One crash where pilots were in conflict with the stall prevention system – could have been pilot training and experience. Two crashes in a few months with the same pattern of the nose pitching up and down and signs that the pilots were struggling with the controls might be more than a coincidence.
Subsequently we learned that the stall prevention system, known as MCAS, relied on a single sensor for Angle of Attack (AOA) measurement – that’s how steeply the plane’s nose is pitched up relative to the airflow – a critical measurement of how likely the plane is to stall, and lose lift. A single sensor? Was it so reliable that it could be totally trusted, and then fail twice in 5 months? No – of course, there’s more….. much more.
The 737 has been in production for 5 decades, and has gone through a number of stretches, facelifts, control system modernizations, and most importantly in the cutthroat world of airlines for keeping the cost per passenger mile down, engine upgrades. The last engine upgrade from manufacturer CFM on the 737-600, 700, 800 had a distinctive flattened bottom to the air intake, because the 737 sits quite close to the ground.
A few years ago, Pratt & Whitney, engine supplier to subsidized European rival, Airbus, finally perfected a geared turbofan engine which they had been working on for 20 years, allowing the A320 model to gain a huge jump on the 737 in terms of power, noise, emissions, and most importantly, fuel consumption. Boeing was in a difficult corner – CFM had an incrementally improved engine, but no way would it fit on the current series of 737s.
The successor that wasn’t – while touted as the next enhancement of the venerable and trusted platform, the 737 Max had a stretched airframe, different wings, much larger engines with more thrust, longer landing gear, especially the nose gear, resulting in a different center of gravity, an initially higher angle of attack due to the nose gear, and a tendency for the bigger engines to pitch the nose up even more. The trick was how to make it feel and fly as much like the prior model as possible. This is especially important for FAA certification and minimizing pilot retraining.
Enter the “Maneuvering Characteristics Augmentation System” (MCAS) – software additions to the autopilot which used that single AOA sensor to prevent the nose pitching too high during takeoff by trimming the tail surfaces to push the nose back down – what could go wrong?
Of course, Boeing did not entirely trust the single sensor, and there was additional software to monitor the second sensor on the opposite side of the nose for differences large enough to indicate malfunction. The software update announced last week makes an “idiot light” for sensor disagreement a standard feature, as well as limiting the number of times the MCAS will push the nose down. Surprising though it was that checking both sensors was not initially part of the design, what was truly stunning was what a Boeing exec told a group of American Airlines pilots (WSJ):
On Nov. 27, about a month after the first crash, Boeing executive Mike Sinnett told American Airlines’ pilot union that their pilots wouldn’t experience the sort of problems that doomed the Lion Air flight, according to Dan Carey, union president. That’s because American paid for an additional cockpit warning light that would have alerted them to the problem, while Lion Air and most other airlines didn’t.
“This wouldn’t have happened to you guys,” Mr. Carey recalled Mr. Sinnett saying during the meeting. The cockpit indicators would have directed pilots to have the potential problem checked out on the ground. A Boeing spokesman said Mr. Sinnett didn’t recall making that statement, and was unavailable for an interview.
Wait just a cotton pickin’ minnit pardner! You’re telling me those doomed Indonesian and Ethiopian pilots could have had a warning in their faces if the airlines had paid for it?? How could an essential warning for a mission-critical system be an optional extra?? Even the greedy auto makers don’t charge extra for a warning when your anti-lock brakes (no longer optional on most cars) have a problem!
But wait, there’s even more – if you pay for it! Yesiree, Bob, Pilots who really want to know what’s going on with those two sensors, can add another option to display the angles reported by both of them, and THAT, friends, is still an extra cost item as of the initial announcement of the software fix. SO, was safety an optional extra on the 737 Max version 1.0? We report, you decide!
Most images courtesy King 5 News, Seattle, who posted this video describing the announcement of the fix: