Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed

[LDRA]

Originally Posted by infrequentflyer789

Thanks, much more readable



Um, really? So that implies B will continue to kick off "automated stall prevention", directly opposing pilot inputs, based on output of a single sensor with no cross checks?

Pardon the expression, but if not that, WTF are they fixing?

Speculation: Put limit on how much MCAS can command nose down trim. If MCAS trim authority is limited, they can argue aircraft controllable

Edit: Come to think of it, they may be able to get rid of the MCAS all together with nose ballast that shift CoG position. Yes it increases trim drag, which in turn increases fuel consumption. But depending on how much CoG shift is needed to have monotonic AoA control response, it might be tolerable, especially with all the stigma from single point failure

[LDRA]

Originally Posted by FCeng84

jimtx - you are right about MCAS addressing a hand flying, handling qualities issue. MCAS is active only when the autopilot is disengaged. You are also correct that a pilot paying close attention to pitch response will manage the stick (or in this case the column on 737) so as to command the pitch attitude needed to achieve the desired normal load factor / flight path angle. The issue here is that certification rules require that increasing pull pitch controller input must be needed to command ever increasing AOA / normal load factor. Without MCAS there are 737MAX flight conditions where AOA / normal load factor can increase with constant column pull such that the pilot would need to relax the pull force in order to avoid exceeding their target nose up pitch maneuver. This characteristic does not comply with certification requirements and thus MCAS was introduced to assure that a steady increase in AOA / normal load factor requires a steady increase in column pull force.

But MCAS takes ~10secs to trim one increment. Maybe the non-linearity in column to AoA response only occur at low pitch rate?

[Timmy Tomkins]

Originally Posted by Smythe

Part of the certification was required to provide this software, as the aircraft becomes unstable in certain conditions to to the increased size and weight of the engines.

So basically design an unstable aircraft and then use gizmos to make it flyable; what kind of philosophy is that? OK Military fighters are designed that way with fly by wire b ut they are doing a different job with very different risk tolerance levels; for a major manufacturer of passenger aircraft to do this is highly questionable.

[aterpster]

Originally Posted by Timmy Tomkins

So basically design an unstable aircraft and then use gizmos to make it flyable; what kind of philosophy is that? OK Military fighters are designed that way with fly by wire b ut they are doing a different job with very different risk tolerance levels; for a major manufacturer of passenger aircraft to do this is highly questionable.

The 737 MAX is not a FBW airplane. But, the larger engines had to be placed to cause a shift in the CG. No doubt it was a money over modern design decision by Boeing, but SWA was driving it because they wanted to remain all 737.

BTW, it is not basically an unstable airplane.

[icemanalgeria]

Do we need this ?

How many times did a Boeing stall and Crash without this system ?

Do we really need this kind of solution in today’s aircraft ?

[Ian W]

Originally Posted by icemanalgeria

How many times did a Boeing stall and Crash without this system ?

Do we really need this kind of solution in today’s aircraft ?

As the 737 Max8 is the only variant with the issue and has only flown with MCAS that is difficult to say. But the point is academic as the certification authority believes that a system is needed to avoid rapid reduction in stick force in some flight conditions.

[jimtx]

Originally Posted by Ian W

As the 737 Max8 is the only variant with the issue and has only flown with MCAS that is difficult to say. But the point is academic as the certification authority believes that a system is needed to avoid rapid reduction in stick force in some flight conditions.

Yet the certification authority and the manufacturer don't see any need to warn you about avoiding the flight regimes that MCAS was added to protect when in fact you would be without it if you had to disable electric trim for a malfunction.

[DaveReidUK]

Originally Posted by Ian W

As the 737 Max8 is the only variant with the issue and has only flown with MCAS that is difficult to say.

Does the longer Max9 have sufficiently different characteristics that it doesn't use MCAS?

[FCeng84]

MCAS is on all 737MAX variants with slightly different gains when comparing across the MAX derivatives. MCAS is not active at any operational trim AOA and for the most part does not come active until AOA exceeds the stick shaker trigger point. With properly operating AOA sensors a pilot that honors the stick shaker should not encounter MCAS. This limited exposure was probably one of the key considerations when Boeing/FAA addressed documentation and training impacts of this function.

[gums]

Salute!

The problem, FCeng was that the stick shaker and stall warnings were going off once the flaps came up. Then the plane dived until the crew got it back levl or close to that.
So I am feeling the shaker, hearing the horns and the damned plane is diving down even tho we are not near to a stall.
There are few pilots witjhout a few serious and previous flight control problems that would not be befuddled.
So I continue to cut the crew some slack, but they should never have been there to begin with.

The 737 plane is not "unstable", as some feel. But it certainly has a pitch up problem as AoA approaches some value. So it reminds me of my VooDoo time. The jet was statically stable above and below the mach. But once at certain AoA the thing "dug in". With neutral stick the nose would keep rising. After another degree or so of AoA it just kept coming up and we were on a carnival ride, heh heh.

Although the VooDoo had a "pusher", it did not continuously apply nose down trim when approaching the critical AoA. The plane also had a 60 pound +/- back stick ;limiter you had to overcome to increase AoA.

