Tuesday, March 9, 2010
Making sense of tricky landings
When considering an uphill or downhill landing—especially on an extremely short runway—the runway surface’s condition may be the deciding factor. Though landing into the wind but downhill may yield a shorter landing roll, if you have zero braking, you risk sliding off the end of the runway because there’s more gravity than there is traction.
Hiding behind all of these discussions is that V2 thing. Do you really want to come over the threshold carrying more speed than you want, especially if the runway is downhill and/or slick? We’ve already said that a 2% downhill grade is good for at least a 10% increase in length, and landing with more speed has more of an impact than increased weight. Is there a rule of thumb here? Not exactly, but it seems logical to always touch down as slowly as possible. And this brings us to an important factor: the amount of runway left behind on touchdown. We’re talking about piloting skill.
Sloped runways are known for causing perception problems on final, making it difficult to put the plane where you want it: A downsloping runway makes pilots think they’re low when they’re not, whereas an upsloping runway makes them think they’re high. For that reason, there’s a tendency to land long on a downsloping runway—exactly what you want to avoid. The reverse is true on an upsloping runway: Pilots often get fooled and have to nail the throttle at the last second to keep from slamming into the runway.
The problems are further complicated if the runway’s end is sloping either up or down to the threshold. If it’s sloping down so that you’re, in effect, flying down the face of a hill, then you’ll probably feel that you’re low. If it slopes up to the threshold, you’ll feel high. Thus, if you’re flying down a hill to a downsloping runway, you’ll need to fight the illusions and land it short. Truth is, the illusions are a problem only if you’re one of those pilots who aims at the runway rather than at a distinct spot on the runway. If you pick a spot—whether it’s the numbers, a clump of grass or the threshold—and ignore the runway itself, then none of these illusions will be an issue.
The point on the runway where the glideslope intersects the surface stays stationary in the windshield. The part you’ll fly over appears to go down the windshield (or come toward the plane), and the part you won’t reach appears to go up the windshield (away from the plane). All you need to do is keep your reference point stationary in the windshield to keep your glideslope pointed at it. But you have to ignore the rest of the runway to escape the illusions caused by slope.
Note that in a normal landing (not short field), when you hold a point stationary in the windshield, you won’t land on that point: Your flare will take you past it. If the length of the runway may be a concern, or if it’s downhill, then your airspeed control is critical so that you don’t float more than necessary; you may even use a short-field approach so you’re slower than normal over the threshold and have little to no float. This is helpful when landing downhill: If you kill most of your float through airspeed control, then you’ll have less trouble finding the runway when it’s falling away from you during flare. When doing the same thing on an upsloping runway, however, you don’t want to be slow: Keep in mind that the runway can come up to meet you surprisingly fast, so you may need the extra energy.
If you put the plane down within several hundred feet of the threshold, unless it’s a much steeper (or shorter) than normal downslope, then you’re unlikely to need excessive braking. If it’s very steep, then it’s going to require a fairly high wind on the nose to make up for it. And if braking is limited by runway surface (snow, wet grass, quite rough), then landing uphill, as long as the downwind component isn’t too high, should be considered.
A final note: Sooner or later, you’ll turn final to a runway, and think, “This is going to be really close.” This is a situation where the only black-and-white rule applies: If there’s the slightest possibility that you won’t be able to stop after landing, then find another place to land. Assuming you have fuel and you’re not on fire, there are no landings that absolutely have to be made.
Don’t be one of those heroes who breathes a sigh of relief after a hairy landing only to discover that you’ve landed in an area that’s too short to get out of. Bummer!
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