Rob P wrote:@TopCat
https://publicapps.caa.co.uk/docs/33/CA ... on%203.pdf
Page 103
That OHJ diagram and the diagram on page 104 for crosswind join both show the joining aircraft should be going over the upwind numbers at circuit height.
During my PPL course when doing go-arounds I was taught to not go above half circuit height until crossing the upwind numbers to give height separation to any traffic entering crosswind over the upwind numbers (as shown by the Skyway Code diagrams).
So with an 800 ft circuit that gives 400 ft height separation over the upwind numbers, which might be 300 ft if both aircraft had 50 ft altimeter errors where one was under-reading and the other was over-reading. It might be a bit less in bumpy conditions.
If aircraft doing a takeoff followed the same principal in a strong headwind or with a very long runway there would be no collision risk at the upwind numbers, particularly if the aircraft taking off was a high wing aircraft and the joining aircraft was a low wing aircraft so one or both could not see each other through a wing or engine cowling.
i.e. procedural separation.
A problem with that is some pilots habitually choose not to join crosswind over the upwind numbers, and prefer to do it further upwind, and the aircraft that is taking off (or on the go-around) has resumed climbing towards circuit height, so the procedural height separation has been reduced.
Yes there are many forum debates on who is right/wrong with that.
Perhaps that is why in recent years one popular airfield published its join as an OHJ and explicitly stating flying crosswind over the upwind numbers! At least at that airfield there is no doubt.
Some airfields have a bit of mayhem influenced by noise abatement procedures.