The FAA is proposing a new airworthiness directive for certain Boeing 737-300, -400 and -500 aircraft over angle-of-attack sensor vanes that can freeze and potentially affect the stall-warning system. The proposal, published Tuesday, follows nuisance stick-shaker activations reported on MD-90 aircraft that use related AOA sensor hardware.
Frozen Vane Problem
The MD-90 events occurred while the aircraft were accelerating to cruise speed at the top of climb. According to the FAA, recorded flight data and weather information indicated the AOA vanes had frozen because some sensors did not provide enough heat to prevent ice from forming around the vane pivot. Boeing subsequently determined that certain 737-300, -400 and -500 aircraft use AOA sensor part numbers susceptible to the same condition.
“Heavy moisture conditions on the ground can lead to water entering the AOA vane pivot and freezing prior to or during flight,” the FAA said in the proposed rule. The agency said the freezing could occur before departure or after the aircraft is airborne.
Proposed Fix
The proposed AD would require operators to install new external case heaters on the left and right AOA sensors and perform repetitive functional tests and corrective actions when necessary. Depending on aircraft configuration, additional work could include installing a junction box, adding wiring for automated probe heating, modifying circuit-breaker panels and replacing certain insulation blankets.
The FAA estimates the proposal would affect 133 U.S.-registered aircraft. Installation of the case heaters is estimated at $2,125 per aircraft, while additional required modifications could cost as much as $109,883 per airplane in some configurations. Comments on the proposed rule are due Oct. 2. The FAA said an unresolved sensor problem could lead to failure of the stall-warning system and jeopardize continued safe flight and landing.
0 replies