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

Grazebrook, Alvery alvery.grazebrook at airbus.com
Wed Dec 2 14:52:50 CET 2020

Hi Olwen,

If I understand it correctly (and it's not our aircraft, so I'm not
absolutely sure),
a) there is a rule that says the stick force must increase as pitch
increases. The purpose of this is to give the pilot an intuitive feel for
what he is doing to the aircraft's aerodynamics. I assume this is only
measured in a steady state, because if you include rotational acceleration
I can't see how any airframe would achieve this at all moments of a
maneuver or in turbulence.
b) the generation of a vortex to produce lift is not necessarily sudden. It
would just change the steady state position of the centre of pressure. (CoP
is kind of the balancing point on the airframe in flight). I'm guessing
that the engine nacelle is close to lift-neutral when straight-and-level,
and provides increasing lift at increasing angle of attack.
c) the point Peter made about this feature being necessary only for
certification is sort-of valid. The certification rule exists for a reason.
For example, if a pilot is operating in poor visibility and with partial
instrument failure, I could see the "intuitive" feel associated with a
monotonically increasing stick force as a function of pitch might help
reduce confusion, and therefore might make an accident less likely. It
won't change whether the pilot can achieve a particular steady-state pitch
if that's their intention. So you could argue that a known failure to meet
this requirement in an area of the operating envelope that isn't normally
used isn't truly a safety problem, just a training challenge.

Does this help?



** opinions expressed are my own, not necessarily those of my employer.

On Wed, 2 Dec 2020 at 12:11, Olwen Morgan <olwen at phaedsys.com> wrote:

> On 01/12/2020 21:11, Herrell, Dave wrote:
> <snip> ( emphasis in extracted text is my own - OM):
>  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.
> <snip>
> I had actually read this text on the web before Dave quoted it here ...
> and yet PBL has said in a previous posting:
> >>>As far as I am aware, there is no "pitch-up tendency [which] starts to
> lift the aircraft's nose".
> These positions appear to me to contradict each other.
> Two questions:
> 1. Who is right? PBL or the Boeing people who produced the text that Dave
> Herrell quoted?
> 2. If the forces on the stick change rapidly, this suggests, to me at
> least, that the aerodynamic pressure on the relevant control surfaces
> (thinking physics again) changes rapidly at some point, hence the question,
> what causes the sudden change of pressure? Flow separation from the HS,
> possibly??
> And if PBL whinges again about repeating himself, I would respectfully
> remind him that
> (1) Repeating oneself and expressing oneself clearly are two different
> things.
> (2) Repeatedly asking people to say clearly what they mean is a foul habit
> that I acquired by reading about the cantankerous ancient Greek criminal,
> Socrates.
> Still confused,
> Olwen
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