[SystemSafety] "FAA chief '100% confident' of 737 MAX safety as flights to resume"

Herrell, Dave dherrell at mpr.com
Tue Dec 1 22:11:00 CET 2020

One of the clearest, simplest explanations I have found on the web is at  http://www.b737.org.uk/mcas.htm. The data is from pilots and engineers involved with the Boeing 737.

The 737 MAX was produced with several differences from the NG. Many of these differences were obvious such as the new LEAP engines or the larger flight display screens. Some were less obvious but well documented such as the FBW spoiler system. It also now appears that some differences were almost hidden, certainly from the flight crew. MCAS is one such difference.

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.

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."

David Herrell
10 Springbrook Dr
Fredericksburg, VA 22405

-----Original Message-----
From: systemsafety [mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of Olwen Morgan
Sent: Tuesday, December 1, 2020 1:13 PM
To: systemsafety at lists.techfak.uni-bielefeld.de
Subject: Re: [SystemSafety] "FAA chief '100% confident' of 737 MAX safety as flights to resume"

On 01/12/2020 16:47, Peter Bernard Ladkin wrote:
> No. The problem is that, without MCAS, the aircraft does not satisfy 
> the airworthiness regulations and therefore cannot be certified to fly 
> passengers in commercial service.
So, whereas earlier generations of the 737 were airworthy without MCAS, the 737 MAX is not airworthy without MCAS?

If so, this strikes me as being somewhat more than a marginal difference.

And as regards stick force, what is the cause of the reduction in stick force that MCAS was introduced to correct? Reduced aerodynamic pressure on the aft horizontal control surfaces?

In the matter of repeating oneself, let me observe that I am thinking in terms of flight physics, whereas you seem to be thinking in terms of JAR rules. When such differences of perspective exist, it is hardly surprising if a series of exchanges is needed to establish a clear basis of understanding.


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