Pilots are trained to think, talk, and trust numbers — specifically, performance numbers. But somewhat like the good intentions that pave the road to the Underworld, this primrose path has a few land mines, too. For one thing, POH data give the impression of being precise, and indeed, they are. However, they represent a new airplane trimmed to a fare-thee-well with a flawlessly tuned engine, and flown by a highly trained test pilot using every trick in the book to extract every iota of performance allowed by the laws of physics.
Inside Information: If your chariot left the factory before 1979, performance data — in fact, any handbook at all — was entirely voluntary and often creative to the point of being fictional. As Barry Schiff recounts in his book “The Proficient Pilot” (Volume 2), a published range for an early Cherokee 180 evidently assumed that the airplane was first teleported to cruise altitude. It was then flown until the tanks were dry and deftly flown (with the engine stopped and at best glide) down to the destination. There was absolutely no allowance for take-off and climb! Did they actually say that? No!
YOUR OWN POH
Don’t trust performance numbers. If you’re the proud owner of an airplane (even a new one), it would be a good idea do make your own test flights (rather than relying on any such “flights of fancy“), to see what the numbers really are. Schiff’s book has some great pointers as to how to do just that. Don’t launch from an abbreviated space on a hot day with the wife and kids just because the POH says you can.
STRATEGY: See how the airplane actually performs under controlled conditions — when you aren’t operating with your back to the wall, performance-wise. In this example, a la Schiff, you could…
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- Pick a point in the table (or chart) which comes close to what you’ve got that day, (weather-wise) and instead of using the wife & kids, use just you.
- Then take whatever runway length you actually used up, and divide it by that wildly optimistic number in the POH.
Key: That gives you the “reality factor“, which you then apply to whatever it said you’d need at max gross. For instance, let’s say you used up 2275 feet solo, where the book forecast 1750. Your reality factor is 1.3 (2275 is 1.3 times 1750).
- Now have a look at the book’s forecast with the whole show on the road. In this example, the book forecasts 2430 feet used up.
- You should expect no less than 1.3 times that, or about 3160. (It’s actually not linear like that, but it sure beats blind faith!)
NEVER HAVE, NEVER WILL
Another of Schiff’s Proficient Pilot concepts is Maximum Personal Range. This involves a method that, if always followed, absolutely guarantees that you’ll never run out of fuel. Interested? It’s easy. Here’s how: First, top off your tanks … on a level surface. (This method does presuppose you have more than one, and there’s no crossfeeding. You’ll have to do your own research to make sure it applies to you.)
How it works: Fly a test flight and follow these simple steps:
- Use one tank only for taxi, run-up, take-off, climb, descent, and landing.
(Yes, I know the pre-takeoff and before landing checklists in many a POH say to use only “BOTH“. As Schiff diplomatically notes, this might not be possible in “some” training aircraft.) - Use the other tank only after you’re leaned out in cruise, flying one hour (though two is better) at your “average” cruise altitude. Then switch back to the other tank.
- After landing, top off both tanks again and note how much fuel each tank took.
- Divide the number of gallons used to refill the “cruise” tank by the number of hours you flew. This gives you a number for your fuel burn per hour for this “average” trip.
- Subtract the number of gallons used to fill the “climb/descent” tank from the amount of total usable fuel as listed in the POH (yes, we’ll trust it for now).
- From that number subtract one hour’s worth (deduced in step 4) for reserve fuel. The result will be a net fuel volume in, let’s say, gallons.
- Divide that number by the gallons per hour that you just derived for cruise (again, that’s step 4) and you’ll get the MPR.
You now have the self-imposed number of hours beyond which thou shalt not fly! Yessiree, there’s no gospel like something you can believe… Just be sure to check your math.
