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.
0 Comments