What a compressor wants
The compressor is a stack of spinning blade rows that push air into a space of ever higher pressure. Each blade works like a small wing: it only does its job while the air meets it at a modest angle. Ask a blade row to push against too much pressure, or feed it air at the wrong angle, and the flow breaks away from the blades, just as a wing stalls when its nose is pulled up too far.
Stall and surge
A compressor stall is that breakdown on part of the compressor: a pocket of stalled flow on a few blades, which may spin around the annulus. It costs pressure and raises the temperature, and it can grow. A compressor surge is the whole compressor giving up at once. It can no longer hold back the high-pressure air behind it, so that air rushes backwards out of the front of the engine. The combustor's fire goes with it, which is the flame people see spitting out of the inlet or the exhaust, and the noise is a loud bang. The pressure then drops, the compressor recovers, builds pressure again and, if nothing has changed, surges again: a cycle of bangs a few times a second.
Common causes:
- Too much fuel too quickly: the combustor's pressure rises faster than the compressor can speed up to match it.
- Disturbed air at the inlet: a strong crosswind, a bird, gun gas, or flying at a steep angle to the airflow.
- Worn, dirty or damaged blades, which stall sooner than clean ones.
Surge margin, on a compressor map
Engineers draw a compressor map: pressure ratio against airflow, with one curve for each rotor speed. Above a certain line, the surge line, the compressor cannot hold the flow. The engine normally runs a safe distance below it, and that distance is the surge margin. Accelerating is the risky moment: fuel arrives instantly, but the heavy rotor takes seconds to speed up, so for a while the pressure is high for the speed and the margin shrinks. Real engines protect it with bleed valves that dump air at low speed, variable stator vanes at the front of the compressor, and a fuel computer that limits how fast fuel is added (the FADEC guide covers that last part).
Flameout
A flameout is the combustor's fire going out while fuel is still meant to be burning. A flame only survives within a range of fuel-to-air mixtures: too lean (too little fuel for the air) and it blows out, too rich and it is smothered. Engines flame out when the fuel supply is interrupted, when a sudden throttle chop leans the mixture, when the engine swallows heavy rain, ice or ash, or after a surge has blown the flame out of the combustor. In 1982 a British Airways Boeing 747 lost all four engines after flying through volcanic ash over Indonesia; the crew glided down and relit them once out of the cloud.
In flight, the air rushing through the engine keeps the rotor windmilling, which helps a relight: the crew switches the igniters on and the fire usually catches again. On the ground, or at low speed, the starter has to spin the engine up first.
Surge and flameout in the game
The game runs the compressor on its own map and surges it when the operating point crosses the surge line. During a surge the airflow drops to about a third, the combustor pressure roughly halves, thrust collapses and the exhaust temperature climbs. Every bang costs the compressor blades a little life, and a surge that goes on long enough breaks them (the Fatigued challenge). The engine recovers once the operating point has dropped back well below the line for most of a second.
The SURGE MARGIN reading on the instrument panel shows how close you are, and a Low surge margin warning appears below 8 %. With FADEC on, the engine computer never lets the fuel push the engine closer than about 10 % from the surge line, and it cuts the fuel to the minimum during a surge. With FADEC off, the throttle sets the fuel flow directly, so slamming it open is the quickest way to hear a surge.
The game's engine flames out when the mixture goes too lean or too rich, or when the rotor drops below about 12 % speed with the fire lit. Start the engine again to relight it.
Things to try
- Turn FADEC off on the free Training turbojet and slam the throttle from idle to full. Watch the surge margin go negative and listen for the bangs, then pull back before the blades give up.
- With FADEC on, do the same slam and watch how the engine computer feeds the fuel in instead.
- In the Design office, give the compressor fewer stages for the same pressure ratio. Each stage then works harder, and the surge margin shrinks.