Why Air Crash Of Ethiopian And Lion Air Flights Happened?

Boeing 737 Max aircraft which will be discussed below is not a new aircraft and it was launched in year 1968 by leading aircraft manufacturer company Boeing after that Boeing has made many minor upgrades and updates in this aircraft. Two major changes were   done in this aircraft, first in year 2000 when Boeing launched new aircraft Boeing NG or Next Generation, in this aircraft wingspan and overall seating capacity was increased beside these new engines were incorporated and old cockpit was replaced with new glass cockpit. Second change took place recently in year 2017 with launch of Boeing 737 Max aircraft. 

Boeing 737 is a very good aircraft in its performance, aerodynamics, handling  and its rate of climb resembles like a rocket, its performance can be appreciated because it can make take-off on a single engine also. Now the question arises if the performance of the aircraft is so good then why air crash of Ethiopian and Lion Air Flights happened? MCAS was responsible or pilot error?

The main competitor of Boeing is Airbus. When airbus launched its A320 Neo, then Boeing launched its 737 Max aircraft against 320 Neo. In 737 Max Boeing changed its some design features. In normal planes engines of aircraft lies below wingspan as old version of Boeing variants but in 737 Max engines were slightly shifted forward and also in upward. Actually py-lance was redesigned. If you have seen Boeing aircraft, then you have noticed that engines remains close to ground means smaller ground clearance for aircraft, so the bottom part of  engines were kept flat in old variants. But when powerful engines were installed in Boeing 737 Max, company needed more room to fit engines along with sufficient ground clearance. Due to this reason engines were shifted from their standard positions to compensate these changes. This changed designed feature doesn’t affect aircraft normally, but every aircraft in its flight passes through a situation called stall. Stall is an undesirable situation for a flight, actually the lift for aircraft increases with angle of attack proportionally, but after a certain angle of attack lift vanishes abruptly and in this situation aircraft get stalled. Stall is a subject of advance aerodynamics which is considered during design of aircraft. During testing of aircraft Boeing found that stall characteristics have been changed due to changes made for sufficient ground clearance and MCAS system was come into existence to compensate these changes. 
Boeing Aircraft

MCAS stands for Maneuvering Characteristics Augmentation System. MCAS was installed to control maneuvering characteristic of the aircraft. In normal flight take-off when aircraft reaches to its take-off speed control column has to be pulled for the movement of elevators. Elevator is a control surface on rudder for the control of pitch motion of aircraft. When high speed air flow over it this elevator downs the tail or empennage section of aircraft due to which nose of aircraft goes in upward direction and aircraft makes takeoff and starts to climb as engines continuously provides thrust.
During climb period of aircraft the control column has be held in a fixed position until cruising altitude reached. Pilot takes advantages of trim control. Elevators are controlled by hydraulics actuators and horizontal stabilizers are controlled by jack screw mechanism. The motion of horizontal stabilizers is directly controlled by pilots, switches are provided in yoke/control to control motion. The movement of horizontal stabilizer is crucial for whole aircraft as it makes aircraft sensitive to airflow. So according to need and situation pilots deliberately trims aircraft. Trim actually locks control surfaces in fixed position made by pilot. Trim takes aircraft up-to cruising altitude and this will continue even after cruising altitude if pilot does takes control over it. Similarly during landing or decent trim is taken in use. And every aircraft is incorporated with this.
MCAS activates in the situation when pilot by mistake applied air brakes or throttle system get disengaged due to technical failure and speed of aircraft begins decrease and aircraft starts decent motion. When plane starts decent motion, then autopilot system of aircraft starts to nose up the plane to maintain the required altitude. With decrease in speed of aircraft   angle of attack or pitch angle continuously increases up-to point called critical angle of attack. At this point the aircraft would have its minimum velocity and maximum angle of attack, in this scenario plane will not have sufficient speed to continue motion so the plane starts to fall like a leaf. There are many warning system and shakers to warn pilot. Stall recovery maneuvering system is basic training system given to every pilot to recover from stall. In stall recovery pilots take help of elevators, they nose down the plane and with help of gravity and speed is built for airplane. Now, one can smartly question why pilot not increases speed with speed throttles, exactly at this point MCAS is needed. Engines are mounted below the wings and at the time stall it has direction somewhat parallel to pitch angle. In this situation when pilot will give maximum thrust to engine, engine will develop pitch up force and it would not let the plane to nose down. That’s why first pilots nose down the plane and then gently thrusts the plane with pitch angle and they continue the process so that plane don’t face secondary stall. 

In case of 737 Max with changed design feature of airplane, the phenomena of increase in pitch with thrust get increased, to compensate this MCAS was invented by Boeing. In its functioning MCAS system automatically triggers before a plane reaches to critical angle of attack and within time MCAS trims the aircraft and starts to nose down the plane. This was the main purpose to mount MCAS. 
Cockpit View

In both the airplane crashes Ethiopian and Lion Air Flights, both planes appear to have lost altitude in first few minutes of their flights. Why it happened is still in under investigation. There are many sensors and pitot tubes on plane which continuously transfers the data of angle of attack to the flight computer system. If there is fault is this tubes or sensors then MCAS will be automatically triggered. It also may happen that MCAS activates at wrong time. 

Question arises that why pilot didn’t handle situation by taking manual control over plane?

At present time aircraft is a very advance machine; if a pilot makes a mistake then airplane will understand it smartly and will ignore and cancel all the command given by pilots. If situation arises when pilot confidently knows that autopilot is doing something wrong, then automation system can be disabled and manual control over the plane can taken.
I haven’t seen the cockpit of any Boeing aircraft and I also cannot say why the pilot didn’t take control over plane.

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