Boeing's 737 MAX crisis

After the two accidents involving Boeing's 737 MAX series, aviation safety standards will likely change

The grounding of the Boeing 737 MAX series may remind aviation historians of the De Havilland Comet. The Comet was the first commercial jetliner. It was designed by Sir Geoffrey de Havilland, who designed the legendary World War II Mosquito, and the popular Moth range. 

Inducted into service in 1952, the Comet had several crashes. One of the worst was at Dum-Dum airport, Kolkata in 1953. The wings came off at take-off and the plane turned into a fireball, killing all 43 people on board. 

After the Comet was grounded, several design flaws were discovered. The wing profile could cause problems with lift in some circumstances. Stresses on the fuselage during flight were more dangerous than had been realised and metal fatigue was studied in detail. Better aero design resulted as aircraft manufacturers absorbed those lessons. A redesigned military variant of the Comet, the Nimrod, remained in service with the Royal Air Force until the 21st century. 

Boeing’s 737 Max series has been grounded worldwide, following two crashes, with 346 fatalities. The first crash was a Lion Air flight in Indonesia on October 29, 2018 and the second, was an Ethiopian Air crash in Ethiopia on March 10.  Investigators say there are “clear similarities” in the incidents caused by problems with the MCAS, or Manoeuvring Characteristics Augmentation System, which is used in the 737 Max. While the 737 family has been in use since 1967, with over 10,000 variants delivered by Boeing, the Max series underwent major redesign.

This was to comply with modern standards of fuel-efficiency, and make it competitive with the Airbus A320Neo. The 737 is a twin-engine, narrow-body (single aisle) aircraft. The Max series has variants, seating between 140-230 passengers, with ranges of 6,000-7,000 km. The first delivery was in May 2017 and by January 2019, Boeing had 5,011 orders and it had delivered 350. 

The new Leap-1B engines claimed to reduce fuel consumption by 14 per cent in the Max, compared to previous 737s. The Max was touted as 4 per cent more fuel-efficient than the Airbus Neo. The new engines are located further forward, and placed higher on the wings. This changes the aerodynamic characteristics, compared to the previous 737-NG series. The new configuration can result in the nose of the aircraft rising, especially at slow speeds. An aircraft depends on lift caused by airflow under the wings to keep it flying. 

The wings are always at some angle to flight direction. This is called the “angle of attack” (AoA) . Lift depends on the AoA. The maximum lift is attained at the “critical angle of attack”. If the AoA is higher than the critical angle, lift decreases, leading to stalls.  

As the nose rises, AoA increases, and as mentioned above, the 737 Max has a tendency for the nose to rise. The response to exceeding critical AoA is to pull the nose down, until AoA comes back to the safe zone. The MCAS is a sensor-based software system, which is supposed to do this automatically. It calculates air-speed and angle of attack and moves stabilisers in the tail to push the nose down (by pushing the tail up).  

Unfortunately, there seem to be problems with the MCAS. One is that the sensors were apparently misbehaving, during the Lion Air crash, at least.  The MCAS repeatedly diagnosed a stall and pulled the nose down, when the AoA was actually alright. The pilots could do nothing about this and may not even have realised there was a problem until it was too late. 

The MCAS can be disconnected if the pilots realise there’s a problem but two optional safety features were missing on both planes. One is an AoA indicator, which displays the sensors’ readings; the other is a light that activates if the sensor readings disagree with each other. 
 

The “disagree” light costs $80,000 – not much, given that the plane costs over $120 million. 
 

Pilots also say that they have not received enough training on a plane that has changed flight characteristics, and uses a new software system, which may misbehave. Capt. Dennis Tajer of American Airlines, who is the spokesman for the Allied Pilots’ Association, says pilots received just 56 minutes training on an iPad when shifting from the 737NG, to the 737 Max. 

Boeing now faces class-action lawsuits and it has of course, seen spate of cancelled orders. Aviation industry regulations and safety standards may well change as a result of these two incidents. If that happens, at least the deaths will not have been in vain.