DCA17MA022

The failure of the left main landing gear (MLG) due to fatigue cracking that initiated at a corrosion pit. The pit formed in the absence of a required protective cadmium coating the cause of which could not be determined from available evidence. Contributing to the failure of the left MLG was the operator's overhaul limit, which exceeded that recommended by the airplane manufacturer without sufficient data and analysis to ensure crack detection before it progressed to failure.

On October 28, 2016, about 1751 eastern daylight time, FedEx Express (FedEx) flight 910, a McDonnell Douglas MD-10-10F, N370FE, experienced a left main landing gear (MLG) collapse after landing on runway 10L at Fort Lauderdale–Hollywood International Airport (KFLL), Fort Lauderdale, Florida, and the left wing subsequently caught fire. The airplane came to rest off the left side of the runway. As the flight crew was preparing to evacuate, the nearly empty left main fuel tank exploded after the exterior surfaces were heated by burning fuel, which flowed from a broken fuel supply tube in the left engine pylon. The two flight crewmembers evacuated the airplane, and the captain sustained minor injuries during the evacuation. The first officer was not injured. The airplane sustained substantial damage. The first officer, who was the pilot flying the cargo flight, conducted an approach and landing that were stabilized and within specified limits. Although crosswind conditions were present at the time the airplane landed, the weather did not affect the landing. The application of braking by the flight crew did not initiate or contribute to the landing gear fracture. A review of the status of all applicable airworthiness directives and service bulletins for the airplane, powerplants, and appliances found no discrepancies. Metallurgical examination of a 3-by-3-inch fragment of the left MLG cylinder (see images B and C in figure 1) and the mating fracture region on the largest piece of the fractured left MLG (see figure 2) found indications consistent with overstress fracture having emanated from the air filler valve bore. The overstress features emanated from a small thumbnail crack, located at the radius between the cylinder inner diameter surface and the air filler valve bore surface. This thumbnail crack was observed on both mating fracture surfaces of the MLG and the 3-by-3-inch fragment, and it exhibited features (such as crack arrest marks) consistent with a pre-existing crack that progressed to fracture of the MLG. The observed features in the initiation region of the thumbnail crack were found to be consistent with fatigue cracking, as exhibited by the fatigue striations observed during microscopic examination. Later propagation regions of the thumbnail crack revealed features consistent with mixed mode fatigue, overstress, and intergranular fracture. These findings confirmed that once the crack progressed to a critical length, the left MLG cylinder fractured in overstress due to the loads imposed during landing. Figure 1. A diagram of an MD-10-10F left MLG (A), photograph of the 3-by-3-inch fragment from the lower section of the accident left MLG cylinder (B), and closer view of the region pointed out in image B (C). Figure 2. Photographs of largest piece of fractured left MLG pointing to the crack initiation site (top) and a closer view of the crack initiation site, with a yellow dashed line highlighting the mating thumbnail crack (bottom). The most prominent quality of the crack initiation site was a feature consistent with a corrosion pit. No indications of nickel, chrome, or cadmium plating were found at the radius or along the smooth sections of the air filler valve bore, as stipulated by maintenance instructions. The absence of a protective coating, over time, could lead to corrosion pitting. Any corrosion pitting or mechanical damage to the protective coating incurred during maintenance can lead to fatigue cracking. Figure 3. Closer view of the primary fatigue crack region (highlighted by the dashed yellow line) and the region of mixed-mode crack propagation (highlighted by the dashed blue line). The MLG's last overhaul was completed on February 28, 2008, during which the air filler valve bore was inspected per Alert Service Bulletin (ASB) DC10-32A259. This ASB (which was prompted by an MLG collapse on a FedEx MD-10-10F in 2006 and indicated that stray plating, if present, can enhance local corrosion under the edge of the plating in and around the bore) instructed operators to inspect the air filler valve bore for stray nickel or chrome electroplating deposits, corrosion, or cracks to prevent fatigue failure of the cylinder. Although neither stray plating nor cracks were found as part of the ASB inspection, corrosion repair was completed on the accident cylinder, and it underwent brush cadmium plating per maintenance instructions before being returned to service. It was not possible to determine whether cadmium plating applied during the last overhaul did not