Aviation Safety Letter 2/2005

The Chimney Effect

Reprinted from Aviation Safety Ultralight and Balloon 1/1990

The decision to land always involves many factors. This story illustrates what happened when the pilot set up for a landing in a vacant field near a built-up area. The balloon ended up being towed by the chase crew from my neighbour's front lawn. The pilot in this case was a visitor to Canada taking part in a local Balloon Fiesta, so he may not have been entirely familiar with the area and Canadian requirements. Transport Canada discourages the practice of landing balloons close to built-up areas.

Why did this balloon, after a normal approach to the selected field, suddenly change direction and drift toward the row of houses just as it was touching down?

As balloon pilots, there is something we must all keep in mind when flying in light wind conditions on sunny mid-summer mornings: air heated over the dark-coloured roofs of nearby houses flows upward in a rising column. This rising column of air is replaced by cooler air near the ground, and this creates the inflow toward the building. The unsuspecting balloonist can get caught in this airflow when landing close to buildings or trees during periods of rapid daytime heating. This apparently happened to our visiting pilot. During the landing he found himself moving toward the front doorstep of my backyard neighbour. This being Sunday, the absent homeowner was probably at church and missed all the excitement.

In this case the pilot would have had a better selection of fields further along his flight path. The light wind conditions and immediate availability of the chase crew to tow the balloon into the field may have influenced his decision to land at this point. In fairness to this pilot's decision, the field chosen is often used by balloonists, and another balloon landed well clear of buildings about the same time. The problem began during the approach because the balloon was not in position to land near the centre of the field away from the buildings.

As a safety promotion writer I cannot resist the dramatic opportunity to illustrate the effect of daytime heating on this balloon landing. As a fellow balloonist I hope it does not cause undue embarrassment to the adventurous visitor to Canada.

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