Air France pilots warn of risky speed probes
Paris - Air France pilots have criticised investigators of the carrier's fatal jet crash in June 2009 and warned planes should avoid icy weather until tests prove whether their speed sensors can withstand it, a report said on Sunday.
The weekly Journal du Dimanche cited extracts from a report to be submitted to judicial officials this week by two pilots, one the head of the pilots' union SPAF, on the crash of flight AF447 from Rio to Paris that killed 228 people.
In a report last month, the French air accident investigation agency BEA said the jet's Pitot speed probes gave false readings before it crashed into the Atlantic and called for better testing standards for such probes.
Until these are developed, however, "planes are still flying in weather conditions for which the speed sensors are not certified", which runs "contrary to the safety requirements under current regulations", the pilots' report said, according to the newspaper.
Pilots' unions and some of the relatives of victims of June's crash have accused Air France and plane maker Airbus of ignoring longstanding problems with air speed monitors on its jets in the run-up to the June disaster.
The companies insist that their jets met all safety standards, but they have nevertheless replaced Pitots made by the French electronics company Thales with a different model produced by US firm Goodrich.
The report by the pilots, Henri Marnet-Cornus and SPAF president Gerard Arnoux, said this "would lower the level of risk", but further faults were possible since the probes had not been tested for the effects of ice crystals.
The pilots had argued in an earlier report that the freezing of the Pitots had caused the Airbus 330 to crash. The BEA has said they were "one of the factors" in the crash, but not the sole cause.
The pilots also criticised what they called "the short-sighted approach of the BEA concerning the faultiness of the Pitot probes", according to the newspaper.
They said planes should be banned for the moment from flying into patches of icy weather that could cause the speed probes to freeze up and probes should be certified for all weather conditions and all types of plane.
When Pitots are blocked by ice they send false speed measurements to the plane's onboard flight computers, as was the case on the missing flight in June. It sent a string of automated error messages before plunging into the ocean. - AFP
http://www.iol.co.za/index.php?fsetid=1&click_id=24&art_id=nw20100103221508586C174735
Industry Aware Of Pitot Tube Issues Since 1995
Barriers to sharing vital aviation safety information may start coming down this year, but the prevailing lack of transparency has allowed deficiencies to fester in the global air transport system for years.
The recent focus on pitot tube icing highlights that while various stakeholders were aware of equipment shortcomings, no concerted effort was made to understand what was going on or to remedy the situation.
Northwest pilots are among those of several airlines that saw anomalous speed information emanating from the pitot tubes in the noses of the Airbus A330s they were operating. Credit: AIRBUS
“It is scandalous that so far no effective safety data-sharing platform has existed,” says Gunther Matschnigg, the International Air Transport Assn.’s senior vice president for safety, operations and infrastructure. IATA, the FAA and the European Commission are launching such a data-sharing initiative this year, and IATA expects to sign a memorandum of understanding for the project by the end of March.
One problem is that the reporting system in Europe has remained fractured. The European Commission recently launched a drive to create a centralized clearinghouse for information, with the European Aviation Safety Agency (EASA) in the lead.
The history of pitot tube problems illustrates how overdue the measures are. Various stakeholders saw problems throughout the past 15 years, but until now no system-wide analysis of the incidents has been undertaken.
As early as 1995, Airbus became aware of the deficiencies of the speed probes in extreme weather conditions, following defects on Rosemount pitot tubes. “Strong cumolo-nimbi containing a high density of ice crystals can be encountered particularly in the intertropical convergence zone,” Airbus wrote in a technical follow-up, TFU 34.13.00.005, dated December 1995. “In such an icy and turbulent atmosphere, the aircraft air data parameters (pressure dependent) may be severely degraded even though the probe heaters work properly. It has appeared that the characteristics of such an environment could exceed the weather specifications for which the pitot probes are currently certified.”
Airbus subsequently developed more stringent requirements, which were incorporated in new Goodrich devices, and it required the old probes to be replaced in 1996. But that was only made mandatory by French civil aviation authority DGAC five years later, in 2001, and regulators left their certification standards at the lower performance threshold.
A serious incident in 1998 led the German air accident investigation authority, the BFU, to recommend that “the specification for the pitot tubes should be changed so as to allow unrestricted flight operations in heavy rain and under severe icing conditions.” An Airbus A320 on approach to Frankfurt temporarily lost all airspeed data as well as autopilot and autothrottle functions until descending to 10,000 ft.
Even after Airbus ordered the use of more resilient probes, evidence soon emerged that the new devices were also subject to performance irregularities. In an Operator Information Telex in July 2002, Airbus stated that “several single-aisle operators have reported airspeed discrepancy on aircraft fitted with Thales pitot probes PN CN16195AA.”
In 2005, Thales itself launched what it called the Adeline research project aimed at finding alternative technologies that could eventually replace pitot probes. At the time, Thales warned that losing data such as airspeed, angle of attack and altitude could “cause aircraft crashes especially in case of probe icing.”
