AF447 brings revised procedures

First of all sorry for not posting for ages, been very busy with exams.

The investigation is still in process:


After the crash of Air France flight 447 into the South Atlantic on 1st June 2009, over two years after the crash, the investigation is still in process.

A recent report in May from the Air Transport Intelligence read that the French investigators have released that the crew of the Airbus A330 maintained a nose-up inputs to the aircraft even after it had stalled. Also the crew set the engine thrust variously to go-around power and idle as they battled to rescue the jet.

The Bureau d'Enquetes et d'Analyses (BEA) has detailed the last few minutes of the flight (below).

The BEA confirms that the captain had left the cockpit to rest about eight minutes before the emergency having discussed about possible turbulence ahead to the relief crew.

The report revealed that in the cruise the pilots changed course by 12° to the left, and as turbulence increased they opted to reduce speed to Mach 0.8 (around 465kts). Two minutes after this change in course, the aircraft's autopilot and autothrust were disengaged, and the aircraft was taken out of the cruise and climbed from 35,000 feet to 38,000 at a vertical speed of 7,000 ft/min! From here the aircraft entered 'alternate' law, because of this, the aircraft entered a dangerous pitch angle as its angle of attack protection was lost by being in 'alternate' law.

Last six minutes of AF447 taken from flight global.com

The aircraft began to roll right, as the pilot countered this roll with a nose-up left input, the A330's stall warning sounded twice - indicating that the aircraft had exceeded its critical angle-of-attack threshold.

The Primary Flight Display (PFD) on the captain's side showed a large decrease in airspeed from 275kts to 60kts, and the aircraft's angle of attack "increased progressively" beyond 10°.

The pilot made nose-down and left and right roll inputs and the vertical speed decreased to 700ft/min with the angle of attack reduced to 4°. This meant that the displayed speed "increased sharply" to 215kts. The BEA said that 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". The angle of attack continued to increase along with the trimmable horizontal stabiliser which increased from 3° to 13° - where it stayed for the rest of the flight.

When the aircraft reached its maximum altitude of 38,000 feet the angle of attack had increased to 16°.

The captain returned to the cockpit just 90 seconds after the autopilot had been disengaged - by which time the aircraft had began its fatal descent.

When the aircraft passed through back through its cruise altitude the angle of attack had increased to more than 40° with the aircraft descending at 10,000 feet/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°.

According to the BEA: "The [flying pilot] made an input on the sidestick to the left and nose-up stops, which lasted about 30 seconds,". Also just 20 seconds after the captain had returned to the cockpit, the thrust levers were set to 'idle' position, with the engines delivering 55% of N1.

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

F-GZCP - The Airbus A330-203 involved in the accident seen here landing in Paris Charles de Gaulle (LFPG)
© Vincent Edlinger at airliners.net

Air France emphasised the evidence of a technical malfunction on the Airbus A330 after French investigators detailed the final minutes of the flight. The BEA confirmed there was a sudden change in displayed speeds and that that there was a discrepancy between that shown on the PFD and that on the standby instrument system.

The non-flying pilot said that the aircraft had "lost the speeds" and then mentioned that the aircraft had switched to 'alternate' law - a normal response from the aircraft in the vent of unreliable speed information. The BEA did not mention any reason for these inconsistent speed indications.

But Air France said: "It appears...that the initial problem was the failure of the speed probes which led to the disconnection of the autopilot and the loss of the associated piloting protection systems, and that the aircraft stalled at a high altitude." They did not discuss the crew's response to the stall beyond pointing out that the captain, having left the cockpit to rest, quickly returned to address the situation.

"The crew, made up of three skilled pilots, demonstrated a totally professional attitude and were committed to carrying out their task to the very end and Air France wishes to pay tribute to them," said the carrier. "All the data collected must be now analysed. It will only be at the end of this complex task, which requires patience and precision, that the BEA will be able to establish the causes that led to the disaster."

Revision of safety procedures:

This accident has lead to a recent revision of safety procedures. These concentrated on placing greater emphasis on reducing critical angle of attack which is the critical characteristic of a stall rather than using aircraft power to get out of a stall situation and cause minimum loss of altitude. Put simply, when an aircraft is entering a stall, pilots have been concentrating on increasing engine thrust to combat it, rather than concentrating on keeping air flowing smoothly over the wings and maintaining a sensible pitch angle.

A formal document detailing the revisions pointed out that: "There have been numerous situations where flight crews did not prioritise [nose-down pitch control] and instead prioritised power and maintaining altitude."

Operational experience has shown that doing the above can result in the aircraft stalling. The BEA disclosed that the crew maintained nose-up inputs after the stall but has yet to determine the reason or reach any conclusions. They also set the thrust to take-off/go-around power, in line with Air France's standard operating procedure at the time. However the procedure also required them to reduce pitch altitude, roll the wings level and ensure the speedbrake was retracted.

The revised recovery procedure was agreed between the major airframers Airbus and Boeing, some 12 months after the accident, although a source familiar with the investigation stresses that the change was "not prompted" by the accident.

At the heart of revision is the acceptance that applying high-power for recovery is not necessary in all stall situations and could be ineffective, particularly at cruise altitudes where available thrust would be limited and the engines would require time to spool up.

There is also a risk that a crew may not recognise that an aircraft has crossed the point of near-stall to an actual stall and still use the same ineffective technique for recovery.

The new procedure is designed to cover all aspects of a stall - recovering the angle of attack may require reduction of thrust, regain pitch-down authority, as well as loss of altitude. This procedure removes the priority of increased power and also points out that thrust should be applied smoothly, particularly becuase aircraft with under-wing engines have a tendency to pitch up, increasing the angle of attack when power is applied.


Sources:  Flight global - flight information
                 Flight global - 'technical problem' information
                 Flight global - revised procedures
                 Boeing.com - angle of attack
                 Wikipedia - 'alternate' law