On the afternoon of April 4, 1977, a Southern Airways Douglas DC–9 flew through a line of severe thunderstorms over New Hope, Georgia. Hail cracked the airplane's windshield. Both of the DC–9's engines then ingested massive amounts of water and hail from the storm, and subsequently flamed out. The crew tried to dead-stick the airplane to Dobbins Air Force Base, but ran out of altitude and performed a forced landing on a highway instead (see " Safety Pilot Landmark Accidents: Deadly Surprise," August 1998 Pilot). Of the 85 persons aboard Flight 242, 62 were killed in the crash, 22 were seriously injured, and one was slightly injured. Eight persons on the ground were killed, one person was seriously injured, and one person died one month later from injuries.
The NTSB accident report (NTSB-AAR-78-3) said that the factors contributing to the accident included the failure of the company's dispatching system to provide the crew with up-to-date severe weather information; the captain's reliance on weather radar for penetration of thunderstorm areas; and limitations in the FAA's air traffic control system that precluded the timely distribution of real-time hazardous weather information to the flight crew.
The FAA took the latter observation to heart, and by 1980 added center weather service units (CWSUs) to the nation's air route traffic control centers (ARTCCs). The CWSUs are stations manned by meteorologists, located on center premises, and tasked with passing along news of any late-breaking hazardous weather situations to center controllers.
On June 12, 1980, Air Wisconsin Flight 965—a Swearingen SA-226 Metro—was on its way from Appleton, Wisconsin, to Lincoln, Nebraska, with an intermediate stop at Minneapolis. That day, a cold front oriented northeast-southwest was moving over eastern South Dakota and western Nebraska, and a sopping wet, southerly flow of air prevailed over the route. Area forecasts first predicted widely scattered thunderstorms with tops to 45,000 feet. An amendment to the forecast warned of severe thunderstorm activity over the eastern halves of the Dakotas and Nebraska by the afternoon. By 11:55 a.m. Central time, the first convective sigmets of the day began to churn out. The predictions were coming true. By 2:55 p.m., the fourth convective sigmet—Convective Sigmet 42C—stated: "Twenty nautical miles northeast of Sioux City to 10 nm east of Omaha to 50 nm east-northeast of Grand Island. Area of thunderstorms moving 280 degrees at 20 knots, maximum tops 45,000 feet."
The flight crew knew that thunderstorms were forecast but didn't update its weather information during the stop in Minneapolis. It did receive a pilot report that the thunderstorms on the route to Lincoln were dissipating. It did not receive Convective Sigmet 42C or another, preceding convective sigmet. It didn't attempt to receive weather updates en route to Lincoln from ATC, and Convective Sigmet 42C wasn't included on transcribed VOR broadcasts. By looking out the windshield, it must have been obvious to the crew that thunderstorms were in progress, although no deviation requests were made, and the airplane flew in level 2 or greater precipitation rates for at least 50 nm.
What about the National Weather Service, the FAA, and CWSU hazardous weather dissemination system? By the time the storm reached the VIP (video integrator processor) Level 5 or greater status, the severity of the storm system was well known to the NWS forecast office in Omaha, and the CWSU meteorologist in the Minneapolis ARTCC. Unfortunately, that CWSU meteorologist only told ATC supervisory personnel of the storms. The supervisors didn't tell the controllers working traffic at the center. The CWSU meteorologist didn't tell the Omaha approach controllers of the severity of the storms—the controllers who were working Flight 965. These controllers could, however, see rudimentary precipitation symbology ("H" symbols and hashed lines) on their ATC radar displays—displays designed for traffic surveillance, not contouring precipitation returns. Omaha controllers did inform the crew that a sigmet was in effect.
According to the NTSB accident report (NTSB-AAR-80-15) the Metro crew reported "moderate chop, moderate precip" as it requested a descent from 12,000 to 8,000 feet. By this time the flight had been apparently deviating around storm cells, but neither of the pilots knew of the storm severity.
In an apparent effort to avoid the worst of the turbulence, the Metroliner continued to descend. On the way to 6,000 feet, the crew reported "moderate precip with some lightning." At 6,000 feet, the crew told Omaha approach about the turbulence at that altitude: "It's moderate to severe now out here."
Flight 965 asked for lower. "Descend and maintain 3,000. That's as low as I can give ya," came Omaha's clearance. About two minutes later the Metro crew reported that both engines had flamed out. Ten seconds later, the pilots said they had gotten the engines going again. But the airplane was losing altitude fast and crashed in a field in Valley, Nebraska, less than a minute after first reporting the engine stoppages, which presumably occurred because the engines were subjected to a calculated injected water flow more than twice the certification standard. Thirteen of the 15 aboard Flight 965 were killed; two were seriously injured.
The bottom line in these accidents? Others knew of the weather's severe nature, but the pilots were out of the loop. That's the big item. The storms grew so quickly that the word didn't get out in time. The Air Wisconsin accident drove home the idea that much more should be done to enable—and require—ATC to assist pilots in providing accurate and timely information for use in tactical weather decision making. Recommendations to the FAA and NWS to that effect were carried in both NTSB accident reports.
A time long ago
So the Southern Airways crash gave us CWSUs, and the Air Wisconsin crash provided the impetus for more aggressive center weather advisories (CWAs). CWSUs had been around for several months by the time the Air Wisconsin crash happened, but their world was different 20 years ago.
