Sunday, April 1, 2012
Weather Radar: Navigating the Storm
As part of the Shopping for Weather series, Rotor & Wing examines various weather radar options for helicopter operators.
The sixties were great. I wasn’t old enough to fly then, but several of the aircraft I flew were of that vintage—the UH-1, OH-58, C-130 and the B727. (I won’t mention the Cessna 150, TH-55 or the T-37). Many didn’t have radar and the aircraft that did had a single monochrome display that was not integrated into a flight management system. In the industrial heyday of their prime, pilots with starched white shirts and skinny black neckties worked complex radar techniques and interpretations from smoke-filled cockpits with mixed, and sometimes, disastrous results. In my early flying days, helicopter radar was rare, and at the time, I was told airborne weather radar was something reserved “for the big boys.” Techniques such as “skud running” or the use of ADF receivers as a “poor man’s lightning strikefinder” were more common. My first experience with weather radar was in the Coast Guard, and my success consisted of much trial and error. But once you become accustomed to flying with airborne radar, it becomes difficult to fly “blind” without it.
Today, there are several options for both single and multi-engine helicopters that can be forward or retrofit, with costs ranging from $35,000 to $150,000. Many radar or multi-function displays (MFDs) are night vision goggle (NVG) compatible so you won’t lose a minute of what you’ve been missing.
Weather Radar 101
If you majored in auto hobby like me, you can take it apart and put it back together, but how does it work? I’ll compare radar to a flashlight. The “beam” is the light transmitted from the flashlight into space until it dissipates or hits an object, and the light reflected back toward the holder of the flashlight is the echo or return. The closer the object (range) is to the flashlight, the clearer the image (attenuation). Also, the brighter the flashlight, the better the range and attenuation.
As with radar, if there is one object in front of another, you may only see the object at the closer range, such as being positioned in front of a building—you only see the building. Unlike stadium lighting, the flashlight is a pencil-like beam; you are more likely to detect an object by sweeping the flashlight in an arc of 120 degrees. So imagine holding a flashlight at your waist and navigating your way out of dark, unfamiliar building. As you slowly pan the flashlight right-left-right-left, you will probably adjust the angle (tilt) of the flashlight up and down slightly to check for obstacles and illuminate the path.
Excessive tilt down and you only see the ground at your feet (ground clutter) while excessive tilt up and you miss some or all of the obstacles. Once an obstacle is detected, most people would instinctively offset left or right to see if there is anything behind it. Now replace the flashlight with a 10 to 12-inch antenna that is transmitting 4 to 12 Kw of energy and receiving the reflected returns. While greatly simplified, this is the basic premise of operating weather radar. Helicopters such as a Eurocopter AS350 and AgustaWestland AW109 use a 10-inch antenna, while larger helicopters can accommodate 12, 15, 18 or 24-inch antennas. Even a 10-inch antenna will provide a 200 nm range, but the larger antennas will provide better attenuation. When flying in heavy rain you can experience degradation in range or attenuation created by the film of water that forms on the radome.
Today’s radars will typically display four colors, depicting the level of intensity of the reflected return.
In terms of weather avoidance, the higher the moisture content, the greater the reflection or echo and higher probability of turbulence. Hence green for light, yellow for moderate and red for heavy while magenta depicts either severe turbulence or windshear. You can have turbulence without moisture and you can have moisture without turbulence. “Dry” participation such as snow and ice crystals do not reflect much of the transmitted energy back to the antenna and typically cause the radar signal to scatter. Other forms of precipitation will cause the signal to be absorbed and reduce the range and possibly mask the true intensity of the weather ahead. Wet participation such as rain, wet snow and hail reflect quite well. The intensity level of the turbulence can also be assessed by the contour of the returns. Like isobar contouring, the closer the contour and change in color (intensity) the higher the probability of strong turbulence and bouncing yourself and your passengers off the ceiling while you figure this out (I have never done this).
As mentioned with the flashlight example, be wary of an echo presentation with nothing depicted behind it. Occasionally one echo or cell can block another more intense one. It pays to use the tilt feature and offset your heading to see what behind an echo. This begs the reminder that today’s airborne radar’s don’t come with a hood and cape or provide the ability to penetrate weather. Airborne radar is for weather avoidance, and combined with interpretation based on training, experience and common sense works quite well.
One of the most misunderstood features on any radar is the use of the tilt function. Most are calibrated between in positive degrees (tilt up) and negative degrees (tilt down) from 0 to +/-15 degrees. Too much tilt down and you will paint nothing but ground clutter, like having your head in the sand. Too much tilt up and you will either miss the large echo in front of you entirely, or get false returns. The operation of the tilt function is a fluid one depending on the attitude of the aircraft, phase of flight and type of returns that are depicted. If you are on the ground and using the radar to make a go/no-go decision, a high angle of tilt up can aid in determining the location of hazardous weather approaching your location.
