High-altitude take off

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ScottVal

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Scott
I did my second high altitude takeoff yesterday. Airport elevation 6300 feet. I took off from the same airport last October, and everything was fine. Yesterday's takeoff was a little squirrely. After rotating, the aircraft came down and bounced on the runway. When we started climbing again, our vertical speed was only about 100 feet per minute. I didn't want to retract the gear too soon, but when I did I noticed our vertical speed was improving nicely.

Next time, I'll treat it as a short-field take-off, with one notch of flaps, with the brakes on while bringing up the power. And I'll rotate a bit later and climb out at Vx initially, being careful not to raise the nose too much.

Note that this airstrip is paved, length of about 8000 feet. AC is a Piper Arrow, and yes I did lean the mixture, two occupants, OAT around 50F.
-Scott
 
I'm going to mildly disagree with the 'short field' part. For me, short field requires you to get the plane in a high angle of attack at very low speed. This could be counter-productive in the case where you have plenty of runway and speed is what's needed but lacking due to lower power available.
 
If you rotated at the normal indicated airspeed for that weight, there should be no difference other than a lower climb rate/gradient after liftoff. However, what often happens with pilots lacking high DA experience is they try to rotate when the visually perceived speed reaches that associated mentally with normal rotation rather than waiting for the normal indicated airspeed for rotation. With a DA of 7000 feet, you would be rolling over the ground nearly 10 knots faster than at SL to reach normal light single rotation speed, and the runway zipping by that much faster combined with the much longer acceleration time can fool a pilot into rotating early, which can produce the results Scott saw -- settling back and bouncing on the runway.

Another problem is that the pilot rotates normally, lifts off, and then continues pitching up to the normal SL climb attitude. Because of the lower power available at that high DA, the climb rate and thus flight path angle are significantly lower, and that means the pitch attitude for the same AoA is also lower. If you pitch to the "normal" climb pitch attitude, you may increase AoA and thus induced drag t the point that the aircraft settles back on the runway. It's important to make sure you only pitch up enough to maintain the desired climb speed, and do not pitch to the attitude normally seen in climb at SL.

So, for takeoff at high DA's, if you monitor the airspeed during the takeoff roll, rotate only when reaching the normal indicated rotation speed, and then only pitch as required to maintain the desired indicated airspeed for climb, you can avoid the results which Scott aw.
 
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docmirror said:
I'm going to mildly disagree with the 'short field' part. For me, short field requires you to get the plane in a high angle of attack at very low speed. This could be counter-productive in the case where you have plenty of runway and speed is what's needed but lacking due to lower power available.
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Gotta agree, Vy is your friend for high DA airport departures. Terrain may be a concern but don't spend one second longer than you have to at Vx. Also remember that Vx increases with altitude and that the backside of the power curve is lurking out there to get you. I don't know if a NA Arrow has enough "ooomph" to get out of the backside on engine alone (no descent) like the Frankenkota does.
 
if you are going to try Vx, make sure an adjust it for the altitude.
 
I would also go Vy if terrain wasn't an issue. Also, with an 8000 foot runway, you should be able to take off without flaps. A notch of flaps will get you off the ground faster if that's an issue, but once you're off the ground your climb performance will be better without them. Personally, if the runway length permits it, I like to accelerate to Vy in ground effect then climb out.
 
You can use SOFT field technique at high altitude, not short. You'll still settle back to the runway if you hold brakes. What you probably did was over-rotate (real easy), or perhaps the mixture wasn't right. When in doubt, lean the mixture at takeoff power, for maximum RPM or ROP EGT (CS prop). Use a Vy climb. The ONLY exception is to clear an obstacle. Use the correct Vy. It's always less at high altitude than sea level, and a sea level Vy can result in really bad climb performance.

If the runway is paved, it's OK to stop and run the engine to full with the brakes on, but the point is to fine tune the mixture, not to get the extra 20 feet of runway. As for using flaps, READ THE POH, as there is some variation in this. In a 172, 10 flaps will give you a shorter ground run, but a longer climb to 50 feet, exactly what you don't want. In a PA28, you're probably going to want 25 flaps.
 
