Most aircraft engines are limited by the structure of the atmosphere — the higher you go, the less air is available to create power. Turbocharging, or mechanically compressing inlet air to provide more power at altitude, overcomes this limitation by boosting the air pressure to something greater than “natural” levels. Turbocharged engines particularly shine above 10,000 feet MSL, where sea-level (or greater) power is combined with reduced air resistance to provide spectacular true airspeeds.
INSIDER’S TIP: You may have heard of “turbo-NORMALIZED” engines. A turbonormalizer is a device (usually a modification to a non-turbo engine) that boosts engine induction air pressure, but only enough to make up for any loss of air pressure from an increase in altitude. Turbonormalized engines, like turbochargers, allow an engine to develop full power to a high altitude, but because they don’t boost as much as “traditional” turbochargers, turbonormalizers usually do so at lower operating temperatures.
HOW IT WORKS: Turbocharging comes from the spinning of a compressor in the induction system. The compressor itself is connected by a shaft to a similar “pinwheel” in the exhaust system — the “turbine side” of the turbocharger. Exhaust gases spin the turbine as they flow overboard, spinning the shaft and the compressor to provide turbo “boost.”
Since in the simplest description a turbocharger is just a flywheel compressing air in the induction system, it’s logical to assume that failure of the turbocharger would merely cause the engine to act like its naturally aspirated cousins. If the failure occurs at altitude, it would follow, available horsepower would increase to “normal” as you descend. If the turbo dies “down low,” the logic continues, you could fly “naturally aspirated” at low altitudes until the turbo is fixed.
WRONG! In reality a “dead” turbocharger might lead to serious, even deadly problems. It might be the first, most obvious sign of a major engine malfunction.
Let’s look at the causes and implications of various turbocharger failure modes, to emphasize that an uncommanded loss or increase of manifold pressure in a turbocharged engine is always grounds for landing as soon as possible.
If the full force of engine exhaust spun the turbine, the system would create greater compressor boost, which causes an increase in exhaust flow and more turbine output, and more compressor action, etc. There has to be a way to limit the amount of exhaust spinning the turbine, controlling its output to create the correct amount of boost for the altitude and throttle setting. Controlling the exhaust flow over the turbine is the function of the wastegate.
The wastegate is simply a valve that directs exhaust gases either through or around the turbocharger. With the wastegate fully open, most or all of the exhaust bypasses the turbine, and little or no turbocharging occurs. This is the position of the wastegate at idle power and (in most systems) at altitudes very near sea level. As the wastegate closes, more and more exhaust spins the turbine, and constant or increasing amounts of power result. Some turbo systems have a fixed wastegate, making the pilot control boost with the cockpit throttle control. Other devices have a manual wastegate, controlled by a second “throttle” for each engine. More sophisticated turbochargers have one of several automatic wastegate designs with a wastegate controller, reducing pilot workload. Both the wastegate and controller are mechanical valves and, as such, are subject to possible obstruction or “sticking.”
- If the wastegate or controller sticks, the wastegate may not open fully as you descend or reduce power, causing higher-than-expected manifold pressures. This could lead to a dangerous engine overboost on takeoff or the next throttle advancement.
- Conversely, a wastegate or controller stuck in a low-power position may limit takeoff, go-around or climb thrust enough to severely degrade performance.
SYMPTOMS of a sticky wastegate or controller are hesitations or lags in commanded power changes, or uncommanded increases or decreases in power. See these troubles and you should get the airplane checked as soon as possible.
INDUCTION LEAKS OR OBSTRUCTIONS
To work properly, induction air needs to flow unimpeded from the air inlet into the turbocharger and, once compressed, with as few bends and turns as possible to the cylinders. If ice, dirt or anything else contaminates the inlet air filter, or if foreign objects have somehow entered and blocked the induction manifold, then the turbo has to work harder to achieve a given manifold pressure. Increased turbo work means higher induction temperatures (from additional compression), so an obstruction on the inlet side of the turbocharger manifests itself in reduced takeoff and climb performance despite “normal” manifold pressure.
An induction system obstruction downstream of the turbocharger means some of the work done by the turbocharger doesn’t make it all the way to the cylinders. Maximum manifold pressure will be low; takeoff, climb and even cruise performance may suffer. Leaks causes by small holes or loose connections in the induction system will have similar effects.
SYMPTOMS: Suspect induction system leaks or turbo obstructions if manifold pressure doesn’t reach “red line” when commanding full power, if performance isn’t up to expectations, or if manifold pressure drifts irregularly during climb, cruise or descent. You might also see increased cylinder head, exhaust gas and/or turbine inlet temperatures for a given power setting and fuel flow. These indications also warrant inspection as soon as practical.
Exhaust gases are highly corrosive, and dangerously hot. The smallest of exhaust system leaks will quickly erode (grow) under the force of hot exhaust. The resulting vent of hot exhaust gases is like a blowtorch burning, unguarded, inside the cowling. If the leak points toward a fuel or oil line, or an easily damaged accessory, the results can be disastrous.
SYMPTOMS: Your first indication of an exhaust leak might be an uncommanded loss of manifold pressure, as the wastegate closes further and further trying to maintain turbo boost. As we’ve seen before, any unexpected change in manifold pressure warrants an immediate landing for investigation.
The wastegate controller drives wastegate position by altering pressure in a dedicated oil line. If the controller senses insufficient manifold pressure for the selected throttle position (for instance, during a climb), the controller partially closes a valve in the oil line. This increases pressure downstream of the valve, pushing a piston that closes the wastegate and develops the desired manifold pressure. Conversely, if manifold pressure exceeds throttle selection, the wastegate controller opens the oil valve, which drives the wastegate to open more. A drop in manifold pressure might be your first warning of a catastrophic oil loss as the pressure in the wastegate controller drops, opening the wastegate.
SYMPTOMS: An unexplained manifold pressure drop might warn of an oil system failure. Crosscheck oil pressure and temperature indications, and get the airplane on the ground before the engine seizes.
THE BOTTOM LINE: You simply can’t fly a turbo’d engine as if it was a normally aspirated model. Turbochargers can dramatically improve an airplane’s performance at altitude. If a turbocharger doesn’t work as advertised, and manifold pressure changes unexpectedly, you simply can’t tell from the pilot’s seat whether the cause is benign or disastrous. Uncommanded manifold pressure changes in turbocharged engines warrant landing at the nearest airport.