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For battery starting, specifically for the DHC-8 -100/-200/-300 series, there's a note in the procedure that specifies unfeathering and delaying second engine start.

The delay is specified in addition to the generator load limitations and makes reference to the FGC start-up sequence. Does anyone know what this delay accomplishes? Is there something on the secondary busses that's necessary for FGC start-up/self-test?

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  • $\begingroup$ I'm unlikely to know the answer, but others may want to know the specific aircraft/engines to which your question refers. $\endgroup$
    – RetiredATC
    Commented Mar 3 at 18:05
  • $\begingroup$ @RetiredATC Aircraft was mentioned in the tags, I edited it into the question as well. Engine should be the PW123 for -100 to -300 models. $\endgroup$
    – Bianfable
    Commented Mar 3 at 18:15
  • $\begingroup$ The note is in the -102/106/314 manuals, so at least PW120A/121/123E would be affected. $\endgroup$
    – locus
    Commented Mar 3 at 18:59

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The Flight Guidance Computers are powered with DC off the applicable Main DC buses via feeder circuit breakers to the avionics circuit breaker panels. The FGCs also get fixed frequency 115VAC from the inverters in the nose compartment, which are running on 28vdc from the Essential and Main buses.

I can't say with 100% certainty, but it is almost certain that the FGC startup BITE depends on a stable voltage from the applicable DC bus, or possibly the TRUs can be affected by input voltage transients that can affect the 400 Hz 115VAC voltage output to the FGC.

So, running the starter/generator on the other engine in starter mode will cause voltage transients from the current load of the starter/generator, and that can mess up the FGC BITE, so they want you to wait for its BITE to complete before the running engine's starter/generator is loaded up by the opposite starter/generator.

Unfeathering brings the gas generator speed, and hence the accessory gearbox speed, up to ground idle, where it can adequately support the opposite starter/generator with current.

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  • $\begingroup$ Interesting. Our procedures for battery start include waiting for the generator load to be below .4 (i.e. 40%; the generator load can be greater than 1.0 for up to 60 seconds after a battery start). It normally takes a couple minutes to get the battery charged enough for the DC gen load to get below .4, so I guess i wondered if that would be enough time for the FGC self-test/startup sequence. I appreciate your time considering the possibilities. $\endgroup$
    – locus
    Commented Mar 4 at 0:36
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    $\begingroup$ I worked on the -8 program in the early 80s and happen to have an ops manual handy to refresh my memory of the system, but this is about as far as I can go. When you have a black box and they want you to leave things alone while it does its BITE, it's almost always because of some voltage transient that affects the BITE somehow. They were probably getting faults during 2nd engine starts during testing and found if they waited it didn't happen. I suspect it has more to do with the TRU FF AC output than the DC mains. $\endgroup$
    – John K
    Commented Mar 4 at 2:31

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