MAKG1 said:
You can use SOFT field technique at high altitude, not short.
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You certainly can do that, but I recommend strongly against it. That will significantly lengthen your takeoff distances due to the increased aerodynamic drag when holding the elevator full up during the initial portion of the takeoff roll, and when lifting off and flying in ground effect at lower-than-normal liftoff speed.

You'll still settle back to the runway if you hold brakes.
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How would holding the wheel brakes while running the engine up before staring the takeoff roll affect aircraft performance after rotation and liftoff?
 
Regarding the difference since the Oct. take-off. DA, and its effects, can vary greatly based on temp and weight, along with winds & other factors. If you were just marginally heavier, hotter, or had less favorable winds, you could easily been performing 10-15% less than last time. Likely, you had enough oomph to get off the ground, but when you got out of ground affect, you lost the advantage and needed to gain a little speed before you could climb.
 
Ron Levy said:
You certainly can do that, but I recommend strongly against it. That will significantly lengthen your takeoff distances due to the increased aerodynamic drag when holding the elevator full up during the initial portion of the takeoff roll, and when lifting off and flying in ground effect at lower-than-normal liftoff speed.
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I generally modify the technique for that reason. You're not going to pop off the runway at 10 knots, so there is no point to hold the elevator all the way up until it can pop you off. The point is to stay in ground effect, not to get the nose wheel off the ground (like it is for a rough field). Popping a wheelie is definitely a high-drag configuration, so it makes a lot of sense to minimize that.

How would holding the wheel brakes while running the engine up before staring the takeoff roll affect aircraft performance after rotation and liftoff?
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It will make no difference. The point I was trying to make is that it won't help. If he was going to settle back to the runway, he'll do it whether he held the brakes or not.
 
Thanks, guys; I think the point here is to:
1. Taxi into position and add FT with brakes on, and fine tune the mixture.
2. Don't be in a hurry to rotate, and when I do, lower the nose and stay in ground effect until I reach Vx, and then start climbing and gradually accelerating to Vy. But even then, being careful not to raise the nose too much.
 
You don't have to be in position to fine tune the mixture. You can do it in the run-up area, as long as there isn't loose gravel around or something behind you that won't like the extra prop wash.

Stay in ground effect until at least Vx, where Vx is appropriate to the altitude (it's higher than at sea level). You can stay all the way to Vy if there are no obstacles or other reason to get altitude fast early.

I would suggest a #3 for you -- identify an abort point BEFORE starting the roll, and preferably before starting the engine. And stick to it. For that 8000 foot runway, I'd want to be at 50 feet halfway down it in a 172/177/182/PA28, or else land on the remaining runway. This leaves plenty of margin to come to a safe stop even if you've already broken ground. For a shorter runway, you may have to play games like 70% of Vx by half the runway, but it's best to require out of ground effect if you can.

FWIW, the one time I tried this exercise in a 172, it got far above 50 feet by 4000, on an 85 deg day at Lake Tahoe (somewhat worse conditions than what you reported). It was closer to 300. I prefer not to take 172s into the Sierra mainly because of potential winds, but the density altitude performance isn't as bad as you might think.
 
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MAKG1 said:
I prefer not to take 172s into the Sierra mainly because of potential winds, but the density altitude performance isn't as bad as you might think.
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You have many good points that I deleted. Going for Vy rather than Vx is major. In the 'kota Vy at my usual DA's is about 3 to 5 kts lower than book. Dunno if OP has a hershey or taper but suspect the change in Vy will be similar with either wing. Would like to hear from folks with hershey experience.

On another note, 172's can be horrible with a little gusty wind and high DA. The 180 hp aren't bad but the 150 & 160 hp will give a thrill every now and then. I just love hearing the stall warning sound when climbing out at Vy...
 
Best angle of climb puts you closer to the critical angle of attack. When that close to the ground I see no need to get closer to a stall if you have the available runway.

Airspeed is your friend.
 
