Tailwind Takeoff ?

[joegoersch]

Today I took off in a M20J (lightly loaded) from a 2700 ft rwy where they were doing some work on the taxiway.   My choices were

1) intersection takeoff into wind with about 2200 ft usable runway

2) back taxi and try to turn around on 65 ft rwy (and risk taxiing on soft wet grass adjacent to runway) or

3) full length take-off with 5-6 kt tailwind.

some trees a little past runway, nothing horrible...

#3 seemed most prudent, no big obstacles, low (negative) density altitude, plenty of power...but #2 probably would have been fine too. When I got home I looked in POH to calculate when I'd be at 50 ft AGL using 6 kt headwind vs 6kt tailwind.  But much to my surprise NO TAILWIND PERFORMANCE NUMBERS in POH for takeoff.  Never noticed this before...

Thoughts ?  Anyone choose differently ?

[cbarry]

If I'm not mistaken, the turning distance on the typical Mooney is just over 40 feet.  Option 2 sounds like it would have been feasible, but I agree that taking the chance of running a tire off into soft conditions doesn't sound like much fun.  Also, I suspect that the POH contains no tailwind performance information just simply due to something that wouldn't normally be suggested or possibly for liability purposes.  There are several good apps that can come in handy such as "Takeoff Calc" that allows you to configure various takeoff conditions that may be helpful.

[PTK]

You considered most key variables: lightly loaded, low DA, no big obstacles and plenty of power. All good.

Keeping in mind that a tw takeoff will use a lot more runway to get enough lift, because it takes distance to cancell out the tailwind, I wouldn't have selected this option. Also considering that more often than not a tw is stronger at two or three hundred feet agl it's not fun to find out that the plane doesn't want to climb after lift off! 

I would have chosen your option 2. 65 feet should be plenty of room to turn.

[joegoersch]

2 minutes ago, PTK said:

You considered most key variables: lightly loaded, low DA, no big obstacles and plenty of power. All good.

Keeping in mind that a tailwind takeoff will use a lot more runway to get enough lift  because it takes distance to cancell out the tailwind I wouldn't have selected this option. Also I would consider what the tailwind is doing at 200 or 300 feet AGL. More often than not it will be a stronger tw and not much fun to find out that the plane doesn't want to climb after lift off! 

I would have chosen your option 2. 65 feet is plenty of room to turn.

POH says 41 ft turn radius, (82 ft diameter).  Granted that is without breaks so you could do a little better, but I'm not sure you could stay on pavement...

[PTK]

51 minutes ago, joegoersch said:

POH says 41 ft turn radius, (82 ft diameter).  Granted that is without breaks so you could do a little better, but I'm not sure you could stay on pavement...

My home field is 50 ft wide and it's tight. With braking I can manage turning it with the main just barely droping off the edge of the pavement. That's why I think 65 feet should be doable. 

On a broader note to select a tw takeoff is exactly that: a choice. And turning radius just doesn't seem an important enough factor imo to force myself into a takeoff with tw. 

[Guest]

I've done down wind take off's in my RV4 in some 10-15 knot quartering winds on a big runway, and was surprised at the lack of stability and the distance required.  I've only done it solo, 1400 pounds with 160HP.

I'd never consider a downwind departure in other airframes.

Clarence

[Bob_Belville]

At 5-6 kts. I would prefer 2200' upwind to 2700 downwind. Hands down. And it's not just the take off run but also the climb gradient. My M20E performance chart has sea level, light load, zero wind ground roll as 575'. I would think you'd be off the ground half way to the end. 

And I agree that by pumping the brake turning would have been no problem.  

[Vance Harral]

The effect of a tailwind is much larger than many people think at first glance.

The takeoff performance charts for a Cessna 172 indicate takeoff distance decreases 10% for each 9 knots of headwinds, but increases 10% for each 2 knots of tailwind.  A more general rule of thumb states that a tailwind which is 10 percent of the takeoff airspeed will increase the takeoff distance approximately 21 percent, and that matches the 172 with a normal takeoff rotation speed of 55 KIAS pretty closely.

