The Right Tilt
Honeywell's latest improvements to its current-generation weather radars include auto tilt to relieve pilots of having to make constant antenna adjustments to spot thunderstorms plus an upgrade to improve reliability for long-range flights.
As transport pilots know, the additional duty of constantly adjusting the radar antenna tilt is just another chore to manage. In addition, adjusting antenna tilt properly to spot storm cells and avoid frying into them requires some training and skill. The FAA has said training in the use of weather radar at airlines needs to be improved. Mistakes can and do lead to unpleasant encounters with turbulence. Avoiding such abrupt surprises will not only keep passengers and crews from injury in a high-G upset, it avoids a situation that could lead to an accident.
Some misidentified "clear air" turbulence encounters at cruise altitude actually involve thunderstorm cells bubbling up from below. Focusing the radar beam on what is happening at cruise level can totally miss these hazardous cells.
There may be very little in the way of rain to reflect the radar energy at cruise altitude, even though there could be plenty of turbulence and even hail. Hail, however, only returns 3% as much energy as raindrops. A thunderstorm below an aircraft's flight level and its true strength will be revealed best by directing the antenna beam to the freezing level where much of the rain is located, according to Stephen D. Hammack, manager of technical marketing for radar products at Honeywell Aerospace.
To make sure tilt is adjusted properly without any need for pilot involvement, Honeywell has created an auto tilt feature for its RDR-4B radar. Auto tilt can also be retrofitted onto the earlier RDR-4A model. Lance Okada, a Boeing 737-300 fleet standards captain for United Airlines, says auto tilt "reduces crew workload tremendously." The system is now certified for use on both Airbus and Boeing aircraft.
The manual technique used by most pilots is to set the radar antenna in "ground park" by lowering the beam until the display shows a little ground return at the outer edge. The radar is then set to detect thunderstorm cells. As the aircraft flies toward the cells out ahead, the thunderstorms "walk" into the beam and are picked up.
However, this technique does not work when a passenger jet is flying over water because there will be little ground return, even with high seas. "You almost have to memorize the correct tilt settings over water," Hammack says.
Tilt should also vary depending on the height of the terrain out in front of the aircraft so that ground return is minimized. For example, if an aircraft is flying into an area with flatlands off to the left of course, and mountains off to the right, the correct tilt for one would not work for the other. If the tilt is set correctly to minimize ground return over the flatlands, the antenna beam will be aimed too low for the mountains, and the display will be filled with ground clutter. And if the tilt is set to minimize ground return over the mountains, it will be too high to detect storms properly over the flatlands.
Honeywell has solved this with the auto tilt feature by dividing the 180-deg. area ahead of the aircraft into five sectors. The tilt can be varied five times during the 4-sec. sweep from left to right, for example. So each sector has the correct setting for the terrain involved based on elevations contained in the EGPWS database. The antenna sweeps at 45 deg./sec., and tilt adjustments from one sector to the next are just a few degrees at most.
Pilots typically fly at cruise with one radar set on long range (e.g., 150 naut. mi.) and the other set on short range (e.g., 30 naut. mi.). Several airlines teach pilots that once a storm is detected by auto tut, they should select manual and adjust the tilt to measure the top of the cell.
There are about 340 auto tilt systems in revenue service on Boeing 737s, 747s, 767s, 777s and the Ilyushin 11-96. This includes aircraft at United Airlines, Singapore Airlines, Japan Airlines, US Airways and Rossia, an airline in Russia. Supplemental type certificates (STCs) are also in process for the Boeing 757 and 767. Airlines pay about $6,000 per aircraft for auto tilt.
This Aviation Week & Space Technology pilot-editor saw the auto tilt function demonstrated on the Honeywell Convair 580, an aircraft that rolled off the production line right after World War II. The flight out of Boeing Field over Puget Sound was on a beautiful day, unfortunately, with nary a thunderstorm in the sky to demonstrate the auto tilt features. Seeing the Enhanced Ground Proximity Warning System (EGPWS) used in action for the first time proved quite interesting, however.
The system provides warnings well in advance of any terrain conflict by looking ahead and comparing the aircraft's flight path with a digital terrain database. The warning times are much improved over earlier generation GPWS systems. We flew to Port Angeles, Wash., and made an approach to William R. Fairchild International Airport there.
Then Honeywell test pilot Markus Johnson pointed the nose toward the high terrain of Olympic National Park where mountains rose to several thousand feet in front of us. The EGPWS map showed the elevations we had to be concerned about in red and yellow (at or above our altitude). EGPWS is designed to provide caution alerting up to a minute before a terrain conflict and to provide stronger warnings closer in.
This worked well on our approach to the high terrain with plenty of warning to pull up before closing in on the hazard. During a second pass on the same terrain, we employed the older GPWS in which there is no reference to a terrain database, just data from a radar altimeter looking straight down. When flying toward a precipice, as we were, the GPWS algorithms had little chance to spot rising terrain. The warning time was barely enough in this case, but we were VFR, and Johnson pulled up so we could head back to Boeing Field.
Another improvement that Honeywell has made to its RDR-4A and -4B weather radars is to make the antenna drive redundant. This project started out to meet the needs of Cathay Pacific Airways on its transpacific flights where radar outages have a greater chance of showing up on a long overwater leg. Previously weather radars have used a single azimuth and single elevation motor in the antenna drive, even though some aircraft such as the Airbus A340 are equipped with two receiver transmitter (R/T) units. As Honeywell notes, if either of the single antenna motors fails on the previous design, the radar is out of service.
Honeywell calculations show that if an aircraft makes 60,000 flights per year with an average duration of 5 hr. and with an azimuth motor providing 25,000 hr. mean-time-between-failure (MTBF), the airline will probably experience 19 cases of motor failure per year. These failures would cause an antenna outage and the loss of weather radar for the rest of the flight.
By installing the dual drive motors with the 25,000-hr. MTBF, Honeywell estimates there will be fewer than one antenna outage per year (0.03 per year). Since a single turnback due to loss of weather radar can be quite expensive, this $16,000 option (catalog price) can pay for itself quickly, Honeywell says.
About 200 of these dual drive upgrades have been installed on aircraft operated mostly by Cathay Pacific but also by Japan Airlines, Singapore Airlines, ANA and Rossia. While most of the supplemental type certificates so far have been for widebodies such as the Boeing 777 and 747 and the Airbus A340 and A330, an STC is in process for the narrow-body A320.
Honeywell's radar upgrade with dual antenna drive motors should cut out-ages to one per year on an aircraft
DIAGRAM: By providing two azimuth motors and two elevation motors on the antenna drive, Honeywell estimates antenna outages on an aircraft will drop to 0.03 per year.
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By David Hughes
, 7/26/2004, Vol. 161 Issue 4, p47-50