NTSB Identification: LAX04FA150.
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Accident occurred Tuesday, March 09, 2004 in Oakdale, CA
Probable Cause Approval Date: 01/31/2006
Aircraft: Ercoupe 415-C, registration: N93707
Injuries: 2 Fatal.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The airplane broke apart in flight while maneuvering. The pilot and a passenger were on a local flight and did not return. The wreckage was located strewn over an area of grassy hills. The wings and empennage sections were found separate from the main fuselage. The owner of the airplane had flown with the pilot just prior to the accident flight and reported that the pilot had been doing aileron rolls. The airplane had flown about 4 hours since its annual inspection. Prior to that time, the airplane was damaged during a hard landing and sat, unrepaired, for 2 years. Post accident examination of the airframe revealed extensive corrosion through the wing walk areas and wing spar center section, with corrosion affecting at least 20 rivets that secure the web to the upper spar cap. A Safety Board metallurgist determined that the area of the wing spar center section corrosion could not have been detected through current inspection methods and procedures. The critical buckling stress on the upper spar cap was several times as high as the estimated stress carried by the upper spar cap in straight and level flight, which indicates the failure most likely occurred in a steep turn or other high load maneuver. The collapse of the spar could only occur when enough rivets had corroded to allow the disconnected part of the spar cap to reach its critical buckling strength. Ercoupe Service Bulletin (SB) No. 18 was issued for the reinforcement of the center section belly skin as a direct result of cracks occurring in the center section belly skin at the rearmost rivet attachment in the center section belly skin outer stiffeners. Service Bulletin 31 was issued in July 2000, and required inspection within the next 5 hours of operation or at the next 100-hour or annual inspection, whichever occurred first. The SB was intended to detect corrosion and prescribed certain visual methods of inspection. AD 2002-26-02 was intended to detect and correct corrosion in the wing center section, which could result in failure of the wing center section structure during flight. The AD contained three methods of accomplishing the inspection: install inspection openings; use a scope and a light source; or remove outer wing panels to gain visual access to the wing walkway box structure. AD 2003-21-01 was issued in response to additional reports of corrosion on airplanes found during compliance with AD 94-18-04 R1. The actions of the AD were intended to prevent wing damage caused by a corroded wing outer panel structural component. The AD called for installation of inspection openings in the outer wing panels and inspection of the area for corrosion and corrosion damage. It also called for repetitive inspections every 12 months. Comparison of the inspection requirements in the Service Bulletins and AD's to the area where the corrosion was present in the accident aircraft revealed that none of the inspection methods would likely have found the corrosion. All of the current inspection methods are done visually and there is no requirement for nondestructive inspection (NDI) or nondestructive testing (NDT).
The National Transportation Safety Board determines the probable cause(s) of this accident to be: the in-flight separation of the main spar due to corrosion in the wing center section not detectable by inspection methods described in manufacturer's service bulletins and FAA Airworthiness Directives. Full narrative available
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