Vestibular Illusions  The Leans  Coriolis Illusion  Somatogravic Illusion  Graveyard Spiral  A condition called the leans, is the most common illusion  during flight and is caused by a sudden return to level flight  following a gradual and prolonged turn that went unnoticed by  the pilot. The reason a pilot can be unaware of such a gradual  turn is that human exposure to a rotational acceleration of 2  degrees per second or lower is below the detection threshold  of the semicircular canals.  [Figure 17-4]  Leveling the wings  after such a turn may cause an illusion that the aircraft is  banking in the opposite direction. In response to such an  illusion, a pilot may lean in the direction of the original turn  in a corrective attempt to regain the perception of a correct  vertical posture.  The “coriolis illusion” occurs when a pilot has been in a turn  long enough for the fluid in the ear canal to move at the same  speed as the canal. A movement of the head in a different  plane, such as looking at something in a different part of the  flight deck, may set the fluid moving, creating the illusion  of turning or accelerating on an entirely different axis. This  action causes the pilot to think the aircraft is performing a  maneuver it is not. The disoriented pilot may maneuver the  aircraft into a dangerous attitude in an attempt to correct the  aircraft’s perceived attitude.  For this reason, it is important that pilots develop an  instrument cross-check or scan that involves minimal head  movement. Take care when retrieving charts and other objects  in the flight deck—if something is dropped, retrieve it with  minimal head movement and be alert for the coriolis illusion.  As in other illusions, a pilot in a prolonged coordinated,  constant-rate turn may experience the illusion of not  turning. During the recovery to level flight, the pilot will  then experience the sensation of turning in the opposite  direction causing the disoriented pilot to return the aircraft  to its original turn. Because an aircraft tends to lose altitude  in turns unless the pilot compensates for the loss in lift,  the pilot may notice a loss of altitude. The absence of any  sensation of turning creates the illusion of being in a level  descent. The pilot may pull back on the controls in an attempt  to climb or stop the descent. This action tightens the spiral  and increases the loss of altitude; this illusion is referred to  as a “graveyard spiral.”  [Figure 17-5]  This may lead to a  loss of aircraft control.  A rapid acceleration, such as experienced during takeoff,  stimulates the otolith organs in the same way as tilting the  head backwards. This action may create what is known as  the “somatogravic illusion” of being in a nose-up attitude,  especially in conditions with poor visual references. The  disoriented pilot may push the aircraft into a nose-low or  dive attitude. A rapid deceleration by quick reduction of the  throttle(s) can have the opposite effect, with the disoriented  pilot pulling the aircraft into a nose-up or stall attitude.  Graveyard spin  Graveyard spiral  No turning  No sensation.  Start of turn  Sensation of turning  as moving fluid deflects  hairs.  Constant rate turn  No sensation after fluid  accelerates to same  speed as tube wall.  Turn stopped  Sensation of turning in  opposite direction as moving  fluid deflects hairs in opposite  direction.  Endolymph  Cupola  Tube  Figure 17-5.  Graveyard spiral.  Figure 17-4.  Human sensation of angular acceleration.  17-7 