CAPT TURNED OFF MODE C TO CONCEAL THE FACT THAT THEY HAD OVERSHOT THEIR ALT DURING DESCENT.

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VERY FAMILIAR SCENARIO. I WAS TIRED FROM 6 PREVIOUS DAYS OF SETTLING STRESSFUL FAMILY MATTERS. CAPT WAS ON FIFTH CONSECUTIVE DAY ON LINE. IT WAS END OF 10+ HR DUTY DAY. I; THE F/O; WAS FLYING FROM BWI TO HPN; CRUISE ALT OF FL190. ATC; NY CENTER CLRED US TO 15000' WHICH WAS SET IN ALT ALERTER. 200' BELOW SELECTED ALT BUZZER SOUNDED; BUT WAS MISSED BY US BOTH. CTLR CALLED US AT 14400'. I LEVELED FROM A 1000-1500 FPM RATE DSCNT BY 14200'; INITIATED CLB IMMEDIATELY BACK TO 15000'. CAPT WAS WORKING RADIO AND RESPONDED WE WERE AT 15000'; SIMULTANEOUSLY SWITCHING MODE C OFF. ONCE BACK AT 15000'; HE SWITCHED MODE C ON. NO FURTHER MENTION MADE BY THAT ON 4 CONSECUTIVE CTLRS UPON WHICH WE LANDED AT HPN. MY CONCERN IS I CAUGHT CAPT TWICE PRIOR TO THIS OF POTENTIAL ALT BUSTS DURING HIS LEGS TO FLY; AND IN A TACTFUL MANNER MENTIONED WE WERE BOTH PRIME FOR PROBS TO CROP UP FROM FATIGUE. HE DIDN'T TAKE IT TO HEART. WHEN I MENTIONED ABOUT THE ASRS PROGRAM; HE TOOK IT LIGHTLY; PARTLY FROM LITTLE INFO ABOUT IT; PARTLY FROM BELIEF THAT NO SUSPENSION WOULD RESULT AND ONLY A FINE AND A MARK ON HIS RECORD AT WORST. PEOPLE WITH THESE ATTITUDES ARE ALWAYS THE ONES WHO GET ME IN TROUBLE OR WHO NEVER AVERT A PROB I BRING ON MYSELF BUT DON'T CATCH TILL TOO LATE!