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What Happened to Air France 447?

A330 Air France

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.

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:

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.

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.

The Equatorial Mid-Atlantic is Known to be Stormy

Tradewinds flowing toward the equator form the Inter-Tropical Convergence Zone (“ITCZ”). Winds in the northern hemisphere flow toward the southwest, while winds in the southern hemisphere flow toward the northwest. During the northern hemisphere’s winter months, the tradewinds come together, or converge, south of the equator. During the northern hemisphere’s summer months, tradewind convergence occurs north of the equator. At the point of convergence, air is forced up, increasing convection, creating lines of rain and thunderstorms that can extend for hundreds of miles. As depicted below, these conditions were ocurring at the time of flight 447’s loss. Get the current Atlantic Tropical Weather.

Image credit: Tim Vasquez, WeatherGraphics.com ©2009 – All rights reserved. Reproduced by permission. For a comprehensive analysis of the conditions probably encountered by Air France 447, see Tim’s Air France Flight 447: A detailed meteorological analysis.

Depicted above is a radar simulation, based upon weather data in the mid-Atlantic, of the flight conditions probably being experienced by Air France 447. The flight proceeded northeast from the INTOL intersection, picking its way through an area of thunderstorms, several hundred miles wide, just north of the equator on its way to Paris. Its flight in the turbulent area probably lasted ten minutes. The aircraft was almost through the weather as it passed between two of the more powerful cells, or clusters of thunderstorm cells, when it stopped transmitting data.

Thunderstorms over warm water draw in a tremendous amount of most air, which can be lifted by rising air currents (updrafts) to great heights, sometimes exceeding 50,000′. Air France 447 was flying at FL350, or 35,000′. Did the flight encounter relatively warm (above -40 deg. C) super-saturated air, as it passed between the two areas of activity depicted? If so, did the relatively warm, moist air overcome the ability of one or more of the aircraft’s pitot tubes to remain ice free, resulting in conflicting data being sent to the air data computers? The next section discusses known problems with the pitot tubes originally installed on Airbus A-330’s.

Pitot Tube Design

The U.S. Department of Transportation, Federal Aviation Administration (FAA) published an Airworthiness Directive (AD) on May 18, 2004, related to several Airbus models, including the A330. Although the Airbus is of European manufacture, and Air France is the French flag carrier, the FAA participates in regulatory oversight because Airbus jetliners serve cities in the United States, and are operated by a number of U.S. Airlines. The A330, in particular, is operated by US Airways and Northwest. Further note that the publication of an AD does not, of itself, necessarily send a negative message about any particular aircraft design. Airplanes, particularly jet transports, are complicated machines consisting of thousands of components. It would be more noteworthy to find and comment on a modern jet that does not have an AD issued, than to find one which has a number of ADs.

That said, here is the summary of the AD which pertains to the pitot tubes on the A330:

“This amendment adopts a new airworthiness directive (AD), applicable to certain Airbus Model A300-600, A310, A319, A320, A321, A330, and A340-200 and -300 series airplanes, that requires a one-time inspection to determine if certain Thales pitot probes are installed, a check for certain part numbers and serial numbers of the affected pitot probes, and cleaning of the drain hole of any affected pitot probes if obstructed. This action is necessary to prevent obstruction of the air intake of the pitot probes, which could result in misleading information being provided to the flightcrew. This action is intended to address the identified unsafe condition”.

Note that this AD, issued five years prior to the accident, only requires inspection of the pitot tubes, so the cost of compliance is minimal:

“Cost Impact. We estimate that 758 airplanes of U.S. registry will be affected by this AD, that it will take about 2 work hours per airplane to do the inspection, and that the average labor rate is $65 per work hour. Based on these figures, the cost impact of the inspection required by this AD on U.S. operators is estimated to be $98,540, or $130 per airplane. [Emphasis added].

To read the full text of the AD, download the document 041002.

