Europe Airpost B733 at Dublin on Oct 15th 2014, returned twice over transponder problems

An Europe Airpost Boeing 737-300, registration EI-STA performing flight AB-423J from Dublin (Ireland) to Split (Croatia) with 146 passengers, was climbing out of Dublin's runway 10 when the crew requested to level off at FL070 due to a minor flight tracking problem. ATC requested their heading, which the crew indicated was 100, then approved level off at FL070 and issued delay vectors. The crew subsequently queried whether ATC did receive a proper transponder response, which ATC replied to in the negative. The crew therefore requested to return to Dublin, where the aircraft landed safely on runway 10 about 30 minutes after departure.

The aircraft departed again about 2 hours later but needed to return again due to lack of proper transponder responses.

A replacement Boeing 737-800 registration F-GZTD reached Split with a delay of about 12 hours.

Radar data suggest the aircraft's transponder worked but did not supply altitude data.