Boeing, and FAA oversight

[BDAttitude]

Originally Posted by Peter H

c) A stable aircraft may be uncertifiable because of "feel" requirements. It seems possible that MCAS was introduced to address this issue. I cannot remember if B has commented on this.

Which does mean no less than that the stability margin is not large enough to satisfy the certification requirements.
Stressing repeatedly the FEEL term is just the lucent attempt to disguise that it is actually a stability requirment that is violated and give it a harmless spinn.

[OldnGrounded]

Originally Posted by MechEngr

Quite a few of the astonishing items in that article are false. But it is written as a Gish Gallop, so an equally lengthy response is required to debunk them.
Example: That the 737 MAX was unstable and MCAS corrects that instability. So far, all facts point to False. The 737 Max is stable and MCAS is not needed to correct instability. There are many others, but since it starts off with misinformation, it is not a good sign.

I'm pretty sure that there are at least as many knowledgeable observers who suspect that MCAS is a longitudinal-stability/anti-stall system as there are those who accept B's assurance that it is not. The stick-force gradient explanation definitely doesn't pass everyone's smell tests.

Edit: Oops. I should have read the other responses to MechEngr's post before typing this one -- I probably wouldn't have bothered. I think the point is made that the real purpose (and need for) MCAS remains an open question.

[568]

Originally Posted by MechEngr

Quite a few of the astonishing items in that article are false. But it is written as a Gish Gallop, so an equally lengthy response is required to debunk them.
Example: That the 737 MAX was unstable and MCAS corrects that instability. So far, all facts point to False. The 737 Max is stable and MCAS is not needed to correct instability. There are many others, but since it starts off with misinformation, it is not a good sign.

I think I have asked you this question before, "do you fly 737's and if so how many hours?

Please enlighten us as to what "astonishing items in that article are false".

The MAX is unstable due to various forces on the engine/airframe at high angles of attack at low speed and also at high speed!
MCAS is installed to correct for the above and possibly because of stalling aoa and other aerodynamic forces, which Boeing may have found during flight test.

[Luc Lion]

Originally Posted by OldnGrounded

I'm pretty sure that there are at least as many knowledgeable observers who suspect that MCAS is a longitudinal-stability/anti-stall system as there are those who accept B's assurance that it is not. The stick-force gradient explanation definitely doesn't pass everyone's smell tests.

if I am not mistaken, in one of the multiple previous MCAS threads, a Boeing engineer has explained that MCAS had been designed to comply with 14 CFR Part 25.175.
https://www.ecfr.gov/cgi-bin/text-id...se14.1.25_1145
This requirement is definitely about stick force gradient (or stick force stable slope) and its title is "§25.175 Demonstration of static longitudinal stability."

So the difference between a "longitudinal stability system" and a "stick-force gradient correction system" might be just pedantic.

[BDAttitude]

Originally Posted by Luc Lion

So the difference between a "longitudinal stability system" and a "stick-force gradient correction system" might be just pedantic.

Those are synonymous even for the pedants.
There are two pedants' arguments:
- If an aircraft that has not demonstrated failed to demonstrate "static longitudinal stability" according to the FAR you kindly linked, may it be called "unstable"? Answer: For pedants, physicists and engineers no. It might still be stable but just not stable enough. For Joe Average and just about every journo, yes.
- If a system is augmenting logitudinal stability at high AOAs is this an "anti stall device"? For Boeing it is not, for most others including pedants, engineers and physicists, it is.

[OldnGrounded]

Originally Posted by Luc Lion

if I am not mistaken, in one of the multiple previous MCAS threads, a Boeing engineer has explained that MCAS had been designed to comply with 14 CFR Part 25.175.
https://www.ecfr.gov/cgi-bin/text-id...se14.1.25_1145
This requirement is definitely about stick force gradient (or stick force stable slope) and its title is "§25.175 Demonstration of static longitudinal stability."

So the difference between a "longitudinal stability system" and a "stick-force gradient correction system" might be just pedantic.

Yes, the reference has been cited multiple times. When it has, it's almost always in the context of someone claiming that MCAS isn't an anti-stall/stability-enhancement system -- because the MAX allegedly doesn't need one -- but is "only" to address the stick force requirement.

From now on, I think I'm going to save BDAttitude's post, just above, and paste it in as a response to repeated assertions that "it's not anti-stall."

[Snyggapa]

In amongst the pedantry of whether failure to meet a "stick force gradient (or stick force stable slope) and its title is "§25.175 Demonstration of static longitudinal stability" means that we can call the aircraft unstable or not, I am surprised that there is so little comment about the potential wiring bundle chafing problem identified above, where if I understand it correctly it seems that a live wire could short to the to stab trim control and drive the stab trim to the stops - without the cutoff switches doing anything about it.

