The Electronic Engine Control
Push the thrust levers forward. That is the whole of what the crew does — a two-inch shove of two handles, the simplest gesture on the flight deck. Now consider what that gesture sets in motion. In the second it takes, something has to decide how much more fuel to spray into the fire and how fast to ramp it in, re-angle rows of compressor vanes so the airflow stays smooth, crack a bleed valve to keep the compressor from surging, and watch the turbine temperature climbing toward a limit that would ruin it — and it has to get every one of those right, together, before the engine hurts itself. The pilot asked for more push. Everything else is somebody else's problem.
That somebody is the Electronic Engine Control, and it is worth pausing on what it is up against. The engine it minds makes up to 74,100 pounds of thrust, runs hotter inside than most metals can survive, swallows several tons of air a second, and must be defended every instant against overspeed, overtemperature, compressor stall, and a dozen other ways to tear itself apart — all at once, all continuously, on every engine, every flight. No human hand could ride that in real time. This does, and never blinks. (Remember the PMA from the electrical chapter — the little alternator that feeds the EEC from its own private generator so a main-bus blackout can never dark it? This is what that was protecting.)
Call it the engine's brain, and mean it literally: every decision the engine makes flows through the EEC, and the engine does nothing without it. It is a full-authority digital engine control — full authority over every lever the engine has: fuel flow, the variable vane angles, the bleed valves, which igniter fires, the reversers, the idle speed. The crew moves one thing and states an intention — give me this much push — and the EEC does everything else required to honor it without hurting the engine. That is the bargain this whole section unpacks: one simple request in, and a torrent of precise, protective decisions out.
Two ways to run: normal mode and alternate mode
For the EEC to turn a thrust request into the right commands, it has to be fed the signals it needs — and it runs on many at once: air pressure and temperature read at several points around the engine and the airframe, the two spool speeds, the thrust-lever position, and the atmospheric picture from the airplane's air data.
It guards that last one especially hard. The outside static pressure is what lets it match a thrust request to the right N1 for the day's air, so it refuses to depend on a single source for it. Normally it reads static pressure from the airplane's air data system — but each engine also carries two static ports of its own, low on the fan cowls, an independent read for when the main system goes quiet.
As long as it is getting the signals it needs, from wherever, it runs one way: normal mode. Lose the signals it depends on and it can no longer do the full computation, so it drops to a simpler way of running: alternate mode. Those are the two modes, and the difference between them explains most of how the engine behaves when things go wrong.
Normal mode is where the engine lives on essentially every flight. Here the thrust lever does something subtler than most people assume: it does not command a fan speed. It commands a thrust — it says to the EEC, "give me this much push" — and the EEC then works out, for today's air, exactly what N1 will deliver that push. That is why the very same lever position gives a different N1 on a hot day than a cold one: the EEC is constantly adjusting N1 to hold the thrust you asked for, whatever the atmosphere is doing. Push the levers fully forward and you get maximum rated thrust and not a pound more — the EEC keeps a computed ceiling and will not let the engine climb past it no matter how hard you shove.
Alternate mode is the fallback for when those signals go missing. Strip the EEC of the air data it needs and it can no longer reliably turn a thrust request into the right N1 — so it stops trying. It falls back on a fixed, pre-set schedule where the lever angle simply is an N1 command: this lever angle equals this much N1, the same regardless of conditions. The lever now commands a fan speed directly, not a thrust. An advisory tells the crew the engine has gone to alternate, and an amber ALTN light comes up on the mode switch. It still flies, still answers the levers — it is just running off a simpler map that does not know today's air.
There is a catch in dropping onto that fixed map, and handling it is why alternate mode comes in two flavours, soft and hard. Consider what happens at the instant an engine drops into alternate mode in flight. The lever is sitting wherever the crew last set it for the thrust they wanted — but nothing guarantees that the lever angle matches the N1 the alternate mode fixed schedule assigns to it. If the engine simply snapped to the scheduled N1 the instant it reverted, the thrust could lurch up or down with nobody having touched a thing.
