Turbo Failures: Heat, Oil, Plumbing

This story originally appeared in Aviation Consumer Magazine

For many, turbosupercharging is a must, while pricey upkeep is the price to pay for high-flying efficiency and safety. But even the best-maintained turbos have little tolerance for careless operation, although these days, advanced electronic engine monitors are a welcomed backstop.

But no matter how gently you finesse them, turbos work hard. They spool to impressively high RPM and generate glowing-red heat within tightly packed, cooling-starved engine cowlings. Neither are keys to longevity and eventually, you’ll be faced with turbo component repairs.

Still, recognizing subtle symptoms early can get you to a shop before the turbo fails hard, plus doing some troubleshooting on your own might save you some shop labor. Here’s a basic rundown to get you started, with input from experienced techs and turbo shops who have seen it all.

For pilots of normally aspirated birds, there might be some mystery surrounding aircraft turbochargers, but don’t overthink it. While all are critical, the typical aircraft turbo can have as few as five or even three moving parts.

Think of a fan driving a fan. The engine exhaust gases drive a turbine wheel (supercharger) that’s shafted to a compressor wheel, ultimately compressing air into the engine’s induction plumbing.

Veteran aircraft techs know the Garrett and Rayjay brands, but these days Hartzell Aviation holds a huge piece of the market after acquiring Kelly Aerospace. Bring your repairs to a shop that’s familiar with the system that’s installed, and choose a third-party overhauler that sees a lot of them.

California-based Main Turbo (www.mainturbo.com) is one respected name spoken around maintenance shops for years. This FAA Repair Station offer exchanges, can rebuild your existing turbo and can help with troubleshooting. We asked company principal Gary Main if the damage he sees in turbos is generally caused by operator error. While that might be the case in at least some failed turbos, Main said that digital engine monitors have tamed the complexity of managing a turbo, unlike the old days of basic analog single-probe temperature monitors on turbocharged engines. For the typical pilot dealing with an unhealthy turbo, by far the most common symptom Main spoke of is decreased manifold pressure. More on critical altitude in a minute.

“When turbos come through the shop we see everything from units needing inspection for oil contamination to ones that are simply timed out,” he said. Excessive heat aside, he also pointed out that the biggest problem turbos face is not getting enough oil (main bearing health is at risk) because of restrictions in an oil line or check valve.

Worth mentioning is that it’s difficult to completely flush contaminants from a turbocharger because once it’s embedded in the turbo’s main bearing material, no amount of flushing is going to rid it. Without a teardown and inspection, you really don’t know what the main bearing is going look like.

Symptoms can vary widely, but turbo teardowns generally reveal common failure modes. It’s important to understand the differences in operation among systems, especially for the wastegate. That’s a butterfly valve in the exhaust pipe upstream of the turbo that routes air either down the remainder of the tailpipe or diverts it into the turbo, or some mixture of the two. When closed, it all goes into the turbo. Some wastegates are fixed, where the pilot controls power with the throttle. Others are manual, with a second control in addition to the throttle that the pilot must use in setting power. The majority in service are automatic, where a controller senses the pressure of the air in the upper deck of the induction system and uses engine oil pressure of the turbocharger. There might be low oil pressure, a sluggish turbo, a pressure relief valve opening at too low of a boost pressure or a gummed-up wastegate/controller. Even scratching the surface of shop-level troubleshooting, it’s clear why turbo repair work can generate big invoices.

Still, the majority of problems show up as a bootstrap condition below the critical altitude, with over-speeding and over-temping evident by turbine blade tip erosion and wheel shroud pucker. Blade tip erosion can actually be caused by spinning too fast or too hot—or both. The blades on the tip of the turbine wheel have it rough because the EGTs are at their highest speed when they strike the blade tips on the exhaust side of the turbocharger. If that speed is reduced because of tip erosion, you might see a decrease in manifold pressure at higher altitudes.

And as you’d expect, those steel blade tips stress to the point of eventually coming apart (the tips actually fling off small pieces) because over-speeding will allow the centripetal forces to build up higher than the stressed blades can handle. Turbo experts say that this blade damage generally happens at higher altitudes because the thinner air requires the turbo to spin faster to pump a constant manifold pressure. So the faster the turbine spins, the greater the tip speed.

Some manifold gauges on high-flying turbocharged airplanes have blue markings representing the manifold pressure limits at that altitude to prevent over-speeding. As for heat, temperatures higher than the manufacturer’s limits weaken the alloy wheel further and may cause blade tip flinging, even at normal speeds, and the resulting poor performance.

