Wx Watch: The VFR-Into-IMC Accident

Insights on fatal weather scenarios

July 1, 1993

Thanks to generations of flight instructors and reams of safety-related articles and publications, most pilots know the dangers of attempting VFR flight in instrument meteorological conditions. That goes double for taking on low ceilings and visibilities without the benefit of an instrument rating.

Pure cognition, though, doesn't seem to be enough. Accident statistics continue to show a large number of VFR-into-IMC accidents — most of them fatal. So it's not enough to simply know that pressing on VFR into worsening weather can be a prescription for disaster. This knowledge has to be applied during every phase of every VFR flight. As soon as ceilings or visibilities lower, VFR-only pilots must be prepared to alter course and head for better weather or land.

The numbers show the dimensions of the problem. I called up two years' worth (1991 and 1992) of suspected VFR-into-IMC accidents (the National Transportation Safety Board has yet to assign official probable causes for accidents this recent) from the AOPA Air Safety Foundation's accident information database. A total of 145 accidents was listed, with 117 of these involving fatalities.

A closer look showed that 102 of the 145 accidents involved the pilot's taking off into instrument or marginal VFR conditions, without a proper weather briefing or filing an IFR flight plan. Night instrument conditions prevailed during 41 accidents, and only 28 accidents occurred under what most of us would believe to be "classic" conditions. That is, a non-instrument-rated pilot taking off in good visual meteorological conditions, then picking his or her way through gradually deteriorating weather until all visual reference is lost.

That bit of conventional wisdom doesn't always hold true. Any strong ideas that the typical VFR-into-IMC accident ends up with an inadvertent thunderstorm penetration should also be reexamined. Of all the accidents, not one involved flight into convective activity. Instead, low stratus clouds and fog claimed the most victims.

There are many types of stratus clouds. Cirrostratus clouds form at high altitudes — between 18,500 and 45,000 feet msl — and are often the first signs of an approaching warm front. Altostratus clouds, typically found between 6,500 and 23,000 feet msl, are often the next in the progression of pre-warm-front clouds.

But these high-altitude stratus clouds are not the problem. Virtually all the VFR-into-IMC accidents featured low-altitude stratus.

These clouds — all of them occurring from the surface to 6,500 feet msl — are associated with a wide range of weather conditions. It's important to make distinctions among these different kinds of stratus clouds because some are more indicative of extensive instrument weather than others.

For example, one type of cloud, called stratocumulus cumulogenitus, is typical of post-cold-front weather. This type of stratus is primarily created by the spreading out of cumulus clouds. It creates a scattered to broken sky, with patches of blue sky showing through. Vertical development is minimal, so VFR climbs to VFR-on-top conditions are possible. Once on top, it becomes obvious that this type of cloud shield can stretch over vast distances. VFR cross-country flights can be conducted either below (where it's turbulent) or above (where it's smooth) the cloud layer, and the chances of precipitation are usually remote. They're basically good-weather clouds; the cold front has passed, and the clouds are formed by the instability created by the intrusion of a new air mass.

Plain stratocumulus clouds are a bit different. They can form as moist air is lifted and moved across high terrain. They also tend to form after passage of a cold front, and they can cover a wide area. But they create overcast conditions and carry more moisture. Rain- and/or snowshowers are possible, and when the temperature is right, so are icing conditions. Obviously, VFR climbs to on-top conditions are not possible. Depending upon the height of the cloud base, VFR beneath the clouds may not be possible, either.

The other two principal types of stratus clouds — stratus fractus and nimbostratus — are most often found in warm and stationary fronts (or slow-moving fronts of any type) and anywhere near a low-pressure center. Stratus fractus, better known as "scud," can appear in flat patches or layers and have smooth bases or ragged ones. Nimbostratus clouds, by definition, are rain producers. With these two types of clouds, expect widespread low ceilings and visibilities and little in the way of improvement for many hours.

Fog goes hand-in-hand with stratus clouds. Why? First of all, stratus clouds block sunshine and thus help suppress the normal daytime temperature rise. At the same time, the inflow of moisture — signaled by lower level stratus clouds in the first place — produces elevated dew points. The result is a close temperature/dew-point spread. And as we all should have learned in ground school, that's the formula for fog. Anytime the temperature/dew-point spread drops to 5 degrees Fahrenheit or lower, expect fog to form.

A similar process takes place at night, another time when temperatures frequently drop to dew-point or near-dew-point levels.

