744 RB211 Fire Loops - Time Delay?

[N1 Vibes]

Does anybody know if Boeing built in a delay to the Fire indication system on the 744 for the RR application?

I.e. for a full fire indication (whistles and bells) can there be a delay of X seconds between the Loop A discrete and the Loop B discrete setting the Fire indication before annunciation?

And also for a fault indication is there a buffer of X seconds that a fault has to be present before a Fault message is set?

Many thanks,

N1 Vibes

[spannersatKL]

To my knowledge no buffer would need to look at the diagrams before being certain. In Dual loop mode both loop detect then signal to MAWEA and bells/red lights.

Also in this case it isnt a 'fault' its a fire....its real.

In case of a failed loop there is a Status mesage set for example
ENG 1 LOOP B.

In my experience these only set when you test the system (this is done on a daily check using the overhead buttons) and find a failed loop.

It should also be noted that the RB211 has a Turbine overheat system linked to the Fire Warning system.

[N1 Vibes]

cheers spanners,

finally got the answer from Boeing:

A fire detected on one loop is annunciated as a 'full fire', as soon as a fire OR fault is detected on 2nd loop

There is a 20 second buffer in Loop Fault indications.

Many thanks,

N1 Vibes

[SMOC]

N1 Vibes,

So have I got this right,

If you get a fault in one loop and within 20 sec get a fault in the second, this will result in a FIRE ENG _

Also

If you get a Fault in one loop and and 30 sec later get a fault in the other, you will get >DET FIRE/OHT _

Cheers

[banana head]

If you get a fault in one loop and within 20 sec get a fault in the second, this will result in a FIRE ENG _

No - You need an actually FIRE sensed in the first loop, then either a FIRE or a LOOP FAIL sensed in the second loop within 20sec. This is to guard against a LOOP FAIL due damage at the point of Engine Failure.

[N1 Vibes]

smoc/bh,

we initially thought it was 2 loop faults within 20 seconds. But then with some experimenting on a good engine proved bananaheads correct knowledge. It seems though that we had a very interesting defect, to get a full ENG FIRE (red) we needed a chafe through both loops at the same place going to ground.

Interestingly we actually had the softwire of one loop chafing on the connector screw of the other loop - so we thought this was our defect. Utnil we realised the softwire was 'unshieled' at the point of chafing, and we actually had a 2nd chafe in a completely different part of the system that then took both loops to ground.

A precautionary tale - never assume you've nailed an avionics defect when you find ony ONE chafe!

Regards,

N1 VIbes