Soft alternate exists precisely to prevent that lurch. It is the mode you get when the EEC reverts on its own, automatically, having lost its signals. Instead of jumping to the scheduled N1, it freezes N1 essentially where it was at the moment of reversion and slides the alternate schedule in underneath — so the crew feels nothing, no surge and no sag, just the advisory and the light. The N1 it holds may not match the N1 the schedule would assign to that lever angle — but the EEC would rather hold your thrust steady than jump to the map's value. This hold lasts exactly as long as the crew leaves the levers alone.
Hard alternate is where the lever commands N1 directly, the fixed map fully in force. You get it two ways: by manually selecting ALTN on the mode switch, or the moment you move the levers after a soft reversion. At that instant the engine goes to whatever N1 the schedule assigns the lever's current position — which can be a noticeable jump if the lever was not already sitting where the schedule wanted it. The practical rule is clean and worth carrying: an automatic reversion will never surprise you, because it always begins soft; a manual selection can, so it is done deliberately, with a hand on the levers. In both flavours the FMC computes the alternate N1 targets and the autothrottle stays connected.
Now the part that matters most about alternate mode: it gives up overboost protection, and the reason follows straight from what alternate mode is. Without the atmospheric data, the EEC cannot compute the maximum rated N1 for today — and that computed maximum was the whole basis of the thrust ceiling. Not knowing where the ceiling sits, it cannot hold you below it. So in alternate mode, shoving the levers fully forward can push the engine past its rated thrust — an overboost — and a caution flags it when commanded N1 climbs past the maximum. The crew simply has to know that in alternate mode, rated thrust arrives before the levers reach the stop.
It is worth being exact about what is lost and what is not. What is lost is the rated-thrust ceiling — the guarantee that full forward cannot exceed rated. What remains, fully active, are the N1 and N2 redline protections that guard against genuine mechanical overspeed. The engine will still refuse to tear itself apart; it will simply allow itself to be pushed past its rated thrust.
The mode switch matters, so hold the distinction clearly. NORM: the levers ask for a thrust, and the EEC computes the N1 that delivers it for the day and holds you off overboost. ALTN: the levers set a fixed N1 straight off a schedule that does not know the air precisely — it still works, still answers the levers, but the protection against overboost is gone. Fly it the same way; just know the ceiling is not there.
Why does the thrust lever command a thrust in normal mode, not a fan speed?
Because what a crew actually wants is push, and push does not track a fixed fan speed from one day to the next. In normal mode the lever tells the EEC "give me this much thrust," and the EEC works out, for today's air, exactly what N1 will deliver it. That is why the same lever position gives a different N1 on a hot day than a cold one: the EEC is constantly trimming N1 to hold the thrust you asked for, whatever the atmosphere is doing. Shove the levers fully forward and you get maximum rated thrust and not a pound more — the EEC keeps a computed ceiling and will not let the engine past it, however hard you push.
What is alternate mode, and what protection is lost in it?
Alternate mode is the fallback the EEC drops to when it loses the signals it needs — chiefly the outside air data — so it can no longer turn a thrust request into the right N1. Instead of computing thrust, it falls back on a fixed schedule where the lever angle simply is an N1 command: this much lever equals this much N1, the same regardless of conditions. It still flies and still answers the levers; it just runs off a simpler map that does not know today's air. What is lost is overboost protection: without the atmospheric data the EEC cannot compute the maximum rated N1 for today, and that computed maximum was the whole basis of the thrust ceiling. So shoving the levers fully forward in alternate mode can push the engine past its rated thrust — rated thrust arrives before the levers reach the stop. What is not lost are the N1 and N2 redline protections against genuine overspeed: the engine will still refuse to tear itself apart, it will simply let itself be pushed past rated.