Boeing 737 Max8 Aircraft Crashes and Investigations [Part 6] 17

[Sparweb]

This post is the continuation from this series of previous threads:

thread815-445840
thread815-450258
thread815-452000
thread815-454283
thread815-457125

This topic is broken into multiple threads due to the length to be scrolled, and images to load, creating long load times for some users and devices. If you are NEW to this discussion, please read the above threads prior to posting, to avoid rehashing old discussions.

Thank you everyone for your interest! I have learned a lot from the discussion, too.

Some key references:
Ethiopian CAA preliminary report

Indonesian National Transportation Safety Committee preliminary report

A Boeing 737 Technical Site

Washington Post: When Will Boeing 737 Max Fly Again and More Questions

BBC: Boeing to temporarily halt 737 Max production in January

 

[LionelHutz]

A wing doesn't go from producing maximum lift at the critical AOA to no lift the second you go past it. The airflow over the top of the wing doesn't suddenly change from being fully attached to completely separated. The airflow separates more and more as the AOA increases past critical. So, I would say airfoil stall would more precisely be when an increase in the AOA decreases lift, not when the flow over the airfoil is separated from the surface.

Apparently, certification requires the manufacturer to publish the stall speeds for a bunch of different flight configurations (weights and flap positions).

Each AOA in a specified configuration does have a minimum associated speed. You might say it's not the stall speed, but the end result is the same. Go to slow and the plane begins dropping out of the sky unless you either speed up or change the AOA to restore enough lift.

Saying that stall can't happen at 0 speed is being rather pedantic, really worthy of nothing more than being a water cooler discussion.

 

[Sparweb]

...an unstalled wing at 0 kts airspeed is easily demonstrated: In 0g flight the wing produces no lift and is essentially at 0 AOA, so the air happily sticks to wing and flows over it just fine. Or would if it were moving, like at a half knot or so. This is the condition encountered in a parabola or its subset the vertical climb.

An aircraft at any airspeed can do this. Such as the "vomit comet". The aircraft cannot have any lifting forces at the time it is following the ballistic trajectory, otherwise it would be accelerated - not the point of the flight.

My flight instructor also demonstrated this to me. I knew exactly what was going on and all the math - but it was fascinating and memorable just the same.

http://www.sparweb.ca

 

[Alistair_Heaton]

Here is a graph of some common types Cl V AoA note it includes 737-300 wing, the top curve is with flaps down slats out and the bottom one with sharp drop off is a clean wing.

While your correct that it does not go to zero immediately after exceeding Alpha crit depending on the profile it can be quiet a vicious drop off. But the drop in CL is usually enough you start dropping like a brick. Training type aircraft such a Cessna 150/152/172 have extreme placid stall characteristics and its more of a leaf falling towards the ground than a brick falling off the top of a building. Every one thinks Bernoulli makes aircraft fly but there is a fair old lump of Newton reaction force in there as well as most wings have a 3-5 deg angle of incidence built in relative to the hull so in no way is it zero lift, just not enough to maintain altitude or tighten a turn (which is usually where stalls occur). Conversely the piper tomahawk PA38 which I did most of my instructing in was a vicious little bitch stalling. But I quickly realised that this produced a better more prepared pilot post PPL completion after they moved to more benign aircraft types such as the 4 seater C172 which just makes a screeching noise the nose doesn't drop and it wafts towards the ground at a slightly greater rate of decent than normal. The Tommy nose drops by 30 degs you usually get the wing dropping suddenly all very quickly after a couple of buffets.

Designers have a few tricks to keep the airflow attached and move the initial stall point. They stick vortex generators on the top surface and they position them so the wing stalls at the root of the wing instead of the towards the tips or behind the alerions so you don't loose roll control in a stall.

But with all these things if you design the wing for nice easy stalling you loose other KPI's . Commercial aircraft are built mostly for fuel economy, they have two stage stall protection run off the AoA sensors (which is why they were initially added to the design) You have the stick shaker going off at 8% to go to the stall and a stick pusher forcing the nose down at 5%. When you turn the icing protection on they automatically alter the protection Alpha critical to a lower value and the trigger points change correspondingly.

As for the Vne speeds and stall Bob is correct for utility class aircraft you would bend the wings and the engine would cut out way before you reached stall speed. Extra, Steerman and a few others in the unlimited aerobatic class can do it although its extremely unpleasant even attempting it add most pilots will get G-LOC and release the back pressure on the controls before actually stalling. TO be honest outside acrobatic stupidity the main time this may be an issue and does happen is in a spiral dive or spin recovery. Some of these ultra light cat aircraft have quite low Vne with a cruise speed of 80-90 knots and a Vne of 130- 140kts. And once they dig themselves out of a spin they then end up pointing straight at the ground past Vne and pulling back on the stick with a dead mans grip. The poles in the wings bend, aircraft is written off, but most of the time the occupants survive.

This discussion might seem off topic. But there is a strong suspicion that MCAS is a stall protection device. If it is deemed to be one because of dirty stall characteristics then the certification requirements for it is going to go through the roof. We won't get to find out until the other regulators get there hands on the aircraft for test flights. EASA will be testing the flight envelope with MCAS turned off and stall characteristics will part of those tests.

