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TURBOCHARGING SYSTEM FAQ & TROUBLESHOOTING

The following are typical of the questions received and answers given at Kelly Aerospace over the years. The answers given are by no means the only or definitive answers but they are designed to lead you in the direction to remedy a given problem. The goal is to share our experience and to save labor time in troubleshooting. Any actual repair activity must be done in accordance with the appropriate maintenance or service manual from the applicable source. (airframe and/or engine to latest revision). Turbochargers, turbo valves and controls are not typically field repairable.

Frequently Asked Questions:

Q: I notice some small nicks in the compressor blades when inspecting my turbocharger. Can these be dressed out and is there a tolerance?
No, due to the very high speed of the turbine/compressor wheel, balance is very critical. Even the smallest bits of damage to the blades can set up an imbalance causing a vibration which will at best reduce the life of the turbo and at worse, cause turbo operation to cease.

Q: My airplane acts like I have no turbocharger. I loose power as I go up, but take off power seems normal. What might be happening?
The most likely cause is the oil supply to the wastegate. Look first to determine engine oil pressure, assure the pressure reading is accurate and is well in the green. This can be in the form of blocked oil lines, or restrictions in the wastegate capillaries. Check oil lines for kinks or blockage. If the condition persists and the oil has been (or previously been) contaminated by any event, the wastegate is suspect and should be replaced or overhauled.

Q: The turbocharger controller seems to be very slow in operation. Any ideas why?
On Lycoming or Continental powered airplanes there are several things to check before changing a controller. First, be sure the oil temperature is in the green (warm), cold oil temps will cause sluggish operation. Check for leaks in the air pressure sensing lines. Look for oil leakage at the compressor outlet air sensing line to the controller. A loose line, corrupted seal or cracked fitting may be to blame. If the oil is dirty or has had some event contaminate the oil, particles may cause the poppet valve (internal) to stick open. Perhaps the most common fault is proper adjustment. Carefully review the airplane maintenance manual and check all adjustment.

Q: I have noticed some white smoke coming from the exhaust of my turbocharged airplane. I have been told it is probably the turbo. Can there be other causes rather than a bad turbo?
There can be many reasons related to the core engine as well as the turbocharger. Many times a turbo will smoke when new due to excessive preservative. However, if the smoke is light and continual, the best suspect is a weak or failed scavenge pump. This would flood the turbo with oil as the scavenge pump is what assists in removing the oil in turbo operation. It should be noted that this same condition can be caused by a blocked or kinked scavenge oil line as well. Continental engines use a check valve in turbocharger oil drain line, should it fail the turbo will flood with oil. (Refer to Continental service information to inspect the check valve.) The scavenge pump condition is best observed by noticing a drop or two of oil out the exhaust pipe after shut down and heavier white to blue smoke each start up.

If the turbo is truly on it's way to failure, several conditions can be noted as precursor. Oil spots or puddles from the exhaust pipe on the ground increasing in size after each flight or run. Heavier white or blue smoke with a failure to maintain low idle condition. (Turbo blades stop turning due to friction.) Continual light smoke due FOD damage to compressor or turbine wheel blades or other internal damage due to contaminated oil. Any of these conditions should mandate immediate attention with the turbocharger replaced or sent for overhaul.

Q: My airplane has a turbo with a wastegate and controller. I am loosing power before critical altitude. What is the most likely cause?
Unless extreme loss of power is experienced, slight reductions of horsepower are hardly noticeable on single engine airplane. On twin engine airplanes, single engine climb, even slight loss of power can be noticed. The first thing to look for is the adjustment on the wastegate. Many wastgates come pre-adjusted to a nominal setting. Depending on the aircraft, the adjustment can be as simple as one setscrew or very complex linkage. Not reaching critical altitude will often show up after a wastegate or other turbo component is replaced.

Many times "power loss" translates into an indication loss, especially if the loss can not felt. Basic airframe instrumentation such as MAP, oil press, TIT and others should be checked for proper reading. If a true loss is suspected, the lines carrying air or oil to the controller or wastegate should be examined for leaks. Loss of manifold pressure in climb on turbocharged engines can also be caused by the butterfly valve in the wastegate is not closing completely. This can be a carbon build up or a wear related condition. Also remember that leaks in the intake and exhaust system may dramatically affect turbocharger operation.

For more extreme or unusual reductions in power as seen in MAP other things should be looked at. Check the compressor impeller for damage. In some cases FOD, including large ice particles will not only nick or dent the impeller but alter the blade contour reducing efficiency or interfering with the housing. Although more difficult to inspect, the turbine (exhaust side) can also suffer from FOD. Should anything pass through the combustion chamber and exit through the exhaust they have the potential for damage. Engine manufacturers have service information regarding the inspection and detection of this subject.

Q: I have noticed high manifold pressure and the engine is easy to overboost. What are the possible reasons?
Although each turbocharged airplane is a little different, the likely causes revolve around the pressure relief valve or the exhaust bypass valve. The relief valve aneroid bellows may be faulty or out of calibration. The exhaust bypass valve is faulty either due to blockage of oil pressure or a mechanical failure. A poppet type (ball) exhaust bypass valve will sometimes stick on its seat leaving it in the closed position. A malfunctioning rate controller can cause the same conditions if it fails to properly sense a rapid increase in compressor discharge thus leaving the valve closed.

Q: Sometimes when I pass through precipitation or visible moisture and again occasionally when I adjust for cruise flight I experience a loss in MAP. Any ideas why?
Both conditions may relate to the same cause. Many turbocharged airplanes utilize alternate air doors that can "suck open" should restrictions block the normal inlet. Many of these suck open doors are held shut with magnets and the pilot lever simply overcomes by pushing in on the door. In the case of precipitation or visible moisture it most likely that some ice is forming and the door is opening. When open the intake air to the turbo comes from within the cowling where the flow is hotter that outside and has no ram effect so the MAP will change. If the magnet has become weak or the opening lever is not adjust properly, even the slightest inlet blockage will open the door. This can also apply to the cruise set up. If the magnet has become weak or the opening lever is not adjust properly, airflow in the cowl or changes in MAP may be enough to open the door. Of course many airplanes have conventional alternate air doors controlled by linkage. Wear or improper adjustment can allow the door to creep open.

Keep in mind that engine air filters can be the problem for either of the above conditions as well. Paper and pleated air filter may collapse with airspeed or may swell with moisture. Of course is always a concern. Pleated air filters that may otherwise look clean may appear "fuzzy" after a few times being cleaned or blown out. This fuzzy texture provides ample area for very small water droplets to form causing a quick icing condition and filter blockage. If the inlet system is tight and there is a mechanical alternate air door, the inlet hoses or tubes can collapse especially if they have not been changed recently. (As pressure drops, the hose or tube contracts and expands aging them.)