On April 6, a UH-60JA helicopter of the Ground Self-Defense Force crashed in waters near Miyakojima Island of Okinawa Prefecture, killing all 10 crewmembers. Among them was Lt General Yuichi Sakamoto, Commander of the GSDF 8th Division.
A photo in the Yomiuri Shimbun newspaper shows the helicopter wreckage salvaged from the seabed. It shows a main rotor blade that was torn off and appears to have pierced into the engine cover. Using flight recorder data reported on May 24 as a reference, I tried to consider the cause of the accident.
A Similar Accident in the Past
Yahoo! News carried a comment on April 23 that a break around the main rotor head, which caused the crash of a GSDF AH-64D attack helicopter in Saga Prefecture in 2018, should be suspected as a factor behind the latest accident.
According to the comment, a rotor head bolt was broken due to metal fatigue in the Saga accident. As a consequence, a rotor blade was torn off, making it impossible to adjust the rotor angle. Then, the rotor hit the airframe, almost immediately triggering the crash. There is a video showing the crash.
Some four months after the Saga accident, the GSDF estimated that a bolt for a component to fix the main rotor to the rotary shaft was broken. Two possible causes of the bolt breakage are suggested. One was the abrasion of the bolt through the deterioration of the corrosion inhibitor. The other was an original crack in the bolt. However, the Saga report fell short of naming the cause of the bolt breakage.
Considering Metal Fatigue as a Cause
I would suggest metal fatigue as the third po cause of the breakage.
One type of metal fatigue is high-cycle fatigue which arises from about one million repetitions. It is a type of fatigue that causes cracks to expand and break instantly, even with small loads of about one-tenth of the tensile strength of the metal.
Past high-cycle fatigue cases include damage to the fixing part such as bolts on a rotary shaft, damage to a weld for a large pump at Unit 3 of the Tokyo Electric Power Co.'s Fukushima Daini Nuclear Power Station, breakage of a tire hub for a large truck, breakage of a wheel for a high-speed train in Germany, and the rupture of a pressure bulkhead on a Japan Airlines aircraft.
Regular inspection and maintenance of bolts and joints are important.
Make Public Flight Recorder Data
According to a Yomiuri Shimbun newspaper report on May 24, the flight recorder of the crashed helicopter in the latest accident was recovered from the seabed. Moreover, it recorded a situation in which the engine of the aircraft made an abnormal noise and an alarm sounded giving notice of the aircraft's trouble.
The captain and co-pilot were recorded shouting to maintain altitude. Meanwhile, the engine output was decreasing. The aircraft is believed to have crashed into the sea immediately after that. And the voices stopped after the last voice said "Ah."
Assuming an event similar to the breakage of the Saga rotor head bolt occurred on the UH-60JA helicopter, we can imagine what happened. First, there was a decrease in gas turbine engine output due to the abnormal angle of the rotor blades. Then the broken rotor blade damaged the engine cover and generated abnormal engine noise. Next, the remaining rotor blades hit the aircraft and caused it to crash.
Even if you try to identify the cause of an accident without knowledge of high-cycle fatigue, as in the 2018 accident, you may not be able to identify the cause and leave it as it is, and the precious lives of SDF members including the division commander will be lost five years later.
I would like the government to make public the flight recorder's time-series data of aircraft abnormalities. That includes engine output reduction and crash. The data should be shared with the world to help ensure the safety of some 6,000 same-type helicopters worldwide.
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Author: Tadashi Narabayashi
Tadashi Narabayashi is a specially appointed professor at the Tokyo Institute of Technology and a director at the Japan Institute for National Fundamentals.