What was found
(CAN) WHILE ENROUTE AT FL360 THE ACFT HAD AN IDG 1 FAILURE. THE FLIGHT CREW DECLARED A PAN PAN AND DESCENDED TO FL300 TO START THE APU. AFTER A SUCCESSFUL APU START, THE FLIGHT CLIMBED TO 330 AND CONTINUED NORMALLY REF FIM 24-21-00-810-865-A, FAULT CLEARED DURING PROCEEDURE D. SWAPPED GCU NR 1 WITH APU GCU AMM 24-21-19 AND 24-22-03. BOTH SYS CHECK SERVICEABLE. ENGINE RUN CARRIED OUT, NO FAULTS. GCU NR 1: PN 1701321D, SN 1364. MFG: PN: 1701321D, SN 0506.