acceleration, and cannot distinguish inertial forces from the force of gravity. Some examples of this include the inertial forces experienced during a vertical take-off in a helicopter or following the sudden opening of a parachute after a free fall. Illusions caused by the otolith organs are called somatogravic illusions and include the Inversion, Head-Up, and Head-Down Illusions. The Inversion Illusion results from a steep ascent followed by a sudden return to level flight; the resulting relative increase in forward speed produces an illusion the aircraft is inverted. The Head-Up and Head-Down illusions are similar, involving sudden linear acceleration (Head-Up) or deceleration (Head-Down), leading to a misperception the nose of the aircraft is pitching up (Head-Up) or down (Head-Down); the aviator could be fooled into pitching the nose down (Head-Up) or up (Head-Down) in response, leading to a crash or a stall, respectively. Typically, the Head-Up illusion occurs during take-off, as a strong linear acceleration is used to generate lift over the wing and flaps. Without a visual reference, the pilot may assume from the vestibular system the nose has pitched up and command a dive; if this occurs during take-off, the aircraft may not have sufficient altitude to recover before crashing into the ground. Semicircular canals and somatogyral illusions In addition, the inner ear contains rotational accelerometers, known as the semicircular canals, which provide information to the lower brain on rotational accelerations in the pitch, roll and yaw axes. Changes in angular velocity are detected from the relative motion between the fluid in the canals and the canal itself, which is fixed to the head; because of inertia, the fluid in the canals tends to lag when the head moves, signaling a rotational acceleration. However, semicircular canal output ceases after prolonged rotation (beyond ) as the fluid has now been entrained into motion through friction, matching the motion of the head. If the rotation is then stopped, the perceived motion signal from the inner ear indicates the aviator is now turning in the opposite direction from actual travel, as the fluid continues to move while the canal has stopped. In addition, the inertia of the fluid means the detection threshold of rotational acceleration is limited to approximately 2°/sec; angular accelerations below this value cannot be detected. Specific common somatogyral illusions induced by the semicircular canals are the Leans, Graveyard Spin, Graveyard Spiral, and Coriolis. If the aircraft enters an unnoticed,