A Mechanic reports his carrier currently has a Dispatch Deviation Procedure (DDP) that addresses a crew rest compartment configuration that they don't fly on their B737-900's. An MEL for a left pack deferral discrepancy has a dispatch procedure requiring the Forward Cargo Heat Duct Shutoff valve to be manually closed. Normal operation is for valve to be always 'Open'.
Synopsis
A Mechanic reports his carrier currently has a Dispatch Deviation Procedure (DDP) that addresses a crew rest compartment configuration that they don't fly on their B737-900's. An MEL for a left pack deferral discrepancy has a dispatch procedure requiring the Forward Cargo Heat Duct Shutoff valve to be manually closed. Normal operation is for valve to be always 'Open'.
Narrative
I took a turn over from the graveyard technicians that included troubleshooting a wing to body overheat condition. The graveyard technicians had indicated that they could duplicate the problem by running both packs for 15 minutes. The graveyard technicians subsequently narrowed the problem down to a bad M275 overheat loop. I removed and replaced the loop which subsequently fixed the discrepancy.Upon review in our maintenance computer history; for ATA (Air Transport Association) Chapter 21-00 and 26-00; I found the MEL had been changed; which changed the DDP (Dispatch Deviation Procedures) procedure. I confirmed that D488 [Connector] was connected to the left Pack Flow Control valve prior to functional checks of the left pack; which were good.1) I suggest removing the DDP procedure under MEL XX-00 that requires manually closing a valve whose primary purpose is to shut off air to a Crew Rest compartment in case of smoke; reference functional description in IPC (Illustrated Parts Catalog). Furthermore; this valve had no provisions for wiring. The connector on the valve is capped. 2) Simplify/clean-up MEL XX-00. It is four pages long to cover all of our effectivities; but the DDP procedures are scattered throughout the four pages.We currently have a DDP procedure that addresses a crew rest configuration that we don't fly. I spent nearly four hours in the compartment aft of the potable water tank. Access was through the removed outflow valve. The aircraft was located on the pad. I understood from the graveyard technicians that both packs were running during their troubleshooting; but I neglected to ask if they had undone the DDP [procedure]. Additionally; after having read the history in maintenance computer I was under the impression that a different DDP was in effect that didn't require checking the forward Cargo Heat Outflow valve position.
NASA callback
Reporter stated that after replacing the M275 overheat loop for a wing to body overheat condition; he pulled the placard and cleared the MEL for the Left Pack that was originally deferred due to a wing to body overheat condition. But shortly after the B737-900 departed; the flight crew reported getting an Equipment Cooling Overheat and Cabin Differential Pressure issues; but only after the aircraft was airborne and at lower altitudes.Reporter stated that after much trouble shooting effort; he noticed a reference in their Illustrated Parts Catalog (IPC) about a Forward Cargo Heat Duct Shutoff valve in the Forward Cargo compartment. Further research showed the same valve comes from Boeing with the electrical connector capped and stowed; no electrical connection to the valve and the valve is always 'open' with air dumping from the duct into the Equipment Cooling Exhaust Plenum.Reporter stated that because the DDP required the valve to be 'manually' closed when the Left Pack was deferred; under the assumption the aircraft had a crew rest compartment configuration; in order to shut-off smoke from going into the crew rest compartment; he thought the graveyard mechanics had undone the DDP procedure - they had not. Reporter stated when he cleared the MEL for the left Pack; he should have checked the DDP procedures andverified the Forward Cargo Heat Duct Shutoff valve was in the normal 'Open/ position. Reporter stated that because their B737-900's do not have Crew Rest Compartments in the Forward Cargo pits; the DDP procedure to 'Close' the Cargo Heat Shutoff valve is unnecessary and the DDP Manual should be changed.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.