“Cranfield will accept you in the descent…”
I was recently asked why London Control gave the following clearance, “Cranfield will accept you in the descent to altitude 4500 feet. You are cleared to leave controlled airspace in the descent”.
In order to understand this, you must remember that if you are outside of Controlled Airspace (in Class G airspace), ATC will take no responsibility for the safe conduct of your flight. Airfield radar units might offer a Traffic Service to give you advice about other known traffic, or radar vectors and altitude changes to help you establish on an approach, but it ultimately remains your responsibility to avoid other aircraft, hitting the ground or entering restricted areas! They will never say “go direct to..” because that would imply that it is safe for you to fly a straight line to that location; instead, they say “own navigation to ..” which should remind you that navigation is your responsibility and that, for example, you might have to fly a dog-leg around a glider site. Finally, remember that Oxford, Gloucester, Cambridge and Cranfield are all located within Class G airspace whilst Cardiff, Bristol and Bournemouth are located within Class D terminal airspace. For more information on flight information services outside of Controlled Airspace, make sure to read CAP 774.
It’s a totally different story when you are operating inside Controlled Airspace (Class A in the airways, or Class D terminal airspace around large airports). Radar Control Units (including London Control who manage Class A airspace) will assign you to levels and altitudes within Controlled Airspace, and give you radar headings and go-directs to points within Controlled Airspace whenever they wish; you MUST follow those instructions. They can do that because they know that there are no hazards in doing so.
If flying level when you leave Controlled Airspace, London will only ever offer you a Basic Service because they have no remit or capacity to give you warning about other traffic outside of Controlled Airspace. However, Cardiff, Bristol and Bournemouth can sometimes offer a Traffic Service in their local Class G airspace.
If descending out of Controlled Airspace, London Control will never clear you to a level or altitude because it is outside of Controlled Airspace; it would not be safe for them to do so because they have no idea what is there. Instead, they will say, for example, “Cranfield will accept you in the descent to 4500′, you are cleared to leave Controlled Airspace in the descent”. The implication is that, once you are outside of Controlled Airspace, you’re on your own. Note that giving you clearance to leave Controlled Airspace in a descent does not give you permission to leave the frequency; you will be given that permission once you actually exit Controlled Airspace.