First, this accident is still in the field investigation process; until the results of that probe are in, it’s important to avoid speculation. But people who travel by air understandably want to know: is it safe to get on a Boeing 737 MAX plane? The 737 MAX, the doomed model in this crash, and the most popular plane in Boeing history, is flown by (or on order for) close to 40 airlines, according to the Washington Post.
Let’s look at what we know so far. The Digital Flight Data Recorder (DFDR) that was recovered and analyzed indicates some very disturbing circumstances here — ones that should infuriate pilots who fly these planes.
The 737 MAX is an airframe design that dates back to 1967. It is the latest in Boeing’s line-up, includes a “fly-by-wire” (that is, computer controlled) electronic flight control system and highly efficient turbofan engines. Because the engines are also more powerful and heavier than previous versions of the airplane, the tendency for an “underslung” (under the wing) mounted engine to cause the nose of the airplane to rise when thrust is increased, especially at lower airspeeds, becomes more pronounced.
Boeing compensated for the nose-up tendency by programming the flight control computers to apply nose-down stabilizer trim automatically, a function that is invisible to the pilot even when hand-flying the airplane.
This additional Maneuvering Characteristics Augmentation System (MCAS) was added to provide protection against an aerodynamic stall, which mostly occurs at lower airspeed, by programming the computer to stop pilots from raising the nose: in short, it fights pilot input to prevent the plane from stalling.
The trigger for the MCAS system is the angle of attack (AOA) detector, which measures the angle between the wings and the air flow. If the AOA detects an approaching stall, the MCAS is activated. In Lion Air’s situation, it appears that the AOA was faulty, transmitting erroneous data. The pilots unsuccessfully struggled to wrestle control of the airplane away from the computer.
Having spent my entire airline career on Boeing airplanes, I’ve always liked their philosophy of designing the electronics to work in harmony with the pilot rather than protecting the airplane from the pilot. The MCAS system seems to represent the latter.
But here is why 737 pilots (and everyone else) should be angry: The MCAS system was apparently not specifically described in the airplane operation manual. (Boeing Chief Executive Dennis Muilenburg told Fox Business Network on Tuesday that the information was available as part of the manual. CNN has asked Boeing for a copy of the original manuals issued to carriers who bought the Boeing 737 MAX 8, but Boeing has not provided those operation manuals to CNN.) According to experienced pilots, the system was never included in previous versions of the airplane.
How can pilots compensate for a malfunction if they are not aware of how it works? And recognition is key. If crews don’t understand the abnormality, they can’t react via an appropriate emergency checklist. For Lion Air, their altitude of 5,000 feet didn’t leave much time to analyze the situation and then react.
Boeing is now reportedly taking the unprecedented step of not only briefing customers — that is, airlines using the 737 MAX — but also involving the pilots’ unions. And as of November 9, a procedure that involves pilots disconnecting the stabilizer trim switches in the cockpit is now included in flight manuals as a solution to Engine Indicating and Crew Alerting System (EICAS) warning messages that relate to faulty sensors. I worked as an airline pilot for 34 years, and this very rapid response is, in my experience, unprecedented.
Some reports indicate that the AOA detection system may have malfunctioned on previous flights with another crew. Was the malfunction repaired according to Boeing’s procedures? Was it the same malfunction experienced by the accident crew? Did the airplane react the same way on the Lion Air accident flight? Did the malfunction on the previous flight occur at a higher and safer altitude? The answers to these questions are what the ongoing investigation will determine.
Is the 737 MAX a safe airplane? The short answer is “Yes.” Now that pilots are armed with better knowledge of the MCAS system and have a quick solution for related abnormalities, the airplane is even safer.
But Boeing still has a lot of explaining to do.
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