Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Cruise, Drop Of Tail Wind = Aircraft Overspeed  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (2 years 8 months 2 days 3 hours ago) and read 5178 times:

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

8 replies: All unread, jump to last
 
User currently offlineQFA380 From Australia, joined Jul 2005, 2084 posts, RR: 1
Reply 1, posted (2 years 8 months 2 days 1 hour ago) and read 5125 times:

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.

User currently onlineMir From United States of America, joined Jan 2004, 21882 posts, RR: 55
Reply 2, posted (2 years 8 months 2 days 1 hour ago) and read 5100 times:

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
User currently offlineSLCPilot From United States of America, joined Aug 2003, 593 posts, RR: 2
Reply 3, posted (2 years 8 months 1 day 23 hours ago) and read 5056 times:

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...
User currently onlinevikkyvik From United States of America, joined Jul 2003, 10356 posts, RR: 26
Reply 4, posted (2 years 8 months 1 day 23 hours ago) and read 5051 times:

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.



How can I be an admiral without my cap??!
User currently offline26point2 From United States of America, joined Mar 2010, 856 posts, RR: 0
Reply 5, posted (2 years 8 months 1 day 22 hours ago) and read 5021 times:

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.


User currently offlineFlighty From United States of America, joined Apr 2007, 8777 posts, RR: 3
Reply 6, posted (2 years 8 months 1 day 13 hours ago) and read 4842 times:

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.


User currently offlineSLCPilot From United States of America, joined Aug 2003, 593 posts, RR: 2
Reply 7, posted (2 years 8 months 1 day 12 hours ago) and read 4825 times:

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...
User currently onlineMir From United States of America, joined Jan 2004, 21882 posts, RR: 55
Reply 8, posted (2 years 8 months 1 day 11 hours ago) and read 4785 times:

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
Top Of Page
Forum Index

Reply To This Topic Cruise, Drop Of Tail Wind = Aircraft Overspeed
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Use Of Tablets In Aircraft Maintenance posted Wed Mar 28 2012 16:53:16 by withak
What Are The Cruise Speeds Of Various A/C Types? posted Fri Mar 26 2010 14:00:29 by c5load
Cruise Altitude Of Dash 8? posted Thu May 28 2009 07:25:36 by Ps76
Configuration Of Modern Transport Aircraft posted Tue Nov 18 2008 09:52:59 by SpenceSaab
Post-Production Support Of Nearly-Extinct Aircraft posted Mon Nov 17 2008 10:57:26 by 2H4
The Relaity Of Flying, Wind And Lightning posted Sun Oct 5 2008 23:08:52 by F27friend
Aircraft Overspeed Velocity posted Fri Jun 15 2007 06:27:27 by Blackbird
Effect Of Sand On Aircraft posted Tue Jun 12 2007 16:15:38 by OHLHD
Moment Of Inertia & Aircraft Rotation posted Thu Mar 22 2007 22:28:08 by Rush744
Advantages Of A T-tail Vs. A Conventional Tail posted Sat Dec 17 2005 03:15:13 by Thrust
Post-Production Support Of Nearly-Extinct Aircraft posted Mon Nov 17 2008 10:57:26 by 2H4
The Relaity Of Flying, Wind And Lightning posted Sun Oct 5 2008 23:08:52 by F27friend
Aircraft Overspeed Velocity posted Fri Jun 15 2007 06:27:27 by Blackbird
Effect Of Sand On Aircraft posted Tue Jun 12 2007 16:15:38 by OHLHD
Moment Of Inertia & Aircraft Rotation posted Thu Mar 22 2007 22:28:08 by Rush744
Advantages Of A T-tail Vs. A Conventional Tail posted Sat Dec 17 2005 03:15:13 by Thrust

Sponsor Message:
Printer friendly format