Tuesday, March 9, 2010
Making sense of tricky landings
Unfortunately, there are few one-size-fits-all answers. Every situation and airplane will present varying conditions. Parameters to consider include runway conditions, groundspeed (increase or decrease due to wind) and runway slope.
In pondering the best landing conditions, you’re actually considering the energy the plane will have on touchdown, and once on the ground, how to minimize and get rid of that energy. You may remember kinetic energy from high school physics: It’s the energy an object possesses by virtue of its movement. You probably learned that the formula for kinetic energy is ½mv2, where “v” is velocity (in this case, groundspeed). Simply put, once a mass is moving, its energy is affected more by its speed than by its weight. It’s that “velocity squared” thing that affects all aspects of flight, from lift to drag to required runway length. It also affects how hard the plane will be to stop, because the brakes have to exert enough energy to counter the energy of the moving airplane.
To make things more complicated, the brakes’ ability to slow the plane (i.e., ablate the kinetic energy) is affected by how much traction the tires get on the runway surface. In the best situations, there’ll be some slippage when the brakes are applied. But if it’s grass, the available traction is minimal; same thing for wet asphalt. If it’s light snow or ice, all bets are off, and you have to operate as if you have no brakes. This is when trying to understand the energy trade-offs involved in an uphill/downhill landing versus an into/out of the wind landing is critical—and frustratingly complex.
The most common and difficult operational decision faces us when the wind is blowing uphill. In this situation, you know that your groundspeed over the threshold will be lower. But landing downslope means that gravity will be pulling the mass down, making it harder to slow. The question is: How much wind does it take to offset landing downslope? That, in turn, leads to more questions: How much slope is there, and how much effect will it have?
The POHs of some high-performance aircraft (such as the Cessna 180) have rules of thumb that say taking off from a 1% upslope runway (i.e., a 25-foot height gain on a 2,500-foot runway) will have the effect of shortening its length by 5%. This is because of the longer takeoff roll, although some references say it’s shortened by 10% per one degree of slope. Thus, taking off from a 3% slope, the runway effectively will be 15% shorter (going with the more conservative numbers, however, the takeoff roll would be 30% longer, so the effect would be the same as having a shorter runway).
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