Pilots of powered aircraft are reminded that Standardised European Rules of the Air require that aircraft equipped with a serviceable transponder shall operate the transponder at all times during flight. Aircraft without sufficient electrical power supply are exempt from the requirement.
If equipped with a transponder and an electrical charging system, it is in any case good practice to fly with the transponder turned on and with mode C selected.
The regulation is as follows;
SERA.13001 Operation of an SSR transponder
(a) When an aircraft carries a serviceable SSR transponder, the pilot shall operate the transponder at all times during flight, regardless of whether the aircraft is within or outside airspace where SSR is used for ATS purposes.
(b) Pilots shall not operate the IDENT feature unless requested by ATS.
(c) Except for flight in airspace designated by the competent authority for mandatory operation of transponder, aircraft without sufficient electrical power supply are exempted from the requirement to operate the transponder at all times.