And BTW the Steerman is a great aircraft to fly. Your wallet takes a huge hit though. It burns 50 ltrs an hour at 2$ a ltr in the UK. And that's just the fuel. Add on the dry rental price and you talking 500$ an hour. Mucho fun though. But I am a family man these days so don't go near anything that burns petrol or get in a helicopter of any type of fuel.

 

[VEBill]

"There is only one critical angle of attack for each aerofoil."

Wouldn't there be a 2nd one when the aircraft is upsidedown? e.g. A stunt aircraft making a level pass while inverted.

 

[Alistair_Heaton]

I like it, and you are correct

Inverted is beyond my knowledge. I just know that you get poo loads of drag and things can go very dangerous very quickly and you have to put opposite inputs in than usual to recover. And delta wings are a bit of a mystery as well.

There is a new course that all commercial pilots have to do before their first multicrew rating Advanced upset prevention and recovery. Every time we are in the sim we do some form of wake vortex/ mountain wave upset recovery. But as its a new requirement and only applies before you do the first type rating I won't have to do this course which is in a single engine piston aerobatic type.

 

[dik]

AH: "Inverted is beyond my knowledge. I just know that you get poo loads of drag and things can go very dangerous very quickly and you have to put opposite inputs in than usual to recover."

Even with a 747?

Dik

 

[waross]

Are you thinking about the famous barrel roll flybys with the prototype 707?
Barrel Roll

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Alistair_Heaton]

This is where it becomes complicated to visualise

Please see my comments above ref aircraft coordinate systems V ground reference system.

Most aerobatic maneuverers are done at positive G which means lift is still for want of a better word is in the normal direction.

A barrel roll and loop are both positive G manoeuvres so at no point is the aerofoil inverted even though the aircraft is upside down in relation to the ground. Its only when things are negative G that inverted aerofoil performance is occurring.

 

[moon161]

^That's not inverted 1g steady state that would give you a negative alpha. A barrel roll is a positive alpha case all the way around.
Pouring water during a barrel roll: Link

 

[Alistair_Heaton]

This lady is the world champion at aero's and I chose it because her hair is extremely good for seeing what forces are going on and when the aircraft is negative G.

BTW her neck muscles must be like wire rope.

She is pulling between +7 G and -2.5G during this display. Most people would black out. You can see her tense up before the heavy G pulls to trap blood in her head.

https://youtu.be/AYrH_rELbf4

 

[waross]

Does an aircraft lose a lot of altitude during a barrel roll or is it falling more forward than down?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Alistair_Heaton]

It should come out at the same alt as it goes in.

You basically draw a circle round a reference marker with your nose.

Pick a ref point 45 deg off the nose on the horizon. pitch up as you do start the roll input when your at the top your nose should be 45 deg above the horizon above the reference point. This is the lowest G loading of about 0.5g. The continue the rolling as you descend and when you get back to the same picture as you started every thing should be the same again. Gliders you loose maybe 200ft doing one.

The amount of alt gained at mid point is a function of the roll rate.

The main reason why most planes can't fly inverted or under negative G for any prolonged periods. Is nothing to do with the flight dynamics of the surfaces. It because the engines need fitted with special oil and fuel systems to keep them running.

Commercial aircraft can do Barrel rolls as well but its not normal and not trained for. Concorde apparently did one on its test flight after getting the rubber tank protectors fitted with the head of UK CAA test pilot at the controls and the BA Concorde fleet chief pilot in the other seat.

 

[LittleInch]

While all this is good stuff and very interesting, I think we need to remember why we're here discussing this.

The 737 MAX was designed around new, larger, more powerful yet more fuel efficient engines.

They didn't fit in the space available from the 737NG for historical design reasons. Thus the engines were placed further forward and higher than other similar aircraft. This has an impact on the flying characteristics of the plane, especially in high AoA situations whereby the control forces become lower as the AoA increases.

The 737 is a power assisted aircraft, much the same way as power assisted steering on a car. Like a car, the forces being fed back to the operator (pilot/driver) increase as the forces on the item in question increase. Other design (FBW) is to be a fully powered system which divorces pilot / driver input and runs it through a set of computers to decide what to make the control surfaces do. Thus those units need to be very reliable.

So for a car, to take an analogy, as you enter a bend and start to turn, if keeping a constant speed, the force to decrease the turn radius gets harder and harder until something finally gives and the car under or over steers. Same thing with the plane while increasing AoA. As I understand it this is the "rule" for this type of operation. This doesn't happen in the 737MAX and hence they introduced an artificial modification which mimics the same feedback. However the reliability of this needs to be very high to operate when it is supposed to and not when it isn't.

So is it a stall protection device? I think it is very close to that myself (though what do I know?). It still won't prevent a stall if the pilot just keeps pulling back, but at least he or she won't do it by accident as the stick forces reduce, which is counter intuitive and counter "normal" operation.

