Autothrottle — Page 401, Lesson 494

Let's start with the autothrottle disengage light, because that's the first thing on this page, and it's a classic exam favourite. The A/T Disengage light — A/T is short for autothrottle — will flash red if the autothrottle disengages for any reason. So the moment the system drops out, you get a flashing red warning. That's your immediate cue that the autothrottle is no longer controlling thrust. Now, that same light has a test function. The Disengage Light Test Switch has two positions. If you hold the switch in position 2, which is the red filament test position, the light illuminates steady red. If you hold it in position 1, the amber filament test position, it illuminates steady amber. So the switch lets you check both filaments of the light — red and amber — independently. The amber flashing is where it gets interesting, because it's not just a test. A flashing amber A/T Disengage light indicates an A/T airspeed error — meaning the autothrottle is not holding the speed you commanded. Specifically, it flashes if the speed is not held within plus 10 or minus 5 knots of the commanded speed. Note that asymmetry: plus 10, minus 5. And this only applies when all of the following conditions exist simultaneously: you're in flight, the flaps are not up, and the A/T is engaged in either MCP SPD or FMC SPD mode. MCP is the Mode Control Panel, SPD is speed mode, and FMC is the Flight Management Computer. So if you're in flight, flaps out, and the autothrottle is in a speed mode, and the speed drifts outside that plus 10 minus 5 window — you get the flashing amber. There's also an automatic test of that flashing amber function. It runs if the A/T is engaged and three conditions hold: MCP SPD or FMC SPD is the active A/T mode, more than 150 seconds have passed since lift-off, and the flaps are extended. When that test runs, the amber light flashes for 2 seconds, stays extinguished for 2 seconds, then flashes for 2 seconds again. So you see that distinct flash-off-flash pattern — that's the self-test confirming the amber warning circuit works. Now let's move to the Thrust Mode Annunciator panel, the TMA. On the Boeing 737-400, the TMA is located on the centre instrument panel, above the N1 rpm indicators. N1 is the fan speed of the engine, expressed as a percentage of maximum. The TMA displays the active N1 limit reference mode for both autothrottle and manual thrust control. So it tells you which N1 limit is currently governing your thrust. Those N1 limits are also displayed on the N1 rpm indicator cursors, with the reference knobs pushed in. Normally, N1 limits are calculated by the FMC — the Flight Management Computer. But if the FMC N1 limit calculations become invalid, or if either engine's N1 is less than 18%, then the annunciator shows A/T LIM. When that happens, the autothrottle computer takes over and calculates a single N1 limit for the affected engine or engines. So the TMA is telling you who's in charge of the limit — the FMC normally, but the autothrottle computer in the A/T LIM case. That's the whole page: the disengage light with its red and amber filaments and the airspeed error logic, then the TMA and the N1 limit source switching.