Date: 201308
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Robinson R22
Operator: Other Flight School
Operating under: Other 141
Flight plan: VFR
Mission: Training
Flight phase: Final Approach
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Engine Control
Component problem: Improperly Operated
Reporter organization: FBO
Reporter function: Instructor
Reporter qualification: Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument
Second reporter function: Trainee; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Procedure

Narrative: We were cleared to land. My student began his pre-landing checks and disengaged the cyclic trim. The student struggled to push down the trim knob and had to apply a fair amount of pressure to get the knob to go down. When the trim went down he kept his hand there over the trim knob and mixture control which are approximately two inches apart. The student acted as if he was confused about what he had just done then began to pull the mixture full off. I attempted to stop the student from pulling the mixture but realizing it was too late and he was going to do it; I took the controls and entered an autorotation from approximately 100-150 feet and 35 KIAS. After safely setting the aircraft on the ground I notified the Tower that we had landed and requested permission to change frequencies and contact my Company Dispatch to report the event.After speaking with Maintenance personnel we deemed there was no damage to the aircraft and returned to base. I believe the event was caused by the student's momentary confusion after disengaging the cyclic trim and could have been prevented if the student took a moment to think rather than acting on the first thing that came to mind.


Second narrative: During the pre-landing checks I reached around the left side of the cyclic to push the trim down; into the off position. I noticed the trim was stiff to push; but could not see it from my position. After I felt the trim knob lower I returned my left hand to the collective and moved my left knee slightly to the right so that I could visually verify the trim had gone down. At that moment I saw the trim knob down; but I also identified the mixture guard was not on the mixture knob; but rather sitting between the mixture knob and the trim knob. Realizing that the guard was not in its proper position; realizing that was not correct; I became momentarily confused and concerned; thinking the mixture knob was supposed to be out and wondering; had I accidently just pushed the mixture in to the incorrect position as I was pushing the trim. In my self-induced confusion I proceeded to pull the mixture knob out; thinking to myself that I was correcting the problem; when in fact there was nothing wrong other than the mixture guard not being in its proper place. I could hear my instructor yelling; 'Don't pull that'; as I was pulling the mixture knob. The aircraft lost power almost instantaneously. My instructor immediately took the controls and performed a safe; full-down autorotation.The two contributing factors that I could think of would be: 1. My sleep has not been as steady as it was and; 2. The day of the flight I had not eaten since waking up. From this instance forward; those are two factors that I will be mindful of before flying.


Synopsis: R22 instructor pilot took control of the helicopter and made a successful autorotation after the student pilot mistakenly shutoff the mixture control.