CEN13FA122

The pilot’s inadvertent encounter with localized icing conditions and his subsequent in-flight loss of helicopter control.

GPS tracking data revealed that, after departure, the helicopter proceeded westbound about 600 ft above ground level (agl), following a roadway. About 6 minutes after liftoff, when the helicopter was about 3/4 mile south of the accident site, it turned right and became established on a northerly course. The helicopter subsequently turned left and appeared to be on a southerly heading at the final data point. Shortly before beginning the left turn, the helicopter entered a climb, reached an altitude of about 1,800 ft agl, and then entered a descent that continued until impact. Weather observations from the nearest Automated Surface Observing System, located about 7 miles east of the accident site, indicated that the ceilings and visibility appeared to be adequate for nighttime helicopter operations and did not detect any freezing precipitation. Although an airmen's meteorological information advisory for icing conditions was current for the route of flight, and several pilot reports of icing conditions had been filed, none of the reports were in the immediate vicinity of the intended route of flight. Witnesses and first responders reported mist, drizzle, and icy road conditions at the time of the accident. It is likely that the pilot inadvertently encountered localized icing conditions, which resulted in his subsequent in-flight loss of helicopter control. A postaccident examination of the helicopter revealed no preimpact failures or malfunctions. The engine control unit recorded engine torque, engine overspeed, and rotor overspeed events; however, due to their timing and nature, the events were likely a result of damage that occurred during the impact sequence. Evidence also indicated that the cyclic centering, engine overspeed, and hydraulic system warning lights illuminated; it is also likely that their illumination was associated with the impact sequence. Further, the engine anti-ice status light was illuminated, which was consistent with the activation of the anti-ice system at some point during the accident flight.

