Air France Flight 447 crash

         Air France Flight 447 was a scheduled airline flight from Rio de Janeiro to Paris that crashed into the Atlantic Ocean on 1 June 2009, killing all 216 passengers and 12 aircrew.The investigation is still ongoing, and the cause of the crash has not yet been determined, but a briefing released by the BEA on 27 May 2011 revealed that the aircraft crashed following an aerodynamic stall. It further revealed that for about a minute prior to the crash there were inconsistent readings in the pitot tubes. The cause of the faulty readings is yet to be determined, but a theory is that ice formed on the pitot tubes, which would have caused them to freeze, giving inconsistent measurements owing to their reliance on air pressure measurements to give speed readings. Pitot tube blockage is suspected of having contributed to airliner crashes in the past such as Birgenair Flight 301 in 1996.

         The investigation into this accident was severely hampered by the lack of any eyewitness evidence and radar tracks, as well as by difficulty finding the aircraft's black boxes, which were located and recovered from the ocean floor two years later in May 2011.

         The accident was the deadliest in the history of Air France. It was the first fatal accident to befall an Airbus A330 airliner while in passenger service.

DETAILED REPORT ON THE CRASH

          French investigators have disclosed that the crew of Air France flight AF447 maintained nose-up inputs to the aircraft even after the Airbus A330 entered a stall.The inquiry has also revealed that the pilots set engine thrust variously to go-around power and idle as they battled to rescue the jet.

          In an update to the loss of the A330 over the South Atlantic two years ago the Bureau d'Enquetes et d'Analyses has detailed the last few minutes of the flight. BEA said the aircraft climbed from its cruise altitude of 35,000ft towards 38,000ft and stalled, but added that the flying pilot "maintained nose-up inputs" to the controls.BEA confirms that the captain had left the cockpit to rest, about eight minutes before the emergency on 1 June 2009, having discussed with the relief crew possible turbulence ahead of the aircraft.

Recovered black box data and details inferred :

LAST SIX MINUTES OF AF447

          The pilots altered course slightly, about 12° to the left, and as turbulence increased they opted to reduce speed to Mach 0.8.

          About 2min later the aircraft's autopilot and autothrust disengaged, and remained so for the rest of the flight. This would have put the jet into 'alternate' law, meaning it lost its angle-of-attack protection.

          The aircraft began to roll to the right, and as the pilot made a nose-up left input, the A330's stall warning sounded twice - an indication that the aircraft had exceeded a critical angle-of-attack threshold.

          The primary flight display on the captain's side showed a "sharp fall" in speed from 275kt to 60kt, and the aircraft's angle of attack "increased progressively" beyond 10°.

          While the jet had initially been cruising at 35,000ft, investigators stated that the aircraft climbed, with a vertical speed of 7,000ft/min, heading towards 38,000ft.

          The pilot made nose-down inputs as well as inputs for left and right roll. The vertical speed fell back to 700ft/min, the displayed speed "increased sharply" to 215kt, and the angle of attack reduced to 4°.

          In its update the BEA said the non-flying pilot "tried several times to call the captain back".

          There was another stall warning and the BEA said the stall warning sounded again. The thrust levers were positioned for take-off/go-around power but the flying pilot "maintained nose-up inputs".

          Angle of attack continued to increase, it added, and the trimmable horizontal stabiliser increased from a 3° nose-up position to 13° nose-up - where it stayed for the rest of the flight.

          The aircraft reached 38,000ft - its maximum altitude - with its angle of attack having increased to 16°.

          AF447's captain returned to the cockpit - just 90s after the autopilot had disengaged - by which time the aircraft had started its fatal descent.

          As it passed through 35,000ft the angle of attack increased to more than 40° and the A330 was descending at 10,000ft/min. Its pitch did not exceed 15°, its engine power was close to 100% of N1, and the jet oscillated with rolls of up to 40°.

"        The [flying pilot] made an input on the sidestick to the left and nose-up stops, which lasted about 30s," said the BEA.

         Just 20s after the captain returned to the cockpit, said the BEA, the thrust levers were set to the 'idle' position, with the engines delivering 55% of N1.

Measured angle of attack values, the BEA pointed out, are only considered valid when the measured speed is above 60kt. It said that the angle of attack, when valid, always remained above 35°.

          AF447's had turned almost a three-quarter circle to the right during the emergency, and - having descended for 3min 30s - it struck the ocean surface with a ground speed of just 107kt, a nose-up pitch attitude of 16.2°, with a heading of 270°.

BEA stated that the aircraft stalled but that the inputs from the flying pilot were "mainly nose-up". It added that the engines "were operating and always responded to crew commands".

Technical evaluation by FAE

Though the reasons has not been cornered to either technical fault or pilots errors, the first hand reports from blackbox data seems to suggest the error was on the pilot side and also the stall recovery procedure followed was not the appropriate one for the existed condition.

PROCEDURE THAT MUST HAVE BEEN FOLLOWED

1. Following the stall when the aircraft is on recovery, If the engines are set to full power an aircraft with engines mounted in low thrust line will tend to pitch up the aircraft which may increase the risk increased Angle of attack(the same condition persisted in AF447).
2. Therefore it is precautions to set back engine power and allow aircraft to pitch down for a short course 
3. Then gradually increase power and recover from stall.
4. This procedure is followed only in high altitude stall recovery since this procedure involves in loss of altitude which is not possible when in low altitude.

-Nirmal Kumar.U

FAE

 

(The content are based on articles published in various sites and magazines and the above post is on personal evaluation of the author over the crash )