A DHC8 FLC FAILS TO MAKE THE SID DEPICTED TURN AFTER TKOF FROM LGA; NY.

Narrative

DEPARTED RWY 22 AT LGA. FORGOT TO FOLLOW LGA 9 DEP PROC; FLEW RWY HDG INSTEAD OF TURNING L TO 070 DEGS AT 2.5 DME. REALIZED AFTER EVENT THAT DURING CLRNC BRIEF; WAS INTERRUPTED AND NEVER FINISHED THE BRIEF. IT WAS THE THIRD DAY OF A 4 DAY TRIP THAT HAD BEEN FILLED WITH MANY PROBS; MOSTLY MECHANICALS. THIS PARTICULAR FLT INDEED WAS A FERRY FLT DUE TO A MECHANICAL ON AN ACFT WE SWAPPED INTO. THIS CREATED A TIME OF DUTY PROB FOR THE FO AND WE WERE RUSHING TO TRY AND DEPART SO FLC WOULD NOT BE AFFECTED DOWN LINE. CONSIDERABLE ATTN AND DISCUSSION WAS SPENT ON DUTY TIME PROB AND TRYING TO FIGURE OUT IF THIS WAS GOING TO BE A PROB. I FEEL BECAUSE OF THESE DISTRS COUPLED WITH THE FACT NEITHER THE FO NOR MYSELF HAD EVER DEPARTED RWY 22 AT LGA; EVEN THOUGH BOTH OF US ARE EXPERIENCED IN OPS AT LGA; AND THE FACT WE DID BRIEF FOR THE EMER OP (ENG OUT) PROCS AND THEREFORE THOUGHT IT WAS RWY HDG DEP; CAUSED US TO MISS THE TURN. SUPPLEMENTAL INFO FROM ACN 483229: DEP CALLED AT 2000-3000 FT AGL TO ASK WHETHER WE WERE IN A L TURN (LGA 9); BUT THEN REQUESTED WE CONTINUE STRAIGHT OUT.