Much like tapping the brake pedal in a car to disengage cruise control, a sharp tug on the controls of older models of Boeing Co’s 737 used to shut off an automatic trim system that keeps the plane flying level, giving the pilot control.

But Boeing disabled the “yoke jerk” function when it brought out the 737 MAX, the latest version of its top-selling jet — and many pilots were unaware of the change, aviation experts told Reuters.

The difference may help explain why pilots struggled to keep their aircraft climbing after takeoff on two fatal 737 MAX flights less than five months apart that killed 346 people.

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SINGAPORE (Reuters) - U.S. and European regulators knew at least two years before a Lion Air crash that the usual method for controlling the Boeing 737 MAX’s nose angle might not work in conditions similar to those in two recent disasters, a document shows.

The European Aviation and Space Agency (EASA) certified the plane as safe in part because it said additional procedures and training would “clearly explain” to pilots the “unusual” situations in which they would need to manipulate a rarely used manual wheel to control, or “trim,” the plane’s angle.

Those situations, however, were not listed in the flight manual, according to a copy from American Airlines seen by Reuters.

The undated EASA certification document, available online, was issued in February 2016, an agency spokesman said.

It specifically noted that at speeds greater than 230 knots (265mph, 425kph) with flaps retracted, pilots might have to use the wheel in the cockpit’s center console rather than an electric thumb switch on the control yoke.

EASA and the U.S. Federal Aviation Administration (FAA) ultimately determined that set-up was safe enough for the plane to be certified, with the European agency citing training plans and the relative rarity of conditions requiring the trim wheel.

In the deadly Lion Air crash in October, the pilots lost control after initially countering the Maneuvering Characteristics Augmentation System (MCAS), a new automated anti-stall feature that was pushing the nose down based on data from a faulty sensor, according to a preliminary report from Indonesian investigators released in November.

The flight conditions were similar to those described in the EASA document, a source at Lion Air said. The source said that training materials before the crash did not say the wheel could be required under those conditions but that Boeing advised the airline about it after the crash.

Boeing declined to comment on the EASA document or its advice to Lion Air, citing the ongoing investigation into the crash.

Ethiopia’s Transport Ministry, France’s BEA air accident authority and the FAA have all pointed to similarities between the Lion Air crash and an Ethiopian Airlines disaster this month. But safety officials stress that the Ethiopian investigation is at an early stage.

‘NOT PHYSICALLY EASY’

The crashes have also heightened scrutiny of the certification and pilot training for the latest model of Boeing Co’s best-selling workhorse narrowbody, now grounded globally.

In the EASA document, the regulator said simulations showed the electric thumb switches could not keep the 737 MAX properly trimmed under certain conditions, including those of the Lion Air and Ethiopian Airlines crashes, according to the Indonesian preliminary report and a source with knowledge of the Ethiopian air traffic control recordings.

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