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In aviation, a brownout (or ''brown-out'') is an in-flight visibility restriction due to dust or sand in the air. In a brownout, the pilot cannot see nearby objects which provide the outside visual references necessary to control the aircraft near the ground. This can cause spatial disorientation and loss of situational awareness leading to an accident.〔 〕 Pilots have compared landing during brownouts to parallel parking an automobile with one's eyes closed.〔 The brownout phenomenon causes accidents during helicopter landing and take-off operations in arid desert terrain. Intense, blinding dust clouds stirred up by the helicopter rotor downwash during near-ground flight causes significant flight safety risks from aircraft and ground obstacle collisions, and dynamic rollover due to sloped and uneven terrain.〔 〕 Brownouts have claimed more helicopters in recent military operations than all other threats combined. There are several factors which affect the probability and severity of brownout:〔 *rotor disk loading *rotor configuration *soil composition *wind *approach speed and angle Countermeasures to prevent brownout-related accidents include: *Site preparation *Pilot technique *Synthetic vision systems also known as "see and remember" *Upgraded horizontal situation indicator with improved symbology *Aerodynamics such as the "winged rotor" on the AgustaWestland EH101 *Non-visual displays of position and orientation data derived from suitable sensors, such as Tactile Situational Awareness Systems (TSAS) providing information to the pilot through the sense of touch using tactors. ==Sensory illusions== Blowing sand and dust can cause an illusion of a tilted horizon. A pilot not using the flight instruments for reference may instinctively try to level the aircraft with respect to the false horizon, resulting in an accident. Helicopter rotor wash also causes sand to blow around outside the cockpit windows, possibly leading the pilot to experience the vection illusion, where the helicopter appears to be turning when it is actually in a level hover. This can also cause the pilot to make incorrect control inputs, which can quickly lead to disaster when hovering near the ground. In night landings, aircraft lighting can enhance the visual illusions by illuminating the brownout cloud. The visible effects of sand rotor abrasion have been extensively observed in Afghanistan. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Brownout (aeronautics)」の詳細全文を読む スポンサード リンク
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