Short & soft field takeoffs and the 70/50 rule

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Photo by Jordan McQueen on Unsplash

A scenario

Your small airplane has landed in a grass strip, or field, or other confined area, one with which you’re not familiar. There’s a runway of sorts, but it’s muddy in places, or the grass hasn’t been cut for a while. Also it’s not level, like the tarmac runways you’re used to. There’s a bit of an up-slope here, and it definitely slopes down over there. The wind, though light, seems to be blowing in different directions from one minute to the next. And it’s a warm afternoon – one of the warmest of the year so far.

You’d like to fly out, but you’re not 100% sure you can do so, safely, under the conditions at hand.

So what can you do?

The 70/50 rule

You may have heard of something like these, or a variety of one of them:

  • Your takeoff roll will be twice the distance it takes you to reach 70% of your flying speed
  • If you reach 70% of your flying speed before using 50% of the runway, the takeoff will be a success
  • If you haven’t reached 70% of your flying speed by halfway down the runway, you should abort while it’s still safe to do so.

These rules of thumb sound like a really good idea. I want to show you now why they’re actually a really bad idea.

Constant acceleration required

A little school algebra will quickly show you that an object undergoing a constant acceleration does indeed reach 70% of the speed it will have some point at 50% of the distance to that point. So the 70/50 rule would have some applicability to an airplane trying to take off if it had a constant acceleration during its takeoff roll.  Unfortunately the acceleration of a small airplane on a muddy or grass field is nowhere near  constant, for the following reasons:

  • The thrust from the propeller isn’t fixed: thrust typically goes down as you get faster, and the thrust profile depends on whether you have a constant speed or fixed pitch propeller.
  • The drag from an uncertain surface typically will go up, the faster you go.
  • The wind resistance certainly increases as the airspeed goes up
  • The surface conditions, gradient and wind can vary from one part of the takeoff roll to another

Nor does the rule take into account the wind, or limits to your engine power: a stiff breeze can give you 50% (or even 70%) of your takeoff airspeed before you even start to move; but limited power combined with a draggy surface may mean that you don’t ever reach a safe flying speed, even after 100% the distance you have available.

That accident near Meadow Peaks, Idaho

This now-famous youtube video shows a takeoff accident where, because of a combination of environmental factors, loading and engine misconfiguration, the airplane never ever reached a safe flying speed. However, it looks probably to me that 70% of a safe flying speed was reached quite quickly: certainly well before half of the 5000 feet long available takeoff run was used up.

There are many things we can learn from that accident. One thing – not the most important, overall, but relevant to us – is that if the pilot had been using the 70/50 rule to assure himself that a safe takeoff would be possible, it would not have given him the right answer.

Another problem: the 70/50 rule (as usually stated) doesn’t come into play until you’re halfway down the runway. The rule might call for you to stop at that point, but now you’re moving at up to 70% of your takeoff speed and you may no longer have room safely to do so.

If you want further convincing not to put store in the 70/50 rule, recall the last time you practiced a takeoff from a grass runway. A constant acceleration would mean that the time taken to get (say) from 10 to 20 knots was the same as the time taken to get from 20 to 30 knots, and the same as the time taken from 30 to 40 knots, and from 40 to 50 knots. In fact, the indicated airspeed during a takeoff roll on long grass goes something like this:

“10 …. 15 ….  20 …. (this is going well, I’m halfway there already) …. 25 …. 30 …. 33 …. 35 …. 36 …. 37 …. 37-and-a-half …. 38 …. 37 (hit a bit of a rough patch, there) …. 38 …. 39 …. 39 … 39 … 39 … …. 40 … airborne!

An aircraft undergoing truly constant acceleration will keep getting faster, and faster, and faster, and faster. Forever. You already know, if you stop to think about it, the last 10 knots of a takeoff roll take a considerably longer time than the first 10, and the ten after that, were they needed, might never happen at all.

Take-away message: don’t use the 70/50 rule, or any variation of it, to inform any kind of decision about taking off on an uncertain surface.

So what can we do?

Firstly I have to say that there is no way to guarantee a takeoff on an uncertain field is going to be safe.
Things you will want to do to minimize and understand the risks are:

  • Walk the length of the ruway, to observe the conditions. How does it slope? Is the grass a consistent length? Is there a muddy patch at the far end, or wet grass, or a downslope that would make braking difficult or impossible?
  • What does your Pilot Operating Handbook or Flight Manual suggest for a hard-surface takeoff roll? Do you have several times that distance available? Does the book give a figure for grass, or corrections for sloping runways?
  • Can you lighten the aircraft? Offload some fuel? Leave your passengers behind, at least for a trial takeoff? Take out some seats?
  • If there are other pilots around, find out if anyone is familiar with takeoffs from the field. But don’t let someone else talk you into a take-off. They might not stick around to help you out of the trees and they certainly won’t help you pay for the damage.
  • And, naturally, you’re going to use POH recommended technique to minimize drag, for instance by having the nose-wheel just off the ground during the roll.

Working up to it

One technique you could use is to work up to using the full field length: from the threshold start with a short full power acceleration roll, then test the braking beginning at a point where you can be sure you still have room to stop. Then repeat, going a little further and a little faster before you commence braking each time. If you can work up to close to a safe flying speed and still brake with plenty of room to stop then it’s likely you can accelerate for a little longer and further and takeoff, one final time. But, remember, that last bit of airspeed is the slowest to come, and meanwhile you are chewing up the yards waiting for it.

If, instead, you find you can only get to a fraction of takeoff speed before using up all the stopping room, you can taxi back and consider the very difficult choice facing you. If you’re going to fly out you will have to commit to the takeoff past that point, with no plan B. If conditions don’t allow you to reach a safe speed in the remaining space and you still attempt a takeoff, either you will succeed (hurrah!) – or, you will damage the airplane and probably injure yourself. It will be your choice, of course, whether to take that risk, but you should do so eyes wide open, and understand the risks to yourself, your airplane and your passengers.

Is the 70/50 rule good for anything?

Not exactly that – but, if you’re taking off from a known surface – say clean, flat, concrete – then I can see a utility in an ad-hoc performance profiling tool: if each time you takeoff you note the distance at which you reach half your rotation speed. Given some variation for weight, and headwind, that distance should be comparable from one takeoff to the next. If one some particular day it takes you much longer to reach half your rotation speed than you would expect from your previous experience of the same aircraft on a similar surface – then perhaps you should stop and find out why. Who knows – maybe you left the parking brake set? Or maybe you got the winds wrong, and you’re trying to takeoff downwind. Or maybe you have a problem with your carburettor and the engine isn’t developing as much power as it should be. Worth investigating, no?

There are no guarantees

As an aside, let me say that one of the ways that flight training misleads students is by presenting them only with situations that have a safe exit. That’s great for preventing accidents during training but once cut free from the training environment a pilot quickly realizes that the universe no longer guarantees a safe outcome from his or her decision-making. Is it difficult to make a decision in this scenario? Yes. Does the universe owe you a guarantee that you can make the takeoff that you want? No. Taking the wings off and trucking the plane out is still an option.

Overall, you are responsible for the safety of your flight if you choose to attempt it. You must assess and understand the risk through best use of the tools you have at hand. But please make sure you use the right tools.


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