THE "20 000 FT" MAX FLAP EXTENSION LIMITATION
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THE "20 000 FT" MAX FLAP EXTENSION LIMITATION
GENERAL
Why is there a "20 000 ft" Flaps extension ? Many pilots always ask themselves the reason of
this limitation...
· At low velocities air can be considered an essentially incompressible fluid. However, when an
aircraft increases speed and/or climbs to a sufficiently high altitude, the surrounding air
increasingly assumes the characteristics of a compressible fluid.
· It is necessary to either account for these (compressibility) characteristics in the aircraft design or
impose limitations to prevent operation in the region of the operating envelope where
compressibility effects exist. The Mach number at which compressibility becomes significant
ranges from approximately 0.50 to 0.55M.
· For Flaps up, high speed flight the compressibility effects are accounted for in design of the
aircraft structure as well as for handling qualities, performance, etc. However, the high lift
systems are normally intended for use during relatively low speed flight and consequently are
designed using low speed, incompressible aerodynamic data.
· The flap placard speeds (in knots IAS) are established in relation to the altitude where
compressibility effects (Mach Number) could be expected to significantly influence the
aerodynamic characteristics of the airplane. In standard atmospheric conditions, flap placard
speed approaches the compressibility significant Mach number range at approximately 20000 ft.
· The operation of high lift devices is limited to altitudes no greater than 20000 ft. Operation of
high lift devices at altitudes greater than 20000 ft. may place the aircraft into a flight regime for
which it was not specifically designed and may produce unexpected aerodynamic
characteristics.
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