Downslope with a Tailwind
Recently, a few of us were discussing the situation where the runway is
sloped and the wind is blowing down the slope. It is best to takeoff
downhill, but also into the wind. If the wind is calm, the answer is
obvious. It is also obvious if the wind is a steady 25 knots right down the
runway. What about in between? One of us (not I) found a website with the
answer. The URL is on my work computer and I don't have it here at home.
There is an equation that is used to calculate the "break even" wind speed
where the ground run is about the same either way. If the wind is below the
break even speed, then takeoff downhill with the tailwind. The equation
uses the runway slope, the zero-wind ground run distance, the liftoff
speed, and the gravitational constant. The equation sets the break even
wind speed to the runway slope (in degrees) times the liftoff distance with
zero wind and zero slope, divided by 5 times the liftoff speed (in knots).
One place where this equation is useful is at Sedona (Arizona), where the
runway slopes down about 2 degrees on Runway 21, which is the preferred
runway for takeoffs (for noise abatement). The airport is at 4800 feet, and
it can get above 80 degrees in the summer. On a warm day (like today), the
zero-wind zero-slope takeoff run from the POH tables in the POH is about
2000 feet. According to the equation, it is better to take off downhill
with a tailwind until the tailwind component of the wind speed exceeds 12.3
knots. If it is above 12.3 knots, then it is better to takeoff uphill into
the wind. The published rule for Sedona Airport used to be to takeoff
downhill unless the tailwind was greater than 10 knots, but more recent
publications leave it up to the pilot (probably for liability reasons).
Since most of our planes have a takeoff speed of 65 knots (or 75 mph), I
created on a handy chart that is based on the calculation. It can be
printed out and kept in the plane. I submitted a pdf version of the chart
to the MM download. If it doesn't show up within a few days, email me
directly and I will send it to you.
Carl Foster Sundowner 9761L Tucson AZ
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