Thursday, April 1, 2010
Radars, ladars and situational awareness displays are keys to safe helicopter landings in Degraded Visual Environments
Blinding sand and dust clouds churned up by helicopter rotors still cost the U.S. armed services lives and aircraft in ongoing conflicts. Since 2002, the Army alone has lost or damaged 27 helicopters in brownout mishaps, the latest last October when a Special Operations Chinook hit a hidden obstacle on takeoff and crashed with 10 fatalities.
The Air Force, Navy and Marine Corps likewise have suffered losses operating at unprepared sites in dense, recirculating dust. One former Air Force Pave Low pilot summarized, “I’m essentially flying a controlled crash into the ground with no outside reference.”
Better crew training and improved cockpit symbology and flight controls have provided some help in addressing the common threat of brownout. Singly and in partnership, the services and the Defense Advanced Research Projects Agency (DARPA) meanwhile are pursuing advanced see-through, see-and-remember and combination technologies for safe landings in desert dust.
The DARPA Sandblaster initiative culminated early last year in a clear-air demonstration using millimeter wave (MMW) radar to update a stored terrain database and synthetic vision display of the upcoming landing zone. Phase II tests of the Helicopter Autonomous Landing System (HALS II) sponsored by the Army Aviation Applied Technology Directorate (AATD) used MMW radar to see through Yuma dust clouds and generate obstacle symbology.
Last September, the 3D-LZ system integrated by the Army Aeroflightdynamics Directorate (AFDD) and Air Force Research Laboratory (AFRL) used laser radar (ladar) to build and update a dynamic database and cue pilots with a Brown Out Symbology System (BOSS). The 3D-LZ Black Hawk made actual landings in Yuma dust clouds.
Follow-on efforts are under consideration, and all three systems will need repackaging before they can save lives.
The Army formally recognized brownout as a major safety hazard in 2003, and Degraded Visual Environments (DVE) are a core focus of the new Naval Aviation Center for Rotorcraft Advancement at Patuxent River, Md.
Once inside the cloud, helicopter crews denied situational awareness are vulnerable to excessive sink rates, lateral drift and obstacle collisions. Early in Operation Enduring Freedom, an Army CH-47D was destroyed and 16 soldiers injured when the Chinook set a landing gear in an Afghan irrigation ditch.
Helicopters and tilt rotors with high-set rotors, engines and transmissions are also prone to catastrophic roll-overs. During a day training exercise in 2006, a Marine Corps CH-53E Super Stallion drifted in a dust cloud and rolled over on landing with one fatality.
Low-speed flight symbology already helps prevent crashing descents and dangerous drift in dust. Army Apache pilots use AH-64 hover symbology to make brownout landings, and similar cockpit cues have migrated to Air Force cockpits. The Rockwell Collins Common Avionics Architecture System (CAAS) in new Chinooks incorporates symbology developed for the AATD Brownout Situational Awareness Upgrade (BSAU). BSAU velocity vector, acceleration cuing, radar altitude and vertical speed symbology helped test pilots make brownout landings at Yuma in 2004 and appear on the CAAS displays in the operational CH-47F and MH-47G. CAAS is also part of the UH-60M upgrade of the Army Black Hawk, and derivative displays will go into the aging Marine CH-53E and new fly-by-wire CH-53K.
To provide motion cues to pilots in brownout, the Dutch National Aerospace Laboratory is investigating helmet displays, and the Dutch contract research organization TNO has experimented with vibrating belts. TNO’s “Fly Tact” is a vest and belt with “tactors,” vibrating elements like those used in mobile phones.
Cuing symbology also works with integrated flight controls to enhance stability in brownout. The Air Force Special Operations Command upgraded MH-53M Pave Low and HH-60G Pave Hawk helicopters with an Altitude Hold Hover Stabilization system and improved cockpit symbology. Marine MV-22 and Air Force CV-22 tilt rotors have flight path vector displays that let crews make brownout landings manually with cues on the hover indicator or automatically using the fly-by-wire hover-hold function.
The baseline UH-60M Black Hawk now in production has a coupled autopilot, and the UH-60M Upgrade in test introduces fly-by-wire to better stabilize DVE approaches and landings. Boeing Chinook engineers, meanwhile, claim the BAE Digital Automatic Flight Control System in the CH-47F achieves nearly the same results at lower cost. With an automatic departure mode, DAFCS is already credited with saving lives when pilots lost spatial orientation in brownout.
Hover symbology and enhanced flight controls nevertheless do nothing to avoid landing zone obstacles hidden by dust clouds. In 2006, the AFRL tested the Photographic Landing Augmentation System for Helicopters (PhLASH) on a Pave Low MH-53M. Applied Minds Inc., of Glendale, Calif., built a gimbaled, 16 Mpixel camera with an infrared strobe and laser rangefinder to image the landing zone before entering the cloud. The pilot saw a clear picture of the LZ as it was 20 to 30 seconds before landing, geo-registered on the real world with a GPS receiver and inertial measurement unit. The see-and-remember PhLASH had the resolution to spot small obstacles but could not show hazards entering the LZ after brownout occurred.
Brownout initiatives now look to integrate see-through sensors with synthetic vision displays. Though AFRL tests showed mid- to long-wave Forward Looking Infrared (FLIR) sensors had twice the dust-penetrating performance of electro-optical cameras, the 3-to-5 micron or 8-to-12 micron targeting and navigation FLIRs on combat helicopters are essentially blind in brownout. Successful brownout tests have used millimeter wave radar and ladar to paint a landing picture.
