Ich würde den Transponder reparieren lassen...

In Lufträume, für die ein Transponder vorgeschrieben ist, also auch für Flüge über 5000 Fuß darf man mit einem defekten Transponder nicht einfliegen:

NCO.IDE.A.105 Minimum equipment for flight
A flight shall not be commenced when any of the aeroplane instruments, items of equipment or
functions required for the intended flight are inoperative or missing, unless:
(a) the aeroplane is operated in accordance with the MEL, if established; or
(b) the aeroplane is subject to a permit to fly issued in accordance with the applicable airworthiness
requirements.

außerdem:

NCO.IDE.A.200 Transponder
Where required by the airspace being flown, aeroplanes shall be equipped with a secondary
surveillance radar (SSR) transponder with all the required capabilities.