Several Sport Aviation magazine readers commented on the “What Went Wrong” article in the October issue. The article is a review of the NTSB accident report of a Lancair ES pilot who lost control of his airplane while flying in the clouds downwind of Mount Rainier.
The accident itself – a loss of control in IMC – was not, unfortunately, a rare occurrence for general aviation pilots, but what was unusual is that the airplane had a recording system embedded in its flat glass avionics system the preserved key parameters of the flight. What got readers’ attention was that a descent rate of 10,500 fpm was recorded after the Lancair pilot lost control. How can an airplane descend that fast?
The answer is the natural over-banking tendency of an airplane that leads to a spiral dive.
All airplanes that exhibit normal stability have an over-banking tendency that most of us don’t think about much, if at all. The tendency is always there in a steep turn, but we subconsciously correct for it unless we are disoriented.
The over-banking tendency usually begins to show itself in a bank angle of 40 degrees or more. That’s one of the reasons in normal flying the standard is to limit maximum bank to 30 degrees.
As you know, as bank angle increases, lift in the up direction decreases, so you must pull back on the controls to hold altitude. In a bank of 30 degrees or less, pulling up elevator will raise the nose. In a steep bank, pulling back on the controls will steepen the bank. That’s the over-banking tendency.
The reason we don’t worry much about the over-banking tendency in normal flying is that we subconsciously add opposite aileron to defeat the over-bank. For example, when you are flying steep turns for a practice or a checkride you will pull back to hold altitude, but also, at some point, roll in opposite aileron to keep the bank angle from increasing. It is a very natural control input to make.
If you want to see the over-banking tendency in your airplane, roll it into a steep bank of 45 degrees or more and freeze the aileron position while pulling back. The more you pull, the more the bank steepens until you add aileron opposite the bank.
Now imagine that you are in the clouds and are disoriented. You are unsure of the bank angle, or overall attitude of the airplane, but you see the airspeed increasing. The natural reaction is to pull back to raise the nose and slow the airplane. But if the airplane is already in a steep bank and you pull back, the over-banking tendency takes over and the bank angle steepens.
If you’re disoriented you won’t add opposite aileron to keep the bank angle from increasing as you pull. The over-banking tendency will steepen the bank, the nose will go lower, the airspeed will increase, and you will pull harder trying to slow down. The vicious – and typically fatal – cycle of the spiral dive has developed.
In a spiral dive the bank angle increases to, or even beyond, the vertical and the nose keeps dropping. Because the up elevator is steepening the dive instead of pulling the nose up, the g-load does not increase. The NTSB reported that in the Lancair spiral dive loss of control the g-load remained constant at about 1.1 g’s. This rolling dive can lead to the amazing vertical speeds that are observed when pilots lose control in the clouds.
A spiral dive won’t damage or break an airplane because the g-loading is low, but if the pilot emerges from the clouds, regains orientation, and levels the wings, the mere act of leveling the wings can be enough to overload the airframe as the airplane noses up seeking its trimmed airspeed. Add the stick pull of a panicked pilot seeing the ground rush up and the loading will almost certainly be too much for the airframe to tolerate.
That’s why so many eyewitness accounts of loss of control accidents over the years include reports that “the airplane came out of the clouds and then I saw parts breaking off.”
Beech was a pioneer in attempting to explain the over-banking tendency, and resulting spiral dive, in the early days of the Bonanza. When pilots lost control of the slick Bonanza in the clouds Beech calculated that the resulting spiral dive would hit more than 300 mph in very few seconds.
Beech’s advice if you became disoriented in the clouds was to take your hands of the wheel and attempt to hold heading with your feet. The stability of the airplane would keep airspeed in check if the wings were anywhere close to level, and the rudder could do that. Keeping your hands off the controls helped to fight the urge to pull if airspeed was too high. Lowering the landing gear would add drag and help control airspeed.
If the air is turbulent, steering with your feet is going to be tough, but the advice is still pretty solid if you ever lose orientation in the clouds. If a steep bank is allowed to develop, recovery is darn close to impossible. But if you can keep the wings approximately level, the airspeed will behave as you expect it to.