AoA sensor errors on Lion Air flights JT043 and JT610
In 2006 a Boeing 747 taking off from London Heathrow with over 400 people onboard had both stick shakers begin to operate as the aircraft started to rotate. Once in the air, the stick shakers continued to operate, and the pilot’s and F/O’s instruments showed a continuous disagreement in the IAS air speed. The pilots turned back to land at Heathrow, where maintenance engineers diagnosed the problem as an ADIRU problem and replaced the RHS ADIRU. When the pilots tried to take off again, the stick shakers began to operate in exactly the same way, so they aborted the takeoff and taxied to an airport parking area, where they shut down the plane. Data from the flight data recorder did not show anything unusual in any of the sensors or the electronics. The next day, while the engineers were conducting a simulated flight test on the ground by forcing air past the pitot sensors, it was found that the right hand stick shaker was activated even when the AoA vane remained in the horizontal position. The RHS AoA sensor was then replaced and the system was re-tested, showing that the problem was fixed. The aircraft was returned to service, but a ten days later several diagnostic messages started to appear indicating a different type of intermittent “AoA vane” failure on the RHS. When the RHS AoA sensor was replaced again, the problem was fixed again.
The first failed sensor was returned to the manufacturer’s facility in Seattle Washington, where it failed during a test. The problem was found to be a loose main drive gear on the vane shaft that connected the vane shaft to two inductive-type resolvers and an oil-filled damper. The loose drive gear was caused by an improperly torqued set screw that allowed the vane to rotate freely 360° around the main shaft. This caused angle readings that were offset randomly from the zero position, which correlated with the continuous abnormal activation of the stick shakers. A counter-weight was also found to be loose and had a free play of about ±2° rotation.
The second failed AoA sensor was tested at the facility of its different manufacturer in the UK. It also failed its test. The problem was found to be an “open spot” in the older potentiometer-type resolver, which could have been caused by a contaminant particle acting between the brush-type angle contactor and the continuous resistor coil. This “open spot” almost certainly accounted for the intermittent failures reported by the aircraft’s diagnostic messages.
What this incident report shows is that a vane-type AoA sensor is more complicated than just a vane with a resolver on the same shaft. The additional gearing required between the vane and the resolver can be the cause of random offsets in either direction if a gear can slip on its supporting shaft. The offsets can remain constant until sufficient force is placed on the vane to cause the gear to slip on its shaft, which causes a new offset value in the sensor output. This failure mechanism may explain the 22° offset in the LHS AoA reading on the 737 MAX Lion Air JT610 flight of 29 October 2018, which caused the stick shaker to activate and remain activated for the entire flight. While this reported anomalous sensor behavior does not prove that the AoA sensor was at fault in the Lion Air incident, it does provide a working hypothesis that may be tested further.
For further information, see AAIB Bulletin 8/2008, titled G-VHOT, EW/C2006/12/01.