properly bond to the bore surface, was removed during later maintenance, or wore off over time because there is no routine procedure to effectively inspect this area during on-wing maintenance activities conducted since the last MLG overhaul. To be effective, it is critical that a protective coating be uniform, adherent, and complete when applied in areas known to be susceptible to pitting corrosion. Finite element analysis of this MLG cylinder (conducted using the same method, tools, and format as those used in the investigation of the 2006 event) concluded that residual stress levels at the corrosion pit were sufficiently high to promote crack propagation leading to full fracture of the cylinder barrel. Since the accident, the maintenance facility that performed the last MLG cylinder overhaul has introduced a tank dip method for plating air filler valve bores, which provides improved uniformity, adherence, and coverage over the brush plating method. The manufacturer-recommended overhaul limit for the MLG assembly is every 8 years or 7,500 flight cycles, whichever occurs first. At the time of the accident, FedEx's MLG overhaul limit was 9 years or 30,000 flight hours, whichever occurred first, and the left MLG outer cylinder on the accident airplane was 152 days away from its next required overhaul. FedEx reportedly adopted the 9-year overhaul interval used by the previous owner/operator of the first DC10s it purchased. The company produced no documentation or data analysis supporting the longer overhaul interval. Following this accident, FedEx inspected all MLGs in its MD-10-10F fleet (27 in-service airplanes and 54 MLG). Sixteen cylinders were identified as "concerns" and were permanently removed from service. After reviewing its maintenance program, FedEx reverted to an 8-year overhaul limit for MLG cylinders, as recommended by the manufacturer. The NTSB notes that if FedEx had not adopted an overhaul limit that exceeded the manufacturer's recommendation, the fatigue crack in the accident MLG cylinder, which was last overhauled 8.5 years before the accident, likely would have been detected and addressed before it could progress to failure.

About an hour before the accident, at 1653, the KFLL METAR reported wind from 50° at 15 knots gusting to 25 knots, 10 miles visibility, few clouds at 2,400 ft agl, scattered clouds at 3,300 ft agl, broken ceiling at 4,100 ft agl, overcast clouds at 25,000 ft agl, temperature 26°C, dewpoint 20C, and an altimeter setting of 29.99 inches of mercury. At 1753, about 2 minutes after the accident, the KFLL METAR reported wind from 050° at 15 knots gusting to 24 knots, 10 miles visibility, few clouds at 2,600 ft agl, broken ceiling at 5,000 ft agl, overcast clouds at 25,000 ft agl, temperature 26°C, dewpoint 21°C, and an altimeter setting of 29.98 inches of mercury. A peak wind speed had been recorded at 1732 of 060 degrees at 26 knots. The two crewmembers had flown together previously and reported having a good rapport. The departure from MEM was the first leg of the most recent flight pairing. The Captain The captain, age 55, was hired as a flight engineer by FedEx in 2000. At the company, he worked in the Boeing 727 as a flight engineer, a first officer, and a captain, as well as a captain on the MD-11. He had a total flight time of about 10,000 hours (he was uncertain about his time as pilot-in-command) and estimated about 1,500 hours in the MD-11. The captain held a multiengine airline transport pilot certificate, with type ratings in the LR-Jet, Boeing 727, and MD-11. The captain held a Federal Aviation Administration (FAA) first-class airman medical certificate dated June 6, 2016, with a limitation that he "must have glasses available for near vision." The First Officer The first officer, age 47, was hired as a flight instructor by FedEx in 2004. In 2007, he became a flight engineer in the Boeing 727 and became a first officer in the MD-11 in 2012. He estimated a total flight time of 6,000 to 6,300 hours, with about 4,000 hours as pilot-in-command. He estimated a total time of about 400 to 500 hours in the MD-11, with no time as pilot-in-command. The first officer held a multiengine airline transport pilot certificate, with a type rating in the MD-11. The first officer held an FAA first-class airman medical certificate dated September 6, 2016, with no limitations. The airplane, a McDonnell Douglas MD-10-10F, serial number 46608, registration N370FE, was manufactured in 1972 and had an airframe total time of 84,589 total flight hours with 35,606 total flight cycles at the time of the accident. FedEx purchased the airplane on August 21, 1997. The airplane was converted from a DC10-10 to a DC10-10F on July 3, 1999; it was converted to an MD-10-10F on November 2, 2003, when a transport-category standard airworthiness certificate was also issued. It was powered by three General Electric model CF6-6D turbo-fan engines. The airplane's last "C" check was on September 21, 2016. It's last inspection as October 25, 2016, when a