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False stick-shake prompts order to check 747 sensor

False activation of a stick-shaker during take-off has spurred a US directive to check and test certain angle-of-attack sensors on Boeing 747s.

The directive, which covers a range of 747 variants including the -400, requires a general visual inspection to determine whether a specific sensor with a paddle-type vane is fitted.

For those aircraft affected the US FAA is demanding an operational test of the stall-warning system and, if necessary, replacement of the angle-of-attack sensor. The directive also covers the earliest 737 models as well as 727s.

The FAA says it is ordering the work to prevent "erroneous activation" of the stick-shaker on climb-out after departure. Since the standard response to a stick-shaker warning is to lower the nose, the FAA is concerned that a false alarm could lead to a runway overrun or failure to clear terrain or other obstacles.

It has rejected a Lufthansa Technik request to skip inspection of the aircraft in favour of reviewing its maintenance records to determine which sensor type is fitted.

"For purposes of correcting a potential unsafe condition in the angle-of-attack vanes, the FAA considers actual physical inspection of the type of angle-of-attack vanes installed to be the most reliable method of determining what type of vane is installed," says the US authority.

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