Turbocharging

Even if you're primarily a VFR pilot living in the East Coast or Midwest, however, and only fly on CAVU days in late spring, summer and early fall, compressed power can be next to godliness.

Like many of you, I discovered turbos 15 years into my flying career. I learned to fly in the cheapest trainer I could find. The first was an Aeronca Champ and later, a Piper Colt, and supercharging in any form was something I only read about on military and airline aircraft.

I had to wait until I bought my fourth airplane, a new 1979 Mooney 231, to appreciate the benefits of a blower out front. After my first flight in it, I thought I had died and gone to Oshkosh. Everything about that airplane (except the payments and the maintenance) was right out of a dream.

Turbochargers do, indeed, change your perspective on flying high, and for many of us, the positives far outweigh the negatives. Unfortunately, any form of supercharging, like practically everything else worth else having, has a price tag.

Back in the day, prospective buyers could plan on spending an extra 10 to 15% to add turbocharging to a new airplane. In those halcyon times of the late '70s and early '80s, when the industry was selling at least 15,000 airplanes a year, a buyer had a huge selection of turbocharged models to choose from. There were more than three handfuls of personal aircraft available in both normally aspirated and turbocharged trim.

Piper had the Arrow and Turbo Arrow, along with the Dakota and Turbo Dakota, Lance and Turbo Lance, Cherokee Six and Turbo Six, Navajo and Aerostar; Cessna was selling the 182/182RG/210/206 and 310 in normal and heavy-breathing configurations; Beech had the 36 Bonanza and Baron with and without turbos; Mooney offered the 201/231, Socata sold the Trinidad and Trinidad TC; Bellanca was marketing the Viking and Turbo Viking; and Lake sold turbo and non-turbo versions of its little Buccaneer and Renegade amphibians. (Apologies to anyone I missed.) Indeed, it seemed turbocharging was the wave of the future.

Or not. Today, many of the models above are no longer in production, and most of those that survive are represented by a single version, usually the normally aspirated airplane. There's nothing inherently wrong with turbos---in fact, they're more reliable than ever---but the market has contracted and manufacturers have been forced to cut back.

As we all learned in flight school, most engines are rated for max cruise at 75% power. A normally aspirated aircraft engine will develop 75% to about 7,000 to 8,500 feet, depending upon the efficiency of the induction system. Above that height, full throttle power gradually drops off at the rate of roughly five percent per thousand feet. That means you can reasonably expect a properly leaned engine to develop 65% power at 9,500 to 10,000 feet and 55% at 11,000 to 12,000 feet. (Your mileage may vary.)

Turbocharging adds another dimension to flying, a spectrum of sky well above the bottom two-and-a-half miles of airspace that clings to the Earth. A turbo typically contributes at least another 10,000 feet of vertical altitude to an airplane's flight envelope, expanding available cruise levels from roughly 13,000 to 14,000 to 23,000 to 25,000 feet. A turbocharger, not so simply, compresses intake air and delivers it to the engine at a graduated rate.

Most modern turbos employ automatic waste gates that limit boost depending upon altitude and provide sea-level power until the waste gate is fully open. (Some even utilize intercoolers to cool the intake air and provide additional prospective power, but that's another story.)

This allows the engine to develop full- rated horsepower to the critical altitude, the height at which the waste gate is wide open and the engine can still deliver sea level power. On most turbos, that's usually at least 17,000 feet, but often as much as 25,000 feet. Logically, if the engine can still deliver 100% power at 17,000 feet, it should now deliver the recommended 75% maximum allowable cruise power at 24,000-25,000 feet.

Flying high has a number of advantages, and while operation in the mid-teens to mid-20s may offer obvious benefits, a turbo can provide better performance at lower levels, as well. Cruise at 10,000 to 13,000 feet offers 10 to 12 knots better speed, because the engine can still deliver 75% long after the standard breather is down to 55% or less.

The big benefit comes in the aforementioned block of airspace between 13,000 and 25,000 feet. If your airplane is turbocharged but unpressurized, oxygen is mandatory for the flight crew above 14,000 feet and for everyone on board for flights above 15,000 feet. Today's oxygen systems have become so simple and transparent, with the gas dispensed from either a cannula or the microphone stalk on a headset, that you may hardly know you're using O2. Those systems generally are limited to operation at 18,000 feet and below. Full face masks are required above 18,000 feet.

Still, the oxygen requirement automatically excludes most of the normally aspirated general aviation fleet, so there's little traffic to contend within the middle altitudes. File IFR, and direct routings are likely to be the rule, since controllers have sparse conflicting traffic in the middle altitudes.

If you do happen to be using VOR navigation rather than GPS, range is usually excellent, even over high, mountainous terrain. In the southern 48 states, there's nothing higher than 15,000 feet, so even a modest turbocharger will let you fly high above the peaks.

Turbochargers have other duties besides high and fast, however. They provide a knowledgeable pilot the ability to operate out of airports at ridiculous elevations. The key word above is knowledgeable.

