Downwind Takeoffs

[coppercity]

I would join you in the air CT! Not that I am in to punishment but around our area I would never fly if I waited for a calm day without windshear and crosswinds, it's just the joy of flying near mountains in the high desert. That being said I still stay very cautious, pay very close attention to the conditions and always remain vigilant for a go around or an aborted takeoff.

[Jim Meade]

I'm going to have to fly out and visit you for a checkout.

[Flying Bozo]

Nothing changes for a 15 knot tail wind take off except you need 15 knots more speed. I've been there a time or 2 for a take off and a landing, but the extra ground speed doesn't bother me, just the slow full stall guys.

 

That ought to start a discussion.

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Roger thought that his posting would begin a discussion. So here it is!

 

Nothing changes except for a 15 KT more take off speed. Not true. Not true!!

 

1. Your lift off spot on the runway is significantly farther down the runway leaving you less runway ahead to abort in the event that you had an issue.

 

2. Once you do get the thing in the air your angle of climb made good is significantly less than the angle of climb in a no wind condition. This has everything to do with obstacle clearance ahead.

 

3. Since the angle of climb made good is much flatter your altitude above the airport is far less on climb out making a return to the airport almost impossible should the engine sag or have an issue.

 

4. If you think you might be able to make it back to the runway with a slightly sick engine you will find that you now have a 15 knot headwind to buck on the way back

 

There are other factors that might mitigate the above safety trade offs like runway gradient and perhaps a one way runway.

 

Larry CFI, A, G, I

[Flying Bozo]

I hope this doesn't post twice. I tried once and don't see it in the topic.

 

Roger thought that his posting would begin a discussion. So here it is!

Nothing changes except for a 15 KT more take off speed. Not true. Not true!!

1. Your lift off spot on the runway is significantly farther down the runway leaving you less runway ahead to abort in the event that you had an issue.

2. Once you do get the thing in the air your angle of climb made good is significantly less than the angle of climb in a no wind condition. This has everything to do with obstacle clearance ahead.

3. Since the angle of climb made good is much flatter your altitude above the airport is far less making a return to the airport almost impossible should the engine sag or have an issue.

4. If you think you might be able to make it back to the runway with a slightly sick engine you will find that you now have a 15 knot headwind to buck.

There are other factors that might mitigate the above safety trade offs like runway gradient and perhaps a one way runway.

[Roger Lee]

I fully agree with all. Those are all a given, but it still fly's and works if you had no alternative. I prefer not to use over 8 knt tailwind, but have had to use higher a number of times, but it's not my first pick for sure. I'll always pick a headwind given the chance.

 

This is why I said it would start a discussion.

[Ed Cesnalis]

What it looks like:

 

[FlyingMonkey]

Maybe I'm obtuse, but how do you get 15kt winds in opposite directions on the the same runway (09/27)?? :huh:

[Roger Lee]

Mountains, buildings , trees and swirling currents.

[FlyingMonkey]

I see the pics CT posted now...jeez, it looks insane.

[mocfly]

I'm sure that is an ever changing view all the way to altitude.

 

[Ed Cesnalis]

I'm sure that is an ever changing view all the way to altitude.

 

The rotor at the lee of the sierra crest is the E ticket ride. The lee side rotors are what make me laugh when the conversation turns to 30 degree pitch limitations.

[Doug G.]

I fully agree with all. Those are all a given, but it still fly's and works if you had no alternative.

[Doug G.]

I fully agree with all. Those are all a given, but it still fly's and works if you had no alternative.

I am trying to figure out when I would not have an alternative...stay on the ground is always an alternative.

[Jim Meade]

If I have it figured right, with a 60 kias Vx and a 15 knot wind, assuming 800 fpm climb rate, the difference in altitude achieved in 1 nm is 640 feet with a tailwind vs 1066 feet in a headwind, a 6° angle vs a 10° angle. But, I'm not all that confident of my math skills. I may have mixed up the base leg and the hypotenuse. Still, this is a lot of difference. Make one look at the downhill, downwind runway with climbing terrain at the end a little differently.

 

[Dan Kent]

Jim,

I don't quite understand your math. I believe the Vx and the 800 fpm will set your angle. I worked it a bit differently, and assumed flying for 1 minute. That gives you a right triangle with the hypotenuse of 6072 feet (1nm) and a height of 800 feet, so the plane traveled through the air 6020 feet horizontally. To this you either add the 15 kt component (1520 feet) or subtract depending on whether into the wind or down wind to get across the ground distance. This gives you 124% of a nm downwind and 74% of a nm into the wind. So downwind you will climb (800 ft/1.25) 645 feet, and into the wind (800 ft/.74) 1080 feet in a nm for a difference of about 435 feet or about 40% less climb performance.

