Gulfstream Aerospace G-IV, registered JA001G, operated by the Japan Civil Aviation Bureau of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), took off from Obihiro Airport at 11:16 hours local time. Purpose of the flight was to carry out flight inspections to confirm the required performance of aeronautical navigation facilities. The captain was Pilot Flying. While the aircraft was flying south-southwest on Airway V31 toward the Niigata VORTAC at an altitude 9,000 ft, the captain and the first officer sighted a lower layer of clouds but had visually confirmed that there was no cloud on the flight route in the vicinity of Niigata Airport. At about 14:32, the aircraft commenced a descending turn from right above Niigata Airport in order to inspect on the next airway. At this time, the captain and the first officer had checked on an indication on the weather radar display of the aircraft showing a precipitation area, albeit not on their route, and were therefore focusing attention in that direction. They then encountered scattered thin clouds, but this was judged not to be thick clouds which might lead to turbulence or thunder. At about 14:35, as the aircraft entered the thin cloud at an altitude about 6,500 ft, a very loud noise was heard on the radio equipment while the captain, the first officer and the onboard maintenance engineer were listening to it. After three to four seconds, the captain and the first officer received an impact accompanied by a loud popping noise, together with a strong flash from the front of the aircraft. While continuing the flight, the captain and the first officer checked the instruments to check whether abnormality had occurred to the aircraft; consequently, they confirmed that discoloration was found on some of the cockpit instrument displays, and that the airborne DME (Distance Measuring Equipment) display on the captain’s side had disappeared. However, there was no abnormality in the DME display on the first officer’s side and there was no problem with the visibility of the discolored instrument displays; accordingly, they judged there to be no serious abnormality which would affect the flight. The captain, the first officer and the onboard maintenance engineer discussed whether they should continue the flight inspection, and although there was thought to be no hindrance to the flight, they decided to suspend the flight and return to the base of the aircraft at Tokyo International Airport, because they could not confirm damage to the airframe while on board. Thereafter the aircraft landed at Tokyo International Airport at 15:26. On the post-flight inspection, damage was confirmed to the left front fuselage outer skins, the front lower fuselage and the left horizontal stabilizer. PROBABLE CAUSES: “It is probable that this accident occurred because the aircraft sustained damage to the outer skins of its left front lower fuselage as a result of being struck by lightning to the airframe in flight. It is probable that the lightning strike was occurred because the aircraft came close to the electrically charged cumulus clouds in the scattered thin cloud where it was difficult to predict lightning.”