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Short, hot & high takeoff prep

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Short, hot & high takeoff prep

Old 11th Jan 2020, 13:22
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Short, hot & high takeoff prep

Another in my series of dumb questions!

So you are preparing to take off from a short airstrip at a high density altitude. You have run the numbers which say that you have 340m to spare which is 1/3 of the nominal TORA. But the engine is not in its first flush of youth, the surface is rough and there is a flukey tail wind. How do you maximize your margin of error here? Obviously reducing weight is good, but assume that's been done. I suppose, wait until the cool dawn next morning would be another option! But, one thing I wondered about was adjusting mixture for max available power. Normally we lean until revs drop, then go a 'bit' richer. Would it make sense to reduce that 'bit' and stay leaner until you are flying ? Or, maybe, translating the optimal mixture at 1,700 rpm to full takeoff power is not precise enough to make that meaningful?

Otherwise, flaps as for short-field in the POH, fix a point for a 50:70 speed check, full power before releasing brakes (and never mind the grit being blown up) and go for it?
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Old 11th Jan 2020, 14:09
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First off the calculations from the manual always. The condition of the engine and the mixture: find the best lean before taxying (there will always be an argument whether you should then adjust to rich of this) to ensure the plugs remain clean. The only way of being certain the engine is/will achieve the book figure is by performing a static power check having positioned cross wind at the hold. The manual should give you the figures but if a special technique is required then how to do this. Yes, take off leaned at a high and hot altitude but also maybe at sea level if the temperature is abnormally high - i've had to do this in the past even in the UK. Follow the manual short take-off technique. Climb path obstructions/hazards above 50 ft asl should also be calculated.

I've always calculated the decision point using: at 1/3rd of runway expect 2/3rds of the take off speed and the appropriate achieved RPM. This will following an abort give another third to stop plus another 1/3rd for safety. I don't know what a "flukey" tail wind is. In the conditions you describe a tail wind should be a no go (there may be upslope/downslope conditions that could justify taking-off downwind. If this is the case advice from local experienced pilots should be sought, there may be other factors to consider.
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Old 11th Jan 2020, 14:22
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Thanks. For perspective, the occasion I am thinking of had a density altitude of 8,500'

Originally Posted by Fl1ingfrog View Post
I don't know what a "flukey" tail wind is. In the conditions you describe a tail wind should be a no go (there may be upslope/downslope conditions that could justify taking-off downwind. If this is the case advice from local experienced pilots should be sought, there may be other factors to consider.
In this case, a slight downward slope plus surrounding terrain that falls away rather than rises, dictates the takeoff direction.

Last edited by double_barrel; 11th Jan 2020 at 15:04.
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Old 11th Jan 2020, 14:25
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How about restricting fuel onboard and load? Refuel enroute instead of going nonstop.
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Old 11th Jan 2020, 15:20
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The figures in the book are for a level hard runway. Add a third for short dry grass and another third for longer grass.
If the ground is wet/soft put the aircraft back in the hangar.
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Old 11th Jan 2020, 16:00
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Good foregoing information. In addition, mentally prepare yourself that when you break ground, you'll allow the plane to accelerate in ground effect for as long as it is comfortable/practical. There after, clear the obstacles - by only enough to be safe. Allow your airspeed to build, at the expense of excess altitude if needed. If you are airborne, and suddenly worried that you might not clear the obstacle (like the trees at the end of the runway), aim 2/3 their height, and build the speed. Your good common sense to prevent your hitting them. What NOT to do is to climb needlessly high to clear them, and then jeopardizing your airspeed. Just clear the obstacles safely, that's all you need.

When airplane performance is just enough, there are more cases of climbing too much, and then stall spin back in, than hitting the trees because you did not climb enough. When I teach float flying, I'm anal about not aiming for a flight level over the trees at the end of the lake, just clear them with a bit of extra airspeed in your back pocket.
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Old 11th Jan 2020, 16:47
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In addition to above, do not leave ANY runway behind you- consider stopping the engine and pushing the tail as far back as it will go before doing your final power checks.
Alternatively, if all the checks have been done, consider using full power as soon as you have taxied on to the runway heading (rather than stopping and using full power against the brakes, which is not possible in some aircraft, particularly taildraggers)

However, imho, only use these to increase your margin. If you are not confident that you can take off safely without these techniques, wait for better conditions!

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Old 11th Jan 2020, 17:11
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Good advice to be found here......

