What Happened to Air France 447?

6 Jul

June 14, 2009 Legal.com
Updated: June 18, 20, 29, July 7; August 16 and 31, 2009
by Dave Alden

On June 1, 2009, an Airbus A330-200 registration F-GZCP, similiar to the aircraft depicted, being operated by Air France, disappeared approximately four hours into a flight between Rio de Janeiro and Paris.

A330-200 seat chart

Source for the two images above is: http://www.airfrance.fr

The A330 is a modern, wide-body jet, designed for long range intercontinental routes. As preliminary data is being gathered, some are asking whether the Airbus jetliner, known for its extensive automation, is too advanced? Did the aircraft’s sophisticated systens, including fly-by-wire controls, frustrate the flight crew’s efforts in the last moments of the flight, when basic airmanship skills were required? This is only speculation, as the accident investigation is in its early stages at this writing.

Leading Theory

Automated messages uplinked from the aircraft to Air France’s maintenance control indicate that the autopilot disengaged and there were conflicting airspeed readings, just moments before all contact with the flight was lost. Following is a depiction of those messages:

ACARS Air France 447

Did Ice Block One or More Pitot Tubes?

To understand the role of the pitot tubes, following is a simple diagram of an aircraft pitot-static system. The leading theory suggests that a blockage to one or more of the pitot tubes, probably caused by ice (see discussion of weather conditions below), resulted in misleading airspeed information being sent to the pilots and flight control systems. Such misleading information may have led to the aircraft flying too slow, or too fast, as it attempted to maneuver around storms.

pitot static system

Source: FAA-H-8083-15A[1] (2008) Figure 3.2, Instrument Flying Handbook.

Pitot tubes are used to determine an aircraft’s speed through the air. They point forward, to measure dynamic (ram) air pressure. Airspeed is derived by comparing the dynamic air pressure measured by the pitot tube, with static (ambient) pressure sensed by one or more static ports. In transport category aircraft such as the Airbus, there are three pitot tubes, each heated to prevent ice accumulation. Modern jetliners feature computers that interpret the air data, including information sensed by the pitot tubes and static ports, along with air temperature and other measurements, to present accurate information to the pilots and automated flight control systems.

When the data sensed at one source conflicts, beyond tolerance, with data from another source, the computers detect and report the conflict. From the Instrument Flying Handbook published by the U.S. Federal Aviation Administration:

“The pitot tube is particularly sensitive to blockage especially by icing. Even light icing can block the entry hole of the pitot tube where ram air enters the system. This affects the ASI [airspeed indicator] and is the reason most airplanes are equipped with a pitot heating system.

“Indications of Pitot Tube Blockage
If the pitot tube becomes blocked, the ASI displays inaccurate speeds. At the altitude where the pitot tube becomes blocked, the ASI remains at the existing airspeed and doesn’t reflect actual changes in speed.
• At altitudes above where the pitot tube became blocked, the ASI displays a higher-than-actual airspeed increasing steadily as altitude increases.
• At lower altitudes, the ASI displays a lower-than-actual airspeed decreasing steadily as altitude decreases”.

Misleading airspeed indications could have caused AF447 to be flying too fast, or too slow, at the time of its loss. If the aircraft were flying too slow, airflow over the wing would be interrupted, resulting in a loss of lift. If flying too fast, certain critical airspeeds may have been exceeded including maximum turbulent air penetration speed (Mb) or max operating speed (Mmo). The former could cause an aerodynamic (wing) stall which, in a swept wing jet, may be impossible to recover from. This would result in an uncontrolled descent, eventually leading to impact with the surface. The latter could lead to structural failure at altitude.

Documented Maintenance Problems and the Stormy Mid-Atlantic Ocean

For a discussion of the flight conditions likely being encountered by flight 447 at the time of its loss, and known deficiencies of the pitot tubes installed on the Airbus A330, continue reading this story.

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