WING BODY OVERHEAT - B737 NG

[owl-attack]

Hello

I am not able to understand a few things written in the MRG book on 737 (by Pat Boone) and B737 Technical guide by Chris Brady

MRG says:

For a wing body overheat, the leak can be confirmed on the bleed air duct pressure indicator.
What does the book mean by this, What indications am i supposed to see on the bleed air duct pressure indicator if there is wing body overheat. All I know is that once the NNC is followed and respective side bleed and pack switches are selected to OFF position, the BLEED AIR DUCT PRESSURE pointer for that will drop to zero . So what does he mean by a leak can be confirmed on the bleed air duct pressure indicator?

Boeing 737 technical guide says (Chris Brady):

Wing body overheat light on the left side is more critical , because this includes ducting to the APU.

How does leak on left side make it any more serious than right side. Am I correct to say that it is more serious because of more ducting than right thus more chances of airframe damage?

Last question w.r.t the QRH:
The WING BODY OVERHEAT NNC, tells the crew to switch off the TRIM AIR switch if the wing body ovht light stays illuminated after switching all the bleed sources. How will switching off the TRIM AIR switch help?

Kindly help me with this confusion

Thanks

[NSEU]

So what does he mean by a leak can be confirmed on the bleed air duct pressure indicator?

Perhaps the writer expects you to check the duct pressure before you carry out the NNC?

How does leak on left side make it any more serious than right side. Am I correct to say that it is more serious because of more ducting than right thus more chances of airframe damage?

I'm just guessing at what he's thinking, but..
Most of the APU Bleed duct is inside the cabin (I mean in the pressurised, mostly cargo area). Pack bay and wing ducts are outside the pressurised area.

Don't worry, you're not the only one having trouble with this subject :P

http://www.pprune.org/tech-log/559086-wing-body-overheat-737-800-a.html

Cheers
NSEU

[NSEU]

P.S. If the APU is not running, a properly functioning APU check valve should stop engine air getting into the part of the APU duct which runs through the pressurised fuselage area. I assume the APU is only used in flight for special circumstances, so the additional seriousness would only be a problem on the ground or when the APU is run in the air.

I have heard of an APU duct leak on a 767 where the passengers in the aft zone on the left hand side started to get a bit warm ... but there was no cargo fire alarm, perhaps because the duct is in the cargo sidewall and the sidewall panels are taped up for additional fire protection. By this, I mean the tape provides an air seal. E.g. if there was a fire inside your house, you would close all the doors to slow down the progress of the fire (less oxygen, less smoke, etc).

[Capt Quentin McHale]

NSEU,


"but there was no cargo fire alarm"....entirely correct, because the cargo holds have/use smoke detectors, as opposed to overheat detectors on APU and wing/strut ducting etc.


McHale. :)

[NSEU]

"but there was no cargo fire alarm"....entirely correct, because the cargo holds have/use smoke detectors, as opposed to overheat detectors on APU and wing/strut ducting etc.

I'm not saying it won't happen. I'm sure there are materials in the sidewalls which would generate smoke at those kinds of temperatures... dust, paint, plastics, paper items, etc. You'd be surprised what slips past the floor decompression panels on some aircraft.

Smoke will eventually find a way past the panelling.

[Avenger]

From the Engineer information site: This is the view of a technical bod:

When you switch off the affected side bleed and pack, you make the duct on that side pressure-less. The bleedvalve on that engine closes and there is no hot airflow anymore. So even when there was a leak, there will not be any hot-air blowing towards any component. Of course, before the leak was detected, it might have damaged certain parts, but the system is designed to detect it before its too late / parts are damaged beyond safety-limits. The maintenance action on the ground will detect the leak and inspect /repair the parts which where hit by or damaged by the hot air.

An open or ruptured bleed duct is the only thing you have then, and you may continue your flight safely with it. Even if the light remains on, it may indicate a failed detection loop. (happens more than a real leak...)

? View topic - Questions from a pilot ref Wing Body Overheat (http://sjap.nl/forum/viewtopic.php?f=1&t=460)

[owl-attack]

Thank you guys