744 Ignition operation?
 

Greetings all
Would anybody care to offer info on this, please?
Event button print-out with continuous ignition switched ON showed only two of the four ignitors operating. Doing a similar check with nacelle anti-ice selected ON gave similar results. Does anybody know what the EIU memory shot is tracking? Is it the physical operation of the ignitors- (unlikely, since the MM check for operation is to LISTEN for the cracker boxes) or is it just looking for the logic triggers being made/unmade? Additionally, is an event record so instantaneous as to miss the ignitors if they were being flip-flopped or sequenced in some other way?

This was triggered by a query from one of our crews and I can't find anything directly relevant in the MM.
Thanks in advance. http://www.pprune.org/ubb/NonCGI/confused.gif

"Event button print-out with continuous ignition switched ON showed only two of the four ignitors operating."

Most likely, Cornish, the EIU's will only know what positions the ignition control relays (which control the ignitor boxes and, ultimately, the ignitors) and cockpit switch(es) are in. If you have a copy of the Boeing Schematics, for example Chapter 74-31-0x p101 Sheet2 (where x is the engine), this will show some of the ignition inputs to the EIU's. In this diagram you will see that an additional contact on the ignition relay is used to send an ignition status signal to the EIU. As you suspected, the EIU doesn't really know if the ignitors are actually working or not (and an audible check is required)... It's only "guessing" that the other relay contacts and the ignitor boxes are operating normally (i.e until the engine flames out(grin).

The relays are either full on or full off, so there is no cycling of relays which would cause your strange(?) Event recording. I suspect it is an EIU software thing rather than a malfunction of ignition ccts(Sorry, I don't know if this is normal for all 747-400 airlines/engine configs).

But... I hope this helps.
Cheers.
Q.

QAVION
Many thanks. I'd just about come to that conclusion but it's nice to have confirmation. Sorry to be a pain, but do you have any info on what ACTUALLY happens to the ignitors when they are selected on/automatically on for flaps/icing etc? As I understand it, the internal pressures are too high to allow actual spark production until there is a flame-out - then they start to work properly. Do you know if this is correct?
Thanks in advance.

"As I understand it, the internal pressures are too high to allow actual spark production until there is a flame-out - then they start to work properly. Do you know if this is correct?"

Sorry, Cornish...
My training really didn't go that deeply into this kind of stuff. If a car's spark plug doesn't have any difficulties discharging in a combustion chamber, I wouldn't have thought that a 10 joule spark would have had any difficulties(???).

According to my notes, at takeoff thrust, the max px in an RB211 core is a mere 425psia. Perhaps it's not the spark that is the problem here, but the way the fuel reacts with the spark at these pressures/air velocities (just guessing here!).

Cheers.
Q.

Q
Again many thanks. I like the car analogy - afraid my 'past their shelf life' brain cells hadn't taken the obvious parallel !! http://www.pprune.org/ubb/NonCGI/redface.gif
Time to put the anorak away again. :)

CJ, your desire to dig deep to find the answers proves why you're such a good groundschool instructor... I look forward to my next tech. refresher as always.

For Qavion (and any other 744 operators), we have finally nailed the reason for the apparent anomalies with the event record print out. The EIU normally only looks at the #1 ignitors and reports on them. This means that if an EEC has changed the operating ignitor to #2 (flip-flopped on each successive start) that engine will show as ignitor OFF. However, if the stand-by selector or the auto-ignition selector is placed to a specific ignitor, that one will be monitored and reported on in the event record.
Glad that one's finally put to bed !! :)

Imagine if this had been in the log book.....

"Imagine if this had been in the log book....."

"Checked OK on ground test" :)

Thanks CJ.

Cheers.
Q.

The chances of this problem happening again are inversely proportional to the time it took to find the solution