The Asia Pacific Aircraft Storage Facility in Alice Springs, Australia, Oct. 23.

Photographer: David Gray/Bloomberg

On Sept. 15, an Indonesian flight carrying 307 passengers and 11 crew to the northern city of Medan momentarily veered off the runway after landing, sparking an investigation by the country’s transport safety regulator. It found the pilot had flown less than three hours in the previous 90 days. The first officer hadn’t flown at all since Feb. 1.

The incident underlines an emerging risk from the coronavirus pandemic: pilots aren’t getting enough opportunity to fly because airlines have grounded planes and scaled back operations due to a slump in demand for air travel.

“Regular flying keeps your mind in the cockpit,” said Mohan Ranganathan, an aviation safety consultant who was an adviser to India’s Directorate General of Civil Aviation. “Being away from flying for such a long time brings in some complacency. Add loss of income, uncertainty about jobs or the future of the airline, that brings in additional stress. With an increase in stress levels, proficiency drops.”

Analytics company Cirium says almost a third of the world’s passenger jets remain in storage -- parked in the center of Australia and the U.S.’s Mojave Desert. While there’s been a recovery in domestic travel in larger markets such as China, international traffic is way off pre-pandemic levels because of border restrictions and mandatory quarantine, a big deterrent to travelers. Thousands of pilots have been laid off or furloughed, and those still in work are flying a lot less because there’s so little demand.

The return to the skies of Boeing Co.’s Max 737 could add another layer of complexity. The jet was grounded worldwide in March 2019 after two fatal crashes but was last month cleared by the U.S. FAA with an extensive package of fixes.

“For some operators of the Max, depending on where they are and their operational status, the grounding may cause additional challenges,” said Shukor Yusof, founder of aviation consulting firm Endau Analytics in Malaysia. “However this is something that Boeing has already been working on and they have teams to help their customers throughout this process.”