Turbocharging systems get somewhat of bad, but to a degree, deserved rap for requiring more than their fair share of maintenance and pilot workload. They certainly have to deal with more than their fair share of very hot air and have to spin at dazzling rpms to do their jobs.
Moreover, they can be less forgiving of being operated out of their normal operational parameters. Especially those heavily manual pilot involved systems so poplar with the lower end planes and simpler systems and smaller turbos designed to keep prices down and simplicity up, and still leave room in already crowded engine compartments.
Unfortunately, some manufacturer attempts to keep turbo systems simple means little tolerance for operational error, or unsophisticated feedback control systems, which can spell a pain in the wallet.
That said, an owner-operator can easily diagnose most turbo problems and if you understand the system, many are avoidable. In this report on turbochargers, we’ll examine the turbocharger itself and the pressure relief valve, more commonly known as the PRV.
The simple Turbocharger
A turbocharger is really quite simple and typically has only five moving parts in the Garrett and three for the Rajay There is a Rajay parts Web site-RAJAYparts.com, as well as for Garretts.
The problems that plague turbos are few, but generally expensive when encountered. and they pretty much all center around heat control and lack of it and related leaks in the exhaust system and rest of the turbo plumbing. Proper operating techniques and cool TITs (turbine inlet temperature) will keep you out of trouble in this department. As will a recording engine management computer with TIT scales.
As you probably know, a turbocharger is nothing more than a fan driving a fan. The exhaust gases drive a turbine wheel, which is directly linked via solid shaft to a compressor wheel, which compresses air into the induction system of the engine.
Almost all problems with turbos will show up as a bootstrap condition below minimum critical altitude. The most common operational malady that kills turbos before their time is turbine section degradation by either over-temp or over-speed. This manifests itself physically in two ways that you can actually see: Turbine blade tip erosion and wheel shroud “pucker.”
Tip Flings
No, this isn’t a weekend party thrown by the turbocharger. It is the weekend party thrown by your maintenance shop after you send in the check for their repair invoice. Known as blade tip erosion, this problem can actually be caused by either over-temp or over-speed or both.
The wheel blade tips are critical to turbo performance. Exhaust gasses are at their highest speed at the point where they strike the blade tips on the exhaust side of the turbocharger. If that speed is reduced over the tip of the blade, due to erosion, this will decrease turbine efficiency quickly.
Therefore, the slightest bit of tip erosion will start to lower the critical altitude of the engine, which the pilot will immediately notice as lowered manifold pressure at higher altitudes, probably below critical altitude.
Over-speed will allow centripetal forces to build up to a point higher than the wheel blade tips can withstand at operating temperature. The tips actually start flinging off small pieces. This is due to the weakening of the wheel alloy at elevated temperatures, the tips generally being the hottest part on the wheel. These are very exotic steel.
Aircraft that fly at very high altitudes (above FL 240) are most prone to this type of failure. The thinner the air, the faster the turbo has to go to pump the same manifold pressure.
The faster the turbine spins, the greater the tip speed. For instance, if you look at the manifold pressure gauge in a Cessna 414, you’ll note altitude markings in blue on the manifold pressure gauge.
These are the manifold pressure limits at that altitude to prevent over-speed. The turbo’s full-floating bearings are also a consideration but don’t usually fail because of over-speed. The tips usually disappear first, because in the overall scheme of things, they’re simply less tolerant than the rest of the system.
Temperatures higher than the manufacturer’s limits weaken the wheel alloy further and may cause tip flinging even at normal speeds. Either way, the result will be the same: bad turbo performance.
Turbine Kissing
Wheel shroud “pucker” is also caused by over-temp conditions. Pucker happens when the flat face of the shroud gets too hot and warps. With little clearance between the shroud and the back of the turbine wheel, the slightest warp will begin to rub on the wheel, adding friction and slowing it down. Again, this reduces the turbo’s ability to deliver rated MP.
Rajays don’t usually have this problem because their aluminum center housing carries away a lot of unwanted heat. You’ll see it in Garretts, however. Another turbo malady is tongue cracking and splitting.
