Quoted from LukyDuck:
As thermionic previously stated, the most likely cause is that he was flying in the fog/clouds and developed vertigo. It is very easy to lose your sense of right side up vs down when you lose your visual reference and your internal senses are telling you something else. This is something that every pilot can encounter when flying in the clouds, fog or at night.
Below is a good explanation:
"Vertigo is a false sense of movement, causing confusion, disorientation—and, eventually, incapacitation. According to the FAA, vertigo and spatial disorientation (SD) contribute to 15 percent of accidents, typically at night or in instrument meteorological conditions (IMC). Most are fatal, and experienced pilots are not immune. The U.S. Air Force investigated 633 crashes between 1980 and 1989 and referenced SD in 13 percent. Non-instrument-rated pilots have a life expectancy of less than three minutes in IMC, probably because of SD.
Positional sense in space occurs from combining visual and gravitational input, a properly functioning inner ear, and proprioception—brain feedback from nerve receptors. Ever induced a “dead arm” from lying on it? Other than looking, you have no idea where it is. That is failure of proprioception. Your inner ear plays a big role in vertigo. Inboard of your ears is a complex system of three semicircular canals oriented in pitch, roll, and yaw planes. Specialized cells sense movement of fluid inside the tubes and transmit information to your brain. That’s how theme park simulators trick you into believing you are upside down, or, when aboard a stationary train and another train beside you starts to move, why it creates confusion.
Initially pilots experiencing vertigo/SD acknowledge conflict between sensations and instruments; the disconnect then blurs—and finally, incapacitation follows with nausea, visual disturbances, muscle spasms, and panic. Different flying maneuvers provoke insidious, yet compelling and specific, forms of disorientation. Unlike other in-flight emergencies such as cockpit fire or catastrophic engine failure, the spatially disoriented pilot does not perceive there is anything wrong. The aptly named graveyard spiral occurs after a bank; feeling the nose drop, the pilot pulls back to initiate a climb or reduce perceived rate of descent. A tighter turn ensues that magnifies the effect and leads to a stall, overstressing the aircraft or flying into the ground.
The leans occurs after a routine turn with sudden transition to level flight provoking a feeling that one is turning the opposite way. The pilot therefore turns back to the original bank, attempting to correct to a perceived proper orientation. If a pilot turns his head out of the plane of rotation while executing a turn, perhaps looking down at a chart, a rolling sensation may occur and, depending on orientation, erroneous pitch, yaw, or roll inputs may result.
Another illusion convinces pilots they are inverted after a high-performance climb. After leveling off, a lightness in one’s seat is felt while contemporaneous seat-back pressure, caused by continued acceleration, induces continued pitch increase, eventually transitioning to the sensation of inversion."
Pilots are taught to transition to instruments when encountering vertigo and to trust their instruments. Imagine that your senses are telling you you are right side up and your instruments are telling you are in a bank diving at a high rate of speed. It is hard to ignore your body and trust your instruments. You spent a lifetime trusting your senses. The correct thing to do is follow your instruments, level the aircraft and start gaining altitude. Altitude is your friend in a helicopter.
When the pilot flew into the fog, started circling in order to locate his position on the ground he most likely started to encounter vertigo or SD. He did not realize how close he was to the hills and entered into an unknown dive/bank that he was unable to recover from or even realized before the aircraft hit the hillside.
Its happened to me and every other pilot I know. It is a scary feeling when you are as close to the ground as this pilot was. You have to recognize it quickly and climb to safety. Having a second pilot might have prevented this accident as the second pilot might not have been encountering vertigo and realized that they were in a dire situation. The second pilot, as they were taught, would take the controls from the first pilot and place the aircraft in a safe flight envelope.
Many times during my career I have had to take the controls away from my copilot because they did not recognize the dangerous position they were in. With that said, there were times I was flying and my pilot in command took over the controls. There is nothing more scary than watching your rotor blades start hitting a stationary object or the ground coming in your direction faster than you planned.
