Turbocharging and Pressurization: An Analysis of the Benefits, Costs, and Disadvantages
Intrigued by the potential benefits of turbocharging and pressurization, but scared by the bad reputation and horror stories of reduced TBO and high maintenance costs? Don't believe everything you hear! AVweb editor Mike Busch (who has owned, flown and maintained a variety of piston-powered aircraft — normally- aspirated and turbocharged — over the past 30 years) offers a frank and detailed analysis of the costs and benefits.
When I learned to fly on the East Coast thirty-something years ago, turbocharging was a dirty word. Everybody said turbos were expensive, inefficient, maintenance-intensive, problem-prone, shortens TBO drastically, and makes sense only for folks based in the Colorado Rockies. As a young, impressionable airman, I bought it...lock, stock and intercooler.
I bought my first airplane in 1968, a nice conservative normally-aspirated Cessna 182. After flying the Skylane for four years and 1,000 hours, I traded up to a higher-performance retractable, a Bellanca Super Viking, which was also normally-aspirated. Following the Bellanca, I flew a succession of retractable singles including a Bonanza and a Cessna 210. All normally-aspirated.
My turbo initiation
And then nine years ago, I bought my first turbocharged airplane. Not just any turbocharged airplane, mind you. A turbocharged twin. My first twin. A 1979 Cessna Turbo 310, to be exact, with a pair of turbocharged TCM TSIO-520 engines.
I was scared to death that this twin-turboed machine would eat me out of house and home. In fact, I promised myself that if the 310 started showing signs of being a lemon or a hangar queen, I'd sell it in a heartbeat...and buy some nice conservative normally-aspirated single.
Well, here I am nine years and nearly 2,000 hours later. I still own the T310R. Astonishingly, it has proven to be the most reliable and trouble-free airplane I've ever owned. And a truly wonderful traveling machine.
As a result, my attitude toward turbocharging has made a complete one-eighty. My next airplane may not be a twin, but it certainly will be turbocharged. I'd never again consider purchasing a normally-aspirated airplane. That's how strongly I feel about the many benefits of turbocharging.
By the way, most of the discussion that follows really isn't limited to twins: it is equally applicable to singles.
Turbos and ice
If I had to pick one benefit of turbocharging above all others, I guess it would have to be ice avoidance. Prior to buying the T310, most of my scariest flying experiences had been due to airframe icing. Flying a normally-aspirated aircraft in IMC when the freezing level is below the MEA is no fun at all. Even if the tops are below the airplane's service ceiling, trying to top an icing layer in an bird with anemic climb performance is seldom a winning proposition. Trust me, I've been there and tried that.
So when I first got my twin, you can imagine how excited I was about having all that fancy known-icing paraphernalia: boots, hot props, windshield hot plate, heated static and fuel vents, etc.
But in nine years of flying my T310, much of it coast-to-coast trips in all sorts of weather, I can't recall a single flight in which having the boots and other deicing gear offered a decisive advantage.
You know why? Because the airplane is turbocharged!
With turbocharging, there's always some ice-free altitude available...either below the freezing level, on-top, or up where it's cold enough that icing isn't a problem. And with full takeoff power available up to 20,000 feet, getting above the ice becomes do-able.
If I had to make a trip in icing conditions and was given the choice of turbocharging or deicing boots (but not both), it would be no contest. I'd pick the turbo every time.
Of course, having both a turbo and boots is even nicer.
Avoiding the bumps
Thunderstorms are the other great producer of sweaty palms and stained underwear, and turbocharging offers some significant benefits here, too.
Being able to cruise in the high teens and low twenties won't let you top a line of frontal thunderstorms. But when penetrating a field of air mass thunderstorms, FL200 is often high enough to offer a good visual perspective and let you circumnavigate the buildups visually. And personally, I'd much rather use eyeball avoidance than rely on a Stormscope or weather radar.
Even when icing and thunderstorms are not a factor, turbocharging often makes it possible to find smooth air when normally-aspirated aircraft are being badly beaten up by bumps. While turbulence is usually considered more of an annoyance than a serious threat, it can be a major cause of discomfort and fatigue, especially on the long trips I often fly. I'll happily give up 10 or 15 knots of groundspeed to get out of rough air.
Speed and climb
Turbocharging is often touted as a "speed mod" and sometimes it is. I've had the pleasure of catching a few 100-knot tailwinds up in the lower Flight Levels that treated me to 300 knots on the groundspeed readout and enabled me to make it more than halfway across the country nonstop...and boy is that ever fun! But frankly, this sort of scenario doesn't happen often enough to justify turbocharging.
