Unlisted Numbers

Most problems are of our own making and most solutions begin as numbers. From our first day in ground school, we’re told about the importance of good decision-making, the need for continually evaluating in-flight weather, the value of maintaining alternative options, and most daunting of all, our own fallibility. One perennial issue is fuel management. Folks are always managing to run the tanks dry, and it’s usually just a few miles short of their destination. Whether it’s an amended clearance, calculation error, ‘un-forecast’ weather, or some quirk in the fuel system … it happens.

Moving around in three dimensions is just bound to introduce more variables, and unknowns, than in two. So we arm ourselves with backup systems, extra reserves, an alternate airport, and any number of tricks we can play when the going gets tough — like a forward slip, climbing out at Vx, or slowing to maneuvering speed when it’s bumpy. Some of these numbers (like Vne or Vno) are right there at the end of colored arcs on the dial, but some are not. Here are a few numbers that might prove useful someday

Maximum Range Airspeed: Paradoxically, airplanes go further the slower you fly ’em (up to a point). If you have range profile curves in Section 5 of your airplane’s manual, have a look. See how much further you can go on 55% power (or 45%, or whatever the lowest value is). Usually that’s something around 2100 rpm (and 18 inches manifold pressure for controllable-pitch propellers).
Inside Information: Generally, you’ll get even more miles per gallon if you fly at 15% faster than Vy. You’ll be nose high, and s-l-o-w. But if you’re in a miserly mood, or circumstances beyond your control have ‘scrooged’ you, do it. That’s a lot slower than most of us would like.

Optimum Cruise: If you like being miserly just on principle, then this is your airspeed. Optimum cruise is the point where the fuel flow per knot is at a minimum. Generally, this is about one third again as fast as best glide. Hint: Fly at the most efficient altitude (usually about two-thirds of the service ceiling) and follow proper leaning procedures to help out even more. What to do about winds is up to you, but a rule of thumb is to increase airspeed by half the headwind component, and decrease it by a quarter of any tailwind component

Maximum Endurance Speed: To stay in the air the longest, you would slow down still more. Record setting aside, a practical application of this would be if you were confronted with a situation in which it would be wise to wait for the moon to rise on a clear, but dark night, or perhaps for a ground fog to burn off a bit more. This time, aim for a few knots below best glide speed. (That’s even more nose high and slow. And we’re talking bladder-stretching duration here!)

Minimum Sink Speed: This is just above a stall. It’s no news to sailplane pilots — and it could be welcome news for you if your engine quits. Don’t just barrel down at best glide, especially if you’re stuck above a rough surface. Like best glide and best range, which are a few knots apart, best endurance and minimum sink speeds are too — the power-on state being the faster one, in both cases.
Inside Information: If you ever need to make a course reversal without power and with minimum altitude loss, do what glider pilots do during simulated rope breaks. If possible, make your turn toward the wind — at minimum sink speed + 20%. Why + 20%? ‘Cause you’ll be doing it in a 45 degree bank for maximum turn rate/minimum altitude loss! And remember, impact forces increase with the square of velocity, so if you’re going to hit anything (runways included) you’ll be better off if you do it at minimum sink speed.

COMMON SENSE: These numbers are *not* precise, but if you fly conservatively they don’t have to be. We don’t fly to dry tanks, after all. Speaking for myself, I’d go into auto-flail mode if I wasn’t on the ground with an hour’s fuel still aboard. So, to me, anything to widen that safety zone is fair game. Think of them simply as more arrows in the quiver against outrageous fortune.

Editor’s Note:This article originally ran when iPilot was just a baby. Be sure to check the Insider Series Archive for more great stories you may have missed.