Low Altitude Troubles

An email correspondent asked me to explain why low-altitude thermaling is apparently so dangerous. After all, he (and I) have never unintentionally spun at high altitude, so why is it so bad to thermal down low? He suggested the answer might be of more general interest so here it is.

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It's not really "more likely to spin" but surely "more likely to get in to big trouble."

- Thermals form in very small little bubbles and streams down low. These coalesce together between 500 - 1000 feet to form the larger structures we use. Many of the little streams and bubbles die out. The lower 500 feet is (you hope) super-adiabatic which means unstable in both directions. If a little bubble tries, a bigger bubble nearby can push it down and quash it. At any rate, lift down low is much smaller, gustier, rattier and less reliable.

- In the bottom 500 feet you are in the turbulent boundary layer, just like in the last mm of your wing. This means there is lots of wind induced turbulence, lots of little gusts and eddies. Many things that feel like thermals will vanish and turn in to sink and tailwind when you turn. The overall gustiness is greater in general.

- Ground illusions are very strong. Just at the moment when you are really busy you have to deal with a totally different view out of the cockpit. Doing 60 mph heading downwind it is really hard for your brain to realize you're on the edge of a stall.

- Add these up. You're going in to the wind. You feel a gust - which is just a gust, but you don't know it. The glider rises, the vario chirps, you feel g in the pants. Whew! You turn. As you pass 90 degrees you're also getting the "dynamic soaring effect" that the gust is accelerating the glider. But as you hit 180, the gust turns in to lull, which means you're headed downwind and lost 10-20 knots of airspeed, and the nose points down. But since you're headed downwind the trees are going by really fast, you think you have plenty of speed, the nose is pointed down (since you just got dumped), so you pull back on the stick

I don't think the vast majority of accidents resulting from low-altitude thermaling attempts are simple stall-spin entries in the thermal. I think crashes out of thermaling attempts are really sink/gusts. This seems to be for example the consensus of the crash at Ionia -- the gust robbed him of airspeed at the wrong moment, the glider fell out of the sky and just didn't have room to pick up speed.

The stall-spins come from very low altitude maneuvering after the thermaling attempt expires. If you try to thermal at 400 feet, you give up at 300 feet... and 40 knots and pointed the wrong way. Now you have to do a lot of last minute maneuvering to get to the field.

The last minute maneuvering is where the stall/spin happens - lined up wrong, changing fields at the last moment, etc.

Here too the ground illusions set in. You left the thermal doing 40 knots, as you were thermaling. Or, you got dumped on the backside of the gust. Now there will be big downwind illusion on your "downwind" plus a mad desire not to lose altitude -- it's not often you start a pattern at 300 feet and 40 knots. Subconsciously, you will not want to give up another 200 feet to get the airpseed up where it belongs. As you make last minute maneuvers -- let's not call this a pattern -- you are still used to the sight picture of turning on a point that happens high up. That a turn takes radius may come as a surprise. Remember you're under huge stress here. If you don't line up with the field, and you are uncomfortable banking -- now at 100 feet and still 40 knots -- your subconcious will want to use the rudder. And as the nose falls, pull back.. here we go.

In sum, trying to thermal below 500 feet is much less likely to work, because the thermals are much smaller and the wind gusts much worse and more tempting. The biggest danger is that these attempts will lead to a low, tight, slow pattern, and the last minute maneuvering can easily set up the stall spin.

On a cheery note, this sort of thinking sets some safety parameters. It's more dangerous to thermal low if you don't have the landing field exactly mapped out and your approach to it; if the wind is stronger; if the thermal day (super-adiabatic layer) is stronger; if there are trees or hills contributing to low altitude turbulence; and if your stress level and ability to adapt to the different air and ground illusions is weaker.

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Not a cheery thought for a cold winter day, but a scenario worth rehearsing.. The temptation to keep trying and avoid a landout is oh so strong, and all the ways the air is different down low not so obvious.

John Cochrane