director of the Aeromedical Training Institute, discusses spatial disorientation in an article on which most of the following section is based, and from where the following quotes originate.
In darkness, peripheral visual information (or ambient vision) is reduced. “Ambient vision is sensitive to flat planes (i.e. the horizon) and motion cues, and is processed by the preconscious brain. Under day visual meteorological conditions (VMC), the pilot specifically uses ambient visual information…to judge and maintain proper aircraft attitude.”
“At night, much of the ambient visual information is absent. Also, the potential for other visual illusions (false horizon or indistinct horizon) is much higher.” Focal vision must be used to maintain aircraft attitude by reading the flight instruments. Focal vision is processed by the conscious brain, and the conscious brain can quickly become overwhelmed. This state is termed task saturation, and can result in important situational awareness cues (i.e., altitude, descent rate) being missed. The pilots most susceptible to task saturation are those who are not trained to operate in limited visibility conditions, those who have been trained but have not flown in similar conditions for a long time, and pilots with limited experience.
Cockpit tasks are more difficult at night. “Switches are harder to find and placards are harder to read under low cockpit lighting conditions. Again, this puts an increased load on the conscious brain and this, in turn, raises the potential for unrecognized spatial disorientation and/or loss of situational awareness.”
“Instrument training disciplines the pilot in conscious attention management and helps him avoid attention management problems like channelized attention, distraction, and task saturation. Additionally, instrument training disciplines the pilot to ignore false sensory perceptions and believe the instruments.”
According to the Federal Aviation Administration, spatial disorientation and loss of situational awareness cause 15% to 17% of fatal general aviation crashes annually (about 2.5% of the total mishaps). “More significantly, 9 out of 10 spatial disorientation mishaps result in a fatality. Most of these mishaps occur when pilots are flying at night and/or…in instrument meteorological conditions.”
A number of factors can predispose a pilot to spatial disorientation during night flying. These factors can be classified into 3 categories: environmental, psychological, and physiological.
Environmental factors include those factors that reduce the amount of information (usually visual information) that is normally available to the pilot during day, VMC flying conditions. These include: night, IMC, blending of the surface of the water with an overcast sky (for overwater flights), fog, and haze. These factors tend to make the horizon difficult or impossible to distinguish, thus requiring the pilot to revert to the flight instruments in order to maintain level flight.
Richard A. Leland, “Night VFR: An Oxymoron?”, The Journal of Aviation/Aerospace Education and Research, Vol. 9, No. 1, Fall 1999, Embry Riddle Aeronautical University.