Another AD Required Replacement of the Pitot Tubes

Another AD (2004-03-33 Airbus: Amendment 39-13477. Docket 2001-NM-302-AD) required replacement, not just inspection, of the pitot tubes on larger variants of the A330 (the -300 series). Note that this AD calls for the installation of Thales pitot tubes. These are the pitot tubes that are the subject of the AF447 investigation. This AD references Airbus Service Bulletin A330-34-3038, dated November 19, 1996; or Revision 01, dated September 14, 2001; or Airbus Service Bulletin A330-34-3071, dated December 11, 1998; or Revision 01, dated May 30, 2001, depending upon the production modification of the aircraft. The summary of this AD includes the following warning:

“This proposal would require, among other actions, replacement of certain pitot probes with certain new pitot probes. This action is necessary to prevent loss or fluctuation of indicated airspeed, which could result in inadvertent excursions outside the normal flight envelope. This action is intended to address the identified unsafe condition”.

The required maintenance procedure for A330-300 series aircraft is described, as follows:

“For Model A330-301, -321, -322, -341, and -342 Series Airplanes: Replacement of Rosemount Pitot Probes (g) Within 30 months after the effective date of this AD, do the action specified in paragraph (g)(1) or (g)(2) of this AD, as applicable.
(1) For Model A330-301, -321, -322, -341, and -342 series airplanes: Replace the Rosemount pitot probes in zones 121 and 122 with new Rosemount (formerly BF Goodrich) pitot probes, per the Accomplishment Instructions of Airbus Service Bulletin A330-34-3038, Revision 01, dated September 14, 2001. Replacements accomplished before the effective date of this AD per Airbus Service Bulletin A330-34-3038, dated November 19, 1996, are acceptable for compliance with the corresponding action required by this paragraph.

“(2) For Model A330-301 series airplanes: Replace the Rosemount pitot probes in zones 121 and 122 with new Thales (formerly Sextant) pitot probes, per Airbus Service Bulletin A330-34-3071, Revision 01, dated May 30, 2001. Replacements accomplished before the effective date of this AD per the Accomplishment Instructions of Airbus Service Bulletin A330-34-3071, dated December 11, 1998, are acceptable for compliance with the corresponding action required by this paragraph”.

Click here for the full text of this AD.

EASA Initially Declines to Issue AD

On June 9, 2009, the European Aviation Safety Agency (EASA) issued Safety Information Bulletin SIB 2009-17 under the subject heading “Unreliable Airspeed Indication,” wherein the agency acknowledged that, “There have been a number of occurrences of unreliable airspeed indications or misleading air data information”. The bulletin explains that, “The root cause of this may be due to, but is not limited to, inappropriate maintenance, contamination by small objects or materials on the ground or in the air, extreme environmental conditions producing icing outside the certification envelope of the probes or large amount of water ingestion”. Although EASA is empowered to issue airworthiness directives, the agency has, so far, addressed the loss of AF447 with a non-mandatory safety bulletin. The bulletin states that, “The Aircraft Flight Manuals and/or Flight Crew Operating Manuals include procedures for unreliable airspeed indication (Air data system misleading information) and these should be well known by flight crews”. The only recommendation of this bulletin is that flight crews be trained, among other things, “To apply procedures for the safe continuation of a flight with unreliable airspeed indication up to a safe landing”. The full text of EASA SIB 2009-17, as published nine days after the loss of AF447, is here: easa_sib_2009-17. Approximately three monts later, EASA followed through, issuing AD 2009-0195 (summarized in the update on next page).

Similar Air-Data Anomalies Reported

Air Transport World (ATW Daily News Monday June 29, 2009) reported that the U.S. NTSB is investigating two incidents, one on May 21st and the other on June 23, 2009, during which airspeed and altitude information aboard Airbus A330 aircraft may have malfunctioned. The incident in June was described as, “The aircraft entered the cloud tops and experienced light to moderate turbulence. After about 15 sec. it encountered moderate rain that was visible on the windshield. The pilots noted that the cockpit suddenly became very warm and humid and a few seconds later all three airspeed indicators rolled back to 60 kt. and autopilots and autothrottles disengaged, as did rudder limit protection …” In both of these incidents, the aircraft continued to a safe landing.