 

I think I've got the math right.

[Jim Meade]

Our results were very close to the same although your methods was different.

[Dan Kent]

Thanks Jim, I misread your post and thought the delta was 640 feet.

[Ed Cesnalis]

...Flying with a tailwind, the ground speed is sum of airspeed + wind speed..

 

Landing with a tailwind, the ground speed is the sum of the tail wind component + true air speed.

[mocfly]

 

Here's the paragraph that accompanies the data posted.

 

The bottom line is that more runway will be required along with a greater distance to climb over an obstruction at the end of the runway.

 

Suppose a plane requires a ground run of 200 feet to lift off at 40 mph. It climbs out at 500 ft/min at an airspeed of 45 mph. It's operating off a runway that is 1,000 ft long.

 

After making a few assumptions, we can use some basic physics to get at least an idea of the effect of the wind. The big assumption is that the acceleration of the plane is constant during its takeoff run. This is almost certainly not the case, but hopefully the resulting error is not great.

 

So, realizing the possibility for inaccuracy, here are the results:

 

No wind:

Ground speed at liftoff: 40 mph Length of takeoff run: 200 ft. Altitude after 1,000 ft: 101 ft

 

10 mph Headwind::

Ground speed at liftoff: 30 mph Length of takeoff run: 138 ft. Altitude after 1,000 ft: 140 ft

 

10 mph Tailwind:

Ground speed at liftoff: 50 mph Length of takeoff run: 384 ft. Altitude after 1,000 ft: 64 ft

 

15 mph Headwind:

Ground speed at liftoff: 25 mph Length of takeoff run: 96 ft. Altitude after 1,000 ft: 171 ft

 

15 mph Tailwind:

Ground speed at liftoff: 55 mph Length of takeoff run: 468 ft. Altitude after 1,000 ft: 50 ft

 

Note that a 10 mph tailwind almost doubles the takeoff roll, 384 ft as opposed to 200 ft. The sensation felt by the pilot in such a case is that the plane doesn't want to come off the ground! And after it does, it doesn't want to climb like it normally does.

 

Observe in the tabulation above that the altitude gained by the time the plane crosses the end of the runway decreases significantly with a tailwind. With a 15 mph tailwind, it is about half the no-wind value.

 

There is another factor to consider. Recall the earlier discussion about how the wind a short distance above the surface is likely to be stronger than the wind at the surface. As the plane climbs upward from the runway, it will likely be entering the region where the tailwind is stronger. This has two effects.

 

(1) As the plane penetrates the layer where the tailwind speed is greater, the airspeed will drop, momentarily, until the “drift with the wind” phenomenon has time to develop. And because lift is dependent upon airspeed, the rate of climb of the plane will decrease. (2) At the same time, a bit of bumpiness or turbulence may be felt as the plane passes through the shear layer between the slower and faster layers of air.

 

Here's the picture seen by the pilot: After a much longer than usual ground run, the plane seems to gain altitude at a much shallower angle. It's moving fast down the runway, but it doesn't want to climb. Then there is a bit of turbulence, and the plane begins to feel mushy with a tendency to wallow around. A glance at the airspeed indicator shows it to be lower than what it was just a second ago! And the trees are getting closer all the while.

 

Now let's assume the pilot clears the trees and continues the climb. In just 10 seconds or so, the plane will have begun to move with the wind. From that point on, the rate of climb of the plane will be its normal value, but the angle of climb will be less because of the increased ground speed. It's only at this point that the pilot can begin to relax a bit.

 

 

[Jim Meade]

You are right that wind speed tends to increase with altitude, and we all know that it shifts direction, as well. Although the change in direction is not dramatic, it is significant. As a rule of thumb, we think the wind at 2,000 AGL is likely to be at 45° clockwise to the surface wind.

[opticsguy]

some mornings departing the UL strip at Oshkosh they will have you depart with a quartering tailwind. It's also downhill slightly, but it is a scary feeling seeing the grass go by and not getting any lift.

 

The first time it was just me. The second time it was me, the wife, all our camping gear, and 15 gal of fuel (all in the left tank). It meant a 45 degree bank turn 20 feet off the ground, but the CTSW handled it.