Sixteen new Airbus A321neo aircraft will be purchased by Hawaiian Airlines under a memorandum of understanding announced today.

The long-range, single-aisle aircraft will join the airline’s fleet of wide-body, twin-aisle aircraft used for routes between Hawaii and the west coast.

The fuel-efficient A321neo will be used to serve new markets not viable for wide-body service, and can augment existing service, said Mark Dunkerley, Hawaiian president and CEO, in a statement. The additional markets to be served were not disclosed.

The aircraft purchases will take place between 2017 and 2020, and the agreement allows for the purchase of an additional nine aircraft.