How to Calculate Take-Off Distance

First of all, everything I say here is NOT to be used in actual flying. This is for funny reading, and not to be applied in real life.

Here is how it seems that MAF pilots figure out an expected take-off distance:

You are over an airfield that is brand new, and you want to figure out if you can land. First, use manufacturer’s published figures. Let’s say it is 800 feet.

Then we adjust for wind, slope, surface condition, pilot skill and speed, and add a healthy margin for error just to be safe.

The primary concern here is that by the end of our obscenely short airstrip, we are not only going fast, but flying, and safely passing over obstacles.

Once we have looked up the book figure from the manufacturer, we add the correct percentage for slope. Upslope take-off? Bad. Add 10% to 40% or so depending on the amount of slope. Down slope good! Subtract a bunch. Don’t know the amount of slope, well get out your inclinometer and find out!

Then we add of wind. Head wind good! Subtract 10%. Or more. The manufacturer handbook says how much.

Next is surface. If it’s wet and grassy, that’s not so much of a problem for acceleeration, unless the grass is tall. Add another 10% or so according to the manufacturer.

Now, by this point, you have spent about $30 circling the runway 3 times at 1000′ altitude and made all your passengers sick. The good news is, you aren’t even close to being done. You have to decide if the number you came up with is less then 75% of the runway that remains after you remove the distance required for leaving margin, stopping distance, and reaction time. This is where things get fun and totally subjective.

Reaction time: 1 second or 150 feet or so at take off speed.
Margin: 150 feet minimum.
No Coffee: +50′
Missed Lunch: +50′
Need to Pee: +200′

Take those numbers and remove that from the total runway length, and you should be able to take-off and use less then 75% of that number.

You can? Great! One last thing and then you can land and go pee.

Abort point. Who cares about abort point? YOU! You may be able to take-off, but if something happens, at what point can you safely stop the airplane on the runway, or do you become committed to either flying, or running off the end of the runway?

Take the runway length, subtract the required margin, and the subjective reaction time/health factors. That leaves you and accelerate stop distance. Half of that or so should be your safe take-off roll. However, you have to figure in surface. Accelerate-stop distance minus slope factor, wind factor, and then surface factor. What is surface factor? Great question!

Paved: +\- 0
Grassy: ?
Long Grass: – up to 30%
Short Grass: 0
Dirt soft: +??
Dirt hard (best braking action): 0
And then the wild card.
Wet and long grass: ADD UP TO 100% stopping distance.

Factor all that together and you get the point after which you are either committed to fly, or will roll off the end of the runway if you tried to stop.

Still think you can take off there? Great! Now you have to figure out your landing numbers before you can land. But that’s another blog entry for later.

Please note, always refer to the manufacturer’s data for take-off distances and apply plenty of margin for your situation.


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