WIGYCIWYT,
I had meant to mention Penzance as one of the places which is genuine Class 1 as I believe it is a scheduled airline operation, the same as most public transport fixed wing operations. Nearly all TDPs, particularly for older generation helicopters, are variable dependent on weight, temperature (because they affect DA) and most depend on accel/stop distance and have an altitude component either to clear the tail during the flare when decelerating or to give adequate terrain clearance and/or the ability to accelerate to a higher airspeed when using a technique such as the 170/20 TDP you describe. A number of operations have gained specific clearances from the CAA once they have been demonstrated to work. A couple of others which come to mind were the clearance for the AS355 for the early Manchester Police operations and the rooftop clearance for a smaller-than-normal pad for G-RMGN for Robert Maxwell.
In the offshore oil industry many oil companies pay lip service to Class One operations, whilst actually utilising Class 2 procedures. I remember XOM in Cameroun initially specifying Class 1 operations for their trans-Cameroun pipeline helipad operations until they found out what payload that would give them
. Many pilots also receive little or no ground training regarding Class 1 or 2 operations as too many companies (both operators and oil companies) will try to get away with the minimum training time they can
