Incident Overview
A certified flight instructor and a student pilot were practicing touch-and-go landings at night. Shortly after the fifth touch-and-go, the engine lost power. The aircraft subsequently sustained damage when it was landed in trees.
Engine Examination
Examination of the engine revealed dark, sooty deposits on all spark plugs. The spark plugs were reinstalled, and an operational check was performed. The engine started almost immediately and developed power when the throttle was opened. Further inspection identified that the carburetor heat door on the carburetor airbox would not move to the full-on position when the carburetor heat control was activated. The airbox had been improperly modified and/or repaired. No entries were found in the aircraft logs concerning the airbox.
Environmental Conditions
The temperature and dew point were 61°F and 45°F, respectively. According to icing probability charts, these conditions were conducive to carburetor ice. The CFI reported that carburetor heat was used prior to each landing.
Maintenance History
The aircraft had flown approximately 97 hours since the previous annual inspection, which had occurred about five months before the accident.