A long-standing Boeing 737 emergency procedure that the pilots of two MAX jets employed while attempting to avert fatal crashes is under review by the Federal Aviation Administration (FAA), according to the head of a major pilots union.

The basic checklist for handling an issue known as runaway stabiliser trim has remained substantially unchanged since 1967, the earliest days of the 737 jet liner, CNN quoted Dennis Tajer of the Allied Pilots Association, as saying on Friday.

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He said FAA officials told airline safety officials and pilots unions in April of the review.

Such a review is “uncommon” after an accident, said David Soucie, a former FAA safety inspector, adding that hanging the checklist could require pilots to undergo additional training.

The trim on the horizontal stabiliser, on an aircraft’s tail, is one way of controlling how steeply the plane climbs or descends.

In addition to manual control, it can be moved by the autopilot and a new feature in the 737 Max that has come under scrutiny from crash investigators, the Manoeuvering Characteristics Augmentation System (MCAS).

The checklist is intended to give pilots an emergency method to override automated systems and regain control of the plane.

Asked to comment on the review, an FAA spokesman on Friday did not say when the procedure was last revised.

In the wake of the October 2018 Lion Air crash in Indonesia, Boeing issued a bulletin for pilots reinforcing the procedure and revealed to them the presence of the MCAS system.

Since the second 737 MAX crash on March 10 in Ethiopia, Boeing has defended the runaway stabiliSer trim procedure as sufficient.

The two crashes killed 346 people.

The Indonesian and Ethiopian preliminary reports both say the pilots followed the procedure but both the planes ultimately crashed.

Tajer told CNN on Friday that FAA officials also disclosed reviews of two other emergency checklists — for handling inaccuracies known as airspeed and angle of attack — and that the reviews can help determine if the lists contain the details they need several generations of 737 later.

The procedures have not been updated, he said, “since Lyndon Johnson was President and the country was struggling with the Vietnam War”.