So I get the impression that the 737 Max with MCAS has a serious problem up at a high AoA where even neutral control stick/wheel/yoke will not prevent an increasing AoA and stall. Am I right?

Gums sends...

[FCeng84]

gums - no doubt the 737MAX with AOA failed high presented a confusing situation. My understanding is that stick shaker went off as soon as they lifted off and first took to the air. MCAS did not start its thing until they went flaps up. I don't know if standard operating procedures would have been to recognize something is not right when stick shaker commenced right away and would not quit when they were clearly not near stall. If they had gone around the pattern and landed without going fully flaps up they never would have encountered MCAS. I certainly cut the crew some slack - they had clearly more skin in the game than any of us and were doing the best they could to make it home. I have less slack to offer the crew from the day before who flew their full flight with the stick shaker going off, activated the stabilizer cutout to shutdown the misbehaving stabilizer control, and did not (from what I have seen) communicate all of that well enough to those who sat in Row 0 the next day. Hind sight is 20/20, but that airplane should not have gone out with passengers on Oct 29th.

In your last sentence above did you intent to state "737 Max without MCAS" rather than "737 Max with MCAS"? I assume that your question to me is with regard to the characteristics without MCAS. Without MCAS, starting from a trimmed state at a normal condition for a revenue flight I don't think that a pull-up maneuver to higher AOA will get you to the point where you have to push to keep AOA from increasing. The issue that MCAS mitigates is finding any point along the way where AOA can continue to increase at the current level of column pull and thus requires that the column pull be relaxed in order to keep to the desired AOA.

Regards,

FCeng84

[jimtx]

Originally Posted by gums

Salute!

The problem, FCeng was that the stick shaker and stall warnings were going off once the flaps came up. Then the plane dived until the crew got it back levl or close to that.
So I am feeling the shaker, hearing the horns and the damned plane is diving down even tho we are not near to a stall.
There are few pilots witjhout a few serious and previous flight control problems that would not be befuddled.
So I continue to cut the crew some slack, but they should never have been there to begin with.

The 737 plane is not "unstable", as some feel. But it certainly has a pitch up problem as AoA approaches some value. So it reminds me of my VooDoo time. The jet was statically stable above and below the mach. But once at certain AoA the thing "dug in". With neutral stick the nose would keep rising. After another degree or so of AoA it just kept coming up and we were on a carnival ride, heh heh.

Although the VooDoo had a "pusher", it did not continuously apply nose down trim when approaching the critical AoA. The plane also had a 60 pound +/- back stick ;limiter you had to overcome to increase AoA.

So I get the impression that the 737 Max with MCAS has a serious problem up at a high AoA where even neutral control stick/wheel/yoke will not prevent an increasing AoA and stall. Am I right?

Gums sends...

Gums, we don’t know if you are right. It couldn’t be a serious problem in that Boeing suits said airline pilots would never be in that flight regime. And FCeng says it would be serious if you ignored stick shaker. And if you had to disable electric trim Boeing and the FAA give you an abnormal that does not caution about any flight regime. So we are left with real world wind shear escape maneuvers or simulator steep turns where I envision some stick lightning but who knows. Turn the thing off if it’s possible and tell pilots that they might have lower pitch forces in whatever regime they might unlikely, according to Boeing suits, encounter.

[gums]

Salute!

@FCeng from my post

So I get the impression that the 737 Max with MCAS has a serious problem up at a high AoA where even neutral control stick/wheel/yoke will not prevent an increasing AoA and stall. Am I right?

What I am implying is that the MCAS was put in the plane in order to satisfy certification requirements and just old basic aero you would expect from an airliner. i.e. it was not installed or required in previous 737 models.

I have a good plot of the longitudinal pitch authority of the plane I flew 30 years ago. At one point (AoA) the stab looses nose down control authority with a "neutral" stick command. Ditto for my VooDoo, and most of us could feel the stick getting lighter if we were pulling hard. No feedback of any kind in the Viper or Airbus to let you know how close you wereto losing it.

So what I am hearing about the new 737 is that as we get close to a certain high AoA that the plane looses some amount of pitch authority that is normally commanded by the pilot. Worse, it may even enter the "pitch up" mode where no pilot command will lower the nose.

Gums sends...

[edmundronald]

Obviously, I'm not a pilot, but with Gums last remark it looks like some cool heads with access to full aerodynamical modelling tools for this 737 type, and maybe a test plane should look at the situation in detail. The problem is that everybody will now be tied up in legal knots and fingerpointing for an unknown amount of time, as any computed, simulated or real data could end up used in court to decide on the compensation to the souls aboard the lost airframe. One of the absuridties of a system in which saying sorry we made a mistake has become impossible.

Edmund

[BluSdUp]

Lack of redundancy!

Gums
I have to say I enjoy Your analyses.
In the airliner world there is two camps: Airbus and Boeing.
And very few that has ever been in both, at least the ones posting here.
In the Boeing camp there is the sub group of 737 crew that are under the illusion that they fly a modern Boeing.
It is no such thing, as a matter of fact it is a 1956 707 flight deck frame and parts of a 727, that made it a great plane in 1967 when put on the market. And for money making still a great plane, but it was not made for scale up as was the A320.
And the systems was basic. No triple redundancy as generals demanded in modern designs.

