There is a danger that comes with all this, the thing every pilot has heard named and few have had explained cleanly: surge. To see it, remember what the compressor is really doing — it is pushing air the wrong way. Air left alone runs from high pressure to low, the way water runs downhill; the compressor spends its whole life forcing air the other way — uphill, from the low pressure at the mouth toward the great pressure piled up at the back. You can only drive air uphill like that by pouring energy into it, just as you must to pump water up to a tank, and pouring it in is the blades' work. The engine's compressor is that pump. Each rotor blade is a little wing, mounted on a spinning disc. As it sweeps through the air it pushes the air along — speeding it up and setting it spinning — and that is the energy going in. The fixed vanes behind then slow that fast air down, and the slowing turns the speed into pressure, just as we saw. The energy is paid for, by way of the shaft and the turbine, out of the fire — and the blades grip and turn the air to spend it just as a wing grips and turns the air flowing over it. And a wing, as you know better than most, has a limit: hold it at too steep an angle to the oncoming air and the flow can no longer cling to it — it breaks away, and the wing stalls.
The compressor's blades are no different. So long as the air meets them at the angle they were shaped for, all is well. But let that angle go wrong — and it goes wrong whenever the engine is pushed far from the trimmed, steady cruise it likes — and the air breaks away from the blades just as it breaks from a stalling wing. One blade losing its grip is nothing. But the stall can spread, row to row, until the whole compressor lets go at once; and the instant it does, nothing is holding back that two hundred pounds crammed into the rear. The air does what air always does — it bolts for the low pressure. It blows forward, back out through the compressor and the mouth, against the entire flow of the engine, with a bang you can hear in the cabin and sometimes a sheet of flame from the intake. The thrust collapses. That is a surge: not an explosion, but the engine's careful one-way river suddenly running backwards.
Two words get muddled here, so let me separate them. A stall is the local trouble — the air breaking away from the blades of a stage or two, the compressor still turning but working badly. A surge is the whole compressor letting go at once, the flow flooding backward with the bang. The stall is the seed; the surge is what it grows into when it spreads — though pilots tend to call the whole event a "compressor stall," which is close enough.
Picture a crowd trying to get into a concert, funnelled through a single turnstile into a hall that is already packed. At first, people push gently to get moving and they file through. But they get impatient and want to get in fast. They push too hard, cram the hall too full, and a panic sets in: the crowd turns around and bolts back the way it came: the main entrance, the obvious and natural path. The people at the turnstile, who were pushing forward a moment before, are stopped dead and shoved backward. A compressor pushed past what its blades can hold lets go in exactly that way: the orderly forward stream breaks, and the whole flow stampedes back out the front.
The engineers tame it in two ways, both worth knowing because you fly behind them every day. The first is to make the front of the high-pressure compressor adjustable: on the GEnx the first four stator rows can pivot on their own axis — like the slats of a blind, and never spinning round the shaft — so they keep presenting the air to the blades behind them at a tolerable angle whatever the engine's speed, holding those blades clear of the stall. The rows further back stay fixed. The second is a ring of bleed valves between the low-pressure and high-pressure compressors. At low engine speeds and during quick throttle changes, the low-pressure stages up front can pump more air than the core behind them can swallow; the surplus stacks up and back-pressures those front stages, and left alone it would stall them. So the valves open and spill the surplus sideways into the bypass, keeping the air moving cleanly instead of piling up, breaking away, and trying to reverse back out the front.