Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
CE-07-46
September 6, 2007
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin alerts you of an
airworthiness concern regarding the installation of bulkhead universal
fittings in a hydraulic pump port.
Background
The FAA received reports of leaking hydraulic fluid due to improper
installation of bulkhead universal fittings when installed in a
hydraulic pump pressure port. The bulkhead universal fittings were
turned in too far or not far enough causing the o-ring to contact the
fitting threads resulting in oring damage and failure.
While superseding standards and specifications exist, this SAIB refers
to the installation of standard parts that resulted in the reports of
leaking hydraulic fluid. The standard design installation of a bulkhead
universal fitting into a port includes specific procedures to assure
that the fitting is positioned so that the o-ring is located between,
rather than on either of the two threaded portions of the universal
fitting. These installation procedures are applicable, unless
superseded by the Instructions for Continued Airworthiness for a
specific airplane.
Standard designs for installation of a bulkhead universal fitting
(flared, flareless, and straight threaded connectors) into a port
utilize an AN6289 nut with a recess for a back up retainer for the
oring. The use of an AN924 nut should no longer be proposed as a
standard design for a new or modified installation of a bulkhead
universal fitting into a port. The use of an AN924 nut instead of an
AN6289 nut with a backup ring was initially included within the
standard design per AND10064 (for flared tube and straight threaded
connectors) for fuel and engine oil applications only. The use of an
AN924 nut on a bulkhead universal fitting installed in a port became
inactive for design in 1955 via AND10064. Refer to the attached excerpt
from AND10064.
The use of an AN924 nut instead of an AN6289 nut with a backup ring was
initially included within the standard design per MS33566 (for
flareless tube and straight threaded connectors) with nominal use
identified for aircraft engine fluid connections. The use of an AN924
nut on a bulkhead universal fitting installed in a port became inactive
for design in 1975 via MS33566. Refer to the attached excerpt from
MS33566.
Previously approved installations using an AN924 nut that have
acceptable in-service performance remain approved and remain acceptable.
While acceptable performance of the AN924 nut on a bulkhead universal
fitting installed in a port in low pressure hydraulic systems is known
to have been achieved, un-acceptable performance in medium or high
pressure systems is expected. Refer to ARP 4752 Aerospace – Design and
Installation of Commercial Transport Aircraft Hydraulic Systems and
AS4716 (R) Gland Design, ORing and Other Elastomeric Seals for
additional seal information and general rule information that orings
operating above 1500 psi should utilize backup rings.
Installations of an AN924 nut on a bulkhead universal fitting installed
in a port without acceptable inservice performance warrants review and
consideration for a design change.
MS21344 installations and MS33566 installations (with AN6289 nut with
MS28773 backup retainer) of a bulkhead universal fitting in a port is
accepted by the FAA as a standard design for fluid pressures up to 3000
psi.
Fitting design evolution continues. AS33566 retains the use of bulkhead
universal fittings with an AN6289 nut and MS28773 retainer into a port
and consequently is an accepted standard design. The FAA has also
received reports that AS5440 includes information for the design
authority to preempt the use of bulkhead universal fittings due to
their problematic service history. Refer to AS5440.
Recommendations
We recommend that you do the following:
Verify via the current parts
catalog and applicable service bulletins whether the installation
requires an AN6289 nut with MS28773 backup retainer or an AN924 nut
prior to installation. You should not substitute an AN924 nut for an
AN6289 nut with MS28773 backup retainer unless the substitution is
approved.
Install bulkhead universal
fitting in a port in accordance with the manufacturer's instructions
such as the applicable maintenance manual or service bulletin. If the
manufacturer's information does not include these instructions, refer
to the installation instructions found in MS21344 or MS33566 or
superseding SAE Aerospace Standard.
For Further Information Contact
James P. Galstad, Propulsion Aerospace Engineer, 1801 Airport Road
– Room 100, Wichita, Kansas 67209; phone: 316-946-4135; fax:
316-946-4107.