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.