The first two aircraft to undergo the ‘Increment 3’ modification will be upgraded in Jacksonville, Florida, with the remainder of the fleet of 12 to be completed at RAAF Base Edinburgh, South Australia.

The descent was controlled by an auto-flight system that had to be disconnected when the aircraft dipped to 480 ft below the minimum safe level and wouldn’t respond to crew instruction.

Frank Kendall said there would also be a pilot in the cockpit alongside him, but “hopefully” neither would be required to take control of the specially adapted aircraft.

The United Firefighters Union of Australia said the “in-principle” deal with Airservices satisfies its concerns about understaffing, resourcing and working conditions.