Montreal - Last week Boeing's newest generation single-aisle jet, the 737 MAX, made headlines due to two fatal crashes at short intervals - Lion Air Flight 610 on October 29, 2018, killing 189 people, and Ethiopian Airlines Flight 302 on March 10, killing another 157 passengers including crew members.
Both aircraft crashed just a few minutes after takeoff. Similarities between the two accidents raised questions about the safety of the aircraft, and several civil aviation regulators imposed an immediate flight ban for the Boeing 737 MAX series jets in service.
At first, the US Federal Aviation Agency announced that it didn't see any basis for a flight ban. But after reviewing aircraft tracking data from satellites and physical evidence retrieved from the crash site suggested a possible link between the two accidents, the agency ordered airlines to ground all Boeing 737 MAX jets.
One piece of evidence, the horizontal stabilizer of the aircraft found at the Ethiopian crash site suggested that the elevators were tilted upwards, which should have forced the nose of the aircraft down.This was also the case in the Lion Air crash.
Flight 302 crash site
At that point, a problem in the flight control software of the aircraft seemed to point to the cause of both accidents, or perhaps the software itself is was not the only problem. Consequently, the design of the aircraft needs to be looked at.
The new 737 MAX comes with new software called MCAS (Maneuvering Characteristics Augmentation System), which didn't exist on the previous generation of Boeing 737s.
This was required because the aircraft had bigger engines than earlier models, providing up to 14% savings in fuel consumption. They are fitted further forward under the wings, which could potentially cause the plane to stall. In order to prevent this, Boeing engineers developed the new MCAS software.
737NG vs 737 MAX engine
The software tells the flight control system to change its Angle of Attack (AOA) downward if a stall risk is perceived, which the computer initiates automatically. The pilots would have no chance to intervene even if the autopilot is switched off.
On the aircraft, there is a sensor called the “Alpha Vane”. This sensor measures the AOA. There are two of them, on both sides of the plane.
The video below explains the working principle of the system very well.
This seems to be the problem. In the case of Flight 610, a faulty sensor sent false AOA data and MCAS took control of the aircraft. The system concluded that the AOA of the plane was over 40 degrees and pushed the aircraft’s nose down, causing a sharp dive into the Java Sea.
The initial data point out a similar situation in the case of Ethiopian Flight 302. Boeing has promised a software update in the coming weeks. However, Boeing's reputation has certainly been damaged, and the manufacturer will need extra effort to regain the confidence of its customers and travelers worldwide.