In the flow logic of your screen PAI event try to put the chan-endchain. Do the checks inside the module.
Something like this -
PROCESS AFTER INPUT.
chain.
field sflight-carrid.
field sflight-connid.
field sflight-fldate.
module check_flight.
endchain.
here any errors in module check_flight will ensure that after error is displayed all the three fields are still enabled.
Best regards,
Saurabh