Pilots can learn from each flight – even when they’re passengers. I’m writing this post “live” from a Lufthansa Boeing 737 operated as LH3666 from Frankfurt to Geneva. The weather is dry but there is a thick layer of stratiform clouds and the air below is almost saturated. Shortly after take-off, I could observe a really neat materialization of the airflow when the depression over the wing and at the wingtip created a sufficient drop in temperature to make the air condensate…

The wingtip vortex was visible for quite a long time, and I contemplated it as long as it last. Even if I know what it is and were it comes from, the idea that it could be kind-of-alive was hard to refrain. Like many aerodynamic and complex physic phenomena, it did undulate in a smooth motion, a bit like a swimming fish, and flying swan. Who said there’s no poetry in flying ?

We’re now in cruise, flying through blue and smooth skies, under a vivid spring sun. The layer’s top is only a few thousand feet below us, looking like fresh snow, or cream for the most gourmands of you. As it is a stable layer, flying through it was not bumpy and it was not too dense. The whole wing did remain visible at all time, not like in dark, compact and shaky convective clouds.

The interesting thing – and this is where safety and pilot tips enter the game – was to  try guessing the very moment at which we would get on top. Flying on-top is always better and easier than flying IMC and I always look forward to it. Not a lot of information is available to pilots about the top of cloud layers. When a layer is not too dense, the light that comes through often leads to the feeling that the top is really near. The risk in believing so and making decisions on such an assumption is to try climbing above aircraft or pilot limitations.

The worst case would be a “slightly” icy layer. Light aircraft and ice don’t go very well together and a good escape plan is important. If light creates the impression that the tops are pretty close, climbing could sound like a plan. Why going low and IMC if VMC on top is possible ? Getting VMC ensures that the accumulation of ice will stop and the sun could even make it go away. But how far is “on top” ? More than two minutes went by between the moment I started to think “we’re quite close now” and the moment  we did reach VMC. I’m not an expert for Boeing 737 performances (any correction welcome) but I guess that a climb rate of 2′500 feet per minute is not unrealistic. This means that the tops were at least 5′000 feet higher than my initial uneducated, uninformed guess.

Should this occur in a light aircraft let’s say at 8′000 feet, with a planned cruise altitude of 10′000 feet, requesting higher altitude to escape IMC and ice could lead to a climb towards… 13′000 feet. Could the aircraft and pilot sustain that without a turbo, pressurization, or extra oxygen ? And if yes, for how long ? I felt hypoxia effects once and this was hopefully VFR in VMC and an immediate descent was possible. No ice there.

Even worse, what if the layer is icy at higher altitude ? The rules about icing and temperature are quite fuzzy. No icing in positive temperature is clear, but the “no icing below -15°C” is only a rule of thumb, not a physical law. Climbing to escape icing or to regain VMC for comfort is not an option for light aircraft, unless the planned altitude is below 10′000 feet and it is certain that VMC conditions prevail…