Stall speeds in flight

[xfeed]

I'm curious how you would recover the speeds while in cruise if you had a strong head wind that was quickly reducing your speed. For example, if you were in the cruise at FL350 and had a strong head wind of 100+ and your speed started to move into the stall buffer, how wouuld you recover?

Cheers!

[Old Smokey]

I think that you got it a bit back to front, if you were flying into a rapidly increasing Headwind, INCREASING Airspeed would be your problem!

I've experienced a rapidly reducing Tailwind, with rapidly increasing IAS/Mach, IMMEDIATE full thrust lever closure and full speed brake deployment was required to arrest the increasing speed, and then MMO was exceeded. Our company recommendation in such circumstances is to apply the speed brake first, then thrust reduction.

If the opposite were to happen, i.e. rapidly decaying airspeed, apply the maximum thrust possible, and, if this is insufficient to prevent approach to stall, lower the nose (as you would in a stall) until a safe speed is reached.

Regards,

Old Smokey

[Dream Land]

It's much more dangerous when you takeoff in a strong headwind and then turn down wind. :}

[Mad (Flt) Scientist]

Normal stall recovery with application of power would be sufficient, or if not, trading altitude for speed would give both increased airspeed, and a wider acceptable speed range.


A word of caution: "normal stall recovery techniques" that line pilots usually see are demonstrations of recovery for relatively low altitudes, where there is usually plenty of excess thrust available to recover with minimal altitude loss. This is rarely going to be the case for a high altitude stall - in fact, getting behind the "power curve" is very likely - which means that trading altitude for speed is the only reliable recovery technique in many cases; you won't be able to just power your way out of trouble. The key is to get the nose down and reduce the AOA as soon as you know you're in trouble; speed itself won't cause a stall, AOA is the problem, so directly attacking the source of the problem is the surest approach.

[chornedsnorkack]

Of course, if you are descending out of a headwind, it might be hard to get out of stall. After all, you cannot exchange altitude for airspeed! You can only exchange altitude for groundspeed - and even while your groundspeed is increasing, if descending simultaneously causes loss of headwind, your airspeed could decrease...

[xfeed]

Hello,

I appreciate all your responses. I am only a PPL with limited hours so I still have much to learn. Forgive me if I don't see the obvious but here is the situation I am trying to understand:

Flying on a NAT when simply descending from a cruise at FL350 may not be an option I am curious about a strong head or tail wind that may significantly bleed off speed. If you are flying at M.84 in VNAV and you notice that you are moving down into the stall magin area and the N1 is at the maximum to compensate for the strong winds how do you manage to bring back the speed? I understand that you'd want to put the nose down but how much descent would you be allowed to deviate from your assigned altitude?

Could a strong tail wind be more troublesome than a strong head wind in the situation I've described?

Cheers!

[Mad (Flt) Scientist]

Its not a question of "how much descent am I allowed".

If you do stall then you are coming down, period. Possibly in an uncontrolled fashion, if things REALLY go wrong. At least if you put the nose down its a controlled descent.

If you have been placed in a portion of the flight envelope where stall is no longer avoidable then you have an emergency. Anything you can do to escape that situation is a valid course of action, IMO.

Yes you may deviate into causing a conflict - so tell people what you're doing, if you can. TCAS will help keep people out of your way, with any luck. But to put yourself into a situation where you lose control of the aircraft out of a desire to maintain an arbitrary altitude is not the safest course.

For info, this is an extract from an All Operators Message we issued following the Jefferson City CRJ crash (it's referred to in section 'M' of the Operations/Human Performance Group Chairman's Factual Report (http://www.ntsb.gov/events/2005/pinnacle/exhibits/317489.pdf) on that accident.

Do not wait for the onset of continuous ignition or stick shaker before attempting a descent from an altitude where continued operation is not possible. Descend immediately.

In the event that a stick shaker / approach to stall occurs, the crew should expect that a deliberate loss of altitude will likely be required in order to restore the aircraft to a normal energy state and to prevent an aerodynamic stall and possible departure from controlled flight.
I don't think there's anything at all special about the CRJ in this regard; it's basic physics at work.

[AerocatS2A]

In your secenario are you maintaining altitude?

A steady headwind or tailwind will have no effect on your airspeed (I figure you know this, but it is not clear from your question whether you do.)

An increasing headwind will result in an increasing airspeed, so that's not going to bring you closer to your stalling speed.

Only an increasing tailwind will decrease your airspeed and possibly cause you to approach the stalling speed. If this happens you have two options, increase power, and/or lower the nose. If you don't have the excess power available then you must lower the nose, maintaining assigned altitude is not a consideration.

[xfeed]

Thanks AerocatS2A. I was maintaining altitude in my scenario and I got the winds wrong way round. I thought I may have been mistaking the headwind for the increasing tailwind. That was exactly what I was referring to. Does this scenario play out often enough that planes need to leave their assigned altitude when a strong tailwind is heaving them along the north atlantic? I know that this bit of airspace in particular is very controlled in procedures. Eventually you need to stop descending so what if the tailwinds are so strong that you can not maintain any assigned altitude? Maybe this doesn't really happen so much? Just curious.

Cheers,

xfeed

[AerocatS2A]

First up, I don't fly anything above FL250, so maybe one of the other guys can be more helpful.

Second. I'm still not sure that you understand that a steady tailwind has no affect on your airspeed. You could have a 200 kt tailwind and your airspeed will be the same as if you had a 30 kt tailwind.

The only way a tailwind can adversely affect you is if it changes suddenly. Then it is a transient thing; you get an increase in tailwind, you lose a bit of airspeed then the airspeed recovers and you carry on. So it's only while the wind is changing that there may be a problem.

If something like that happens to the point where you are in danger of stalling, then you do whatever is required to prevent the stall. Stalls in swept-wing aircraft are not pretty (so much so that there are stick shakers and pushers to make sure you don't stall) and staying alive trumps any controlled airspace considerations.

[Intruder]

Does this scenario play out often enough that planes need to leave their assigned altitude when a strong tailwind is heaving them along the north atlantic?
No.

That abrupt a decrease in wind would be accompanied by significant turbulence, and would be very short-lived. While it is theoretically possible, it is very rare that an airplane cannot accelerate faster than the rate of decrease in headwind at high altitude. It would be exacerbated if the airplane was flying at maximum altitude for its weight, which would leave it with little excess power and little stall margin. Most of us avoid that situation if turbulence or wind shear is forecast.

[xfeed]

AerocatS2A and Intruder,

Thanks both of you for clearing that up for me. I appreciate your time.

Cheers,

xfeed