Taiwan has grounded all military airplanes for training and drills after a pilot was killed and another is still missing following a mid-air collision of two fighter jets, renewing worries about the island’s aging military hardware.

Rescuers are searching for one of the pilots after the Taiwanese F-5 fighters collided at 3:06 p.m. Monday during training, the National Rescue Command Center said in a text message. The pilot of the other plane was found unconscious at sea and later died of his injuries.

The jets disappeared from radar screens 1.4 nautical miles east of Mudan Township, in the southern county of Pingtung.

The air force has grounded all military aircraft for safety checks, except those on guard or combat readiness missions, air force Chief of Staff Huang Chih-wei said at a briefing Monday night, adding that the military will conduct comprehensive inspections on ejection seats and will strengthen training.

Other pilots training at the time of the accident saw one seat eject following the collision, but the military is uncertain yet whether the ejection was triggered by the pilot or by the impact with the other aircraft, Huang said.

This incident has sparked renewed concern for Taiwan’s aging fighter fleet, especially at a time when the air force is under pressure to intercept almost daily incursions by Chinese military aircraft into Taiwan’s air defense identification zone.

Taiwan manufactured 242 single-seat F-5s and 66 two-seaters under license from Northrop Corp. between 1973 and the mid 1980s, making it at one point the world’s biggest user of the plane, the island’s defense ministry said in a Facebook post in 2014.

Monday’s collision means there have been nine accidents involving Taiwan’s F-5s over the past 20 years, resulting in the deaths of 12 pilots, two of whom are listed as missing, presumed dead, according to the Central News Agency. In the most recent accident, an F-5 fighter crashed and its pilot was killed during a training exercise east of Taiwan in October last year. The pilot, who had ejected and was rescued, later died despite hours of emergency treatment.

The next month, an F-16 was lost off the east coast minutes after taking off from a base in Hualien.

Taiwan plans to retire its F-5 jets in 2026, when the U.S. completes delivery of 66 F-16Vs ordered in 2020, CNA reported in October, citing an unidentified military source.

Last January, the island’s most senior uniformed officer and 12 others were killed when the helicopter they were traveling in went down in bad weather in mountains south of Taipei. Two pilots also died when their helicopter crashed following military exercises in July.