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

 

[moon161]

RE: Is it an anti-stall system

I think this is why 737 max 8 sim models, and until the crashes happened, the existence of MCAS were kept as confidential as possible. With the boeing 'pilot as the ultimate authority' philosophy, stick/yoke feedback is more critical than with an FBW system with programmed attitude, alpha and speed limits because stick feedback is a proxy for the stick feel that a pilot would have with unpowered controls, and would be a cue for the pilot about a/c performance.

If you ask 'what does an anti-stall system do?' Signals impending stall and modifies aircraft configuration or pilot input to prevent passing critical alpha. some answers could be:

Prompts pilot about impending stall (stall horn, stick shaker)
Modifies/prevents pilot input that would stall a/c (alpha limiter, stick pusher)
Modifies wing to change stall characteristics based on alpha or airspeed.(automatic slats passive on rollers like A-4, actuated like F-16, both change wing camber and alpha)
Autothrottle linked to alpha or airspeed to maintain margin on stall speed at 1g, steady state airspeed

MCAS was added as a patch because at high alpha (unknown how high?, how near critical alpha?) the stick feel was not sufficiently increasing in proportion to alpha. Or was it alpha was no longer linearly reponsive to to elevator input? Since it's full hydraulic this begs the question 'couldn't they modify the artificial feel unit instead?'. Taken at face value, since stick feel is a stall avoidance input to the pilot, if it looks like a duck and quacks like a duck, it's a duck.

 

[Sparweb]

You've all got the picture, AFAIK. It's not about the stall or the angle of stall, it's about the slope of the curve approaching stall that's non-linear. Without a feedback system, a pilot could pull back by 1 inch with added 1 pound of force to keep it back, but the next 1 inch back would take <1 pound to hold. If at some point you can pull back another inch with nearly zero additional force, then the control feedback vanishes and the confusion begins. Pull it back 1 more inch, and you will need -1 pound to keep it there, and now confusion rules.

This figure was posted here many months ago but since the topic has come back...
It shows that the feedback is linear but not at high angle of attack. Nose-down is positive in airplane pitching moment terms, so that's why the trend goes down (nose-up). The graph also shows the complication of the MCAS adjustment that puts the feedback force back closer to the original linear trend. As with any non-PID feedback control, using discrete lumps of negative feedback leads to zig-zag trend lines.

http://www.sparweb.ca

 

[waross]

LittleInch.
The point is that there is no force feedback in normal flight. The force on the stick is generated artificially by the force computer.
A dependable AoA signal could have been used to bias the force computer.
The force on the stick relative to the elevator deflection could have easily been adjusted to much less and still been proportional in accordance with the regulations.

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

 

[waross]

Apart from the artificial restrictions on the MCAS, such as avoiding sim training, is there any requirement for a uniform stick feel on all aircraft?
Would the FAA allow an aircraft to fly with 1/2 of the stick force of a similar but different model plane?
Is there any regulatory requirement for the high forces on the stick that MAX pilots experience when fighting off a MAC attack?

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

 

[IRstuff]

To some degree, I would think that one wants relatively uniform behavior across all planes a pilot is certified to fly; otherwise one runs the risk of the pilot expecting one behavior in an emergency, but getting something different, failed to respond correctly.

I have much more trivial issues with cruise control activation across the different cars I drive, because they are different.

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

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

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Sparweb]

25.173
Static longitudinal stability.

[Under the conditions specified in Sec. 25.175, the characteristics of the elevator control forces (including friction) must be as follow:
(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC / MFC , whichever is appropriate, or lower than the minimum speed for steady unstalled flight.
(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in Sec. 25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in Sec. 25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.
(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.
(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.]
Amdt. 25-7, Eff. 11/14/65

Sec. 25.175
Demonstration of static longitudinal stability.

