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Variations in the landing surface

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Variations in the landing surface

Old 4th Apr 2015, 00:56
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Variations in the landing surface

We've had discussions about approaches, the flare, and other elements of landing, which are quite informative. But often those discussions stop at the point where the plane contacts the surface. Being prepared for the surface contact, in what you might expect, and then dealing with it as it happens is also important.

Some of us typically fly off and onto grass, others, pavement or concrete most commonly. What are the variables on those surfaces?

Grass runways can have varied firmness, right down to "you're up to your axles in mud". If in doubt, telephone first. Or, that runway might not have been mowed in some time, and the grass is much longer than you expect. Your arrival will be pleasingly soft and short, but your departure could be challenged. Uncertainty about this can be reduced with a carefully planned and executed touch and go. Carefully in that your time rolling on the surface will be intentionally brief - on, feel with the wheels, and off again. Then, perhaps repeat at several places along the runway to assure uniformity for the length you'll need. Carefully also in that, when you touch, several factors might result in a greater drag than you expect, which will equal a pitch down. The plane is still at flying speed, so the elevator will work, and you can control this - as long as you're expecting it, and ready.

Pavement and concrete, is more predictable. Pot holes aside, it's surface is uniform, and firm. But, standing water can produce a surprise, particularly if braking, or crosswind control becomes important. Standing water can occasionally be found on grass runways, but lucky is the grass runway owner to have a runway so flat and uniform that water could stand on it!

In between these, we might find gravel. Nice to roll on, never has standing water, but hard on the prop blades, and leading edge of the tail, if your technique is poor. If you find yourself stopped on gravel, apply pitch controls to keep the prop as far away from the surface as possible, and apply the power very slowly, so you're rolling before the RPM comes up much. Best not to stop, of you can prevent it.

Then there are the much more variable, and potentially dangerous surfaces, like sand can be amazing fun to fly from (and so too, the places it is common), but it can be a trap. Best to seek local knowledge. Your better chances will be on the wet sand at low tide, but don't plan to stay 'till high tide! (so know the tides for any more than a brief visit). I have used the same touch and go technique to assess the surface firmness, before I commit to landing there.

Similarly snow. It can be near impossible to judge the depth of unbroken snow. On wheels, with no knowledge of the depth, attempting to do it in flight is very risky. Even flying at low altitude over an open area with no surface texture is very dangerous, as you cannot judge your height. Unintended collision with the surface is still alarmingly too common. When on skis, there is a much greater chance of success. But even then, landing into 18" of loose power snow, is fun - trying to gain speed to take off from it is not! It is common in taildragger ski planes to run the landing surface for more than twice the required length, to assure the surface. Many runs can be made so as to pack the snow, and define the runway. Then, for the full stop landing, land a bit short, and stop on your tracks, with your runway for takeoff ahead of you. Get your snowshoes out, you're going to need them! The running the runway also assures you that for snow covered lake landings, there is not water not slush under the snow. That will really spoil your day! This is why tricycle aircraft are less desireable as skiplanes. Snowmobile tracks are fine to land on on skis, but not on wheels. The tracks are not compressed enough to support normal pressure tires. If I know the snow depth to be okay, I always land beside the snowmobile tracks, not on or across them. Snow can also conceal drifts, so you have greatly varying densities of snow along your run.

For Grass, gravel, sand and snow landing surfaces, sizing them up beforehand is best achieved as though they were the opposite gender and you were "interested": Size, Surface, Slope, Surroundings, Security, and Wind.

And then there's water. The hard surface you might land on, which is always moving. If you are landing on water, it is certainly not a runway, and has very little chance of being an aerodrome. It's probably hardly a place intended for aircraft operations at all, so lacks the infrastructure, and those things which reassure you, like a windsock, and people around to help, should something go wrong.

Water surface can be calm, or, ripples, waves and swells - in any combination! Calm would seem to be inviting, but danger, you cannot judge your height, as was the case with the unbroken snow. Ripples with no swells are a delight. You can judge your height, it won't be rough, and there must be a bit of breeze. Waves or swells will be a problem if higher than 8" for most Cessna/Piper sized floatplanes. Waves higher than 12" will result in a pounding ride, and swells though often smooth, will reach up to grab you, and pull you into unstable pitch excursions. And, if you have managed all that, hidden dangers float in the water, or sit shallow on the bottom, waiting to go bump on your hull(s) or worse...

So after you have mastered the approach and the flare, and you're "perfect" landing is jarred with the lurch of a mainwheel gabbling the pavement, or dragging in the long grass, or soft turf, remind yourself that there can be even more challenging surfaces to master!
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Old 4th Apr 2015, 14:49
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All good points, never landed on anything other than concrete/asphalt or grass, it was interesting reading about the other surfaces.

In respect of grass, in the UK of course, PPR is a requirement of most airstrips so you'll likely be talking by phone first anyway. Not sure many strips noise abatement policies would tolerate the numerous touch and go's being advocated ! Do this other than for operational reasons (missed approach) and you might find the operator chewing your ear. Or were you advocating, touching then going to be repeated in the same approach ?
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Old 4th Apr 2015, 16:01
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Flew into Sandown a year or so ago, and on the radio was told to land on the mown part of the runway...
So on Final I noticed the left side was a different colour to the right side, my first thoughts were....
He's mown the grass upwards on the right and downwards on the left, how nice...
OR... Could it be, only the left was mown...
OR... Could it be, only the right was mown...


I had to give a hasty call on the radio to see which bit they meant, as it all looks indistinct from quarter a mile and 70 knots...
(It was the half on the right.)
.
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Old 4th Apr 2015, 17:17
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And then there's water. The hard surface you might land on, which is always moving. If you are landing on water, it is certainly not a runway, and has very little chance of being an aerodrome. It's probably hardly a place intended for aircraft operations at all, so lacks the infrastructure, and those things which reassure you, like a windsock, and people around to help, should something go wrong.
And if you look way into the distance like a lot of today's instructors teach you can land right on top of floating objects.
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