Grass Take-Off and Landing Distances

[MyData]

Here's a question which is probably easy to answer, but is something I can't comprehend at this time.

Looking at a table of take-off and landing factors for a PA28 I notice that in long dry grass the take-off factor is 1.25. This is understandable as the long grass will create friction.

It stands to reason then, that when landing, this friction will bring me to a halt sooner. But this isn't the case, the landing factor is even greater at 1.3.

Why?

For wet grass I can understand why the factors are 1.3 and 1.4 respectively, friction and 'stickyness' on the take off roll, 'sliding' and 'skidding' on landing.

[Ultralights]

after touchdown on a sealed surface and applying the brakes, will slow you down. doing the same on grass, will usually result in a skid, unless the braking is light at a lower speed.

[Genghis the Engineer]

Some aircraft, some conditions, grass gives you a better take-off than tarmac (although not very often!). Some aircraft, some conditions, grass gives you a better landing - particularly long dry grass + smallish wheels.

The conditions can be quite wide and variable - those values are very much "worst case".

G

[bencoulthard]

Go drive your car in a field of dry grass and try braking at varying powers, you will soon see grass is still quite slippy even when dry.

As for take off being less than landing, that is because your power comes from the prop and not the tyres on take off. The extra needed to overcome the grass drag which would be greater at lower speeds and lessens as the aircraft lifts and lightens the load on the tyres.

Ben (first informative post) Coulthard

[FlyingForFun]

I seem to recall that at least some of the publications which list these safety factors also make a note (albeit a very small note) that in some cases landing distances can be reduced on grass, and give the example of aircraft which don't have brakes.

FFF
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