I recently read a forum discussion started by a student pilot who experienced a wing drop on final on a summer day, and he was wondering if it was a thermal or something else. Given how strong and local the phenomenon was, he could not believe it was “simply” a thermal.

Thermals are a rather simple thing: warm ground makes air warmer and thus lighter, creating an updraft. If you ever walked bare-feet on tarmac or concrete in summer, you know that certain surfaces get warmer than other, typically concrete gets warmer than grass. A piece of concrete surrounded by grass will can create a very local thermal.

There is a perfect example of that in Geneva, on final approach to runway 05 grass. As you can see on this picture, the airport is located in a relatively green area. The grass runway in on the left side of the main runway.

Most of the final is flown over the grass area, parallel to the main runway. At approximately 200 feet above ground level, on short final, the approach path is exactly on top of the aero-club and rescue helicopter parkings, which you can see left between the runway and the bubbles.

This relatively small piece of concrete is smaller as 100 x 100 meters, but on warm summer day it creates a rather strong thermal, sufficient to destabilize the finest approach. The effects can range from gaining a few dozen feet – not optimal on short final – to surprisingly strong bank angles.

This third photo has been taken at the very place where the turbulence usually occurs. I personally got bank angles up to 30 degrees, which are quite impressive so close to ground. Gaining a few dozen feet, or a simple stop in descent is more “standard”. However, as the runway is relatively short and climb-out path is obstructed by the trees, landing at the (dispalced) threshold is a must.

So what to do if you get strong bank at slow speed with full flaps on final, or an altitude change that make the landing unlikely ? The answer is simple… exactly the same as jet pilots apply for wind-shear conditions: go-around ! The pilot’s life insurance, one of the sole decisions that is never the wrong one… if decided early enough.