The Transition Level is the altitude at which the altimeter will be switched from standard pressure or QNE to the local pressure QNH.

It is easiest to differentiate between transition altitude and transition level using a convenient memory device. Some people think Altitude=Above and Level=Low or Landing. Regardless of what method is used to remember which is which, the transition altitude will always be depicted on the approach plate for the airport and it will not change. The transition level will be assigned by ATC and will change based upon the local pressure. Generally, the Transition Level will be broadcast on the ATIS, it will also be identified by the controller when they switch from the verbiage FL to thousand with the corresponding local pressure.

For example, N650GG descend five thousand QNH 1017. When the local altimeter setting is given it is also important to read back the altitude clearance as well as the pressure setting. N650GG descend five thousand QNH 1017. Generally, there is a buffer between the transition altitude and the transition level known as the transition level. How should the flight crew set the altimeter when the transition altitude and transition level are the same? For instance, the ATIS advertises the transition level and altitude as 6,000. This is often the case in the United Kingdom. The answer is to treat the transition level and the transition altitude the same as you would in the states. If the controller says, “N650GG descend and maintain FL60 you will use standard pressure as the altimetry reference. If the controller says descend and maintain five thousand, you will use the local pressure reference.

Another generally accepted practice when flying internationally is to switch to the local altimeter when crossing through the transition level. Likewise, when climbing through the transition altitude it is ICAO practice to switch to standard pressure when crossing above the transition altitude. To add to the confusion, it is standard practice in the UK to switch to QNE when cleared above the transition altitude and when cleared below the transition level. Regardless, each operator should have a documented SOP and if ICAO differences exist, a through briefing should be conducted.