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 NW-SE 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 lift-off distance required is equal to the percentage of lift-off 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 "lift-off distance required."

Rule of thumb to determine if the
runway length is adequate

10 times the square root of the percentage of lift-off distance required is equal to the percentage of lift-off 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 lift-off distance," we use 50 percent for the lift-off distance and the square root of 50 is 7.07. Ten times 7.07 equates to 70.7 percent of the lift-off 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.