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Why is it possible to over-boost piston aircraft engines? All turbo car engines have a wastegate that just vents pressure back to the intake, whereas in an airplane the pilot has to look at the manifold pressure gauge to protect the engine.

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The majority of piston airplane engines are based on very old technology, before computerization was available. The vast majority of airplane piston engines are carbureted with manual mixture control, prop control and turbo control and the pilot has to manage all that plus carb heat manually.

New engine technology has been very slow to come into the piston market because certification costs are so high, and the market is so small. It would cost engine manufacturers millions to develop and certify new turbo-normalizing systems for these old engines and as there are probably only a few thousand turbo-normalized avgas piston airplanes in the world they would never recoup the costs.

There are modernized engines on the market with electronic fuel injection, computerized engine management with turbo-control, and single-level operation. These are slowly making inroads into the market but it's going to take time.

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Some of the early, basic, turbocharged aircraft did have a manual waste gate control. "Turbo-normalizing" was designed to allow turbocharging at altitude and was not to be used a sea level. If not used properly, it would be possible to over boost the engine at low altitude but a "blow off valve" was incorporated to prevent damage to the engine.

N93641, 1973 Bellanca 17-31ATC Super Viking control panel showing manual waste gate control below throttle: enter image description here

enter image description here

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It is possible to overboost ANY turbocharged or turbo-normalized engine if handled improperly, aviation or otherwise.

Now a dedicated turbocharged engine generally uses an sutomatic waste gate to control the flow of exhaust gases through the turbo, thereby regulating the manifold pressure available to the engine. They're also a little more ruggedly built that a normally aspirated engine specifically for that purpose. That being said, they do have their limits and smooth throttle inputs are suggested to minimize the risk of an overboost condition below the turbo's critical altitude.

A lot of normally aspirated engines were turbo-normalized, that is fitted with an aftermarket STC turbo kit (The Bellanca above looks to have been fitted with a Rayjay STC turbo kit). Older STC kits for turbo normalizing used manual waste gate controls which had to be adjusted manually and we're not cleared for used below 3000-4000 feet to prevent overboost. Here pilot negligence does present an opportunity for over boosting and damaging the engine through failure to regulate the waste gate and or engage/disengage the turbo at low altitude. There are s number of aftermarket STC turbo kits such as those made by Tornado Alley which do have automatic waste gates to reduce pilot workload such as those found on the Cirrus SR-22 Turbo and Mooney M20TN.

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Why it's possible to over-boost piston aircraft engines?

It's not. Unless the system is malfunctioning or not set up properly.

while in the airplane pilot has to look at the manifold pressure gauge to protect the engine.

This is simply not true. A pilot only has to look at the gauge to set the manifold pressure to whatever power setting he desires.

The turbo will never over boost. All piston turbo aircraft have had over-boost protection since day one. Most often by waste gate.

They are not controlled by computer. They use pressure controllers that sense manifold pressure and divert oil pressure to supply muscle to open the waste gate.

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  • $\begingroup$ PA-44-180T Turbo Seminole has 2 over-boost indication lamps. If you a low and cold and apply full throttle you will two red lights in the cockpit. $\endgroup$ – Andrius Sep 16 '16 at 11:09
  • $\begingroup$ I'm fairly sure the Cessna T182 has a manual wastegate system and it is possible to overboost the engine. The newer T182T has automated it, though. $\endgroup$ – Pondlife Sep 16 '16 at 13:36

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