Maintenance Lapse Identified as Initial Problem Leading to Lion Air Crash

[Double07]

AoA sensor errors on Lion Air flights JT043 and JT610

One may ask how the error in the captain's LHS AoA sensor on flights JT043 and JT610 originated. If the error was it present when the AoA sensor was installed wouldn’t the installation test procedure have detected this error?
In response, I believe it may be possible that the testing process used after the installation of the new AoA sensor prior to the JT043 flight may have been the cause of the 22.5° offset. My reasoning is as follows.

It is known that the range of rotation of the Rosemount 0861FL AoA sensor on the B737NG and B737MAX aircraft is ±110°. The captain’s LHS sensor is the same 0861FL sensor flipped over 180° about the horizontal axis.

However, the report quoted above implies that the vane of the AoA sensor has no such end stops, because it states that if the main gear on the vane shaft becomes loose, then the vane can be rotated completely around 360°. Therefore, the end stops limiting the range of rotation of the AoA sensor must be elsewhere inside the sensor housing. Now, a review of the specifications of many types of resolvers on the internet shows that all resolvers have limitations on the ±angles they can be rotated through. This implies that the resolvers have stops inside them to prevent them from being rotated beyond their maximum angles of rotation. This implies that the maximum angle of rotation of the AoA sensor vane is limited by the stops in the resolver, and not by any stops on the vane or the shaft on which the vane is installed. Therefore, if one applies too much force to the AoA sensor vane to pin it against one of the stops, it is possible that this force can cause a slipping of the main gear on the vane shaft, causing the vane to be offset from the resolver while the resolver continues to read the same value because it is up against the stop inside the resolver. Therefore, an offset can be created between the vane angle and the resolver output angle.

Now, we know from the maintenance records for the aircraft of flights JT043 and JT610 that an installation test was done after replacing the AoA sensor before flight JT043 because the maintenance engineer noted on 27 October 2018 that: “For troubleshooting due to repetitive problem perform replaced the angle of attack sensor in accordance with Aircraft Maintenance Manual (AMM) Task 34-21-05-000-001 and task 34-21-05-400-801 carried out. Installation test and heater system test result good”.

But the Aircraft Maintenance Manual actually specifies TWO types of reference checks that can be performed: 1) A recommended test using a test fixture similar to the one shown below. (Notice that it uses the two tooling holes on the AoA sensor to register the correct angular position). A maintenance technician outside the aircraft sets the AoA sensor vane to the angles 0°, -10°, and °, respectively, and either the same technician, or perhaps a different technician, checks the output of the ADIRU to see if the same angles are provided to the SMYD display. 2) In the absence of a test fixture, a quick check can be done by setting the AoA sensor vane to the angles 0°, -100°, and +100°, the latter of which are the end stops of the vane travel. The output of the ADIRU is again checked to see if the same angles are provided to the SMYD display.

The first reference check cannot cause an offset in the vane-to-resolver output angle. However, the second reference check CAN cause an offset in the vane-to-resolver output angle if the technician setting vane angle applies too much force while setting the vane against the end stop. Specifically, if the last angle to be tested is +100°, then the AoA sensor output will be offset in the positive direction as observed in the JT043 and JT610 flights. This offset will not be observed during the test because the resolver output remains pinned at its +100° output value. Only if the last angle to be tested is different from the +100° end stop setting will an offset be observed in the AoA output during the test.

One further observation. Several posters have commented that the captain’s LHS AoA sensor that had an offset of 22° on flights JT043 and JT610 appeared to have a higher random noise on it than the F/O’s RHS AoA sensor. This may be the result of a defective viscous damper inside the captain’s LHS AoA sensor. This may indicate that the replacement LHS AoA sensor installed on flights JT043 and JT610 was, in fact, a reworked AoA sensor, which may explain why investigators want to review the procedures at the AoA sensor rework facility in Florida as well as the AoA production facility in Minneapolis. And if the sensor was a reworked sensor, perhaps the gearing between the vane and the resolver was not torqued high enough to prevent offsets from being induced by pressure on the vane against the end stops.