When I started my helicopter pilot training at Whiting Field (US), I practiced my landings on clean, square, cement pavement. Conditions hardly differed when I made my first operational landings as a Navy helicopter pilot. When I approached a ship’s landing spot, my only concern was not dropping my helicopter into the sea. I had no idea that my landings could become more challenging until I started to fly missions in support of Amphibious Operations or Special Operations. In fact, during my first landings on unprepared, non-paved landing zones (LZ), I experienced something I initially considered to be light Foreign Object Damage (FOD), but it was not. During some training with a United Kingdom Navy Squadron, I discovered Army pilots had to deal with this phenomenon every day. They called it ‘brownout’. Imagine an approach at night, with little natural illumination, to an unknown landing area in the middle of the desert only defined by given coordinates. Add to that talcum powder dust that begins to pick up at 50 feet and envelops your cockpit and cabin at 20 feet above the ground. The best way to describe a true brownout approach is to ask you to close your eyes at around 25 feet above the ground with near zero air speed and try to land. Believe me, Afghanistan was – in that regard – the nightmare of my Squadron.
Landing in brownout conditions has been described as ‘far and away the most dangerous thing you can do as a helicopter pilot’, and it is costing the military significant amounts of money and – more importantly – lives. One pilot described this phenomenon as ‘essentially flying a controlled crash into the ground with no outside reference’.1 The issue of helicopter brownout has long been a known problem, but it has become a really expensive problem for NATO forces during military operations in Afghanistan and Iraq. Since 2002, the US Army alone has lost or damaged 27 helicopters in brownout mishaps. NATO helicopters from all Nations and services have suffered losses operating at unprepared sites in dense, recirculating dust. The US Air Force Institute of Technology stated that the US Department of Defense (DoD) attributes over 100 million USD in total costs per year to brownout mishaps, and it found brownout accounted for 65 per cent of non-hostile fatalities during hover and low speed flight.2 In the overall Operation Enduring Freedom, the US DoD attributes one third of all helicopter mishaps to brownout. Consequently, strengthening pilot awareness through improved flight displays for low speed manoeuvring is a top priority for all military forces using helicopter support in an effort to prevent brownout mishaps. That is why brownout has recently become a more prevalent research topic than in the past.
The Brownout Phenomenon
Technically speaking, helicopter brownout is the dangerous phenomenon often experienced when performing take-offs, approaches, and landings in dusty environments, where sand or dust particles get swept up in the rotor downwash and obscure the pilot’s vision of the terrain. Brownout develops due to the inherent nature of the helicopter rotor system, which takes air in and accelerates it downward at a vector resulting from the angular deflection of the rotor blades. This accelerated air is known as the rotor downwash. During flight at high altitudes, the rotor downwash dissipates easily into the surrounding air. However, as the helicopter hovers near to the ground at relatively low airspeeds, the downwash makes contact with the surface terrain and creates a cushion of air in between the helicopter and the ground. This reduces the air entering into the rotor system and is known as ground effect. The start of brownout is typically expected to begin when the aircraft enters in ground effect (IGE), which occurs at an altitude approximately equal to the diameter of the main rotor.
Rotorcraft pilots flying in moderate desert environments have learned the brownout effect during landings can instantly transform unlimited visibility to complete blindness, i.e. from Visual Meteorological Conditions (VMC) to Instrument Meteorological Conditions (IMC). While this problem is prevalent in dry, dusty, and sandy areas, a similar phenomenon can also occur in snowy conditions and is known as a ‘Whiteout’.
Further research on the technical causes of brownout came up with additional factors driving brownout severity.
Rotor Disk Loading. This is the ratio of a helicopter’s mass to the lifting area of the main rotor disk, usually expressed in pounds/square inch or kilograms/square meter. A helicopter in a hover must produce a downward thrust equal to the mass of the helicopter, thus the heavier the helicopter, or smaller the rotor disk diameter, the higher the disk loading. Helicopters with higher disk loading produce more thrust and hence faster rotor downwash velocities, and are typically expected to generate more severe brownout as a result.
Rotor Configuration. Despite this principle of rotor disk loading, tandem rotor configurations experience more severe brownout than single rotor helicopters. This particular phenomenon has been further examined by Phillips and Brown, who applied an Eulerian simulation of simplified landing manoeuvres to predict the formation of the dust cloud under different rotor configurations.3 Their findings showed that tandem rotorcraft, such as the CH-46 ‘Chinook’, generate more dense and longer lasting dust clouds in comparison to the single rotor configuration.
Blade Tip Design. Another factor in brownout may be blade tip design. Pilots of the Leonardo ‘Agusta Westland’ EH-101 reported that its blade system, developed by the British Experimental Rotorcraft Program (BERP), produces a ‘donut effect’ of clear air around the aircraft reducing the brownout effect.4 Though specific causes for the phenomenon are not known, the manufacturer Agusta Westland, attributes the phenomenon to advanced blade tip design of the BERP blades. A similar blade tip design is used for the Lynx helicopter; however, it does not experience the same ‘donut’ effect. Studies were conducted comparing the UH-60 and the EH-101 to investigate reasons for differing brownout performance with no conclusive evidence found. One possible explanation for this could be related to the airframe design of the EH-101, rather than the blade tip design.
