Performance, Turbocharging and FADEC - ATPL 021 — Airframe, Systems and Powerplant

A piston engine's power is ultimately a count of air molecules: how much air mass reaches the cylinders each second decides how much fuel can burn there. This closing lesson of the module follows that idea to its consequences: why power fades with altitude and heat, how the turbocharger and supercharger fight back, the vocabulary of boosted engines from waste gate to critical altitude, the discipline of moving the engine's levers in the right order, and the FADEC computer that takes all of it over.

What sets the power

The parameters that move a piston engine's power output, petrol and diesel alike, all work through the mass of charge each induction stroke captures:

Ambient pressure — lower pressure means thinner air and a lighter charge: power falls as the barometer does, and above all as the aircraft climbs.
Temperature — hot air is thin air: a hot day cuts power exactly as extra altitude would.
Density altitude — the two effects combined into one number: the altitude at which the standard atmosphere would match today's actual density. It, not the altimeter reading, is what the engine feels.
Humidity — water vapour is lighter than the air it displaces and burns nothing: humid air costs a further small slice of power.
Exhaust back pressure — anything resisting the exhaust's escape leaves burnt gas crowding the cylinder, spoiling the next charge: higher back pressure, lower power.

The diesel feels the same losses through its own logic: less air mass means less fuel can burn cleanly, so its power ceiling falls with density just as the petrol engine's does.

Normally aspirated, and the case for boost

A normally aspirated engine breathes at ambient pressure: its manifold can never hold more pressure than the atmosphere outside, so its power tracks the density of the day and falls away steadily with every foot of climb. That is the requirement for power augmentation in one sentence: to go on delivering sea-level charge to the cylinders where the atmosphere no longer supplies it, the induction air must be compressed, and turbocharging is the standard way of doing it.

The turbocharger

Turbine — sits in the exhaust stream and is spun by energy that would otherwise be thrown away, which is the turbocharger's whole economic argument.
Compressor — a centrifugal compressor on the turbine's common shaft, squeezing the induction air to more than ambient pressure.
Waste gate — a valve in a bypass around the turbine: opened, it lets exhaust escape without working the turbine, reducing boost (the margin of manifold pressure above ambient); closed, it forces everything through the turbine for maximum boost.
Waste-gate actuator — the muscle that positions the waste gate, typically oil-operated under a controller that holds the commanded manifold pressure automatically: as the aircraft climbs, the actuator closes the gate progressively to keep the boost constant.

Ground-boosted and altitude-boosted

Two philosophies use the same machine: an altitude-boosted (turbo-normalised) turbocharger restores sea-level manifold pressure at altitude but never exceeds it: at sea level the waste gate stands open and the engine behaves normally aspirated, the boost arriving only as the climb thins the air. A ground-boosted turbocharger compresses beyond sea-level pressure from the runway upward, giving more than normally-aspirated power everywhere, and the engine must be designed for the higher pressures it will live with.

Critical altitude and its relatives

Critical altitude — the altitude up to which the turbocharger can maintain the rated manifold pressure: the waste gate reaches fully closed there, and above it power falls with the atmosphere.
Full-throttle height — the altitude at which the throttle must be fully open to hold a chosen boost: each power setting has its own, and climbing above it means accepting less than the chosen boost.
Rated altitude — the full-throttle height for the rated power: the highest altitude at which the engine still delivers its rated output, which is the critical-altitude story told from the throttle's side.

Turbo lag and the intercooler

Turbo lag is the delay between opening the throttle and receiving the boost: the turbine and compressor must spin up, and they are driven by an exhaust flow that itself only grows as the engine's power grows, so the response builds on itself rather than arriving at once. The pilot feels it as a pause, then a swelling of manifold pressure, and smooth, anticipating throttle movement is the working answer. The intercooler answers a different side effect: compression heats the charge, and hot charge is both thinner (undoing part of the boost) and closer to detonation. The intercooler is a radiator between compressor and cylinders that cools the compressed charge back down: denser air, wider detonation margin.

The supercharger

The supercharger is the turbocharger's older sibling: the same idea of compressing the induction air, but driven mechanically from the crankshaft through gears instead of by an exhaust turbine. The differences follow from the drive: the supercharger responds instantly with no lag and needs no exhaust plumbing, but it spends shaft power to do its compressing, power the propeller never sees, while the turbocharger's energy comes from exhaust heat that was leaving anyway. Its boost is also tied to engine rpm rather than scheduled by a waste gate.

Setting power in the right order

Boosted or not, an engine with a constant-speed propeller has three levers, and the order of moving them protects the engine. The danger is high manifold pressure on low rpm: big charge, slow cylinders, maximum time at pressure, the detonation recipe. So the rule runs:

Increasing power — mixture rich first, then rpm up, then manifold pressure up.
Decreasing power — manifold pressure down first, then rpm, then the mixture leaned.

Each move keeps the rpm ahead of the boost, and the propeller lever's phase schedule from the previous lesson slots into this rule.

Take-off power and maximum continuous power

Take-off power is the highest output the engine may deliver, and it is time-limited, typically to five minutes: the temperatures it generates are tolerable as a burst, not as a diet. Maximum continuous power is the highest output with no time limit, the ceiling for the climb and for any emergency that outlasts the take-off allowance.

Starting in the deep cold

Extreme cold compounds the start: the oil thickens until the starter fights it and the first minutes of running starve remote bearings; the battery delivers a fraction of its warm current exactly when cranking demands more; the fuel barely vaporises, inviting the over-priming that floods intakes and starts carburettor fires; and a cold-soaked engine's clearances are wrong until the metal warms. The answers are external preheat for engine and oil, external power for the cranking, patient priming, and gentle running until temperatures reach their operating ranges.

FADEC

A FADEC (full-authority digital engine control) replaces the pilot's direct mechanical links to throttle, mixture, ignition timing and boost with a computer. The principal difference from traditional manual controls is in the name: the computer's authority is full, meaning every engine parameter is set by it and there is no manual reversion behind it. The flight-deck controls shrink to match: a single power lever commanding what the pilot wants, with the FADEC deciding how mixture, timing and boost deliver it, plus its displays, test switches and backup-supply controls.

Full authority is also full responsibility: the FADEC runs the engine at its efficient best continuously (no forgotten leaning, no wrong lever order, the whole previous section automated), protects it by limiting every parameter before an exceedance can develop, and contains failures, degrading gracefully and keeping the engine running on its remaining resources. Because there is no manual fallback, redundancy is designed in everywhere: dual computing channels each able to run the engine alone, duplicated sensors and data paths feeding them, and independent electrical supplies, typically a dedicated alternator with a battery backup, so that the failure of the aircraft's electrical system is not the failure of the engine.

What the exam asks

The definitions (normally aspirated, critical altitude, full-throttle height) are recall; the waste-gate logic, the lever order and the FADEC redundancy reasoning are understanding marks. The overview, following the learning objectives:

Describe the effect of ambient pressure, back pressure, temperature, density altitude and humidity on power

Explain 'normally aspirated' and the requirement for power augmentation

Describe the turbine, compressor, waste gate and waste-gate actuator

Explain altitude-boosted versus ground-boosted turbochargers

Define critical altitude, full-throttle height and rated altitude

Explain turbo lag and the function of the intercooler

Explain the supercharger's purpose and its differences from the turbocharger

State the correct order of engine controls when increasing or decreasing power

Define take-off power and maximum continuous power

Describe the start problems of extreme cold weather

Describe FADEC: the difference from manual controls, the flight-deck controls, its full authority with failure protection, and its redundancy in power and data