FF SAFETY: AF 447 : Investigation (PART 4)

Today is the fourth in a five-part series exploring the loss of Air France Flight 447 and the lessons firefighters can learn about effective teamwork, communication and decision making in a high risk environment.

Air Speed and Pitot Tubes

The A-330’s computerized flight systems are critically reliant on reliable air speed data.  This information is gleaned from “pitot tubes”,  bullet-shaped devices that extend outside of the aircraft near the nose which are crucial in measuring air speed.  There are three on the A-330 for system redundancy and to provide data for a backup standby instrument system.

Blocked pitot tubes can render air speed calculations unreliable.  They have been the cause of previous incidents with other types of aircraft and airlines.  On the Airbus, blocked tubes would create a scenario where the air speeds were unreliable or absent and where normally available systems and protections would be eliminated because of the invalid speeds.  Despite the Airbus trademark systems of redundancy, critical functions would be lost and for that period, the survival of the flight would be based on the ability of the flight crew to maintain the aircraft in its safe flight envelope.

It is now posited that AF-447 flew into an area of ice crystals, and despite the fact that the pitot system was heated to prevent clogging, the crystals blocked the tubes and subsequently interfered with flight system operation.

There is a documented history of pitot tube blockage resulting in unreliable or inaccurate air speeds on the A-330.  Prior instances had been reported or documented and unreliable air speed indications are a frequent enough occurrence to require training for coping with them.  That training would center on flight with unreliable air speeds and controlling the aircraft manually at high altitudes.

Organizational/Human Factors

As the Captain reentered the cockpit, a catastrophe was unfolding:  the aircraft was falling at a rate of 9,000 feet per minute and the speed was dropping below 100 knots.  Neither copilot summarized the situation making his attempt at assessment even more impossible.  The nose of the aircraft was pitched up and the angle was increasing.  He had sesconds to diagnonse the problem and solve it.

When the Captain had originally left the cockpit for his rest break he had confirmed that the right-seat copilot was qualified to replace him as a relief pilot.  After his departure, this “acting captain” continued to fly the airplane.  The question arises as to whether this was the best use of resources.  In emergency environments is it better to have the commander removed from the point-of-contact so that he/she can attempt to gain a full picture and prescribe effective action?

The official reports remark on the evident lack of “synergy” between the two pilots in the cockpit during the emergency.  When the unreliable speeds occurred the pilots were aware of it but failed to call out the procedure to deal with the problem.  This raises a potentially serious organizational issue.  The pilots had been trained for the Unreliable Indicated Air Speed Emergency Maneuver but only at lower flight levels which called for adopting a pitch attitude of 10-15 degrees.  Documentation describing the speed problems with the A-330/340 fleet had been sent to flight crews.  For events where loss of life is the outcome of improper action is it essential to train realistically?  Does the dissemination of written material without follow-up or skills testing meet the readiness test?

The BEA report also states that the lack of training for manual flight at high altitudes “likely contributed to the inappropriate pilot inputs and surveillance.”  During much of the emergency the pilots failed to call out speed, pitch, altitude or vertical speed.  The pilot not flying was apparently observing available aircraft performance data and repeatedly requested that the nose be lowered. The flying pilot would make a pitch-down input but the majority of inputs continued to be pitch-up.

Sixty-two seconds after the beginning of the event, all three speed indications became valid again, showing 183 knots.  Thereafter, they fell dramatically, coinciding with a rapid loss of altitude as the aircraft fell to the ocean surface.

Tomorrow- FF Safety: Air France Flight 447 Disaster: Conclusion (Part Five)

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