Autothrottle — Page 401, Lesson 494

I want to walk you through the autothrottle annunciation system on the Boeing 737-400. We're starting with the A/T Disengage light, which is your primary warning that the autothrottle has dropped out. Here's the key distinction you need to remember: the light has two colors and two behaviors. If the A/T disengages for any reason, the light flashes red. That's your immediate, unmistakable warning that the autothrottle is no longer controlling thrust. But the light can also show a steady red or a steady amber, and those come from the test switch. When you hold the Disengage Light Test Switch in position 2, which is the red filament test position, the light illuminates steady red. When you hold it in position 1, the amber filament test position, it illuminates steady amber. So the test switch lets you verify both filaments are working. Now, the flashing amber is a completely different message. Flashing amber indicates an A/T airspeed error — meaning the autothrottle is failing to hold the speed you commanded. Specifically, it flashes amber if the speed is not held within plus 10 or minus 5 knots of the commanded speed. Note that tolerance: you can be 10 knots fast, but only 5 knots slow before it warns you. 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 SPD means the speed is set on the Mode Control Panel; FMC SPD means the speed is commanded by the Flight Management Computer. So if you're in flight with flaps out and the autothrottle is holding a selected speed, and it drifts outside that plus 10 minus 5 window, you get the flashing amber. There's also an automatic self-test of this flashing amber function. It runs if the A/T is engaged and three conditions are met: 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 can recognize that test pattern — it's a specific on-off-on sequence. Now let's move to the Thrust Mode Annunciator panel, the TMA. On the 737-400, this panel sits 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. In other words, it tells you which source is currently governing the maximum N1 you're allowed to use. The N1 limits are also shown on the N1 rpm indicator cursors, but only when the reference knobs are pushed in. Normally, the N1 limits are calculated by the FMC, the Flight Management Computer. But there's a failure logic you need to know. When the FMC's N1 limit calculations become invalid, or if either engine's N1 is less than 18%, the TMA annunciates A/T LIM. At that point, the autothrottle computer takes over and calculates a single N1 limit for the affected engine or engines. So the annunciation A/T LIM is your cue that the FMC has stepped aside and the autothrottle computer is now setting the limit. Let me tie that together. You have two annunciations to read: the A/T Disengage light, which tells you about engagement and speed-holding, and the TMA, which tells you which computer is setting your N1 limit. Red flashing means disengaged. Flashing amber means an airspeed error outside plus 10 minus 5 knots, under those specific flight conditions. And A/T LIM on the TMA means the FMC's N1 calculation is invalid or an engine is below 18% N1, so the autothrottle computer is now in charge of the limit.