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ScottVal said:
I did my second high altitude takeoff yesterday. Airport elevation 6300 feet. I took off from the same airport last October, and everything was fine. Yesterday's takeoff was a little squirrely. After rotating, the aircraft came down and bounced on the runway. When we started climbing again, our vertical speed was only about 100 feet per minute. I didn't want to retract the gear too soon, but when I did I noticed our vertical speed was improving nicely.

Next time, I'll treat it as a short-field take-off, with one notch of flaps, with the brakes on while bringing up the power. And I'll rotate a bit later and climb out at Vx initially, being careful not to raise the nose too much.

Note that this airstrip is paved, length of about 8000 feet. AC is a Piper Arrow, and yes I did lean the mixture, two occupants, OAT around 50F.
-Scott
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Fly the airplane, not the numbers.
 
ScottVal said:
Thanks, guys; I think the point here is to:
1. Taxi into position and add FT with brakes on, and fine tune the mixture.
2. Don't be in a hurry to rotate, and when I do, lower the nose and stay in ground effect until I reach Vx, and then start climbing and gradually accelerating to Vy. But even then, being careful not to raise the nose too much.
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Why wait? Why not set mixture during runup? The POH states setting the mixture at 1800 (iirc) and not full throttle, even at high alt.

As curiousity, where were you flying, which high altitude airport?
 
Cpt_Kirk said:
Fly the airplane, not the numbers.
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When it comes to airspeed, I disagree - Fly the numbers. Lots of problems happen when pilots used to 3-digit field elevations fly by the seat of their pants at high airports, rotating too early and attempting to climb too steeply (which does not work).

Use the same indicated airspeeds you do normally and you should be OK - Just don't try to pull back more to get more climb, it won't work.

Also, with 8000 feet of pavement, I wouldn't bother with either a short or a soft technique. Lean the mixture when you run up, and on the roll, rotate where you normally do, retract the gear when you normally do, climb at Vy (which will be a bit lower at a higher elevation), and just be ready for everything to happen a lot slower than usual.
 
I would go with the longer ground run and more speed on a long runway.

All I can add is no matter what elevation a short field is, when you lift off, aim right for the tops of the trees or any obstacles. You only have to clear them by a foot or two. Speed is paramount. You can always do a pull up if you have the speed.
 
flyingcheesehead said:
When it comes to airspeed, I disagree - Fly the numbers. Lots of problems happen when pilots used to 3-digit field elevations fly by the seat of their pants at high airports, rotating too early and attempting to climb too steeply (which does not work).

Use the same indicated airspeeds you do normally and you should be OK - Just don't try to pull back more to get more climb, it won't work.

Also, with 8000 feet of pavement, I wouldn't bother with either a short or a soft technique. Lean the mixture when you run up, and on the roll, rotate where you normally do, retract the gear when you normally do, climb at Vy (which will be a bit lower at a higher elevation), and just be ready for everything to happen a lot slower than usual.
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Well, I'm glad you didn't bite my head off when you disagree unlike most people on here.

I didn't mean ignore the numbers, but realize that everything will be different from what you're used to at SL. Listen to what the airplane is telling you.

ClimbnSink said:
Don't overyoink it.
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+1
 
And if there mountains right near by and you need to cross a ridge go ahead and do a spiral climb over the airport until you are certain you have sufficient altitude to cross the ridge. Taking into account the flow of wind over the ridge
 
Jeanie said:
And if there mountains right near by and you need to cross a ridge go ahead and do a spiral climb over the airport until you are certain you have sufficient altitude to cross the ridge. Taking into account the flow of wind over the ridge
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Very important point - always establish required (1,000' or better depending on conditions) clearance prior to approaching a ridge. People also point out that a 45 degree approach line should be used. I find in practice the approach line is frequently determined by terrain and the 45 degree angle may not be practical at every crossing point. As always, YMWV.

And something that is coming out of some IFR accidents over the mountains is don't fly over the highest terrain in the area even though you may have established clearance. It seems that strong wind and the highest terrain tend to form some ugly downdrafts that seem to reach above the mountain tops. Stick with the traditional VFR crossing points even when you've established higher altitudes (folks in the flight levels excepted of course)
 
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