SEATTLE (Reuters) - Once global regulators clear Boeing Co’s 737 MAX to fly again after deadly crashes, airlines which have put their fleets into mechanical hibernation since March will scramble to begin the biggest ungrounding effort in history.

FILE PHOTO: An aerial photo shows Boeing 737 MAX airplanes parked on the tarmac at the Boeing Factory in Renton, Washington, U.S. March 21, 2019. REUTERS/Lindsey Wasson/File Photo

Quickly reintegrating Boeing’s 737 MAX, a fast-selling model because of its fuel efficiency, longer range, and passenger capacity, is crucial for optimizing airlines’ routes and improving margins after having to cancel thousands of flights.

Global airlines have warned of a major hit to profits due to the mid-March grounding of the MAX following two fatal crashes.

International regulators are meeting on May 23 to review Boeing software and training plans, though doubts remain over how quickly foreign authorities will clear new flights.

In the United States, following regulatory approval of a Boeing software fix and new training, airlines will have to run through an FAA-approved checklist, industry officials say.

Such work, which will vary by operator, includes cycling the engines, changing and filling fluids, removing covers from the engines and running routine electrical and hydraulics checks.

Each airline will also have to upload Boeing’s new software for an anti-stall system implicated in the crashes and complete additional pilot training.

Jason Goldberg, spokesman for the Allied Pilots Association, said it will take about a week to prepare American Airline Group Inc’s MAX fleet to fly, not counting the extra training.

“While the planes are in storage there is of course maintenance that can be done. But as far as steps to put it back into service, none of the work is going to be done until the aircraft is cleared to fly.”

Airlines likely halted standard daily systems checks while the planes were in storage, said one former Boeing test pilot. Similar to a modern car, when mechanics restore power to the aircraft, built-in testing equipment runs checks on sensors gauging the health of aircraft systems from hydraulics and to fuel - known as a ‘BITE’ test - that would signal failures.

Another issue is pilot training.

Boeing said on Thursday it was in the process of submitting a plan on pilot training to the U.S. Federal Aviation Administration (FAA) for approval, after which each airline will develop its own FAA-approved training program.

Southwest Airlines Co, the world’s largest MAX operator, has agreed a 30-day window with its 10,000 pilots to implement new MAX training, said Mike Trevino, spokesman for Southwest Airlines Pilots Association.

“If it’s computer-based training, that won’t be difficult to deploy. The pilots can do it at home,” he said.

A draft report by an FAA-appointed board of pilots, engineers and other experts concluded that pilots only need additional computer-based training, rather than simulator time, though other regulators and some pilot groups have argued for more.

Southwest is expected to upload the new software at a facility in the California desert where its 34 MAX jets are parked, while American Airlines is expected to install the software at its Tulsa, Oklahoma maintenance facility.

Southwest and American have scheduled MAX flights as of Aug. 6 and Aug. 20 respectively. If the jets are not cleared to fly by then, the airlines will be forced to again cancel more than 100 daily flights.