runway length
Operating at mountain airstrips presents us
with various passenger loads and different density altitude conditions for
nearly each takeoff. These factors combine to provide a loss of
performance, creating concerns about whether or not the runway is long
enough for takeoff.
Airstrip runs NWSE on right
side
You might not be too concerned about landing at
this airstrip. There are unobstructed approaches from either end.
But, when it comes time to depart, you might have some doubts about
the runway length. We have a rule of thumb that can determine if the
runway length is adequate for the takeoff; although, it will not
guarantee rate of climb after the takeoff. The POH (pilots operating
handbook) should be consulted to determine the rate of climb.
The rule really is quite simple. It states: "Ten
times the square root of the percentage of liftoff distance required
is equal to the percentage of liftoff speed that should be attained
in that distance."
Because airplanes stop better than they
accelerate, we can easily accelerate to the halfway point of a
runway and determine if there is sufficient performance to continue
the takeoff. If there is insufficient speed, we can easily stop in
the remaining half of the runway. (This really doesn't work well on
downhill runways.)
So we will use the halfway point of the
runway for "liftoff distance
required."
Rule
of thumb to determine if the
runway length is adequate
10 times the square root of the percentage of
liftoff distance required is equal to the
percentage of liftoff speed that should be
attained in that distance


Remember, this rule of thumb does not guarantee
that the rate of climb will be sufficient to clear any obstacles after
takeoff, but it does guarantee there is sufficient runway for the
takeoff.
Mark the halfway point on the runway. This
might require you to walk the length and count your steps, then walk back
and determine a distinguishing characteristic or place a flag or marker at
this point.
Using the rule, "10 times the square root of
the percentage of liftoff distance," we use 50 percent for the liftoff
distance and the square root of 50 is 7.07. Ten times 7.07 equates to 70.7
percent of the liftoff speed should be obtained at the halfway point to
guarantee takeoff in the remaining half of the runway. If you have the
speed, continue. If you do not have the speed abort.
