More specifically, you are frozen on pp18 unless you consistently bid for all available Heathrow commands and you are eligible to bid. In practice BALPA have recently determined that this means you must be bidding for all LHR commands every year from pp16 onwards, and you cannot change aircraft type from pp13. (If you change aircraft you will be frozen on it for 5 years, during which time you are no longer eligible to bid for commands on other aircraft and so your pay will be frozen at pp18). The terms are a lot more restrictive than most people realise.