Coupled with the inability to stop it or to use the now tiny manual trim wheel , how many people fancy their chances of winning that argument with the aircraft ?

Just because it hasn't happened yet (205 million safe hours) doesn't mean that it won't happen as things age and fatigue (think pickle forks)

[OldnGrounded]

Originally Posted by Snyggapa

In amongst the pedantry of whether failure to meet a "stick force gradient (or stick force stable slope) and its title is "§25.175 Demonstration of static longitudinal stability" means that we can call the aircraft unstable or not, I am surprised that there is so little comment about the potential wiring bundle chafing problem identified above, where if I understand it correctly it seems that a live wire could short to the to stab trim control and drive the stab trim to the stops - without the cutoff switches doing anything about it.

Coupled with the inability to stop it or to use the now tiny manual trim wheel , how many people fancy their chances of winning that argument with the aircraft ?

Just because it hasn't happened yet (205 million safe hours) doesn't mean that it won't happen as things age and fatigue (think pickle forks)

If the FAA has the same view of the risk, what might that mean for all those NGs?

[megan]

How did we get here from there? What .b737.org.uk has to say.

CFM powered aircraft - Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas, pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition.LEAP powered aircraft - MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

LEAP powered aircraft - The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.

MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance longitudinal stability characteristics with flaps UP and at elevated Angles of Attack (AoA). The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control Computer (FCC) using input data from sensors and other airplane systems.

The MCAS function becomes active when the AoA exceeds a threshold based on airspeed and altitude. MCAS will activate for up to 9.26 seconds before pausing for 5 seconds. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers (for the same AoA above the activation threshold).

After AoA falls below the hysteresis threshold (0.5 degrees below the activation angle), MCAS commands nose up stabilizer to return the aircraft to the trim state that existed before the MCAS activation.

The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

To summarise; MCAS will trim the Stabilizer down for up to 9.26 seconds (2.5 deg nose down) and pause for 5 seconds and repeat if the conditions (high angle of attack, flaps up and autopilot disengaged) continue to be met. MCAS will turn the trim wheel. Using electric pitch trim will only pause MCAS for 5s; to deactivate it you need to switch off the STAB TRIM CUTOUT switches.

Notice the commonality between the purpose of the STS and MCAS? The way I read it, because the LEAP is larger than the CFM it has a greater influence

Thread on the STS B-737 Speed Trim System

[BDAttitude]

Re 248:
Well, you know, if the wire bundle is babied well enough, it might not become a problem. On the other hand, a fatality might be just a carelessly executed safety wire pig tail (or any other ridge, or forgotten screw driver) away, as we have just learned in the EMB thread.
These rules are there for a reason. And a company which consideres itself epitome of civil aerospace engineering would be honoured to comply.

[Luc Lion]

Originally Posted by Snyggapa

I am surprised that there is so little comment about the potential wiring bundle chafing problem identified above, where if I understand it correctly it seems that a live wire could short to the to stab trim control and drive the stab trim to the stops - without the cutoff switches doing anything about it.

Coupled with the inability to stop it or to use the now tiny manual trim wheel , how many people fancy their chances of winning that argument with the aircraft ?

No.

I am a bit too lazy to look for the link to the wiring diagram, but I remember that the cut-off switches remove power from the whole system early in the diagram and undoubtedly before the wiring bundles carry the command to the tail section.

The hazard involved with the bundle is similar to the MCAS hazard : the pilots must cut-off the faulty command early if they want to save the day.

[Snyggapa]

Originally Posted by Luc Lion

No.

I am a bit too lazy to look for the link to the wiring diagram, but I remember that the cut-off switches remove power from the whole system early in the diagram and undoubtedly before the wiring bundles carry the command to the tail section.

The hazard involved with the bundle is similar to the MCAS hazard : the pilots must cut-off the faulty command early if they want to save the day.

Not what the leak above suggests, which implies that I guess a permanent live for another part of the system runs in close proximity to the stab trim after the cutout switch - so if the two chafe and short, you get an uncontrolled runaway that you can't stop.

"In one instance, engineers found a hot power wire that was too close to two command wires running to the jet’s moveable horizontal tail, or stabilizer, one for commanding the tail to swivel to move the jet nose-up, the other to move it nose-down. The danger is a short that causes arcing of electricity from the hot wire to the command wire.

“If a hot short occurs between the power wire and either the up or down command wire, the stabilizer can go to the full nose-up or nose-down position,” the engineer said.

Furthermore, the electrical power in that wire could circumvent the cutoff switches in the cockpit that, in the event of such a stabilizer runaway, are used to kill electrical power to the tail. Theoretically, the pilots could be unable to shut it off."

