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POH versus reality

Making takeoff calculations meaningful

Have you helped your students see the practical significance of computing takeoff distance? If not, then it’s possible their takeaway is that the exercise is a silly preflight task we do only as required by FAR 91.103 but, otherwise, a waste of time. As an examiner, I routinely see this calculation dismissed as such. Here are some suggestions to make teaching the takeoff distance performance calculation meaningful.
Photography by David Tulis.
Zoomed image
Photography by David Tulis.

Know the mechanics of the chart.

I hope it goes without saying but it’s important to ensure that your student does this calculation regularly and has a facility with the chart or table. Each incorporates pressure altitude, temperature, aircraft weight, and headwind/tailwind component to arrive at an estimate of the takeoff roll and, often, the total distance to climb over a 50-foot obstacle as well. Be sure that the conditions on which the performance is predicated, such as lift-off speed and flap configuration, will be the same during takeoff.

Remember why we make this calculation.

On an FAA knowledge test, candidates need to demonstrate the ability to interpolate among values on a table, but that skill isn’t terribly practical here. We are trying to prove that the airport runway is long enough for a safe departure so, instead of interpolating between 927 feet and 1,023 feet, just use the latter, more conservative choice and move on. And instead of computing an accurate estimate of the headwind component, just use zero winds and plan on taking off into a headwind to improve performance.

Make it practical.

I asked a recent private pilot candidate to prove to me that the runway at Sewanee’s Franklin County Airport (UOS), with trees at the end, is long enough for our use. He computed the distance to climb over a 50-foot obstacle and argued that it was less than the available runway. I pointed to the trees and told him that they have been surveyed at about 90 feet tall and he sat puzzled. It took some doing but he was finally able to modify his earlier estimate for the non-50-foot obstacle.

Notice reality.

On a commercial exam earlier this year, the candidate estimated a ground roll of 750 feet for a short-field effort. As we began the takeoff roll in the airplane, the wheels were still on the ground one stripe past the 1,000-foot markers, but I’m the only one who noticed. That is a significant departure from anticipated performance that could be because of improper technique or an engine that might be nearing a date with an overhaul shop.

Add a safety buffer.

There are myriad reasons why an aircraft may not achieve book performance beyond what we’ve already mentioned: changing winds, a sloping runway, a dragging brake, to name a few. Adding 50 percent to 100 percent of that estimated distance to clear any obstacles present is common. For example, if the charts predict 1,400 feet to clear any obstacles, ensure that the runway length is at least 2,800 feet long. It’s easy math that encourages safety.

My favorite part of a practical exam occurs at the end when I debrief the candidate and instructor on ways to improve preparation. Much of the day focuses on determining whether a single applicant has achieved the knowledge, skill, and decision-making ability to earn the certificate or rating. In contrast, if the flight instructor puts suggestions into practice, the debrief will have improved the flight training experience for many current and future students. Remember that the big prize is not a temporary certificate but, rather, guiding students toward a long, happy life flying airplanes. 

Catherine Cavagnaro teaches aerobatics at UOS and is the Gaston Swindell Bruton Professor of Mathematics at Sewanee: The University of the South.

Catherine Cavagnaro
Catherine Cavagnaro is an aerobatics instructor ( and professor of mathematics at Sewanee: The University of the South.

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