B737 Igniters
 

Hello Ladies and Gents.

I always thought until now that in the case of a loss of both engines, after applying the memo items and setting the Start Switches to Flight you would get dual Igniter sparking in the engines while trying to recover the situation.
Though, as the L - Igniters are powered by the respective AC transfer Bus these will not be powered as there will not be any AC available from the engines (unless of course APU is available). Only the R - Igniter will be available through the inverter (AC SBY Bus). Is this correct?
So putting the Start Switches to Flight or selecting the R - Igniter with Start Switches in CONT has the same effect?

Theoretically, yes...

However, moving the start switches to FLT is easier to use (two switches being used instead of three), easier to remember as well as you have to consider the fact that if you start the memory items quite soon and obviously depending on the type of engine failure, you will probably still have both generators supplying the electrical power to all the busses, so relight with 2 ignitors is much more likely than with a single one.

Additionally, 737 can depart with R igniter U/S as per the MEL, whereas then the L igniter has to be wired to the AC standby bus.

I think moving the start switches to FLT is just bulletproof solution to cover many possible scenarios and to ensure the highest probability of a successful engine relight.

I think your statement is incorrect in the sense that:

• ONE IGNITOR for each engine must to be powered by DC battery power through the AC Standby Bus;
• if the RIGHT IGNITOR is not working, prior to departure aircraft maintenance procedures MUST be applied to ensure LEFT IGNITOR is powered by AC Standby Bus;
• if the LEFT IGNITOR is not working, prior to dispatch no actions are required provided RIGHT IGNITOR works properly, to ensure operational integrity in this case the switch is left in the BOTH position (to avoid using LEFT).

As per FCOM Start Switch to FLT "provides ignition to both igniters when engine start lever is in IDLE", and in some cases a power source is still available through APU, eg on ETOPS configured flights. Therefore it would be unthinkable to init yourself to battery ignition by selecting RIGHT.

You are correct. I was trying to make the same point, but your writing is much more precise.