CAPT OF A DC9 OVERSHOT ASSIGNED ALT DURING CLB DUE TO THE DISTR OF GEAR RETRACTION PROB WHICH HE WAS AIDING THE FO IN RUNNING THROUGH THE QRH CHKLIST. AFTER NOTICING THE OVERSHOOT HE RETURNED IMMEDIATELY TO ASSIGNED ALT.

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WE HAD JUST DEPARTED PHL; WHEN DURING CLB CAPT NOTICED AN UNSAFE R GEAR LIGHT ILLUMINATED. WE NOTIFIED ATC OF THE PROB; REQUESTED VECTORS AND CONTINUED CLB TO 8000 FT MSL. AFTER PERFORMING THE QRH PROC; WE DETERMINED THAT WE COULD CONTINUE TO DEST ALTHOUGH AT A LOWER ALT AND AIRSPD. ATC WAS NOTIFIED; AND WE WERE GIVEN VECTORS TO CONTINUE A CLB TO 10000 FT; THEN A CLB TO 12000 FT. PASSING 11000 FT; THE FO MADE HIS '11000 FT FOR 12000 FT' CALL; AND CAPT (PF) ACKNOWLEDGED '1000 FT TO GO.' THAT IS WHEN THE GEAR DOOR OPEN LIGHT CAME ON. THE CAPT HAD THE AUTOPLT ENGAGED WITH APPROX 500 FPM CLB; WHEN HE CALLED 'I STILL HAVE THE ACFT; QRH -- GEAR DOOR OPEN LIGHT ON.' THE FO'S ATTN WAS DIVERTED TO FINDING THE PROPER CHKLIST; WHEN THE ALT BELL SOUNDED. THE CAPT THOUGHT THAT HE WAS PASSING 11000 FT FOR 12000 FT; BUT WAS ACTUALLY PASSING 12300 FT. HE IMMEDIATELY DISCONNECTED THE AUTOPLT AND DSNDED TO 12000 FT. LESSONS LEARNED: 1) CARDINAL RULE DURING EMERS IS -- FLY THE ACFT FIRST. THE AUTOPLT WAS INTENTIONALLY ENGAGED TO SERVE AS A 'THIRD PLT' ALLOWING THE PF TO STAY IN THE DECISION MAKING PROCESS; BUT THE ALT CTL WHEEL WAS LEFT IN THE 500 FPM CLB. 2) CAPT MOMENTARILY LOST SITUATIONAL AWARENESS. FORTUNATELY; THIS WAS A VMC CLBOUT. DURING IMC IN A HIGHLY CONGESTED CORRIDOR LIKE THIS; THE RESULTS COULD HAVE BEEN DISASTROUS. SUPPLEMENTAL INFO FROM ACN 414504: ATC NEVER INQUIRED ABOUT ALTDEV. DURING SUBSEQUENT CALL TO MAINT CTL IN MILWAUKEE; PROB WITH GEAR WAS HANDLED SATISFACTORILY AND FLT WAS COMPLETED WITHOUT FURTHER INCIDENT.