When Using NEXRAD Can Be Dangerous

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."

PA-32-260
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.

An NTSB meteorologist conducted a time and data comparison between the National Weather Service local radar and NEXRAD mosaic images, which would have been transmitted to the airplane via a weather subscription service. The accident pilot had a valid subscription to a weather service and a GPS device for its display. The Owner's Manual for the GPS device specifically states that NEXRAD weather data should be used for "long-range planning purposes only," and not to "penetrate hazardous weather," as the "NEXRAD data is not real time." The NTSB meteorologist determined that the NEXRAD mosaic images were up to eight minutes, 22 seconds out of sync when compared with real-time radar. The data indicator in the cockpit would have shown a one-minute time delay, referring to the time it took for the data to be processed by the subscription service for transmission, not the true age of the data.

AS350 B3
Also mentioned in the Safety Alert is the March 25, 2010, accident involving a medical helicopter in which the commercial pilot and two flight nurses were killed. The Eurocopter AS350 B3 crashed near Brownsville, Tenn., while trying to get to its home base before a severe thunderstorm arrived. Night visual meteorological conditions prevailed for the flight, which operated on a VFR flight plan. Investigators learned that during the flight, the cockpit display received a NEXRAD image, which indicated it was about one minute old, even though the depicted weather conditions were older. The image indicated that the severe thunderstorm was about seven miles away from the helicopter's home base, when it actually was crossing over the home base at the time the image was received on the helicopter. The accident location was about 2.5 miles east of the helicopter's home base.

PA-34-220T
Although not mentioned in the Safety Alert, another accident indicates that the issue highlighted by the NTSB has been around for quite some time.

On June 25, 2006, a Piper PA-34-220T impacted terrain after an in-flight breakup near Tafton, Penn. The certificated private pilot, a pilot-rated passenger, and a second passenger were fatally injured. The flight was in instrument conditions on an IFR flight plan. The airplane departed Greensboro, N.C., destined for Sanford, Maine. The pilot was flying to Maine with his father, a retired airline pilot, who was in the right front seat, and his sister in a rear seat. The pilot had received two weather briefings from the FAA's flight service contractor. The second time, the pilot and the weather briefer discussed thunderstorm activity along the route of the flight, and the pilot subsequently filed an IFR flight plan.

When the pilot was handed off to Wilkes-Barre air traffic control, the airplane was at 7,000 feet and a groundspeed of 180 knots. ATC informed the pilot of an area of severe weather located six miles ahead of the airplane's flight path, and asked if the pilot had weather radar available. The pilot replied that he had a GPS receiver with weather radar, and observed weather at his one o'clock position. Air traffic control indicated that the weather was at the pilot's twelve-thirty to one o'clock position, four miles away. The pilot asked ATC if he could fly direct to the Kingston VOR, and he was subsequently cleared to the RAGER intersection. There were no further communications from the airplane. Radar data indicated that the airplane descended slightly to 6,900 feet, and had a groundspeed of 162 knots. Then, it was down to 6,400 feet and a groundspeed of 137 knots. The airplane's last radar target showed 5,300 feet, and a groundspeed of 137 knots. A witness near the accident site stated that he heard the sound of an engine "revving up and down." He didn't see the airplane, but heard a "muffled pop" and then no longer heard the engine noise. He then saw debris falling from the sky. Another witness saw small pieces of debris, similar to insulation, falling from the sky for about to five to 10 minutes.

Weather radar images obtained from National Weather Service radar at Binghamton, N.Y., depicted echoes reaching "moderate" intensity over the accident site, with echoes of "strong" intensity within five miles east of the accident site and "strong to very strong" echoes south and moving north. When the airplane's flight path was plotted onto the weather radar images, it entered a developing area of "moderate to heavy" intensity echoes consistent with a convective cell or thunderstorm, about the time of the accident. The co-owner of the airplane confirmed to investigators that the GPS receiver was able to receive and display weather information through a weather subscription service. A representative from the device manufacturer stated that the NEXRAD weather radar images updated automatically about every five minutes.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, N.Y. 10602-0831.