SEA03LA076

After initiating their descent the crew heard a "thunk" sound from the left side of the aircraft, followed by the gearbox LOW OIL PRESSURE warning light illuminating. After consulting the Quick Reference Handbook, the left engine was shut down, the crew declared an emergency, and a single-engine landing was made at the destination airport without incident. Subsequent examination by maintenance personnel revealed that the #1 engine drive shaft had failed at the universal joint connection area on the gear side of the engine, resulting in substantial damage to the firewall. The left engine's accessory drive shaft assembly, including the forward and aft universal joints, were sent to the NTSB Materials Laboratory Division, Washington, D.C., for examination. The examination revealed that the accessory drive shaft failed due to deterioration of bearing number 1 in the aft fork of the joint. This failure led to frictional heating for some time and off-axis rotation that eventually allowed the spider journal to slip out of the bearing cup at bearing number 3, leading to the rapid separation of the joint. The cause for the number 1 bearing failure is unknown.

On April 17, 2003, about 1530 Pacific daylight time, a Fokker F27MK 600, N730FE, registered to Federal Express Corporation, and operated by Empire Airlines as a 14 CFR Part 121 cargo flight en route to Seattle, Washington, sustained substantial damage to the firewall following a #1 engine failure while in cruise flight near Port Townsend, Washington. The flight continued to Seattle where a single-engine landing was made without incident. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The two airline transport pilots were not injured. The flight departed Ketchikan, Alaska, as Empire Flight CFS 8202, at 1247 Alaska daylight time.

According to the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the pilot reported that just prior to starting the descent into the Seattle area, the crew heard a "thunk" sound from the left side of the aircraft and the gearbox LOW OIL PRESSURE warning light illuminated. The pilot stated that the crew consulted the Quick Reference Handbook, then elected to shut down the left engine in accordance with the Precautionary Engine Shutdown checklist. The pilot reported that after the engine had been shut down and the checklists completed, the crew declared an emergency and proceeded to the Seattle-Tacoma International Airport where a single-engine landing was made without incident. The NTSB investigator-in-charge was subsequently notified by company maintenance personnel that an initial examination indicated that the #1 engine drive shaft had failed at the universal joint connection area on the gear side of the engine, resulting in substantial damage to the firewall.

The accessory drive shaft assembly includes a universal joint at each end of the drive shaft. Each universal joint consists of two forks aligned perpendicular to each other in the plane of rotation and a center unit assembly. Each fork has two lugs, each of which contains a bearing outer race, called a cup. The center unit assembly includes a center body, called the spider, with four journals that comprise the bearing inner races, each having 21 cylindrical rolling elements and a cage. The cup fits over the spider journals and rolling elements. A grease seal is located at the inner end of the cup between the cup and the spider, with spring clips inserted in grooves in the fork lugs, which retain the cups within the forks. Grease is applied to the bearings in each joint using a zerk fitting on each joint.

A Senior Materials Engineer at the NTSB Materials Laboratory in Washington, D.C conducted an examination of the accessory drive shaft including the forward and aft universal joints. (Refer to Attachment 1)

The results of the examination revealed that the forward joint of the drive shaft was fractured through its aft fork, with the fracture features and deformation consistent with overstress fracture in bending and rotation.

The drive shaft exhibited circumferential score marks concentrated near the aft end. Damage was consistent with contact with an adjacent structure, with the shaft rotating counterclockwise relative to the structure.

Tinting consistent with heating was observed on all components of the aft joint. The output shaft, which included the aft fork of the aft joint, had multiple damage locations and was not connected to the spider, the center unit assembly. Two bearing cups contained their rolling elements, cages, and washers, while only one rolling element from the other two cups was submitted. All four grease seals were missing and the grease zerk fitting on the forward side of the spider was fractured. One bearing cup was fractured and deformed with most of the cup missing. One retaining cup was fractured with a portion missing, and a second retaining cup was nearly straightened, consistent with deformation at high temperature.

Witness marks were observed on the forward fork of the aft joint. The marks corresponded to contact and wear with a lug from the aft fork at the bearing 1 location and contact with spider journal number 3. An additional mark was observed on the interior of the fork in one zerk fitting access hole, corresponding to contact with the missing zerk fitting portion (refer to figure 4).

The aft fork of the aft joint exhibited wear damage near the horizontal axis of the fork. The location of the wear corresponds to off-axis contact with the grease valve retaining screw head (refer to figure 5). The damage was consistent with the spider shifting toward the bearing 1 location and either rotating counterclockwise or shifting toward the bearing 4 location, all relative to the aft fork. The location and orientation of this mark, combined with the location of the wear mark at the bearing 1 location on the forward fork indicates that the aft fork lug corresponds to the bearing 1 location.

Although heat damage was observed throughout the aft joint, the bearing 1 location of the output shaft showed the greatest level of heating. A darker level of heat tinting and missing coating in the vicinity of the bearing 1 location was evidence of greater heat damage in that area.