Hi,

 

This is related to Boeing 737s, but could apply to other aircraft of course.

 

Simple question - as per title of the thread: the engine ignition selector switch has 'Left Ign', 'Right Ign' and 'Both' options. What in fact is the correct usage, as per real world flying, when starting engines? I had once thought you switched to L Ign when starting the left engine and R Ign for the right (but then what was 'both' used for?), but that is not the case I think.

 

Googling found this (Wikipedia):

 

• Ignition Switch - Activates the magnetos by opening the grounding or 'p-lead' circuit; with the p-lead ungrounded the magneto is free to send its high-voltage output to the spark plugs. In most aircraft the ignition switch also applies power to the starter motor during engine start. In piston aircraft engines, the battery does not generate the spark for combustion. This is accomplished using devices called magnetos. Magnetos are connected to the engine by gearing. When the crankshaft turns, it turns the magnetos which mechanically generate voltage for spark. In the event of an electrical failure, the engine will continue to run. The Ignition Switch has the following positions:
  1. Off - Both magneto p-leads are connected to electrical ground. This disables both magnetos, no spark is produced.
  2. Right - The left magneto p-lead is grounded, and the right is open. This disables the left magneto and enables the right magneto only.
  3. Left - The right magneto p-lead is grounded, and the left is open. This disables the right magneto and enables the left magneto only.
  4. Both - This is the normal operating configuration, both p-leads are open, enabling both magnetos.
  5. Start - The pinion gear on the starter motor is engaged with the flywheel and the starter motor runs to turn the engine over. In most cases, only the left magneto is active (the right p-lead is grounded) due to timing differences between the magnetos at low RPMs.

OK, fine, but all instructions/checklists I've ever found suggest that 'Both' is not the normal startup position - they say select Left OR Right and leave the selector there. Is that wrong (and Wikipedia correct??). In a real 737 (etc.), will the right engine actually start with the selector set to Left Ign the whole time? According to the above, why would it?

Thanks,

 

Martin

737s don't have magnetos.

 

Piston engines normally run in the "both" position and use "R" and "L" during runup to check the health of the mags.

 

regards,

Joe

No, 737s don't have magnetos, of course! Bad example to print (from Wikipedia), but you see the gist.

 

OK, so L & R just for 'testing'; I am doing the 'right' thing in leaving the ignition on 'Both' it seems then.

 

Thanks

 

 

In a real 737 (etc.), will the right engine actually start with the selector set to Left Ign the whole time?

 

Yes, either one will start both engines. "Ign L receives power from the respective ac transfer bus, while Ign R receives power from the standby ac bus and is also used for inflight starts on standby (battery) power so this system is required for dispatch." -from Bill Bulfer's 737NG Cockpit Companion. BTW highly recommended reading. Also, quite a few airlines use an alternating policy where on, say, even numbered days, you start with Ign L and on odd numbered days, you start with Ign R. This evens out wear and tear on the ignitors. It is unusual that you would use the "Both" position.

That sounds familiar, and was probably what I was struggling to remember! I was clearly associating L & R Ign with L & R engines... Thanks for the explanation.

Also I think the FLT position on the start switch bypasses the L/BOTH/R selector and energizes both sets of igniters.

 

scott s.

.

The crew usually switches to the alternate set of igniters. If it's in the R they switch it to the L and next crew will switch to R. Helps with wear and tear and also helps identify a bad set in case of a fail to start.

thanks..

AFAIK BA uses L outbound base and R inbound

 

Cheers,