On August 20th, 2007, a regularly scheduled China Airlines flight landed at Naha Airport in Japan. The Boeing 737-800 had just stopped, and as all passengers readied to leave the aircraft, fuel began leaking from the right-wing tank and caught fire The captain rushed to evacuate everyone from the jet, and only minutes later, the aircraft was engulfed in flames.
In this case, the 165 people on the aircraft evacuated in time, and no one died or was severely injured. While this incident was rare, and all that was wounded was the aircraft, a straightforward maintenance oversight led to a devastating ending for the 737 that day – a fallen washer.
The China Airlines Flight 120 aircraft registered B18616 had taken off from Taiwan Taoyuan International Airport at 08:14 am local time and landed at Naha Airport at 10:27 am. As the aircraft taxied and stopped at Spot 41, ground controllers were greeted with the Sight of fuel pouring from an area near the second engine pylon.
The captain recounted that he heard “Fire! Fire!” through a headset and looked out the window to see smoke. He quickly evacuated passengers and crew and shut off the fuel supply to the engines. He and his first officer escaped through the right cockpit window. After everyone had evacuated, a large explosion resounded in the middle of the aircraft, ultimately burning most components of the 737, which had been first delivered to China Airlines in 2002.
In a recount in the Japan Transport Safety Board (JTSB) safety investigation report released two years later in August 2009, the captain insisted there were no alarming signs before the incident. said:
“I found no indication of abnormal fuel consumption during the flight, either. Everything was normal until the accident occurred.”
Damage to the aircraft
The resulting damage from the fire was significant. According to the JTSB report, the cockpit and nose landing gear were intact, but the cabin interior was destroyed in the forward fuselage. The lower fuselage near the wing roots was burnt entirely, as well as the cabin interior aft.
The left wing was almost completely burned and lost its original shape. The right wing was also burned, and the fire also consumed the CFM56-7B26 engines.
A breakdown of what led to the disaster
Many minor incidents led to the aircraft having a deadly end. According to the JTSB, the final culprit was that a slat on the right wing was punctured, creating a hole that led to the fuel leakage. Slats are “extendable, high lift devices” on wings that increase aircraft lift when it flies at low speed, like take-off and landing, SKYbrary Aviation Safety explains.When the jet retracted the slats after landing, the “track can” that housed the no.5 slat was pierced.
But how did this happen? To help prevent these slats from moving out of position after being retracted, a ‘downstop assembly’ is a component attached to the back end of the slat. It contains a bolt, two washers, two downstops, a sleeve , a stop location, and a nut.
Diagram of the downstop assembly. Photo: FAA
In July, a few months before the incident, China Airlines performed a C-check that requires airlines to readjust the nut on the downstop assembly so that it doesn’t unscrew and allow for a washer to fall – to avoid precisely what happened. check was introduced after two previous cases were reported in 2005 where the nut detached from the downstop assembly.
According to Kylan Dempsey, a US-based journalist who analyzes plane crashes, the maintenance process is “done by feel” because the downstop assembly is in a hard-to-reach area. He explained that in this particular C-check:
“While fumbling around inside the track can, the maintenance engineer accidentally bumped the downstop assembly and knocked the washer off the bolt,” Dempsey said in a Medium article. “For whatever reason, the maintenance workers didn’t hear it tumble down into space inside the leading edge, so they finished attaching the nut and closed everything up again without noticing the missing washer.”
As the washer fell into the empty space of the track can, the slat retracted and pressed against it, ultimately piercing it and creating a hole. “sufficient attention to this.”
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Following the incident, the Republic of China Civil Aeronautics Administration grounded all 14 of the remaining Boeing 737-800s of China Airlines, Mandarin Airlines, and the air force.
While the losses were slim compared to other well-known plane crash disasters, China Airlines said they would compensate each passenger NT$1000 ($33.08) per kilogram of lost luggage and a maximum of NT$20,000 ($661.82) for checked-in baggage and another maximum for carry-on luggage.
Many recommendations were made by the JTSB and the American Federal Aviation Administration (FAA) as it was a Boeing-made aircraft. The JTSB requested that the FAA supervise Boeing to ensure the company made clear maintenance job instructions, including the accessibility of job areas, to avoid future errors.
Further, the Civil Aviation Bureau of Japan (JCAB) issued an airworthiness directive on August 23rd, 2007, to instruct all Japanese operators of Boeing 737-7 and 8 jets to conduct ongoing inspections on the downstop assembly. Eventually, all models of the 737 type were instructed to be checked.
In August 2008, a year later, Boeing changed the design of the downstop assembly and requested all previous models be fitted with the new hardware.
The smallest missteps can make the largest impact. The China Airlines flight 120 echoed the importance of seemingly insignificant procedures that are vital to the aircraft’s ecosystem. Extreme action was taken after the incident because while no one was hurt on August 20th, 2007, that may not be the case in the future.
Sources: Medium, JTSB report, SKYbrary