Aircraft Turbochargers 101

Flying at the higher level

As the market leader in aircraft turbochargers, HET continues to leverage on past industry success to manufacture the highest quality turbocharger systems under the AeroForce name. New turbocharger development programs are a part of our ongoing commitment to propel our customers and yours to a higher level.

AeroForce turbochargers offer advanced product design, simulation and component testing, and new-generation manufacturing, while providing superior support for the broadest catalog of FAA certified turbocharger products available. No one else in the industry comes close to delivering the turbocharging systems experience and capability as we provide.

Let’s climb together.

 

The Pilot’s Guide to Turbochargers

We talk to a lot of pilots who aspire to one day own a turbocharged airplane. Why? Well, first of all they want the capability to climb higher where the ride is smoother and fuel efficiency is at its peak. Secondly, anything with a turbo on it is just cool.

While making that turbocharged dream come true is a wonderful accomplishment, it does come with some added responsibility. Unlike the normally aspirated engine you’re used to flying behind, turbochargers require a bit more understanding and finesse when it comes to proper operation. Unfortunately, the right ways to operate a turbocharged engine is not often part of the typical aircraft checkout.

A few examples of the things you need to look at for optimum turbocharger operation starts with proper fuel mixture, CHT and EGT management and continues through proper flight planning to avoid over temping or thermal shock. As with most sophisticated aircraft systems, good piloting technique is critical to optimum system reliability and performance.

Believe us, turbochargers are not hard to operate you just have to know how.

 

Read the Aircraft Turbocharger POH

Well, before you make that first flight in your new turbocharged whatever you want, take time to read and understand the Pilot’s Operating Handbook (POH) and/or the engine operating instructions. While overall it’s all very important for safe operations, one area that experience says you need to concentrate on is the section(s) covering the proper post-flight cool down procedures for your aircraft’s turbocharger.

The number one enemy of the turbocharger system is heat build-up and that heat comes in the form of the combination of a lack of 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 in the absence of adequate oil flow. The second is the development and build-up of abrasive carbon deposits in the oil.

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 AeroForce 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.

And while improper pre-shutdown cooling is one of the most common mistakes pilots routinely make, turbocharged, inter-cooled, high-performance engines require constant monitoring of the systems throughout all phases of flight.

 

Anatomy of A Turbocharger System

One thing that will help you understand the need for proper operation is to understand that the turbocharger is not a stand-alone component: It’s just one part of a very sophisticated system, which consists of the turbocharger(s), controller, pressure relief valve, intercooler 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 operate at speeds over 100,000 RPM and at temperatures exceeding 1,650º (F). It’s a highly complex piece of engineering that operates in a very harsh environment.

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 (or maintains) the engine’s power output at higher density altitudes.

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.

In HET’s considerable experience, the majority of the turbocharger returns we receive from the field are found not to be issues with the unit itself but in most cases they are problems with the system’s installation, inadequate pre-lubrication or other operational issues.

Typically those operational issues include an inability for the aircraft to reach its critical 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.

Along with these operational items you may also encounter 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.

We 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.

To help you and your mechanic get a better understanding of common turbocharger performance issues, HET has created an Aircraft Turbocharger Troubleshooting page.

The bottom line is: Knowing the engine’s typical operating parameters and recognizing differences can help eliminate costly repairs down the road.

 

But Speaking of Aircraft Turbocharger Repairs…

Okay, so even with proper operations, your trusted turbocharger is starting to show 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 overhaul or exchange it for a new or rebuilt unit?

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 it, in essence, has to meet the standards of a brand-new unit. When we do a rebuild at HET, we inspect all the original ‘reusable’ parts and if they meet new tolerances, use them in the rebuild. But, critical parts such as the highly stressed turbine wheel assemblies, bolts, lock plates and nuts, piston rings, thrust bearings, seal rings, retaining nuts and so on get replaced 100% of the time.

But, you ask, if rebuilt and brand-new turbochargers are basically the same, what about a “field overhauled unit”?

The biggest difference is that overhauled units are not required to meet ‘new’ standards, so they are usually less expensive. In most cases, the typical field overhaul shop will reuse a lot 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 that also includes replacing all the required parts as indicated by the instructions.

The operative statement here is: “follow the latest instructions and manuals…” So, you need to talk to 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, center housings, piston rings, thrust bearings and other components I mentioned earlier.

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.

 

Recommended Service Facilities

With all their complexity and the variety of levels of service, it can be difficult for owners and mechanics to determine which overhaul shop is right for them. We’ve tried to make it easy by initiating a Recommended Service Facility (RSF) network.

Participating turbocharger repair and overhaul facilities have agreed to follow the HET overhaul manuals and only use genuine HET parts. Currently in the U.S. our RSF shop is Quality Aircraft Accessories in Tulsa, OK and Fort Lauderdale, FL and, in Canada, it’s Aero Recip. 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 them.

To help make aircraft owner/operators easily recognize if their turbocharger has been overhauled by an approved RSF, HET recently started issuing new product data labeling tags for these units.

As HET’s President, Keith Bagley explained it, “This initiative is the next natural progression in our ongoing RSF program. It gives a voice to our RSF shops while indicating our endorsement for the great work that they do supporting our customer base.”

 

Rebuilt vs. Overhauled Turbocharger

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, along with a full 24-month or 1,000 hours, whichever occurs first, warranty.

New, factory-overhauled, field-overhauled, rebuilt – there are certainly a lot of options. And, while there is a lot to consider when making your decision, it often boils down to cost.

For example, let’s look at a turbocharger used on a number of popular twin Cessna models (406610-9025). According to Brett Benton, President of Quality Aircraft Accessories, the cost for the various options are as follows:

• Factory new: $3,075.00
• Rebuilt: $2,875.00
• Overhauled: $2,475.00

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.