With a little understanding, care and effort, you can achieve a lot more reliability from your aircraft’s turbocharger.
It seems like every time I get around a group of pilots, the talk soon turns to turbochargers. And, seemingly, every pilot in the group has his/her own turbocharger horror story.
I guess it’s understandable. As the Director of Product Support for Hartzell Engine Technologies (HET), I have the advantage of working around the design, manufacturing, rebuilding and testing of turbochargers every day, so I know a lot about these mystery systems. And I also know, contrary to hangar talk, that, when operated and maintained properly, the turbocharger is among the most reliable pieces of equipment under the cowling.
With apologies to pilots and mechanics, my opinion is that a lot of what pilots perceive to be wrong with turbochargers has to do with a combination of them not really knowing how to properly operate their aircraft, and the considerable possibility that their mechanic doesn’t know how to properly troubleshoot and maintain the unit. Put those two together and it’s no wonder turbochargers have gotten a bad rap.
In HET’s considerable experience, the majority of the turbocharger returns we receive from the field are found to be free of issues with the unit itself. In most cases, the problems are with the system’s installation, inadequate pre-lubrication or other operational issues.
Typically, those operational issues include: an inability of the aircraft to reach altitude, the system’s inability to reach the maximum-rated manifold pressure, a surging or dropping off of manifold pressure when climbing or descending, and oil leaks from the compressor or turbine side of the turbocharger.
Again, because few pilots ever take the time to fully comprehend the individual components of their aircraft, they don’t really understand that the turbocharger is not a stand-alone component; it’s just one part of a very sophisticated system that, when properly manufactured, installed, operated and maintained, will run to normal engine TBO.
The turbocharger system consists of the turbocharger(s), controller, pressure relief valve, and wastegate, along with the exhaust/intake assemblies leading from and to the engine.
And, while the turbo itself seems like a simple device, it is in fact a very precise component that can be called upon to operate at speeds over 100,000 RPM and at temperatures exceeding 1,650º (F).
Contrary to popular belief, although the turbocharger does convert waste energy (in the form of hot exhaust gasses) into additional ‘power’, the system is not a power source itself. The compressed gasses produced by the turbocharger increase the density of the fuel/air mixture, which in turn, increases the engine’s power output at higher density altitudes.
As I stated earlier, your aircraft’s turbocharger system is very complex and operates at extreme speeds and temperatures – two conditions that just go looking for problems. But the good news is, more times than not, you will get indicators of some pending problem before it becomes critical.
Typically, you will get nuisance-related maintenance items, such as a bit of extra oil consumption, a slight leak, a bit of blue smoke from the exhaust, or possibly oil collecting at the exhaust outlet. These are just some of the visual indications that something is not performing as it should.
I routinely remind pilots that a thorough pre-flight is the time to look for these clues. Keep in mind that some of the warning signs are very subtle and easy to miss. Take your time. If you see something you’re not used to seeing, you need to get it looked at by a mechanic.
Another highly-effective practice is for owner/operators to be very proactive in their maintenance. Since the turbocharger shares the oil system with the engine, you should routinely change the oil at 25 or 35 hours. And, like other parts of the engine, the condition of the oil can provide an indication of the health of the turbocharger.
During the oil change, the owner or mechanic should take the opportunity to give the entire turbocharger system a good inspection. Keep the air filter clean and check the security of the intake and alternate air systems to prevent FOD (foreign object debris) from entering the compressor or from robbing the compressor of air. Also, take time to visually inspect all clamps, hoses, ducts and related components of the intake/exhaust system.
The bottom line is: Knowing your engine’s typical operating parameters and recognizing differences can help eliminate costly repairs down the road.
Before All Else Fails, Follow The Instructions…
One simple way you and your mechanic can help ensure that your turbocharger system gets you to TBO is to read and follow the Pilot’s Operating Handbook (POH). In particular, I mean the section that covers the proper turbocharger cool-down procedures for your aircraft.
The number one enemy of the turbocharger system is heat build-up, and that heat comes in the form of lack of oil, lack of cool oil, hot exhaust and poor power management. As a general rule, all turbocharged engines should be ‘idled’ for a specific time and power setting to cool the system before shutdown after landing or engine maintenance operations.
Properly following this procedure will prevent two bad things from happening: The first is the continued high-speed rotation of the turbo without adequate oil flow. The second is the development and build-up of abrasive carbon deposits in the oil. (a.k.a. coke).
