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As I understand it, and I have a Ph.D. in aerospace engineering so I think I understand it pretty well, the problem with the 737 MAX is that the new engine nacelles change the pitching moment of the aircraft at high angles of attack. Certification requirements require that the pilots stick force increase as angle attack increases. In the 737 MAX, the engine nacelles change the shape of stick force curve so that the stick force actually starts to decrease the more the pilot pulls. Boeing implemented MCAS to solve this problem by automatically inputting up to 2.5 degrees of nose down trim using the horizontal stabiliser. Previous posts have noted that MCAS is only supposed to be activated when certain conditions are met and the flight crew can manually override and even disable the automatic trim inputs if they are aware of what's going on. In the earlier Lion Air crash, the MCAS was apparently reading angle of attack from a faulty sensor and so was making erroneous nose down trim inputs. It did this on at least three flights before the accident flight, but the flight crews were able to disable it and complete the flights without incident. The fourth crew was not able to do this. The big question about the Ethiopian flight is whether there was a similar hardware failure that similarly caused erroneous inputs from the MCAS or if there is another and different failure mode.

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25 minutes ago, VMA131Marine said:

In the 737 MAX, the engine nacelles change the shape of stick force curve so that the stick force actually starts to decrease the more the pilot pulls. Boeing implemented MCAS to solve this problem by automatically inputting up to 2.5 degrees of nose down trim using the horizontal stabiliser.

 

Got it, so it's counterintuitive control feedback and not some nose-up tendancy near AoAcritical. Boeing is trying to artificially mask misleading control forces with the MCAS system.

 

I understand the AoA measurement for MCAS is from a single sensor too so there is no voting or redundancy in it.

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1 minute ago, Jamie @ Sovereign Hobbies said:

 

Got it, so it's counterintuitive control feedback and not some nose-up tendancy near AoAcritical. Boeing is trying to artificially mask misleading control forces with the MCAS system.

 

I understand the AoA measurement for MCAS is from a single sensor too so there is no voting or redundancy in it.

Yes, exactly!

 

From what I have read, the MCAS gets its input from a single AoA sensor but it switches inputs between two of the AoA sensors every time the system resets.

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4 hours ago, bzn20 said:

The second grounding was for the engine mount pins  . That was found to be bad practices ,not IAW the DC-10 Maintenance Manual but fitting the engines in the hangar in service ,which is human error . That was lifting the engine with a forklift putting one pin in and the engine was hanging by one pin and then forked up to locate the second pin which overloaded the first pin , taking the whole engine weight .

 

American Airlines Flight 191 Chicago O'Hare

 

Aa191_ohare.jpg

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bzn20, that was my point. Conflict between desire for new and commonality with previous version, necessary for certification purposes and as a selling point is bound to produce more compromises with every new version. With B737 Boeing was more than successful, after all this type is in production for more then fifty years. The only other airliner I can think of that had been produced for about as long is Antonov An-2, although there is hardly any other comparison between types. In my opinion B737's successor is long overdue. Obviously Boeing has other ideas. Cheers

Jure

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3 hours ago, Stu_davros said:

The modified airframe does not require a brand new type certificate, most aircraft types evolve like this over time.

It seems to have evolved past the point of the basic airframe. The 737 is one of the the greats but the latest iteration is killing people. Boeing tried to compensate with software. Read  bma131marine and bhouse's comments again. 

Military aircraft are long designed to be unstable compensated by computers. Perhaps Boeing has slipped up on this one. 

Edited by noelh

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I gotta say this discussion is totally fascinating with the amount of information being shared, although it's sad that human lives were lost in order to bring the topic to light (publicly).

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On 3/14/2019 at 10:36 AM, hovis said:

This won't have any bearing on the Poseidon will it?

 

None what so ever, the Poseidon is based on the Max's predecessor, the 737-800 which does not use the system causing this grief.

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Fascinating stuff indeed, and thanks everyone for the information and insights on the subject, albeit as @whiskey rightly points out, the outcome and consequences of it is human tragedies in the hundreds, for those gone and those left behind to mourn.

I'm sorry for them, as I'm sure we all are. 

 

As a bit of a duffer, what is the Poseidon?? 

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18 minutes ago, rob Lyttle said:

As a bit of a duffer, what is the Poseidon?? 

It's the replacement for Nimrod the RAF are getting.

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On ‎3‎/‎14‎/‎2019 at 10:25 PM, noelh said:

Perhaps Boeing has slipped up on this one.

You can say that again. Perhaps it SHOULD have had a new type cert. The CofG isn't where it should be ( hardly a small thing) and that's the quick fix reason for the problems .  From what I've read there doesn't seem to be any info passed to crews that the software will do things their previous 737 didn't , flick switch on checklist and forget . So what were the pilots expected to do when everything they did produced a different reaction to the 737 NG or any other planes they'd flown ? It's unbelievable what Boeing has done , it's so far away from flight safety for a manufacturer as you can get. Have to wait for the Black box results  .

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I do not Claim any Expertise in the Topic, but with around 350 MAX's delivered and flying, it's hard to believe that the MCAS is a General Problem - otherwise the grounding would have occured Long before due to Pilot complaints about erratic/unpredictable/dangerous behaviour. Apparently it takes one or more other preconditions to be met, like the faulty AoA sensor on the Lion Air plane. I haven’t followed that examination - has it been found out why the preceding Lion Air crews were able to cope with the defective sensor/its effects - when supposedly the procedure to disengage MCAS was not publicized in the manual - while the final crew weren’t? Could the sensor have deteriorated to a point that its readings‘ effects on MCAS could not be compensated any more?

In the end, it could well be that a relatively „trivial“ (for lack of a better expression, in connection with some 350 casualties) reason like a faulty batch of sensors or a design fault in it is to blame. Possibly reliance on two sensors could mitigate some effects, but what if both are faulty...

Edited by tempestfan
Adding some thoughts

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Reading through the articles on this issue, it seems Boeing wanted to hasten this plane into market. Boeing is a large corporation and has influence with regulators and government. So it did not fully test new systems and forced FAA to give it favorable ratings.  The manuals from Boeing it seems did not fully explain the features and pilots were not fully aware of these new features. 

 

Its unfortunate but when a corporation's profits become bigger than many developing countries budget, its overwhelming influence invariably breeds corruption.

 

A lot of issues came through in development of 787 as well. The most dangerous one was its lithium batteries overheating to a dangerous level. 

Edited by stalal

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1 hour ago, tempestfan said:

but what if both are faulty

It should have a back up sensor ,maybe 3 in total would be better . Then like some Autopilot systems that have 3 streams of info and if one drops it's out voted by the  other 2 all done automatically . Hope that's not confusing ,bit difficult to describe . have 2 out of action would be unusual ,3 is belts and braces .

I'll add I'm airframes not avionics so if there is anyone on here who is might like to comment.

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