"The detection system on the 747 ( and from mono's post seemingly the 767 and 757) relies on bleed air flow to draw in air from the cargo compartment for sampling. What if for some reason this bleed air flow failed or had to be switched off e.g. if you have a centre bleed duct leak on a -400 you have to isolate that portion of the duct and as a result you lose the bleed air flow. What would be your actions now that you no longer have any cargo fire protection ?"
Loss of vacuum is sensed by a pressure switch and an Advisory message "}CARGO DET AIR" and an associated Status message is generated on your EICAS displays.
If you had these messages pop up on departure (and the switch was proven to be operating correctly), you would not be allowed to proceed unless your cargo bays were empty.
As for what to do in the air.... I guess this is the part where you start earning your money
It would probably come down to probabilities. What are the odds of you having to isolate the Centre manifold
and having a cargo fire. Come to think of it, having to isolate the Centre manifold is already pretty serious, isn't it?
Any help?
Cheers.
Q.