Uphill/Downhill 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).

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.

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.

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!