These crashes were not merely due to any pilot errors or miscommunication. The real reason behind them involves far more convoluted details.
In order to understand the real reason of these crashes, it is necessary to roll back in time (in 2010) when Airbus was planning to upgrade their A320 aircraft.
Jet fuel is expensive and is one of the major contributing factors in terms of cost incurred to the airline.
Thus, improving fuel efficiency is one aspect which the aircraft manufacturers look forward to and make it their unique selling point.
This was the case with Airbus back in 2010. Therefore, Airbus came out with an updated version of the A320 family which came to be known as A320 NEO where NEO stands for "New Engine Option".
The updated version - Airbus installed new engines which had a larger diameter compared to its predecessors thus improving the fuel efficiency of the aircraft by 15%.
Interestingly, the engines could be installed on what was basically the same airframe because the ground clearance was sufficient enough to accommodate an engine with a larger diameter.
As a result airliners switched to buying A320 NEO airplanes more than any variant of Boeing 737.
Therefore, Boeing was forced to do some similar modifications to its single-aisle 737 as well to give competition. But there was a problem!
Unlike the Airbus A320, the ground clearance of Boeing 737 was much smaller. As a result, incorporating an engine of a larger diameter was not feasible.
The possible solution for incorporating a larger diameter engine would be to jack up the aircraft, but that will involve huge costs which Boeing did not wanted to incur.
Moreover, this process would also reduce the commonality with its current fleet which in-turn will result in high costs incurred by the airlines.
Nevertheless, in a March 2011 interview with Aircraft Technology, Mike Bair, then the head of 737 product development, said that re-engineering was possible.
“There’s been fairly extensive engineering work on it,” he said. “We figured out a way to get a big enough engine under the wing.”
So the engineers mounted the engines of larger diameter by installing them at a higher vertical level compared to its predecessors and shifted the engine in the forward direction.
By doing so, the aerodynamics of the aircraft changed significantly compared to the previous ones.
The problems associated with this modification:
1. Because of shift in the placement of engines in vertical direction, the thrust centreline changed as well.
2. Also because of this shift of engine nacelle in the front direction, when the power was increased, it resulted in lift generation by the engine nacelles as well particularly at high angle of attack.
As a consequence, the engines started developing a pitch up moment (clockwise direction), and allowed the 737MAX aircraft at an angle of attack to go to an even higher angle of attack.
Although the correct step here for Boeing would be to modify the airframe so as to compensate for the this erratic pitch up behavior, they in-turn decided to develop a system which could bring the nose back to level position as the former step would again have involved huge costs.
So, Boeing decided to create what is known as the Maneuvering Characteristics Augmentation System (MCAS) which would certainly have been less expensive.
2. Also because of this shift of engine nacelle in the front direction, when the power was increased, it resulted in lift generation by the engine nacelles as well particularly at high angle of attack.
As a consequence, the engines started developing a pitch up moment (clockwise direction), and allowed the 737MAX aircraft at an angle of attack to go to an even higher angle of attack.
Although the correct step here for Boeing would be to modify the airframe so as to compensate for the this erratic pitch up behavior, they in-turn decided to develop a system which could bring the nose back to level position as the former step would again have involved huge costs.
So, Boeing decided to create what is known as the Maneuvering Characteristics Augmentation System (MCAS) which would certainly have been less expensive.
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| Source: https://www.flickr.com/photos/dirkpons/48010786076 License URL:https://creativecommons.org/licenses/by/2.0/ Credit: Dirk Pons |
Function of MCAS?
MCAS is responsible for pushing the nose of the plane down when the system thinks that the aircraft might exceed the critical angle of attack and go in what is known as aerodynamic stall (sudden loss of lift).
So it commands the aircraft's trim system to lower the nose as well as pushes the pilot's control columns (these are the things which helps pilot control the aircraft manually).
The MCAS is directly connected to what is called as the Angle-Of-Attack (AOA) Sensor which is responsible for giving pilots the information regarding what angle of attack the aircraft is flying at.
In both the crashes, it was first the angle of attack sensor which gave wrong readings (due to in-servisibility) to the MCAS. The system when it senses a reasonably high angle of attack prompted the aircraft to nose down along with pushing the control columns forward.
This resulted in plane going in a nose dive and since it achieved high velocities as a consequence, the pressure on the control surface (elevator) was too large which was nearly impossible for the pilots to overcome it by taking manual control.
But this manual control in the first place can be gained only when the pilot knows how to disengage the MCAS.
It was found out that pilots were not given any information that the new 737MAX incorporated a MCAS.
Apart from that even though the AOA sensor was faulty, the MCAS system too had numerous errors in its algorithm.
A potential solution could have been the pilots taking over the plane when AOA sensor gives faulty readings and keep MCAS as a redundant system instead of AOA sensor directly reporting to the MCAS.
In conclusion, had the AOA sensors were serviced along with the pilots having the knowledge on MCAS and how to disengage in case it activates, the situation would have been much different.
It is these chain of events which contributes to a bigger event.
MCAS is responsible for pushing the nose of the plane down when the system thinks that the aircraft might exceed the critical angle of attack and go in what is known as aerodynamic stall (sudden loss of lift).
So it commands the aircraft's trim system to lower the nose as well as pushes the pilot's control columns (these are the things which helps pilot control the aircraft manually).
The MCAS is directly connected to what is called as the Angle-Of-Attack (AOA) Sensor which is responsible for giving pilots the information regarding what angle of attack the aircraft is flying at.
In both the crashes, it was first the angle of attack sensor which gave wrong readings (due to in-servisibility) to the MCAS. The system when it senses a reasonably high angle of attack prompted the aircraft to nose down along with pushing the control columns forward.
This resulted in plane going in a nose dive and since it achieved high velocities as a consequence, the pressure on the control surface (elevator) was too large which was nearly impossible for the pilots to overcome it by taking manual control.
But this manual control in the first place can be gained only when the pilot knows how to disengage the MCAS.
It was found out that pilots were not given any information that the new 737MAX incorporated a MCAS.
Apart from that even though the AOA sensor was faulty, the MCAS system too had numerous errors in its algorithm.
A potential solution could have been the pilots taking over the plane when AOA sensor gives faulty readings and keep MCAS as a redundant system instead of AOA sensor directly reporting to the MCAS.
In conclusion, had the AOA sensors were serviced along with the pilots having the knowledge on MCAS and how to disengage in case it activates, the situation would have been much different.
It is these chain of events which contributes to a bigger event.
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