A Mooney will be a little different than those numbers, but not substantially so.  My M20F POH shows a ground roll of 834' at gross weight, sea level, standard conditions.  If you apply the 172 numbers with 6 knots of wind, that becomes 778' with a headwind, but 1084' with a tailwind - a difference of 300'.  But as mentioned above, the climb gradient in ft/nm once you take off also suffers.  If you're trying to get over the canonical 50' obstacle, the delta is closer to 500'.  That's with a 6 knot wind, which a lot of pilots would consider "light and variable".  You can do the numbers yourself with 10 knots.

All that said, I don't think your downwind takeoff was "unsafe", since the intersection upwind takeoff is close to a wash at sea level on a standard day.  Good to know the turn radius, though.  41 feet is a little more than one wingspan, so that's one reference point to understand the turn radius while taxiing.

[MB65E]

I would take a direct 5kt tail wind over a 10-15kt crosswind from the left any day in the M20E. I'd do almost any under wind from the right side. However, I pay close attention to my winds from the left side. 

With the fact you were under 3000ft of RW, I'd try next time for the headwind. 

Sounds like you thought of everything before you departed vs blatantly taking off down wind. 

I plan on working off my dads strip next summer with a heavier weight. (2700 ft grass at 800msl). It has a pretty big slope in it on one side up into trees. I'll Probably always favor the slope. 

-Matt

[carusoam]

Take off without proper information has killed too many pilots.  It is that serious.  A simple mistake took the life of an MS pilot a few years ago.  He departed from a shortened runway.  He could have back taxied... or done a few other things... but he didn't the trees come up quickly at the end of short runways.

1) it is not hard to look up the turn radius.  Know which direction is tighter than the other.

2) what is the worse thing that could happen?  Shut down, and push the plane until you get it turned around.

3) take off on a short runway with a tale wind, you will be in the trees with a larger tale wind as you climb up.

4) only your POH will have the data you need for such a calculation.  Otherwise the estimates will leave 2,000' of error margin.

5) Get to know your plane.  Use an app and a WAAS gps like CloudAhoy.  Actually measure and calculate your real T/O distance with your loading.  Compare to your POH.

6) Always add density altitude into your calculation.  It can be worse than a tale wind.

7) look up Patrick.  He will remind you of how a simple error like this... 

8) no data?  Means do not try.  Get your own data on a longer runway for your use next time.

9) what was your abort plan?

Get a feeling of how disrupting a case of bad judgement can be..?

i probably missed something.  But I'm not going to miss taking the opportunity to educate people of the issue encountered by Patrick.

Your situation was awfully close to Patrick's...

apologies if I missed something.  Erring on the side of caution.

Best regards,

-a-

[carusoam]

Think about tail winds as a physics acceleration problem.  You only need to accelerate 6 more knots to cover the T/O speed.  Add in some gust buffer.  So instead of rotating at 65, you are rotating at 75... how long does it take to accelerate the extra 10?

how much distance do you cover in that amount of time at 65.  100' per second or so? Is it going to take another five or ten seconds?   Quickly, 500' or 1000' have gone by....

how strong is your engine running? 

the importance of using real data has been shown.  Don't be afraid to get real data for your plane. Using assumptions can lead to terrible results.

words like wish, hope and seems are not in the POH.  If you hope you have enough room...

or you wish you could accelerate better, or #3 seemed like a good idea....

have a plan B.  Know when to abort when you are proven wrong.  On a short runway, this takes incredible skill.

i'm glad you asked this question.  It makes me think, hard! 

Best regards,

-a-

[Hank]

I've never tried to turn around in a 65' runway, not sure I've ever been on one. Turning around on a 75' runway is easy, with room to spare using full rudder, no brakes,so I would imagine that adding some inside brake would make the 65' runway turnaround doable.

The thing to know, though, is which way does your plane turn best? Try it on a runway and see, the ramp is too wide, you won't notice the difference. My C turns around much better to the right. Run the left main close to the grass, throttle to idle, wait a bit to slow down, full up yoke and right rudder to the floor; give it a little bit of throttle to keep from stopping, straighten out and back to idle while releasing controls. This generally puts me 6-8' inside the other edge of the runway. On 40' wide runways, I always go to the bulge at the end, which is somehow always on one side and not centered on the runway . . .

