THE INSTRUCTOR AND TRAINEE DURING SLOW FLT PRACTICE LOST AUTH OVER THE THROTTLE INPUT TO THE CARB AND WITH PARTIAL PWR LOSS MADE A FORCED LNDG IN A CORN FIELD. THERE WAS NO INJURY OR DAMAGE.

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FLT LESSON BEGAN XA30 LCL WITH EMER PROC PRACTICE AND SHORT FIELD AND SOFT FIELD TKOFS AND LNDGS AT AN UNCTLED ARPT 10 MI N. HAD STUDENT CLB AND LEVEL OFF AT 2800 FT MSL FOR SLOW FLIGHT. STUDENT REDUCED PWR TO 1700 RPM; ACFT (1973 BE23-C) SLOWED AND BEGAN A DSCNT. PROMPTED STUDENT TO INCREASE PWR; HE INCREASED THE THROTTLE; BUT ENG DID NOT RESPOND; RPM REMAINED AT 1700 RPM. WE TRIED EXERCISING THE THROTTLE FROM 1700 RPM TO FULL PWR SEVERAL TIMES; BUT THERE WAS NO RESPONSE. I MOVED THE THROTTLE FULL TRAVEL; ENG RPM DROPPED TO 1300. PROCEEDED WITH PARTIAL PWR LOSS CHKLIST ITEMS AND FORCED LNDG ITEMS AS PER POH. LANDED THE ACFT IN CORN FIELD (STALKS 4 INCHES TALL; SNOW 2 INCHES DEEP) WITHOUT DAMAGE TO THE CREW; THE ACFT; OR OTHER PROPERTY. ACFT HAS HAD 3 OCCURRENCES/ COMPLAINTS (WHICH WERE FIXED) IN THE PAST YEAR INVOLVING IMPROPER CARB SETTINGS; MIXTURE AND CARB HEAT RIGGING. THROTTLE RIGGING DID NOT APPEAR TO BE AT THE CAUSE OF PREVIOUS PROBS. A SISTER ACFT (1979 BE23-C) OPERATED BY OUR COMPANY HAS HAD CONSISTENT PROBS WITH CARB SETTINGS AND ADJUSTMENT; THROTTLE; CARB HEAT; AND MIXTURE CTL RIGGING. SVC PERSONNEL (4 A AND P'S AND IA'S; BEECH FACTORY TRAINED WITH 20 PLUS YRS EXPERIENCE EACH) HAVE HAD DIFFICULTIES SETTING CARB AND ENG CTL SETTINGS. ON THE SISTER ACFT (1979 BE23-C) CURRENTLY IN OVERHAUL; THE THROTTLE; CARB HEAT; MIXTURE CABLES HAVE BEEN FOUND TO HAVE BEEN STRETCHED; SOMETIMES BINDING IN THE CABLE SLEEVE. AS THE ENG WAS RUNNING SMOOTHLY PRIOR TO THE PARTIAL PWR LOSS; I BELIEVE THE PROB MIGHT BE SIMILAR TO WHAT WAS FOUND IN THE 1979 BE23-C. BEFORE THE SLOW FLT DEMONSTRATION THE ENG RAN SMOOTHLY AND RESPONDED TO 11 OR 12 PWR ADJUSTMENTS DURING THE PREVIOUS HR'S FLYING. CONSIDERING CURRENT WX CONDITIONS (CLR WITH THIN BROKEN AT 3000 FT IN THE SW QUADRANT; TEMP 6 DEGS F AT THE SURFACE) I SHOULD HAVE BEEN CLOSER TO AN ARPT DURING THE SLOW FLT AND STALL PRACTICE. WHILE THE OUTCOME WAS SAFE; BY BEING CLOSER TO AN ARPT; I COULD HAVE REDUCED THE RISK CONSIDERABLY. MY DECISION TO GO FURTHER (10 MI) FROM THE ARPT WAS SO I COULD HAVE THE STUDENT PRACTICE GND REF MANEUVERS ON THE WAY BACK. GIVEN THE BASE ARPT CLASS D AIRSPACE AND POOR GND REF POINTS ON WHICH TO PRACTICE; I CHOSE TO GO FURTHER S.