Aircraft Accident/Incident Report

Elfrida, Arizona 85610
Tuesady, February 15, 2000 20:24 MST

The aircraft collided with ground obstructions and nosed over during a night forced landing on a dirt road following a catastrophic engine failure during cruise. A large hole was found in the engine case on each side of the upper spine between the numbers 1 and 2 cylinders. FAA AD 99-09-17 was issued on April 22, 1999, and mandated compliance with Continental Mandatory Service Bulletin (MSB) 99-3, which required visual and ultrasonic inspections of the crankshaft for cracks; these inspections had to be done by Continental factory service representatives. The inspection was performed on May 4, 1999, 220 hours prior to the accident. To perform the inspection, cylinders 1 and 3, and all 4 counterweights on the crankshaft had to be removed. This engine's crankshaft has 4 counterweights, which are installed on hangar blades extending out on each side of the Nos. 2 and 5 crank cheeks (CC). The weights are retained on the blades by pins, which are held in the bores by pin retaining plates on each side of the weight. These plates are slightly elliptical on one edge of the circumference, which forms an ear that results in an interference fit within the bores of each weight. The plates are held in place by snap rings that fit into separate grooves machined into the weight bores. Three versions of MSB 99-3 were eventually issued. 99-3A was issued on April 22, 1999, and was current when the compliance inspection was performed. Versions 99-3B and 99-3C were issued on July 6 and 27, 1999, respectively, and superceded 99-3A and each other in sequential order. An additional Service Bulletin, SB 00-3, was issued on February 2, 2000 (13 days prior to the accident and 9 months after the crankshaft inspection was performed), and provided detailed and specific procedures for the reinstallation of crankshaft counterweights, including the correct orientation and installation of the counterweight pins, retainer plates, and snap rings. The principal difference between MSB 99-3A, and versions 99-3B and 99-3C, is that versions B and C incorporate the guidance on counterweight pin retainer plate and snap ring installation found in SB 00-3. Versions 99-3B and 99-3C specifically emphasize a dimensional measurement between snap ring ears as a means to determine that they are fully seated in their grooves. SB 00-3 and versions B and C of SB 99-3 specify that the pin retainer plates and snap rings must be installed in specific orientations within the weight bores. Review of the precursor service bulletins to SB 00-3 disclosed that it was the first one to detail and emphasize the criticality of the retainer plate and snap ring orientations within the bores, and provide a measurement between the snap ring ears when the ring is fully seated within the weight bore. The engine overhaul manual does not go into specific detail regarding the orientation of the pin retainer plates and snap rings, nor does it provide a measurement of the snap ring ears to verify full seating of the ring. All other references that would have been available to the mechanic at the time of the inspection did not address the criticality of the correct counterweight retainer plate and snap ring installation. During the inspection process on May 4, 1999, the operator's mechanic prepared the engine by removing cylinders 1 and 3 and the associated connecting rods. When the factory representative arrived to perform the visual and ultrasonic inspections of CC2 and CC5, the mechanic removed the counterweights. Following the inspection, the mechanic reinstalled the weights, connecting rods, and cylinders. The mechanic would have had to perform all the work through the 5.5-inch cylinder base openings in the crankcase, and the weight bores, including the positions of the retaining plate snap rings, would have been perpendicular to the opening. According to the mechanic who performed the job, the work was inspected by a company inspector at each stage and signed off on the work order. The oil pan was removed from the engine and the sump contents were in part identified as follows: 1. A crankshaft counterweight (in pieces) identified as having been mounted on crank cheek 2 as the leading weight. 2. 2 Crankshaft counterweight pins. By part number, one pin was for the No. 2 leading weight and one was from the No. 3 trailing weight. 3. 4 Counterweight pin retaining plates 4. 5 counterweight retaining plate snap rings Of the four recovered counterweight pin retaining plates from the oil sump pan, two were bent and distorted and two were not damaged or distorted. For the recovered pin retaining plate snap rings, three were distorted and two were not; the undistorted snap rings were of the correct diameter. During disassembly of the engine, the condition of the three remaining counterweight assemblies were documented as follows: The No. 1 trailing weight on crank cheek 2 remained attached to the blade, with the pin retainer plates and snap rings correctly installed. The No. 3 trailing weight on crank cheek 5 was partially retained on the hangar blade by only the forward pin. The aft pin, and, the retainer plate and snap ring from one side of the weight were missing; these parts were found in the oil sump. The forward pin was in place and retained by its associated retainer plates and snap rings. One snap ring was installed correctly and one was not. The No. 4 leading weight on crank cheek 5 remained on its hangar blade. All snap rings and retainer plates were intact and installed correctly. Examination of the engine found no evidence of oil starvation or exhaustion, or any other internal anomaly beyond the separation of the No. 2 leading counterweight that would have led to the catastrophic destruction of the engine.

The company mechanic's failure to correctly install one or more of the crankshaft counterweight pin retaining plate snap rings, specifically ensuring that the snap rings were fully seated in their grooves. This led to the complete separation of the No. 2 leading counterweight and the partial separation of the No. 3 trailing weight from the crankshaft and a resulting internal catastrophic engine failure. A factor in the accident was the lack of definitive procedural guidance in any of the engine manufacturer's reference material, available at the time, on the correct installation of these components.