Aircraft Accident/Incident Report

Hesperia, California 92345
Sunday, December 10, 2006 17:55 PST

The emergency medical services (EMS) helicopter was performing a cross-country repositioning flight from a hospital back to its base during dark night conditions back over a routing that the pilot had flown 5 times that day and also earlier in the evening when they had transported a patient to the hospital. Visual meteorological conditions predominantly prevailed along the route of flight; however, analysis of the weather reports disclosed conditions consistent with broken to overcast clouds having bases at 4,000 feet msl in the vicinity of the accident site. An AIRMET had been issued for the area for IFR conditions, with mountain obscuration, precipitation, mist, and fog. The helicopter was equipped with a satellite-based tracking system that reports the helicopter's GPS location to the operator's ground base while the system is in operation, and the data for the accident flight was reviewed. The route of flight proceeded toward the apex of a mountain pass, which is the main transition route from one side of a mountain range to the other, where the helicopter's base is located. The tracking data indicated that the helicopter appeared to follow a major highway in the lower portion of the pass. The highway makes a large "S" shaped path as it gains in elevation toward the top of the pass, which is about 4,200 feet mean sea level (msl). The route along the highway is away from a well-lit major city area that has a well-defined light horizon, toward rising and dark terrain with no ground reference lights other than vehicles on the highway. Once at the top of the pass as the highway turns toward the northeast, the upper desert communities on the other side of the mountain range once again provide a well-lit and clearly defined horizon. Near the upper end of the pass, the helicopter's satellite derived flight track showed that it inexplicably diverged toward the east, away from the highway, instead of continuing to follow the highway into the upper desert valley. The helicopter collided with terrain about 0.7 nautical miles east of the highway at 4,026 feet msl. The accident site was located in a small ravine, near the base of a 100-foot tall electrical transmission tower that was located along the ravine's east ridge. During subsequent examination of the airframe structures, flight control components, and engines, no pre-impact anomalies were found that would have precluded normal operation prior to impact. While the operator was in the process of equipping its helicopter fleet with night vision goggles, the accident helicopter had not as yet been equipped with any enhanced night vision devices. The helicopter was equipped for instrument flight, including a 3-axis autopilot. The first fire department responders to the accident site reported that the area was covered by what they described as "intermittent waves" of fog that would suddenly form and then dissipate, which made it difficult to locate the wreckage.

The pilot's inadvertent encounter with instrument meteorological conditions and subsequent failure to maintain terrain clearance. Contributing to the accident were the dark night conditions, fog, and mountainous terrain.