Tuesday, February 21, 2012
Murphy’s Laws Of Aviation
Why what you learned in flight school may be open to question
Under some circumstances, tanks can develop air bubbles that can supplant more fuel than you'd believe. Once in Reykjavik, on a delivery to Europe in a new Malibu, the fuel total didn't look right after refueling for the leg to the UK. I went out to the airplane, popped off both caps, and the level looked to be right to the top. I shook the wing vigorously, the level dropped on both sides, and the truck came back and added 15 more gallons. That's almost an extra hour of range.
As a result of such realities, we also subtract 5% from available fuel, especially when we have four or more tanks aboard. (The more tanks, the more the possible margin for error.) In other words, 300 gallons becomes 285 gallons. That's to allow for any possible overboarding during climb or filling to inconsistent levels.
Most pilots understand not to trust everything they read in flight manuals regarding cruise speed and fuel burn. No, the manufacturers probably aren't lying to you about either figure, but they determine performance and fuel consumption based on absolutely optimum conditions.
There are a number of reasons that manufacturer's specs may not be readily attainable by us mere mortals. First, the flight manual's cruise speeds are derived by company test pilots with thousands of hours in type, aviators who know how to milk every knot of performance from a given airplane. These pilots fly new, well-polished, perfectly rigged aircraft with finely tuned engines, all vents closed, usually operating at max allowable aft CG, in smooth air and ISA conditions. You have about three chances of duplicating those circumstances in the real world: slim, fat and none.
Fuel burn specs are also real but optimized, a result of leaning to the exact EGT in the handbook, often utilizing extremely precise flight test instrumentation that has been calibrated to eliminate any error.
Altimeters introduce a whole new variety of potentially dangerous variations, and not always of their own making. Nearly every altimeter has some error, and pilots only add to the problem.
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