Officials noticed the panel was missing from the Boeing 747, one of two jetliners used by Prime Minister Shinzo Abe for foreign trips, after it flew from Tokyo to its base on the northern island of Hokkaido on Thursday, a spokesman for the Japan Air Self Defence Force (ASDF) said.
The part, measuring 38 cm (15 inches) by 20 cm, has not been found. It is an access panel near the pylon connecting one of the engines to the right wing.
"There was no sign of it on the runways so it's going to be very difficult to locate," the spokesman said.
A recent string of incidents involving U.S. aircraft based on Japan's southern island of Okinawa prompted Japan's Defence Minister Itsunori Onodera on Tuesday to ask his U.S. counterpart James Mattis to implement measures that would prevent any recurrences.
Those incidents, which included a window falling from a helicopter onto a school playground last month, have fuelled public opposition to the U.S. presence on the strategically located island at the edge of the East China Sea.
The air force would conduct an investigation to determine why the panel had fallen off Abe's plane, the ASDF spokesman said.
The two government jumbos left Tokyo on Friday carrying Abe, government officials and Japanese business representatives on a six-day trip to Eastern Europe, including stops in Estonia, Lithuania, Serbia and Romania.
Abe did not board the 747 that lost the panel.
Japan has ordered two Boeing 777 planes to replace the ageing jumbos after March 2019. It also plans to switch to ANA Holdings for maintenance work from rival Japan Airlines Co..
(Reporting by Tim Kelly; Editing by Paul Tait)
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