F-35 Fleet Is Grounded by US Air Force Over Ejection-Seat Worries
The US Air Force has grounded its fleet of F-35 jets temporarily, citing a potentially faulty component in the ejection seat that could endanger pilots in an emergency, a concern that also grounded other types of military planes that are used in training.
www.bloomberg.com
So it's the explosive cartridges. Sounds similar to the Takata airbag recall.