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

Olwen Morgan olwen at phaedsys.com
Thu Dec 3 18:23:40 CET 2020

Rummaging around the internet, I found at:


the following text:

Control authority is a function of the local dynamic pressure (the 
product of air density and speed squared), geometry (the product of 
control surface area and lever arm), local angle of attack𝛼α, the angle 
of deflection𝜂ηand the relative flap chord. Generally, the control 
surface works fairly linearly in a range between−15°<𝜂<15°−15°<η<15°for 
a relative flap chord of 25% rsp.−25°<𝜂<25°−25°<η<25°for a relative 
flap chord of 15%. Interpolate for values in between. Note that flap 
effectiveness is proportional to the square root of the relative flap 
chord, such that a flap of 15% needs twice the deflection angle to 
effect the same control authority than a flap of 60% relative chord.

I haven't a clue whether this is correct but it does suggest that 
control authority depends on local aerodynamic pressure. So I ask again, 
assuming that reduced stick force reflects reduced forces acting on the 
HS and elevators, then it would appear that local aerodynamic pressure 
is reduced. And if so, according to the quoted text, presumably control 
authority will have been reduced.

So, in 737 MAX with MCAS not in operation, when the stick force reduces 
while the nose pitches up, how close is the overall system to a 
trajectory through its phase space that enters a deep stall?

If nobody here can answer this, then I'd suggest that we're all 
wallowing in our own ignorance-induced BS.


On 03/12/2020 16:06, Grazebrook, Alvery wrote:
> Sorry - that was badly worded  - the MAX (they say) isn't unstable. I 
> guess it's more like critical damping - not enough damping leads to a 
> higher risk of an overshoot, but still ultimately stable.
> Cheers,
>     Alvery
> On Thu, 3 Dec 2020 at 16:03, Grazebrook, Alvery 
> <alvery.grazebrook at airbus.com <mailto:alvery.grazebrook at airbus.com>> 
> wrote:
>     Hi Olwen,
>     Authority and Stability are different things. Authority still
>     exists in an unstable aircraft. The problem is that the authority
>     also includes positive feedback. The loss of authority in a
>     high-tail aircraft is caused when the tail is so much in the
>     turbulence of the wing that it doesn't matter what you do to the
>     elevator, it has no significant effect. The MAX problem is
>     different. No loss of authority, but loss of stability. This
>     occurs when the centre of lift is in-front of the centre of
>     mass. At this point, a movement of the elevator will produce a
>     rotation effect that increases with increasing AoA. It's quite
>     possible to correct this by reversing the movement, but it is
>     likely that a human will over-correct.
>     Cheers,
>         Alvery
>     On Thu, 3 Dec 2020 at 13:57, Olwen Morgan <olwen at phaedsys.com
>     <mailto:olwen at phaedsys.com>> wrote:
>         On 03/12/2020 12:43, Dewi Daniels wrote:
>         >
>         >
>         > That isn't what is meant by a deep stall. A deep stall is
>         one in which
>         > the elevators have no authority,
>         If the aircraft is in a condition where there is much reduced
>         force on
>         the column due (presumably) to much reduced aerodynamic
>         pressure on the
>         HS and elevators, does this not constitute, if not loss of
>         authority,
>         then at least much reduced authority?
>         Once again, I'm trying to get at what kind of flight physics
>         is going on
>         with the 737 MAX airframe.
>         olwen
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