Busted Boost

Turbocharger failures are hardly everyday events. Some are emergent and some are routine. Heres how to tell the difference.


If you fly a turbocharged airplane, you know a different reality than the poor slugs who have to muddle along in the weather gasping for manifold pressure above 12,000 feet. Pilots with boost climb faster, fly higher and have more options in

Airplane Engine Turbocharger


making altitude-related weather decisions.

Nothing is for free, however, especially if you own an airplane with one or a pair of turbochargers. Youll pay more for overhauls and routine maintenance, and you may burn bit more fuel. You also know-or should know-a different reality with regard to potential system failures. Although theyre as simple as a bag of rocks, turbochargers are like any other mechanical device, so they occasionally fail, presenting sometimes perplexing symptoms you wont see from a normally aspirated engine. Lets consider some possibilities.

Scenario One: Hot Side

Heres the setup: In cruise flight at 17,000 feet, you hear a bang or a thump, the manifold pressure drops, taking the oil pressure and fuel flow with it. You also notice some smoke in the cockpit. Catastrophic crankshaft or rod cylinder failure? Not likely if the prop is still turning and theres no fundamental, heavy running vibration. How about a cylinder departure or failure? That might produce some smoke, but it shouldnt immediately tank the oil pressure nor would there be a dramatic drop in manifold pressure.

This actually happened to the pilot of a factory-new Cirrus SR22 Turbo near Reno, Nev., in July 2007, the result of what appears to be one of the rarer turbo failure modes: A catastrophic failure of the turbochargers hot-side turbine wheel.

Subsequent investigation revealed that due to a manufacturing fault on the compressor side of the turbo assembly, the turbine wheel failed and departed the shaft and the airplane. The turbo was manufactured by Kelly Aerospace, which has since corrected the faulty machining assembly that apparently caused the failure. (Two other Cirrus turbo models had similar in-flight turbine failures within weeks of the July incident, and a subsequent recall by Kelly applied to Columbia and Mooney aircraft as well.)

The low-time Cirrus pilot did a masterful job of handling his in-flight emergency and landed without further incident at Reno, Nevada. Shortly before landing, the engine either failed entirely or the pilot shut it down intentionally.

I suspect the latter, which begs the question: Is the intentional shutdown the right way to handle this kind of emergency? Im not one to argue with a successful outcome, but some additional fine-point knowledge reveals why the shutdown may not be necessary.

Hot side failures in turbine wheels-again, theyre rare-are likely to take the shaft and bearings out, meaning the engine will puke much of its oil but should retain enough to run for quite some time. Thats because turbocharger oil systems are equipped with pressure-controlled check valves that close when engine oil pressure drops below a prescribed value, about 10 PSI in the Cirrus setup. Thats enough pressure to provide essential lubrication for the engine, although not much supplemental oil cooling. In the Reno incident, the engine-a turbonormalized Continental IO-550-N-had about two quarts in the sump, according to the NTSB report. The rest was gooed all over the belly.

But thats not to say theres no argument for shutting down the engine with this type of turbo failure. With that much oil spewing around hot exhaust pipes and cherry red turbo housings, theres a real risk of fire. There may or may not be enough air volume to blow a fire out in the enclosed engine spaces so its not impossible that fire could escalate and burn through a fuel line or a structural component.

This is what happened in a handful of Cessna 300 and 400 series accidents and incidents, in which breached turbocharger exhaust components induced lethal engine compartment fires.

Whatever the case, an exhaust breach or blowout is an immediate emergency landing situation, not a precautionary landing scenario. From the altitudes turbocharged airplanes tend to fly, theres usually an airport within practical range, but this is still a get-down-now scenario. If that means an off-airport landing, so be it. Any kind of engine fire is not to be trifled with. And from the mid-teens to the low 20s, the ground is at least five minutes away, so theres no time for delay.

Scenario Two: Exhaust

Just as turbochargers require more oil plumbing, so do they require more exhaust plumbing-crossovers, bends, clamps and so on. These also represent failure points, although as with turbine failures, these are rare.

The symptom? Perhaps none or possibly a noticeable decline in manifold pressure signaling that either the compressor is slowing because it has less exhaust flow due to the breach or the pressurized upper deck air is venting overboard because of an induction leak.

But which is it? It may be impossible to tell, but associated indications may be a hot, burning but not oily smell and smoke in the cockpit. If you have a carbon monoxide detector, an unexplained spike in cabin CO in a single may signal an exhaust leak. This is cause to declare an emergency and get on the ground without delay. If theres no evidence of fire, an off-airport landing may not be necessary. With fire, it might be the only survivable choice.

Scenario Three: Oil Leak

Turbocharger-related oil leaks are normally more of a maintenance issue than an in-flight immediate action issue. Youll see oil weeping from fittings or hoses and maybe running down the belly or out the cowling after a rainy flight.

In flight, a significant oil leak related to the turbocharger oil lines may be noticeable without being emergent. A faint hot oil smell in the cockpit may indicate a leak large enough to be a concern without representing a fire risk. This is unlikely to show on the engine instruments as either falling oil pressure or rising temperature and may not affect the turbocharger in any way.