However, the effort to deliver an alternative approach to providing airspeed information was shelved in 2008.
In addition, FAA certification standards and those in Europe have diverged over time. EASA issued a notice of proposed amendment (NPA) to harmonize its standards with the more stringent FAA levels only last October.
Even that step does not appear to go far enough. The NPA has been rejected by Airbus for several reasons, but mainly because “the [Technical Standard Order] icing conditions are not sufficiently conservative” and “the TSO does not require the probes to be tested in ice crystal or mixed-phase icing conditions.”
The FAA also tests probes only in conditions expected to be encountered at altitudes of up to 40,000 ft. and temperatures of -40C. All of the severe icing incidents known occurred at temperatures between -45 and -65C.
Differences persist in regulatory authorities’ actions on the pitot tube matter. In August 2009, the FAA issued an airworthiness directive mandating that Thales pitot tubes on A330s be replaced with Goodrich models. It permits one Thales probe of the latest standard to be left in one of three pitot tube positions. A proposed EASA directive preceded the FAA’s action, but the European safety agency never made its recommendation final and now signals it believes problems extend beyond the Thales design.
Still, Gerard Arnoux, head of the Syndicat des Pilots d’Air France union and an Airbus A320 captain, contends that Airbus should have blocked the use of Thales AA-standard probes long ago based on all of the information the aircraft maker had available.
“We know that the Goodrich probes are far better than the Thales tubes, but Airbus kept Thales devices flying,” Arnoux says.
Concerns Persist About A330 Speed Indications, Pitot Tube Icing
A330 incidents point to need for better weather information, flight control changes
Recommendations to improve aviation safety in the wake of the crash of Air France Flight 447 last June and other Airbus A330 incidents are inadequate, several stakeholders warn.
Pilots and industry officials fear that unless more is done, system-wide failures will persist, with dangerous consequences for passengers.
The calls to do more would impact Airbus, airline operations and European regulators. They also signal that further equipment and operational changes may emerge in the coming months as investigators learn more about both what has caused a spate of A330s to briefly lose speed information and the crash of AF447. The Air France A330-200 encountered severe weather over the Atlantic enroute from Rio de Janeiro to Paris before it crashed into the ocean, killing all 228 onboard. A final report on the accident is not due until the end of this year.
The long history of one key problem, that of pitot tube icing, is moreover raising the question as to why preventative action was not taken earlier.
“The AF447 accident could have been avoided,” says Gerard Arnoux, head of the Syndicat des Pilots d’Air France (SPAF) union and an Airbus A320 captain. He claims that the June 1 crash was the result of “collective failure” by Airbus, Air France and regulators such as the French civil aviation authority (DGAC), its air accident organization —the BEA—and the European Aviation Safety Agency (EASA). Arnoux asserts that all failed to act to address known safety deficiencies.
That pitot tube performance standards do not meet real-world conditions has been known since the mid-1990s, according to a time line of incidents compiled by Arnoux. Those deficiencies—including blockage of the speed sensors due to icing—had triggered individual actions by regulators or manufacturers but no concerted effort to address shortfalls (see article below).
That is now changing. The BEA has called for a complete review of pitot tube certification standards and a scientific assessment of the meteorological conditions that cause problems.
Accident investigators are still sifting through the debris of AF447, but they hope to find more once the search for wreckage resumes in February. Credit: MAX POP/LANDOV
The BEA is indicating that other changes to aircraft design or procedures could emerge as a result of the scientific review. In addition to pitot tubes, design changes to engine inlets and other aircraft parts could be warranted, says BEA director Jean-Paul Troadec.
BEA officials first want to learn about the size and shape of ice crystals formed in the extremely cold high-altitude conditions, which can block pitot tubes and lead to erroneous speed information being provided to pilots and flight control systems. Currently, the probes are certified only to handle temperatures of up to -40C. EASA says climate change could be contributing to more extreme weather conditions at high altitudes that have not previously been encountered by aircraft.
Arnoux believes the BEA recommendations “do not go far enough.” If it cannot be demonstrated that pitot tubes can function despite large quantities of ice crystals at an altitude of 41,000 ft. and temperature of -70C without disruption, then operational changes need to be made, he argues. If the probes fail to meet that standard, then Airbus aircraft should “be limited to flying outside of clouds at these altitudes,” Arnoux says. Such a move would have huge operational implications for airlines on many long-range routes by reducing the efficiency of their Airbus wide-body fleets.
Aircraft compare data from three pressure probes, but an accident investigator notes that when two pitot tubes freeze and provide anomalous speed information, the data from a third functioning probe can be rejected.
The BEA says it has examined 13 significant events from five airlines but notes that data on another 40 events could not be reviewed owing to missing data. BEA data shows many of the events examined occurred in environmental conditions outside the certification standard for pitot tubes and outside the Airbus requirement, which itself is far more stringent than demanded by certification authorities.
