History of Flight
On December 7, 2012, about 2234 central standard time, a Cessna 501, registration N212M, was substantially damaged during recovery from an unusual attitude after departing from Detroit Lakes Airport (KDTL), Detroit Lakes, Minnesota. The two pilots and four passengers were not injured. The airplane was operated under 14 CFR Part 91 on an instrument flight plan with an intended destination of Bessemer Airport, Alabama. Night instrument meteorological conditions prevailed at the flight level of the accident.
After takeoff from Runway 31, the airplane entered IMC about 1,000 feet above ground level (AGL). The autopilot was engaged at about 3,000 feet mean sea level (MSL). Following an ATC clearance for a left turn on course, the pilot rotated the heading select bug to command a left turn. While climbing through approximately 7,000 MSL with the autopilot engaged, the airplane rolled left and transitioned into a nose-down descent. Both pilots reported that their attention was focused away from the attitude indicators just before the unusual attitude began.
The pilot disengaged the autopilot and attempted recovery, observing the airplane in a nearly inverted, left-turning spiral with a steep nose-down attitude. Recovery was executed using the standby attitude indicator, turn needle, and heading indicator. Both wings sustained structural damage during the recovery.
Radar track data showed a climbing left turn after departure, level-off at 6,000 feet MSL for about 30 seconds, then a climbing right turn to about 7,200 feet MSL, followed by a descending right spiral with a descent rate exceeding 10,000 feet per minute, continuing to about 3,500 feet MSL. The airplane then transitioned to a right climbing turn through 4,800 feet MSL and subsequently climbed with wings level.
Personnel Information
The pilot, age 62, held an airline transport pilot certificate with single-engine and multiengine land ratings. He reported 10,161 total flight hours, with 810 hours in the accident airplane make and model. The acting copilot, age 50, held a commercial pilot certificate with single-engine and multiengine land ratings. He reported 1,498 total flight hours, with 273 hours in the same make and model.
Aircraft Information
The Cessna 501 (serial number 501-0280) was manufactured in 1977. It was registered with the FAA on a standard airworthiness certificate and certificated for single-pilot operation. Total airframe time as of December 5, 2012, was 3,612 hours.
Meteorological Information
At 2213, the KDTL weather station reported wind 140° at 3 knots, visibility 10 miles, broken clouds at 1,900 feet AGL, temperature -5°C, dew point -9°C, and altimeter 30.03 inches. Pilots reported light snow at takeoff. Upper air soundings indicated moderate or greater rime icing likely from 2,000 feet through 14,000 feet MSL.
Avionics Testing and Examination
FAA personnel examined the airplane and found about a quarter cup of water in the forward avionics bay and openings in the nose radome quick-release latches. About ten weeks after the accident, the NTSB, FAA, and an avionics technician conducted testing. Snow was observed in the avionics bay, as the airplane had been parked outdoors during snowstorms. The nose radome quick-release latches were installed per Sierra Industries STC SA8437SW.
Avionics systems were tested per Sperry maintenance manual procedures. With ground power, the attitude systems, autopilot, and flight director showed no anomalies. The heading system tested normally, but the #2 heading system exceeded the recommended warning flag retraction time (9 minutes vs. 5). The #1 vertical gyro, switching unit, autopilot computer, and flight director computer were removed for further testing.
The #1 vertical gyro and switching unit were tested by an overhaul facility with FAA oversight. The gyro became erect and operational within limits, output signals were verified, and internal inspection showed no moisture damage except two small corrosion areas on the circuit board base. A precession test was within specifications.
The flight director computer (Z-500) was tested at Honeywell; no corrosion or water damage was found. It passed acceptance testing with no short or blown fuses. The autopilot computer (SPZ-200) showed black spots on the chassis and boards near connector pins, but no moisture damage. Acceptance testing revealed two tests marginally out of tolerance. No short or blown fuses were found during manual functional testing.
Additional cold-temperature testing of the autopilot computer at an overhaul facility revealed no anomalies. A technician noted previous failures of a fault isolation transformer in SPZ-200 autopilot computers that could cause uncommanded roll, but no such failure was replicated in these tests. Moisture conditions were not introduced during any testing.