Regular spark plug changes as a set - along with the harness.

They had to be cleaned, gaps set and then individually tested for leakage and operation on a special test rig. The ignition system was critical in getting the required power output and smooth running. Did you ever notice piston engined aeroplanes turning into wind and running engines at full power before entering the runway? The pilot would set high power and then switch off each magneto in turn to see if there was an RPM drop ("Mag Drop"). If there was, the aircraft doesn't fly - it goes back to the flight line to have an ignition system change.

megan (#31),

Sorry for the delay in this reply - but thank you for your full explanation !

Danny.

If the aircraft doesn't have a mag drop it means something is wrong, equally if the drop is too large there is something wrong. The permissible drop is spelled out in the pilot notes. For the particular version of Merlin I checked, the drop is to be no more than 150RPM.

The Lancaster is airborne, presumably on an air test... :ok:

Scheduled to appear at the Scampton Air Show today, together with 4 Spitfires (the latter may be a tad optimistic).

Before the mag drop check at high power, a check would be made at low power, which involved switching both mags off together and straight back on again. The engine should cut out briefly, and pick up again. If it does not, you have a live mag. This was done both before and after a flight - if a mag was live, the engine could fire when the groundcrew were turning the engine over by hand, with nasty results. (the groundcrew on Shackleton Squadrons liked to line up the props- looked nicer.) Live mags were rare- only saw one in 2500 hours on Shacks.

Long time ago but I seem to remember the the mag. check was carried out at zero boost, before the rpm were up in the propellor constant speed range. An rpm drop was essential but not to exceed 50. If there was no drop you knew that you had a live mag. and you knew which one. (Banter) see your 2500 hours on Shacks Oxenos and raise you 800 on Lancs as well!

O.K. I Fold

Saw the Lancaster flying today at Scampton.

Accompanied by any Spitfires ?

Hurricane and Griffon Spitfire apparently.