COVINGTON, KY, USA
N47AE
ATR 42
While taxiing for takeoff, the right main landing gear collapsed. Examination of the landing gear revealed that the barrel lugs (used to attach the swinging lever to the main landing gear barrel) had failed. The fracture of the outboard lower lug exhibited a fatigue crack emanating from corrosion pits in the bore of the lug. The bushings for the lower lugs (replaced during overhaul) did not have the proper interference fit. During overhaul, the bushings did not have the proper applications of corrosion preventatives applied to the surface of the bushings as specified by the manufacturer's procedures. During the course of the investigation, nine landing gears with various numbers of total cycles since overhaul or manufacture were inspected. The inspections revealed torn or missing rubber sealant used on bushing flanges, corrosion in the lug bores, and corrosion on the lug faces.
On November 20, 1996, at 1050 eastern standard time, an American Eagle, ATR-42, N47AE, operated by Simmons Airlines, Inc., sustained minor damage during taxiing for takeoff when the right main landing gear collapsed. The airline transport pilot shutdown the airplane on the taxiway and deplaned the 41 passengers. There were no injuries to the 3 crew members or the 41 passengers. The CFR 121 flight was scheduled to depart Cincinnati/Northern Kentucky International Airport, Covington, Kentucky, and land at O'Hare International Airport, Chicago, Illinois. Visual meteorological conditions prevailed and an instrument flight plan was filed. The pilots reported that the aircraft was cleared to taxi to runway 27 for departure. While taxiing on taxiway K, the right main landing gear failed causing the airplane to list to the right. The engines were shut down and the crash, fire, and rescue personnel were dispatched to the scene. The crew secured the aircraft and the passengers were deplaned normally. The failure of this main landing gear was similar to an ATR 42 landing gear failure that occurred on April 3, 1996, in Traverse City, Michigan. In both incidents the lower lugs of the barrel had failed and the swinging lever had separated from the aircraft. Maintenance records indicated that last main landing gear (MLG) overhaul was performed in April 1991, by Dowty Aerospace Aviation Services (DAAS). At the time of the accident the landing gear had accumulated a total of 23,000 landing cycles, of which 11,152 cycles were since last overhaul. DAAS indicated that the main landing gear barrel, P/N D60260-2, was replaced in October 1988, with a new barrel, and that by the time of the overhaul this barrel had accumulated a total of 5,159 landing cycles. Therefore, the barrel had a total of 16,311 cycles (5,159 + 11,152) at the time of separation. The right MLG assembly (P/N D22686040-2, S/N MN22), the bushings, and the lower lugs were sent to the National Transportation Safety Board's (NTSB) Materials Laboratory Division for examination. Visual inspection of the landing gear revealed that a rubber sealant, PR 1422 B2, was applied to the flange of the bushings and the outboard surfaces of the lug. The purpose of the sealant was to prevent moisture and corrosive materials from getting in between the bushing and the bore of the lug. The aircraft operator reported that during regular phase inspections of fleet aircraft, it was not uncommon to find the sealant compromised. Inspections of the bore of the lug revealed evidence of corrosion. Examination of the outboard lower lug revealed extensive corrosion pitting on the bore surface in the grease gap between the bushings. Scattered corrosion pitting was found at other locations of the lug bore, the bore chamfer and the lug outer face. The lower lugs of the barrel were examined. The inboard lower lug contained features typical of overstress separation. The examination of the outboard lower lug revealed that approximately 80 percent of the of the surface on the aft section of the outboard piece was smooth and flat, and contained crack arrest markings typical of fatigue. The fatigue cracking emanated from an origin area located in the bore of the lug at or adjacent to the chamfer between the bore and the outside face. The fatigue fracture plane was oriented approximately 25 degrees below the horizontal center line of the lug. A scanning electron microscopic (SEM) examination of the origin area of the fatigue crack revealed that the fatigue fracture features stemmed from two adjacent corrosion pits. Together the corrosion pits at the origin area measured 0.077 inch wide on the bore surface of the lug. Adjacent to the origin area, the fracture surface was covered with what appeared to be corrosion deposits. However, away from the origin, the fatigue portion of the fracture contained well defined fatigue striations. Outside of the fatigue region, the fracture surface contained features typical of overstress separation. The bushings for the lower lugs of the barrel had been replaced when the MLG assembly had been overhauled in April 1991. During the examination of the bushings, it was determined that the bushings did not have the proper interference fit. The specified interference fit was 0.144 mm (min) to 0.193 mm (max), but the measured interference fit of the bushings was 0.099 mm to 0.124 mm. An energy dispersive spectroscopy (EDS) examination of the outside surface of both bushings, as well as in the corner between the outside diameter and the flange, revealed no evidence of cadmium plating. The manufacturer's specifications require cadmium plating during overhaul. The overhaul procedures specified by Messier-Dowty, the landing gear manufacturer, were: 1) to plate the bushings with 5 to 7 microns of cadmium plating. 2) to apply a primer paint to the bushings and allow it to dry. 3) to apply a wet coat of primer paint prior to press fitting the bushings into the lug. DAAS reported that the procedures used during overhaul were not in strict accordance with the manufacturer's procedures, but were an acceptable practice. The procedures used by DAAS at the time of overhaul included: 1) to plate the bushings with 5 to 7 microns of cadmium plating. 