Many moons ago a group of US Army H-34s was on maneuver in Europe. They flew into a canyon in trailing formation. The lead helicopter entered into a thick haze and the lead told the following helicopters to stop and do a 180. The trailing helicopters exited the canyon but the lead helicopter was totally socked in. He brought his helicopter to a hover (no indication of airspeed) and tried to assess his situation. He had no indication of forward speed and no indication of vertical speed. However his Artificial Horizon indicated that he was in a climb and banking to the left. The rigging of the helicopter caused this. With high collective there is a bias in the controls to counter tail rotor thrust and the disc is tilted to the left. The helicopter will also try to hang under the disc due to the 3-degree forward tilt of the mast, which gives a tail low attitude. Which instruments should he believe. He tried turning on pitot heat thinking the pitot tube was frozen and that didn’t work. So there he is with no visual contact with the ground or the canyon walls and conflicting indications from his instruments. He eventually crashed killing all aboard.
