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A. Airbus A330-200--------------------------------------------------------------------------------------------------------------42
Pilot Comments----------------------------------------------------------------------------------------------------------------46
Conclusion----------------------------------------------------------------------------------------------------------------------47
Recommendations -------------------------------------------------------------------------------------------------------------47
B. Boeing B-777-300------------------------------------------------------------------------------------------------------------47
Pilot Comments----------------------------------------------------------------------------------------------------------------51
Conclusions ---------------------------------------------------------------------------------------------------------------------52
V. Lessons Learned ------------------------------------------------------------------------------------------------------------------52
A. Simulation---------------------------------------------------------------------------------------------------------------------52
B. Aircraft Flight Test -----------------------------------------------------------------------------------------------------------52
C. General--------------------------------------------------------------------------------------------------------------------------53
VI. Conclusions/Reccommendations--------------------------------------------------------------------------------------------53
Revision 14.0 4
1.0 INTRODUCTION
Modern airliners are equipped with many systems designed to protect the aircraft and its
occupants from harm. These systems range from simple warning devices to complex envelope
protections. The modern Fly-By-Wire (FBW) flight control system with their flight envelope
protection features have the potential of offering significant safety benefits over the protection
features of aircraft with conventional flight control systems.
The addition of various protection systems has tended to improve airline accident rates over
the years1. Occasionally however, some of the very systems designed to protect the aircraft
have contributed to accidents. This opposite effect of the onboard safety systems seems to
be the result of inadequate or incomplete design, or the occurrence of unanticipated events. In
those cases where the safety system itself was causal to an accident, the flight crew was often
unable to counter the effects of the system.
This paper presents a discussion of the evolution of aircraft protection schemes and lessons
learned, along with design recommendations for aircraft systems.
2.0 ARGUMENT
Aircraft protection systems should be designed so that the aircraft is fully protected without
limiting pilot authority. However, pilot authority does not necessarily mean that the pilot has
the ability to select an inappropriate system configuration or operate the flight controls so as
to jeopardize the structural integrity of the aircraft. Rather, the pilot in command must have
the authority to obtain maximum available system and aircraft performance, in conjunction
with safe operation of the aircraft, under all flight conditions.
Aircraft protection systems should be designed to allow pilots easy access to the normal
operating envelope of the aircraft and its systems. A straightforward and intuitive
disengagement scheme must always be available to allow the pilot increased control authority,
up to structural or aerodynamic limits in an emergency situation.
3.0 ON-GROUND SENSING
An example of an early protection system is the on ground sensing function provided by squat
switches. Early squat switch systems retained full pilot authority by providing full pilot
override. Aircraft on-ground sensing systems are an example of a protection system that can
either limit or retain full pilot authority, depending upon their design.
On-ground sensing was provided in earlier designs, many of which are still in service, by squat
switches whose purpose was to prevent inadvertent activation of ground spoilers and engine
1 The Impact of Automation on Accident Risk, Airbus Industrie, 14 September, 1998.
Revision 14.0 5
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,
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