A more detailed look at the 737MAX and MCAS

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A more detailed look at the 737MAX and MCAS

#1 Post by Boac » Fri Mar 29, 2019 9:16 am

Some 'technical insight' please:

Premise 1:
The MAX, whilst to all intents and purposes is a 'stable' aircraft, to satisfy certification requirements it requires a system to modify the stick force/alpha curve at high alpha due to the nose-up pitch force coming from the forward re-positioned engines.

Premise 2:
This is derived from the output of a single AoA vane through software. When this vane 'detects' a certain high alpha, it triggers a modifcation to the pitch control system via 'MCAS'.

Premise 3:
Previously the output of the 'active' vane was alone required to activate MCAS. Due to recent accidents, the software will now 'compare' the outputs of both AoA vanes and 'warn' the crew if they differ excessively.

Premise4:
It is, to me, unknown at this point what crews are expected to do then. I assume ( yes, I know!) they will be instructed to disable the aircraft electric trim system, thus disabling MCAS.

OK - with me so far?

Query:
Aircraft, whilst in practical terms the aircraft is now not 'certifiable', but still 'stable' and quite flyable (albeit with a much increased workload), are the crew now required to land at the nearest suitable airfield? Under the current 'Non-normals' for the 737 up to the NG, there is no requirement for any further actions once the electric trim is disabled. As far as the FAA/CAA/Boeing are concerned, the flight can continue. Believe me, having done the exercise a few times in the sim, it is bloody hard work! HOWEVER,it is do-able. The problem now is that the MAX is deemed to be 'unfit' for flight without the magic MCAS.

What now?

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Re: A more detailed look at the 737MAX and MCAS

#2 Post by barkingmad » Fri Mar 29, 2019 9:49 am

I suppose we'll have to wait til Mr Boeing has formulated a lawyer-proof "fix" and had it approved by this same erudite profession. Maybe some pilots will also be involved with the process having presumably been ignored and/or excluded from the initial design and aporoval process.

The manual trim scenario certainly was a strenuous but survivable situation for pilots of average build and strength which leads me to wonder if we're drifting into the anthropological differences between the operators' nationalities and (gawd forbid) diminutive members of the fairer sex.

Of course controls forces are supposed to be within the certification regs (Aaaah BCARS!!) but looking at some of the unfit snowflakes who have licences but can't open or shut a cargo door makes me wonder.

The Shackleton certainly required one to eat both the plate of cornflakes and the box but BCARs didn't apply to that wonderful museum piece.

Back to the thread, it will be interesting to view the thoughts of PILOTS in this forum and to compare them with the final and ultimate bodge issued by a once great aircraft manufacturer who seems to have lost their way?

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Re: A more detailed look at the 737MAX and MCAS

#3 Post by ian16th » Fri Mar 29, 2019 10:07 am

Can a former fairy ask a question please?

Has 'trimming' the a/c come to mean moving a control surface?

My understanding is that 'trimming' meant moving a trim tab, a sort of fine control.

Moving a control surface, rudder, elevators and ailerons is called controlling the a/c, the job that pilots do.
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Re: A more detailed look at the 737MAX and MCAS

#4 Post by barkingmad » Fri Mar 29, 2019 10:10 am

Pax jets evolved to move the entire tailplane slab to trim the aircraft leaving sometimes relatively ineffective elevator(s) to deal with the transient pitch changes required in 'normal' flight.

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Re: A more detailed look at the 737MAX and MCAS

#5 Post by k3k3 » Fri Mar 29, 2019 2:11 pm

The elevators are used for short term pitch inputs, if the need for it stays then the stab moves so the elevators are streamlined with the stab and there is no stick force required.

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Re: A more detailed look at the 737MAX and MCAS

#6 Post by ian16th » Fri Mar 29, 2019 3:19 pm

To summarise, instead of moving a small surface, the system makes a small movement of a large control surface.

Is this correct?
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Re: A more detailed look at the 737MAX and MCAS

#7 Post by Boac » Fri Mar 29, 2019 3:46 pm

Yes - and on most airliners. NB On Boeing this is only to 'trim' the system, not to make the initial control input. I am not aware of any airliner that has an 'all flying tail' where the whole tailplane is moved by the control column, but it is very common on supersonic fighter aircraft as it mainly eliminates shock wave effects.

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Re: A more detailed look at the 737MAX and MCAS

#8 Post by ian16th » Fri Mar 29, 2019 8:44 pm

Boac wrote:
Fri Mar 29, 2019 3:46 pm
I am not aware of any airliner that has an 'all flying tail' where the whole tailplane is moved by the control column, but it is very common on supersonic fighter aircraft as it mainly eliminates shock wave effects.
The Valiant had a flying tail! Operated by a separate switch, a switch that was blamed/suspected for more than one accident.

Search for XD869 at TOP.

Not proven but strongly suspected.
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Re: A more detailed look at the 737MAX and MCAS

#9 Post by Boac » Fri Mar 29, 2019 10:15 pm

No more airline drivers to comment on post #1?

