Article By: Peter Garrison
Brought to you by: www.flyingmag.com
The cautious thing to do is to land at the next opportunity and get more fuel. But that is not always possible or convenient. There may be no intermediate place with suitable weather; you may have told someone to meet you at a certain time. And there is always the reluctance to lose time, and to give up altitude and then to have to claw it back — a reluctance so strong that many a pilot has run out of fuel rather than overcome it.
|It ought not to be true, but it is: In every pilot’s life there comes a moment when he wishes he had a little more fuel.
Perhaps the headwind was stronger than forecast; the gauges have dropped below a quarter sooner than you hoped they would; the descent and climb for an en route stop to drop off a passenger used up more fuel than you expected; you took a detour around weather; or your planning was careless in the first place. Whatever the reason, you find yourself in that awkward spot: a certain distance from your destination, with a certain amount of fuel and with a nagging worry about where those needles will be pointing when you arrive.
But nothing can be done about extreme pigheadedness. Let us stipulate that there are situations in which a pilot of normal maturity, competence and regard for safety might feel concerned, even conflicted, about his or her remaining fuel, but in which a decision to continue might depend on rational analysis rather than, say, how lucky he or she was feeling that day. These are the situations in which it is not irresponsible to “stretch” range.
How far an airplane can go on a given amount of fuel is principally determined by four factors: propeller efficiency, fuel consumption, speed and wind.
The role of wind is obvious. Any headwind, and even a side wind, increases the time to fly. The chart of wind components is familiar to pilots, though roofers, who have to cut their two-by-fours to match the run and rise of rafters, are more likely to remember the precise numerical relationships. But in the era of GPS no chart is needed; the wind component is obvious from the groundspeed.
Your flight time will be lengthened in roughly the same proportion as the headwind component stands to your airspeed. If you cruise at 150 knots and the wind component is minus 15 knots, your flight time will be increased by about one part in 10; it will take you 66 minutes — actually, 66 minutes and 40 seconds — to go as far as you would normally go in an hour. That is not likely to be a problem. But a component of 30 or 40 knots might be. To maximize your range you want as little headwind as possible, and so you should pick an altitude — if you haven’t already done so — where the wind component is least.
A headwind component works against you in two ways. First and more clearly, it increases the time needed to go a certain distance. Less obviously, it complicates the choice of a speed to fly.
Speed is a pilot’s most powerful tool for increasing range. The amount of speed you get in exchange for a given fuel flow — in other words, your miles per gallon — varies across the speed range. It is worst at very high and very low speeds, owing at the high-speed end to parasite drag and at the low-speed end to lift-related induced drag. Parasite drag increases with speed, and induced drag increases with slowness; they are equal at the speed for minimum drag. This is the speed at which the least power is needed to stay aloft, and therefore it is the speed for greatest endurance. It is typically about a third greater than the clean stalling speed.
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