Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
NM-08-24
May 22, 2008
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin alerts you of an
airworthiness concern on all Boeing Model 737 airplanes relating to the
potential failure of certain potable water couplers during freezing
weather conditions. The failure of the couplers could result in water
spraying and freezing around the outflow valve, rendering the valve
inoperative. If enough ice forms, the performance of the outflow valve
will be very limited. This could result in difficulty controlling cabin
pressure.
Background
We have received reports of several instances of potable water
systems leaking due to damaged couplers around the potable water
storage tank. These instances occurred during winter seasons. When the
potable water tank is not properly drained during overnight parking in
below freezing temperatures, the potable water lines can freeze. The
frozen water expands and breaks the couplers. During flight, the area
around the couplers and water lines remains above freezing (35-40°
F). Therefore, the freezing is taking place on the ground with
long-term exposure to sub-freezing temperatures.
Consequently, during morning flights, the pressurized potable water
lines spray water, which creates ice around the outflow valve. If
enough ice forms, the performance of the outflow valve will be very
limited. This could result in difficulty controlling cabin pressure.
Recommendations
We highly recommend that operators adhere to Boeing Aircraft
Maintenance Manuals (AMM), specifically 737NG AMM 12-33-01 and 737NG
AMM 12-33-02. These sections provide instructions on draining the
potable water tanks if the airplane is to be grounded overnight during
freezing conditions.
For Further Information Contact
Nick Wilson, Aerospace Engineer, 1601 Lind Avenue SW, Renton, WA
98057-3356; phone: (425) 917-6476; fax: (425) 917-6590; email:
nicholas.wilson@faa.gov.
For Related Service Information Contact
The Boeing Company, P.O. Box 3707, Seattle, WA 98124, or through
the following website:
https://www.myboeingfleet.com/ReverseProxy/Authentication.html