The Transition Altitude is the height where aircraft change from operating their altimeter by reference to an altitude derived from a local pressure setting, expressed in feet (normally above mean sea level), to an area-wide standard pressure setting that allows the flight to be conducted using an internationally agreed set of Flight Levels.

At lower levels, it is important for a pilot to know the vertical distance from the ground in order to ensure safe separation from terrain and other obstacles. This requires aircraft to use a local pressure setting to derive their altitude above mean sea level. The UK Transition Altitude is not currently consistent across the UK and varies between 3,000 ft and 6,000 ft depending on location and type of airspace. This was set in the 1950s when aircraft instrumentation and air traffic control systems were much less accurate and aircraft performance was lower.  

Modern commercial aircraft reach these sorts of altitudes extremely quickly on departure, and the change in operating techniques as the aircraft passes through the Transition Altitude adds to the complexity and workload for pilots at a critical stage of the flight.  

Also, for pilots of most light General Aviation aircraft, there will be one less pressure setting to have to consider. This will assist in the drive to reduce airspace infringements as confusion over which setting to use, and therefore the indicated height of the aircraft, has led to aircraft infringing lower levels of controlled airspace. As such there is a safety benefit to be derived from the proposed changes both inside and outside controlled airspace.