August 2009 Update: Europe Proposes Airworthiness Directive

On August 10, 2009, the European Aviation Safety Agency (EASA) issued notice of its proposal for an airworthiness directive, stating, “Occurrences have been reported on A330/340 family aeroplanes of airspeed indication discrepancies while flying at high altitudes in inclement weather conditions”. If adopted, the airworthiness directive would require replacement of the Thales pitot tubes currently installed on the A330/340 fleet with another Thales pitot tube, which is believed to improve airspeed indication in heavy rain conditions. The full text of the proposal is here: easa_pad_09-099_1.

Then, on August 31, 2009, EASA issued AD No. 2009-0195, summarized as follows:

Occurrences have been reported on A330/340 family aeroplanes of airspeed
indication discrepancies while flying at high altitudes in inclement weather
conditions. Investigation results indicate that A330/A340 aeroplanes equipped
with Thales Avionics pitot probes appear to have a greater susceptibility to
adverse environmental conditions than aeroplanes equipped with Goodrich
pitot probes.
“A new Thales Pitot probe P/N C16195BA has been designed which improves
A320 aeroplane airspeed indication behaviour in heavy rain conditions. This
same pitot probe standard has been made available as optional installation on
A330/A340 aeroplanes, and although this has shown an improvement over the
previous P/N C16195AA standard, it has not yet demonstrated the same level
of robustness to withstand high-altitude ice crystals as the Goodrich P/N
0851HL probe. At this time, no other pitot probes are approved for installation
on the A330/A340 family of aeroplanes.
“Airspeed discrepancies may lead in particular to disconnection of the autopilot-
and/or auto-thrust functions, and reversion to Flight Control Alternate law.
Depending on the prevailing aeroplane altitude and weather environment,
this condition could result in increased difficulty for the crew to control the
aeroplane.”

The full text of the European AD requiring replacement of the A330 pitot tubes with either an improved Thales part number, or Goodrich pitiot tubes, can be found here.

Underwater Search for Wreckage

Clues to help determine the cause of the crash are being sought by ships and submersibles in the mid-Atlantic. The New York Times on June 19, 2009 published an illustration depicting the sea floor and describing the search. The two most sought-after items: The flight data and cockpit voice recorders.

Status of Investigation: August 2009

As of July 7, 2009, the flight data recorder and cockpit voice recorder have not been located. Because the ‘pinging device’ is only expected to last 30 days, and that time has expired without the recorders being found, it is likely that they will never be recovered. There are no witnesses, either from aboard or outside the aircraft. The accident reminds us of a bygone era, when an aircraft departed and later was  simply declared overdue and presumed lost. Were it not for the routine maintenance data automatically uploaded by the aircraft, we would have even fewer clues to the loss. Thankfully we have weather data from the mid-Atlantic Ocean, to correlate with the information uploaded by the aircraft. Our prayers for those lost and condolences to their families and loved ones.

About the Author

Dave Alden is a line pilot for a U.S. Part 121 airline, and an attorney who practices aviation law from offices in Sacramento, CA.

Further Updates

Since this article was written in the months immediately following the accident, additional evidence has been recovered and findings made. Further, the U.S. FAA updated Airworthiness Directive 2004-03-33, pertaining to Thales pitot tubes in 2009.  See Air France 447 Recorders Found for more.


Aviation’s Ironies

Johanna Ganthaler, an Italian woman vacationing in Brazil, missed AF447 but caught another flight the following day. She was killed in an automobile accident driving home from the airport, when her car swerved into an oncoming truck in Kufstein, Austria.

Five passengers decided to miss BOAC 911 on March 5, 1966, which crashed with the loss of all aboard. Cubby Broccoli, Harry Saltzman, Ken Adam, Lewis Gilbert and Freddie Young were in Japan scouting locations for the James Bond film, You Only Live Twice. Source: Wikipedia, BOAC 911. Note the final irony: That prophetic flight number.

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