Originally Posted by
FCeng84
(1) If the flight crew uses their pilot commanded electric trim (thumb switches) they will not stop trimming for more that 5 seconds until the column force has been trimmed to (or close to) zero.
(2) Repeated events of the automatic stabilizer control running the stabilizer away from trim when starting from an otherwise trimmed, relatively steady flight condition will be recognized by the flight crew as errant behavior of the automatic stabilizer control system and that the flight crew response will be to activate the stabilizer cutout switches to disable further automatic stabilizer control commands.
(3) The impact of an errant AOA signal feeding into MCAS would be acceptable at the expected failure rate based on assumptions (1) and (2) above
FC; those seem to be reasonable compliance requirements for the PSCP for a 25.672 system, the FAA SEA TAD will likely be looking at some criteria along those lines. Note that JT610's data provided in the preliminary report by Indonesia's NTSC shows that the prior flight (DPS-JKT) had an apparent DC offset of the L and R AOA from the get go, as did the accident flight. Prior to the earlier flight from DPS, maintenance reportedly replaced the AOA sensor, and that along with the offset looks like the probe was incorrectly installed/calibrated before departure DPS. That would result in an additional area of reinforcement needed for that particular MEDA issue.
DPS-JKT: AOA error prior to takeoff, constant difference between AOA's. Crew recognise a problem exists to pitch force, Capt sees trim being applied automatically against FO's elevator input, and Capt selects cutout on Stab. Capt then reverses cutout, problem re-occurs, and cutout reselected, nose down trim stops. Crew use trim wheel for rest of flight, and operate non RVSM, manual flight. Stall warning is on from nose wheel lift off until landing in JKT. partial writeup in logbook, not whole story.
JKT accident flight: wash-rinse-repeat, except the stab is not selected to cutout. crew fight the trim with pickle switches nose up, and outside of 5 sec window, auto trim giving nose down. Crew lose plot by the looks of it, and stab trim finally runs away in the end. (if the pickle switch [split ARM-CMD series switching] being used was intermittent at the end, then a runaway of the trim would result, until such time as the cutout switches were employed, or the other pickle switches were used). Throughout this situation, the stall warning system, and multiple EICAS messages are adding to competing cognitive tasks.
Sensor validation by voting with the other AOA output and inhibiting function with a detected difference, or comparison of AOA to pitot/static-attitude-inertial/GPS data to determine validity would be needed to avoid reliance on crew detection and correct response. In the end, a failure of the system will still be a possibility, however remote, and crew timely intervention to isolate the system is needed to be reinforced. There is an inherent reticence to alter system states on the aircraft ( a good thing in normal RPT ops) but the training matrix is filled with warm fuzzy time expenditure on LOFT etc vs dealing with vignettes of HQ problems, time critical detection and response events. We continuously squander training resources to appease training programs that fail to meaningfully train. HF stuff can be done in a classroom or in front of a cardboard bomber, HQ stuff needs to be done in a sim, procedural stuff needs at least PTTs to be undertaken effectively. The travelling public are being sold lemons by the nonsensical Part 61/JAR/EU FCL requirements.
On the missing information to the crew, the same problem in a Cessna, Learjet, Citation, or B747 would exhibit in the same manner, that there is an undesirable state, which is readily apparent to the crew, and which needs to be responded to promptly, and to which mechanism to provide intervention exists, being a off switch, cutout, or CB depending on the flavor of the ride. The crew detected the fault in the DPS-JKT leg, and actioned correctly. The actual reason why is not needed at that time, only the detection of an undesirable state, and the intervention necessary to achieve a satisfactory outcome. In both accident flights, it appears that the crew did not achieve a state of affairs where they had detected the condition and from that, from training or knowledge, continued to effective intervention.