Operators of Boeing MD-90s are being instructed to fit external case heaters on angle-of-attack sensors to prevent improper activation of the stick-shaker.

The order features in an airworthiness directive, effective from 2 February, newly adopted by the US FAA.

It states cites incidents in which the stick-shaker activated at airspeeds above the thresholds of the stall-protection system on the type.

The directive is aimed at preventing ice formation between the angle-of-attack sensor vane and its face-plate which, it states, could immobilise both vanes.

“If both vanes become immobilised, the stall-protection system could become unreliable or non-functional, which could result in loss of control of the [aircraft],” it adds.

Operators have been ordered to install external case heaters on the sensors. Boeing issued a service bulletin in November 2015 describing procedures for installing such heaters and carrying out functional tests.

Source: Cirium Dashboard