The Ups and Downs of Turbulence

To understand the whole story, you need to look at a Vg Diagram — that's the one you most likely last saw in your private pilot study materials. It's a graph that illustrates the stall lines, the limit loads, ultimate limit loads and Vne as the "envelope" of the aircraft — and most of us have never seen an actual one for our aircraft! Va falls at the point where the 'positive limit load' line intersects the 'stall' line on the positive 'g' portion (upper) of the diagram. And we are told that if we are flying at that speed, the airplane stalls before it exceeds the limit, and thus can sustain no damage.

But, there is a problem with that. Look at the bottom portion — the negative 'g' part — of the Vg diagram. There is a similar point there where the 'negative limit load' (which is far less than the positive) intersects the stall line — effectively a "negative Va." In virtually ALL cases, this is a significantly slower speed than Va on the positive 'g' side.

So, here's the question. Have you ever experienced only positive, or "upward" turbulence (i.e. nothing to the negative or downward side)? I know that I surely haven't. In serious turbulence, the bumps go both ways. What does this mean? Since no negative Va speeds are published — anywhere — you MUST decelerate to a speed SLOWER than Va to be truly protected! How much slower? Since nothing is specified here, just fly as slowly as you can and still have decent aircraft control — then fly out of the turbulence as soon as practical.

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Alan C. Davis, Master CFI - Emeritus/CFI AIMEI Gold Seal / FAASTeam Representative. Davis is a also board member of SAFE (Society of Aviation and Flight Educators)._