More than 630 Hawaiian Airlines pilots began voting Monday on whether to authorize their union leaders to conduct a strike if they cannot reach a contract agreement with the company. The strike authorization ballot will close May 17, the day before the airline’s annual shareholders meeting.

Before any strike could take place, the National Mediation Board would have to release the two sides from mediation and then offer binding arbitration.

If either party rejects arbitration, a 30-day cooling-off period would begin, after which both parties could exercise what is referred to as “self-help” — a strike by the union or a lockout by the company.