This also occurs because of over temp conditions and will cause a drop in turbo performance. The tongue is the end of the housing “scroll” gas path that directs the exhaust gases onto the tip of the turbine blades.
When the tongue cracks and splits, the exhaust gas velocity at the turbine blade tip drops and turbine speed goes south with it. This is much more common on Rajays but has been known to happen on Garretts, too, especially the model in the Cessna T-210.
Garrett does allow up to a 3/4-inch long crack here, but splitting will begin to decrease the efficiency of the turbine and the tongue will eventually have to be repaired.
Coking
Another problem encountered on turbochargers is center section housing overheating. This happens more often at engine shutdown rather than in running operation.
The oil seals in the turbo are like small, stationary piston rings that sit in machined grooves in the turbine shaft, but don’t turn with it. To seal, these rings coke up with hot oil just enough to seal in the oil that comes out of the bearings, which is at little to no pressure.
The idea here is to keep oil from leaking out of the turbine side of the shaft. Once it’s done its cooling and lube job, the oil then drains down to the scavenge chamber or fitting where it’s sucked or drained back to the engine sump.
When an engine is stopped before the center section is cool-and cool is relative, here, it’s still plenty hot no matter how long you idle before shutdown-the oil in this section bakes until it turns into a hard brown or black solid.
This excessive coking eventually binds up the turbine shaft and, again, causes poor turbo performance. In extreme cases, it can freeze the shaft completely. Either symptom is cause for “de-coking” the turbo.
Most aircraft manufacturers of turbocharged aircraft have a de-coking procedure in their maintenance manuals. This is the only way to free up the shaft short of pulling and disassembling the turbo. Basically, this involves merely tapping on the turbine shaft to dislodge excess coke.
Note: When the manual says “lightly” tap the turbine shaft, they mean lightly. Anything more will break the sealing coke and cause oil leakage into the turbine area.
When that happens, it looks like skywriting but has serious consequences, called oil starvation. So be careful when performing this procedure.
Coking is one of the reasons the aircraft manufacturers and your shop tell you that it’s a good idea to idle the engine for a three to five-minute cool down period before engine shutdown. There are experts who will tell you unnecessary, nut as long as the manufacturer recommends it we think cool down makes sense.
This is generally done at 800 to 1000 RPM and full-rich idle mixture, if your idle is adjusted correctly. If you prefer, ground lean during the cool down. In either case, the cool down idle period will help prevent the heavy coking that may otherwise provide your shop with the funds for another weekend party-for them. Make it a point to allow a cool-down period unless you have a long taxi back to the hangar that will also allow a cool-down.
FOD
The next most common cause of turbo failure is FOD or foreign object damage, the bane of jet engines as well as turbochargers. FOD in the turbine wheel is generally caused by a broken valve or debris left in the induction or exhaust system after maintenance.
Broken turbine blades or tips are the usual result of this type of failure. Compressor blade damage is usually from bad alternate air door sealing or something upstream in the induction system coming apart. Either of these problems will show up as substandard turbo performance and will require further inspection for possible engine damage.
One of the least common but hardly unheard of turbo problems is turbine wheel rub. Wheel rub (other than on a puckered shroud) is most often the result of deposits building up in the turbine housing to a point where the wheel begins to drag on them and lose speed.
This can be caused by an engine that burns excessive oil, is constantly run too rich or a turbo that has a bad turbine or compressor section oil seal. Inspecting the inside of the exhaust system upstream of the turbo will identify the culprit here.
Worn bearings can also cause this but the problem will usually show up first in compressor wheel rub rather than turbine wheel rub. This is because the clearances in the compressor section are smaller than those in the turbine section and extra play will cause contact there first.
Two problems rarely encountered are excessive clearance between the turbine wheel and housing or compressor wheel and housing. These happen because of improper buildup at the overhaul/repair accessory shop and not while the engine is in operation. They show up, as do all the other sources, as premature bootstrapping, also called the tail wagging the dog.