But a massive failure-say a fractured fitting, which Ive see in the incident reports a couple of times-represents the same fire risk as a turbine failure that takes out the bearings and seals. It wont take long to pump all but a couple of quarts overboard, so you need to get on the ground without delay and watch for smoke and fire. Close the heater and cabin vents before this happens, which should be a standard response to an engine fire, along with reducing power to the minimum required or shutting down the engine if a serious fire ensues. In a twin, you always have the option of caging the engine-thats why you have two, after all.

Scenario Four: Induction

Induction or upper deck failures are probably the most common turbocharger faults. Or perhaps a near second to wastegate and controller faults that arent immediate safety concerns. Significant and sudden induction leaks will definitely get your attention, but theyre not likely to cause either damage or a total power failure.

They occur when a hose or clamp on the pressurized upper deck air induction side lets go. This dumps all or most of the boost and if youre at high altitude-say above 12,000 feet-the manifold pressure will tank and with it goes a significant amount of power. The fuel flow will drop, but the oil pressure and temperature wont change.

Ive suffered three such failures, all under identical circumstances, one in a Piper Navajo and two in the same Mooney 231. All three occurred in the high teens and right at the top of climb. In the twin, the props went out of sync and the airplane yawed, suggesting an engine failure. But once my heart rate returned to near normal, I could see the engine still had normal oil pressure and was running at about 17 inches MAP, having dropped from 30 inches. Because Id seen it before, the induction failure in the Mooney was less startling, but clearly the airplane wasnt going to maintain 17,500 feet comfortably.

If the upper deck air goes away, youre back to flying a normally aspirated engine with low-compression pistons. If you descend low enough, youll recover most of the manifold pressure, meaning a normal, non-heroic landing should be possible.

Knowing this, should you continue the flight to your destination? Not really. For one, any boost failure should be taken seriously and just because you think its a benign failure, doesnt mean that it is. Just because you didnt hear something break or smell something burning is no take-it-to-bank guarantee that you dont have an exhaust breach or a turbine failure thats wiped out the oil system. Second, if it is an upper deck failure, an as-soon-as-practical landing will allow the usual easy fix-a broken clamp or parted hose. In our Mooney, the first incident was caused by a broken clamp and so was the second. After that, we wised up and proactively replaced all of the clamps-10-year-old original equipment. The problem never recurred.

Induction leaks in turbocharged airplanes can sometimes be diagnosed on the ground before takeoff. If you notice higher-than-normal manifold pressure at idle, that could be because the engine is drawing in air through a busted hose or clamp. The additional unwanted air may cause the engine to run lean enough to produce roughness or stumbling.

Scenario Five: Other Stuff

Some turbocharger failures live in that nether world between maintenance shortfalls

Cirrus SR22's Turbo


and parts that break because they were ready to break or because they werent maintained correctly. And that gets us to controllers and wastegates.

All modern turbochargers on new production airplanes have automatic variable wastegates to control boost. They work by controlling the volume of exhaust gas routed to the turbine, dumping overboard whatever isnt needed to deliver the commanded manifold pressure. Some older airplanes have manual variable wastegates and some even have the equivalent of no wastegate at all, a fixed wastegate with a relief valve to protect against overboosting.

Because wastegates live a hard life in a stream of superheated exhaust gasses, they get cranky with age. They can become fouled with exhaust byproducts or corrosion and fail closed or partially closed. When this happens, if the wastegate fails closed, the engine may revert to an unstable closed-loop induction state-so-called bootstrapping. With no wastegate to control turbine speed, the turbine winds up, producing more boost and power and yet more exhaust flow, followed by more boost and so on.

The wastegate-if there is one-is controlled by the turbo controller, which samples upper deck air and tells the wastegate how much to open and close. Turbo controllers do fail, but rarely. If bootstrapping occurs, savvy mechanics will look at the wastegate first. A sticky wastegate can cause variations in manifold pressure that might mimic bootstrapping, but they arent the same as bootstrapping.

Regardless of the cause, bootstrapping or MAP variations arent an emergency condition and dont represent worse things about to happen. Boost will be difficult to set and wont be stable, but the airplane wont be unflyable. In fixed waste-gate systems, such as the Mooney 231s TSIO-360, some bootstrapping isnt unusual.

Worn-out turbochargers, like old athletes, may just be unable to perform as they reach the end of their service lives. Turbine blades eroded by exhaust gas are less efficient and wont deliver the same boost they did when fresh from the factory. When a turbocharged engine that normally doesnt use much oil begins to use more, after the usual suspects have been ruled out, it could be that turbocharger bearings and seals are letting a little oil pass through. This will burn up and pass out the exhaust. Like a wastegate failure, its not a safety-of-flight issue, but the turbocharger will soon need an overhaul.

Like any other components or systems, turbochargers do fail, although its unclear how often it happens because like engine failures in twins, if the airplane lands without incident, no ones the wiser except the owner and his mechanic. The NTSB or FAA wont take note. And if you handle your own turbo failure correctly, you can keep it that way.

Paul Bertorelli is editor-in-chief of sister publication Aviation Consumer.


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