Metal Chip Possible Super Puma Crash Factor
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Frances Fiorino fiorino@aviationweek.com
As a result of its continuing investigation of the failure of the epicyclic reduction gearbox module on the Eurocopter Super Puma AS332L2 that crashed Apr. 1 into the North Sea killing the 16 people on board, the U.K. Air Accidents Investigation Branch (AAIB) said it is possible the aircraft might have been pulled from service -- if the relevance of a metal chip found in the module days prior to the crash had been diagnosed differently.
The AAIB has so far determined that the failure of the module of the main rotor gearbox (MRG) resulted in a rupture of the gearbox case. This led to the main rotor head and upper section of the MRG separating from the aircraft. All 16 people onboard the flight from Aberdeen, Scotland, to the Miller oil platform in the North Sea were killed.
The July 16 accident update noted that the MRG is fitted with three magnetic chip detector plugs, which are designed to detect and retain any chips of magnetic material shed from the gears or their bearings.
Data from the accident helicopter (G-REDL) indicates that a chip was detected at 12:51:15 (local time). In the following 1 min. 43 sec., three other chips were detected. Although recorded by the Helicopter Usage Monitoring System (HUMS), the flight crew would not have been aware of these detections, according to the AAIB.
Before the accident, on Mar. 25, the HUMS detected a metal chip while the aircraft was in flight. Workers removed and examined the magnetic plug, but initially found no particle. But during a 25-hr. maintenance check the same day, a chip was discovered - but deemed a type of particle that did not require further investigation of the module. In addition, no other contamination was detected in a check of the oil system.
The discovery of the chip, however, led to daily inspections of the gearbox chip detector and, over the next 25 flight hours, maintenance downloaded and analyzed HUMS data each time the helicopter returned to base. As no abnormalities were discovered in the period, the checks ceased on Mar. 31, the day prior to the accident. This, says the AAIB, "served to reinforce the view that a correct diagnosis of the chip had been made."
The report notes there is "evidence of damage throughout the epicyclic module that is consistent with operating for a period while contaminated with sizeable debris."
Investigators, in concern for the processes employed in identifying and assessing the relevance of the chip, issued two safety recommendations pertaining to AS332L2 and EC225LP helicopters. They ask that the European Aviation Safety Agency and Eurocopter:
*Review procedures to ensure that correct identification of the type of magnetic particles found with the oil system of the power transmission system is maximized.
*Review the design, operational life and inspection processes of the planet gears used in the epicyclic module of the MRG.
The AAIB's two recommendations issued July 16 follow four earlier ones made in the course of its continuing investigation.
File photo of a Super Puma AS332L2--not G-REDL--from Eurocopter
