3/25/97
AC 20-128A
(8) Damage to or inadvertent movement of aerodynamic surfaces (e.g.. flaps,
slats, stabilizers, ailerons, spoilers, thrust reversers, elevators, rudders, strakes, winglets, etc.) and
the resultant effect on safe flight and landing.
c. Safety Analysis Objectives It is considered that the objective of minimizing
hazards will have been met if:
(I)
have been taken;
The practical design considerations and precautions of Paragraphs 7 and 8
(2) The safety analysis has been completed using the engine/APU model
defined in Paragraph 9; .
(3) For part 25 transport and part 23 commuter category airplanes, the
following hazard ratio guidelines have been achieved:
(i) Single One-Third Disc Fragment. There is not more than a I in 20
chance of catastrophe resulting from the release of a single one-third disc fragment as defined in
Paragraph 9a.
(ii) Intermediate Fragment. There is not more than a I in 40 chance of
catastrophe resulting from the release of a piece of debris as defined in Paragraph 9.
(iii) Multiple Disc Fragments. (Only applicable to any duplicated or
multiplicated system when all of the system channels contributing to its functions have some part
which is within a distance equal to the diameter of the largest bladed rotor, measured from the
engine centerline). There is not more than I in 10 chance of catastrophe resulting from the
release in three random directions of three one-third fragments of a disc each having a uniform
probability of ejection over the
360
0
(assuming an angular spread of
0103
0
relative to the plane of
the disc) causing coincidental damage to systems which are duplicated or multiplicated.
NOTE: Where dissimilar systems can be used to carry out the same function (e.g. elevator
control and pitch trim), they should be regarded as duplicated (or multiplicated) systems for the
purpose of this subparagraph provided control can be maintained. The numerical assessments
described above may be used to judge the relative values of minimization. The degree of
minimization that is feasible may vary depending upon airplane size and configuration and this
variation may prevent the specific hazard ratio from being achieved. These levels are design goals
and should not be treated as absolute targets. It is possible that anyone of these levels may not
be practical to achieve.
Par 10
21
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