Helicopter engine operation was controlled by a Triumph Engine Control Systems, formerly Goodrich Pump & Engine Controls, model EMC-35R Engine Control Unit (ECU), serial number JG09ANU1247. One corner of the ECU housing was broken out, exposing a portion of the underlying circuit board. The non-volatile memory components related to the primary and reversionary governors was downloaded. The total ECU and engine operating times were 1,196.80 hours and 1,003.06 hours, respectively. The ECU did not provide for continuous recording of engine parameters. However, the unit did record engine fault and incident data. The ECU incorporated primary and reversionary governor systems, which provided redundancy for engine operation. ECU data was organized into engine history, last engine run faults, time stamped faults, accumulated faults, and incident data. Time data associated with each fault or incident corresponded to the engine run time. Upon logging of a fault or incident event, the ECU also recorded 12 seconds of pre-event data into non-volatile memory. The last engine run fault data files associated with both the primary and reversionary governors did not contain any fault codes. The time stamped fault data file associated with the primary governor did not contain any fault codes. The time stamped fault data file associated with the reversionary governor contained a total of 21 faults. The most recent fault was recorded at 733 hours ECU operating time. Because the current engine operating time was about 1,003 hours, the most recent fault was recorded about 270 hours before the end of data, which was well before and not relevant to, the accident flight. The reversionary governor accumulated fault data file contained only faults also recorded into the time stamped fault data file. Three incidents were contained in the snapshot data file. The first was recorded at 1,003:06:16.344 (hh:mm:ss.sss) engine operating time and consisted of a high engine torque event of 110 percent. The associated 12 seconds of pre-event data were unremarkable, with engine speed, rotor speed, and engine torque parameters within normal limits. The collective pitch parameter was about 58 percent and the fuel flow about 452 pounds per hour (pph) during this timeframe. Cyclic and anti-torque pedal positions were not recorded by the ECU. The second snapshot was recorded at 1,003:06:22.873 and consisted of a high power turbine event of 108 percent. An engine overspeed parameter is set due to this event. In addition, the collective pitch parameter had decreased to 32 percent and fuel flow to 36 pph at this time. The third snapshot was recorded at 1,003:06:22.920 and consisted of a high rotor speed of 109 percent. The loss of subsequent data was consistent with a loss of electrical power to the ECU at impact. In addition, the engine and rotor overspeed events, in conjunction with a decreasing fuel flow and collective pitch, was consistent with the rotor system being aerodynamically driven above 100-percent, such as in the descent prior to impact. Light bulb filaments from the caution and warning panel were examined by the investigator-in-charge at the NTSB materials laboratory. The filaments associated with the cyclic centering, engine anti-ice, engine overspeed, and hydraulic system exhibited stretching consistent with illumination at the time of impact. According to the aircraft flight manual, the cyclic centering annunciator light will be illuminated when the helicopter is on the ground and the cyclic stick is not centered. The engine anti-ice annunciator will illuminate when the engine anti-ice system is activated. Engine overspeed annunciation is provided when an overspeed condition is detected by the ECU. Hydraulic system indication is provided when the system pressure decreases below 650 psi. The normal hydraulic system operating pressure is 1,000 psi. An autopsy of the pilot was completed at the Mercy Medical Center, Mason City, Iowa, on January 3, 2013. The pilot's death was attributed to multiple blunt force injuries sustained as a result of the accident. FAA Civil Aerospace Medical Institute toxicology testing was negative for all substances in the screening profile. The Mason City Municipal Airport Automated Surface Observing System (ASOS), located about 7 miles east of the accident site, at 2053, recorded weather conditions as: wind from 300 degrees at 8 knots; 8 miles visibility; broken clouds at 1,700 feet agl, overcast clouds at 3,300 feet agl; temperature -3 degrees Celsius; dew point -5 degrees Celsius; and an altimeter setting of 30.05 inches of mercury. At 2100, the recorded conditions included a wind from 310 degrees at 9 knots, broken clouds at 1,700 feet agl, and overcast clouds at 3,300 feet agl. At 2105, the wind was from 310 degrees at 10 knots, with broken clouds at 1,500 feet agl and overcast clouds at 2,000 feet agl. At 2110, the wind was from 310 degrees at 12 knots, with overcast clouds at 1,500 feet agl. The Forest City Municipal Airport Automated Weather Observing System (AWOS), located about 8 miles northwest of the accident site, at 2055, recorded conditions as: wind from 300 degrees at 9 knots; 10 miles visibility; overcast clouds at 1,000 feet agl; temperature -2 degrees Celsius; dew point -3 degrees Celsius; and an altimeter setting of 30.04 inches of mercury. At 1955, about one hour before the accident, the observation included a note of unknown freezing precipitation. However, this notation was not included in the subsequent observations. The MCW terminal forecast, issued at 1959, expected wind from 250 degrees at 6 knots, 5 miles visibility in light snow, and overcast clouds at 1,400 feet agl. Satellite imagery depicted an overcast layer of stratiform clouds over the region with cloud tops near 11,000 feet. The regional radar mosaic for Iowa did not depict any significant meteorological echoes in the vicinity of the accident site about the time of the accident. However, the radar scan sampled the airspace from about 6,630 feet to 15,100 feet over the accident site. Any echoes below this height would not have been detected by the weather radar. Pilot reports (PIREP) filed between 1500 and 2400 over Iowa indicated light to moderate rime ice ranging in altitude from 3,500 feet msl to 8,500 feet msl. These reports ranged from Sioux City, at the western end of the state, to Dubuque at the eastern end of the state. The closest PIREP was over Spencer, Iowa, about 70 miles west of the accident site where a pilot reported light rime icing during climb at 6,400 feet msl. This was about 23 miles west of the accident flight intended destination. An Airman Meteorological Information (AIRMET) advisory for icing was current for the route of flight. AIRMET Zulu was issued at 2045 and was in effect until 0600. It warned of moderate icing conditions below 10,000 feet msl, with icing conditions expected to continue through 0900. Witnesses and first responders reported mist, drizzle, and icy road conditions at the time of the accident. One first responder reported observing a police car slide through a roadway intersection due to the slick conditions while responding to the accident site. The accident aircraft was a Bell Helicopter model 407, serial number 53959. The helicopter was configured for helicopter emergency medical services (HEMS) operations. The FAA type certificate required one flight crew member (pilot) and permitted operations under day or night visual flight rules (VFR). The helicopter was not certificated for intentional flight into known icing conditions. The operator noted that the helicopter was equipped for instrument flight; however, it was not certified for flight under instrument flight rules (IFR). The helicopter was equipped with heated pitot and static ports; however, the rotor blades were not equipped with ice protection. The helicopter was powered by a Rolls-Royce Allison model 250-C47B turboshaft engine, serial