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This is a followup on question: Why don't short runways use ramps for takeoff?

There are multiple answers to that question that answer why an uphill ramp at the end of the runway does not make sense. But what about going the other way?

Downhill ramp

I would imagine a downhill ramp at the beginning of the takeoff roll could give the aircraft more energy on takeoff (converting potential energy to speed), and that could indeed improve takeoff performance. This would be useful at places where extending the runway is not possible.

How the aircraft would get up there is another question, but worst case scenario it could always back-taxi on the runway before turning around and taking off.

Is this practice ever used? If not, what are the reasons?

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  • $\begingroup$ Back-taxi up the hill, turnaround at the top to come down? Sure, that is done at places where the terrain requires that. Jaffrey, NH comes to mind. $\endgroup$ – CrossRoads Aug 1 at 19:01
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    $\begingroup$ There are a number of strips around the U.S. that are uphill-downhill strips. Near us is McKenzie Bridge State Airport in Oregon. You takeoff to the west, land to the east, regardless of the wind. There have been fatal accidents involving pilots that ignored that, choosing to operate into the wind. $\endgroup$ – Terry Aug 1 at 19:41
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    $\begingroup$ Can you elaborate on why you think this would be different? Most of the issues mentioned on the other question would still apply here, as if you switch directions to follow the wind, now the ramp is at the departure end again. $\endgroup$ – fooot Aug 1 at 20:00
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    $\begingroup$ At least one does... $\endgroup$ – Dave Aug 1 at 20:30
  • $\begingroup$ With the sloped end(s) of the runway, the requirement for 35' clearance on takeoff/go around becomes a limiting factor to overcome. This idea reminds me of the dented sterns on carriers used for carrier quals with first time students. getting below the threshold is not a good thing on short final. $\endgroup$ – Mike Brass Oct 5 at 6:56
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Many of the arguments against using a ramp from the other question is valid for this as well. Any ramp means a height difference between the ramp itself and the runway, for it to actually make a big difference I suspect a fairly long slope from the ramp to the runway is needed, and this will obstruct the runway for landing aircraft. If the ramp is above ground, aircraft taxiing for take-off would also have to overcome the upward slope, meaning that more fuel is needed for taxi.

You'd also need a ramp at the opposite end; the runway would basically look like a stretched out 'U'. A possibly unintended consequence of this is that the aircraft would at some point reach the point where the runway begins to slope upwards again, resulting in the aircraft either having to abort or use additional power to get airborne.

To avoid such a case you would have to stretch out the runway so this isn't an issue, and if the goal is to reduce the runway length the current design is more efficient. Alternatively you could create a separate runway for each direction, but then you'd be faced with potential crashes whenever a pilot tries to land in the wrong direction. This would also result in difficulties for airports located in built-up areas where there simply isn't available room for additional airport infrastructure.

If you keep the ramps on ground level and the runway center below you'd also need working drainage pumps whenever it rains or snows. If not, the runway would quickly be reduced to a pond with very wide roads leading into it. Depending on where in the world you are, water might seep in from the surrounding terrain as well, making reliable drainage even more important. This is generally avoided by building runways with a slightly raised centerline so that water runs out to the sides so that you can use "passive" drainage systems such as pipes, ditches, pools etc where no machinery is required.

There are some airports such as Lukla where one end of the runway is substantially higher than the other, but due to surrounding terrain it is only possible to land and take-off from opposite directions. Landing aircraft can use the upward slope for an additional braking effect, while aircraft taking off will get a speed boost from the downward slope.

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    $\begingroup$ I'm picturing some kind of hydraulic lift used to raise and lower the ramp at each end so that it can be reconfigured depending on the wind... ;-) $\endgroup$ – Lnafziger Aug 2 at 2:29
  • $\begingroup$ Well, but then you end up adding a lot of complex machinery which would cost a lot of money to maintain and operate, just to save a bit on the take off distance. The runway, ramp and slope would still have to be adapted to the largest aircraft you would expect, meaning that smaller aircraft might not even need the ramp at all. According to community.infiniteflight.com/t/… a fairly common regional jet requires a runway of around 3500 ft while a 747 needs up to 7000. $\endgroup$ – bjelleklang Aug 2 at 13:09
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    $\begingroup$ @bjelleklang, I think Lnafziger was being somewhat sarcastic.... (I know, it's hard to tell over the internet) $\endgroup$ – FreeMan Aug 2 at 13:40
  • $\begingroup$ Surely you could provide for passive drainage on a sloped-at-both-ends (lengthwise) runway. I can't imagine that part being much more difficult than for a normal/flat runway. Snow is going to be a problem no matter how you do it, because as long as the temperature remains below freezing it has a tendency to remain where it falls (unless whatever it lands on has a very strong slope; enough so that it would likely make the surface useless as a runway) but that doesn't seem to be more a problem in the case of what OP proposes than for a normal runway. $\endgroup$ – a CVn Aug 4 at 19:18
  • $\begingroup$ It depends on how it is constructed. If the ends/ramps of the runway are above surface the center would be closer to the surface, and that would make it easier. If the center is 10 meters below the surrounding surface passive draining would be more difficult, and draining in general would be much more important. $\endgroup$ – bjelleklang Aug 5 at 6:01
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it could always back-taxi on the runway before turning around and taking off.

So ... that would limit the steepness of the slope significantly, and thus the potential benefit.

Most other aspects are already covered by Bjelleklang's answer: Landing aircraft would either have to clear the obstacle before being able to touch down, or they'd fall over the edge if they don't brake in time (rather than, as these days, get stuck in the grass behind the runway end)

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Here's an example of a sloped up runway, midway down looking southwest. The end is definitely sloped up. We're typically off the ground well before that tho, the runway is 3110 feet long.

Taking off from that end towards the northeast, going downhill to start will help with the takeoff acceleration. enter image description here

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    $\begingroup$ I'm not sure this anecdotal reference actually answers anything... $\endgroup$ – T.J.L. Aug 4 at 23:10
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A "relaxed definition" of this concept is in use today on aircraft carriers.

Notice the ramp in question is essentially a mechanical device to increase the total thrust force on the aircraft, which increases acceleration. This is also exactly the same as pitching an airborne aircraft down, or launching a model aircraft by throwing it. RATO can help too.

The energy input for higher rate of acceleration: F = ma total = ma thrust + ma gravity, steam catapult, thrown with hand etc.

The result is higher air speed for a given distance as compared with no "ramp".

The practicality of building such a device would be questioned. Generally, it's landing distance that is limiting.

But many soaring birds take it one step further, they simply jump off a cliff (disclaimer: attempting without certified wings may be hazardous to your health).

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