Brownout (aeronautics)

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 (as of 2005). There are several factors which affect the probability and severity of brownout: • 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.{{cite news ==U.S. military experience==

Several coalition military aircraft were lost due to roll-overs while executing dust landings during the Gulf War period of 1990–91. In the decade between then and Operation Enduring Freedom, the U.S. Army recorded over 40 cases of brownout condition accidents during training at the Fort Irwin Military Reservation National Training Center in California, and other various sites. Since 1991, there have been over 230 cases of aircraft damage and/or injury due to unsuccessful take-offs or landings in a dust environment. Although the majority of the incidents occur during landings, there have been a significant number of incidents occurring during take-offs as well. For the more than 50 brown-out incidents with damage reported to date during Army military operations in the 2001–2007 time frame, 80 percent were during landings and 20 percent during takeoffs. Helicopter brownout is a US$100 million per year problem for the U.S. Military in Afghanistan and Iraq. The Army cites brownout in three out of every four helicopter accidents there. is not visible in the large brownout dust cloud that it created during training. Brownout accidents destroyed or severely damaged four AH-64D Apache Longbows in the first three weeks of the 2003 Iraq invasion, while only one had been lost in combat in the same time period. The tandem seat Apache has a narrower stance than the UH-60 Black Hawk, making it more susceptible to rollover if the pilot begins to lose roll attitude control from the brownout. But at night, the Apache's infra-red vision system provides improved visibility when dust obscures the moonlight—the Blackhawk's night vision goggles only amplify available visible light. Brownout is a particular concern for the U.S. V-22 Osprey tiltrotor aircraft, which was deployed for combat in Iraq in September 2007. The high proprotor disk loading creates a high-velocity downwash, which stirs up the dust cloud from a much higher altitude. This can be a problem while hovering during personnel insertion and extraction via hoist or rope.{{cite news ==Partial list of related accidents==

Civilian • August 18, 2001, Vinton, California, USA: A Rocky Mountain Holdings, Aerospatiale AS355F1 (N53LH) MEDEVAC helicopter was damaged in dynamic rollover after an aborted takeoff at a remote location, with no injuries. The helicopter was substantially damaged. The pilot experienced brownout after lifting off approximately off the ground. • August 12, 2002, FOB, Operation Enduring Freedom: A U.S. Air Force, 347th Rescue Wing, HH-60G Pave Hawk helicopter departed slowly without sufficient power to climb out of the dust cloud generated on take-off from its own rotor wash. The pilot tried to land and hit a sand berm. The six aircraft occupants evacuated without serious injury. • March 23, 2003, U.S. Army Aviation Base Camp, Central Iraq: A U.S. Army 6th Cavalry Regiment, AH-64D Apache Longbow helicopter crashed on takeoff on the unit's first day at that base camp. • April 5, 2003, Camp Thunder Road, Kuwait: A U.S. Army 101st Aviation Regiment, UH-60 Black Hawk helicopter collided with a sling load during a pickup attempt in brownout conditions. The three injured crew members pulled the two seriously injured pilots from the burning wreckage before it was destroyed by fire. ==See also==