Wing body overheat 737-800

[737ngpilot]

In the QRH there are 3 options for these lights turning on...one option is

Both WING-BODY OVERHEAT lights illuminated:

TRIM AIR switch . . . . . . . . . . . . . . . OFF

The other two optins are well explained in our system manual but not the two lights simultaneously ...how do both lights come on due to trim air..

[NSEU]

how do both lights come on due to trim air..

A leak/rupture of the pneumatic ducts upstream and downstream of the trim air valve could possibly flood the belly airconditioning bay with hot air (where the overheat loops are). Closing the trim air valve might help with the overheat problem if the rupture was downstream of the valve.

I stand to be corrected :P

[vapilot2004]

Closing the trim air valve might help with the overheat problem if the rupture was downstream of the valve.

...and prevent the need to shut off both bleeds, leading to a descent to MDA/FL100 and possibly landing at nearest suitable airport.

[737ngpilot]

That make sense , and came to the same conclusion myself as well...I was wondering if somebody had a more definite answer, like a schematic

[vapilot2004]

Not a schematic, but shows the flow:

BLEED SOURCE (R or L ENGINE)====BLEED AIR VALVE====PACK VALVE====CHECK VALVE====TRIM AIR PRESS MOD.VALVE AND SHUTOFF===MIX VALVE====DISTRIBUTION.

There are 3 TRIM AIR consumers:FLT DECK, FWD CABIN, REAR CABIN.

PACK VALVES are upstream of the TRIM AIR pressure modulating and shutoff valves. Pack bleed plumbing, while not entirely enclosed, is more or less sequestered within the pack machinery. Raw bleed-fed TRIM AIR plumbing, however, travels around the A/C BAY in order to get to all three post-mix manifold distribution ducts.

These 'outlying' areas are monitored by the WING BODY OVERHEAT sensing loops consisting of the L & R AC BAY loops, and further aft, the KEEL loops. Packs have their own dedicated overheat sensors.

While the above describes the flow and physical layout, I am not 100% positive that this is the whole story behind the reasoning for the procedure.

[NSEU]

I was wondering if somebody had a more definite answer, like a schematic

The pnematic/aircon "schematics" should be in one of your manuals, but it won't necessarily tell you where the components are. You would be better off with a few diagrams from the maintenance manuals.

In the maintenance manuals, there are diagrams showing the location of the overheat loops in Chapter 26. The aircon components will be shown in Chapter 21. I was using these to come up with my earlier deduction

[737ngpilot]

NSEU and vapilot 2004..
Thanks ..i have gone to our traing department with this question and they are going thru the maintenance manuals, but my impatience cause me to post the question ...

[vapilot2004]

You're doing the right thing NG

Engineers or long-time wrenchies are two reliable sources I go to when it comes to system design questions, particularly now in the era of dumbed-down manuals and need-to-know training philosophy. Our outfit is fairly small and the flight training department consists of a gal that arranges for outside training/currency checks in her spare time!

Favor? Would you mind passing the word when you have an answer you like? I am curious what the story is if different than offered here. Cheers!

[737ngpilot]

This is what I have learned so far.....The main Trim Air Valve (controlled by the Trim Air switch in the flight deck) is located in the right pack bay. From there, the hot air splits off and flows to the respective Zone Trim Air Modulating Valves.

The FWD and AFT Cabin Zone Trim Air Modulating Valves are located in the Right Pack bay, and the Flight Deck Zone Trim Air Modulating Valve is located in the Left Pack bay. By the way, all of these valves and ductings are in the front of both pack bays. The trim air ducting for the Flight Deck comes off of the main Trim Air Valve in the Right Pack Bay, passes through the center of the aircraft and into the Left Pack Bay where the Flight Deck Zone Trim Air Modulating Valve is located and attached to the duct. The area in the middle of the bays where this duct passes through is not completely isolated. The Wing Body Overheat sensor loop runs around the whole pack bay on its respective side.

[vapilot2004]

Excellent. So it sounds like my engineering sources and NSEU's books are spot on for the reasoning. Nothing magical here, it's all down to plumbing!

Thanks for the update!

[RVF750]

Occasionally I stay behind after work and if the line are doing a daily I ask to open up panels like AirCon bay doors so I can just sit and look at the plumbing.

I guess I'm either too sceptical to believe manuals or just need to see the metal in front of me to understand stuff.

You can learn a lot sat on the ramp under the aircraft!