On Sept. 15, an Indonesian plane 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.

In its preliminary report, Indonesia’s National Transportation Safety Committee said the pandemic has made it harder to maintain pilot proficiency and flying experience. The Lion Air aircraft involved was an Airbus SE A330, one of 10 in the carrier’s fleet. Because Lion Air doesn’t have a simulator for the A330, its pilots are trained at third-party facilities in Indonesia, Malaysia and Singapore. COVID-19 travel restrictions have made those harder to access.