Most pilots associate spatial disorientation with a loss of control in instrument conditions. Unfortunately, pilots can fall prey to visual illusions and spatial disorientation in visual conditions as well.
On August 8, 2003, the pilot of a Cessna 340A was killed when the aircraft crashed shortly after takeoff from Eastern Sierra Regional (BIH) Airport in Bishop, California.
On the day of the accident, the pilot met the airplane's owner and a passenger at Upland Airport in Upland, California. They left Upland at 7:15 p.m. local time and arrived at Bishop around 8:30 p.m. The pilot dropped the passengers off, refueled, and taxied out for the night flight back to Upland (sunset was at 8 p.m.).
Witnesses saw the airplane taxi onto Runway 12, accelerate, rotate, and climb to 200 to 300 feet above the ground. The aircraft stopped climbing and rolled into a left turn with a "wings vertical" attitude. It then descended into terrain half a mile east of the airport.
Although the moon was 89 percent illuminated, Runway 12 is oriented toward the White Mountain Range, which is mostly unpopulated and has few lights to provide reference to the ground.
The pilot had logged 1,300 hours of total time, 113 of which were in Piper Seneca and Beechcraft Duchess multiengine airplanes. He held a commercial pilot certificate as well as flight instructor certificates for single-engine land, multiengine land, and instrument instruction. No record was found to indicate that the pilot had ever flown a Cessna 340A prior to the day of the accident.
The NTSB determined the probable cause of this accident was the pilot's in-flight loss of control due to a "somatogravic" illusion and/or spatial disorientation. Factors included the dark lighting conditions and the pilot's lack of familiarity with the airplane.
A somatogravic illusion occurs during a rapid acceleration or deceleration. Acceleration creates the illusion of being in a nose-up attitude, which, in turn, can lead the pilot to push the yoke forward into a dive attitude. Deceleration has the opposite effect, possibly causing the pilot to pull back on the yoke and precipitate a stall. Somatogravic illusions are intensified when the pilot has no outside visual references, such as at night or in instrument conditions. Many pilots treat operations in dark night conditions the same as instrument conditions because of the lack of outside references.
The best way to overcome sensory illusions like these is to trust and rely on the aircraft's instruments in all low-visibility conditions — IFR or VFR. Start with the attitude indicator, then cross-check the airspeed indicator, vertical speed indicator, and the turn coordinator.
For more information about spatial disorientation and visual illusions, read the AOPA Air Safety Institute's Spatial Disorientation Safety Advisor.
Accident reports can be found in ASI's accident database.