Boeing omitted flight-control features on the 737 MAX commercial jetliner that were included in an earlier version of the system designed for a military tanker, according to a news report.

Citing unnamed people familiar with the matter, The Wall Street Journal reported that the anti-stall system for the US Air Force's KC-46 Pegasus tanker limited its ability to point the nose of the plane down to avoid a mid-air stall. The tanker is designed for mid-air refueling of fighters and other aircraft.

Investigators believe the MAX's Maneuvering Characteristics Augmentation System (MCAS) repeatedly pointed the nose of the commercial planes down, and they went into a fatal plunge, killing all aboard commercial flights in Indonesia and Ethiopia.

The system used in the military aircraft pointed the nose of the aircraft down only once and therefore was easy to override in an emergency.

The anti-stall system used in the military air tanker was developed about 10 years prior to the MCAS used in commercial jets, but the decision not to replicate it on MAX aircraft doesn't mean that Boeing was negligent, an analyst believes.

"It comes down to what customers required and what the marketplace dictated," Robert Mann, president of R.W. Mann & Co, an aviation consulting firm in Port Washington, New York, told China Daily. "The airlines wanted a new plane common to the MAX's predecessor, the Boeing 737 NG, so it wouldn't require a new certificate and additional pilot training. The airlines wanted a derivative — and that's what they got."

However, he said basing the MAX on prior designs proven in service doesn't validate the use of a single data vane in the commercial jet's anti-stall system or suggest a conspiracy to put profit above safety.

"This isn't a conspiracy any more than the holes in Swiss cheese are a conspiracy," Mann said.

In a statement, Boeing said: "The KC-46 tanker does use a Maneuvering Characteristics Augmentation System flight control law. However, the architecture, implementation and pilot interface of the KC-46 tanker MCAS are different than that of the 737 MAX. The systems are not directly comparable."

The Air Force's KC-46 tanker is based on Boeing's wide-body 767 jet used by commercial airlines. The military version included wing pods that increased lift and to raise the nose of the plane under some flight conditions. To handle the problem, Boeing developed MCAS software to point the nose of the tanker down when necessary and to meet FAA standards, the Journal said.

The MAX's engines were larger and more fuel-efficient than previous 737 engines. The increased weight meant the engines were placed closer to the fuselage, and that caused the plane's nose to rise in certain flight conditions, increasing the risk of a mid-air stall and creating questions about certification of the commercial airliner by the FAA.

In general, the MAX's larger engines had the same effect on the aircraft's aerodynamics as the refueling pods on the military tanker. Boeing solved the problem with the MCAS.

The horizontal stabilizer — the control device at right angles to the airplane's tail, is moved only one time by the anti-stall device on the military plane, allowing the pilot to regain control of the aircraft by pulling on the controls, or yoke.

In the MAX, the anti-stall device remained active, and overriding the system is therefore more complicated. MAX designers determined pilots would quickly realize the MCAS was misfiring, creating a condition known as "runway stabilizer" and use established cockpit procedures to counteract it.

China was the first nation to ground MAX jets last March after crashes on Oct 29, 2018, in Indonesia and March 30 in Ethiopia killed 346 passengers and crew. Investigators have focused on the anti-stall device that may have erroneously pointed the nose of the planes down to gain speed to prevent a mid-air stall and into a fatal plunge.

A single sensor used in the commercial version of the anti-stall system may have provided erroneous information and may have told MCAS the plane was about to stall. There are expected to be two sensors in the redesigned system to determine the plane's "angle of attack", or angle of the plane's nose in flight, and the anti-stall system will turn off only once rather than repeatedly. Boeing has not yet submitted the updated software to the FAA for review.

Boeing hopes the MAX jets will return to service in the fourth quarter of this year, but it's unlikely that foreign regulators will adopt the Federal Aviation Administration's findings as they have in the past.

Different regulators in different countries may impose different requirements, and the jets may be cleared for commercial service in some countries but not others. That could create a staggered return to service for the jetliner and could create scheduling problems.

Returning the jets to service in the fourth quarter may be too optimistic. Southwest Airlines, which operates more MAX flights than any other US airline, has canceled use of the jet through Jan 5.