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Re: A more detailed look at the 737MAX and MCAS

#10 Post by Slasher » Fri Mar 29, 2019 11:46 pm

My brain is mush after 2 sims during darkness smorning Boac but I'll try posting when I'm alive again. It's been a tough few days.

Ian - the stabiliser on a jet is deemed a secondary flight control, the same as what yaw dampers and flaps are too. Just that it's a very powerful chunk of metal influencing the longitudinal axis, especially at high IAS. The stabiliser aerodynamically is a more efficient way of elevator trimming than tabs as there's nothing sticking out in the airflow. This has a relationship on ANM/1000kg at Mach. This also means the stick will always be in the neutral position when correctly trimmed out.

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Re: A more detailed look at the 737MAX and MCAS

#11 Post by ian16th » Sat Mar 30, 2019 9:39 am

Slasher wrote:
Fri Mar 29, 2019 11:46 pm
This also means the stick will always be in the neutral position when correctly trimmed out.
So is the position of the stick not in relation to the actual position of the control surface?
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Re: A more detailed look at the 737MAX and MCAS

#12 Post by boing » Sat Mar 30, 2019 11:00 am

Generally true. On light aircraft the trim did (does?) alter the stick neutral position but on larger aircraft the idea is to have no offset of the elevators themselves (as explained in previous posts). You always want full travel of the elevators available. Sometimes you will need every bit of elevator travel. For instance, if the trim jammed in the higher speed cruise position you will need very high elevator movement and force at the slower speed of the approach for landing.

That looks as though it may have been the problem with the MCAS. It applied so much nose down trim the elevators could not prevent the aircraft descending.

The stabiliser will always be changing its angle relative to the airframe depending on the trim setting but this is a relatively infrequent movement. To provide short term pitch control the elevator will be moving frequently relative to the stabilser. Once the desired elevator movement is completed there will probably be some residual elevator force required to hold the new required pitch which is not something you want to be holding for a long period of time so this force is removed by re-trimming the stabiliser.


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Re: A more detailed look at the 737MAX and MCAS

#13 Post by Slasher » Sun Mar 31, 2019 12:39 am

ian16th wrote:
Sat Mar 30, 2019 9:39 am
Slasher wrote:
Fri Mar 29, 2019 11:46 pm
This also means the stick will always be in the neutral position when correctly trimmed out.
So is the position of the stick not in relation to the actual position of the control surface?
Sorry Ian I should've said "...when the elevator is correctly trimmed out." This is why I had so much trouble with my briefings to cadets (not that you're a cadet mate!) by not including such info to get the true message across. Thus I put you under the incorrect impression this applied to all primary flight controls which it doesn't. Rudder trimming for example results in a repositioning of the rudder's neutral position and you'd feel it in the rudder pedals. There's no such thing as a "flying fin" of course.

Since my head is full of the A320 and I haven't flown a 737 since 2004 I can't recall every detail about the type and rely somewhat on the 737 destructors here in Honkers for info.
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Re: A more detailed look at the 737MAX and MCAS

#14 Post by ian16th » Sun Mar 31, 2019 9:46 am

Thanks to Y'all.

Makes me pleased I don't need to get in a flying m/c and go anywhere.

If I wanted to, I can't start from here anyway!
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Re: A more detailed look at the 737MAX and MCAS

#15 Post by AtomKraft » Wed Apr 17, 2019 2:24 pm

The reason the THS is trimmable, is to allow the aircraft to take off safely whether the Cof G is forward or aft.

If, for any reason the THS is not in the correct place during any phase of flight, one is in the *sh*t*.

And will remain so until the situation is corrected.

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Re: A more detailed look at the 737MAX and MCAS

#16 Post by Undried Plum » Wed Apr 17, 2019 3:33 pm

Slasher wrote:
Sun Mar 31, 2019 12:39 am
There's no such thing as a "flying fin" of course.
Buggah! :((

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Re: A more detailed look at the 737MAX and MCAS

#17 Post by Slasher » Wed Apr 17, 2019 11:45 pm

Yeh ok Plum - no flying fins except on the SR-71. ;)))
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Re: A more detailed look at the 737MAX and MCAS

#18 Post by AtomKraft » Thu Apr 18, 2019 2:53 am

And the Vigilante....🤓

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Re: A more detailed look at the 737MAX and MCAS

#19 Post by Slasher » Thu Apr 18, 2019 9:22 am

Yeh yeh ok. I take it back. 😔

Atom another purpose why the stab is trimmable is it allows a greater envelope of CG range. Also on Airbuses there is a ECAM warning if the T/O trim is set outside the T/O range and if there is a mismatch between actual trim setting and what's inserted in the box. This becomes known when the T/O Config button is pressed on the ECP (simulates takeoff) as part of the pre-dep checks while taxying.
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Re: A more detailed look at the 737MAX and MCAS

#20 Post by AtomKraft » Fri Apr 19, 2019 2:35 am

Yeah slash. I agree. It's mainly for taking off with that wide cg range. Although it help to reduce drag in the cruise too of course.
In fact, the trimmable stab is what makes transport jets so useful. Without it they could not be loaded in so many different configs from MTOW to almost empty, and with pax all over the place.

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