N358MNCessna T310R

The flight instructor in the retractable landing gear, multi-engine airplane reported that, while performing aerial survey training for a newly hired pilot, the airplane was slowed to the mission airspeed of 117 knots, which meant reduced power and the use of more than 15° of flaps. He then pulled the circuit breaker to silence the landing gear warning horn to ensure clear communication, but did not pull the landing gear motor circuit breaker. The pilot receiving instruction reported that he did not know that the instructor had pulled the circuit breaker.

The flight instructor further reported that, while returning to the departure airport, the landing checklist was "accomplished" 15 to 20 miles from the airport. He observed that the pilot was "falling behind on power and descent management" and the airplane was high and fast for the approach. The instructor advised the pilot to correct but then began to assist the pilot when it "was taking longer [than anticipated.]" The airplane decelerated into the white airspeed arc, and the instructor moved the landing gear selector switch into the down position and added full flaps. He added that, since the flaps were added shortly after the landing gear selector switch was put into the down position, he could not tell by the flight characteristics that the landing gear had not extended. Neither he nor the pilot observed that there was no green down and locked position light indicator. The airplane landed with the landing gear retracted.

The flight instructor added that, while configuring the airplane on the runway, he observed that the landing gear warning horn and landing gear motor circuit breakers were tripped.

The airplane sustained substantial damage to the fuselage and both engines.

The director of maintenance reported that, during a postaccident examination, they lifted the airplane onto jacks and performed an emergency gear extension. He added that they then did three electrical landing gear retractions and no defects were found.

In a photo provided by the flight instructor, the landing gear warning horn and landing gear motor circuit breakers are immediately next to each other.

The airplane manufacturer pilot operating handbook contains a note in section titled "Landing Gear Warning Horn", which states,

Do not pull landing gear warning circuit breaker to silence horn as this turns off the landing gear control relay, thus the landing gear cannot be retracted.