[cbarry]

7 hours ago, joegoersch said:

POH says 41 ft turn radius, (82 ft diameter).  Granted that is without breaks so you could do a little better, but I'm not sure you could stay on pavement...

When turning radius is talked about,  in most cases it's simply jargon to mean the smallest circle/distance that a vehicle can be turned within.  The technical meaning of radius, like you state, is one half of the diameter.  This is probably one of those wrongly adopted word useages that we have been using since the stagecoach days...

[wrench]

Would this work? Keeping numbers simple here.

Chart exampel: 0 knot wind TO distance = 1200 ft

Rotation speed = 60kts

Tailwind= 6kts

Tailwind increases ground speed at rotation by 10% and would add 10% distance to 1200 ft takeoff for 1320 ft total.

The above is for debate/conversation purposes only. Looking for expert comments here.

[BillC]

I operate out of a 50' wide runway and turn around with ease in my Ovation. (did it in my J as well.) In the Ovation, the turning radius is 40' to the right and 48' to the left with no brakes. To aid in the turn, there is a rivet line on top of the wing that is above the main gear. Using that, I know where the gear is at all times and can start the turn tight against the runway and not have a problem.

[jackn]

I really don't like tailwind take-offs in my Mooney. In the summer, when the control tower is in operation and tailwind is 5 kts or so, we'll get assigned rwy 28 just to fit in with the jet traffic. It may be me, but doing so in a Cessna or Bo doesn't seem too bad. Maybe it's the laminar flow wing, but it feels I'm eating up a lot of runway getting off.

[chrisk]

Check your POH.   You may find the newer aircraft have a chart for take off performance with a tail wind.  

 

Also, the tail wind take off usually comes with the up hill vs down hill take off.   i.e are you better with a down hill take off with a tail wind, or an up hill take off with a head wind.  http://www.pilotworkshop.com/tips/downslope_runway_performance.htm

 

 

[epsalant]

I've never tried to turn around in a 65' runway, not sure I've ever been on one. Turning around on a 75' runway is easy, with room to spare using full rudder, no brakes,so I would imagine that adding some inside brake would make the 65' runway turnaround doable.
The thing to know, though, is which way does your plane turn best? Try it on a runway and see, the ramp is too wide, you won't notice the difference. My C turns around much better to the right. Run the left main close to the grass, throttle to idle, wait a bit to slow down, full up yoke and right rudder to the floor; give it a little bit of throttle to keep from stopping, straighten out and back to idle while releasing controls. This generally puts me 6-8' inside the other edge of the runway. On 40' wide runways, I always go to the bulge at the end, which is somehow always on one side and not centered on the runway . . .

I never would have guessed asymmetrical turn radius. I'll try this. Just the nature of hand-built machines?

Sent from my XT1254 using Tapatalk

[Vance Harral]

4 hours ago, wrench said:

Tailwind increases ground speed at rotation by 10% and would add 10% distance to 1200 ft takeoff for 1320 ft total.

No, acceleration isn't linear.  As noted in my Cessna POH example above, a 6 knot tailwind - which is about a 10% increase in rotation speed - results in a 30 percent increase in takeoff roll.

[epsalant]

Check your POH.   You may find the newer aircraft have a chart for take off performance with a tail wind.  
 
Also, the tail wind take off usually comes with the up hill vs down hill take off.   i.e are you better with a down hill take off with a tail wind, or an up hill take off with a head wind.  http://www.pilotworkshop.com/tips/downslope_runway_performance.htm
 
 

Oh! Now I see it. I missed the tailwind dashed line before. Cold, lightly loaded (30 gal 170 lbs pilot, no pax not bags), just trees 3/4 mile past runway 3000 feet way more than enough.

Not saying it was right decision, but no chance of sinking into mud/prop strike etc with tailwind choice. Especially with strong wind gradient, though, climb with tailwind could be anemic.

Huge difference between now (0° C) and summer (hot). I'm glad we're talking now !!! Really good to review all this before a hot, heavily-loaded summer day! Yesterday probably any choice okay. Sometime the right choice might be life or death!

