Dispatched with a deferred right engine prsov (pressure shut off valve); a B757-200 diverted to a cl...

Narrative:

The right engine prsov (pressure shutoff valve) was deferred inoperative/closed and the left engine bleed was driving the left pack (rh pack off per MEL). In cruise at fl 350 the captain felt a pressure change in his ears and asked if I felt the same. We both looked at the pressurization panel and noted that the cabin altitude was climbing at 500 feet per minute and the cabin differential pressure was decreasing. After discussing the situation we opened the isolation valve; turned on the right pack and turned off the left pack. The cabin altitude decreased to normal after having reached approximately 8000 feet. We contacted dispatch and maintenance control and explained the situation. All parties agreed a divert was the best option. At the time; there was a large line of thunderstorms in the valley area and to continue to the coast with possible loss of the remaining pack and need to descend would put us in the weather. We made an uneventful divert. Within 15 minutes of block-in; maintenance discovered the cause of our problem; a hose clamp for the left pack ducting had come loose. We were given another aircraft and continued.

Original NASA ASRS Text

Title: Dispatched with a deferred right engine PRSOV (Pressure Shut Off Valve); a B757-200 diverted to a close airport when the operative pressurization system associated with the left PRSOV malfunctioned and a large area of thunderstorms loomed enroute to their destination.

Narrative: The right engine PRSOV (Pressure Shutoff Valve) was deferred inoperative/closed and the left engine bleed was driving the left pack (RH pack off per MEL). In cruise at FL 350 the Captain felt a pressure change in his ears and asked if I felt the same. We both looked at the pressurization panel and noted that the cabin altitude was climbing at 500 feet per minute and the cabin differential pressure was decreasing. After discussing the situation we opened the isolation valve; turned on the right pack and turned off the left pack. The cabin altitude decreased to normal after having reached approximately 8000 feet. We contacted Dispatch and Maintenance Control and explained the situation. All parties agreed a divert was the best option. At the time; there was a large line of thunderstorms in the Valley area and to continue to the coast with possible loss of the remaining pack and need to descend would put us in the weather. We made an uneventful divert. Within 15 minutes of block-in; maintenance discovered the cause of our problem; a hose clamp for the left pack ducting had come loose. We were given another aircraft and continued.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	853539
Time
Date	200909
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Pneumatic System
Person 1
Function	Pilot Flying
Experience	Flight Crew Last 90 Days 240 Flight Crew Total 6400 Flight Crew Type 3960
Events
Anomaly	Aircraft Equipment Problem Critical Inflight Event / Encounter Weather / Turbulence

Dispatched with a deferred right engine prsov (pressure shut off valve); a B757-200 diverted to a close airport when the operative pressurization system associated with the left prsov malfunctioned and a large area of thunderstorms loomed enroute to their destination.