Here's what CWSU personnel duties were:
Prepare written briefings three times a day. These briefings were posted in the CWSU work area, and those interested could stop by and read the latest information—including any sigmets relayed from the National Severe Storms Forecast Center (NSSFC, the predecessor of today's NWS Storm Prediction Center, or SPC).
Give one verbal briefing a day to supervisory personnel (facility chiefs, team supervisors, and weather and flow coordinators).
Record telephone weather briefing messages three times daily.
Prepare CWAs. These come out:
- When an update or supplement to an existing convective sigmet is warranted, or
- When the CWSU meteorologist believes that observed or forecast conditions meet sigmet or airmet criteria. If meteorologists at the NSSFC/SPC concur, then a sigmet or airmet is issued as part of the CWA, or
- If a CWSU meteorologist believes that a CWA is required and time is of the essence to ensure the safe flow of traffic, that meteorologist can take the initiative and issue a CWA without consulting the severe storms center (NSSFC/SPC). In this regard, the CWA fulfilled the fast-response mandate suggested by NTSB recommendations stemming from the Air Wisconsin accident.
CWAs now: Doppler and distribution advantages
So, CWAs did exist way back when. What's the difference between then and now? Well, the same basic issuance rules listed above still apply. In the Air Wisconsin case the supervisors evidently failed to pass the word along to the appropriate controllers, so a critical link in the hazardous weather dissemination system suffered a fatal break.
The big difference today is that we're now blessed with the nation's ground-based network of Nexrad Doppler weather radars. These radars use narrow beams of radar energy that yield much higher resolution of storm features than did the radars of 20 years ago. They contour precipitation gradients very well, in color, and this permits better observation of hazardous storm signatures, such as hooks, scallops, pendant shapes, and steep precipitation gradients—all signs of a severe thunderstorm. These radars are set up so as to cover virtually all of the nonmountainous areas of the contiguous United States.
What did the CWSUs have in 1980? They had access to radar imagery from continuously scanning NWS radars, but the presentation, according to the accident report, was "not adequate to determine storm intensity." More relevant is that the Grand Island, Nebraska, radar unit wasn't in the system used by the CWSU, so there was "no means to determine accurate weather echo intensity and location…in eastern Nebraska."
Apart from this limited radar information, CWSUs relied on faxed-in radar summary charts that could be up to two or three hours old by the time they were received. Along with dated satellite photography, teletyped text describing surface reports, and other aviation weather products generated in 1980, this was the extent of the CWSU's information base.
Today, weather information travels much faster, via modem. Weather radar imagery still isn't superimposed on ATC radar, but it is presented on centrally located display screens on the center floor. Controllers can swivel their seats around for a look at the latest storm returns, then mentally superimpose this information on their air traffic radars. It's not an ideal setup, but it's certainly an improvement over the bad old days.
As for distribution, CWAs are now broadcast over all air traffic control frequencies the moment they are issued. The first broadcast will be the only broadcast, except for the broadcasts made over VORs that are set up to disseminate HIWAS messages. Those broadcast weather information continuously for the area within 150 nm of a HIWAS VOR. Of course, flight watch (on 122.0 MHz) can always provide you with timely in-flight updates of weather events, including any CWAs.
Radar lies
Here's another lesson we can learn from the Southern Airways and Air Wisconsin accidents: radar lies. When there's heavy precipitation, radar signals lose their strength and are absorbed and scattered in a phenomenon known as attenuation. When this happens, only some of the radar energy makes the return trip to the airplane's radar antenna. The result: The radar can't "see" through strong precipitation echoes. Instead of seeing a true picture of the situation ahead, the pilot may see a band of return, and what appears to be a clear area behind those returns. Of course, the "clear" area isn't clear at all. Instead, it can contain the heaviest precipitation and the worst of the thunderstorm.
In the Southern Airways case, investigators believe that the crew was tricked into trying to cross a line of storms by flying through what it thought was a narrow spot on the cockpit radar display. In fact, this narrow spot was really the zone of highest precipitation and highest attenuation, which masked the reality behind it—a Level 6 storm full of hailstones.
The use of radar for storm avoidance is a subject best treated in a separate, more expansive venue. Taking a formal course in airborne weather radar's use and interpretation is a wise investment, and a necessity for any radar-equipped pilot who routinely flies on instruments.
For now, suffice it to say that radar is not a sure antidote for flying around thunderstorms. The best way to circumnavigate thunderstorms is to keep buildups in sight and avoid them by at least 20 nm. When using radar, be sure to give precipitation echoes at least as wide a berth, and don't be tempted to fly toward radar "shadows"—apparently precipitation-free areas behind any radar echoes.
Flying smart
CWAs are another reason to file IFR—or use VFR flight following—when flying cross-country. You'll hear CWAs the moment they're published and be the first to know that something bad is out there. Just remember that CWAs can address very localized areas, and so the usual identifiers used to define the boundaries of an airmet or sigmet—certain VORs, for example—may not apply.
To learn more about CWAs, and to see if any are in effect, call up the Aviation Weather Center's "CWSU Corner" on its Web site ( www.awc-kc.noaa.gov/awc/cwsu-corner.htm).
The moment you hear a CWA, consider checking with flight watch to learn more about the weather and how it affects your route of flight. With this information, you're ready to make your tactical decisions. Together with a good preflight weather briefing, CWAs can help make flying as surprise-free as possible. For this early warning system, spare a thought for the unfortunates over New Hope and Valley just 20-odd short years ago.
Links to additional information on weather radar resources can be found on AOPA Online ( www.aopa.org/pilot/links/links0008.shtml). E-mail the author at [email protected].
Related Articles