The psychological downside to turbocharging is that it may fool a pilot into thinking he's invincible. Many years ago, while delivering a new Bellanca Turbo Viking from the factory in Alexandria, Minn., to California, I stopped in Denver for the overnight on the way West. It was early July, and the following morning offered chamber-of-commerce weather conditions. If the Earth had been flat, I could have seen Hawaii. I decided to drop in to Leadville, Colo., elevation 9,927 feet MSL, the highest municipal airport in America.

After a cup of coffee with the manager and some deep-breathing exercises, I climbed back into the energetic T-Viking for the remainder of the trip West. The temperature was about 74 degrees when I pushed the throttle forward for takeoff. The Bellanca surged ahead with what seemed its usual enthusiasm, but when it came time to fly, what had been 1,500 fpm at sea level turned out to about half that at 10,000 feet MSL. True, the density altitude was up to nearly 13,000 feet, but I assumed the twin turbos out front would take care of that problem. The moral is that you can turbocharge the engine, but you can't turbocharge the wing and the prop.

In most reasonable situations, compressed power will make a big difference in both climb and cruise. I got a graphic taste of what a turbo can do for speed in 1994 when I set eight world-class C1C city-to-city speed records between Los Angeles, Albuquerque, Dallas and Jacksonville in a new Mooney Bravo. I flew the trip at FL250 all the way with the world's fastest refueling stop at Dallas Love Field. Average speed for the full, seven-hour-nine-minute-cross-country dash from LAX to JAX was 300.1 mph, but one of the intermediate legs, Los Angeles to Albuquerque, worked out at 338.4 mph.

Of course, one major benefit for my record flight was good weather and excellent winds aloft. Fully half of the world's weather tops at 18,000 feet MSL or below, so the higher you fly, the better the ride. If you operate at or above FL180 very often, you'll most often be cruising in smooth air and sunshine.

Additional altitude can sometimes seem like an intangible asset---until the engine quits. A turbo can provide a thicker buffer of altitude in the event of an engine problem, and while there's no guarantee you'll always make the right choice of an emergency landing site, no matter what your cruise altitude, remember that the available space increases as the square of altitude. If you're flying an airplane with an L/D of 10 to one and cruising a mile above Kansas when everything suddenly becomes quiet, you'll have a theoretical emergency radius of 314 statute miles to pick a landing spot. If you're flying 20,000 feet above Kansas, your available landing area increases to just over 5,000 square miles.

There are a few negatives to flight at high altitude, but very few. Fuel burn will inevitably increase, partially because of the need to keep the engine cool. TBO also may take a hit. Mooneys are out of production these days, but when both the 201 and 231 were on the line, the 201 burned about 11 gph at max cruise, and my 231 consumed more like 13 gph.

To be of any value up high, turbo models must offer their occupants oxygen systems, and these add weight, expense and (sometimes) minor discomfort. As mentioned above, modern O2 systems have alleviated much of that problem, but oxygen will still dry out your mouth, may congest your sinuses and presents a minor fire hazard.

Oxygen and oil can be combustible, so you need to advise anyone wearing lipstick or oil-based makeup to remove it before flight. (Those T-shirts that read "Remove Before Flight" take on special meaning in a heavy-oxygen environment.)

On top of that, refilling an O2 tank isn't cheap, primarily because it requires the attention of an A&P mechanic. Total system weight is typically 35 to 40 pounds, subtracted directly from payload. If your system is portable, you might substitute medical oxygen, which is usually cheaper, but make certain the bottle will be inside the cabin where it's warm. Medical O2 contains more humidity, which could freeze the regulator if exposed to extreme cold temps.

By definition, icing may be another consideration of flying high, even in summer when it wouldn't be a hazard for normally aspirated aircraft. Many turbo models aren't approved for anti-ice systems, much less de-ice, either pneumatic boots or TKS. On many of those airplanes, the most exotic anti-ice equipment available may be pitot heat.

Plane & Pilot worked with Barry Doctor of Weather Service International in West Palm Beach, Fla., to define the risks of icing, and we'll have more to say about that in next month's issue.

The condition of the heater may not seem a critical item in summer unless you happen to be launching for cruise at FL240. Standard temperature at that height is -27 degrees F. Even if you're departing Palm Springs in July, yo­­­­u may need a properly functioning heater at high altitude.

Pilots who fly at 10,000 feet usually plan their descents for 500 to 700 fpm and assume they'll actually gain a little speed during descent. Flying high, say at 20,000 feet, you may actually lose speed, because you may drop out of your tailwind. Descents demand more time and attention if you have three or four miles of altitude to lose, and gradual descents may be impractical because of terrain considerations.

The airlines start down as much as 150 miles from their destination, partially because of their greater speed, but also a function of the need to lose 35,000 to 40,000 before landing. If you're flying at 25,000 feet and approaching an airport near sea level, even a 700 fpm descent would demand 35 minutes. If your airplane is a Mooney, 210 or Bonanza, that could translate to starting down 100 miles out.

The value of turbocharging is very much in the eye of the beholder and the size of his bank account. Few things in aviation are free, but if you're willing to absorb the higher price of acquisition, additional license requirements (instrument rating above 18,000 feet), shorter TBO, extra fuel burn and higher maintenance cost, a turbo may be just the ticket.