On takeoff with a nose-low attitude, a mid-range tilt up will keep you out of ground clutter and looking ahead. In cruise flight, set the tilt so you just start to paint some ground clutter. While in cruise flight I typically set the range out 20-80 miles at low altitude and work the range and tilt together to determine the location, height and intensity of weather ahead. I also like to use FMC/GPS/IRS data to determine wind direction, velocity, cross track, and cell movement to develop my Plan A, Plan B and Plan C for avoiding weather. The same technique can be used on decent or approach when using the radar for weather avoidance, adjust the tilt up slightly to avoid painting ground clutter. Most contemporary radar systems allow you to either manually adjust the gain, or leave it in an automatic setting. If you are not familiar with gain—leave it in the auto mode. I would compare manually adjusting radar gain as similar to manually adjusting squelch on a radio. On older radar systems, there is benefit in adjusting gain to eliminate “ghosting” in the weather mode and sea clutter in the sea search mode.
Another common mistake with radar is not turning it on. Pilots sometimes forget, encounter weather and get behind in the weather avoidance business. This may be caused by the fear of leaving the radar on after landing. Some radar systems are connected to the transponder. If configuring the radar is not currently part of your checklist or flow item, consider developing procedures or best practices for testing and configuring the radar for different phases of flight. Pilots also like to estimate the size of the cell or precipitation ahead. A simple rule of thumb is to estimate 100 feet for each degree of tilt per nautical mile. Although probably not something you would consider attempting in a helicopter, never try to out climb a thunderstorm.
Doppler radar is a terrific tool for detecting windshear in both ground and airborne based systems. Doppler doesn’t detect the location of moisture but detects motion, and in the case of Doppler radar, the motion of updrafts, downdrafts, microbursts and even mountain waves. The back of your 10-9 page will indicate if the airport you are operating near has Doppler. Airborne weather radar can also be equipped with Doppler to depict turbulence and windshear. Windshear systems are categorized as either reactive or predictive. Reactive windshear will indicate you’ve encountered windshear, while predictive windshear will indicate presence of windshear prior to encountering it. These systems are most commonly found on Air Carrier aircraft.
Who’s on Point?
Ground mapping is a feature inherent to many airborne radar systems that allow you to paint various features on the ground using the signature of the returned echo. Most radar systems are capable of depicting recognizable features such as coastlines, significant terrain and man-made objects such as bridges and large buildings. Certain radar systems have advanced ground mapping capabilities that enable off-shore navigation and compliance with AC-90-80B, approval of offshore standard approach procedures, airborne radar approaches and helicopter en route decent areas. Airborne radar can be a tremendous asset for navigation in defined situations. The radar imagery can be combined with other navigation data from flight management computers, GPS or inertial reference system, providing an integrated solution, depicted on an MFD. Heliports or runways can be defined or highlighted though the use of radar reflectors, designed to provide precise depictions. To avoid collision and also assist in being located by SAR aircraft, many vessels carry and display a radar reflector to enhance the appearance of their echo. Many manufacturers incorporate GPWS/TWAS, radar and map functions into an integrated MFD unit or a head-up display (HUD).
Where Can I Get One?
There are several radar manufacturers in the marketplace, but two of the largest for weather radar are Honeywell and Rockwell Collins. Both manufacture standalone or completely integrated systems for a variety of aircraft. Honeywell’s lowest cost system, the RDR 2000, has a catalog price of $31,199 and weighs in at just 10 lbs. It has many features such as Target Alert providing automatic alerting of weather hazards regardless of range setting. Moving up to the Primus 660/880 brings the cost to $77,998-$119,284 and features REACT—Rain Echo Attenuation Compensation Technique—which helps depict one cell or echo blocking another from view. It also comes with Auto Tilt, Doppler Turbulence Detection and ground mapping. The Primus 700 Series can be found on large helicopters such as the S-92 and AW139.
In addition to the airborne weather and ground mapping functions found on other Honeywell systems, the Primus 700 provides sophisticated mapping and navigation features such as sea clutter reduction, sea search for locating small targets and target de-confliction in a variety of sea conditions.
Honeywell is also developing advanced technologies that apply their radar technology to integrate a helicopter Cable Warning and Obstacle Avoidance (CWOA) that will be capable of identifying hazardous cable, wires, towers and other potential CFIT obstacles. It will also support the ability to “see through” degraded visual environments, or DVEs, such as sand, dust, weather, or brownout landing conditions. A Honeywell spokesperson reports positive results from the first test flight.
Dan Toy, principle marketing manager at Rockwell Collins, said the company is currently offering the RTA-4100 series of weather radars with MultiScan capability for the helicopter market. It is a solid-state radar system providing clutter-free weather detection out to 320 nm and features certified turbulence detection using Doppler measurement techniques. It will also support up to four independent display solutions, each with an independent range capability, as well as dual independent mode and gain setting inputs for pilot and copilot controls. As light as 15 lbs depending on type selected, the RTA-4100 MultiScan radar offers a significant weight savings over many of the systems that are currently available for helicopters.”
While radar was not readily available for helicopters in the past, and if it was, it was heavy, complicated and provided limited benefits. Today’s helicopter radar systems provide fully automated detection and display capabilities for both airborne weather avoidance and ground mapping navigation use. Many vendors offer radar training as well as practical hands-on experience in simulators and flight training devices.