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 (as of 2005).[6]
There are several factors which affect the probability and severity of brownout:[6]
Countermeasures to prevent brownout-related accidents include:
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.[10] In night landings, aircraft lighting can enhance the visual illusions by illuminating the brownout cloud.[11]
The visible effects of sand rotor abrasion have been extensively observed in Afghanistan.[12]
|
Further information: Sensory Illusions in Aviation |
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.[13]
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.[14] Brownout accidents occur close to the ground and at low airspeed, giving these accidents a higher survivability than other types. However, there have been deaths in military accidents in Iraq and Afghanistan, and nearly all of those were preventable.[15]
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.[16]
The CH-47 Chinook has had a relatively high frequency of brownout accidents. As of 2007[update], nine Chinooks were lost in action in Afghanistan, and at least two were caused by brownout, which likely played a role in several other incidents.[17] According to the Project on Government Oversight (POGO), 12 of 41 U.S. Army brownout accidents between 2002 and 2005 involved CH-47s. Data compiled by POGO from government sources show the Chinook flew 7 percent of all U.S. Army helicopter flight hours between 2003 and 2005 but accounted for 30 percent of all brownout-related accidents.[18]
Brownout is a particular concern for the U.S. V-22 Osprey tiltrotor aircraft,[19] 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.[20] Initial operational experience indicates that although the dust cloud is larger with the MV-22 than it is with the CH-46 it is replacing, pilots report regaining visibility near the ground, allowing them to use visual references prior to landing.[21]