Keep the comments/thoughts coming !!! Thanks to all!

Sent from my XT1254 using Tapatalk

[carusoam]

Wrench,

the acceleration isn't linear.  The plane is going fast covering ground while slowly accelerating to the higher speed... the disaster in waiting.

When selling cars the companies used to use a term of accelerating to passing speed. The car would drop down one gear to maximize it's ability to accelerate while going pretty fast already.  Fuel economy and small engines killed off the passing speed argument with 55mph speed limits.

A machine's ability to accelerate while going fast already is hard to come by.  The acceleration is slow and the ground is going by pretty fast while waiting for it to happen.  

 

As for the turn radius being different in one direction or the other.  Mooney's are built that way with precision.  It is a design compromise that comes with the ability to fold the gear up.  Why the chose one direction over the other was based on where the pilot sits.  I don't recall exactly...

 

Another example of extended T/O runs. Losing 200rpm of prop speed causes my plane to go from 800' to 1200'.  10% less hp causes a 50% increase in T/O distance.  Real data, Hard to believe... it is in the POH and I collected data with a skyradar to prove it to myself.

Best regards,

-a-

[Bob - S50]

Check my results vs a POH with tailwind charts, but how about a little physics?

Let's make some assumptions.  SL takeoff, standard day, full gross (so I can use the numbers from my POH).  You can redo this using whatever numbers you want.  My POH says under those conditions, ground roll is 793'.

D = 1/2 * AT²  and V = AT

So I know V (65 knots) and D (793') at takeoff.  Rearrange the second formula to A = V/T and substitute the result into the first equation and I get D = 1/2 * VT (one of the T's cancels out).  That is the velocity in feet/second.  (65 knots * 6076'/nm)/3600 seconds/hour = 109.7 ft/sec.  I now fill in values in the derived formula to get:

793 = .5 * 109.7 * T

Solve for T and I get 14.46 seconds to get to takeoff speed (if your plane and performance is exactly the same as the one used for POH testing).  I can now substitute in either equation to find A.  109.7 = A * 14.46.  This yields a value of A = 7.59 ft/sec²

With a 6 knot tailwind we need to essentially accelerate to 71 knots ground speed to get 65 knots of airspeed.  That is 71 * 6076/3600 = 119.8 ft/sec

Filling in the known values in equation two, we get 119.8 = 7.59 * T.  Solve for T and we get 15.78 seconds.  An extra 1.32 seconds.  Now put the known values into equation one:

D = 1/2 * 7.59 * 15.78²

We get 945 feet.  That's a 152' increase in ground roll for a 6 knot tailwind.

Not a physics teacher, and I made some assumptions such as constant acceleration, but hey, it should be pretty close!

[Andy95W]

Just going straight by the performance charts posted above, the telling thing to me is the magnitude of difference between a headwind and tailwind.

At a density altitude of 2000' and a weight of 2900 lbs., a 10 knot headwind decreases takeoff roll by only abut 150'.  The same 10 knot tailwind increases takeoff roll by about 750'.  Wow.

[cliffy]

OK you buried me back at D=1/2????   :-) :-)   but I have no grounds to doubt you. All I know is tailwind T/Os are bad and I once watched a 737 leave PHX during a T storm with a 30 kt tail wind! I was next in line and said we wouldn't accept the runway.

[joegoersch]

2 hours ago, chrisk said:

Check your POH.   You may find the newer aircraft have a chart for take off performance with a tail wind.  

 

Also, the tail wind take off usually comes with the up hill vs down hill take off.   i.e are you better with a down hill take off with a tail wind, or an up hill take off with a head wind.  http://www.pilotworkshop.com/tips/downslope_runway_performance.htm

 

 

Being conservative, using 1000 ft MSL, 10 kt tailwind, 10 degrees C, 2600 lbs (I had 100 msl,  6 kt tailwind, 5C and 2400 lbs) this chart shows  over the (50 ft) trees by about 2800 feet (before end of runway) with tailwind and over the trees in the other direction (headwind) by about 1800 feet, I think even intersection takeoff may have been better.  But given trees half mile from runway (at least), academic.  But I'll definitely  consider this drastic performance reduction nex time !!!