Tuesday, March 9, 2010
Making sense of tricky landings
Because we’re talking about gaining enough kinetic energy to take off, we can assume landing will be affected the same way, only in reverse: We have to get rid of that same amount of energy, and upslope will help. Our runway acts as if it’s 15% to 30% longer, depending on the reference you use. Factory POHs favor 5% per one degree of upslope.
What about a tailwind? The same POHs say that a takeoff/landing in which the tailwind is 10% of the takeoff speed (which we’ll assume is the same as the landing speed) increases the roll by 20% (the same effect as shortening the runway). So, if the ground roll normally would be 600 feet (average for Cessna 172/Piper Warrior types), and you have a 50 mph touchdown speed and 5 mph tailwind, then your new ground roll will be 720 feet. Another way of looking at it is that the 2,500-foot runway now is 2,000 feet long because of the tailwind. If it’s also sloped down 3%, then it’s only 1,400 feet, which could be a big deal. If it’s sloped up, however, even with the tailwind, it’s 2,600 feet long, so the upslope offsets the tailwind.
|Downsloping Terrain On Approach||Approach Terrain Level With Runway||Approach Terrain Level With Runway|
| || || |
|Pilot incorrectly perceives approach is low||Pilot correctly perceives approach is accurate||Pilot incorrectly perceives approach is high|
|The effect of sloping approach terrain on pilot perception.|
Now we toss in a real kicker: runway surface. In a normal situation (level, no wind), a grass runway landing requires less length and braking because of the drag of the grass on the wheels. Ditto for soft runways. But what happens if you’re moving faster, either because you’re going downhill or because the wind is behind you? Well, you may need some braking. Suddenly, the runway has gotten shorter because your tires lack the traction they have on dry asphalt. For that reason, it’s always assumed anything but asphalt will take more distance. Short grass is good for 20% more runway; tall grass, 30% more; and wet grass, 30% to 40% more!
It gets really interesting if you’re landing downslope and the runway has light snow or ice. You have to assume that zero braking and the force of gravity won’t ever go away. Even if it magically stops, the plane still will have a tendency to roll downhill, and if there’s zero braking (ice, so no traction), it might never stop moving. In that case, you’re just a passenger unless there’s enough headwind to stop the aircraft. The heavier the plane, the more noticeable this will be. If there’s a lot of snow (several inches), then the drag of the snow will help slow the airplane and make up for any gravity-induced tendency to continue rolling/sliding, but your available braking still will be minimal.
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