service check, an "A" check, and a "B check" were completed. Left Main Landing Gear The most recent overhaul of the left MLG occurred February 28, 2008, by Hawker Pacific Aerospace (HPA) Repair Station No. RJ3R817L in Sun Valley, California. It was installed on the accident airplane March 31, 2008. Aircraft time at installation was 74,340.02 total flight hours and 29,953 total cycles. Time since installation was 10,249.03 flight hours, 5,653 cycles, and 3,133 days (8.58 years). At the time of the accident, the MLG overhaul limit at FedEx was 9 years or 30,000 flight hours, whichever occurs first; the left MLG's time remaining until the next overhaul was 152 days (most restrictive). The Boeing maintenance planning document recommends the restoration (overhaul) of the MLG assembly every 8 years or 7,500 flight cycles, whichever occurs first. A FedEx representative indicated that the company adopted the 9-year MLG overhaul limit used by the previous owner/operator of the first DC10s purchased by FedEx. The most recent service of the MLG full shock strut was performed at the airplane's last "C" check in September 2016, in Mobile, Alabama (this work included draining and replacing the strut fluid and nitrogen charge; it does not include a requirement to do a borescope inspection of the air fill valve bore). In addition, aircraft log writeups revealed a pilot report dated February 6, 2014, indicated a hard landing that occurred at Newark Liberty International Airport, Newark, New Jersey. Maintenance personnel downloaded and reviewed FDR data and performed structural inspections in accordance with manufacturer specifications. No defects were noted during the inspections, and no additional work was required. Maintenance History of the Left MLG Outer Cylinder Of the 3 airworthiness directives and 14 service bulletins (SB) accomplished during the left MLG's last overhaul, the work associated with Alert Service Bulletin DC10-32A259 was identified for further examination based on findings in the NTSB's investigation. Maintenance records indicate that no stray nickel or chrome was found during the initial inspection of the air filler valve bore, but corrosion was found and removed per ASB instructions. The MLG cylinder was then brush cadmium plated per the CMM (see Additional Information). The overhaul was completed on February 28, 2008. All dimensions related to the fill valve bore rework were documented in HPA work cards indicating the air filler valve bore was oversized within manufacturer limits, and all recorded dimensions were within manufacturer limits with one exception. The ASB and CMM state that the radius of the air fill valve bore at both the inner diameter wall and thread junction should be maintained at 0.015 to 0.030 inch during rework and that findings be recorded. After rework, the thread junction radius was recorded as 0.030 inch and the inner diameter wall was recorded as 0.090 inch. No additional records were found that addressed the exception. When asked about this exception, a Boeing representative indicated that this type of discrepancy had been brought to the manufacturer's attention previously and that larger radius findings had been approved for continued service. The airplane came to rest off the left side of runway 10L near the A4 taxiway. The outer cylinder of the left MLG was fractured midway through the air filler valve bore on the aft side. The outer cylinder of the left MLG shock strut fractured around its entire circumference; the upper portion of the cylinder remained intact and attached to the airplane at the forward and aft trunnion lugs (figure 1 shows a diagram of an MD-10-10F MLG and figure 2 shows the upper portion of the accident MLG). No anomalies were noted with the trunnion lugs or trunnion bolt attachment points. Figure 1. Diagram of MD-10-10F MLG (right shown) Figure 2. Upper portion of the left MLG attached to the airplane. The inboard side lugs and bolt and nut for the retract actuator attach points and the fixed door attach points remained intact. On the aft side of the lower section of the fractured gear, about half of the upper charging bore was visible (see figure 3). On-scene examination identified features consistent with fatigue on the fracture surface of the outer cylinder in the area of the air filler valve bore (see Tests and Research for more information). Figure 3. Aft side of the lower section of the left MLG cylinder and fill valve. The lower segment of the landing gear was folded aft into the left side inboard flap. The lower portion of the gear remained attached to the MLG side brace assembly. The left MLG truck beam remained attached to the lower portion of the cylinder and shock strut at the pivot pin. The truck beam exhibited a crack running aft from the No. 1 wheel position axle to a 3-inch-long by 2-inch-wide hole on the top side near the pivot area (see MLG diagram in figure 3). Three pieces of the left MLG outer cylinder were found on the runway. One piece measured about 12 to 14 inches long by about