Britain's Civil Aviation Authority (CAA) has banned all Boeing 737 Max flights in UK airspace after a second fatal crash of the type near Addis Ababa in Ethiopia last Sunday killed all 157 people on board.
Meanwhile, Boeing has promised to issue a software update for an under-fire part of the 737 Max flight control suite.
Since last weekend's fatal accident, a bunch of countries including China, Indonesia, Malaysia, Singapore, Australia, France and Germany have suspended airliners from flying the 737 Max in their territories.
A CAA spokesman said: "As we do not currently have sufficient information from the flight data recorder we have, as a precautionary measure, issued instructions to stop any commercial passenger flights from any operator arriving, departing or overflying UK airspace."
The Register understands that all three Max sub-models – the Max 7, Max 8 and Max 9 – are affected.
The first 737 Max incident was Lion Air flight 610, which crashed 12 minutes after taking off on a scheduled flight from Jakarta on 29 October, killing 189 people. Investigators concentrated their efforts on the jet's Manoeuvring Characteristics Augmentation System, or MCAS, which appeared to repeatedly force the airliner's nose down before its final, fatal dive.
The CAA confirmed to The Register that its ban means flights which are currently airborne will be able to enter UK airspace and land at their destination as planned, but will not be able to take off again. At the time of writing, the ban appeared to affect four 737 Max 8s heading for British airports; two from Turkish Airlines and two from TUI Airways (née Thomson).
Both of the Turkish aircraft turned around and began heading home as this article was being written. The CAA ban document (PDF) states that UK-registered 737 Maxes "must not be flown anywhere" while foreign-registered 737 Maxes "must not be flown in UK airspace".
Boeing said in a statement: "Safety is Boeing's number one priority and we have full confidence in the safety of the MAX... It is also important to note that the [US] Federal Aviation Administration is not mandating any further action at this time, and based on the information currently available, we do not have any basis to issue new guidance to operators."
Software patch in the works
To understand what the kerfuffle is about, you need to know what "trimming" and "angle of attack" mean in aviation, and a little bit about the 737's flight control systems, including MCAS.
Flying an aeroplane successfully means keeping it trimmed. In straight and level flight, an aeroplane is properly trimmed if it will fly straight and level without the pilot having to hold the controls to keep it on course. If you are turning, or climbing, or speeding up or slowing down, the trim tends to change. In modern airliners there are software systems that detect these changes and automatically adjust the trim, making the pilots' job easier.
MCAS is software that was added to the 737 Max series in order to help prevent the aircraft from stalling after getting out of trim. Stalling happens when the airflow's angle of attack over the wings becomes too high. This Boeing handout (PDF) explains angle of attack (AoA).
At its simplest, MCAS adds nose-down trim to the 737 Max if it decides that the AoA is getting too high, as this website explains with excerpts from Boeing publications. If MCAS is fed erroneous data from the aircraft's AoA probe, it may wrongly decide that the aircraft is at risk of stalling and start winding on nose-down trim, operating in 10-second bursts.
In a statement published yesterday evening, Boeing said it will release a software update for the 737 Max no later than April which would prevent MCAS from winding the trimmers to the hard limits of their travel:
For the past several months and in the aftermath of Lion Air Flight 610, Boeing has been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer. This includes updates to the Maneuvering Characteristics Augmentation System (MCAS) flight control law, pilot displays, operation manuals and crew training. The enhanced flight control law incorporates angle of attack (AOA) inputs, limits stabilizer trim commands in response to an erroneous angle of attack reading, and provides a limit to the stabilizer command in order to retain elevator authority.
Around 20 737 Max 8s were airborne over the UK and the European continent when the ban was announced. Aviation geeks can look them up on flight tracking websites by using the ICAO codes B37M, B38M and B39M, which correspond to the B737-700 Max, 737-800 Max and 737-900 Max. A total of 387 Boeing 737 Maxes have been delivered to the world's airlines so far. ®