A Boeing Co. 737 cargo plane operated by a Qantas Airways Ltd. subsidiary suddenly depressurized during a flight last year after a misplaced insulation blanket clogged up the main outflow valve, Australia’s air-safety investigator said.

Cold air rushed into the cockpit and the two-person crew suffered pain in their ears when the jet lost cabin pressure descending through 8,000 feet (2,400 meters), the Australian Transport Safety Bureau said in its report on the July 6 incident. The Express Freighters Australia flight was heading to Melbourne from Perth and landed safely.

Inspectors found several insulation blankets in the aircraft’s rear cargo bay were either loose, installed incorrectly or missing. One became lodged in the primary pressurization outflow valve and stopped it closing, according to the ATSB. The freighter probably wasn’t properly inspected after maintenance work 15 months earlier, the ATSB said.

A fleetwide inspection found unsecured or missing insulation blankets in four of the company’s five Boeing 737 freighters. The Qantas division implemented periodic visual inspections after the incident, according to the report.