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

Grazebrook, Alvery alvery.grazebrook at airbus.com
Wed Dec 2 15:55:12 CET 2020


Hi Olwen,

As Peter pointed out, I don't mean "Pitch", I mean "angle of attack".
Sorry, sloppy of me.
Cheers,

Alvery


On Wed, 2 Dec 2020 at 13:52, Grazebrook, Alvery <
alvery.grazebrook at airbus.com> wrote:

> 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?
>
> Cheers,
>
> Alvery
>
> ** 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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>
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