UK investigators worry about RVSM after Atlantic airprox
David Morrow, London (07Jun01, 14:56 GMT, 536 words)
UK accident investigators are urging regulators to review operating procedures in reduced vertical separation minima (RVSM) airspace after a clear air turbulence encounter caused a serious airprox incident over the North Atlantic last year.
Shortly after hitting the turbulence a New York-bound THY Turkish Airlines Airbus A340-300, cruising at FL360, suddenly pitched up and climbed rapidly to FL384. The uncommanded manoeuvre resulted in the A340 passing through FL370, an altitude assigned to a Canada 3000 Airbus A330-200 which had been above the THY jet and overtaking it slightly to one side at the time.
UK accident investigators probing the incident are warning that there is no formal procedure in place for avoiding the risk of collision in such circumstances. The accuracy of modern navigation systems, says the Air Accidents Investigation Branch report into the 2 October incident, is such that there is a greater chance that overtaking aircraft will be separated only vertically and, in RVSM airspace, by just 1,000ft.
It stresses that the speed of the A340’s climb – initiated by the aircraft’s automatic protection systems after it hit the turbulence – was so sudden that the A330 crew might not have been able to initiate evasive action. The report describes the event as a “serious loss of separation” and says the A340 pilot estimated the aircraft to be horizontally separated by 1nm (1.8km).
This has disturbing implications for the RVSM safety case, says the report: “If the intruder aircraft continues its climb there can be no guarantee that an aircraft directly above it could respond in sufficient time to avoid a collision.”
It adds: “It is not clear whether the European or Oceanic [RVSM] safety case studies and models have taken account of the risks of clear air turbulence coupled to the response of sophisticated flight-control systems such as those fitted to the Airbus fly-by-wire aircraft series.”
There is no formal procedure listed in the North Atlantic Minimum Navigation Performance Specifications Airspace operations manual for limiting the risks while overtaking at RVSM altitudes. The report states that lateral offset flying should be considered as a way of providing an effective safety barrier.
“The collision risk could have been reduced if the commander of the overtaking aircraft had been permitted (and expected) to temporarily increase the lateral separation between the two aircraft before they reached the line-abreast position,” it says.
It points out that simulations have indicated that a lateral separation of 1.5nm (2.8km) would be enough to prevent an aircraft’s traffic collision-avoidance system (TCAS) from generating a resolution advisory to the crew, and adds that flying a 2nm lateral offset is already an approved procedure used to mitigate wake turbulence problems.
“There would appear to be a safety case for extending this contingency procedure to overtaking, particularly in regions where turbulence of any kind is evident or forecast.”
Recommendations made in late November last year urged the CAA to request a review of overtaking procedures in RVSM airspace as well as suggest the development of a standardised lateral offset procedure. A spokesman for the CAA says: “We have put forward these recommendations to the appropriate international authorities. It’s really out of our hands now and we can’t say how or when the issues will be addressed.”
Source: Air Transport Intelligence news