Ignoring winds (which always hurt more than help over the long run), turbocharging offers modest speed benefits. A normally-aspirated Cessna 310 cruises at about 180 knots at 6,000 feet and 75% power. At the same power setting, my T310 will do 195K at 12,000 feet (where supplemental oxygen isn't required) and 215K at 20,000 feet.
But I don't fly it that fast because doing so would shorten engine life and burn a lot of fuel (more on this later). I prefer to throttle back to 65% power or less and cruise at 185K down low or 205K up high. Flown that way, the turbo gives me a small speed advantage, but nothing that gets me too excited. That's why speed rates rather low on my list of turbocharging advantages.
Ironically, improved takeoff and climb performance in high density-altitude situations is perhaps the benefit most often cited for turbocharging, but it comes at the very bottom of my list. This might be a big deal for folks who live at higher elevations or who do a lot of flying into mountain airports. I just don't happen to be one of them. When I fly to Colorado or Wyoming, my destination is usually a big airport with a 10,000+ runway. So takeoff and climb performance isn't usually a major issue.
Come to think of it, I do recall a few "exciting" takeoffs in the Bellanca at full gross on hot summer days from South Lake Tahoe or Albuquerque or Santa Fe, despite the generous runway lengths. That doesn't happen anymore now that I fly a turbo.
Reduced TBO is and increased engine maintenance is probably the most frequently cited disadvantage of turbocharging. For example, the published TBO for the IO-520-MB in a normally-aspirated Cessna 310 is 1700 hours, while the TSIO-520-BB in my T310 is rated at only 1400 hours.
But what does published TBO really mean? Damned little, actually. Some TCM engines virtually never make it to TBO, while other engines often make it well past TBO.
Take my T310, for example. When my engines reached their 1400 hour TBO, they were still running beautifully and showed every indication of good health. So I continued flying and checking. And flying and checking. Finally, at 1900 hours, the #6 cylinder in the left engine started losing compression due to an exhaust valve leak. So I pulled both engines and had them majored.
The inspection reports from the engine shop were quite interesting. Except for worn exhaust valve guides, all twelve cylinders from the two engines at 500 hours past TBO were still within new limits! The cranks, cams, bearings and gears all looked like new, too.
With the benefit of hindsight from the teardown inspection, those engines could have been re-valved and run for another 1000 hours. So much for published TBOs.
What does it cost?
How about the increased maintenance costs associated with turbocharging? Even if you accept the reduced TBO theory (which I don't, based on experience), the costs really aren't as high as many think. Let's try to quantify them.
A normally-aspirated IO-520-MB factory reman has a street price (exchange) of about $17,000 and has a published TBO of 1700 hours; this computes out as a reserve-for-overhaul of $10 per hour. A turbocharged TSIO-520-BB reman sells for about $21,000 and has a TBO of 1400 hours; reserve-for-overhaul is $15 per hour. So figure the overhaul cost penalty for turbocharging at $5 per hour (per engine).
Now let's be pessimistic and say that you don't fly your turbocharged engine at reduced cruise power settings (the way I do), and therefore assume that the turbo engine burns an extra gallon-per-hour more than its normally-aspirated sister. That adds $2 per hour (per engine) to operating costs.
Now let's be even more pessimistic and assume that because you run your engine so hard, the turbocharger, wastegate and controller all need mid-TBO overhauls (mine didn't). That costs about $2,800, so adds another $2 per hour (based on a 1400-hour TBO). What the heck, let's even toss in an extra $1 per hour for exhaust system repairs!
.Using this worst-case analysis, the additional cost of turbocharging comes to $10 per hour (per engine). It certainly isn't more than that, and very likely is less.
If we're talking about an airplane in the Bonanza or 210 class (which costs $100 to $150 per hour to fly), the additional $10/hour cost of turbocharging is chump change. The same is true of the $20/hour cost for a twin (which costs $200 or $300 per hour to fly).
How about the poor fuel economy that critics of turbocharging often cite?
Well, it's true that most normally-aspirated engines have a 8.5-to-1 compression ratio and most turbocharged engines have only a 7.5-to-1 ratio. The turbocharged engine is a bit less fuel-efficient (which is why we tossed in that extra 1 GPH in our cost analysis).
But looking at engine efficiency doesn't tell the whole story, because it ignores the fact that airframes are much more efficient up at the higher altitudes that turbocharging allows. By throttling back from 75% to 65% power and climbing from 6,000' to 12,000', my T310 can fly 5 knots faster than a normally-aspirated 310, and do it at lower fuel flow. If I'm willing to put on a cannula, I can climb to FL200 and beat the non-turboed 310 by 25 knots with no fuel flow penalty.
