UK CAA ban B737 MAX8 operations in UK

[pholling]

Originally Posted by daz211

At least two US flights have reported that an automated system seemed to cause their Boeing 737 MAX 8 planes to tilt down suddenly.

In reports filed last year in a database compiled by NASA, the pilots said that soon after engaging the autopilot on their planes, the nose tilted down sharply.

In both cases they recovered quickly after disconnecting the autopilot.

There are two entries into NASA's ASRS database regarding 737MAX uncommanded pitch downs. However, it is highly likely that these two entries are exactly the same aircraft on exactly the same flight. They are both in the same month and the descriptions contained pretty much mirror each other. I don't mean the incidents mirror each other, I mean the descriptions of the discussions between the two flight crew members are basically mirror images. This means it is highly likely that it is the Captain and First Officer on the same flight. Also, this would not be an MCAS issue, since it is occurring when the autopilot is engaged. Though the logic is similar between the autopilot pitch trim logic and MCAS pitch trim logic, they are, in fact, different systems. The end result of bad alpha information will be the same, the aircraft will apply nose down pitch trim, and the fix is similar – if all else fails isolate the automatic stab trim. The autopilot stab trim runaway condition is a corner case on all 737s and it identical in the MAX to the NG. Pilots familiar with the NG, but having very little time in the MAX, should have not problem with this condition, it will have been well trained. The issue is that in the NG the runaway condition really only exists when the autopilot is on. In the MAX it can exist in all automation conditions. This can easily lead the the "What is the aircraft doing?" question. If you are at cruise altitude there is a lot of time to figure things out, but 500' AGL, your insurance policy is quite thin. This type of reaction is, unfortunately, quite common in accidents.

Remember Boeing developed the MCAS to give the 737MAX the same handling qualities in turns as the 737NG, ie eliminate the nose-up tendency that the LEAP engines create. The aircraft should be flyable with out it, and is most definitely flyable with the electric stab trim disabled (this would have been a test case). The issue is that it will fly differently. Systems that allow two different aircraft to handle the same are quite common these days. In fact the modern FBW systems are design to do just that. The 737 is a legacy and so any systems that does this will have been built on top of other systems which may not have been designed with that function in mind.

There are a number of questions that are not answered in the public domain, but should be asked

1. Why does the MCAS system only make use of one of the two AoA sensors? – You don't have triple redundancy, i.e. you can't tell which one is right if they disagree, but you don't actually need the system to fly the aircraft so a disagree could have made the system go INOP
2. Why is the 'AoA disagree' flight-deck alert an option on the MAX? If the airline doesn't buy that option you get things like 'IAS Disagree' instead. This can confuse the flight crew as to what the issue is. Note: In the case of the Lion Air accident the MTC log has the AoA disagree, but the crew only saw IAS disagree. This makes diagnosing issues more difficult
3. Why was there not a limit on stabiliser travel on MCAS? – In the extreme Boeing could have relied upon the stick-pusher to counter critical AoA deviations
4. What was Ethiopian's stab runaway training regime and had it incorporated MCAS failures in light of the LionAir accident? – Note: most airlines that already had 737NGs have not ordered/received MAX specific simulators, so training in that area would have to be 'kludged'.
5. What was the actual basis for suspending operations of 737MAXs? Keep in mind 737s were not grounded after the spate of rudder hard-overs in the 1990s. Training and procedures were updated and over time a modification was developed and installed that minimised the likelihood of a hard over occurring. Also, Airbus FBW aircraft were not grounded after Air France and Air Asia crashes that while that had different initial faults had almost exactly the same human factors failure modes. Again training and procedures have been modified and the US (others?) has mandated training specifically in high-altitude stall recovery to minimise the chance of those accident chains from happening.

There are a number of other things that need to be asked about the drive to minimise differences training on the MAX. Though those get a lot more complex. In some cases the answers to some of the above might be driven by the legacy of the 737 and the way airlines operate them. The 737 has not stood still since its development. Even the NG has had significant back-end avionics architecture changes. However, since airlines may operate mixed fleets it is quite common for the new capabilities brought by these changes to be disabled to prevent confusion/minimise the differences.