Originally Posted by
LEOCh
Thanks Ian. This confusion is very widespread in the current general (outside pprune) consensus of current MAX issues. Not only in the general media, but even in articles/content generated by commercial pilots and industry members (including current NG pilots), MCAS is almost ubiquitously described as an antistall system operating somewhat like a stick pusher operating on the stabilizer instead. This is required because of a putative dangerous pitch up tendency at high AoA.
We are very unlikely to get the actual MAX stability data from Boeing but the Cm versus alpha data is more likely to be something on the left (similar to that suggested in Bjorn's corner at Leeham news). But many reasonably technical writers are describing something more like on the right. This seems unlikely because certification requires the aircraft to be safely flyable without stability augmentation.
This seems to be a double edged sword for Boeing going forward. There is a lot of semi-informed consensus now that the re-engine was totally unacceptable because the unaugmented plane is dangerously unstable, and will remain so even with an improved MCAS2. But demonstrating that the unaugmented plane is fairly safe given pilot knowledge of it's decreased stability margin (over the NG) makes a lethal failure of MCAS1 look even worse, as it the problem it was needed to solve was more certification and sales critical than safety critical.
LEOCH, you are absolutely correct.... For the record:
MCAS normalises a stick force gradient that does not fully meet the applicable certification standard § 25.173 Static longitudinal stability, sub paras a through d. It is not a stall prevention mechanism. Stall warning requirements are specified in § 25.207 Stall warning and for handling in § 25.203 Stall characteristics. Compliance with the latter is exhibited in demonstration compliant with § 25.201 Stall demonstration. The lifting body effect of the engines is a non linear effect, at a modest relative inflow angle, they will develop lift, at high angles that lift increment will not occur, and inertial forces will dominate the aircrafts behaviour, weight is still forward of the Cp, the plane will pitch down at the break #.
The non linear stick force gradient issue is not permitted to be so significant that the failure of the augmentation system precludes flight within the operational envelope, and specifically up to the stall. § 25.672 Stability augmentation and automatic and power-operated systems.
In simple terms, the control force in part of the operating envelope, outside of normal flight conditions experienced by the RPT pilot, but within the full flight envelope did not meet the standard that was set half a century ago, in a time where the automation and instrumentation would have made it unacceptable to fly for a period of time an aircraft that had say the same control forces as a Lancair 360, and which are still more applicable to IFR operation than a Pitts or an Extra. The Lancair, Pitts and Extra can easily be flown by instruments, it is just undesirable for long term comfort, and therefore the system safety. To remove the issue, Bill Boeing added the MCAS, which is a variant of the STS that has been there for years on the SLUF, dealing with a similar issue in a small part of the envelope around retracting flaps and initial acceleration, e.g., 3rd segment.
Bingle 1.0 is understandable in part, the guys didn't have much heads up other than the info in the tech log, unless there was a side bar discussion by engineering or the preceding sectors crew in what they found. Bingle 2.0 highlights the fact that a crew that had been briefed on the issue, still had the same sort of outcome. Crew 1.0 did contain the problem for a period, with the Captain frequently retrimming the aircraft against the MCAS input. On handing over the plane to the FO apparently, that trim intervention got lost and the trim ran down under MCAS to a bad outcome.
When we understand why the first crew could not reconcile the stab motion with an effective run away and flick the cut out switches, we will have a better knowledge of humans making decisions under uncertainty, and with cognitive and temporal stressors. When we understand how a crew briefed on the problem emulate the same outcome, then maybe we will be able to have a safer flight deck than we apparently have now.
The stability issue is an irritant, the cure as implemented led to 2 trained crews losing it.
For the news media etc, please stop referring to MCAS and stall prevention, it has precious little to do with stall.
Reference:
§ 25.672 Stability augmentation and automatic and power-operated systems.
If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with
§ 25.671 and the following:
(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.
(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failures of the type specified in
§ 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.
(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system -
(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;
(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and
airplane configurations) which is described in the
Airplane Flight Manual; and
(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.
[Amdt. 25-23, [url=https://www.law.cornell.edu/rio/citation/35_FR_5675]35 FR 5675 Apr. 8, 1970]
# A nacelle develops lift from a mix of normal circulation theory lift, and some vortex structure lift. The main lift effect will arise from the vortex structure, and that is non linear, with a startup at modest AOA of the nacelle, to a peak around 15 AOA or thereabouts, and a slow drop off of lift to AOA around 25. At higher AOA, the vortex structures fail and lift drops off, with drag being the main component affecting the resultant force couples to the aircraft structure. Normal circulation related lift is limited in total derived force. As an aside, the vanes often placed on the nacelles affect the shedding structure off the nacelle with slat/Kruegers extended at modest to high AOA. They result in a substantial recovery of CsubL for the section of the wing behind the nacelle, and give something nice to watch on humid days.