The normally-aspirated airplane is more efficient than the turbo only if you force both airplanes to fly at the same low altitude. And that's simply unrealistic.
Why the bad rep?
If turbocharging is such a panacea, why does it have such a lousy reputation? There are some good reasons.
For one thing, turbocharged engines are far more vulnerable to abuse in the hands of a ham-fisted pilot. You know, the kind that slams the throttle in and out, doesn't bother to lean accurately, runs tanks dry, etc. A normally-aspirated engine can tolerate a certain amount of such abuse, but a turbocharged engine can't. Turbos need TLC.
So if your airplane is used for training or rental use and flown by lots of pilots, you probably don't want a turbo. But if you're the sole pilot and you make a real effort to treat your engine with care, you'll probably have excellent luck with turbocharging, just as I have.
Some engines use turbocharging to gain additional sea-level horsepower, rather than simply to maintain sea-level performance at altitude. Highly ground-boosted engines like the 325 hp TSIO-520 found in late-model T210s and P210s and many RAM-converted twins have a dismal record of making published TBO, much less going beyond it. The same is true of the 225 hp TSIO-360 in the P337.
All other things being equal, the higher the MP redline, the poorer the longevity of a turbocharged engine. The best candidates for good engine longevity are "turbo-normalized" engines like the 285 hp engines in my T310 (red-lined at a very conservative 32" of MP).
If you fly a highly ground-boosted turbo with a MP redline of 38" or more, one of the best things you can do for engine longevity is simply to "derate" the engine. Fly it at lower power settings and it'll last far longer.
Some lower-cost turbocharged airplanes like the Piper Turbo Arrow, Mooney 231 and Piper Seneca II use the problem-prone Continental IO-360-series engine coupled with a fixed-wastegate system that makes the turbocharger work hard even when you don't need it. These installations rarely make TBO and usually require a mid-term turbo overhaul. Fixed-wastegate engines also demand high pilot workload because the manifold pressure tends to be quite unstable.
Fortunately, Cessna never produced an airplane fixed-wastegate system. However, the T337 and P337 do use the troublesome Continental TSIO-360 but with an automatic wastegate.
Back in the '70s, it was all the rage to hang aftermarket turbochargers on all sorts of normally-aspirated engines. Rayjay made STC'd kits to turbocharge a wide variety of airplanes. Most of these installations were a real disaster, and proved extremely unreliable and maintenance-intensive. Avoid them like the plague.
I used to advise staying away from all aftermarket turbo conversions. But nowadays, I make an exception for the turbo-normalizing conversions to Bonanzas and Cardinals done by FliteCraft Turbo in Pagosa Springs, Colorado (phone 970-731-2127, FAX 970-731-2524, email firstname.lastname@example.org). These conversions are every bit as good as any factory turbo installation, and better than most.
There have also been serious problems with aftermarket intercoolers that many turbo owners add to factory-turboed airplanes.
In general, intercooling is a good idea, because it allows a turbocharged engine to breathe cooler air, thereby improving detonation margins, lowering CHTs, and increasing efficiency. The problem usually isn't with the intercoolers themselves, but with the fact that the STCs often don't require a proper flight manual supplement with revised performance charts.
Because the engine is breathing cooler, denser air, the MP must be adjusted downward to compensate, often by several inches. But many owners wind up installing an aftermarket intercooler and then trying to fly using the original factory performance data. Doing this, it's easy to believe that you're cruising at 70% power but actually be cruising at 85% power instead. You can imagine what this does for engine longevity.
If you're flying an airplane with an aftermarket intercooler, you'll need to reduce the MP shown in the POH performance charts by 1 to 3 inches, depending on altitude. The higher you fly, the more adjustment you need to make.
In general, fuel flow is an excellent indicator of power output. If you find your airplane is burning more fuel than the POH calls for, it's very likely that you're pulling more horsepower than you think you are.
Oxygen versus pressurization
To take maximum advantage of turbocharging for getting above the ice and bumps (and for catching that occasional 100-knot tailwind), we need to climb up to the high teens or low twenties. And that means we must either breathe supplemental oxygen or we must have a pressurized aircraft.
Until the early 1980s, flying high and unpressurized meant wearing an oxygen mask. And frankly, oxygen masks are a real pain in the patoot.
Personally, I find masks seriously uncomfortable. When I wear one, my glasses usually fog up and my mustache always becomes drenched with perspiration.
