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Cold Weather Operations

Boeing, 737-8Q8
(SDR) # 20130123003

While the aeroplane was climbing through 35 000 feet, the crew found the pressurization to be fluctuating and the outflow valve to be unresponsive. During a descent to see if the issue would be resolved, the outflow valve went to full open and the aeroplane rapidly depressurized. The crew initiated a rapid decent and oxygen was deployed for the crew and passengers. The aeroplane returned to its origin and landed without any further issues where there were no reported injuries to crew or passengers.

After arrival, maintenance inspected the aeroplane and immediately found a substantial amount of ice around and in the outflow valve. This ice would have restricted the movement and control of the valve. Including the outflow valve, additional ice was found around the drains and tail skid. Further inspection revealed that a potable water 4-way coupling had released in the aft section of the aeroplane causing water to be sprayed directly onto the outflow valve from the inside of the aeroplane, as seen in the attached figure.

The water coupling was re-secured for correct installation and all ice accumulations and obstructions were removed, making the aeroplane serviceable.

Ice accumulation in the outflow valve of a Boeing 737.

Transport Canada Comments:  

As stated by the operator, the reason for the 4-way coupling failure was due to a frozen water line where upon review of this event, it was discovered that Boeing had published fleet team article 737NG-FTD-38-07001. 

To summarize this article, Boeing has determined that operator diligence towards ground handling in cold weather operations in reference to Aircraft Maintenance Manual (AMM) 12-33-01/02 is essential to prevent such events. Also stated is that Boeing Service Bulletin 737-38-1057 is available for the addition of shroud/drains and heaters for the 4-way coupling along with the replacement of various solid tubing with either flexible non-heated or heated hoses.

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