Remember that the system’s lubricating oil is coming directly from the engine’s oil system, so shutting down the engine immediately stops the flow to the turbocharger. Most Hartzell turbochargers use full-floating hydrodynamic bearings. Hydrodynamic bearings provide outstanding performance in constant-speed applications, like we have in aircraft installations, but if the turbocharger is still turning at a high rate of speed when oil flow is cut off, these bearings can be damaged. In addition, any stagnant oil remaining around the extremely hot turbine shaft will overheat and ‘coke’ or burn.
But why is the turbo still spinning at a high rate of speed after landing? Well, it all goes back to proper planning and power management, which includes a normal descent at reduced power settings to stabilize and maintain engine temperatures.
As the aircraft enters the landing phase, engine power is reduced even further and then taxiing is done at low power settings. By the time you’ve reached the ramp, all engine temperatures should be stabilized and the turbocharger should now be turning at its slowest speed.
But, and I can’t stress this enough, even then, depending on the type of system installed, additional cooling time may be required prior to engine shut down.
(HET has produced an informative, short video on the critical importance of proper turbo cool down. Check it out at: www.hartzell.aero/eo2yU)
And, while improper pre-shutdown cooling is one of the most common mistakes pilots routinely make, turbocharged, intercooled, high-performance engines require constant monitoring of the systems throughout all phases of flight.
And all that starts with proper fuel mixture, CHT and EGT management, including proper flight planning to avoid over-temping or thermal shock. As is often the case, good piloting technique is critical to optimum system reliability and performance.
Out With The Old…
Okay, so even with proper operations, sooner or later your turbocharger is going to start showing signs of distress. What are your options?
Turbocharger systems rely on very sophisticated components with tight tolerances and little room for error. Because of that, if your turbocharger needs maintenance the question becomes whether to have it overhauled or exchange it for a new or rebuilt unit?
The cost of a factory-new turbocharger is only slightly higher than a rebuilt unit and there is no core return required. So, you get all new parts and it’s hassle free. HET also offers factory-rebuilt units on an exchange basis for most of our models.
Pilots and mechanics often ask what the difference is between a new and a “factory rebuilt” turbocharger. Other than the fact that one is all-new and the other is ‘rebuilt’ there is really no functional difference.
In fact, the regulatory definition of a rebuilt turbocharger is, in essence, that it has to meet the standards of a brand new unit. When we do a rebuild here, we use some of the original parts, but many of the parts get replaced with new – especially critical parts such as the highly-stressed turbine wheel.
So, if rebuilt and brand new turbochargers are basically the same, what about a “field overhauled unit”? Overhauled units are not required to meet ‘new’ standards, so they can be less expensive. In most cases, the typical field overhaul shop will reuse more of the original parts than HET typically does. There’s nothing wrong with that, as long as they follow the latest manuals, service bulletins and service instructions for overhauling our turbochargers, valves, controls and wastegates, and replace all the required parts as indicated.
The operative statement here is: “Follow the latest instructions and manuals…” So you need to talk with the overhauler and ask if they have and comply with all the manufacturer’s latest instructions before you consider their shop.
Also, ask them for a detailed list of all the components they change during an overhaul. If they won’t supply one, move on to another shop. And if they do give you a list, pay close attention to whether or not they change critical components like the turbine wheel assembly.
Components like the turbine wheel are subject to fatigue and thermal stress. And the effect of fatigue damage is cumulative, and not always obvious, which makes it very hard to detect during an inspection.
For that reason, HET’s overhaul manual requires replacing critical components with new parts, which is the prudent course of action. It has proven to be the best way to help ensure that critical parts will make it through to the next overhaul cycle.
Hartzell has also initiated a Recommended Service Facility (RSF) program. Participating turbocharger repair and overhaul facilities have agreed to follow the HET overhaul manuals and only use genuine HET parts. Currently, Quality Aircraft Accessories of Tulsa, Oklahoma (www.qaa.com) is an HET RSF, so if you’ve grown fond of your turbo and don’t want to trade it in, it would be a good idea to consider QAA.
Another point to consider when making the new vs. rebuilt vs. overhauled decision is that new and, often times, rebuilt units will offer the benefits of having more new-generation components that are manufactured using the latest equipment, materials, and techniques. Plus, new and rebuilt units are covered by HET’s warranty.
So, while the resulting new-generation turbocharging systems may not give you any more performance than the original units, the much-higher degree of precision will help ensure that, along with some judicious operation and preventative maintenance by you, today’s units will deliver years of safe, trouble-free performance.