Metal Contamination
The last thing to mention here, and perhaps the most important, is bearing contamination. Any time 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 bearing replacement.
An overhauled turbo is even better. Metal contamination because of the failure of some other component in the engine, is by far the most common cause of turbo failure. As you can see, most of the problems encountered with a turbocharger will occur in the high temp section, not the cooler compressor side.
This illustrates the importance of proper operation. The lower the temps, the longer the turbo lasts. The only problems you’ll see with the compressor section are FOD and wheel rub from a worn or contaminated bearing.
PRV
The pressure relief valve is simple in concept. It’s nothing more than a relief valve set to relieve upper deck pressure at a preset limit. Periodically check it. If you see signs of exhaust stains from the last check, then you are overboosting the turbo. Stop that.
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 manufacture.
This pressure reference is needed because of the absolute pressure drop as the aircraft climbs. The altitude bellows adds pressure to the back side of the pop-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 pop- off valve. If the spring breaks you’ll leak a lot of upper deck air and have poor turbo system performance. This will show up as 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 tear down inspection, so don’t do it. Routinely keep an eye on the MP gauge during throttle up.
The aircraft manufacturer’s PRV check is strongly recommended as routine item during inspections or when you suspect there’s something not quite right with the system, as an easy system check.
As you can see, “not quite right,” can manifest itself in a number of very similar ways. But in the end, turbo systems are so simple that it doesn’t take too long to find the problem, assuming you know what to look for in the first place.
Diagnosing Bootstrapping
What is bootstrapping, exactly, and how can you tell if it’s happening? Bootstrapping 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 depends on altitude and RPM.
A climb now will cause you to lose about an inch of manifold pressure for every 1000 feet gained. If the engine bootstraps below the minimum critical altitude-as stated in the maintenance manual-an exhaust leak, induction leak or poor turbo performance is the culprit.
Pressurizing the exhaust system with about 10 PSI of shop air and spraying some soapy slurry on joints and suspected problem areas will highlight leaks.
Any leaks that will blow away the bubbles need to be fixed. A leak that holds a bubble is not a concern.
A note of caution: Before you pressurize the induction system, be sure to disconnect any manifold pressure or upper deck lines that could damage an instrument or bellows with too much air pressure. And limit the pressure to 10 PSI, which is about the difference you get at altitude when in cruise and is more than enough to find leaks.
If there are no significant leaks, the culprit will be the turbo or wastegate controller. Try swapping in a known good controller and repeat the altitude test. If this fails, the turbo is likely the problem. Check for coking and turbine deposits before you send it out for inspection or repair.
Last but not least, obstructions are very unlikely. But check for them before you yank the turbo and send it out. A rag in the induction, a plugged air filter or some sort of blockage in the exhaust is a possibility. It has happened, but not often.
One sneaky and often overlooked item that will affect critical altitude is the alternate air door in the induction system. If this door has a bad leak, it can lose the ram affect of the normal induction intake, and/or let in hot air.
This will lower the critical altitude in some aircraft by almost 10,000 feet. Check this system for a good seal and if applicable, a strong and properly rigged latch mechanism.
As tempting as it is, never get your turbo worked on by an auto shop. Standards and materials are not the same.
Remember, slow, smooth application of the throttle will help prevent over boosting, as it’s easy for the turbo to overshoot as it catches up. And be aware of temperatures. Unfortunately, the original makers of the Rajay and Garrett turbos are no longer making these products. The rights to make the turbos and parts has been picked up by other players in the market. It appears the components, overhauling and exchange parts are available at HartzellEngineTech.com,(877) 359-5355, as well as independent, small businesses found on many Websites.
Turbos and several other product groups seem to continually be acquired from other companies (it’s a small market) so W suggest that if you are in the market for components to give Hartzell a look to see what accessories they have acquired. Your maintenance shop can do the same thing if you prefer that route.
This article originally appeared in the March 2013 issue of Light Plane Maintenance magazine.
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