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Danger of the deadly dust

Posted 17 October 2017 · Add Comment

The harsh climate and environment of the Middle East and North Africa imposes unique challenges for military aviators and particularly for the operators of rotary wing and vertical lift aircraft. Jon Lake reports.

Flying through a dust- and sand-laden atmosphere at low level may be unavoidable, but it can wreak havoc on a helicopter.
Sand will strip and abrade the leading edges of rotor blades and, since today’s advanced, lightweight rotor blades are constructed from fibreglass composite material, they are particularly vulnerable to abrasion and damage.
To counter this, most helicopter rotor blades are fitted with a hard metallic abrasion strip, which prevents the leading edge of the rotor from being worn down too quickly.
Sand and dust also affect a helicopter’s powerplant, with a high risk of rapid engine wear and subsequent power deterioration, and with some chance of glass build-up in high temperature sections of the engine.
There are some solutions to the problems caused by ingestion of sand and dust, but these, themselves, tend to lead to an inherent loss in inlet pressure and, sometimes, auxiliary power.
To combat the ingestion of particles, there are three basic types of inlet protection system. These include integrated inlet particle separators, as well as after-market inlet barrier filters and vortex tube separators, which tend to be offered as a retrofit option.
Typical inlet barrier filters comprise a pleated filter element (pleated to increase surface area), which need to be changed every few flights. It has been calculated that the filter for one helicopter would remain effective for little more than eight minutes in a cloud with 0.5 grams of dust per cubic metre, equal to about 22 dust landings.
Vortex tube separators use a helical vane to spin particles to the periphery, where they are removed via a scavenge flow using a high-speed blower fan. There are no filters to change and minimal inhibition of airflow into the engine air intake.
The typical inlet particle separator creates the least drag, whereas inlet barrier filters provide the highest particle removal rate (at the expense of some loss of power) and vortex tube separators achieve the lowest pressure drop.
But, probably the biggest obstacle to rotary wing operations in sandy and dusty environments, lies in the tendency of helicopters to kick up clouds of dust or sand when hovering close to the ground – especially landing or taking off, when a dense cloud of particulates will be thrown into a vortex by the rotor wash of the hovering helicopter.
This forces aircrew to operate in what is known as ‘brownout’, with the dust cloud obscuring the external visual references that a pilot requires to hover or manoeuvre accurately, to judge his position over the ground or his rate of movement fore and aft or laterally, and even to judge his height or rate of descent.
But brownout is also disorientating – since in the huge swirling mass of particles some will be moving in different directions, which can confuse the pilot as to where the ground is, whether the aircraft is level and even whether it is stationary or drifting toward an obstacle.
Undetected lateral movement when trying to land is particularly dangerous. If the vertical rate of descent is higher than intended, a hard landing will result. However, most helicopters have a strong landing gear that will absorb this.
Similarly, the landing gear will normally cope with unexpected fore and aft movement, since it is already designed to allow rolling landings.
But most helicopters have a high centre of gravity, and any sideways movement can result in a dynamic rollover.
Operating in a degraded visual environment (DVE), whether as a result of environmental conditions (sandstorm or snowstorm) or a brownout, is inherently dangerous. Some experts maintain that operating in a DVE has claimed more military aircraft and lives than enemy fire since 2001, accounting for 400 aircraft losses and 152 lives, at a cost of $1 billion.
Michael Hirschberg, executive director of the American Helicopter Society International, calculated that brownout has been a “significant contributor” in the loss of 400 vertical flight (and 600 personnel) aircraft, while Colonel Matthew Hannah, of the US Army’s aviation systems project office, has said that 25% of the 383 Class A and B US military flight accidents between 2002 and 2015 related to operations in degraded visual environments.
According to Hannah, 56% of these accidents occurred in brownout conditions, costing “approximately nine to ten people per year because of DVE brownout-related accidents”.
The use of prepared landing surfaces at forward operating bases and other regular helicopter stops can reduce brownout landing losses by reducing the amount of loose sand or dust that can be displaced by rotor downdraft.
Alternatively, pilots can keep their aircraft moving forward until its wheels touch the ground, making what resembles a roll-on landing and keeping dust cloud behind the helicopter until it lands. This requires a long flat/smooth landing zone (LZ) without surface hazards and is dependant on surface winds, while also being better suited to aircraft that can land in a more nose-up attitude, like the H-60 and Chinook.
Sometimes, the best option will be to abandon the landing attempt, or to hover high enough to be able to blow the dust out of the way before attempting to land, but this may not be an option, especially in critical military operations.
Technology can help. Workload reduction and improved situational awareness can help in brownout conditions, and improved military helicopter handling qualities and reduced pilot workload are seen by many as forming an important part of any solution. Improved head-up, head-down and helmet-mounted displays, now being developed by the Army Aeroflight Dynamics Directorate in California, will also play an important role.
Some have suggested using commercially available helicopter terrain awareness warning systems (HTAWS), which provide visual and audio warnings if an aircraft is in danger of striking terrain. However, these rely on digital map databases, while military helicopters often fly in unmapped combat zones, and also routinely fly so close to the ground that the system would be issuing near constant warnings.
To properly solve the problem, a pilot needs to be able to see the outside world, or a reasonable approximation of it, to land safely.
Obtaining a visual picture in such conditions requires some kind of sensor – either two-dimensional like infrared video, image intensified video, passive millimeter wave (MMW), or synthetic aperture radar (SAR); or three dimensional as with 3D active MMW, laser detection and ranging (LADAR), sparsely populated radar array, active acoustics or stereo imaging.
Preloaded data may also play a part, including digital maps, terrain and vertical obstruction databases, while station-keeping equipment can provide range/bearing from other aircraft, which may not be in visual contact.
Various solutions have been trialled in recent years, mostly using active sensors, and most notably by the various US service branches – particularly the Army and the special operations community.
In 2013, Sierra Nevada Corp’s helicopter autonomous landing system (HALS), was tested in five US Army UH-60L Black Hawk helicopters in Afghanistan in a year-long trial.
HALS was based on a 94 GHz radar using a frequency whose range is claimed to be “basically unaffected” by smoke, fog, dust, sand or dry snow, and which was reportedly judged to represent the most mature “bang for the buck”. The radar picture is fused with US Department of Defense-sourced digital terrain maps, producing a 60-degree by 60-degree 3D colour image of the outside world, including hazards down to one foot in size.
HALS will eventually be coupled with a brownout symbology system (BOSS) still being developed by the Army, which will overlay speed, altitude, wind direction and even flight control command cues on the picture.
The US Special Operations Command’s Sierra Nevada-supplied DVE pilotage system (DVEPS) was successfully flight tested at Yuma in June 2015. This combines information from millimetre wave, laser scanner and forward-looking infrared sensors to create an accurate, real-time terrain image.
The system has since been selected by the Army to meet its Brownout rotorcraft enhancement system (BORES) requirement for fielding on approximately 300 Sikorsky UH-60M/V Black Hawk and Boeing CH-47F Chinook cargo, utility and medevac helicopters.
 

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