SEATTLE, Jan 9 (Reuters) - The Federal Aviation Administration has approved Boeing's ( BA ) largest variant of its best-selling 737 MAX jet, the MAX 10, to move to the second phase of flight testing on the plane's long-delayed certification campaign, according to ‌a source familiar with the program.

The company and FAA declined to comment.

Boeing ( BA ) executives have previously said they ​expect to finish certification this year for the MAX 7 and 10, ‍the smallest and largest variants of the popular single-aisle jet. ⁠The company has ⁠more than 1,200 orders for the MAX 10 in its backlog, according to aviation data analytics firm ‌Cirium.

Boeing ( BA ) has faced delays in ⁠the certification of ​its MAX 7 and MAX 10 ​models due to an engine deicing issue. The U.S. ​planemaker has also faced delays in certifying its ⁠widebody 777X jet. In October, the ‍FAA gave ​Boeing approval to raise 737 MAX production to 42 ‍planes per month, ending a ⁠38-plane cap in place since January 2024. 

The MAX 7 is a ‍shortened version of the two types already in service, the MAX 8 and 9, which have accumulated tens of thousands of flight hours.

Beginning deliveries of the MAX 10 is widely seen by industry analysts as critical to increasing Boeing's ( BA ) revenue and cash flow.

DEICING ISSUE UNRESOLVED

The FAA approval did not apply to ​the MAX ‍7, according to the source.

Phase two of the FAA flight testing involves the aircraft's avionics, propulsion and other parts of the design, but the ​deicing issue has not been resolved. 

The MAX 10 competes with Airbus' A321neo and risks losing more market share the longer it is delayed. This week, Alaska Airlines ordered 105 MAX 10 airplanes and Alaska CEO Ben Minicucci said he is confident the MAX 10 will be certified this year.

"It's progress, but until (the MAX 10) is certified, it's ​not," said Scott Hamilton, an aerospace analyst and principal with the Leeham Company.  Until there is a clear path to certification, Boeing ( BA ) cannot start production at its Everett, Washington, plant, he ‍said.