ANCHORAGE, AK, USA
N4198T
CESSNA 320
THE PILOT SAID THAT DURING FLIGHT, HE ENCOUNTERED AN OVERVOLTAGE CONDITION & TURNED OFF THE ELECTRICAL MASTER SWITCH. HE NOTED THAT THE VHF RADIO CIRCUIT BREAKER HAD TRIPPED & THE TURN & BANK INDICATOR SMOKED. HE SAID THAT ABOUT 10 MILES FROM THE AIRPORT, HE TURNED THE MASTER SWITH BACK ON & TRIED TO RESET THE CIRCUIT BREAKER, BUT AGAIN HE NOTED SMOKE IN THE COCKPIT. HE SAID THE LANDING GEAR EXTENDED VERY FAST & HARD, BUT IT DID NOT LOCK. WHEN HE ATTEMPTED TO FOLLOW THE EMERGENCY GEAR EXTENSION PROCEDURE, THE CRANK DID NOT WORK. THE MAIN GEAR COLLAPSED DURING LANDING. AN EXAM OF THE ELECTRICAL SYSTEM DISCLOSED A CHARRED & EXPANDED BATTERY. THE DOWN LIMIT SWITCH FOR THE LANDING GEAR MOTOR HAD BROKEN FROM ITS MOUNT BY THE UPPER BELL CRANK TRAVELING BEYOND THE DOWN LIMIT; A TEARDOWN OF THE LANDING GEAR ACTUATOR ASSEMBLY DISCLOSED EVIDENCE OF OVERTRAVEL. THE ALTERNATORS, VOLTAGE REGULATORS & OVER VOLTAGE RELAY ALL TESTED WITHIN ALLOWABLE LIMITS.
On May 28, 1993, at 1410 Alaska daylight time, a retractable gear, wheel equipped Cessna 320 airplane, N4198T, registered to the Pilot-in-Command, and operated by Aeromap US INC. of Anchorage, Alaska, experienced a gear collapse upon landing at Merrill Field, Anchorage. The business flight, operating under 14 CFR Part 91, last departed Merrill Field for a local photo map mission and the destination was Merrill Field. A visual flight rules flight plan was filed and visual meteorological conditions prevailed. The airplane received substantial damage and the Airline Transport Certificated Pilot-in-Command, the sole occupant, was not injured. According to the Pilot-in-Command, he experienced smoke in the cockpit and then a radio failure. The flaps did not function normally and several attempts were needed to properly extend the flaps. When the landing gear was lowered, the gear would not indicate down and locked. An emergency gear extension was attempted through the use of the emergency gear handle. The gear handle would not engage and turned without resistance. The airplane landed and the left main gear collapsed completely and the right main gear collapsed partially. Examination of the airplane's landing gear system showed that the down limit switch was broken from its mount by the upper bell crank traveling beyond the down limit switch. Disassembly of the landing gear actuator assembly showed that the internal gear mechanism had traveled beyond the gear teeth on the "half moon" gear. The manual gear extension rod actuates a gear which operates the "half moon" gear. Further examination of the landing gear actuator assembly showed that the landing gear motor was rated at 5000 RPM. According to Cessna the motor should have been a 6000 to 6500 RPM motor. The flap motor is the same size and shape and is rated at 5000 RPM and when installed, it would cycle the landing gear system slower. The motor mounted on the landing gear actuator would not turn and was visibly overheated. Examination of the airplane's electrical system showed that the airplane's batteries were expanded and charred. The cables in the battery box showed signs of overheating. No other wires on the airplane were burned, charred, or shorted. The alternators, voltage regulators, and the over voltage relay were removed and tested and the components tested within allowable limits. According to Ben Jacobsen, the Airframe and Powerplant mechanic who tested the components, the over voltage relay "popped" at 31 volts. According to the Cessna Service Manual the over voltage relay should open at 30 volts +/- 1 volt. According to Mike Grimes of Continental Motors and Brian Finnegan of Cessna Aircraft, a short in the airplane's batteries would cause an increase in current. According to Ohm's Law if the current increases the voltage will drop. This condition would cause a voltage drop at the main bus. The voltage regulators are designed to sense the voltage drop and excite the alternators to maintain the required voltage. This will increase the output of the alternators. When this output exceeds the recommended voltage, the over voltage relay is designed to open. The over voltage relay is a electromechanical device and does not operate until an over voltage condition develops. According to Brian Finnegan and Mike Grimes, this electromechanical device may not be required to work for many years and its ability to operate at the time of the over voltage condition is dependant on many factors. The subsequent test of the over voltage relay after its removal from the airplane does not conclusively show that it operated at the time of the accident. Examination of the aircraft flight manual shows no emergency procedure dealing with an over voltage condition. The manual states that the over voltage relay will take the alternator off line should an over voltage condition develop. The emergency procedure states that "When an electrical system malfunction occurs, the following action should be taken. Satisfactory operation may resume after one or more of the following steps.. (1) Turn battery switch OFF then ON. (2) Switch to STBY regulator. (3) Turn battery switch OFF then ON. (4) Turn battery switch OFF and emergency power switch ON. (5) Switch back to MAIN regulator." There is no warning in the aircraft flight manual which warns about potential failures of electrically operated components if they are used after an electrical problem.
AN UNDETERMINED FAILURE THAT RESULTED IN AN OVERVOLTAGE CONDITION OF THE ELECTRICAL SYSTEM, OVERHEATING OF THE BATTERY, OVERDRIVE (OVERTORQUE) OF THE LANDING GEAR EXTENSION SYSTEM, A DISCONNECT OF THE EMERGENCY EXTENSION SYSTEM, AND A SUSBSEQUENT COLLAPSE OF THE MAIN GEAR WHEN IT COULD NOT BE LOCKED IN THE EXTENDED POSITION.
Source: NTSB Aviation Accident Database
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