Safety Sense Leaflet 07: Aeroplane Performance
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Old 11th Jan 2020, 18:37
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My spine ran cold (does that really happen?) when I read the question. My answer as a fairly timid pilot these days is "don't land there in the first place". As described, this has a very strong chance of creating an NTSB (/AAIB/...) report no matter what you do.
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Old 11th Jan 2020, 21:06
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Originally Posted by The Ancient Geek View Post
If the ground is wet/soft put the aircraft back in the hangar.
I second that. When still a very inexperience pilot, I once took an airplane with a cruise prop out of a 2000-feet long golf-course quality but wet field, did not do any speed checks during the take-off run and barely made it over the fence. I felt my legs shaking during climbout. (And that was a bit below 2000' MSL.) I've tried to avoid shaking legs since.
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Old 12th Jan 2020, 16:33
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Thanks. I like the importance of the right mental state for not trying to climb out too steeply. In fact, before I started flying, an acquaintance was killed by hitting trees on take off. I had always assumed that she just flew into them. On reflection a panicked pull back followed by a stall-spin is more likely.

Originally Posted by n5296s View Post
My spine ran cold (does that really happen?) when I read the question. My answer as a fairly timid pilot these days is "don't land there in the first place". As described, this has a very strong chance of creating an NTSB (/AAIB/...) report no matter what you do.
As I begin the transition from student to pilot, I want to try to ensure that I become better/safer rather than letting skills and knowledge decay. Personally, I like to do that by challenging myself while managing risk. I found in other fields that it's easy to become complacent and assume it will be ok, while allowing experience and diversity of experience to shrink. So I want to very carefully keep pushing the envelope. Given my environment that means learning to use dubious strips safely. I hope that makes sense? After all, nothing is risk free, I believe that 30% spare take off distance would be luxury for a big jet!? I posted here to help learn all I could from the experience. Thanks to all.


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Old 12th Jan 2020, 17:25
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My 02 cents

1) In the situation described you need every bit of power so do full power runup and lean to max RPM or MP. Don't enrichen the mixture a bit leave it at the max power setting you can't hurt the engine and enrichening he mixture will reduce power

2) Lifting off is only the first problem you face. The next and equally important issue is how will I climb to a safe altitude to proceed on course. You must have pre- planned climb out flight path that will take you over level or preferably descending terrain for your climb out. If you are in a valley airport plan to climb on the windward and sunny side of the valley where there will be rising air

3) Flukey winds are often caused by localized thermal activity near the runway. Waiting a couple of minutes before starting the takeoff will often turn that tailwind into a head wind

4) Finally takeoff planning starts with landing planning. Most light aircraft will use significantly less distance for landing than takeoff particularity in a hot and high context, so before landing you need to have already thought about how you are going to do the departure
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Old 12th Jan 2020, 19:34
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An interesting discussion with lots of good advice. I routinely tow gliders* out of a 2800’ grass strip at 3700’ ASL.

The highest density altitude I have experienced, on a 34° day, was slightly over 7000’. The point of my post is to mention, for the benefit of those who think Compton Abbas is a high-altitude airport (), that you can read about hot and high takeoffs for ever, but until you’ve actually experienced one, you can never appreciate the magnitude of the performance reduction.

* For hot days, we have a graph of maximum takeoff mass vs. temperature, which is level until 21° and then slopes downward quite rapidly.

Concerning the comments about setting max power, I used to fly a 182 towplane and although I never did any proper tests, I am fairly sure that maximum static thrust was obtained about 200 rpm lower than max fine pitch.
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Old 12th Jan 2020, 19:51
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Since I instructed in British Colombia my students were inevitably going to experience hot and high airports. My home field was at near sea level but I would simulate a high DA take off by restricting takeoff RPM to the % power you would get at 6000 ft. They were invariably surprised at how doggy the aircraft felt. It was also interesting that for many the climb rate would improve 25 % to 50 % when they kept the ball in the middle and nailed the Vy pitch attitude
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Old 12th Jan 2020, 20:18
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Flew light aircraft for 15 years in Southern England, then moved to SA. In transitioning my license to the local version, my first hot and high departure (in the Karoo @ 3,500 AMSL, 30C) with my instructor sat next to me in a smartish 172R left me dry mouthed. Numbers done twice as per POH and checked again. Safety factors added. But still your mind does not quite accept what is happening and boy 1300m of firm gravel runway never quite seemed enough.

Pilot DAR and BPFE are, as ever, spot on with their post lift off advice

One other gem I would like to offer is do your power check on the firmest bit of ground you can find (common sense I know anyway). Here in SA many bush strips have a concrete area where you can get your power check and mixture setting sorted avoiding (well maybe minimising potential for) foreign object damage to prop and aeroplane whilst playing with the mixture knob.
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Old 12th Jan 2020, 20:20
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" I believe that 30% spare take off distance would be luxury for a big jet"
Big jets must be able to climb out with one engine inoperative, and do not suffer from carb Ice.
They also can successfully climb after hitting the approach lights.