There are other things coming out into the open but if they haven't really got a reliable solution for the key underlying issue yet then it makes you wonder if they ever will.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

According to Boeing;
" MCAS is NOT a stall prevention system.
It is only there to prevent unexpected stalling."
If you can not say that convincingly you will not make it in Boeing management.
After some contradictory information about how the 737 elevators work and study of the elevator control diagram I believe the following is correct:
With the hydraulic system working properly, there is no feedback whatsoever on the elevator linkage.
It is full hydraulic power. The only force on the control cables is the force required to operate a hydraulic valve against the centering springs.
As the elevator moves the valve moves so although there is no force feedback there is position feedback.
The force on the stick is generated by the force computer which uses hydraulic pressure to apply a graduated force to the stick to mimic a direct mechanical feedback.
In the event of a complete hydraulic failure, There will be a force transmitted back to the stick via the linkage.
This increases the force on a fulcrum in the linkage system. The fulcrum is movable and is connected to the elevator trim tabs.
With a complete hydraulic pressure failure the trim tabs are able to move the elevator against the normal operating hydraulics. As the rudder moves due to trim tab action, the valve moves with it and so there is still position feedback as well as force feedback from the direct action of the trim tabs.
This is not that important in regards to the crashes as the hydraulic systems were functioning and the force computer had ample hydraulic pressure available to generate the extreme forces on the stick.
It would have probably made the plane more flyable if they had adjusted the proportional band of the force control computer so that the forces were still proportional but about 50% of what they are now in the Max and probably in the NG.

In perfect hindsight, how about this for a memory item.
In the event of repeated, unexpected nose down trim caused by a failure of the MCAS system, turn off the main STAB TRIM switches.
Restore trim manually.
In the event that the aerodynamic forces on the stabilizer are too great to allow manual trimming the following procedure may be used to trim the stabilizer electrically.
WARNING
When the STAB TRIM switch is turned on, MCAS will reactivate in 5 seconds and/or will activate 5 seconds after the column switch is released.
To trim electrically, the STAB TRIM may be turned on and a correction made with the column switch within 5 seconds. The STAB TRIM switch must be turned off within 5 seconds of releasing the column stabilizer switch.
Alternatively. The column switch may be held in the direction that it is desired to move the STABILIZER TRIM, and the trim activated by momentarily turning the main STAB TRIM on and off.
IN THE EVENT OF UNWANTED, REPEATED NOSE DOWN TRIM EVENTS THE MAIN STAB TRIM SWITCHES MUST NOT BE LEFT ON FOR MORE THAN 5 SECONDS.
The pilots in the second crash may have been about to figure this out. All except the 5 second "Gottcha". With more altitude they would probably have survived.
Unwilling martyrs to the correction of a flawed system, both mechanical and regulatory.

That information, shared with pilots in a timely manner would have avoided the second crash, and so would have avoided the groundings and worldwide re-certification.
That may well have triggered sim training and cost Boeing a few hundred million in rebates, while saving a few billions and a reputation and the loss of future sales.
Any potential customer weighing the choice between Airbus and Boeing will most likely go airbus.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LittleInch]

I'm very happy to be proved wrong in my understanding of how the elevators work and the force feedback, but this doesn't quite gel with the requirement for MCAS in the first place.

My understanding was that the issue was that the stick force required to increase AoA did not increase with AoA as required by the certification rules.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

This is my understanding. The MCAS original design intent was to provide the correct "feel" to mimic the 737NG's stick, which would minimize the amount of retraining.

On top of that was the part that FUBARed the two planes, which was to force the nose down if there was excess AoA, based on issues encountered during testing.

Because MCAS depended on only one of the two available AoA sensors to determine its behavior, a single point failure would cause the MCAS to do what it did.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[dik]

...and it seems to get more interesting:

"Internal emails from Boeing staff members working on the 737 MAX were made public earlier this month have revealed new safety problems for the company's flagship 777X, a long-range, wide-body, twin-engine passenger jet, currently in development that is expected to replace the aging 777-200LR and 777-300ER fleets, reported The Telegraph.

Already, damning emails released via a U.S. Senate probe describes problems during the MAX development and qualification process. The emails also highlight how Boeing employees were troubled by the 777X – could be vulnerable to technical issues."

Dik

 

[thebard3]

Seeing that management changes have only been made recently, it seems likely that the 737 MAX issues would be systemic and therefore have metastasized into other programs. My only wonder is how was the 787 spared (or was it)?

Brad Waybright

It's all okay as long as it's okay.

 

[Alistair_Heaton]

It wasn't, the batteries are still causing issues on the 787.

There is also issues with the lightning protection of the composite panels.

 

[thebard3]

I'm aware of the battery problem, but I think that is discrete and different from the problems with the 737 MAX and (alledgedly) the 777X programs where it seems that Boeing obfuscated and obscured information from the FAA for systems that they knew were not what they were claimed to be, and apparently realized was poorly designed, at least in the 737's case. I guess I wasn't privy to the composite lightning protection design until you just pointed it out. It seems Boeing needs to organize a new department of "what to do, what to do..."

Brad Waybright

It's all okay as long as it's okay.