European safety regulators have opened a formal consultation on a common transition altitude for the continent, a need driven in part by the development of functional airspace blocks.
The transition altitude is the height at which altimeters are set to a single pressure standard, ensuring there is no vertical discrepancy in altitude between aircraft assigned to a specific cruising level. Preliminary impact assessments by Eurocontrol, released in December 2011, considered three options and concluded the most favourable would not involve fixing a specific threshold but prescribing common criteria for determining a transition altitude above 10,000ft (3,000m).
This would require establishing a coordinated approach between neighbouring functional blocks or other sections of airspace, but also allow air navigation services "some flexibility", said the assessment.
The analysis examined fixing a common transition altitude, at 18,000ft, and found the single threshold "appears to be less favourable" than the third option, simply retaining the current system, owing to short-term costs and "implementation challenges".
However, the differences in results were insubstantial, and the European Aviation Safety Agency is seeking comments on all three options by 29 May.
"In establishing functional airspace blocks, various member states have encountered difficulties related to the fact that transition altitudes are not harmonised in Europe," says EASA, adding that the present provision is "rather outdated" and fails to account for terminal airspace congestion and modern aircraft performance.
It also highlights the potential for confusion and error arising from the variety of transition altitudes used, the need to change altimeters during critical phases of flight, and the fact some transition thresholds "do not adequately take into account" terrain clearance and minimum safe altitudes.
EASA cites an airprox incident in December 2005, between Scandinavian Airlines and Norwegian Boeing 737s, which underlined the possible hazards associated with a low transition altitude.
"Considering the lack of progress in harmonisation over the last decade, it is quite likely that harmonisation can be achieved only by means of European Union legislation," it says.