Enough Runway Available? Maybe

In the 1970s the member companies of the General Aviation Manufacturers Association (GAMA) developed a standard format for the pilot operating handbook (POH) for light airplanes. Before that each manufacturer provided a lot or a little performance data and the way the data was presented in the book was unique to each company.

You might think that the FAA requires comprehensive performance data for all airplanes, but that’s not true. For large airplanes that weigh more than 12,500 pounds the FAA requires extremely detailed information on runway requirements for both landing and takeoff. But for small propeller airplanes, those weighing less than 12,500 pounds for takeoff, performance data is advisory, not an FAA requirement.

The only FAA certified and approved data for a small propeller airplane are the limitations, including limiting airspeeds, maximum weight, CG limits, engine operating limits and so on. That information is in Section II of the standard POH. By FAR pilots must observe the limitations, but other data, such as runway takeoff or landing length, is advisory and not an FAA requirement.

It’s been my experience over many decades that the major airplane manufacturers have been diligent and honest in the advisory performance data included in the POH. If you carefully study the conditions that apply to the takeoff or landing situation the airplane will perform by the book. But if a condition is different than shown in the POH, such as air temperature, weight, runway slope, water on the runway, wind gusts or other factors, your experience will be different.

Even more important is the fact that the test pilots who collected the performance data flew the exact profile described in the conditions. That didn’t happen because test pilots are perfect, but because they keep repeating the test until they hit every test point perfectly.

For example, I can look in the POH and see that on a standard day, which means the temperature is 15 degrees C, the runway is at sea level, there is no wind, and the altimeter is 29.92 hg, my airplane, at max takeoff weight, can take off and clear a 50-foot high screen in 2,100 feet.

Did the test pilot wait for those conditions and then find a 2,100-foot runway with 50 foot tall trees at the end to conduct the test? Of course not. He used a very long runway. And then he repeated the test takeoff until his liftoff speed was exactly 86 knots indicated, as the book says, and the airplane had accelerated to 94 knots at 50 feet above the runway.

Since the atmospheric conditions and runway elevation were not exactly standard day during the test the results were interprolated from other tests under various conditions. In that way a relatively few actual tests can be calculated to cover a broad range of conditions to create a graph covering most circumstances.

Do I believe the test flight measurements and performance calculations are accurate? Yes, as much as possible. But would I attempt a max weight takeoff on a 2,100-foot runway with tall trees at the end? No. And neither did the test pilot.

Landing runway required data is an even a bigger concern because there is so much pilot technique involved. The POH says I can land and stop on a 2,400-foot runway at maximum weight with 50-foot trees at the runway threshold under standard day conditions. To do that the test pilot adjusted power to maintain an 800 fpm descent which is approximately a 3 degree glidepath. He maintained the target approach speed exactly. And he used maximum braking after touchdown.

What the conditions in the POH don’t say is that the test pilot barely flared for landing, if at all. He couldn’t hit nosewheel first in a successful test, but all three could touch at once. And were the trees really there and the runway that short for the test? No, of course not. The runway length noted was the result of as many test landings as it took to get all of the conditions exactly right.

My point is that POH data isn’t a lie, but it can mislead. The results reflect a perfect world that doesn’t really exist in normal flying. And the published number contains no margin. The test pilots collected the data in what is actually a laboratory setting. Which makes sense. If they had used a real 2,400 foot runway with real 50 foot tall trees on the threshold they would have used up a few prototype airplanes trying to get the test just right.

If you fly people for hire the landing runway distance in the POH can’t be more than 60 percent of the actual runway. That means when the book says I need 2,400 feet to get over those tall trees and stopped the actual minimum runway I could use would be 4,000 feet. Would I expect to land and stop on that tree-blocked 2,400 foot runway? Maybe, but not every time. Would 4,000 work? Yes.

It’s essential to understand what the POH is telling us, but equally important to add our own comfortable margins. Even the STOL pilots at competitions don’t take off and land on a sidewalk. They use a long open runway and a measuring tape to find the results, not an airplane hanging in a row a trees at the end of the runway.

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5 Responses to Enough Runway Available? Maybe

  1. Kayak Jack says:

    I’m a wuss pilot. One of my jobs in the Air Force was “Aircraft Crash Recovery Officer”. That job included picking up chunks of planes and pilots out of the sea and snow and sod. A few memorial services later, it sinks in very deeply that if you have little to no margin for error, something bad is going to jump up and snap you in your personal empennage one of these times.

    I include fudge factor for sod instead of hard surface, soft sod, wet grass, tall grass, etc. and then add in another 50% just for me. I’m a wuss. But – a 76 year old wuss.

  2. Glenn Darr says:

    If your “gut” tells you things might get a bit dicey, you had better listen to that feeling! I am a 69 year old “wuss” pilot and have had 27 successful years in the left seat of my plane.

  3. It’s essential to understand the figures/numbers in the POH, but I never found it difficult to meet those numbers and particularly on landing. Like all maneuvers it takes practice and not just to make a good landing. It takes practice under all safe conditions. If you only fly on calm days and have an instrument rating for “just in case”, you are a prime example of an accident looking for a place to happen.

    I always made short field landings with a steeper descent profile, full flaps, landing on the mains, nose high. Immediately let the nose down, get on the brakes and then full up elevator. It will appreciably shorten landings. “Don’t do this with out an instructor” as it takes more precise speed, power, and attitude control, with a much shorter flare time. Pull that elevator too soon and you are likely to break something including your neck.
    Short field take offs are pretty much standard…follow the POH.

    I firmly believe all pilots should be able to be told when to pull the power and where to set down on the runway from most any point in the pattern, like they do at Oshkosh. To me it’s one of the most important elements in flying. Get in a few spot landing contests where they don’t allow jockeying the power.

    • Bill Tomlinson says:

      Roger, you and I sing from the same hymn sheet!

      I especially agree with what you say about taking part in spot-landing contests. The school I learned to fly added short-landing contests to the spot-landings, which is an even better idea.

      I do quibble slightly with your dismissal of short fields as “pretty much standard”. There are some tricks.

      Do a solo circuit before loading up – that gets the engine properly warm, which can make the difference between clearing the trees and leaving your – umm – personal empennage in the treetops.

      If the runway is REALLY short and/or there is an unfavourable wind, think about splitting your load: fly half your pax to a neighbouring, easier strip, then go back for the other half.

      Few really short strips have fuel supplies, implying that you will need to refuel elsewhere, either on your way in or on your way out. Do it on your way out because that way you will be lighter.

      Better not to take a retractable into a rough strip (most short strips are rough, and vice versa). I have read too many stuck-landing-gear post mortems where the probable cause was banging a wheel into a pothole and damaging some part of the retraction mechanism.

      Last, and most important, if the aircraft-type permits, do both landing and take-off with the doors unlatched. That way, if the worst happens you won’t have to contend with trying to get out of a door which has jammed. (In case anybody is thinking that is far-fetched, it happened to a friend of mine: he and two passengers were cremated alive while they struggled to open the doors of a C206).

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