June 1, 2002
By Bruce Landsberg
Bruce Landsberg has served as executive director of the AOPA Air Safety Foundation since 1992.
Not many pilots who tangle with a thunderstorm en route and crash their aircraft while unconscious are able to talk about it. Two unfortunate souls found out the hard way that forecasting is art as well as science—and those who overestimate the technical accuracy of weather prognostication do so at considerable risk, especially when it comes to the exact location of convective weather.
At 4:15 p.m. an instrument-rated private pilot called flight service for a trip from Albuquerque, New Mexico, to Alva, Oklahoma, in a Cessna 421B. The pilot was advised of thunderstorm activity, building in intensity, from Raton, New Mexico, to Garden City, Kansas, with some scattered storms in the Oklahoma panhandle. The weather was moving east-southeast at 25 knots, but no sigmets had been issued for the route, and the pilot filed an IFR flight plan.
It seemed a typical summer afternoon with scattered storms and, hopefully, a few holes where a pilot might slide through without too much trouble. With a pressurized cabin-class twin equipped with on-board radar, this was a normal flight operation.
The pilot and two passengers departed Albuquerque in good VFR conditions. At 5:50 p.m. the flight checked in with Albuquerque Center, climbing to 21,000 feet. Fourteen minutes later, the pilot requested and descended to 19,000 feet. At 5:55 p.m., a convective sigmet was issued for isolated severe thunderstorms located 10 miles north-northeast of Las Vegas, New Mexico. The sigmet was valid until 7:55 p.m. The storms, each 20 miles in diameter, were moving from 010 degrees at 15 kt, with tops above 45,000 feet. The sigmet lay just north of the Cessna's route but was not passed along to the pilot. In forecasting terms this is irrelevant, but to pilots and the legal system 20 miles may mean the difference between an uneventful flight and a disaster. In the dry air of the Southwest it is also unlikely that such weather would pass unnoticed from an aircraft flying in the clear.
The pilot reported to the accident investigator that there were "some clouds" approximately 50 to 60 miles in front of the airplane. This indicates very good visibility. He further stated in his written report: "I then turned on the on-board radar and could see precipitation to the left and to the right of our path. However, the center area, probably 15 to 20 miles wide, was clear on the scope. This was the area that I had planned to go through the clouds. They did not appear to be dark thunderstorms, only clouds like I had flown through many times before. Before we had reached those clouds, about 10 miles before, is when we encountered hail." The conventional wisdom used by most airlines is to fly no closer to severe thunderstorms than 20 miles. That means if two storms are adjacent, the no-fly zone between them should be 40 miles wide. That demands a big hole and many of us might be tempted to cheat.
Cloud appearances can be deceiving. If the sun is shining on the cloud from the west, when flying eastbound toward the cloud, it will appear white. Thunderclouds appear dark when the cloud is between the viewer and the sun as an afternoon storm approaches from the west. Cloud color is determined by sun position and has little bearing on what might be inside. Clouds must be regarded with suspicion, regardless of their color.
The right front-seat passenger held a VFR private pilot certificate without a multiengine rating. He reported that "the on-board radar was on and did not indicate strong storms directly in our flight path. It showed rain on both sides, but just a small ring of rain in front." Radar aficionados know that hail does not show up well on radar and may easily understate the strength of a return. Attenuation, or the loss of return signal strength, occurs as the aircraft gets into precipitation. Radar energy is absorbed by the rain on the trip away from the antenna and only a little is reflected back. This tends to understate the strength of the weather ahead and may not show what is behind the closest return. This masking paints a far more benign picture than what is actually in front of the aircraft. The scenario is similar to what happened to Southern Airways Flight 242 in 1977 when a DC-9 turned into the worst part of a thunderstorm as the pilots misread an attenuated radar (see " Safety Pilot Landmark Accidents: Deadly Surprise," August 1998 Pilot).
The Cessna 421B pilot continued, "It first appeared like rain, then within one minute of first encounter [of hail], a large piece of ice broke through the left windshield." The pilot passenger further reported that the "large [approximately 2 inches] hail blew out the pilot's windshield and dazed the pilot." The pilot put his head in his lap to avoid further injury and "he [the passenger] took over the controls" and attempted to "keep the wings level."
Imagine the confusion, noise, windblast, sudden temperature drop, and loss of cabin pressure. In a rapid decompression situation, the procedure is to immediately don an oxygen mask and then descend—fast. The onset of hypoxia, even in the mid-altitudes, can be sudden and even a slight delay can lead to incapacitation.
The passenger stated that he "never felt dizzy or lightheaded." Nevertheless, the 421 crash-landed in a field with all occupants unconscious. The passenger in the backseat did not remember the crash. "When I came around, all was quiet." He attended to his two friends in the front seats, then opened the cabin door and walked about three-quarters of a mile to look for help. Finding no one, he returned for his cell phone and contacted the sheriff's department in Tucumcari, New Mexico.
An Air Force C-130 was already homing in on the 421's emergency locator beacon. In darkness and heavy rains, it took emergency personnel approximately four hours to reach the site and two hours more for rescue helicopters to pick up the injured.
This pilot either had vague understanding of the hail hazards of convective weather or elected to wriggle through a tight opening. And how many of us have flown close to storms without ever encountering hail? Being in the wrong place at just the wrong moment might be considered bad luck. But Lady Luck smiled very brightly after the initial bad deal. For two pilots to become unconscious, have the aircraft descend, unguided, into an open field, and land relatively gently borders on miraculous.
The pilot sued the FAA and National Weather Service for failure to warn about the weather. According to the lawsuit, NWS "inaccurately forecast" the area of the sigmet and the center controllers failed to advise the pilot of hazardous weather that impacted operations within 150 nm of the sector in accordance with established procedures. The government settled out of court, but in my opinion pilots need to understand that forecasting is an inexact science. What we see out the windshield takes precedence over any forecast, warning, or lack thereof. If you misinterpret that, it really isn't the government's fault.
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