2) to apply a coat of Mastinox D-40 prior to press fitting the bushings into the lug. DAAS reported that they did not use a first coat of primer paint on the bushings since overhaul procedures for similar types of landing gear did not specify using a primer coat. DAAS used Mastinox D-40, an anti-galvanic corrosion product, as a substitute for wet primer when used with cadmium plating. However, Messier-Dowty regarded Mastinox D-40 as mainly a lubricant which did not have the protective effect of the primer coat on the cadmium plating. DAAS reported that as of May 31, 1996, they changed their overhaul procedures in order to adhere to the procedures specified by Messier-Dowty. The ATR-42's landing gear which had failed in the April 3, 1996, incident had also been overhauled by DAAS and had been installed on a Simmons Airlines aircraft. The two incidents shared the following similarities: 1. The outboard lug of the main landing gear had separated with a fatigue crack that emanated from a corrosion pit in the bore of the lug. The fatigue fracture plane was oriented approximately 25 degrees below the horizontal center line of the lug. 2. The bushings did not have the proper interference fit. 3. The application of corrosion preventatives to the bushings was not performed according to the manufacturer's specification during overhaul. After the first occurrence of landing gear failure on April 3, 1996, DAAS identified 59 landing gears that had been overhauled using the above mentioned DAAS procedure. Messier-Dowty requested that DAAS check the recorded interference fit for the 59 landing gears. Twenty of the 59 overhauled landing gears had interference fits which caused concern, and those landing gears were preventatively examined for cracks using an ultrasonic inspection procedure developed by Messier-Dowty. Later this procedure was issued as Service Bulletin 631-32-132. (See Attachment) Messier-Dowty, in conjunction with DAAS and the affected aircraft operators, inspected the 20 landing gears overhauled by DAAS for similar fatigue cracks. Results of these inspections found that five of the inspected gears contained possible crack indications. These gears were sent to DAAS for further examination, and it was determined that no cracks existed. Due to an anomaly in documentation at DAAS, the second landing gear, MN22, had not been listed in the 59 landing gears list, but should have been. Thus it had not undergone the ultrasonic inspection before it failed. As a result of the second occurrence, Simmons Airlines conducted a voluntary fleet wide inspection of the landing gears. The ultrasonic inspection method developed by Messier-Dowty and incorporated into the Service Bulletin 631-32-132 was used. The inspection indicated that nine of 54 landing gears had possible cracks in the lugs. The nine gears were sent to DAAS for further examination. It was determined that none of the nine gears had cracks. The inspection of the nine landing gears did reveal, however, that corrosion discrepancies existed in the landing gear. The nine gears represented a cross section of landing gears at various stages of landing cycles and time since overhaul, and not just at the normal overhaul intervals. Also, one of the landing gears had been overhauled by DAAS, six had been overhauled at another maintenance facility, and two had not reached their overhaul cycle. Common discrepancies to the nine gears included: 1. Cracked or missing sealant around the bushing flanges. 2. Corrosion in both inboard and outboard lugs. 3. Corrosion on inboard and outboard faces. Messier-Dowty issued the Service Bulletin 631-32-132 in January 1997, which formalized the ultrasonic inspection procedure that indicated the presence of cracks in the lugs of the landing gear barrels. Messier-Dowty also issued Service Bulletin 631-32-133 (See Attachment) in February 1997, which specified the following: 1) Perform a detailed visual inspection and a fluorescent penetrant inspection of the barrel lower lugs (outboard and inboard) to detect any possible presence of material defect. 2) Increase the bore size of the lower lugs by 0.015 inch during repair or overhaul whether corrosion was detected or not. 3) Apply a new protective treatment scheme to the barrel lugs and bushings. 4) Accomplish a product improvement by installing stainless steel bushings in the swinging lever. Messier-Dowty issued the Service Letter 631-32-134 in February 1997, which stressed the importance of proper lubrication to the hinge points on ATR-300,400,500 series and ATR-72 landing gears. As a result of the two landing gear failures, the Direction Generale DeL'Aviation Civil (DGAC) of France issued an Airworthiness Directive (AD) 96-294 (B) on January 15, 1997. The AD was issued, "To prevent a fatigue failure of the main landing gear leg caused by corrosion linked to the quality of refurbishment performed during the overhaul or repair... ." (See AD 96-294 (B)) It incorporated Messier-Dowty Service Bulletins 631-32-132 and 631-32-133 as part of the AD. As a result of the two landing gear failures, the Federal Aviation Administration (FAA) issued AD 97-04-09, which called for similar measures mandated in the DGAC Airworthiness Directive. (See AD 97-04-09) It incorporated the ultrasonic inspection specified in Messier-Dowty Service Bulletin 631-32-132, but did not incorporate the terminating actions specified in the Messier-Dowty Service Bulletin 631-32-133. Parties to the investigation included the FAA, the Bureau Enquetes-Accidents (BEA), Simmons Airlines, Messier-Dowty, DAAS, ATR, Alenia, and the Air Line Pilots Association. The aircraft and landing gear parts were released to Simmons Airlines, Inc.
improper use of procedures by maintenance personnel at an overhaul facility during overhaul of the landing gear, allowing corrosion to occur on the barrel lugs of the landing gear, which subsequently led to fatigue cracking of the lugs and eventual collapse of the gear.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