This is really nothing more than a valve set to relieve upper deck (the tube that carries the pressurized air from the turbo to the throttle plate) pressure at a preset limit. In the world of turbo upkeep the PRV is something that needs periodic inspection, particularly for signs of exhaust stains because this could be evidence of over-boosting the turbo—not a key to longevity. The only complicated thing about the PRV is the altitude compensating bellows found in some of them. The bellows are generally filled with dry nitrogen to a precise absolute pressure, which is then sealed at the time of manufacture. This pressure reference is needed because of the absolute pressure drop as the aircraft climbs. The altitude bellows add pressure to the back side of the blow-off valve when at altitude to keep the PRV setting close to what it was set at on the bench at near sea level. Without it, the pressure differential on the upper deck side would open the relief valve too soon.

Some of these bellows have the spring on the inside and some on the outside. Either way there’s a spring and bellows to hold down the blow-off valve. If the spring breaks, you’ll leak a lot of upper deck air and likely see a lower-than-published critical altitude.

On the other hand, if the bellows ruptures, you’ll probably never see it until you do the aircraft manufacturer’s PRV test or suddenly find you can over-boost quite a bit on a cold day. Cold temp over-boost may very well require some engine teardown inspection, so don’t do it. Routinely keep a sharp eye on the manifold pressure gauge during throttle up.

You’ll hear the term bootstrapping in the world of turbo repairs, and it occurs in a turbocharged engine at and above critical altitude. The wastegate is fully closed and can’t pump any more exhaust gases through the turbine unless you increase the RPM. In other words, boost is no longer constant, but it depends on altitude and RPM. You might see a 1-inch loss of manifold pressure for every 1000 feet of climb, pointing to an exhaust or induction leak. One troubleshooting technique is to pressurize the exhaust system with shop air (usually not any more than 10 PSI) and spray soapy slurry on the system’s joints and suspected problem areas. Any areas where the bubbles get blown around are suspect for leaks.

If there are no significant leaks, the culprit will be the turbo or wastegate controller. On the shop level, one option is to swap in a known good controller and repeat the altitude test. Sometimes the problem is something you’d least expect, including a blockage in the exhaust system—everything from a clogged air filter to a maintenance rag (we’ve seen it more than once)that got sucked into the induction hardware.

We’re told one overlooked problem that affects critical altitude (resulting in the lowering of the critical altitude by a sizable margin) is the induction system’s alternate air door. A major leak means a loss of ram affect and sneaking in hot air. Part of thorough, regular inspections includes checking the rigging of the door’s latching mechanism and making sure there’s a good seal. It’s an area that gets overlooked more often than it should.

It’s as nasty as it sounds and can easily trash bearings and other pieces. Anytime the engine has made metal from a blown cylinder, bad valve or guide or any other reason, the turbocharger should be pulled and sent out for inspection and repair. Shops tell us metal contamination from the failure of some other component in the engine is by far the most common cause of turbo failure.

V-band couplings—or more simply, the clamps used to secure the engine’s tailpipe to the exhaust housing—are a well-known maintenance concern on turbocharged engines. Like the FAA and NTSB, mechanics have seen plenty of clamp failures over the years, so it’s no surprise that the FAA has adopted AD 2023-09-09, a means to keep a focused eye on potential pipe clamp failures on a variety of turbocharged airplanes and helicopters.

The bands are secured with a t-bolt and trunnion mechanism that is either riveted or spot-welded in place, and over time, the spot-weld can generate cracks leading to the failure of the clamp. The clamp can fail prematurely if someone over-torqued the t-bolt. There have been plenty of fatigue failures from stress-corroded spot-welds in multi-segment couplings. While some clamps are backstopped with safety wire, that won’t do much good with transverse band cracking and total failure of the spot-weld.

As with the engine market in general, shops unanimously tell us prices for turbocharger components have risen sharply, and supply can’t keep up with demand. AeroForce, under the Hartzell Aviation umbrella, has become the largest manufacturer of turbos, controllers, wastegates and pressure relief valves for a wide variety of engines. Hartzell’s new owner, Arcline Investment Management, has increased prices for parts across the board—in some cases by more than double.

While pricing varies by model, expect to pay up to $7000 or more for an overhauled exchange turbo for a big-bore Continental engine. Shops tell us some customers have been waiting for nearly six months for replacement turbos. Pick a shop that keeps a healthy inventory of components and it will be a quicker turnaround getting your turbo overhauled.

In the interim, keep up with inspections, keep the temps down and catch problems early.