Clear-air Phase II tests of the Sandblaster brownout system in January 2009 showed synthetic vision integrated with millimeter wave radar and fly-by-wire flight controls could indeed bring pilots to safe brownout landings. Limited by safety rules on the AFDD Black Hawk, the Sandblaster system took the helicopter to a 25-foot hands-off hover in simulated brownout at Moffett Field, Calif., and showed landing zone obstacles on a head-down display.
Actual brownout landings await notional Sandblaster Phase III trials, but DARPA program manager Derek Tournear said, “There was nothing in the Sandblaster system that would have prevented us from going all the way to the ground.”
Under DARPA contract, Sikorsky Aircraft integrated its own point-in-space flight control software with a Honeywell SLEEK (Sensor-driven Localized External Evidence Knowledge) terrain database and Sierra Nevada Corp. radar. The fixed-azimuth radar could spot obstacles from 1,000 feet slant range to touch-down. The SLEEK processor overlaid return symbology on Digital Terrain Elevation Data from the National Geospatial-Intelligence Agency.
Merging stored data and real-time returns generated a cartoon-like synthetic vision display including obstacles. The 360-degree presentation gave pilots all-round situational awareness in approach and hover. Spotting an obstacle on approach, pilots could “beep” the radar elsewhere and move their landing to a safe area. A wire detection test at the end of the demonstration also showed the see-through sensor could reveal power lines.
The 94 GHz band gave the Sandblaster radar an antenna about a third as large as a 35 GHz alternative.
Under a Cooperative Research And Development Agreement, the Army AATD and Sikorsky evaluated a 94 GHz see-through Helicopter Autonomous Landing System. In March 2009, the BSAU test Black Hawk with Sierra Nevada Corp. HALS II radar flew into Yuma landing zones alone or behind a UH-1 helicopter to test the ability of the radar to penetrate brownout dust. While Sandblaster used a fixed-angle, fixed-azimuth radar to update its SLEEK data base, HALS II provided two scanning angles to spot obstacles on landing or enroute. Obstacles appeared as generic shapes, but their presence was clear. The HALS II see-through radar painted the LZ from 6,000 feet to touchdown.
Image quality of mechanically scanned radars is generally limited by how fast the antenna can scan. The rolling drum technology in both the HALS II and Sandblaster radars supports fast scans without blurring. Sierra Nevada Corp. is also working on electronically scanned arrays, and a more advanced HALS III radar may fly on a production Black Hawk in 2010.
AFRL brownout researchers concluded that laser radar could provide far better spatial resolution than MMW radar to spot landing zone obstacles.
The 3D-LZ collaboration by the AFRL and AFDD integrated ladar with an intuitive Brown Out Symbology System. The ladar updated a dynamic navigation database that showed pilots color-enhanced obstacles. BOSS cues developed by AFDD consolidated and improved elements of BSAU symbology to give pilots essential flight parameters with reduced workload. The 3D-LZ landings touched down at 0 to 1 knots, with descent rates less than 50 feet per minute. Test pilot and AFDD flight projects office chief Lt Col. Steve Braddom noted, “They weren’t the typical aggressive landings you see in brownout.”
The demonstration system leveraged commercial EBAIR (Eye-safe Burns Active Infra-Red) survey ladar technology from H.N. Burns Engineering Corp., of Orlando, Fla. The 3D-LZ ladar was about 30 percent smaller than the commercial version, and when mounted on the AFDD Black Hawk could gimbal 30 degrees down for landings or from 5 degrees up to 55 degrees down for sling-load simulations. Compared to MMW radars, the narrow beam ladar provided 50 times higher resolution with a single pulse. Small ladar scanning steps, meanwhile, provided 20 times higher resolution in a full frame of data. Maximum range of the ladar was 2,000 feet.
The 3D-LZ ladar populated and updated a dynamic navigation database for real-time vertical and horizontal situation displays through approach, landing and departure. Though ladars in dust are more prone to signal attenuation and backscatter than radars, the system used a dust preprocessor to keep false returns from cluttering collected navigation data.
AFRL brownout lead researcher Walt Harrington observed, “We were truly amazed with the results we got. … The imagery was not corrupted in any sensible way by false alarms.”
Depending on size and distance, the 3D-LZ system selectively enhanced obstacles with false color on natural color terrain. Pilots were able to detect obstacles 18 inches high.
The AFRL was to award critical Small Business Innovative Research contracts in February to demonstrate multi-function ladar for brownout landings, cable warning and obstacle avoidance. Under a separate Navy contract, Burns Engineering and Areté Associates, of Northridge, Calif., are developing signal processing techniques for see-through ladars.
Rockwell Collins previously licensed commercial-off-the-shelf fiber optic laser technology from Optical Air Data Systems, based in Manassas, Va., for the LandSafe air data system tested on a Marine CH-53E. EADS Deutschland is likewise applying scanning ladar to the see-and-remember HELLAS-Awareness System for brownout and obstacle detection.
To provide a true multi-purpose helicopter sensor, AFRL researchers envision 3D-LZ laser technology integrated with navigation FLIR. Sandblaster program managers, meanwhile, consider Sandblaster “sensor-agnostic” and compatible with a mix of imaging technologies. And DARPA is working with the U.S. Special Operations Command and Army aviation leaders to transition the technology to a production brownout aid.