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View Full Version : B747 Classic - Standby Ignition vs Captns Hdg info.


Flight Detent
11th Feb 2002, 13:12
Hi Guys,. .I've been searching around for some time regarding this query -. .Why is it, on the Classic B747, powered by the P&W engines, when the Stby Ign. is selected by the FE, the Captn. loses all his heading info, BUT, on the RR powered ones, he does not?. .Thanks for any info! <img src="confused.gif" border="0">

spannersatcx
11th Feb 2002, 19:09
If you look at the CB for STBY IGN (1) it is also labelled underneath MHRS (magnetic heading reference system) as the 2 are powered through the same cb at a guess when STBY IGN is selected there will be insufficient power for the MHRS.. .I don't think the CB is exclusive to JT9 powered a/c it's on the CF6 ones I deal with and I'm pretty certain it is on the RR, maybe it's just the way the JT9 (old a/c!) are wired.

Hope that helps <img src="wink.gif" border="0">

18-Wheeler
11th Feb 2002, 21:12
Both the P&W and GE's lose the Captain's heading information when going to standby ignition.. .I hear on the grape vine that in the early days of the Rollers, Cathay told Boeing to make them so that this would not happen, and it stayed that way for all Roller powered 747's after that.

mustafagander
12th Feb 2002, 14:39
As I recall it, the stby ign on P&W exceeds the amp hr rate available from the battery, but RR is just below it. I vaguely remember that P&W was 20 joules and RR 4 joules.

Flight Detent
13th Feb 2002, 15:59
Hi all,. .Thanks for that, yes, I know the P&W and RR are wired differently, and therefore breakered differently, but I just didn't know why.. .The high and low joules values are ground and flight start for the P&W and GE(some), but the RR changes over from high to low automatically at 38% N3.. .Anyway, thanks again!. .(tks 18wr)

18-Wheeler
14th Feb 2002, 23:17
Doing my last CGK - JED - CGK (now in JED as I type this) then off to Bali for the break, Flight Detent.. .Cop ya there perhaps?

Flight Detent
16th Feb 2002, 13:43
re: 18-wheeler - na, stayin in KL!. .Cheers.

QAVION
20th Feb 2002, 05:31
Seem to recall that this relationship caused a considerable delay on one of our 747 RR aircraft transitting through an airport somewhere in the South Pacific. At some time during the preflight, the skipper found that his navigation displays weren't working properly, so he called for the engineers. The engineers were scratching their heads for some time until one of them noticed that the Stby Ign Switch was in the wrong position. Apparently the F/O had left it in the wrong position after he had done his preflight checks (grin).

Now, apart from the Stby Power switch, it's the first thing we look at if the Captain's instruments aren't working properly.

Cheers.. .Q.

Flight Detent
21st Feb 2002, 11:03
Hi Q,. .I'm just wondering what the FO is doing with the Stby Ign switch?. .Cheers, see ya!

Flight Detent
21st Feb 2002, 11:09
Hi again Q,. .having read your post again several times, you must be flying for AP, if so, I'm again confused because those RR 238's hdg info, is as I mentioned earlier, non responsive to the position of the Stby Ign sw, I've operated those acft., I remember! ??

QAVION
22nd Feb 2002, 13:31
"you must be flying for AP, if so, I'm again confused because those RR 238's hdg info, is as I mentioned earlier, non responsive to the position of the Stby Ign sw, I've operated those acft., I remember! ?? "

As Maxwell Smart would say.... "Several possibilities" here, Flight Detent... <img src="smile.gif" border="0">

1. My recollection of an incident which someone once told me about when I was still bright-eyed and bushy-tailed is not so good (Fortunately I wasn't there at the time). I've been in the same company for 27+ years.

2. Because this was such a l-o-n-g time ago (well before AP had any of our RR 238's), the aircraft in question may have had a combined StbyIgnMHRS CB (just above the F/E's drawer). AP probably has two CB's (being a later model) and the result of selecting Stby Ignition may have a different effect.

2. The finger was only pointed at the F/O, because he was the closest at the time :)

4. I don't fly, so I may have overstepped my mark commenting on pilot procedures <img src="eek.gif" border="0">

Hope this helps/clarifies things?

Rgds.. .Q.

[ 22 February 2002: Message edited by: QAVION ]</p>