The grounding of Boeing’s 737 MAX aircraft has produced one of the most bizarre situations in the history of aviation. And what’s driving it, partly, is that modern airliners aren’t just airplanes, but veritable flying rivers spewing gigabytes of data on everything from cabin temperature to engine vibration, all of it recorded and quite a bit of it continuously analyzed.
And that’s how we got to the point that while world airlines and regulators finally slapped a universal grounding on the MAX series, three U.S. airlines—Southwest, American and United—continued to fly the airplane after the second crash in Ethiopia earlier this month. Of the three, only Southwest has explained in detail why it still has confidence in the airplane. In this Wall Street Journal report, Southwest CEO Gary Kelly said the airline’s internal data reviews revealed no anomalies in the MAX that would compromise flight safety. And you can bet they eyeballed the data channels having to do with stab trim, AoA and airspeed sensors, all of which are tied into the airplane’s MCAS stall-protection subsystem.
Southwest has 34 MAX 8’s and according to the company’s website, the airline has accumulated 88,000 hours in 41,000 flights. As airline Big Data goes, that’s a mere droplet, but it’s more than any other U.S. operator has. It’s also more than Lion Air and Ethiopia combined. China’s big three airlines have 97 MAX airplanes and may or may not have more operational hours spread among the three.
But Southwest has been the most forthcoming in explaining why it continued flying, even though it now supports the grounding. Not that it has a choice. Southwest’s data-driven confidence raises some tantalizing questions. In the journal interview, Kelly didn’t say if the data showed any MCAS activations during routine flights, but the data has the granularity to do that, and then some. My guess is they didn’t see any MCAS-active events. Boeing said it didn’t originally recommend specific training on MCAS because it thought pilots would rarely see it active.
Just to refresh, MCAS—for Maneuvering Characteristics Augmentation System—is a background subsystem that automatically adds nose-down stab trim at high angles of attack and load factors. It provides envelope protection against stalls and enhances control feel by increasing stick force to offset the MAX’s tendency to pitch up because the heavier engines are mounted further forward than in previous 737 models. It’s active only when the airplane is being hand flown with the flaps up. Flap deployment or autopilot engagement inhibits it.
Knowing that, is it possible that because U.S. pilots are trained to use the automation routinely—including the autopilot—they simply never remotely got near the MCAS threshold? It’s fashionable to complain about automation eroding piloting skills and the magenta line kids now have kids of their own, but we didn’t drive the accident rate to near zero by practicing hand flying.
Long bony fingers have been pointed at the AoA sensors in the Lion Air accident, but no official findings have been published yet. It could very well be the sensors are simply a distractor not at all related to the crashes. That awaits further revelations, but Boeing has said it reworked the airplane software to accommodate input from both AoA sensors, rather than just one, as was the case when the airplane was certified. While we’re waiting, the second question is if the sensors were faultily designed or fabricated, wouldn’t Southwest’s data have picked up such a fault? Or did Lion Air and Ethiopia get the only two bad ones in the batch? Kelly didn’t say either way, although he did say he thought Southwest pilots were trained well enough to counter a runaway trim abnormal, which a faulty AoA might trigger. The Journal report quotes American Airlines’ director of safety saying they didn’t see anything in the data, either.
There’s a quite natural tendency to believe both MAX crashes were due to something the pilots did or didn’t do. I’ve thought and said the same myself. But the failure mode may be more insidious than some of us are willing to admit and perhaps a little more difficult to diagnose than we might imagine.
That enigma has resulted in 350-plus spanking new airplanes being grounded that at least two airlines believe are perfectly safe to fly. Like I said, it’s one of the most bizarre turns in aviation history.
Note to Readers: No, it’s not something you said. Because of persistent denial of service attacks against AVweb, we’re moving the site to another platform. The commenting section will be unavailable for a time. I apologize for the inconvenience, but the site will be better for it in a week or two. Thanks for your patience.
