The Federal Aviation Administration has issued an emergency order addressing the risk that faulty angle-of-attack inputs could cause Boeing 737 Max horizontal stabilisers to put the aircraft into a difficult-to-control dive.
"Possible erroneous angle-of-attack inputs on Boeing 737 Max aircraft… can potentially make the horizontal stabilizers repeatedly pitch the nose of the airplane downward, making the aircraft difficult to control," says the FAA in a 7 November emergency airworthiness directive.
Effective immediately, the FAA orders US operators to revise flight manuals "to give the flight crew horizontal stabiliser trim procedures to follow under certain conditions", the order says.
The FAA gives operators three days to make the updates.
The directive marks a major regulatory response following the 29 October crash of a Lion Air 737 Max 8 (registration PK-LQP), which was operating flight JT610 when it crashed into the sea, killing all 189 people on board.
"The FAA continues to work closely with Boeing, and as a part of the investigative team on the Indonesia Lion Air accident, may take further appropriate actions depending on the results of the investigation," the agency says. "The FAA has alerted foreign airworthiness authorities who oversee operators that use the 737 Max of the agency’s action."
Boeing did not immediately respond to a request for comment.
An initial review of the Lion Air 737's flight data recorder revealed that the aircraft operated its last four flights with faulty airspeed indications, according to Indonesia's National Transportation Safety Committee.
The investigation remains open.
Boeing responded on 6 November with an "operations manual bulletin" that addressed instances of "erroneous input" from angle-of-attack sensors on the 737 Max.
Boeing's bulletin "directed" airlines to follow "existing flight crew procedures" intended to address such instances, the company has said.
The FAA's order targets the latest generation of Boeing's best-selling 737. The company delivered the first 737 Max in May 2017 and has since handed over 219 of the type, according to Boeing's website.
Boeing has unfilled orders for 4,783 additional 737 Max, it says.
Some airlines stress that their crews have already been trained in the procedures highlighted in Boeing's service bulletin.
"WestJet has received the Boeing bulletin and is following its guidance, which recommends emphasising established procedures that have been used and trained on WestJet’s existing 737NG fleet as well as the 737 Max," the Canadian airline tells FlightGlobal.
Malfunctions of airspeed and angle-of-attack indicators occur occasionally, but crews typically deal with such problems without incident, tapping fundamental skills learned while piloting small aircraft in their pre-airline years, says former National Transportation Safety Board member John Goglia.
But, though pilots may be highly-trained to operate technology-laden jetliners, many have less experience flying without such systems, particularly if those pilots hail from countries with less-developed general aviation industries, he says.