Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
NE-10-32
May 7, 2010
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) alerts you,
owners, operators, and certificated repair facilities of potential
problems associated with accessory gearbox bevel gear fractures on all
Pratt & Whitney JT8D series engines due to forward movement of the
constant speed drive (CSD) bearing bushing. At this time, the
airworthiness concern is not an unsafe condition that would warrant
airworthiness directive (AD) action under Title 14 of the Code of
Federal Regulations (14 CFR) part 39.
Background
Since 1988, we have received 31 reports of fractured CSD accessory
drive bevel gears in JT8D series engines related to forward movement of
the CSD steel bushing. Eight of the 31 fractures were uncontained by
the main gear box (MGB) and resulted in in-flight shutdowns. Three
resulted in nacelle uncontainment. None resulted in damage to the
aircraft. Due to the low energy of the uncontained part and the exit
path, no nacelle uncontainment event will result in a hazard to the
aircraft. However, with no corrective action, there is a risk of having
further in-flight shutdowns and nacelle uncontainments.
The running position of the accessory gear box drive shaft bevel gear
and the CSD accessory drive bevel gear is determined by the axial
position of the respective bearings, and the bearing axial position
relative to the respective steel bushings. The current bushing that
houses the bearing on the CSD shaft opposes the axial load that is
generated, with a small outside diameter lip and three pins. Pratt
& Whitney determined that the axial load can overcome the bushing’s
retention features, causing it to shift forward and misalign the CSD
shaft and bevel gear. This increases the stresses in the roots of the
gear teeth and causes cracking in the root. The cracks can propagate
into the web of the gear and result in fracture.
To correct the problem, Pratt & Whitney improved the retention
features in the gearbox rear housing assembly by redesigning an
aluminum repair bushing. The aluminum bushing is installed into the
gearbox rear housing assembly, and then the steel bushing is installed
into the aluminum bushing. A large shoulder feature increases the axial
load retention by 13 times that of the previous design. The new
aluminum bushing-to-gearbox rear housing assembly fit is also tighter
to increase axial retention.
Recommendation
We recommend installing the redesigned aluminum bushing using Pratt
& Whitney Alert Service Bulletin A6495, dated December 17, 2009,
when the engine or module first goes to a maintenance base that can do
these procedures regardless of the scheduled maintenance action or the
reason for engine removal.
For Further Information Contact
James Gray, Aerospace Engineer, Engine Certification Office, FAA, 12
New England Executive Park, Burlington, MA 01803; e-mail:
james.gray@faa.gov; phone: (781) 238 7742; fax: (781) 238-7199.
For Related Service Information Contact
Pratt & Whitney, 400 Main St., East Hartford, CT 06108; phone: (860) 565-7700; fax: (860) 565-1605.