US aerospace giant Boeing has reported that its 737 MAX single-aisle airliner would remain grounded for longer than the company had hoped. The grounding of the aircraft was imposed after two fatal crashes, in late 2018 and early 2019, which killed a total of 346 people. A system fitted to the 737 MAX, the Manoeuvring Characteristics Augmentation System (MCAS), has been implicated in both crashes.

“We are informing our customers and suppliers that we are currently estimating that the ungrounding of the 737 MAX will begin during mid-2020,” said Boeing in its statement. “This updated estimate is informed by our experience to date with the certification process. It is subject to our ongoing attempts to address known schedule risks and further developments that may arise in connection with the certification process.”

The company also noted that the regulatory authorities were “rightly” applying “rigorous scrutiny” in their review of the aircraft’s flight control system. The same standards were also being applied by the Joint Operations Evaluation Board, which established the requirements for the training of pilots.

“Returning the MAX safely to service is our number one priority, and we are confident that will happen,” assured Boeing. “We acknowledge and regret the continued difficulties that the grounding of the 737 MAX has presented to our customers, our regulators, our suppliers and the flying public.”

The 737 MAX is the fourth generation of its hitherto renowned 737 single-aisle airliner family. The previous generations were the original – the 737-100 and 737-200 – models; the classic – the 737-300, 737-400 and 737-500; and the next generation – the 737-700, 737-800 and 737-900ER.

The MCAS was developed for the 737 MAX series because this has different aerodynamic characteristics to all previous 737 types. The 737 MAX is powered by two CFM LEAP-1B engines, which are the biggest power plants ever fitted to the 737. The LEAP-1B has a fan diameter of 69 inches (175 cm), whereas the CFM56-7B engine, which powers the 737NG family, has a fan diameter of 61 inches (155 cm). For reasons that go back to the original design of its original model in the early 1960s, the 737 has a relatively short undercarriage and the LEAP-1B, unlike all previous power plants, could not be fitted under the aircraft’s wing (even with the MAX being given a longer nose wheel strut). So new pylons had to be designed, which position the LEAP-1B engines further forward and higher than the CFM56-7B engines on the 737 NG. This affected the thrust line of the engines, and moved the aircraft’s centre of gravity forward.

The result was that, in certain circumstances, the nose of the aircraft could pitch up. The MCAS was developed to automatically counteract any such pitch-up, the system relying on angle of attack sensors to ascertain the aircraft’s attitude. If the angle of attack became too great, threatening to stall the aircraft, the MCAS would activate flight controls to push the aircraft’s nose down. This would be done without any input from the flight crew. Until after the first of the 737 MAX crashes, pilots flying the aircraft had no idea that the MCAS existed.