On October 28, 2016, at 17:51 eastern standard time, the left Main Landing Gear (MLG) of N370FE, FedEx flight 910, an MD-10-10 freighter (F), failed, then collapsed, during rollout after landing in Fort Lauderdale-Hollywood International Airport. Damage to the wing and subsequent rupture of fuel lines and tank, led to a post-crash fire.
FedEx operates two different DC-10 types in its fleet: the DC-10-10F and the DC-10-30F; both were converted to the MD-10 series. The MD-10-10F is a short-range version because it has no auxiliary fuel tanks. All FedEx’s MD-10-10Fs previously flew for other airlines, mostly passenger.
According to the report for FedEx 910, accident number DCA17MA022, the approach and landing were uneventful; the aircraft did not sustain a hard landing; pilot error was not to blame. The post-accident investigation report revealed that a major contributor to the accident was the left MLG outer strut cylinder failed due to an overstress fracture: “a small thumbnail crack, located at the radius between the cylinder inner diameter surface and the air filler valve bore (AFVB) surface,” located at the top of the strut.
In Memphis, July 28, 2006 (ten years prior), FedEx 630, accident number DCA06FA058, an MD-10-10F, crashed on rollout with a near identical left MLG failure. The timeframe for both accident aircrafts’ MLG, between overhaul and accident, matched: FedEx 910’s left MLG was overhauled 8 years, 8 months prior to its accident; FedEx 630’s left MLG was overhauled 8 years, 6 months before its accident.
Following the 2006 FedEx 630 accident, FedEx, Boeing and the Federal Aviation Administration (FAA) acted quickly. FedEx immediately inspected all MD-10 MLG cylinders for defects per Boeing’s Alert Service Bulletin (ASB) DC10-32A259. Boeing also issued a revision to the Component Maintenance Manual (CMM), requiring video probes and eddy current inspections of the air filler valve bore. The FAA issued Airworthiness Directive 2008-09-17 supporting the ASB and CMM revision. FedEx was the only domestic MD-10 large operator, with thousands of MD-10 flights per year. Neither McDonnel-Douglas’s aircraft design nor FedEx’s maintenance program were the cause. The National Transportation Safety Board (NTSB), however, never looked beyond the MD-10.
Both investigations found that the MLG cylinders failed because stray nickel plating was introduced into the AFVB. According to FedEx 910’s accident report, nickel plating with a thickness of 0.008-inch thickness “results in a stress factor increase of 35%”. During the left MLG cylinder’s in-service lifetime, a load event “compressively yielded” the AFVB material, “causing a residual tension stress”. In layman’s terms, the nickel-plating process somehow weakened the cylinder metal’s integrity in the AFVB area. The residual stress led to fatigue cracking.
There are two troubling issues about these accidents. First, is that both accident reports DCA06FA058 and DCA17MA022 were written in a field accident (FA) report format, similar to a General Aviation report – not in a standard major accident (MA) report format. In other words, there was minimal deserved attention to these accidents. Neither accident had a hearing; they had an inadequate number of investigative groups assigned. The reports’ recommendations never mentioned looking at MLG cylinder overhaul processes. Considering the fact that these were two Title 14 Code of Federal Regulation (CFR) Part 121 aircraft, the reports barely registered on the major accident radar.
By comparison, the NTSB blue cover major accident report from December 18, 2003: AAR-05/01 (DCA04MA011), FedEx flight 647, an MD-10-10F, crashed in Memphis under similar circumstances, yet the report highlighted pilot error. However, FedEx 630’s accident report did refer to AAR-05/01, stating, “The fracture of the right main landing gear of this airplane [FedEx 647] initiated from the same air filler valve bore hole location as N391FE [FedEx 630]” A search of the NTSB.gov website’s Document Management System found no Maintenance or Structures Group factual reports in the dockets for any of the three accidents; these reports would have given the investigative groups’ individual factual findings. How, with almost identical events just three years later, was FedEx 630 not a major accident?
The second concern was the NTSB’s failure to look beyond the three FedEx accidents. How far did investigators go to prevent possible MLG cylinder failures in other MD-10 aircraft, e.g. Orbis Air’s MD-10? Did both a Maintenance investigator and a Structures investigator take their investigatory groups to examine the MLG overhaul process used by the FedEx contractor or interview the technicians performing the procedures? The DCA17MA022 factual report only spoke to Boeing’s DC-10 CMM revision and the ASB DC10-32A259, actions limited to the DC10 … and only the DC10. Did investigators give serious consideration that similar MLG failures could occur on other airliners, e.g. B777, A300 or MD-11?
Both FedEx 630 and FedEx 910 had resulting fires; both accident aircraft’s cabin areas were engulfed in flames and emergency exits were blocked by the fire. If either aircraft was a passenger airliner, a high fatality rate would have resulted. It is likely that all MLG overhaul facilities used similar plating procedures with similar results when overhauling wide-body MLG for passenger airliners. This suggests that a similar incident would have destroyed a passenger airliner with catastrophic results. Why were the two accidents not major accident investigation reports and why focus just on the MD-10’s MLG?
In 2006, FedEx 630, should have had a qualified Maintenance Investigator, with both air carrier and repair station experience; he/she should have led an investigatory group to explore the MLG’s overhaul facility to determine what issues – at the overhaul facility – if any, may have affected safety at the time, e.g. labor issues, quality control issues, management issues. In the wake of FedEx 910, both Structures and Maintenance Investigation groups, again, with experienced investigators with Title 14 CFR Part 121 and Part 145 backgrounds, should have conducted onsite inspections of the MLG overhaul facility.
Instead, the investigation mischaracterized information, e.g. the overhaul requirements. DCA17MA022’s factual report stated, the “manufacturer-recommended overhaul limit for the MLG assembly is every 8 years or 7500 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.” Flight hours and flight cycles are not comparable. In reference to a landing gear component’s airworthiness, flight cycles are relevant; the landing gear is employed every single flight cycle. The landing gear is not used every flight hour, so flight hours do not affect landing gear integrity. FedEx probably received a time exemption from Boeing. Any cargo airliner’s years-to-cycles ratio is less than a passenger airliner’s; passenger airliners average more flight cycles per year. An air carrier-experienced maintenance investigator would have caught that.
Lessons Unlearned? One airline lost three of the same model aircraft to the same contributing factor. It is disturbing that two of these accident investigations were treated as inconsequential and ended up in a field accident report format. It is more troubling, however, that the investigations were treated as less-than-important as a passenger airline accident when one considers that information found in cargo airline accidents is critical to the safety of passenger airlines and the travelling public.