"Tony Cox" wrote in message
oups.com...
[...]
Thinking about it after posting, in practice I think it might get even
more complicated. Ever worked out how accurately you can "spot"
a landing?

I know what you're saying, but all of the landing calculations are subject
to the variations you're describing. I think the basic calculations ought
to be able to be done independent of those, and you can apply the same
wiggle-room at the end that you have to for the usual landing distance
calculations.

A 3 degree slope sounds pretty steep to me. Are you sure it's not 3%?

I'd like to be definitive, but my airport guide is in the plane,
and I don't seem to be able to find the info through google.
I *thought* it was 3 degrees (and I really ought to know,
having been based there 6 years), but I may be wrong. Anyway,
3% works out as 2 degrees.

Well, 3 degrees is 50% steeper. That's a pretty big difference.

Interestingly, Airnav (which basically just republishes A/FD data) does
publish a runway gradient for Temple Bar, but not Boulder City. Though, it
does specifically warn against takeoffs from runway 33 at Boulder City due
to the gradient. Odd that they wouldn't publish the number.

Anyway, the published number has a very specific FAA definition, but it is
essentially just a percent grade. So if you've got a published number for
Boulder City that's called runway gradient, that's most likely what it is.

However you cut it, a 3% grade is certainly significant. Didn't mean to
imply otherwise. It's just that 3 degrees would have been even more
dramatic.

[...]
I've heard that too, and Sedona airport (KSEZ) goes as far
as to mandate which runway to use depending on wind (I think
they have a slope of 1.5 *units*, but their runway is so long it
hardly makes a difference to us little guys).

Driggs, Idaho is also 1.4%, and at 7300 feet seems quite long. But between
the 6200' airport elevation and the slope, both takeoffs and landings can be
interesting, especially done downwind. Sedona is lower at 4800', but the
runway is only 5100' and the gradient steeper at 1.9%. I'll bet things can
get interesting there too. I think even a little guy would want to take
care there.

I think the takeoff calculation ought to be more straightforward.
You can use the whole runway, for a start, and all one needs
to do is to work out how much acceleration you sacrifice by
running up the grade. Clearly, braking effects are irrelevant.
Still, it'd take a bit to figure the math.

Well, the landing distance calculation would make the same assumptions about
touchdown point and speed that one would make for normal landing distance
calculations as well. I agree that in general, takeoff calculations ought
to be more reliably repeatable than landing calculations, but that's not
unique to the question of sloped runways. So as a starting point, I'd say
one ought to just make those same assumptions and acknowledge that there's
some inherent error in being able to replicate the theoretical numbers.

I just feel more comfortable landing uphill. Perhaps its because
on approach you have a bigger target to hit!

Well, the steeper the hill, the safer you ought to feel. But there's
uphill, and then there's UPHILL. Distance to stop increases with the
square of your airspeed, so even a little extra speed translates into a lot
of extra landing distance. The difference between landing with a headwind
and landing with a tailwind is proportional to *twice* the wind speed (since
it's added as a tailwind and subtracted as a headwind). If your landing
speed is 60 knots and you've got a 10 knot wind, that's almost a DOUBLING of
landing distance comparing headwind to tailwind (70 knots is 40% faster than
50 knots, giving you a 96% increase in stopping distance).

You'd need a pretty good hill to halve that doubled landing distance to get
back to breaking even.

Pete