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.
The tragic sequence of events on both flights began when a false reading from an AOA sensor activated a new flight-control system on the MAX — a piece of software called the Maneuvering Characteristics Augmentation System (MCAS) — that was designed, if the angle of attack got too high, to automatically push the jet’s nose down uncommanded by the pilot.
Although every 737 has two exterior AOA sensors, one on each side of the cockpit, Boeing designed MCAS to take a signal from only one of them on any given flight, switching to the other sensor on the next flight.
However, the AOA failures in each crash appear to have different causes. The separate failures, four months apart, underline the vulnerability of these critical sensors.
On the Lion Air jet, the sensor had given false readings on previous flights and at least one AOA vane was replaced. On the crash flight, the AOA vane feeding MCAS gave a false reading, off by 20 degrees from the vane on the other side, even as the jet taxied on the ground. The faulty vane maintained that difference throughout the 11-minute flight.
On the Ethiopian Airlines jet, the AOA vane feeding MCAS was reading accurately until just after take-off, when suddenly it veered off by 75 degrees, an impossible reading. This suggests the vane may have been sheared off by a bird strike.
Boeing is expected to deliver the finalized redesign of the MCAS software to the FAA next month, seeking clearance to allow the 737 MAX fleet to return to passenger service. The updated MCAS will take input from both AOA sensors, rather than one. If the readings differ by 5.5 degrees or more, MCAS will not activate.
In addition, the MAX cockpit will now include a standard warning light that will illuminate when the two angle-of-attack sensors disagree. Because of a Boeing software error, this light wasn’t working on the two crash flights. And airlines can now opt to add, free of charge, angle of attack data to the primary flight display.
A separate change to MCAS, unrelated to the AOA, will ensure that if activated the system will respond with a less forceful nose-down command, one small enough that the flight crew can always counteract it by pulling back on the control column.
But even if the updated MCAS is no longer a danger, faulty AOA sensors will have to be a separate safety focus because the measurement from these sensors is fed into the flight computer and used to calculate other parameters.
Boeing’s bulletin to airlines a week after the Lion Air crash told pilots that on the MAX “an erroneous AOA can cause some or all of the following indications and effects”:
- Continuous or intermittent stick shaker
(which means the pilot’s control column shakes alarmingly to warn of a possible stall)
- An indication that the aircraft is approaching its minimum speed limit
- Increasing nose down control forces
- Inability to engage autopilot
- Automatic disengagement of autopilot
- An alert that the Indicated Air Speed sensors disagree
- An alert that the altitude sensors disagree
- An alert that the AOA sensors disagree
- A light warning of a difference in hydraulic pressure applied to the control columns of the captain and the first officer.