FedEx Express Flight 910

NTSB narrative

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.

Verbatim NTSB analysis and probable cause from the NTSB dataset