Narrative:

I was giving the owner of the aircraft a BFR. He was PIC, as he met all currency requirements and had a valid medical, which I did not possess. He had preflted the aircraft before I arrived, so we departed, did air work at altitude, including stalls, slow flight, steep turns. We descended, did some ground reference maneuvers, and I pulled the power to simulate an engine failure. When I saw he had his field made, we attempted a go around, but the power would not come up sufficiently to maintain flight, so we landed in a plowed field. FAA investigation determined a foreign object was lodged in the venturi of carburetor causing loss of power. Debris was discovered inside the cowling from bird activity. A more thorough preflight might have discovered this debris and the forced landing might have been avoided.

Related reports:

Google
 

Original NASA ASRS Text

Title: C172 INSTRUCTOR AND PLT GETTING A BFR HAD AN ACTUAL ENG FAILURE WHILE PRACTICING FORCED LNDGS. THEY LANDED IN A PLOWED FIELD.

Narrative: I WAS GIVING THE OWNER OF THE ACFT A BFR. HE WAS PIC, AS HE MET ALL CURRENCY REQUIREMENTS AND HAD A VALID MEDICAL, WHICH I DID NOT POSSESS. HE HAD PREFLTED THE ACFT BEFORE I ARRIVED, SO WE DEPARTED, DID AIR WORK AT ALT, INCLUDING STALLS, SLOW FLT, STEEP TURNS. WE DSNDED, DID SOME GND REF MANEUVERS, AND I PULLED THE PWR TO SIMULATE AN ENG FAILURE. WHEN I SAW HE HAD HIS FIELD MADE, WE ATTEMPTED A GAR, BUT THE PWR WOULD NOT COME UP SUFFICIENTLY TO MAINTAIN FLT, SO WE LANDED IN A PLOWED FIELD. FAA INVESTIGATION DETERMINED A FOREIGN OBJECT WAS LODGED IN THE VENTURI OF CARB CAUSING LOSS OF PWR. DEBRIS WAS DISCOVERED INSIDE THE COWLING FROM BIRD ACTIVITY. A MORE THOROUGH PREFLT MIGHT HAVE DISCOVERED THIS DEBRIS AND THE FORCED LNDG MIGHT HAVE BEEN AVOIDED.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.