Geneva, 12 April 2006. Skyguide takes note of the report by the Air Accident Investigation Bureau (BFU) regarding the screen failure at the Zurich control center on 11.11.2003. While skyguide respects the work and the report by the BFU, it nevertheless disagrees with the BFU on some fundamental points in the statements and conclusions. The display system affected was replaced by a newly developed system in May 2004, as had been planned for a long time.
The essential points on which skyguide does not agree with the BFU report are the following:
1. Establishment of the causes The work of the specialist on the system triggered the failure of the screens, but despite what the BFU states, this cannot have been the cause of it. Skyguide's view is that it is highly probable that the cause lies in a latent design fault in the system, which manifested itself on this occasion for the first time. With the aim of reproducing and identifying the system fault, the intervention conducted was repeated in the test system in a long-term test. It was only after three months and approximately 120,000 repetitions of the intervention, equivalent to some 500 years of operation, that a comparable system fault occurred for the first time.
2. Criticism of 'untested intervention' The BFU uses the term 'untested intervention' in the report, meaning a system intervention which is not described in the maintenance manual. However, the fact is that the intervention conducted is defined in the system manual, and that it was deliberately created by the system manufacturer for the purpose of being able to undertake preventive or corrective maintenance tasks during operation. Skyguide in no sense considers the procedure used to be an 'untested intervention'. The procedure is the controlled application of a control function intended and documented for the purpose. Similar maintenance work had already been carried out hundreds of times before the incident without any problems.
3. Criticism of 'inadequate knowledge' by the technical staff The argumentation and statements by the BFU with regard to the qualification of the technical staff are not justified in skyguide's opinion. The specialist for the system in question, for example, has comprehensive knowledge and has known the system since it was developed. His years of experience and his service at the provider and at skyguide have also equipped him optimally with a thorough familiarity with the operational and system-related processes involved.
Development since 11.11.2003 - In April 2004, a second air environment display system, independent of the main system, entered service in Zurich. Redundancy in the event of the failure of the main system is thus guaranteed. - The maintenance procedures have been completely revised. The maintenance procedures were assessed in September 2005 in the context of an ISO audit by an independent unit, and approved. - The system affected by the failure was replaced by a newly developed system in May 2004, as had been planned for a long time.
Screen failure on 11 November 2003 On 11 November 2003, the radar screens at the area control center in Zurich (handling overflights, arrivals and departures) went blank. The situation was entirely normalised after 20 minutes. The air traffic controllers reacted swiftly and immediately went into emergency mode. This meant that the traffic notified was stopped and the existing aircraft in the airspace controlled by skyguide from Zurich was handled by radio. The safety separations were maintained, and safety was guaranteed. Aircraft which were landing could be handed over to the tower, whose radar screens were not affected by the failure. The Air Accident Investigation Bureau (BFU) and the media were informed. The final report by the BFU was published on 12 April 2006.
Skyguide's stance on the BFU report can be found on the skyguide homepage.
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