View Full Version : 737 APU fire warning horn
With battery power only does the APU fire warning horn sound? Had a sim with total power failure and seem to remember a simulated faint horn. What is the rationale behind this?
hetfield
28th Sep 2010, 10:41
Don't know 737.
On 727 and A300/310 on bat only, no horn just the red light.
Jetro6UL
28th Sep 2010, 12:23
Yes, IRS on DC warning horn (on the ground).
de facto
28th Sep 2010, 13:49
yes the IRS on Dc horn on the ground.
I guess thats why if you do a battery start , the supplementary procedure is to align the IRS AFTER the engines are started and gens ON bus.
I guess thats why if you do a battery start - actually it is because you cannot align the IRS on DC regardless of the horn!
de facto
28th Sep 2010, 14:24
Obviously that is the reason but if one put the irs on align then start the engines all it does is getting a horn. Thats what i meant...
OK..my take on this one is...
The APU fire detection system is powered by the battery bus, and the APU fire extinguishing system is powered by the hot battery bus.
Therefore my answer to you is yes.
EW73
J31 MAN
29th Sep 2010, 09:25
The warning horn you hear is the ground crew call horn NOT the APU fire warning. It is in the nose landing gear bay and is to warn that the ADIRU's are on DC power. For the NG at least.
Thanks for the answers so far...so are we saying Boeing went to the trouble of utilising an APU fire warning horn....in the wheel well to let us know we can't align the IRS.
It obviously does the job.....also tells the entire airport that the guy doing the cockpit checks has got his check sequence wrong:O
Just curious as to why they wouldn't consider the on DC light enough of a warning.
Is EW73 onto something? The "Therefore my answer to you is yes" has thrown me :confused:
J31 MAN
29th Sep 2010, 09:41
The APU fire warning horn has got nothing to do with all of this. It is in the MAIN wheel well. The ground crew horn in the nose wheel well is the one you heard. It sounds automatically if the ADIRU's are left on battery power i.e when you had power failure and you were on the ground, the horn comes on after 20 seconds.
Loss of equipment cooling on the ground also brings the warning horn on.
Gas Bags
29th Sep 2010, 09:53
RMC,
The info is in the previous posts combined.
There are two seperate horns. The APU fire warning horn in the R/H MLG wheel well, and the ground crew call horn in the NLG wheel well.
The APU fire warning comes off the battery bus. This is why when you start a cold ship you can carry out an APU fire test with the battery switch selected on (the APU fire warning horn in the R/H MLG wheel well will sound during this test).
I believe the warning you are referring to during your sim session came from the ground crew warning horn in the NLG wheel well, or you would have had the associated fire anunciator and master caution lights come on in the cockpit as well as having heard the horn.
A power failure with the IRS's aligned will cause the warning horn in the NLG wheel well to sound after a time delay.
Hope this clarifies for you.
GB
Oops, sorry J31 posted as I was typing....he pretty much sums it up.
AH! Those fun days of waiting until F/O was in the stbd wheel-well and .....................:)
Just curious as to why they wouldn't consider the on DC light enough of a warning. - indeed it is to those awake, but the warning is to let anyone outside know the AC or cooling has dropped off line. It is not very obvious inside the cockpit.
leaving the FO down there for a sector to find out if it does actually come on at FL360 - greetings fb - now, that's an idea worth developing..............the 411A style of crewing:ok:
Spendid Cruiser
1st Oct 2010, 03:25
RMC,
Had the same the other day in the aircraft. We were sans APU and the GPU just died. The horn sounds immediately. It isn't anything to do with the APU. I often hear that alarm when doing a walk around and an adjacent aircraft is doing a fire test and is a completely different noise from the wheel well.
The faint beeping you can hear from the flight deck is am external warning to the ground engineer that the aircraft is on battery power and that the battery is being depleted. I don't even think there is any specific relation to the IRS.
IFixPlanes
1st Oct 2010, 06:27
...We were sans APU and the GPU just died. The horn sounds immediately. ... immediately??? :=
The horn (in conjunction with ADIRUs on DC) have a time delay of 20 seconds. :E
J31 MAN
1st Oct 2010, 09:08
It's all in the FCOM system description section under Equipment Cooling and Flight Management, Navigation...........