This is a copy of the CAA stuff;
Phase 1 of the CAA’s phased approach for the further carriage of Mode S requires that Mode S transponders replace the use of Mode A/C in all circumstances where transponders are currently mandatory. This proposal was considered to be the minimum next step that should be taken towards the phasing out the use of conventional Mode A/C SSR in UK airspace. This proposal was submitted to and subsequently accepted by the Secretary of State for Transport, and thus enters into force in the Air Navigation Order (ANO) from 31 March 2008.
This measure does NOT affect the voluntary carriage of transponders.
A transition period until 31 March 2012 is being provided to permit time for owners of affected aircraft currently equipped with Mode A/C transponders to have them upgraded to Mode S.
Owners and operators of Mode A/C transponder equipped aircraft are exempted from the ANO until 31 March 2012 by the General Exemption, Official Record Series 4 No. 670.
The exemption only applies if:
the aircraft is equipped with SSR equipment, installed on or before 31 March 2008, which includes pressure altitude reporting capable of operating in Mode A & Mode C and capable of being operated in accordance with such instructions as may be given to the aircraft by the ATC unit;
the aircraft is not operating under IFR within EHS airspace; and
written notice of the aircraft registration and aircraft serial number has been given to the CAA.
Ahhh!
This set of statements does not apply to the VOLUNTARY carriage of transponders - so if there is not a compulsory requirement to carry Mode S then none of the above is applicable.... because it applies to the transition arrangements of compulsory transponder carriage!
As usual with this whole debacle the CAa being less than straight forward (or even honest and transparent!).
Rod I've had a browse through the LAA site and not been able to find the applicable guidance. I'll drop John Brady a note to see if he can post something suitable.
Many thanks for the help!