ADEQUATE 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.
You might not be too concerned about landing at this airstrip (the airstrip runs NW-SE on the right side of the creek) because of 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.
RULE OF THUMB – 50/70
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." Egads, that sounds complicated; but it really is quite simple.
Airplanes stop better than they accelerate. It is possible to land at an airport where there is not sufficient runway available for the takeoff.
With this in mind we are going to mark the half-way point on the runway. If the airplane accelerates to this point and has the required performance, continue the takeoff. If the airplane arrives at the half-way point and does not have the required performance, abort the takeoff. The airplane can be stopped in the remaining distance, providing the airstrip is not downhill or has a wet surface.
We are going to use the half-way point of the runway for the "liftoff distance required." Keep in mind that this rule of thumb does not guarantee that the rate of climb will be sufficient to clear any obstacles that are present after takeoff, but it does guarantee that there is sufficient runway available for the takeoff.
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 halfway 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. This 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 the takeoff and wait for better conditions, or off-load some weight.