TAWS vs MCAS. An engineering adaptation in philosphy?
 

TAWS developed to counteract CFIT. I am a private pilot, and engineer(not aero). At the time of development, the TAWS systems out there would provide Warning, and with later advancements, advice on how to resolve the CFIT (warble, + "terrain, pull up, terrain, pull up"). However, there was no overtaking of the controls to manually take the pilot out of the loop, and force the column aft. As I think of the times, about 40-ish years ago, anything like this MCAS idea where the plane knows better than the pilot what control inputs to make, it was up to the pilots to operate the plane, and the TAWS to provide additional information on how best to operate the controls(pull up, avoid CFIT, everyone lives).

MCAS developed to resolve the design changes from excess lift in climbing caused by the engine location and thrust, as well as nacelle added lift. yes, it is a completely different set of circumstance than TAWS. Yes, it is 'needed' for certification to meet the control deflection force criteria.

What if -- something like the LEAP engine or similar had been developed for an earlier airframe, say the 727. Would Boeing have the philosophy to implement a control deflection system that overtook the pilots responsibility? Having grown up into aviation at that time, I can think that the 'old guard' pilots union would be up in arms, and defiant of anything like this. Given the results, and the later development of TAWS, why would Boeing not simply enhance the stall warming similar to "stall, push forward, stall, push forward", and leave the management of the controls to the pilot, where it belongs? Guessing the FAA/CASA would not allow that kind of change in control forces, but this seems to be a 'fix' to meet a regulation, and not a 'fix' to meet a clearly defined fault.

The fix may be worse than the fault(control force change).

I think you may have forgotten about the stick pushers on various early jet high tail aircraft.

I have not.

Keep in m mind that the MCAS is not a stall prevention device; it is a stability augmentation device. The engine installation apparently led to a much reduced stick force gradient approaching the stall in some situations. All the MCAS was supposed to do was to artificially enhance the stick force. At 0.6 degrees per activation, that might have made sense. I cannot imagine ever needing 2.5 degrees of stabilizer movement to enhance any stick force gradient.

It's also worth noting that the elevator feel shift mechanism and the speed trim system have been doing this on the 737NG for quite some time. The MCAS appears to have been an extension of that philosophy, rather badly bungled.

it is a stability augmentation device

This sort of philosophy has been around for a long time ... for instance on a few of the piston to TP conversions where a variable downspring arrangement produced an SAS outcome typically to address problems with the miss. The usual simple fix for a system U/S was to limit the power (ie thrust) for the miss to keep the normal prop force (cf as well as nacelle added lift) problem under control.

There are reports in the literature dating back to the 40s and 50s on the basics of normal propeller forces.

Then respectfully
How does this fit into your presumption that pilots of that era would of been up in arms, if the aircraft had a system that overided the pilot?

Stick pusher - forcefully shoves the stick forward independently of the pilot
MCAS - trims the aircraft nose down independently of the pilot

I noted in the Boeing CFD modelling, they do not consider the center wing area for lift, so it appears they do not model the nacelles for lift either.
Might this be why the software fix was 0.6 degrees but in flight test, it was actually 2.5 degrees?

Airbus does use the center wing lift area in their models, but I dont know about engine nacelle.

Having flown 2 x stick pusher equipped aeroplanes (ATR and Avro RJ) the pushers could be over ridden with moderate strength. And also could be easily disabled. And had comprehensive fault detection clearly annunciated to the pilot..

Compare that to MCAS which appear to have none of these attributes.

https://www.nytimes.com/2019/04/11/b...-faa-mcas.html

Sully has spoken:

I cannot believe my eyes. The nacelles combined are almost as wide as the body! Can you supply more evidence to torture me?

Speculation, but modelling the wingtips is directed mostly at estimating lift and drag, since this is a critical point in determining fuel consumption. The B737 NG is a well tested design, so there may have been little need to examine the wing box area. The effect of the engines on stability may not have been fully thought through, since the horizontal stabiliser has sufficient authority in most flight regimes. The issues with MCAS relate to dynamic pitch up forces, not overall stability. Perhaps a 'comedy' of errors, under time-pressure? There should be plenty of evidence uncovered by subpoenas, to provide more details of who/when/where/what was decided.

The issues with MCAS relate to dynamic pitch up forces, not overall stability

Such is within the province of stability assessment. I think it more likely that the assessments were done adequately but the outcome management strategies might not have been well thought through ?

Absolutely true. My point was the sequence in which the modelling and flight testing uncovered the different stability issues. I could have worded it better, but was responding to a previous quote.

ec - just as a reminder, Boeing introduced MCAS (in part or in whole) to allow the MAX to fly and "feel" enough like previous 73s that it would not require crews to go through special training in transitioning to the MAX.

And that was a selling point to customers, not just the regulators.

Word on the street is, Boeing gave, e.g. SWA, a $1-million-per-aircraft "performance guarantee" that no sim time would be required to transition from NG to MAX. Just "iPad ground school."

A software patch connecting existing hardware (AoA sensors, trim system, THS, flaps) was the cheapest and easiest way to do that. Especially without screwing up "normal, everyday, straight-and-level" stability and handling.

(That's not a defense of MCAS, just an explanation).

Purely from the point of view of preventing a CFIT or a stall, modern planes already do have equivalent systems - audible warnings so that the pilots can correct things themselves. "Pull up, TERRAIN" vs. "STALL, STALL (rattle-rattle-rattle)."

Of course, if the RAs fail or the AoA sensors fail, the systems fail.

From what I have seen MCAS is supposed to stop working when either the pilot moves the control column or trims in the opposite direction.
It is supposed to be disabled by simply flipping the stab trim cutout switches.

Happy to be corrected, but that sounds awfully similar to the stick pusher setup

So says https://aviationweek.com/commercial-...432ab9ddf59858

To me it begs the question how light a weight, and how can you manage to be very light and have a full aft CoG.. I wonder what the constraints would be if the MCAS was done away with ie impact on weight or CoG.

indeed... very low fuel and high payload... maybe.?