Boeing 737 MAX airplanes parked in Moses Lake, Washington File photo: VCG

Boeing's troubled 737 MAX aircraft gained another boost on Tuesday as Kazakh flag carrier Air Astana said it planned to buy 30 of the planes, which were grounded in March.

Air Astana said it had signed a "­letter of intent" to buy 30 of the 737 MAX-8 model, with the order to be finalized in the coming months.

Boeing said the planes would "serve as the backbone" of Air Astana's new low-cost carrier FlyArystan.

On Monday, Boeing announced at the Dubai Airshow it had sold 10 of the aircraft to Turkey's SunExpress, in the first firm order since the 737 MAX was taken out of service after two crashes that left a total of 346 people dead.

Before this week's sales, the last ­commercial landmark for the troubled model was in June when Boeing secured a letter of intent from British Airways parent IAG to buy 200 of the planes.

The grounding of the 737 MAX has dragged on far beyond initial expectations as Boeing had to upgrade systems and faced tough questions from regulators and politicians.

The crisis is one of the most serious in the US manufacturer's 103-year ­history, and has already cost it tens of billions of dollars, amid multiple investigations by US authorities and complaints from victims' families.

SunExpress said its purchase was in addition to a previous order for 32 of the aircraft.

"We have full confidence that Boeing will deliver us a safe, reliable and efficient aircraft," the airline's CEO Jens Bischof said.

"However, it goes without saying that this requires the undisputed airworthiness of the model, granted by all relevant authorities."

Boeing's embattled CEO Dennis Muilenburg faced a grilling from US lawmakers in late October.

Many of the questions focused on the Maneuvering Characteristics Augmentation System, an automated system that the pilots of the doomed Lion Air and Ethiopian Airlines planes were unable to control, resulting in crashes.

Boeing's entire global fleet of almost 400 MAX planes has stayed out of service ever since.