C172 PLT ENCOUNTERS WINGTIP STRIKE DURING GAR IN SMF AIRSPACE.

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I RECENTLY WAS INVOLVED IN A NEAR CATASTROPHIC INCIDENT. IT STARTED OUT AS A VERY UNEVENTFUL FLT FROM SQL TO AUN. THE FLT WAS VFR AT DUSK AND PLANNED FOR APPROX 1 HR AND 15 MINS. FLT SVC WAS RPTING CEILINGS IN THE BAY AREA AT 1500-3500 FT AND VISIBILITY AT 4-10 MI. OVER THE SACRAMENTO VALLEY; CEILINGS WERE 4500-6000 FT AND VISIBILITY WAS 10 MI. AS WE NEARED AUN; THERE WERE LOW; SCATTERED CLOUDS AND SPOTS OF LOW VISIBILITY. ON APCH TO AUN; A GAR WAS INITIATED; DUE TO A GROUP OF LOW CLOUDS WHICH BEGAN TO COVER THE RWY. APCH WAS TO RWY 7; WHICH IS RECOMMENDED AT NIGHT; DUE TO HIGH TERRAIN E OF THE FIELD; AND UPHILL RWY SLOPE. AS THE GAR WAS INITIATED; CARB HEAT WAS UNINTENTIONALLY LEFT ON; WHICH REDUCED AIRPLANE PERFORMANCE. AS THE AIRPLANE CLBED OUT; THE R WINGTIP CONTACTED WHAT WAS BELIEVED TO BE A TREE BRANCH. THIS BRANCH WAS NOT VISIBLE; DUE TO THE LOW VISIBILITY AND DARKNESS. AIRPLANE CONTINUED FLT WITHOUT ANY OTHER NOTICEABLE PROBS. AS THE DOWNWIND TURN WAS INITIATED AGAIN; THE LOW CLOUDS WERE NO LONGER COVERING THE RWY; SO ANOTHER APCH WAS ESTABLISHED; AND THE AIRPLANE LANDED WITHOUT FURTHER INCIDENT. AFTER LNDG; I CAME TO FIND OUT THAT A HELI; WHICH HAD JUST RECENTLY LANDED AT AUN; TRIED TO NOTIFY US VIA SAC APCH (WHICH WE WERE USING FOR FLT FOLLOWING) THAT THE VISIBILITY WAS NOT GOOD AT AUN. WE DID NOT HEAR THE HELI; ON THE SAC APCH FREQ; NOR DID SAC APCH NOTIFY US OF THE HELI'S CALL. THIS INCIDENT COULD HAVE BEEN PREVENTED BY ANY OF THE FOLLOWING: 1) SELECTING AN ALTERNATE ARPT AT THE FIRST SIGN OF CLOUDS WHICH REDUCED VISIBILITY. BUT; BECAUSE VISIBILITY WAS VERY GOOD UP UNTIL WE REACHED THE ARPT; I HAD NOT GIVEN MUCH THOUGHT TO THE FACT THAT WE MIGHT NEED AN ALTERNATE. 2) IF WE WOULD HAVE BEEN PAYING MORE ATTN TO RADIO CALLS; WE WOULD HAVE HEARD THE WARNING FROM THE HELI AND COULD HAVE HAD MORE TIME TO WORK AN ALTERNATE PLAN. 3) IF THE HELI WOULD HAVE ESTABLISHED POSITIVE CONTACT WITH APCH CTL; THEY COULD HAVE ENSURED THAT WE GOT THE INFO. 4) IF PROPER GAR PROCS WERE USED; THE CARB HEAT WOULD NOT HAVE BEEN LEFT ON.