Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
NE-07-41
August 3, 2007
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin alerts you, owners,
operators, and repair facilities of Pratt & Whitney JT9D-7A and -7J
series engines and Principal Maintenance Inspectors in the FAA Flight
Standards District Offices (FSDOs), of improper maintenance of the high
pressure turbine (HPT) 2nd stage vane assemblies. An uncontained engine
failure due to a fracture in an HPT 2nd stage disk can potentially
occur if maintenance is performed improperly. In an extreme case, the
engine may separate from the airplane.
Background
On October 22, 2004, a Boeing 747 freighter experienced an engine
separation event. A teardown of the engine showed that a significant
portion of the HPT 2nd stage disk rim failed, causing an imbalance that
overloaded the forward and aft engine mounts. The root cause of this
disk failure was the incorrect installation of the 2nd stage vane
assembly, causing it to lean back and contact the disk during
operation. This resulted in circumferential grooves and scoring, from
which several cracks initiated and grew until the disk failed.
It is likely that the HPT 2nd stage vane bolts were not properly
torqued to the correct value at the last maintenance visit. This could
have occurred for several reasons: old bolts were reused, different
lubricant used, bolts were torqued in an incorrect order, or the torque
wrench was not within calibration. Incorrect installation of lock-wire
can also lead to the bolts becoming loose over time.
The engine manual contains a warning since 2000 stating that:
“Improperly seated second stage vanes can cause lean back of the second
stage inner shroud and result in contact with the second stage hub.
Contact can cause wear in the hub web and eventually result in second
stage hub fracture and liberation.”
The FAA is currently reviewing fleet data to identify whether there is
a suspect population beyond this event. As no additional population has
been identified at this time, the airworthiness concern has not been
determined to be an unsafe condition that would warrant airworthiness
directive (AD) action under Title 14 of the Code of Federal Aviation
Regulations (14 CFR) part 39.
Recommendations
Each operator should:
1. Review their HPT 2nd stage vane installation procedures to ensure they use only P&W or similar FAA-approved procedures.
2. Remove immediately from service any engine suspected to have improperly torqued HPT 2nd stage vane bolts.
3. At next exposure, inspect the HPT 2nd stage disk visually and remove
the disk from service if there is any sign of circumferential disk
scoring.
Additionally, P&W is developing an on-wing borescope inspection to check for leading indicators of 2nd stage vane leanback.
For Further Information Contact
Keith Lardie, FAA, Engine and Propeller Directorate, Engine
Certification Office, ANE-142, 12 New England Executive Park,
Burlington, MA 01803-5213; phone: (781) 238-7189; email:
keith.lardie@faa.gov.
For Related Service Information Contact
Pratt & Whitney, 400 Main Street, East Hartford, CT 06108; phone: (860) 565-0140; email: help24@pw.utc.com.