BUT , from there to patching on a very forceful MCAS system WITHOUT telling anyone, depending in a SINGLE AOA vane input is in hindsight incompetent if not criminal.
As You say, lets cut the poor crew some slack. As for Boeing , non! They let us down!
It now looks like a fact that if the crew does NOT use the Cut Out Switches, the outcome is most likely a loss of control.
( I could argue that if I forgot this I would aggressively, re-trim back to in-trim condition and eventually select F1 and be surprised that I solved the problem and returned to land!? Boeing argues I would do the do the cut out!)
All they had to do was add a second AOA vane and this would never happened , but in true Boeing 737 style it is all about milking that old cow. ( I have to say from the 757/767 up they have great planes)
I love the 737-800, but she has some quirks that would defiantly be designed out if it was new.

The other day a saw a short warning that a 737 operator had some airspeed unreliable instances , one was with birdstrike and the AOA vane was damaged. This was a 737-800 so no MCAS, and now we know a bit more, but it will happen again on the MAX and Boeing better have it right as next time , if fatal she is grounded.
Lets hope we get a quick safe and REDUNDANT fix.
Regards
Cpt B

PS
Was there an Engineer on the jump seat or not? Anyone?
Ds

[Bergerie1]

BluSdUp,

There is a second AoA vane. See here:-
B737 controlability-questions & surprises.

[BluSdUp]

Bergerie1
Thanks, I am tempted to delete my latest post, but lets leave it there to demonstrate the Human Factor at work.

Now lets see if I can back paddle me out of this one!? ( Now I know how Boeing feels, not pleasant)

"Sure, now I remember, there is two, always was!"
" Fact still remains it is wired as a single source for the MCAS" ( ie no redundancy.)

[Ian W]

Originally Posted by Bergerie1

BluSdUp,

There is a second AoA vane. See here:-
B737 controlability-questions & surprises.

Well there is a second AoA vane but the MCAS and stick shaker system only uses one at a time. The AoA vane being used swaps with the other AoA vane with Weight on Wheels. It appears both AoA vanes are fed to the ADIRUs so that is where the 'unreliable airspeed' comes from.
Boeing seemed to have expected that the crew would switch off the Stab Trim, if it repeatedly added nose down, even if it canceled on 'manual' trim button press, but then started again, and again and again. One crew did switch off stab trim, the accident crew did not.

[FCeng84]

gums - 737MAX does not have an issue with pitch control authority. The pilot has plenty of pitch authority via the column to the elevator to initiate and recover from any pitch maneuver provided MCAS either does not function or functions with the design limited authority of 2.5 degrees stabilizer at low speed and less as speed increases. The characteristic that MCAS mitigates is one that would probably not have been deemed worthy of correction for a high performance military airplane design. The need, without MCAS, to relax the column pull to keep the AOA from increasing with constant column position would have probably been deemed "a feature". It is my understanding that there is not on any 737 model a risk of a locked in pull-up or locked in stall regardless of how the maneuver is flown. I further believe that starting from a trimmed pitch condition, any pull-up maneuver followed by releasing the column back to its detent will result in recovery to lower AOA / normal load factor. I don't think that a column push is needed to recover from a maneuver initiated via column pull provided one starts from a trimmed condition.

What MCAS strives to maintain is a monotonic, deterministic steady state pull-up pitch response as column pull is increased. The plot of steady AOA / normal load factor vs. column pull should be monotonic. The pilot should be able to target any desired AOA / normal load factor within the normal flight envelopes for those parameters by ramping the column aft to the appropriate position and holding it there. No pilot compensation required to achieve and maintain the targeted load factor during the typical demonstration maneuvers of 1 or 3 knot per second stall entries or wind-up turns.

[infrequentflyer789]

Originally Posted by jimtx

Yet the certification authority and the manufacturer don't see any need to warn you about avoiding the flight regimes that MCAS was added to protect when in fact you would be without it if you had to disable electric trim for a malfunction.

This is my big question too. Although in fact I can see that a system could be needed to meet certification but not need to be explained to pilots because it isn't significant in effect or likelihood, as far as I can see the certification itself also requires such a system to be visible to pilots.

FAR 25.672 (a)
A warning [...] must be provided for any failure in the stability augmentation system

So where is it, what does it say? "I'm sorry Dave, something has failed, something you don't know about, it's not important really" ? Really?

Then there is 25.672(b), which says the pilot must be able to deactivate the system or override it with normal control movement. You can't deactivate something you don't know exists (except by accident by deactivating something else), and MCAS cannot be overridden by column position.

Looking at the NG speed trim description I can fit it to 25.672, despite it having a high-AOA trim-down function (which MCAS is probably the b*****d offspring off) - because that function is part of speed trim, there to enable speed trim function to work at high AOA, and therefore there is a fail-warning, and speed-trim can be overridden by the column (column cutouts). MCAS doesn't seem to fit, to me.