The UK and Irish aviation authorities have begun a consultation period on declaring a common transition altitude throughout British and Irish airspace, both controlled and uncontrolled.

The UK Civil Aviation says: "All parties have agreed that a common transition altitude, significantly higher than those that apply today, was a requirement to facilitate future safety, environmental and [air traffic management] capacity benefits."

A UK/Irish working group is being set up "to define the revised common transition altitude, identify the issues associated with that transition altitude and, to devise an implementation plan", says the CAA.

The transition altitude is a published height above sea level at which pilots climbing to their cruising level change their barometric altimeter datum from the regional pressure setting to the common international standard setting of 1013.2hPa.

This means all altimeters above that altitude read the same at any given level, despite constant natural meteorological changes in atmospheric pressure, or the passage of the aircraft from a region of high pressure to low pressure or vice versa.

Above the transition altitude, altimeter readings are communicated as flight levels, not as height or altitudes. During descent, pilots change the altimeter datum back to the regional pressure setting when passing through the transition level, the lowest flight level above the transition altitude.

Transitional altitudes are local, regional or national and vary considerably between about 3,000ft (915m) and 18,000ft. The USA and Canada have a common altitude of 18,000ft.

Europe is seeking to drop disparate national transition altitudes and standardise. Ideally it should be done throughout, but regional change would be a start, and some Nordic countries are examining that possibility.

In August 2009 Flight International argued in favour of a transition altitude change: "To understand why change is desirable it's worth looking at why Europe has its current system of very low transition altitudes, unlike the long-established North American standard of 18,000ft.

"It's because the International Civil Aviation Organisation advice on transition altitude is to make it as low as possible above 3,000ft.

"ICAO standards are frequently reviewed for continued relevance, but the advice on transition altitude has not been changed in the light of modern aircraft performance.

"In 1944, when ICAO standards were first set, all aircraft flew relatively low because pressurisation was rare and power was piston-based. Airliners cruised at 8,000ft or less, and many did so until the 1960s when jet power began to dominate.

"Under those circumstances, with most movements at low level, it made sense to transition to a common altimeter pressure setting as early as possible in the climb without compromising terrain clearance too much.

"But in a world where cruising at more than 25,000ft is standard for turbine-powered aircraft, priorities have changed. So forcing pilots to fiddle with their altimeters close to the ground while busy with arrivals and departures no longer makes sense."

Source: Flight International