All these characteristic conditions were present in Alabama and Georgia during the afternoon of June 25, 1991. An 80-hour, non-instrument- rated private pilot took off from the Tuskegee (Alabama) Municipal Airport, bound for Rome, Georgia. At the time, a center of low pressure was moving slowly eastward across Alabama, and a surface trough and a stationary front were stretched across the pilot's proposed route of flight. Rain and drizzle had fallen all during the day as well as the night before, and there were widespread IFR ceilings and visibilities.

When the pilot arrived at Tuskegee, marginal VFR conditions prevailed. An airport employee asked him how he got in, and the pilot said he had flown his Piper Warrior VFR on top, then descended through a hole in the clouds. When he took off for Rome at about 3:30 p.m., the weather hadn't improved. The accident report didn't say whether the pilot received a complete weather briefing or not but did note that no flight plan had been filed.

At about 5 p.m., a caretaker at a college in Mount Berry, Georgia, thought he heard and felt a collision near his mountain home. The sound was so loud that he heard it even though he was wearing headphones and listening to his stereo. He went outside to investigate but assumed that the sound was caused by a fallen tree and didn't continue to search.

One of the caretaker's duties at the college was to track weather patterns, so he was accustomed to making weather observations. He reported that 2 inches of rain had fallen during the day and that when he went out to investigate the sound, the mountain was obscured in clouds and mist. The accident report listed the nearest reported weather conditions as being 800 feet broken, with a visibility of 3 miles.

Eleven days after the flight was reported missing, the wreckage was found. The Warrior had crashed 150 yards from the caretaker's house. The airplane was destroyed, and the pilot and his passenger were dead.

On December 29, 1991, a cold front had passed through the Southeast, leaving marginal VFR conditions and low stratocumulus clouds in its wake. The non-instrument-rated pilot of a Beechcraft Musketeer departed the Gadsden (Alabama) Municipal Airport at about 1 p.m. on a cross-country flight to Valdosta, Georgia. Earlier, the pilot had called flight service for weather information. But he did not ask for, and was not given, a standard weather briefing. He was told that the local area would remain marginal until at least 2 to 3 p.m. The pilot said he'd call back for more information, but he never did.

The airplane struck trees and then mountainous terrain at about 1:30 p.m., near Lineville, Alabama. The pilot and his passenger were killed. The wreckage was at an elevation of 1,700 feet msl; nearby weather observations logged the ceiling as 1,200 broken, with visibilities at 5 miles in rain. Local personnel reported that the mountain was obscured in cloud throughout the day, with visibilities of roughly 50 feet.

Though both these accidents took place in the Southeast, they're very typical of VFR-into-IMC accidents all over the United States. In addition to a fair representation of accidents in the Southeast states, the record shows many other VFR-into-IMC accidents in Alaska, Oregon, and California. Areas in and near the Rocky and Appalachian mountains are other very common sites.

This should come as no surprise because stratus and fog are very common in high terrain and coastal regions. In the Southeast, lows and fronts draw moisture from the Atlantic Ocean and Gulf of Mexico. When this moist air flows inland and up rising terrain, expect widespread low stratus, fog, and rain. Coastal California is famous for its fogs and stratus — caused mainly by moist, cool marine air flowing up coastal mountain ranges and, in northern California, a chronically narrow temperature/dew-point spread. These same types of conditions create the stratus and fogs so common to many portions of Alaska.

The weather scenarios behind VFR-into-IMC accidents should serve as powerful lessons. Here are a few that all pilots — but particularly those who are non-instrument-rated — should keep in mind in order to avoid inadvertently flying into IMC:

  • Always obtain a complete preflight weather briefing and file a flight plan.
  • Keep a close eye on the temperature/dew-point spread. If the spread has been narrowing rapidly over the past few hours, heed the warning and reconsider the flight.
  • If flying in or to a coastal area, be mindful of the synoptic picture. An onshore flow of marine air, coupled with a nearby low or front, is a recipe for fog and low stratus.
  • In mountainous areas, be wary of any upslope winds, and remember that mountain obscuration can occur without the help of any nearby weather systems.
  • Watch out for any slow-moving low or front. Low ceilings and visibilities are likely to persist for days.
  • Night flight carries a special warning. If the weather has been marginal during the day, expect it to worsen at night. The temperature will most likely drop and may well reach the dew point.

Of course, the biggest factor in avoiding an inadvertent IMC encounter is good pilot judgment. An understanding of the weather is another essential ingredient in making good decisions. But as we've seen in the two accidents mentioned earlier, there's evidence that some pilots appear monumentally blase in this department.