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Old 12th Jan 2020, 20:38
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Concerning the comments about setting max power, I used to fly a 182 towplane and although I never did any proper tests, I am fairly sure that maximum static thrust was obtained about 200 rpm lower than max fine pitch.
Max thrust in an airplane with a constant speed prop will always be obtained at maximum RPM
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Old 12th Jan 2020, 21:26
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Not a dumb question at all.

This one made it.

This one could take off, but never got out of the ground effect till impact. You need "spare" to get out of ground effect.

Last edited by Pilot DAR; 12th Jan 2020 at 22:10. Reason: typo
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Old 12th Jan 2020, 22:29
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Max thrust in an airplane with a constant speed prop will always be obtained at maximum RPM
Generally yes, and this is what you should expect, and operate to, though it is possible for the governor or fine pitch stops to be set a little too fine. I have experienced this a couple of times, and learned to set a slower RPM by experience, to get the greater thrust. For reference, when I did 39 precisely measured takeoffs in the C 182 amphibian for noise testing, I gathered the data to know. Most prominently, all other factors being exactly equal, the passing altitude at 2 km from brake release was 60 feet higher at 2700 RPM, than 2500 RPM. Yes, it was higher, but a 60 foot difference after 2 km, is bugger all. As MT had told me, run the prop more slowly, the thrust is the same. MT's STC on the 182 is at 2500 maximum, and I was aiming for approval at 2700, thinking there was bags more power. With the learning that day, I accepted EASA requiring 2500 RPM as a limit, for reasons of noise compliance. I had occasion to takeoff that plane from Split, Croatia, on a hot day. It was a sea level departure, but it was really hot, and I might have been one pound over gross. The performance was poor, and I had to fly down the coast for twenty minutes to gain altitude to turn inland over the coastal mountains. But safety was not a concern, as I had the sea to follow, without having to climb, if I could not climb.

The other factor, which is somewhat type specific, is for airplanes with a stabiliator, rather than a horizontal stabilizer, and elevator. Though I have experienced the problem in Cessna Cardinals, I have found the problem more pronounced in Piper Cherokees. What happens is a pilot desperate to get off the ground in hot and high conditions will lift off to early, with too much stabilator deflection. The plane is on the verge of being stalled, but will fly in ground effect. 'Problem is that the highly deflected stabiliator is creating a lot of drag itself' It's sort of stalled too, so really can't raise the nose any more (not that you'd want that anyway). but it seems to be enough drag to prevent the plane accelerating so as to climb out of ground effect. I was right seat in a friend's Piper Arrow when he did this on a very hot day. We were stuck in ground effect, and the plane was not accelerating any more. Though we had lots of runway (more than 3000 feet), it just kept going by. We were both so used to flying out of this runway, the alarm bells sounded too late. As he was drifting off the runway to the left, and landing back was obviously not going to be possible, I retracted the gear. Doing that was enough, that the reduced drag, allowed the plane to accelerate, and slowly climb out of ground effect. Alarmed by this experience, I borrowed a Cherokee during the winter, and took it out to the ice. With 20 miles of lake under me, and ten miles wide of "runway" I was able to experiment, and reproduce the effect, though it was neither hot, nor high. But I could stick it in ground effect, with landing back being the only option.

So, in Cherokees and Cardinals, soft field technique is okay, though understand your limits, so as to not get the nose too high!
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Old 13th Jan 2020, 01:07
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This one could take off, but never got out of the ground effect till impact.
He was above tree-top height, so say at least 50 feet - well out of GE. He might have been able to make it with a super-shallow climbing turn over the tree-free terrain after the runway. But, once again, best not to be there in the first place (or if you're already there, take off at first light when the DA is lower).
As I begin the transition from student to pilot, I want to try to ensure that I become better/safer rather than letting skills and knowledge decay.
Pushing the envelope while sitting in an armchair, or in a discussion forum like this, is an excellent idea. Pushing the envelope in an actual flying machine otoh is an exceptionally bad one. People like BPF and Pilot DAR have already experienced a lot and even when they do something that afterwards they're not so sure about, they're drawing on an awful lot of experience. My strong advice would be to continue pushing the envelope in theory, but in practice just go out and fly as much as you can. You'll experience plenty of "oh s***" moments, that way - trust me - without ever being in REAL danger. As for me, I have 2000 hours, which I regard as just enough to be dangerous.

(For some reason this post reminds me of the old maybe-joke, "If at first you don't succeed, maybe skydiving is not the hobby for you.")
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