Lion Air flight JT610 crashed into the sea off Indonesia’s island of Java on October 29, minutes after taking off from Jakarta, killing all the 189 on board. Indian pilot Bhavye Suneja was one of the two in the cockpit of the ill-fated aircraft.
On Tuesday, Boeing issued a bulletin to all airlines and pilots who operate the 737 MAX. Based on the bulletin, it appears that the nose of the Lion Air aircraft pitched down during the climb-out phase in response to an erroneous input from the sensor. With its nose pitched down, the aircraft probably dived into the sea at high speed.
For a safe climb-out, the aircraft’s nose is pitched up at a small angle. This puts both its wings at an acute angle with respect to the oncoming airflow. This angle between the wing and the oncoming airflow is called the ‘Angle of Attack’ (AOA). Setting the aircraft at an optimum AOA is crucial during the climb-out phase. If the AOA is too low, the aircraft won’t climb out fast enough. If the angle of attack is too high, then its speed decreases and the aircraft could enter an aerodynamic stall. AOA sensor inputs are then crucial because it forewarns a pilot about a possible stall due to a high AOA.
“The Indonesian National Transportation Safety Committee has indicated that Lion Air flight 610 experienced erroneous input from one of its AOA (Angle of Attack) sensors,” said a statement released by Boeing on Wednesday.
Under the subject heading, the Boeing bulletin says: “Uncommanded Nose Down Stabilizer Trim Due to Erroneous Angle of Attack (AOA) During Manual Flight Only.”
“Uncommanded” means an action that isn’t initiated by the pilots—that is, an action taken independently by aircraft systems (the stabilizer trim system, in this case).
The bulletin says: “In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds.” Simply put, it means that if there is an erroneous AOA input—one that indicates the aircraft has a high AOA and so is close to a stall—the aircraft system would respond by automatically bringing the aircraft nose down so as to increase the airspeed and prevent a stall.
A problem would occur if the AOA sensor input is erroneous. That is, the input says the AOA is high, when it in reality it is not. The aircraft system that handles the nose up and down movement (called stabilizer trim system) will put the nose down in response to the erroneous AOA sensor. The only way to prevent this, is for the pilot to intervene and manually deactivate the system. The Boeing bulletin says it is possible that the system will continue to put the aircraft nose down, right up to the limit, unless it is deactivated.
Capt Mohan Ranganathan, air safety expert, said, “If all AOA sensors are erroneous, it puts a big question mark on the air-worthiness of the aircraft. If only one AOA sensor is faulty, then the pilots should be able to identify which is the faulty one by comparing each sensor’s inputs with the standby horizon instrument.”
Ranganathan further said that the “DGCA should consider a dedicated crew for Boeing 737 NG and Boeing 737 MAX because it appears that the systems on both these aircraft are different.” In India, Jet Airways and SpiceJet are the only Boeing 737 Max operators. Currently, both airlines operate only one Boeing 737 Max each.
from Times of India https://ift.tt/2Pe9ASt
