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This is the explanation on the Engine Start Switch in the 737 series FCOM. What might be the reason why the automatic activation range of the AUTO (or OFF) mode in flight limited to "between idle and 5%," rather than simply to "below idle" as in the CONT mode? Is there any good systemic reason for the limitation?

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The reason to go automatically to ignition below idle N2 is to try to avoid a flameout. If the engine has slowed down to be at or below 5% N2, it is way too slow for combustion to happen - you don’t add fuel to a normal start until about 25% N2, by comparison. So “below idle in flight” almost is a valid characterization of when AUTO will turn on the igniters, it is just that the bottom of that range stops not at zero but at a slightly higher value. Ignition isn’t going to accomplish anything between 0% and 5% N2 anyway. (Probably can’t accomplish much at 6% or 7% or etc either, but those are probably seen only briefly on the way to, or from, near zero, so the engineers picked 5% as their cutoff.)

Note also that some switches are

  • GRD-OFF-CONT-FLT

While others are

  • GRD-AUTO-CONT-FLT

So, for the switch with an “AUTO” position, there is no other “OFF” position, so there needs to be a cue to the system to turn the igniters off, rather than to let them run continuously on a shut-down motor. And rather than saying simply that OFF happens at 0.0% N2, they’ve specified a range, so that the igniters can be turned off for a shut-down but still slowly turning (windmilling N1) motor.

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  • $\begingroup$ Thanks, but for an engine shut-down, you need to pull the engine starter switch to "CUTOFF" anyway, so it makes no difference whether the AUTO (OFF) mode provides igntions below 5% or not. $\endgroup$ – lemonincider Feb 24 '18 at 11:22
  • $\begingroup$ @lemonincider That’s true, and I can’t think of a case you’d intend to leave a shut-down motor with the start lever in idle for any great length of time. Maybe it could happen inadvertently with distraction & task saturation & trying to sort out an arrival, approach, and landing. In any event, if you did, the AUTO mode would then work like OFF even with a small residual N2. $\endgroup$ – Ralph J Feb 24 '18 at 11:29

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