[Luc Lion]

Thanks.
I stand corrected.

[568]

Originally Posted by OldnGrounded

Yes, the reference has been cited multiple times. When it has, it's almost always in the context of someone claiming that MCAS isn't an anti-stall/stability-enhancement system -- because the MAX allegedly doesn't need one -- but is "only" to address the stick force requirement.

From now on, I think I'm going to save BDAttitude's post, just above, and paste it in as a response to repeated assertions that "it's not anti-stall."

OAG,

If you take a peek back at the locked thread on the ET accident, I mentioned that if MCAS was described as an "anti-stall" device, then the MAX would have to have a new type certificate as it differed from the previous 737 variants.

[OldnGrounded]

Originally Posted by 568

OAG,

If you take a peek back at the locked thread on the ET accident, I mentioned that if MCAS was described as an "anti-stall" device, then the MAX would have to have a new type certificate as it differed from the previous 737 variants.

Yup. I remember. Probably most here who were raised in the Anglophone world remember this, from Walter Scott's poem, "Marmion" (even if some of us didn't know the origin):

"Oh! What a tangled web we weave when first we practice to deceive"

[568]

Originally Posted by OldnGrounded

Yup. I remember. Probably most here who were raised in the Anglophone world remember this, from Walter Scott's poem, "Marmion" (even if some of us didn't know the origin):

Know it well.

Just looking back at some posts in this thread and I was hoping that we keep the MCAS on a 'straight and level" and the main title thread as our diligent mods will want to keep the threads "on point".
The wiring bundle issue isn't going to go away as the regs have changed before the MAX was put into production.

Will be interesting to see what transpires.

[BDAttitude]

Given the data available from the NG fleet I could see a rewiring mandated for the next C or D-Check. It shouldn't be too much of a hassle, especially in the later case.
However delivering planes and issuing CoA to already manufactured but non-conforming planes ... that's a horse of different color, especially in today's regulatory atmosphere.
I guess we are seeing one of the risks in stockpiling not deliverable planes materializing. If only anybody had warned them.
They'd better save themselves a lot of time and money and start fixing them NOW.

[fdr]

Originally Posted by Snyggapa

Not what the leak above suggests, which implies that I guess a permanent live for another part of the system runs in close proximity to the stab trim after the cutout switch - so if the two chafe and short, you get an uncontrolled runaway that you can't stop.

"In one instance, engineers found a hot power wire that was too close to two command wires running to the jet’s moveable horizontal tail, or stabilizer, one for commanding the tail to swivel to move the jet nose-up, the other to move it nose-down. The danger is a short that causes arcing of electricity from the hot wire to the command wire.

“If a hot short occurs between the power wire and either the up or down command wire, the stabilizer can go to the full nose-up or nose-down position,” the engineer said.

Furthermore, the electrical power in that wire could circumvent the cutoff switches in the cockpit that, in the event of such a stabilizer runaway, are used to kill electrical power to the tail. Theoretically, the pilots could be unable to shut it off."

That's a fairly untidy situation. Applying opposite trim would have power to both sides of the trim motor, would be interesting to see how the logic circuits would work with that. TBC would be able to identify the potential power source for the offending wiring, and ascertain if they can be isolated safely without shedding power down to a dark plane. The plane can fly dark, not much fun and not great for pax experience, but it ain't necessarily over until the singing is done. (Take an iPad, Stratus S2, and an iridium go.... and Amex card, don't leave home without them)

[Off topic slightly, but FYI the following show what is available as information available from a completely independent system on an aircraft. That is 1090 ADSB IN info and a synthetic view of traffic, terrain, runway etc having spent 1/2 a million on "modern" systems, in the last couple of years, the only system that really impresses me is that below. No iPad, no fly IMHO. The iPad gives better information than TCAS II Ch 7.1 for my money. Add to the fact it uploads FPL to the APFD's, and we can modify the current path from the iPad, it's pretty neat. The SA enhancement is impressive in high terrain areas, better than the dual TAWS EGPWS/GPWS system for SA]

[Twitter]

Well, they’ll have plenty of time to change the wiring, while they dismantle the tanks and remove the FOD. In fact they have had plenty of time to put in some alternatives to MCAS, which were derided when proposed on pprune a year back - due to the unacceptable delay incurred...

[OldnGrounded]

Originally Posted by 568

Know it well.

Just looking back at some posts in this thread and I was hoping that we keep the MCAS on a 'straight and level" and the main title thread as our diligent mods will want to keep the threads "on point".
The wiring bundle issue isn't going to go away as the regs have changed before the MAX was put into production.

Will be interesting to see what transpires.

Oh, yeah, you're right about staying on-topic. I misunderstood your intent.