A PIPER PA28 IN CRUISE AT 5000 FT DECLARED AN EMER AND DIVERTED DUE TO THE CABIN DOOR OPENING.

Narrative

DURING CRUISE FLT; THE CABIN DOOR POPPED OPEN. AN IMMEDIATE HIGH LEVEL OF NOISE AND A ROLLING MOMENT SIMILAR TO THAT EXPERIENCED WHEN AN ENG FAILS. A SEEMINGLY INADEQUATE CHK OF THE CHK OF THE DOOR BY THE PIC. ALTHOUGH THE DOOR PASSED A 'THUMP TEST' (BUMPING THE DOOR WITH A CLOSED FIST) AND THE DOOR LOCK MECHANISM ENGAGED; THIS WAS A FALSE AND INADEQUATE LOCKING. THE PIC DECLARED AN EMER WITH ZZZ APCH CTL AS THE FLT CHARACTERISTICS WERE DRAMATICALLY AFFECTED BY THE TRAILING OPEN DOOR. LNDG WAS EFFECTED AT ZZZ; UNITED STATES. ZZZ TFC WAS USING RWY 31; WHEN THE PIC ANNOUNCED AN EMER ON UNICOM AND HE WAS LNDG STRAIGHT IN ON RWY 13 (OPPOSITE DIRECTION) DUE TO HANDLING INSTABILITY OF THE ACFT; ALL ACFT IN THE ZZZ TFC PATTERN DIVERGED AWAY FROM THE ACFT IN DISTRESS AND ALLOWED A SAFE AND UNEVENTFUL LNDG. ONCE SAFELY ON THE GND AND CLR OF THE RWY; THE DOOR WAS EXAMINED FOR DAMAGE; THE LATCHING SYS TESTED AND THE DOOR WAS RE-SECURED. THE FLT CONTINUED UNEVENTFULLY TO ZZZ1 ARPT AND FINALLY TO ITS HOME BASE; ZZZ2 ARPT. A MAINT SQUAWK WAS MADE CONCERNING THE DOOR AND WILL BE EXAMINED FURTHER AND REPAIRED AS NECESSARY BY THE AMT.