Airbus and BEA officials stress, however, that the loss of pitot tube data alone should not cause a crash, and that pilots are trained to safely operate an aircraft when those problems occur.
The extreme weather conditions AF447 encountered have triggered calls by pilots for improved access to weather information. The main Air France pilots union, SNPL, is urging that better meteorological data be provided and that studies be undertaken to improve detection of adverse weather in flight. In addition to AF447, another Air France flight on the Brazil-Paris route recently declared a Mayday in flight when encountering severe turbulence, but it then continued on to Paris without further incident.
In a written response to the BEA’s request for a review, Airbus states that it “fully supports the corresponding recommendation from the BEA that the agencies conduct an industry review to assess all appropriate criteria with which any new equipment have to comply.”
An industry official says it is not just the pitot tubes that need to be looked at, but also, more broadly, the way speed information is processed on the A330, noting the 2008 inflight upset of a Qantas A330-300 (Flight QF72). He argues that the flight control system on A320s has greater fault redundancy than does the widebody’s.
To help address problems that occurred on QF72, when one of the air data inertial reference units (Adiru) provided faulty data to the flight control system, Airbus has modified the flight control software to filter information better.
“The purpose of the modification is to enhance the robustness of the flight control computer against Adiru abnormal parameters,” Airbus says in a written statement. The fixes are being applied to A330s and A340s.
Nevertheless, Airbus dismisses the notion of a link between the AF447 and QF72 incidents, pointing out that the two airlines used Adirus from different manufacturers.
More work is ongoing to assess what further changes may be needed, Airbus says. A new flight control computer software standard is due this year, safety authorities say.
Some regulatory action is already being taken as a result of the AF447 crash. For instance, flight data and cockpit voice recorder standards are headed for an update. In March, a safety committee of the International Civil Aviation Organization will convene to consider enhancements to flight-data recording. The BEA briefed ICAO in November on its proposal to extend to 90 days from 30 the minimum operational life for the beacons attached to cockpit voice and flight data recorders and the proposal to introduce recorders that would be ejected on impact.
The BEA has rejected the idea of streaming flight data recorder information via satellites to ground terminals as an alternative to data recorders. Troadec says such an approach would overload communications links, particularly considering that modern recorders monitor 1,300 data points.
The BEA also would like to a see a third beacon attached to aircraft, probably the aft fuselage, to help locate wreckage.
Starting in February, the BEA plans to direct a third search phase for AF447 debris and the cockpit voice and flight data recorders. Airbus and Air France are contributing to the €10-million ($14.4-million) search costs, but Troadec insists the agency will have the lead and not be compromised by the industrial financial participation.
This month researchers are supposed to help refine the search area to improve the chances of finding wreckage using imaging sonar.
Airbus submits patent for airspeed error monitoring
Airbus has submitted a US patent application for an independent means of monitoring an aircraft's pressure-based airspeed measurement for errors.
While the process is mainly geared at determining pitot system errors on the takeoff roll, giving pilots the ability to abort a takeoff before flight speeds are attained, Airbus in the 3 December filing also notes that "failure to detect these erroneous measurements can have unfortunate consequences during flight".
Airspeed data has been a key focus in the unexplained crash of an Air France Airbus A330 off the coast of Brazil on 1 June. According to French investigators, 10 of the 24 automated messages transmitted by the aircraft in its final 5min of communication have been attributed to an inconsistency of speed measurements.
Both Europe and the US have issued airworthiness directives on certain Thales-built pitot tubes that are used on the A330 and A340 in the aftermath of the crash. "Depending on the prevailing airplane altitude and weather, this condition, if not corrected, could result in reduced control of the airplane," said the FAA in its 3 September mandate.
Pitot tubes, also known as pitot sondes, provide air pressure data used for to compute airspeed. Investigators have not tied potential problems with the devices to the Air France disaster at this point however.
Airbus in its patent application notes that "pitot sondes are easily blocked by dust, insects or any other matter foreign thereto. An error in speed measurement results therefrom. This type of error can have catastrophic consequences if it is not detected".
Of particular concern to the airframer is warning pilots of an airspeed inaccuracy while the aircraft remains on the ground during the critical takeoff phase, though the technology would also aid crews in understanding related anomalies in cruise.
The proposed airspeed monitoring system would compare changes in measured airspeed over a short period of time with changes in ground speed as computed by the accelerometers and gyros in the aircraft's air data and inertial reference unit (ADIRU). Airbus says the ADIRU has a "storage zone" available where the computations can be performed.
While airspeed and ground speed cannot be compared directly, Airbus notes that over a "very short period", normally significant factors like wind speed, changes in altitude, air temperature and angle of attack will be negligible, allowing for a direct comparison of changes in speed rather than speed itself.
Once the speed differences exceed a pre-set threshold, the ADIRU system would alert the pilots with visual and/or aural warnings.
Source: Air Transport Intelligence news