Static longitudinal stability must be shown as follows:
(a) Climb. The stick force curve must have a stable slope at speeds between 85 and 115 percent of the speed at which the airplane--
(1) Is trimmed, with--
(i) Wing flaps retracted;
(ii) Landing gear retracted;
(iii) Maximum takeoff weight; and
(iv) 75 percent of maximum continuous power for reciprocating engines or the maximum power or thrust selected by the applicant as an operating limitation for use during climb for turbine engines; and
(2) Is trimmed at the speed for best rate-of-climb except that the speed need not be less than 1.3 VSR1.
(b) Cruise. Static longitudinal stability must be shown in the cruise condition as follows:
(1) With the landing gear retracted at high speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1, nor speeds greater than , nor speeds that require a stick force of more than 50 pounds), with--
(i) The wing flaps retracted;
(ii) The center of gravity in the most adverse position (see Sec. 25.27);
(iii) The most critical weight between the maximum takeoff and maximum landing weights;
(iv) 75 percent of maximum continuous power for reciprocating engines or, for turbine engines, the maximum cruising power selected by the applicant as an operating limitation (see Sec. 25.1521), except that the power need not exceed that required at / ; and
(v) The airplane trimmed for level flight with the power required in paragraph (b)(1)(iv) above.
(2) With the landing gear retracted at low speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1,nor speeds greater than the minimum speed of the applicable speed range prescribed in paragraph (b)(1), nor speeds that require a stick force of more than 50 pounds), with--
(i) Wing flaps, center of gravity position, and weight as specified in paragraph (b)(1) of this section;
(ii) Power required for level flight at a speed equal to (VMO + 1.3 VSR1)/2 and
(iii) The airplane trimmed for level flight with the power required in paragraph (b)(2)(ii) of this section.
(3) With the landing gear extended, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1 , nor speeds greater than VLE, nor speeds that require a stick force of more than 50 pounds), with--
(i) Wing flap, center of gravity position, and weight as specified in paragraph (b)(1) of this section;
(ii) 75 percent of maximum continuous power for reciprocating engines or, for turbine engines, the maximum cruising power selected by the applicant as an operating limitation, except that the power need not exceed that required for level flight at VLE ; and
(iii) The aircraft trimmed for level flight with the power required in paragraph (b)(3)(ii) of this section.
(c) Approach. The stick force curve must have a stable slope at speeds between VSW , and 1.7 VSR1 , with--
(1) Wing flaps in the approach position;
(2) Landing gear retracted;
(3) Maximum landing weight; and
(4) The airplane trimmed at 1.3 VSR1, with enough power to maintain level flight at this speed.
(d) Landing. The stick force curve must have a stable slope, and the stick force may not exceed 80 pounds, at speeds between VSW and 1.7 VSRO with--
(1) Wing flaps in the landing position;
(2) Landing gear extended;
(3) Maximum landing weight;
[(4) The airplane trimmed at 1.3 VSRO with--
(i) Power or thrust off, and
(ii) Power or thrust for level flight.]
(5) The airplane trimmed at 1.3 VSRO with power or thrust off.

Amdt. 25-115, Eff. 8/2/2004

http://www.sparweb.ca

 

[waross]

Thank you Spar.
I recognize the effort you have expended.
May I impose on you and ask for an explanation of this:

(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots

When an aircraft is cruising in a trimmed condition and the force on the stick is basically zero, how is this reg applied?


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

 

[Alistair_Heaton]

You change the speed by 6 knts then measure the force on the stick.

 

[waross]

Thanks Alistair.

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

 

[LittleInch]

Ok, I got this wrong. The issue as I see it now isn't so much the feel of the stick and elevator ( which is more artificial than I anticipated ) as the fact that as you get to about 10 degrees AoA the position of the stick is no longer proportional to what the airplane wants to do.

i.e. up to about 10 degrees as you pull the stick back for every inch you get a similar effect in terms of AoA. The issue is that above 10 degree this effect changes and for every further inch back the plane stops responding in the same way and the elevator has a lower effect than before. Hence for the same amount of stick movement the AoA increase faster.

So to make the plane respond in the manner sparweb quotes, they needed to counter the lift being generated by the engine nacelles and the way they chose was to use the large stabiliser to counter this and allow the elevator to be the sole/ main input into increasing AoA.

My main point is that this is so fundamental to the design of the aircraft that given its age of design and lack of the required reliability of the control systems compared to true FBW machines, that there may not be a solution i its current form.



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

 

[waross]

How about this:
AoA sensors and reliability:
How about a two point calibration check and agreement check before each flight?
WOW! Over the top. Completely unrealistic, right?
That may be a lot easier and a lot simpler than you think.
When the plane leaves the gate, both AoA sensors will be pointing straight down. Easy to automatically check the position each time the plane leaves the gate. Easy to check agreement.
Next check: As the plane approaches takeoff speed, the AoA sensors will be horizontal. Again easy to check the position and agreement.
A third check may be done to compare the point at which the AoA sensors begin to move. That will check mechanical binding and freedom of movement.
An automatic, two point calibration and freedom of movement check each time the aircraft leaves the gate.

