B737 Stall

[Adsto]

First post, be nice.

Am converting onto 737NG. The AFM talks in vague terms about the stall discussing speed trim system (STS), elevator feel shift (EFS), stick shaker and natural buffet. With these things going on I'm wondering what characteristic I will see if you went beyond the stick shaker into the stall? That is, what exactly is EFS, how does it work, and what will it look like to me the pilot.

Also the FCTM talks about trim continously into the stall (stop trimming and recover at stick shaker). What's this about? I thought we were supposed to stop trimming at 1.3Vs in stalls?

Detailed technical answers appreciated, am keen for more than the dumbed down Boeing speak.

Thanks in advance,

:ok:

[CaptainSandL]

Hi Adsto,

Welcome to PPRuNe. Difficult first question, as I doubt anybody but the dozen or so Boeing experimental test pilots have any experience of stalling a 737. The closest we get is in an air test where we decelerate at 1kt/sec to the stick shaker and then recover immediately.

As for what you will see, the FCTM gives the usual characteristics:
• Buffeting, which could be heavy.
• Lack of pitch authority.
• Lack of roll control.
• Inability to arrest descent rate.

All of which I am sure we knew already, but as it is co-written by the four most senior Boeing 737 TP’s, then we should regard it as the best authority on the subject.

A more detailed handling description is probably omitted because you don’t actually need this info. I am sure that the logic is that they tell you what the symptoms of the stall are and expect you to recover immediately, preferably in the manner described in the FCTM.

You could try a full stall next time you are in the sim, but be aware that it will probably not be a very accurate model.

Regards

S & L

[BOAC]

Adsto - could it be that the stick shaker triggers at 1.3Vs and therefore the FCTM is correct? I cannot find any reference to it.

I can remember doing 737 airtests a while back when we were supposed to go to the actual stall at 1kt/second but we never achieved a 'classic' stall as you run out of back stick before Vs at that rate of deceleration..

[Terraplaneblues]

Elevator Feel Shift, only operates when stick shaker is active & AOA is 8° to 11° more than TAI biased stick shaker AOA.
Not inhibited at low altitude or on ground.

Then sends approx 850 psi to the sys A side of feel actuator. Increases column feel forces up to four times nominal feel.

Reason is to ensure pilots cannot easily override auto stab nose down pitch.

The airplane will attempt to increase speed and try to prevent any pilot action from letting it do so.

Flight deck effect is a very heavy control column pitch wise and stabilizer auto trimming airplane nose down.

Hope this answers your question.

[CaptainSandL]

BOAC,

737-300 at 40T, stick shaker triggers at approx 1.09Vs with flap 5 & gear up and approx 1.07Vs with flap 30 & gear down.

Are you sure you had to go to the "actual stall"? Sounds a bit risky, even for an air test.

S & L

[BOAC]

Thanks for the Vs figures. 1.07 is not much when you look at a Vs of around ?110kts? If my fading memory serves me right (going back to 1988) we went to the first proper 'symptom' of the stall (excluding stick shake), but Tempus Fugit may be playing tricks! IIRC We always found that at 1kt/sec you did not have enough back stick to keep the nose high enough to reach anything other than a gentle 'mush out', and never got to stall speed. I assumed the point of it was to try to ensure there were no 'nasty' asymmetries waiting to bite - but we never found out:D

[XPMorten]

http://www.xplanefreeware.net/~morten/jpgs8/stall.jpg

http://www.xplanefreeware.net/~morten/jpgs8/738stall.jpg

[BOAC]

It has always puzzled me why stick shake speeds are so low - as per the 'note'. I would have thought that having them 10% higher would still avoid inadvertent operation and give even more warning of an unsafe speed - after all, in normal ops, no-one should be anywhere NEAR them? What is the logic behind the slim margin?

[Mad (Flt) Scientist]

Stick shaker speeds have to be relatively near the stall in order to allow reasonable manoeuvring capability at typical operational speeds. If they were set too far from the stall any moderate turn in the takeoff or landing configs might set off the shaker.

Also, its supposed to represent aerodynamic warning for a naturally stalling wing, which doesn't occur a huge amount before the stall.

Finally, psychologically, if shaker was at 1.20Vs (silly example) and was going off 'all the time' as a result, people would start ignoring it as a nuisance - a 'Chicken Little' effect.

[Adsto]

Hey thanks for all the responses,

Terraplanblues mentioned "Then sends approx 850 psi to the sys A side of feel actuator. Increases column feel forces up to four times nominal feel." which I understand. The FCTM recommends trimming throughout the approach to stall manoeuvre, right up until stick shaker occurs. If the purpose of EFS is to multiply nominal (ie held) force from say 10 pounds to 40, why then does the FCTM recommend trimming? This would allow EFS to only multiply say 1 pounds by 4 for 4 pounds? Or 0 x 4 = 0?

I know these are hypothetical discussions, but I really don't see why we should continue trimming into shaker. The table that XPMorten showed indicated a trim speed.

Thoughts?

[CaptainSandL]

Interesting point about trimming all the way to the stick shaker or buffet, we use the table shown in XPM's post, which equates to 1.3Vs. Perhaps this is at the CAA's insistance since it is their Flt Test Schedule? Maybe the Boeing guys do trim all the way back to Vs. One for the Flight Testing forum I think.

[Terraplaneblues]

Hello Adsto

I would guess trimming is recommended so that the column is nulled. In that condition back in the tail a small roller (feel roller) sits in the "V" in the middle of the cam dwells on the feel cam, because when the EFS operates & increases the feel force by 4 times it could attempt to pull your arms out if the roller was out of the "V" by a good way.

The step up in feel by 4x would tend to null a column that was being held back to maintain altitude at decreasing speed. That is it would move forward away from you, so trimming would take away the surprise as the EFS operates.

Hope this answers your question

[Loose rivets]

We used to take the 1-11 through to the push (and the Claxon!!) until we had access to the Sim at Dublin. (several years)

I know that it's a very different aircraft, but around that time the 737 was introduced into the UK, and I guess that there was no sim for that either, what happened with this operator's training?

[I.B. Pilot]

It has been my understanding that we trim all the way to the stick shaker in order to simulate the most realistic scenario. One in which the pilots is busy looking at things other than his airspeed indicator, not realizing he is slowing down too much, all the while triming in order to relieve control pressures.

[Empty Cruise]

Re. the stick shaker margin...

As far as memory serves me, the shaker has to be installed when the buffet margin to stall is less than 7%. This would fit nicely with a range of shaker activation of 1,09 to 1,07 * Vs. Will see if I'm able to find an FCOM reference on it.

BOAC - your experience of not being able to achieve Vs with flight idle when decel. at 1 kt/sec. because you'd run out of up-autority - could that be the reason why no pusher is required to be installed??? :confused:

Brgds to all fm
Empty :)