UK cockpit crew representatives are seeking assurances on several aspects of the Boeing 737 Max’s redesign, including scenarios relating to the angle-of-attack sensors and the potential need for two pilots to turn the trim wheel if the jet is out of trim.

Pilots union BALPA has formally responded to a US FAA notification detailing proposed changes to the Max intended to address design weaknesses linked to the fatal Lion Air and Ethiopian Airlines accidents, weaknesses which subsequently led to the type’s grounding last year.

Sourcing both angle-of-attack sensors, rather than one, to activate the MCAS system – which automatically pushes the nose down to avoid a high angle-of-attack situation – is “clearly an improvement”, says the union.

But it argues that Boeing ought to have pursued a “preferable” three-sensor system, as used by Airbus A320s, which allows a computer to compared sensor data and eliminate, by ‘voting out’, a spurious sensor reading that does not match the other two.

The Max redesign prevents activation of MCAS if the two angle-of-attack sensors disagree, but the union queries how the aircraft would respond if both sensors produced erroneous – yet insufficiently different – readings.

“Is this viewed as an extremely improbable event?” it asks.

Max c Boeing

Source: Boeing

One aspect which emerged during the inquiry into the Ethiopian accident was the crew’s difficulty in correcting the aircraft’s out-of-trim condition using the trim wheel at high airspeed.

The FAA’s notification states that a revised checklist for manually trimming the horizontal stabiliser will note that a “two-pilot effort” may be used.

“Requiring both crew members to turn the trim wheel simultaneously in a non-normal scenario is extremely undesirable,” says BALPA, adding that it “goes against all philosophies” of having flying pilot operating the controls and a non-flying pilot reading the quick-reference handbook.

The union points out that the Max has a smaller-diameter trim wheel, which enables larger display screens in the cockpit, and wants “assurance” that a single pilot can still turn the wheel at airspeeds of perhaps 40kt beyond the maximum operating limit speed.

It also suggests that Boeing should revisit procedures such as the “rollercoaster” manoeuvre to mitigate significant retrimming problems.

This manoeuvre involves trimming nose-up by repeatedly pulling on the control column until the nose is far above the horizon, and trimming as column back-pressure is released – a similar principle to easing pressure on a fishing line while rapidly winding the reel in order to maintain a pull on the catch.

BALPA uses the British Midland Boeing 737-400 accident at Kegworth in 1989 to underline some of its remarks about the Max redesign, notably regarding the reduced certification scrutiny associated with derivative developments.

The Kegworth accident resulted from the crew’s responding to an engine failure by inadvertently shutting down the wrong engine.

UK investigators noted the modernised aircraft featured solid-state electronic engine indicators, rather than the previous electromechanical hybrid displays, and remarked that the change would have required “different techniques” to interrogate the information – skills the pilots might not have acquired before being subjected to a stressful emergency in the cockpit.

“It is strongly felt that all future substantial aircraft design changes should result in certification as a new type with a commensurate level of training required for pilots,” says BALPA in its comments to the FAA.

The union adds that the Kegworth investigation highlighted the “importance of an ‘attention-getting’ facility”, and BALPA wants clarification as to whether Max pilots will receive an automated call-out alerting them to activation of MCAS, ensuring that they are aware of its being triggered.

BALPA also says it “strongly supports” the proposed mandating of an ‘angle-of-attack disagree’ alert on the 737 Max, and argues that the optional angle-of-attack gauge should be a standard fit on the type.