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A long time ago I read an article (in Popular Science, probably) of a plan for an airport runway constructed on a grade, so that one end was ten stories higher than the other. The idea was to save gas: at takeoff, the plane would be going downhill and get a speed boost from gravity. When landing, the plane would be going uphill, and take less engine power to stop because gravity is slowing it down.

Did anyone ever research or actually test this plan? Does it seem practical?

Shawn V. Wilson
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    Well, you have Lukla Airport in Nepal with a 11% grade at 1,700 feet long which is like that because it is in the mountains. With a height of about 190 feet, this would basically be the equivalent of a 20 storey building. – SMS von der Tann Aug 31 '16 at 19:14
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    I don't think you would save any notable amount of fuel. Granted, taking off from a down sloping runway would perhaps take a few seconds less compared to a flat runway, but the engines would still be running at near full power for the entire duration of the flight. The take off roll is only responsible for a fraction of the total fuel consumption, making any difference here hardly noticable. – 60levelchange Aug 31 '16 at 19:24
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    Planes land and take off into the wind. If you wanted to build a sloping runway, you would have to build another, parallel, runway sloping the other way, otherwise the single runway would be sloping in the wrong direction whenever the wind came from a certain direction. – 60levelchange Aug 31 '16 at 19:26
  • On landing, planes don't really use much engine power to stop. Its mostly braking action. (Sure, thrust reversers are available, to reduce stopping distance, but the major deceleration comes from brakes) – abelenky Aug 31 '16 at 19:37
  • @J.Hougaard Are you sure about planes always taking off into the wind? LAX's runways all go east-west; I've seen countless planes approach the airport heading westward, and have never seen one departing the other way. – Shawn V. Wilson Aug 31 '16 at 20:22
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    I take that back. My wife used to live under the flight path of LAX and she HAS heard planes going both directions! – Shawn V. Wilson Sep 01 '16 at 05:33
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    @ShawnV.Wilson Yes, I am quite sure .. that's the kind of thing they teach you at ATC school :) – 60levelchange Sep 01 '16 at 15:46
  • @J.Hougaard I believe you, but Carlo's answer refers to "one-way in / one-way-out runways". Do they switch the "one-way" direction depending on the wind? – Shawn V. Wilson Sep 01 '16 at 20:33
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    @ShawnV.Wilson That depends entirely on the local procedures. Obviously, if there is a mountain at one end of the runway, that is not really an option – 60levelchange Sep 02 '16 at 04:21

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Yes, it does work, but cannot really be called practical.

The first reason is wind: It helps to take off and land with a headwind. If the runway slope is on one end, this has to be the end where the take-off run starts and where the landing run ends. Since both are in the same direction, this scheme now needs twice the runway length, the first half for landings and the second half for take-offs.

The next reason is the magnitude of what can be saved. The energy from the height change $∆h$ can be translated into a speed gain $∆v$ by this formula: $$∆v = \sqrt{2\cdot g\cdot ∆h}$$

Let the ten stories be a height of 40 meters, and your speed gain is only 28 m/s. This was significant in the age of propeller aircraft, but jets need much higher speeds to get airborne - 150 knots or more. In sane units this is 77 m/s, and since the energy is proportional to the speed squared, the 40 m slope will save only 13% of the energy needed for take-off. It is simply not worth it.

Now look at the operational consequences: The landing has to be performed such that the aircraft has slowed down to 28 m/s when it reaches the foot of the slope. If it is still too fast, it will overshoot and roll down the other side, and if it is too slow, it needs to run up the engines in order to climb up the slope, expending the energy it hopes to save at take-off.

No, practical is not what I would call this.

Peter Kämpf
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    Not to mention the wasted energy climbing up the parallel taxiway. –  Aug 31 '16 at 19:43
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    @ymb1 I faintly remember reading about this. The whole terminal building was on a hill, and all runways would end sloping up to it. Would save some taxi fuel, too. – Peter Kämpf Aug 31 '16 at 19:45
  • That's right; I left out that detail about the building. The landing runway came to the "south" side of the building, and the takeoff runway led to the "north". So you really could have runways in both directions. – Shawn V. Wilson Aug 31 '16 at 20:25
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    Personally, I didn't like the idea of a building directly in the flight path, nor that the runways would be ramps that a plane could fall off. That's why (in my Question) I put them on the sides of a hill instead. – Shawn V. Wilson Aug 31 '16 at 20:28
  • @ShawnV.Wilson: You are free to modify my description, and I thought of the terminal being offset from the runways. Placed on a gently sloping hill, there are no ramps, and I changed all occurrences of "ramp" in my answer to "slope" because it describes better what I thought of. – Peter Kämpf Sep 01 '16 at 08:00
  • How about all the fuel needed for driving pax and cargo up that hill? Probably a lot more than what's being saved by the airlines. – Rob Vermeulen Jul 30 '17 at 14:15
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It's never been planned as such due to the usefulness of flying approaches into the opposite end of the runway when winds favor so.

That being said there are a number of inclined one-way in / one-way-out runways in the world, mainly in remote and mountainous areas. The most notorious of which is Courchevel airport in the French alps. Like Gustaf III on St Barts, Courchevel requires special training and a logbook endorsement to fly approaches into.

Romeo_4808N
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Not practical for econoomic advantage, but there is an advantage if you are building airstips in the mountains - the strip doesn't have to be so long! many one-way strips in New Guinea with slopes up to 17%. As in the alps, commercial operations restricted to pilots with five trips under supervision, preferably under different weather conditions. An exemption, which I had, was supposed to require 1000 hrs in PNG and endorsement into 50 strips. I didn't get to that many . Wind direction irrelevant, accept what it is and compensate. On the steeper strips, full power at touch-down or you won't get to the top.

bleeding knuckles
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