Waypoints

Altitude is your friend in more ways than one. There are critical moments in flight when you'd give about anything to have back a little more altitude. But beyond the safety cushion of a few thousand more feet in which to work things out when they go wrong, altitude also makes our flights more efficient — up to a point.

Because true airspeed increases with altitude, you are almost always better off flying higher on cross-country flights — assuming headwind components do not increase dramatically with altitude. In addition, your airplane burns less fuel at higher altitudes, allowing you to stay there longer than would otherwise be possible. As a result, true airspeed seems almost too good to be true — like something dreamed up by a slick door-to-door salesman: "Buy today and I'll throw in the Miraculous Airspeed Booster that not only allows you to fly faster, but on less gas! Think of yourself, madam, flying your Airknocker 150 right up in the flight levels on no more gas than I can put in this measuring cup!"

Well, you get the idea.

And yet despite the magic of true airspeed, few pilots seem to really understand the potential. Often when we run an aircraft report on a turbocharged airplane, we get e-mails or letters from pilots who question the airspeeds in our spec boxes at the end of the article. Unless you read carefully, we seem to be reporting that the airplane cruises faster than its VNE, or the speed that you should never exceed. Take the New Piper Malibu Mirage, for example. Its maximum cruise is about 215 knots true airspeed. It achieves this speed at its maximum operating altitude of 25,000 feet. Meanwhile, the design's VNE is 198 knots indicated airspeed. It's easy to become confused if you overlook the difference between KTAS and KIAS. Remember, because the atmosphere at 25,000 feet is much less dense than at sea level, the airspeed indicator at maximum cruise will show only about 130 to 140 knots — right around the airplane's maneuvering speed of 133 knots indicated airspeed. So, technically, you can rack the airplane around up there at its full design limits and not do any damage — possible, but not exactly good form, particularly if you have passengers on board trying to enjoy lunch in the back.

Think of your airspeed indicator as being calibrated to be accurate at sea level on a standard day when the barometric pressure is 29.92 inches of mercury and the temperature is 59 degrees Fahrenheit. Under those conditions, your indicated and true airspeeds are equal. Go blazing along, brushing the waves at 100 KIAS, and you can bet that the flight computer will tell you that your true airspeed is also 100 knots — assuming no error in your airspeed indicator or pitot-static system and assuming you can work the flight computer at wave-top heights without plunging into the sea to become Shamu food. Climb to 1,000 feet, though, and fly at 100 KIAS and your true airspeed will have rocketed up to a blistering 102 knots. Meanwhile, your nonturbocharged engine will be making less power and, as a result, burning less fuel. Hey, a free two knots.

Step on up to 10,000 feet and maintain that same 100 KIAS and your true airspeed will increase to about 120 knots.

It's easy to estimate true airspeed if you know your altitude and indicated airspeed. As a rule of thumb, true airspeed increases over indicated airspeed by about two percent per thousand feet. If you're cruising at 10,000 feet at 140 KIAS, take two percent of your altitude in thousands of feet. In this case, that means two percent of 10, which equals 0.2. Multiply that by the indicated airspeed of 140, which yields 28 knots. Add that to the indicated airspeed to learn that the true airspeed will be about 168 knots. Remember that anything other than normal lapse rates for pressure and temperature will change the results, your mileage may vary, past returns are not a guarantee of future performance, and parking is prohibited in the red zone.

Meanwhile, because there is less oxygen at 10,000 feet, your normally aspirated engine won't be able to burn as much fuel as it could in the denser air down low. As a result it produces less power. Since the engine can't burn all that fuel, there's no sense dumping in the same amount that you did at sea level, so lean the mixture to reduce the fuel flow.

Save gas, go faster. What a deal. But wait, there's more: Order before midnight and we'll send you this Amazing Turbocharger that allows you to continue making 75-percent power all the way up to 25,000 feet! You heard right, ladies and gentlemen, twenty-five thousand feet! Slip on your oxygen mask, crank up the pressurization, glide along at breathtaking speeds above the weather — leaping entire continents in a single bound (well, OK, maybe two or three single bounds).

If you have a need for speed in a piston airplane, nothing will do like a turbocharger. By compressing the air going into the engine, the turbocharger makes it more dense, much like it is at sea level. With more oxygen molecules to aid in combustion, the engine can make more power all the way up into the flight levels. However, more power demands more fuel, so you won't see the decrease in fuel flows that you do in a normally aspirated engine. But without turbocharging, your engine begins making less than its rated power as soon as you leave sea level; if it's warmer than 59 degrees, you won't even see 100-percent power at sea level. By 7,000 or 8,000 feet, you'll be down to just 75-percent power. It is at about that altitude where the turbocharged airplane starts to become more efficient. From there on up, the turbo airplane takes the lead in speed because it can maintain that power level to a much higher altitude. As the airplane climbs, the less dense air causes less drag, allowing the aircraft to go faster and faster. Meanwhile, the indicated airspeed falls because there are fewer air molecules entering the pitot tube and striking the diaphragm that moves the airspeed indicator needle.

You can dazzle your pilot friends at the next cocktail party by reminding them that it takes a pressure of about 34 pounds per square foot entering the pitot tube to cause the airspeed indicator to read 100 knots at sea level. As you climb, it still takes 34 pounds per square foot to show 100 knots, but with fewer air molecules available as you climb, the airplane will have to go faster and faster to gobble up enough of them to create that pressure in the pitot tube. No doubt, such fascinating trivia will endear you to all of your friends who happen to be physics professors; everyone else will be rolling their eyes and leaving you for the drink line.

Be assured that turbine airplanes are not immune to the laws of physics. Like a normally aspirated engine, turbine powerplants also begin to lose power as they climb. However, their enormous amounts of horsepower allow them to propel their airplanes to much higher altitudes where, once again, triple-digit true airspeeds and lower fuel burns save the day. The 700-shaft-horsepower Pratt & Whitney engine in the Socata TBM 700, for example, burns a sheik-pleasing 30 gallons per hour idling at sea level. But, at 26,000 feet the fuel burn is only about 55 gph, yet the airplane is tooling along at 300 KTAS. You can see why it's important to get to altitude fast in a turbine airplane.

So, remember the next time you plan a cross-country flight, higher is usually better. Also, remember that what sounds too good to be true usually is, unless it's true airspeed.