Originally Posted by
Ian W
For a safety system it makes no sense to have a 'single point of failure' this is a sine qua non of any safety related engineering. MCAS is obviously a safety system and (despite disparaging comments here) Boeing engineers will not have deliberately chosen to make it unreliable. So this begs a question.
First there are a LOT of 737's flying; from Wikipedia:
A large number of those 737 (if not all) will have the same AoA feeding similar ADIRU and will get an Unreliable Airspeed alert plus stick shaker if AoA disagree. Out of 50 or so Max 8 they have already had two errors only months apart. If that was the rate in the whole fleet there would be continual unreliable airspeed reports and no engineer in their right mind would hang a safety system onto a single AoA if they were that unreliable. From that one can only assume that AoA is normally very reliable to the extent that failures are very rare on previous 737 models.
So the question is - why are the AoA systems on 737Max8 failing at a rate higher than acceptable? Is this just a sad coincidence that when a safety related system is designed to use the AoA output - two failures arise in 6 months -or- is there something different about the Max that is leading to AoA problems?
Even 'ramp rash' could be affected by the different position of the engines when push crews and catering used to NG have to work with the Max.
Any thoughts?
There are at least two incidents involving B38M operated by SWA, and perhaps, one more related incident involving Sunwing Airlines.
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Incidents 1 & 2:Southwest Replaced Flight-Control Sensors of the Kind Implicated in Lion Air Crash
Investigators are examining how Boeing heeded earlier warnings about flight-control-sensor failures similar to the one implicated in 737 MAX crash
During the three weeks before Lion Air Flight 610 plunged into waters off Indonesia, Southwest Airlines Co. replaced two malfunctioning flight-control sensors of the same type that has been publicly implicated in the crash, according to a summary of Southwest maintenance records reviewed by The Wall Street Journal.
Both U.S. maintenance issues involved a Boeing Co. 737 MAX 8, the same model that crashed last month in Indonesia. The sensors measure whether the jetliner is angled above or below level flight. Those sensors, or related hardware, needed repairs in the Southwest instances, according to the summary document. The document also indicates Southwest pilots reported they couldn't engage automated throttle settings, similar to cruise control on a car.
A Southwest spokeswoman said the sensors didn't fail and were removed as a precautionary measure as part of a troubleshooting process. She said at least one was repaired...
[note: similarity to what had been reported anonymously on NASA's ASRS database.]
Incident 3 :Sunwing 737 Max suffers spurious indication incident
Canadian investigators have disclosed that engineers replaced an air data computer on a Sunwing Airlines Boeing 737 Max 8 after the crew received spurious indications from the aircraft's instruments. Transportation Safety Board of Canada says the aircraft had been cruising at 35,000ft when the crew received "erroneous" indications on the captain's side. The first officer's instruments, and the standby indicators, were functioning normally and the first officer took control of the aircraft.
It descended to 25,000ft as a precaution, in order to clear instrument meteorological conditions, but "as it passed 28,000ft" the weather radar and TCAS both failed. The aircraft was some 50nm north-west of Washington DC at the time. The crew transmitted a "pan pan" urgency call. The safety board says a left-side inertial reference system fault light also illuminated. he flight continued to Toronto for a safe landing without further incident.
The Canadian TSB reported the left ADIRU was replaced before the plane returned to operation.===================
Lion Air PK-LQP had various failures during its last four flights. The first flight DPS-MDC had Air Speed Indicator trouble. The second flight MDC-DPS had IAS problem persisted. AOA vane was replaced at DPS by the MX. The third flight, DPS - CGK, still had IAS problem but it also had additional problems: CAPT stick shaker, CAPT ALT AND STS trimming the opposite way [as the CAPT described it - actually it was the ghostly MCAS awakened from its slumber], but at least FO instruments were still reliable. The FO got the control and flied the AC manually and landed in CGK.
On the last fateful flight CGK-PGK, the problem got even worse, NONE of the instruments agreed, and of course MCAS compounded the crisis by faithfully following its mandated duty - executing a NOSE DOWN infinite loop.
I think problems which gotten worse over time seemed to point to a much deeper problem than just AOA vanes. Like what had happened to Sunwing, Lion Air PK-LQP problems might have been caused by the identical source: its left ADIRU.