Now what do we do with this signal?
There will be a signal coming to the stick force generator from the force computer. A suitably conditioned AoA signal may be used to bias that signal and continue to increase the stick force with increasing AoA.
How do we know which AoA sensor to use?
Easy, use both. A circuit as simple as two diodes may be used to select the higher of two signals.

Alternately, and remember that the stick shaker and alarms will be going off to alert the pilot that he is approaching a stall.
He is already at a bad AoA. Use both signals compounded. The AoA signal conditioners can be set so that the primary sensor leads the secondary sensor by a few degrees.
This added force may violate the proportional force rule, but the pilot has already continued to increase his AoA despite alarms, the stick shaker and the increased stick force. More stick push-back may not be a bad thing and may be accepted by the regulators.
It has a lot of KISS going for it.

Bird strikes:
That may be easy to recognize.
A flock of birds.
A thump.
Immediate, increased stick force.
The plane is still controllable.
A three position switch, left, both and right.
Cut out either but not both AoA sensors.
Let the pilot fly the plane. KISS
And, by the way, restore the stabilizer control circuits to the original NG configuration.
Step 1.
Approval in principle from the regulators.
Step 2. World wide headlines:
MCAS IS GONE
REGULATORS APPROVE MAX 8 FIX WITHOUT MCAS
PRODUCTION TO RESTART IMMEDIATELY

Step 3. Rewire the controls on the existing fleet back to NG configuration may start within days.
Production may restart slowly and efficiently.
Think of any objections as engineering challenges to be met rather than reasons that it will not work.




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

 

[Alistair_Heaton]

There is multiple solutions now they have ditched the iPad training.

Only problem is which one is quickest to get through certification and costs the least to retro fit to 500 airframes already built.

 

[waross]

It looks as if Boeing is pursuing band-aids on band-aids on band-aids, with MCAS being the first band-aid.

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

 

[dik]

AH: "Only problem is which one is quickest to get through certification..."

Looking through my dad's old log books; he was a pilot for the RCAF during the last war. There are numerous certifications for different aircraft, and it appears the time involved was a day or less... planes must have been a lot simpler back then...

Dik

 

[Alistair_Heaton]

They could fly a new plane after reading the pilot notes in the bar over lunch in the 2nd ww. And swap between single engine spits one day and multi engined bombers the next.

And they weren't simple aircraft to pilot. But system wise they were. But they had a large brutal failure rate compared to today.

Certification in the context I was using is the aircraft not the pilot type rating.

 

[dik]

AH: what bar... at the beginning of the war it was de-icer fluid... pure ethyl alcohol... then the air force denatured it by adding a toxic additive... and it came in 40 gal drums... with the oxygen masks... they sobered up pretty quick.

Dik

 

[SnTMan]

Weren't flying paying passengers, either.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Alistair_Heaton]

Seems there is a clash of Regulators now over the wiring loom possibility of short circuits in the tail.

FAA and Boeing say its fine as they are and have a powerpoint presentation to prove it.

EASA says move them.

FAA is going to release a statement next month on the subject. Which I presume won't occur until there is some form of financial result declaration at the same time.

 

[debodine]

Alistair:

They have a PowerPoint proving all is fine? Well, then <sarcasm on> I just don't see how EASA has any reason to gripe! <sarcasm off> FYI I truly enjoy your posts and since I caught a whiff of your sarcasm in this one, I thought I would just push the noise level up a bit.

 

[Alistair_Heaton]

Thanks, it seems to be a feature of being Captain. Those that don't seem to have healthy dose of it seem to suffer health wise. Must act as a stress release.

I found the site when the Miami bridge collapsed. And found good intelligent educated comments, which is somewhat of a rarity on the internet these days. PPrune when I started flying was the same but in the last 10 years has rapidly lost its professional educated members and seems to be mostly populated by wannabies, spotters, self loading freight, reporters and walts. I haven't gone near it in years. The rotary section is still populated by those that know what they are talking about. And rotary pilots levels of sarcasm is leagues ahead of fixed wing.

Having not practised as a mech eng for 17 years now I felt I wasn't in position to add anything. This thread came up and I felt I could add to the discussion with technical input. So I joined up.

Its actually very good for me to interact with designers and those that are practising. I have learned a lot from the discussions as well. Thanks to you all with a special mention for Sparweb who has corrected my deviations from course and provides the raw regulation for certification.