Technical Requirements and Solutions to Brownout
A Research and Development (R&D) push has resulted in technical solutions to mitigate the effects of a Degraded Visual Environment (DVE) on rotorcraft during low-altitude manoeuvring, particularly during landings and take-offs. Based on visual cues indicating drift, height above terrain (HAT), descent rate, ground speed, attitude, slope, terrain features, LZ location, obstacle clearance, and moving obstacle detection, the pilot is provided with more intuitive and salient information, thus increasing aircraft orientation awareness and strengthening decisions for controlling the aircraft. With this new technology, if available, pilots are principally able to hover, land, and take-off helicopters without outside visual references while immediately recognizing non-intentional aircraft movement.
- Low-speed flight symbology already helps prevent crash descents and dangerous drift in dust. US Army Apache pilots use AH-64 hover symbology to make brownout landings, and similar cockpit cues have migrated to US Air Force helicopter cockpits.
- The Rockwell Collins Common Avionics Architecture System (CAAS) in new Chinooks incorporates symbology for the Brownout Situational Awareness Upgrade (BSAU).5 This program is also part of the UH-60M upgrade to the US Army Black Hawk, and derivative displays will go into the aging US Marine CH-53E and new fly-by-wire CH-53K.
- US Marine MV-22 and Air Force CV-22 tilt rotors have flight path vector displays that allow crews to make brownout landings manually with cues on the hover indicator or automatically using the fly-by-wire hover-hold function.
- Boeing Chinook engineers, meanwhile, claim that the Digital Automatic Flight Control System (DAFCS) in the CH-47F achieves the desired effect at lower cost. With an automatic departure mode, the DAFCS is already credited with saving lives when pilots lost spatial orientation in brownout.
- Brownout initiatives are now looking to integrate see-through infrared sensors with synthetic vision displays. Some tests showed medium-to-long-wave Forward Looking Infra-Red (FLIR) sensors had twice the dust-penetrating performance than electro-optical cameras. The current 3-to-5 micron or 8-to-12 micron FLIRs mounted on attack helicopters for targeting and navigation are essentially still blind in brownout.
- The United Kingdom MoD, in collaboration with Leonardo Company, conducted a research program called All Condition Operations and Innovative Cockpit Infrastructure (ALICIA) looking at future designs and configuration regarding cockpit layouts and the Human Machine Interface (HMI). One of the purposes of this program is exploiting ways to assist the aircrew in take-off, approach, and landing operations in the presence of re-circulating sand and dust. ALICIA yielded many innovative ideas concerning a suite of cockpit design concepts and technology solutions to be universally applied across multiple military and civilian aircraft platforms, both fixed wing and rotorcraft.6
When implementing technological solutions, it is important to take human factors and cultural mind-sets into consideration. In particular, will organizations recognize the requirement for more intense Instrumental Flight Rules (IFR) training in comparison to regular landing procedures? To make the new technologies seem natural to the pilots, solutions should be intuitive and as easy to use as possible, but acclimation to the new technologies can also be improved through more robust training.
The helicopter, by nature, is an unstable platform that forces pilots to continuously operate their controls to gain and maintain stability based on visual or other sense references. Helicopter operations such as externals, fast roping, and rappelling require the aircraft to maintain a hover for extended periods of time, and hovering requires an active outside scan and a visual ground reference. Without immediate corrective input to the controls, the position of the helicopter can only be maintained for a very short period of time. Unintentional drift may develop causing the aircraft to strike an obstacle or hit the ground with excessive rate of descent or airspeed.
Although many aspects of helicopter flight can be performed using only an instrument scan, landing and hovering cannot. Standard instrumentation in most of the current helicopter models does not yet provide the fidelity or adequate feedback for drift and height above terrain meaning that pilot inputs are still essential to keep control of those fundamental parameters during landings and take off. Even the Automated Flight Control Systems (AFCS) available in some legacy airframes still rely on the pilot’s hands-on control.7 All of this makes the aircrew particularly susceptible to brownout.
As long as the technical solutions mentioned above have not yet been introduced and sufficiently proven their reliability, pilot training remains indispensable when it comes to mitigating brownout effects. Most NATO nations, therefore, have improved their helicopter pilot training by incorporating different landing techniques and skills addressing brownout situations. Respective training objectives are now integrated into military helicopter pilot student guides and defined in greater detail in national flight training instructions. In particular, they contain procedures and indicators for pilots to determine – even in the last moment – whether it is recommended to execute a shallow approach rather than a steep approach in order to avoid or mitigate brownout. The different landing and take-off techniques to limit the brownout effect are also described in an extensive Technical Report published in 2012 by the NATO Research and Technology Organisation (RTO).8 Among other manoeuvres, these landing techniques are part of the simulated and live training requirements for all NATO helicopter pilots prior to joining an operation.
Brownout mishaps still cost NATO in aircraft and crew lives in ongoing conflicts. However, troops are being withdrawn from Afghanistan and Iraq to areas where brownout is less probable. That is why investments in cockpit and sensor technology addressing DVE conditions will probably be de-prioritized in favour of other pressing issues competing for the same limited defence budgets. More importantly, when helicopter units do not deploy they spend less time in the air, training opportunities become scarce, and skills consequently degrade.
However, while waiting for technology that hopefully will vanquish the brownout problem for rotorcraft crews, the most practical way ahead remains continuous training. Apart from improved cockpit symbology, better crew training has proven to mitigate the risk related to brownout. Considering training is therefore even more vital than technology. The Alliance and Nations should not wait until the next conflict emerges, but invest permanently in pilot training specifically addressing the brownout phenomenon.