FILE PHOTO: Domhnal Slattery, Chief Executive Officer of Avolon, delivers a speech during the delivery of the first A330neo commercial passenger aircraft for TAP Air Portugal airline at the Airbus delivery center in Colomiers near Toulouse, France, November 26, 2018. REUTERS/Regis Duvignau

DUBLIN (Reuters) - The head of aircraft lessor Avolon, which has committed to a fleet of 148 Boeing BA.N 737 MAX jets, said he believed the planemaker's new June to July horizon for getting the grounded aircraft back in the air was a "worst case scenario."

Boeing said on Tuesday it does not expect to win approval for the return of the 737 MAX to service until mid-year due to further potential developments in the certification process and regulatory scrutiny on its flight control system.

“The best news I’ve heard on the MAX grounding was yesterday, when Boeing said June-July,” Avolon chief executive Domhnal Slattery told a conference in Dublin, adding he hoped it marked the beginning of the end of the MAX crisis.