A PA28-181 ON CLB AT 1500 FT HEARD A 'THUMP.' INSTRUCTOR TOOK CTL BELIEVING ENG PROB OR BIRD STRIKE. PROP SPINNER FOUND MISSING FROM ACFT.

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THE ACFT DEPARTED RWY 33 AT XA15 FOR A PRACTICE IFR TRAINING FLT. AFTER PASSING THROUGH 1500 FT MSL; THE ACFT WAS TURNED ONTO THE L XWIND LEG TO CONTINUE WITH A WBOUND DEP TO SIMULATE VECTORS FOR THE VOR 15 APCH AT ZZZ. STUDENT WAS SEATED IN THE L SEAT AND AT THE CTLS. AFTER PASSING THROUGH 2300 FT MSL; A HARD THUMP WAS FELT IN THE COCKPIT. I TOOK OVER CTL OF THE ACFT AND IMMEDIATELY TURNED BACK TOWARDS THE ARPT WHILE ENRICHING THE MIXTURE AND APPLYING FULL CARB HEAT. THE ENG RAN SMOOTHLY AND ALL ENG INST GAUGES WERE IN THE GREEN. I EXAMINED THE FLT CTLS AND ALL WERE OPERATING NORMALLY. THERE WERE NO VIBRATIONS OR SHAKING EITHER BEFORE OR AFTER THE ANOMALY. AFTER EVALUATING ALL THE INFO PRESENTED TO US; I DECIDED TO CONTINUE THE FLT IN THE ARPT VICINITY. DURING THE POSTFLT WALKAROUND; WE DISCOVERED THAT IT WAS THE SPINNER THAT WAS REMOVED FROM THAT ACFT INFLT. TRACES OF PAINT WERE FOUND ON THE BACK OF THE PROP BLADE; BUT NO DAMAGE WAS DONE TO THE PROP. ALL SPINNER SCREWS WERE ALSO INTACT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED WHEN THEY HEARD THE 'THUMP' IT IMMEDIATELY WAS SUSPECTED A CYLINDER BLEW; BUT ENG INDICATIONS WERE NORMAL. RPTR STATED THAT THEY THOUGHT OF A POSSIBLE BIRD STRIKE BUT CTL SURFACES WERE OPERATIVE. ON TAXI TO THE LINE AND ENG SHUTDOWN; AND BEFORE EXITING THE AIRPLANE; A TECHNICIAN ADVISED THAT THE PROP SPINNER WAS MISSING. THE PROP HAD A FEW SCRATCHES AND HAS NOT BEEN EVALUATED. IT MAY BE REPLACED.