Tuesday, September 4, 2012
When Using NEXRAD Can Be Dangerous
The NTSB says don’t rely on NEXRAD without having this information
Last year, a Piper Cherokee Six experienced an in-flight breakup and crash, likely due to a delay of NEXRAD weather data.
The NTSB recently issued a Safety Alert regarding a problem it sees with the increasingly popular in-cockpit display of NEXRAD (Next Generation Radar) mosaic images. The Safety Board wants pilots to understand that what they think may be real-time, or almost real-time, imagery of potentially hazardous weather may be so old its use for weather avoidance could be dangerous. The Safety Board noted that radar data from multiple ground sites are processed into a single image for cockpit display. Each time the mosaic image is updated, however, data from all of the sites may not be refreshed.
The Safety Board emphasizes that even though a weather subscription service may say that it updates every five minutes or so, the images can be much older because they're based on radar data, which wasn't refreshed, and because it takes time to process the data to create a mosaic image. In extreme cases, says the NTSB, the actual age of the oldest NEXRAD data in a mosaic image can exceed the age indication in the cockpit by 15 to 20 minutes. This means if an on-screen display advises the pilot that updates are sent every five minutes, what the pilot sees could really be up to 25 minutes old.
The Safety Board says pilots need to understand that a common perception of radar images transmitted to cockpit displays being five minutes old isn't always correct. It says pilots need to remember that in-cockpit NEXRAD displays show where the weather was, not where it is.
Weather radar images are available in so many forms, on so many devices, that for many pilots and aircraft operators, they're now part of the background noise. Not too long ago, weather avoidance through the use of color weather images was restricted to those few who could afford onboard radar. It's commonplace now for pilots of all levels to be saturated in color weather radar, from the morning weather on TV, to a preflight weather briefing on the computer, to anywhere on a cell phone and on a handheld, laptop or panel-mounted device in the airplane. This saturation, with so much of the imagery actually being real time, may create an expectation of "real time all the time."
An accident mentioned in the NTSB's Safety Alert involved a Piper PA-32-260, which experienced an in-flight breakup and crash near Bryan, Texas, on December 19, 2011. The instrument-rated private pilot and four passengers were fatally injured. The Part 91 personal flight was being conducted in instrument conditions on an IFR flight plan. The flight originated from the Clayton County Airport (4A7), Hampton, Ga.
The airplane had made a fuel stop at Jackson, Miss., and was en route to Waco, Texas. An initial review of air-traffic-control tapes revealed that the pilot was diverting around an area of thunderstorms at the time of the accident. The pilot last reported that he was in "bad" weather and was going to try to get out of it. Following that transmission, radio contact was lost. The airplane had been on a heading of about 230 degrees and at an altitude of 8,000 feet MSL. It turned south to a heading of 193 degrees, then two minutes later, turned back to a southwesterly heading. Then, it made a right turn toward the northeast and began descending at a rate of about 600 feet per minute. The descent continued, increasing to 840 feet per minute, until radar contact was lost. A witness, located in her house, heard a sound resembling an explosion. The witness reported that at the time she heard the noise, the rain was falling as a light drizzle. However, by the time she and her husband got outside to see what the explosion was, the rain started pouring down. The witness' husband located the main airplane wreckage approximately 450 feet southwest of their house.
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