Narrative:

Flight instructor was talking student through a trim stall while student was performing maneuver. After approach confign was established (full flaps, no power, gliding), student applied power to create trim stall occurrence. As power was applied, the C172 began to shake or vibrate moderately. Full power was not able to be achieved. Prior to this happening, there were no noises or explosions to indicate any trouble. Carburetor ice or contaminated fuel was originally suspected. C172 uses automatic fuel. The C172 would only develop between 1600 and 1700 RPM maximum with moderate vibration of the entire airframe. For this aircraft, maximum throttle resulted in a constant descent of between 200 and 400 FPM flying straight. Corrective actions taken were: carburetor heat applied for icing if present, emergency checklist for engine trouble performed and fuel/air mixture leaned in an attempt to gain more RPM's. The C172 was headed back toward ZZZ airport via safe landing fields (not direct) while these attempts to resolve the engine problem were being tried. At ZZZ1, the C172 had been flying approximately 8 mins and the condition had not improved, nor had it gotten any worse with the engine and vibration. Good oil pressure was maintained throughout the entire time. With an altitude of around 1800 ft MSL, populated areas and mountainous terrain was all that lay ahead with no more suitable landing fields. The flight instructor decided to declare an emergency and land the aircraft in the field at ZZZ2, us. The student squawked 7700 and talked with ZZZ2 approach control while the instructor circled the field and made a soft/short field landing. A normal shutdown of the system and engine were performed after the aircraft stopped. No apparent damage was caused to the aircraft by the landing and no personal injuries occurred. Also, no property damage occurred. The aircraft wings were removed and the aircraft hauled back to the airport. The FAA responded and investigated. The aircraft is to be inspected by mechanics/ai to determine the cause of the rough engine and loss of power.

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Original NASA ASRS Text

Title: C172 INSTRUCTOR PLT AND STUDENT HAD AN ENG MALFUNCTION THAT REQUIRED AN OFF ARPT FORCED LNDG.

Narrative: FLT INSTRUCTOR WAS TALKING STUDENT THROUGH A TRIM STALL WHILE STUDENT WAS PERFORMING MANEUVER. AFTER APCH CONFIGN WAS ESTABLISHED (FULL FLAPS, NO PWR, GLIDING), STUDENT APPLIED PWR TO CREATE TRIM STALL OCCURRENCE. AS PWR WAS APPLIED, THE C172 BEGAN TO SHAKE OR VIBRATE MODERATELY. FULL PWR WAS NOT ABLE TO BE ACHIEVED. PRIOR TO THIS HAPPENING, THERE WERE NO NOISES OR EXPLOSIONS TO INDICATE ANY TROUBLE. CARB ICE OR CONTAMINATED FUEL WAS ORIGINALLY SUSPECTED. C172 USES AUTO FUEL. THE C172 WOULD ONLY DEVELOP BTWN 1600 AND 1700 RPM MAX WITH MODERATE VIBRATION OF THE ENTIRE AIRFRAME. FOR THIS ACFT, MAX THROTTLE RESULTED IN A CONSTANT DSCNT OF BTWN 200 AND 400 FPM FLYING STRAIGHT. CORRECTIVE ACTIONS TAKEN WERE: CARB HEAT APPLIED FOR ICING IF PRESENT, EMER CHKLIST FOR ENG TROUBLE PERFORMED AND FUEL/AIR MIXTURE LEANED IN AN ATTEMPT TO GAIN MORE RPM'S. THE C172 WAS HEADED BACK TOWARD ZZZ ARPT VIA SAFE LNDG FIELDS (NOT DIRECT) WHILE THESE ATTEMPTS TO RESOLVE THE ENG PROB WERE BEING TRIED. AT ZZZ1, THE C172 HAD BEEN FLYING APPROX 8 MINS AND THE CONDITION HAD NOT IMPROVED, NOR HAD IT GOTTEN ANY WORSE WITH THE ENG AND VIBRATION. GOOD OIL PRESSURE WAS MAINTAINED THROUGHOUT THE ENTIRE TIME. WITH AN ALT OF AROUND 1800 FT MSL, POPULATED AREAS AND MOUNTAINOUS TERRAIN WAS ALL THAT LAY AHEAD WITH NO MORE SUITABLE LNDG FIELDS. THE FLT INSTRUCTOR DECIDED TO DECLARE AN EMER AND LAND THE ACFT IN THE FIELD AT ZZZ2, US. THE STUDENT SQUAWKED 7700 AND TALKED WITH ZZZ2 APCH CTL WHILE THE INSTRUCTOR CIRCLED THE FIELD AND MADE A SOFT/SHORT FIELD LNDG. A NORMAL SHUTDOWN OF THE SYS AND ENG WERE PERFORMED AFTER THE ACFT STOPPED. NO APPARENT DAMAGE WAS CAUSED TO THE ACFT BY THE LNDG AND NO PERSONAL INJURIES OCCURRED. ALSO, NO PROPERTY DAMAGE OCCURRED. THE ACFT WINGS WERE REMOVED AND THE ACFT HAULED BACK TO THE ARPT. THE FAA RESPONDED AND INVESTIGATED. THE ACFT IS TO BE INSPECTED BY MECHS/AI TO DETERMINE THE CAUSE OF THE ROUGH ENG AND LOSS OF PWR.

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.