Coastal radar navigation from the last millennium
25 Years ago, GPS was in its infancy as far as helicopters were concerned. Indeed, DECCA was still the primary navigation tool employed by helicopters involved in Oil and Gas and SAR both in the U.K. and Ireland. DECCA had moved on from the MK 19 roller map to the RNav1 and later the RNav2 computers but they were point-to-point navigation systems. There were no moving maps in helicopters in the 1990s. There was also no electronic nav kit in the back of civilian SAR S61s.
Coastal Navigation and let-down procedures for SAR using a point-to-point RNav2 computer was fraught with danger. Particularly when using DECCA as it was prone to atmospheric interference on that dark and stormy night when you would be quite possibly using it in anger.
For this reason, your position had to be confirmed against a separate source and this was the radar. Putting a waypoint on a piece of rock or a headland confirmed your position. If the waypoint appeared under an appropriate shaped radar return that cross referenced with the paper map you were holding you were where the DECCA RNav thought you were and let-down could proceed safely.
Back in 1995 the SAR contract was operated by Irish Helicopters and they had a waypoint on Black Rock as an IP for a let-down to Black Sod. It was called something different then to the one CHC use now but it was on the rock for the reasons I give above.
It was to confirm your position in an S61 using a generally good but possibly unreliable DECCA nav system and a good distance out from Black Sod to allow height loss after over flying the rock.
It seems that a legacy from a past generation using past generation kit was left in use and the seeds of this tragedy were sown in the early 1990s.
I hope this may shed some light on how this came about from a historical perspective and why there was a waypoint on Black Rock and not mid channel.
Last edited by XA290; 16th Apr 2017 at 10:19.