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-   -   B737 Upset Recovery nose low high AoB (https://www.pprune.org/questions/636549-b737-upset-recovery-nose-low-high-aob.html)

Pin Head 4th Nov 2020 02:46

B737 Upset Recovery nose low high AoB

Effectively a spiral descent with lots of energy available but why do the PLI's appear on during the recovery and we then have to respect not to pitch up too quickly.

Always good to ask a stupid question.



B2N2 4th Nov 2020 05:30

Iím assuming in order not to over stress the airframe.
Iíve been told itís Boeingís philosophy that overspeeding is preferable over exceeding G limits.
Have not heard it from the horses mouth rather from sim instructors.

Pin Head 4th Nov 2020 08:10

ok, but then again PLI's come on for stick shaker activation only in low speed flight regime so that makes no sense???

Cornish Jack 4th Nov 2020 08:44

It might make sense as a read-across from the investigation into cross-over speed investigations after the rudder hard-over problems? The Boeing test flight analysis flights demonstrated the requirement to avoid any natural use of elevator to hold height, as it exacerbated the problem. Nothing whatsoever to back up that thought, just a first reaction.

safetypee 4th Nov 2020 21:42

The general design and certification principles would normally require an increase in stick force to minimise over-stressing the aircraft ('g' wt), similarly for higher speeds (Q).

The PLI is related to the stall margin defined by AoA, not solely at slow speeds but also at higher speeds with applied 'g'; PLI could be shown during a high speed / high 'g' turn (spiral dive).

Thus for a spiral dive recovery, levelling the wings should provide greater stall margin as 'g' is reduced - PLI moves up / disappears. With further pull up the speed could reduce, reducing the stall margin; thus PLI appears / repositions on the pitch scale - respect the PLI.
A dynamic situation involving speed, 'g', attitude and above all else AoA.
Beware rolling 'g', do not trim into a turn.

sycamore 5th Nov 2020 07:22


eckhard 5th Nov 2020 07:32

Pitch Limit Indicator

A dynamic yellow symbol superimposed on the pitch scale of the ADI to show the pitch attitude at which the stick-shaker will activate.

safetypee 5th Nov 2020 08:15

PLI - Pitch Limit Indicator; as per eckhard.

Origin was the indication of the maximum pitch attitude which is available to maximise climb performance in wind-shear; displayed on the attitude indicator.

The difference between aircraft datum (zero deg) and max pitch angle (PLI), is an indication of AoA margin for the intended use of PLI - normally stick-shake for wind-shear but over time systems and operations corrupt Ö Ö

The value of PLI - AoA margin, should reposition with change of speed, 'g'.

With more recent depictions / interpretations and switchable computations (EFIS / HUD); the PLI could indicate the wind-shear limit, EGPWS pull up, stall recovery, depend on aircraft type, operator procedures and training.

Similar to some EFIS speed tape low speed awareness symbology.

mustafagander 5th Nov 2020 09:05

I wonder if PLI appears to remind you that rolling G can really hurt your aircraft?
As I recall it, retired 5 years now, Boeing says level the wings, then pull to raise the nose.

sycamore 5th Nov 2020 13:38

Thanks guys.....

B737900er 10th Dec 2020 08:34

The PLI appear to when in a low-speed scenario; You respect them so you don't end up in a stall. As mentioned above, rolling G can really hurt the aeroplane and its recommended to 'Unload the wings first', by doing this it reduces wing loading (increases stall margins) and lowers the AOA. Once the pitch is appropriate then you can roll the wings and adjust thrust accordingly.

If you don't respect the PLI during the recovery you will get the stick shaker.

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