Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

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Aero-Tips 07.31.06

Turbocharged engines develop more power than natural air pressure allows by compressing incoming air. So if the turbo fails, just fly it as a normally aspirated engine to a place where it can be fixed, right? Well...

Turbo design

Most general aviation turbochargers consist of two parts, a turbine and a compressor. The turbine is in the exhaust manifold and the compressor in the induction manifold; exhaust gases spin the turbine, which in turn rotates the compressor to increase air flow.

Most also include a wastegate to control turbocharger output. The wastegate determines the amount of exhaust gas that spins the turbine by opening or closing to adjust the percentage of exhaust that bypasses the turbocharger. Commonly a turbocharger uses engine oil to automatically adjust the wastegate-increasing oil pressure through a wastegate actuator closes the wastegate and increases manifold pressure, while decreasing oil pressure results in a drop in manifold pressure.

Turbo failures

An unexplained drop in manifold pressure may be a result of:

• Oil loss. Oil loss reduces pressure in the wastegate, reducing turbo output. Reduced manifold pressure may be your first indication of a catastrophic oil loss... and impending engine failure.
• Compressor failure. If the compressor itself should become damaged, for instance, by foreign object ingestion, it may restrict the flow of "normally aspirated" air (I've heard stories of shop rags and loose bolts being left in the engine compartment, entering the induction manifold through an alternate air door and then being ingested by the turbo). Power will drop below natural-air levels. If the compressor breaks up, metal shards may enter the engine itself, causing more damage or total engine failure.
• Exhaust leak. An exhaust leak can reduce turbine output, dropping manifold pressure. Exhaust gasses may run as high as 1650 degrees F/899 degrees C or higher; if the leak is directed near a fuel or oil line it can cause an engine fire.

There are other less catastrophic scenarios that can result in an uncommanded loss of manifold pressure in turbocharged engines (example: a pressure controller out of calibration). Trouble is, from the pilot's seat there's no way you can distinguish the benign scenarios from the danger of imminent engine failure or explosive fire.

Aero-tip of the day: An unexplained loss of manifold pressure in turbocharged airplanes is grounds for landing at the very nearest runway, whether or not repair services are immediately available. Do a precautionary engine shutdown if flying a twin. Be prepared for a possible engine fire or failure.

FMI: Aero-Tips