Hi Jim,
Can you clarify more on 'the TODRH can be displaced in the vertical plane, permitting the origin of the departure/arrival surface to be divorced from the surface of the site?
I have found reference to this in appenix 14 but cant quite stitch it all together. The obstacle clearance gradient generally starts at the edge of the FATO and rises out at an angle which creates the initial obstacle surface. This however really doesn't suit a Class 1 helicopter as no obstacles are permitted in the clearway in the direction of flight as their is a requirement for 35ft clearance of the ground OEI until passes the end of the TODRH which is where the aircraft has reached Vtoss and climbing away at 100ft/min 2.5min power.
Is what you are saying is that you can raise the origin of your departure surface, lets say climb the helicopter vertically to 20ft, then start your cat a departure procedure from there? In your approach back to the site you would end up in a 24ft hover instead of a 4 ft hover for example, or in the event of an engine failure arrive over your pad at 20ft and then accept a vertical descent? This is what we 'practically' do in confined area operations but I class this as Cat B as this procedure is not covered in our AFM. Cat B is not approved for inner city operations.
Or, are you relating to increasing you TODP to give clearance of obstacles in your flight path as per the AFM?
Regards
Matt