smartt1982
Topic Author
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### Cruise, Drop Of Tail Wind = Aircraft Overspeed

Hi,

Was in cruise today and we had a tailwind of approx. 30 Knots, it was slightly bumpy but all of a sudden the tailwind suddenly dropped off and the speed tape rapidly increased towards the limit speed/clacker. There appear to be any increase in headwind or anything, just that the tailwind dropped off.

The guy I was flying with said that the drop of tailwind was acting like a virtual change in energy, hence the sudden trend to over speed.

Any thoughts on this, any explanation on why this occurs?

Cheers

QFA380
Posts: 2013
Joined: Fri Jul 22, 2005 4:38 pm

### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

If you have a tailwind of 50 knots, airspeed of 250 knots, you have a groundspeed of 300 knots. If suddenly the tailwind drops to 10 knots, you still have a groundspeed of 300 knots but have an airspeed of 290 knots. Of course the plane will slow down but for a moment you will nudge the red as your momentum keeps you going.

Mir
Posts: 19395
Joined: Mon Jan 05, 2004 3:55 am

### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

 Quoting smartt1982 (Thread starter):The guy I was flying with said that the drop of tailwind was acting like a virtual change in energy, hence the sudden trend to over speed.

It can be thought of that way, sure. I'd tend to think of it as the same as a windshear event near the ground, though instead of a drop in indicated airspeed putting you too close to a stall, you've got an increase in indicated airspeed putting you too close to the overspeed limit.

-Mir
7 billion, one nation, imagination...it's a beautiful day

Slcpilot
Posts: 598
Joined: Thu Aug 07, 2003 3:32 am

### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

It's the same concept with the toy gliders that fly almost 500mph. They go across a shear from a tailwind to still air, and then turn around and fly from the still air into a headwind and repeat! They are actually doing closer to 550mph at times without a motor. It's called dynamic soaring. Birds do it to, just not as fast.

Cheers!

SLCPilot
I don't like to be fueled by anger, I don't like to be fooled by lust...

vikkyvik
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Joined: Thu Jul 31, 2003 1:58 pm

### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

 Quoting smartt1982 (Thread starter):There appear to be any increase in headwind or anything, just that the tailwind dropped off.

Those are, in effect, the exact same thing. If it makes more sense, remove the words "tailwind" and "headwind", and just talk about "wind". So you could say you went from +30 knots wind to 0 knots wind, which would have the same effect as going from 0 knots to -30 knots, or from +15 knots to -15 knots.

 Quoting smartt1982 (Thread starter):The guy I was flying with said that the drop of tailwind was acting like a virtual change in energy, hence the sudden trend to over speed.

Ummm, I suppose one could look at it that way. Simplistically, what's happening is that you have two forces in equilibrium - thrust from the engines, and drag from the air. When you're in steady, level flight with the tailwind, your thrust and drag are equal. When the tailwind suddenly stops, your airplane is suddenly moving past the air 30 knots faster, which equals more drag. Since your thrust hasn't increased at all, your drag is now greater than your thrust, and the airplane slows down until thrust and drag are equal again. That can't happen instantaneously, so for a little while you'll be closer to overspeeding than you were.
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26point2
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### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

What sort of plane and at what sort of altitude?

With over 8000 hrs flying jets at high altitude I have never seen winds aloft "suddenly" drop off, at least not in the sense that the plane would overspeed. I have seen quite frequently mountain wave activity that will cause rhe plane to quickly overspeed however. Perhaps this is what happened.

Flighty
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Joined: Thu Apr 05, 2007 3:07 am

### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

Right, if you have a 200mph tailwind that suddenly stops (you can tell I am not a pilot), if your ground speed was 400mph, you can't suddenly snap to 200mph. That would be like an impact. You will coast from 400 (200mph airspeed) down through 375mph (375 mph airspeed) back down to 200mph ground and airspeed.

Given that I have ridden in a DC-10 going over 700 mph ground speed, I wonder if theoretically it could break the sound barrier in this situation. Probably so.

Slcpilot
Posts: 598
Joined: Thu Aug 07, 2003 3:32 am

### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

 Quoting 26point2 (Reply 5):With over 8000 hrs flying jets at high altitude I have never seen winds aloft "suddenly" drop off, at least not in the sense that the plane would overspeed. I have seen quite frequently mountain wave activity that will cause rhe plane to quickly overspeed however. Perhaps this is what happened.

Our experience is almost identical, and I think you hit the nail on the head with the notion of mountain wave. I have alternated the thrust levers from near flight idle to max power in a fairly short period of time over the Sierra Nevada. One of the strangest events was flying at 55% N1 at 3 degrees nose low near Mmo for over a minute on the way to Calgary one evening. We were in sustained lift at FL350.

Cheers!

SLCPilot
I don't like to be fueled by anger, I don't like to be fooled by lust...

Mir
Posts: 19395
Joined: Mon Jan 05, 2004 3:55 am

### RE: Cruise, Drop Of Tail Wind = Aircraft Overspeed

 Quoting Flighty (Reply 6):Given that I have ridden in a DC-10 going over 700 mph ground speed, I wonder if theoretically it could break the sound barrier in this situation. Probably so.

Not possible. The speed of sound is dependent on the air around the airplane (i.e. at altitude), so things like wind and temperature will factor into the equation. So while you might be going faster than 768mph (speed of sound under normal conditions on the ground) in terms of groundspeed, in relation to the air around you you'll be traveling at some percentage of the speed of sound (probably in the 75-85% range).

-Mir
7 billion, one nation, imagination...it's a beautiful day

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