FAA cautions airlines on maintenance of sensors that were key to 737 MAX crashes

Following two deadly crashes of the Boeing 737 MAX, both of which were initiated by a faulty reading from a single angle of attack sensor, the Federal Aviation Administration (FAA) has cautioned airlines, aircraft-maintenance companies and manufacturers that the sensors are vulnerable to damage and must be carefully maintained.

“It is imperative that all operators are aware of the criticality of AOA sensors and the potential for damage during normal operations, maintenance procedures, servicing procedures, and any other procedures around an aircraft,” states the FAA notice, which was issued last week.

The angle of attack is the angle between an airplane’s wing and the oncoming air flow. If the angle gets too high, above about 14 degrees, the air stream that has been flowing around the contours of the wing will suddenly detach from the wing surface. When this happens, the plane will lose lift and begin to fall – a condition known as stalling.

The sensors are small vanes sticking out from the fuselage that rotate in the air flow.

The FAA notice recommends that all personnel involved “review current procedures identified in their appropriate operational, maintenance, or servicing manuals … around AOA sensors.”

The notice suggests that the failure of the AOA sensors on the Lion Air MAX in Indonesia last October and then the Ethiopian Airlines MAX in March is one focus of some of the various independent reviews of the crashes. The two disasters together killed 346 people.

The FAA typically issues such safety alerts after specific in-flight incidents or reports from airlines of a recurrent problem. In this case, no such reason is given.

The notice, first reported Tuesday by Aviation Week, applies to all airplanes with an AOA vane, not only the 737 MAX. An FAA spokesman said that the notice “was issued as a reminder and is not connected to any specific findings.”

However, though international protocol precludes the agency from saying anything about the continuing accident investigations, what happened on the two MAX crash flights is almost certainly what has spurred the notice.