New detection and presentation technologies promise a dramatic reduction within 10 years of the number of accidents caused by weather

Weather continues to be a force majeur in aviation safety - an unpredictable phenomenon that can have all too predictable consequences if it is not understood or respected. A multi-front effort is under way to increase flightcrew awareness and understanding of weather in a bid to improve safety.

US efforts to reduce weather-related accidents are part of an aviation safety initiative launched in 1997 with the goal of reducing the accident rate by a factor of five within 10 years. Between 1983 and 1995, 33% of US air carrier accidents were weather related, says the National Transportation Safety Board, while between 1990 and 1996, 33% of commercial-transport serious injuries were caused by turbulence.

Figures like these make weather a natural target for safety improvement initiatives. One of the largest of these is the multi-faceted weather accident prevention project that is part of NASA's larger aviation safety programme. The project's goal is to develop technologies that will enable, by 2007, a 50% reduction in the factors that cause weather-related accidents, and a 50% reduction in turbulence-related accidents.

Elements of the project include new weather products, datalinks to send the information to the aircraft, and displays and decision-aiding tools to help a pilot interpret the information. The first results of the programme, which runs to 2005, have begun appearing.

Onboard radar remains the primary weather sensor available to pilots, and recent years have seen the introduction of enhancements such as predictive windshear. The latest advance arrived late last year, with the delivery to Qantas Airways of the first Boeing 747-400ER equipped with Rockwell Collins' WXR-2100 MultiScan radar. The fully automatic MultiScan essentially eliminates ground clutter to provide a more complete picture of the weather.

Refined presentation

"Radar is the last raw data sensor in the cockpit. MultiScan refines the weather presentation," says Steve Paramore, weather radar marketing manager. The radar scans at two tilt angles to collect short-, medium- and long-range weather information. This is stored in a database and, over several scans, digital signal processing removes the clutter to give a weather picture extending from inside 20km to 600km (10nm to 320nm).

The radar scans left to right at a lower tilt angle for short- and long-range weather, then right to left at a higher angle for medium-range weather. Each cycle takes 8s, the display is updated after each 4s scan and clutter suppression is effective after five scans, says Paramore. Whereas a conventional radar can be used to track weather from around 220km to within 40-60km of the aircraft, the MultiScan's dual tilt and clutter suppression allow a pilot to see both close-in and long-range weather, he says.

A key feature of the MultiScan is overflight protection, which is designed to prevent the crew inadvertently penetrating a thunderstorm top that has fallen below the radar beam and off the display in the high-altitude cruise. The lower, "look-down" scan keeps the thunderstorm top in the picture longer and, when the radar return begins to fade, the MultiScan tracks and displays the stored digital image until the storm is safely behind the aircraft.

The MultiScan will be certificated on every Boeing model by the end of June and on every Airbus type by year-end. Paramore says 80% of sales are of the new radar. "Version 2" is in development and will introduce storm top prediction. The radar will make a vertical "figure 8" scan around the thunderstorm to collect growth and storm top information, to help decide if there is a turbulence threat. "We have disconnected the antenna scan from the display update, which gives us a lot of options," he says. The software update will be available at the end of 2004.

Further weather radar enhancements are being developed under NASA's turbulence prediction and warning systems (TPAWS) programme, part of the weather accident prevention project. TPAWS expands on predictive windshear capability, used below 2,000ft (600m), to detect convective turbulence above 5,000ft and provide the pilot with 90-200s warning, says project lead Jim Watson.

"We use existing hardware and new software algorithms that kick in above 5,000ft. The results are excellent," says Watson. "It can't pick up clear air turbulence, but it can pick up turbulence away from clouds, in air that is still moist." Manufacturers are beginning certification efforts, and TPAWS should be in the fleet by 2007, he says.

Turbulence warning

Crews will be presented with a turbulence warning tailored to the aircraft's dynamics and phase of flight. "The hazard to passengers at a particular turbulence level is different in a 747-400 versus a 717," says Wilson. How the information will be displayed "is still being worked on", he says.

The second element of TPAWS is the use of the aircraft itself as a turbulence sensor. Information from onboard sensors will be datalinked to the ground, mainly for maintenance use, but also for weather forecasting. Flights test are planned for this year, and Wilson sees the capability as "promising" for deployment by 2007.

The third element is longer term, and involves tying the radar into the aircraft's flight management system to mitigate turbulence. "It would trigger a turbulence encounter mode in the autopilot, which would not try to hold altitude and speed so stringently, which just makes the ride worse," says Wilson.

NASA has evaluated laser-radar (lidar) turbulence sensors under the TPAWS project, but Wilson says the system "does not look very viable". The problem is getting enough power out of a sensibly sized lidar to measure turbulence significantly ahead of the aircraft. "What lidar offers above radar is not competitive," says Wilson.

Under another NASA weather accident prevention project element, the Aviation Weather Information Network (AWIN), work has been under way to integrate onboard radar information with uplinked weather images. This included co-operative research agreements with industry.

Two of these, the enhanced weather radar (EWxR) and aviation weather awareness and reporting enhancement (AWARE) programmes - both with Rockwell Collins - have been merged into the airborne hazard avoidance system (AHAS), which will be flight tested in August/September this year in NASA's Boeing 757 testbed.

The EWxR project combined weather radar data with Nexrad images uplinked to the aircraft and presented on the same display. "EWxR takes in ground-based weather and onboard weather and does tracking and hazard assessment," says programme manager Kevin Kronfeld. "It tells the pilot where the storm cells will be in the future, and which will affect the flight."

AWARE started out as an internet-based pre-flight planning tool to help general aviation pilots assess the weather along the planned flight, but it has been tested as an onboard decision-aiding tool on NASA's 757, says AWIN project manager Paul Stough. AHAS combines the two systems and is looking at how to fuse data on board the aircraft, he says.

"AHAS brings EWxR and AWARE together," says Kronfeld. "The pilot gets tactical weather from the radar, while strategic weather is datalinked to the aircraft." Onboard algorithms assess the uplinked weather for hazards. Because of the latency inherent in datalinked weather, the information will be used for planning, he says. AHAS is a broader programme than EWxR, says Stough, and will encompass radar, lidar, other aircraft sensors including TPAWS and information datalinked from other aircraft as well as from the ground.

Also under AWIN, Honeywell has developed the weather information network (WINN), which become operational in the USA last year. The network uplinks Nexrad mosaic images and other text and graphical data for display, using airborne telephone or satellite communication links. The WINN underwent an in-service evaluation on a United Airlines Airbus A320.

Satellite service

American Airlines, meanwhile, has conducted tests of Rockwell Collins' satellite weather information service (SWIS) using two Boeing 777s equipped to receive graphical weather via high bandwidth satellite link. Further flight tests are planned for September, when a final decision on the uplink is also scheduled.

Under AWIN, which runs until 2005, NASA will continue development of next-generation weather presentation and decision-aiding technology, "We would like a system that knows the capabilities of the aircraft and the pilot, the phase of flight, that will search the weather products, pick out the relevant ones and present them in a way that is easy to interpret," says Stough.

A flight demonstration is planned for 2005. Stough says the AWIN programme will meet its objectives if the technology developed "provides adequate information during the en route phase of flight to allow a 25-50% reduction in accidents attributed to [weather] situational awareness".

Source: Flight International