What was found
(CAN) DURING SLINGING OPERATIONS WITH THE PILOT REDUCED DOOR REMOVED AND RT SLIDING DOOR IN THE OPEN AND LOCKED POSITION. THE RETAINING NUT PN, ASN 52320BH060N BECAME LOOSE AND BACKED OFF OF RETAINMENT PIN 350A25-1348-21. THE SLIDING DOOR SWUNG DOWN AND TO THE RT DEPARTING THE ACFT. MINIMAL DAMAGE WAS OBSERVED TO THE THE FUSELAGE AS THE DOOR LANDED IN A LARGE PILE OF BUCKBRUSH. THE ACFT WAS INSPECTED FOR DAMAGE, REPAIRED AND RETURNED TO SERVICE WITH A NEW NUT INSTALLED. THE REASON FOR THE NUT BACKING OFF COULD NOT BE DETERMINED AS THE NUT WAS LOST DURING DOOR DEPARTURE.