Masks also interfere with communications. You can't use your normal headset microphone, and those in-the-mask microphones sound about as intelligible as using a speakerphone from across the room.
In other words, oxygen masks suck!
In the early 1980s, the FAA approved the use of cannulas for breathing supplemental oxygen in-flight. This proved to be a tremendous boon for turbocharged-but-unpressurized aircraft. Cannulas are extremely comfortable, so much so that it's easy to forget you're wearing one. Cannulas allow you to breathe normally, communicate normally, even eat and drink in-flight. And so-called "conserving" cannulas, coupled with calibrated vernier flowmeters, permit you to stretch your oxygen supply by a factor of two or three compared to a mask.
Cannulas solve many of the problems associated with breathing supplemental oxygen, but not all of them. Cannulas are approved for use only up to FL180; some of us have been known to push this limit a bit, but I can personally testify that a cannula does not provide adequate oxygen much above FL200.
Furthermore, some folks simply don't do well on 100% oxygen, period. It tends to dry out the mucous membranes of the nose and throat, and prolonged breathing gives some people middle-ear problems. Others suffer from "altitude sickness" that is a mild form of "the bends."
Supplemental oxygen can also be problem if you carry a lot of passengers. That oxygen bottle can last a long time if only one or two people are breathing from it, but four or six can empty it pretty fast. Furthermore, some passengers simply don't care to fool with oxygen paraphernalia (cannulas or masks), and other passengers (particularly children and infants) can't easily be persuaded to use oxygen.
So if you tend to carry passengers or if you're one of those folks who have a problem breathing through a tube, you may want to give serious thought to a pressurized aircraft.
What does pressurization cost?
The costs involved are significant, however. Pressurized aircraft are more expensive to buy, more expensive to operate, and more expensive to maintain.
Let's look at a specific example. According to the latest Blue Book, a pressurized 1980 Cessna 340A sells for an average retail price of $265,000. Its unpressurized counterpart, a 1980 Cessna T310R, has an average retail price of $168,000. So the pressurized airplane commands nearly a $100,000 premium.
At the same time, the 340A weighs about 500 pounds more than the T310R (but has no more useful load), burns 3 gallons-per-hour more, and cruises a few knots slower at most altitudes.
Maintenance of the pressurized bird is also more expensive, but not for the reason you might think. The pressurization system itself requires very little maintenance and seldom gives any trouble. When it does, the fix is usually quite simple: repairing a bad door seal or cleaning a sticky outflow valve.
But pressurization makes certain other maintenance tasks much more difficult and time-consuming. Installing a GPS antenna, a fuel totalizer, or a multi-probe EGT system, for example, is a far more difficult job on a pressurized airplane because of the need to bring wiring through the pressure vessel. Changing an engine control cable or a fuel line are also far more labor-intensive for exactly the same reason.
It's difficult to quantify the additional maintenance cost of pressurization in terms of a dollars-per-hour figure. Most routine maintenance operations are no more difficult on a pressurized airplane. But certain functions that involve penetrating the pressure vessel can be a great deal more difficult, and therefore expensive.
In addition, pressurized airplanes tend to have more engine problems than non-pressurized ones. Again, this is not the fault of the pressurization system. It's simply because pressurized airplanes tend to spend a lot more of their life flying at high altitudes than non-pressurized ones. A Cessna T310R pilot will think twice before climbing up to the flight levels simply because doing so requires him to use oxygen, while a Cessna 340A pilot will climb up there without thinking twice about it.
At high altitudes, the turbo works harder and the engine runs hotter. In the long run, this means that all other things being equal, a pressurized airplane will generally experience worse engine longevity than its unpressurized sibling.
Of course, all other things don't have to be equal. A pressurized airplane in the hands of a pilot who pays meticulous attention to proper powerplant management (warm-up, cool-down, power settings, leaning, temperature control) can have very good luck with engines. And a turbocharged-but-unpressurized airplane in the hands of a ham-fisted pilot can prove to be a maintenance disaster.
If you use your airplane as a serious traveling machine—especially if you fly long trips in instrument weather like I do—you should seriously consider turbocharging.
If you pay careful attention to powerplant management, use conservative power settings, avoid troublesome engines (e.g., highly-boosted, fixed-wastegate, or most aftermarket add-ons), and be careful with aftermarket intercoolers, I think you'll find—as I did—that the benefits of turbocharging far outweigh its very modest costs. If you don't do much flying to high-altitude airports, the greatest benefits of turbocharging lie in avoiding icing, thunderstorms, and turbulence.
And sooner or later, you'll catch one of those 100-knot tailwinds and put a big smiley-face in your logbook.