Stall vs. overspeed
 

I would like to ask pro pilots to explain one thing. What is considered a bigger threat when flying an airliner: Overspeed or stall?

What is more serious condition? What is harder to recover from? What time in training and simulated session is given to each situation?

When I read about crashes like Birgenair 301 or AF 447 I have a perception that pilots didn't recognize stall condition and it may something to do with stall/overspeed confusion. In Birgenair case the pilot faced overspeed warning with stick shaker at the same moment. At AF447 there was no overspeed alarm as far I know but pilots ignored stall alarm for 54 seconds without taking proper action and pulling back the stick for the majority of descend led to fatal ditching.

One more question. How is the stall alarm triggered? (I mean what sensors are taken and evaluated to get the stall alarm in the cockpit.)

In this thread lot-boeing-767-severe-turbulence-unreliable-airspeed there is again a situation of overspeed and stick shaker at the same time.

Can anyone explain and answer my questions above?

Pretty much all aircraft you have to do something if you are stalling.

Some types the over speed limit is linked to some test requirement eg alot of turboprops are limited by the windshield bird strike test. You can go another 50-100knts faster before you will have issues with either structural failure or the prop tips going supersonic. If this is the case there isn't alot of issues with the aircraft overspeeding by 5-10knts.

Other aircraft have a limit due to the airflow over the wing going supersonic once over a certain airspeed once this happens it changes the performance of the wing and is to be avoided.

Now as you increase altitude your Indicated airspeed decreases but so does your maximum speed because as the temperature decreases so does the speed of sound.

You can get to the point where the stall speed is the same as the maximum speed. We call this coffin corner and are trained to avoid it.

I am not a swept wing jet pilot myself which the coffin corner mainly effects so my description may be a bit out. My TP is limited by its wind screens and its pretty well known that the airframe is good for another 50knts over the max we are allowed to fly.

- not so - it can affect all aircraft at high altitude. In simple terms, it involves MCrit and V stall 'coinciding', and since all 'high-level' aircraft have both..................... If my memory serves me right, I think 'Coffin Corner' came to prominence with the NA B45 (straight-wing) bomber, and it would be of interest to you to know that I believe the U2 (straight-wing) has/had a very small IAS margin at cruise altitude - something like 2 or 4 kts, I think.

To answer to OP - Stall. There is always a safety 'margin' on the Mach/IAS limit on any aircraft, but once you are stalled..................

Thanks for that BOAC.

Not something I get very worried about at sub FL250, hence me not being quite up to speed on the subject.

To my knowledge an overspeed stall happens as the center of lift on the wing moves aft with speed, once it passes the wing the aircraft stalls as the tail is still flying. Airliners are not designed for this. This is approaching the trans-sonic range. Aircraft designed for this balance the wing and tail lift config to transition/ sonic flight.

- back to the books, I fear:)

Enlighten me. From the books or oh-**** literature?

Well, to start with "once it passes the wing the aircraft stalls as the tail is still flying."

1) just the tailplane holding the a/c up?
2) A/c do not 'stall' when the shockwave forms and moves over the wing

"No-****". Which book is that from?

Correct me if I'm wrong but, as the CP moves aft with increasing M No, isn't an increasing amount of up elevator required to remain level? That corrective force would decrease tailplane lift (unless it was a delta wing, of course) and, in most cases, make it negative. Isn't the problem then the point where insufficient up elevator deflection is available to stop a dive developing?

You are correct but the Cp does not 'fall off the back of the wing' as suggested leaving only the tailplane to support the a/c. Modern a/c do not run out of 'elevator' in this situation since they generally have moving tailplanes.and have an 'auto' trim function with Mach No.

How we got to an 'overspeed stall' (whatever that is) I do not know. The OP asked about an 'ordinary' stall and Mmo/Vmo:ugh:

Perhaps he is thinking of shock-induced separation.

Another way i can think of which might be the same as LM's comment which is term I haven't heard of before.

Stall speed is linked to the amount of G you are applying.

If you take the Stall speed in level flight and times it by the root of the G you will get the stall speed. So as you increase the G the higher the stall speed which may be above your overspeed.

As for how they measure it.. There are two main ways. A little flap on the leading edge which gets lifted in the airflow and the more modern method of having an angle of attack sensor which measures the AoA and sets off the alarm at a pre determind limit which is changed if you have iceing limits.

- indeed, not 'overspeed stall' - but the OP wasn't. Hence my puzzle.

I thought shock-induced separation was when Mrs found you in bed with the Floozie from No 30............................:p

Did you mean 31 (or maybe 23)? :E

NO! I live at 31........................:mad:

Now then - you don't think I would do that do you?

A fellow Lightning Pilots' wife................????

......mind you...........


PM shortly mate.

I am grateful for the answers given by pro's, I am bit oriented now.

What is still a bit mystery to me what is the reason behind some strange behavior of pilots in cases like Colgan Air or AF 447 with common denominator pulling stick in a stall situation. It seems to me that there is some pattern in this and it would be worth to know. (Apparently this question is discussed ad nauseam on this forum.)

What was the state of mind, knowledge and routine of the pilots? Why would pilots act like this? How this can be prevented? Is there a problem in design - pilot/machine interface of modern a/c which may increase this risk? Is there any chance to discover personality traits which may lead to such behavior in advance?

The reason why I ask - I am in a consulting/training business for general managers and aviation case studies are perfect examples for my students.

Pali,

PM for you.

One more question. If I understand right, the correct action in the case of UAS on AF447 would be to apply pitch & power. Is there a possibility that when PF commenced climb from FL 350 he actually just screwed up pitch & power maneuver?
And one more thing - how stable is a modern airliner like A330? If a pilot wouldn't touch SS and throttle after AP and auto throttle was disengaged what would happen? If the a/c would fly further without any input from pilot for few minutes what would be the possible outgo? I have a strong feeling that flight would end at CDG in the morning...