The battery of the underwater locator beacon (ULB) on board the ill-fated Malaysian Airlines Flight MH370 expired 15 months before the plane disappeared.

According to the interim report by an independent investigation team on the plane's disappearance, the engineering crew had reported a computer glitch as the reason for failing to replace the battery.

"Interviews were held with the MAS engineering technical records staff to determine why the ULB was not replaced before the expiry.

"It was revealed that the engineering maintenance system (EMS) was not updated correctly when the SSFDR (solid-state flight data recorder) was replaced on Feb 29, 2008," read the report.

"Since the system was not updated, it did not trigger for the removal of the SSFDR for the replacement of the ULB battery when it was due (in December, 2012)."

Oversight

The investigators said this "oversight" was not discovered until information about the ULB was requested.

"Subsequently, MAS Engineering Technical records carried out a fleet-wide record inspection for the ULBs to ensure all records for other aircraft are updated accordingly."

The ULB is designed to emit a "ping" every second upon the aircraft's submersion in water for at least 30 days.

The sound can be detected by up to a distance of 1.5km with a special underwater microphone used by search and rescue teams.

Previously, the Department of Civil Aviations (DCA) had stated that the battery was scheduled to be replaced on June 14, 2014.

The 584-page interim report was released today by the investigation team headed by former DCA chief Kok Soo Chon.

The investigation team is required to provide an annual report on the progress of investigations on a missing aircraft if a report could not be finalised.

In a televised address at 3pm today, Kok stressed that the purpose of the investigation is to solely to prevent similar incidents from happening in the future.