Or would this pilot fly his POH right into the trees?
Although this little vignette is entirely fictional, pilots and their passengers have survived scenarios like before - and, too often, not survived.
So what's wrong with this picture?
This pilot started out as one of the luckier ones - at least he had a POH for his 1981-vintage airplane. The average general aviation aircraft is already in the "over 30" crowd, but prior to March 1, 1979, an aircraft "owner's manual" was optional, and all many owners had to go by were a few placards. After that date, if an airplane or rotorcraft had no flight time (in other words, if it was new), CFR Title 14 Part 21.5 required the holder of an aircraft type certificate to provide the owner with a flight manual.
But as it turns out, even having one and following it to the letter has led more than one pilot, along with their loved ones, down the garden path. Here's why.
Pilots deal with numbers all the time. Usable fuel and never-exceed speed values are numbers. When a controller wants us somewhere, we won't usually hear something like "Oh, climb up to about 6,000 feet or so and turn west." From our first hours with an instructor, we learn that any final weight-and-balance point alighting outside the weight and moment envelope isn't just a foul, it could take us out of the game for good! Numbers represent precision, and performance numbers in the POH are part of the gospel handed down to all. Although earlier manufacturers' performance figures were vague or optimistic to the point of fiction, we have come a long way.
The problem is that most of us didn't go to test pilot school, and there aren't any tables of correction coefficients for nicked propellers, bug-splattered wings, sloppy airspeed control, or increased ground rolls from low tire pressures. Nor, because of variability, can we apply any corrective increments based on a ratio of an engine's time in service to its TBO - or increased takeoff distance based on runway gradient. No, there's no concordance in Section Five of these bibles, and there's still very much room for improvement in the presentation of these data.
Let's face it: In a free-market economy, good news sells. The best way that someone with an aircraft to promote can achieve sales success is by hiring a test pilot to derive performance data using a new, perfectly trimmed airplane with a spanking new engine - and by using every trick in the book. Did I really intend to use the word trick? Yes. As Barry Schiff describes in The Proficient Pilot, Volume 2, a published range for an early Piper Cherokee 180 presumed that the airplane was first teleported to cruise altitude, flown until the tanks were bone dry, and then adroitly flown at best glide to the destination.
The point is, don't trust performance numbers. Why accurate weight and balance data are required for general aviation, but not realistic performance data, I'm not qualified to say (although I am qualified to wonder out loud). Both are critical. And fortunately, there are ways that you can still make use of performance data to avoid uncertain flights of fancy.
One tried and true way is simply to apply all corrective factors, whether for density altitude, tailwinds, power setting, etc., and then double the result. Another is a little more sophisticated, and frankly, more worthy of interest for the clever people that we pilots are. (This one also comes from the same insightful book.) It is inadvisable to risk one's investment in the gene pool by launching from an abbreviated runway, uphill, on a warm day at max gross takeoff weight. But if the pilot in our example had instead rehearsed on another, longer runway having about the same upslope, factored in the rehearsal conditions as accurately as possible (temperature, weight, wind, configuration, climb speed, etc.), and measured off what it took versus what the POH said it would take, he would have had a "reality factor" to apply to that wildly optimistic number in the POH.
Let's say that on a previous weekend, he had picked some takeoff conditions that came reasonably close to those at his home 'drome. His uphill ground roll uses up 2,000 feet; the book had said 1,140 for a level runway. That's one and three-quarters as much. Let's see, take the original 1,685 feet times 1.75.... These kinds of factors are not linear, but this sure beats blind faith.
So much for not flying into the trees on takeoff. Now what about that landing, and the flight in between? Knowing your fuel burn, topping off or using a calibrated pipette on level wing tanks to quantify fuel before takeoff, keeping a flight log, and landing with more than the FAA-mandated minimum fuel all are great. Then again, dogmatically doubling your fuel requirements won't put too much of a crimp in Cherokee passenger-miles, either, although that is a tad over-conservative. You could perform other "POH proportioning" rehearsals to derive similar reality factors for other performance figures.
For example, start with the total usable fuel figure from your POH. (This should be accurate.) Let's say that's 40 gallons. Now instead of a $100 hamburger, go for the $100 spreadsheet. Carefully top off, start, and taxi; throw in an extra five minutes for clearance delivery if you often fly IFR. Take off, climb to a favorite cruise altitude, and allow the aircraft to reach cruise speed. Immediately follow that by a normal descent, pattern entry, and landing. Then taxi back in. Hop out, top off (just as carefully), and note the difference. Note how long it takes, too. Let's say it was eight gallons and 40 minutes, for 8,000 feet. Repeat that for as many altitudes as you want. Call those your "up and down" numbers.
Then, do it all again, but instead of coming right back down, actually go somewhere this time, at a chosen altitude, noting your power settings and using proper leaning procedure. Note the exact elapsed time between the start of cruise and the start of your descent. When you land, again top off the tanks, and note the number of gallons consumed. Subtract the previous "up and down" fuel burn from this post-cruise fill-up. This difference is your "cruise" fuel. Divide that by elapsed time at cruise altitude, and you have a good rate for gallons per hour in cruise.
Let's say the fuel meter reads 32 gallons, and your time in cruise was three hours. So you used 32 minus eight, or 24 gallons in three hours, for a cruise consumption of 8 gal/hr. The tank holds 40, so subtracting the eight "up and down" gallons gives 32 or enough for four hours cruise, and four hours and 40 minutes total. Then subtract one inviolate hour for a fuel reserve. What you now have, at least for one altitude and cruise configuration, is something you can use as a good ballpark number to guarantee that you'll never run out of fuel - provided you properly lean the mixture.
The moral for our best of all possible worlds is to use safety margins. "Trust, but verify." Don't put your back to the wall and buy in at the list price. These are just a few of the ways that you can invest in reality. Save blind faith for the place with the stained glass windows.
Jeff Pardo is an aviation writer in Maryland with a commercial pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,100 hours in 12 years of flying. An AirLifeLine mission pilot, Pardo also has flown for the Civil Air Patrol.
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