basic protection scheme often involves the use of a
stick pusher. Most modern aircraft that use stick pushers provide for the crew to manually
deselect the system. Earlier designs do not always provide this capability. Without pilots
having the authority to disable the system, crews were forced to hold excessive amounts of
backpressure to counteract a malfunctioning system.
Obviously, stall warning and stall prevention systems are very important and aircraft that use
stick pushers do so for stall warning and because of undesirable stall characteristics. The
most extensive over-ride capability is provided by the Embraer EMB-145, where a single push
button on the yoke disables or disengages the stick pusher, the auto-pilot, and the elevator
trim system. The Canadair CRJ has a toggle switch next to the captainÕs knee that can be used
to deselect the stick pusher. Since the pilot has the ultimate responsibility for the safe
conduct of the flight, the ability to over-ride a malfunctioning system is of utmost importance
4.1 Recommendation
To maintain pilot authority over a malfunctioning stall warning/protection system, a method
of over-riding the system must be provided.
5.0 ENGINE CONTROLS
In general, Full Authority Digital Engine Controls (FADEC) systems provide engine
protection but sometimes limit pilot authority. The ability to aggressively command engine
thrust provides a level of aircraft protection while retaining a high degree of pilot authority.
Typical FADEC systems allow the pilot, in an emergency, to simply slam the throttle full
forward and receive the maximum allowable thrust (note the word Òallowable, not ÒavailableÓ).
Maximum allowable means the maximum thrust for existing conditions programmed into the
FADEC by aircraft/engine designers. Maximum thrust can be the maximum certified, but
could be the minimum certified thrust and might be the thrust setting allowed to meet engine
warranty provisions. This has become a standard design feature of FADEC controlled
　
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