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Skyvector lists the gradient for KMCI as 0.3%. What is the rule of thumb for how large a gradient needs to be before you always take off downhill and always land uphill like they do at Lukla? Assume winds are not a factor.

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    $\begingroup$ actually the reason for only taking off in one direction is terrain. you won't be able to drop to the tarmac after clearing the hill behind it $\endgroup$ Dec 3, 2014 at 16:32
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    $\begingroup$ A runway with an uphill grade is equivalent to a shorter runway with no grade. You might ask in Physics for the formula. $\endgroup$
    – rbp
    Dec 3, 2014 at 17:01
  • $\begingroup$ @ratchetfreak: Well, VNLK has 12% gradient and that is itself steeper than both descent and climb gradient of most planes, considering the 9 200 ft altitude. $\endgroup$
    – Jan Hudec
    Dec 3, 2014 at 20:09
  • $\begingroup$ 12% is 6.875 degrees! wolframalpha.com/input/?i=12%25+in+degrees $\endgroup$
    – rbp
    Dec 5, 2014 at 14:25

3 Answers 3

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What does your POH/AFM says?

Ignoring winds and other factors, it depends on effective runway length, not just runway length. If you have a runway long (really long) enough, you can takeoff/land in either direction.

As mentioned here:

... advantage or disadvantage of a sloped runway is that a 1% runway gradient — an increase or decrease in altitude of 10’ for every 1000’ of runway length — is equivalent to a 10% increase or decrease in effective runway length ...

Example:

Runway length is 1500' with 3% up-slope gradient

  • Landing uphill on it will give us an effective runway length of almost 2000’ (1500’ x 1.3 = 1950’).
  • Landing downhill on it will give us an effective runway length of just over 1000’ (1500’ x 0.7 = 1050’).

Just for reference, the runway (6: uphill; 24: downhill) at Lukla Airport has 12% gradient and is 1500' long.

Effective Runway Lengths (using the formula above)

 6: 1500' x |(1+1.2)| = 3300'
24: 1500' x |(1-1.2)| =  300'

Hence, hypothetically speaking, if runway 24 is 48,400' long with no wind, A380 can takeoff too.

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    $\begingroup$ Except that you can't land downhill on anything steeper than your descent gradient at all. And you can't take-off uphill from anything steeper than your climb gradient at all either. And in multi-engine you obviously wouldn't attempt to take off uphill steeper than your one engine inoperative climb gradient. Normal descent gradient is 5%, Lukla has 12% (and Courchevel has 18%). $\endgroup$
    – Jan Hudec
    Dec 5, 2014 at 19:17
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    $\begingroup$ @JanHudec Good Point. That's why I said hypothetically, not theoretically or practically. Even with no wind, there are many other factors too. I also started the answer with Ignoring winds and other factors.... $\endgroup$
    – Farhan
    Dec 5, 2014 at 19:24
  • $\begingroup$ By the way, how do you justify the absolute value? The rule does not say any such thing. The effective length for takeoff on 10% uphill/landing 10% is 0 according to the rule and for steeper gradient it therefore should be negative (so it is minus 300' for landing on VNLK 24) $\endgroup$
    – Jan Hudec
    Dec 5, 2014 at 19:40
  • $\begingroup$ @JanHudec I did that because length is a scalar quantity, but I do agree with your example. $\endgroup$
    – Farhan
    Dec 5, 2014 at 20:12
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I happened to be looking at some takeoff distance charts, so I looked into this. As Tyler says, the answer will depend on your airplane's performance. With good brakes and plenty of runway, a small plane may not care as much as a larger one. The following applies to the PC-12.

I know you said to assume winds are not a factor, but it may help to roughly relate the effects of slope and wind. It turns out that 4% of slope will either help or hurt about the same amount as 10 kt of wind (within about 7%). Both provide around 10% benefit or 25% penalty on takeoff distance, and landing distance is affected by slope a bit less and wind a bit more. This would provide a good sense of how much a slope will affect the runway length you need. That penalty will certainly start to hurt as slope increases.

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There is no firm guideline. It depends on wind conditions and other factors like the surrounding terrain.

I have landed at Jaffrey only uphill, which has a 0.9% grade and a bad surface, but people can and do land there downhill. It is very steep for an air strip. If the wind was strong enough I would have no problem landing downhill there, but with moderate winds I would always go uphill.

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  • $\begingroup$ OP wrote "Assume winds are not a factor." $\endgroup$
    – rbp
    Dec 3, 2014 at 21:44
  • $\begingroup$ Well, if there is no wind and no terrain you would normally land uphill on a graded runway if it has a listed gradient. $\endgroup$ Dec 3, 2014 at 22:14
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    $\begingroup$ OP wanted to know how a grade figures into the aeronautical decision-making to land downhill or take off uphill, and if there's a rule-of-thumb. for example, runway 01 is 2500 feet long and downhill, and you're approaching from the south. Would you make a straight in, or circle at .5%? 1%? 5%? $\endgroup$
    – rbp
    Dec 4, 2014 at 14:14

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