Originally Posted by
JRBarrett
I do not have access to a 737 Max AMM, so do not know the post-installation functional checks required after replacing a faulty probe. Most probes contain a guide pin which inserts into a matching hole in the fuselage to insure that the probe body is physically oriented correctly in relation to the airframe, but on all aircraft models I work on, installing a new probe also requires a post-install rigging check to insure that the output position data is correct. Typically this is done with a rigging adapter that attaches to the probe and fuselage, containing a calibrated angle scale and pointer, and a mount to attach a calibrated digital protractor. The check usually involves rotating the probe to specific angles, while checking the generated data to insure it is accurate.
IF the 737 Max requires a rigging check when an AOA probe is replaced, the question is: did the Lion Air engineers perform one, or did they just replace the probe and sign it off? A rigging check would have revealed any inaccuracy in either the new or old probes. If no rigging check is required in the AMM procedure for replacing an AOA probe, then that is on Boeing, not Lion Air.
Based on an old 737 AMM and zero personal hands-on experience, it looks like 737 has something very similar. AMM refers to a "calibrator" which is to used in system performance test after installation. Stall warning system is to be tested with various flap settings rotating the vane until shaker triggers.
I don't have MAX AMM so don't know if that is different - if it is, then that itself could be a source of error (particularly if the difference isn't documented...).
It doesn't look easy to misfit it by 20 degrees (which was apparently the disagreement) - 8 mounting holes (so 45 apart), plus indexing holes. However, there are two compatible parts, one Rosemount one Conrac?, only one calibrator but an adapter is required for one type of vane. Sometimes when things don't fit people use a hammer as an adapter, I can't imagine that on an aoa vane but never say never.
Lots of questions - did they fit it properly, did they test it properly, did the have all the correct test kit, is the MAX different in some way (and did they know), and also, was the sensor replaced more than once (to cause the previous problems as well) with same error made each time.