Voir Wikipedia en anglais sur http://en.wikipedia.org/wiki/Brownout_(aviation)
Brownout par analogie à whiteout voir http://en.wikipedia.org/wiki/Whiteout_(weather)

Le “brownout” a été la principale cause de pertes d’hélicoptères en Irak et en Afghanistan.

In aviation, a brownout (or brown-out) is an in-flight visibility restriction due to dust or sand in the air.[1] In a brownout, the pilot cannot see nearby objects which provide the outside visual references necessary to control the aircraft near the ground.[2] This can cause spatial disorientation and loss of situational awareness leading to an accident.[3] Pilots have compared landing during brownouts to parallel parking an automobile with one’s eyes closed.[4]
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.[5] Brownouts have claimed more helicopters in recent military operations than all other threats combined.[6]