Robinson Helicopter R44 N981RR accident near Santa Barbara, California

Probable cause

A total loss of engine power during cruise flight due to the failure of an engine piston rod for reasons that could not be determined due to extensive damage.

NTSB analysis

The commercial pilot was conducting a local sightseeing flight in a helicopter with paying passengers. During cruise flight, the engine lost partial power. The pilot performed troubleshooting steps, and while he was maneuvering to conduct a forced landing, the engine lost all power, and he immediately initiated an autorotation during which the helicopter struck a building and then landed hard. The helicopter was subsequently destroyed by fire. Multiple witnesses reported seeing an object fall from the helicopter as it flew over a highway just before the accident.

Postaccident examination of the engine revealed that the No. 3 cylinder head assembly and piston were missing; they were found the following day. Examination of all the studs and through-bolts for the detached cylinder head revealed that they exhibited failure signatures consistent with ductile overstress fracture. Although some slight fretting and oxidation were observed on the cylinder flange, machining marks were also present, indicating that the movement was relatively limited and not consistent with a fatigue fracture of the cylinder attachments.

Examination of the No. 3 cylinder's rod, piston, cylinder skirt, and flange revealed damage signatures indicating that the cylinder assembly was forced out by the end of the piston after the rod separated from the piston boss as the crankshaft continued to rotate. The rod end and its bushing were obliterated as the engine continued to operate, and fire damage precluded a full examination of the rod and, thus, a determination of what caused it to fail.

Bushings in several of the engine's connecting rods were found displaced, and their ends were tinted blue, indicating that they were abnormally heated, which could have led to the bushings shifting. However, the abnormal heating likely happened after the connecting rod failed because heat tinting was also present on all of the wrist pins except for the pin from the failed rod.

The engine oil filter, which sustained extensive thermal damage, contained charred remnants of bronzelike material, which was similar in appearance to the bushing material. This evidence indicates that a bushing failure had either taken place before the event or that it occurred secondary to another failure event to the piston rod. Other failure scenarios, such as a defect in the rod assembly or mechanical damage, could also have led to the observed failure; however, because the bushing and rod end were not located, it could not be determined what led to the rod's failure.

About 3 1/2 months after the accident, the engine manufacturer published a mandatory service bulletin (SB), compliance of which was mandated by a Federal Aviation Administration airworthiness directive (AD), for the detection and replacement of connecting rods with nonconforming small end bushings because continued use of the nonconforming bushings could lead to a connecting rod failure. However, maintenance records showed that the engine was rebuilt by the manufacturer during a period not applicable to the SB. The manufacturer also issued an SB that recommended the inspection of the bushings any time a cylinder head was removed; however, a cylinder had never been removed from the engine since the rebuild.

Although the shifted bushing condition observed on several of the connecting rods may have been secondary to the connecting rod failure, bushing shifting can cause connecting rod fractures. According to an AD issued by the Civil Aviation Safety Authority of Australia (CASA), all the manufacturer's engines that were new, factory rebuilt (such as the accident engine), or factory overhauled during the same calendar year as the accident engine were susceptible to premature wear of the connecting rod bushings. According to CASA, the development of this premature wear condition was relatively slow and could be detected through regular oil and oil filter inspections in accordance with the manufacturer's recommendations. Maintenance records indicated that these inspections had been regularly conducted on the accident engine at the required times. During these inspections, maintenance personnel should have been able to detect the excessive wear associated with bushing shift before the connecting rod fractured but failed to do so.

Text from the NTSB. Open the NTSB docket for WPR17LA097 for the full record.