The Needle and the Ball
By Richard L. Collins
October 2006
 I was having lunch with Bob Buck earlier this year and he posed an interesting question. Can any correlation be made between loss of control (in clouds) accidents and the switch from turn and bank indicators (needle and ball) to turn coordinators? Bob has been dealing with instru-ment flying for a long time, as his airline career started on TWA DC-2s and ran through 747s. He's 92, and has thus been retired for 32 years, but his interest in things like this is strong and objective. And, he doesn't particularly like turn coordinators.
As I thought about this, my interest expanded to glass cockpits. While the turn and bank indicator reigned supreme for 46 years and the turn coordinator for the following 40 years, both instruments are effectively gone as far as new airplanes go. The only turn coordinator to be found in new airplanes is in those that still have rate-based autopilots. Then, it is not in view, it is behind the panel. The glass cockpits have a rate of turn indicator, usually atop the HSI, and they have a slip-skid indicator, usually atop the attitude indication. The implication is that this information, especially the turn rate, is quite secondary in nature. There is also a suggestion by some that the larger attitude display on the glass will contribute to safety because it is more obvious to a pilot's peripheral vision.
To begin, what did the old turn and bank tell us? Not what the name implies. The needle told only of yaw rate. The ball told only of coordination. If the ball was in the middle and the needle was deflected, it did tell of bank. The Brits called them slip-skid indicators, which is a good description. Because the needle showed yaw, a turn and bank was hard to fly in an airplane that had an active yaw response to turbulence. It would sit there and wave at you.
In a turn coordinator, which was designed to both replace the turn and bank and to provide information to autopilots, the gyro in the instrument is tilted at about 35 degrees of the longitudinal axis of the airplane. That way it can see both roll and yaw. It is damped and offers a much calmer display in turbulence.
The challenge in times of trouble is to keep the wings of the airplane level. Either one of these instruments can be used to do just that, as well as to make turns at an acceptable level of bank. Some thought the turn coordinator looked so much like the artificial horizon that pilots might think it also has pitch information. In fact, for a while some airplanes had placards by the TC that said "no pitch information."
If the larger attitude indication in a glass cockpit is thought to be better, we have actually been there and done that. Up until the mid to late '60s most of the gyro instruments used were war surplus. The artificial horizon was bigger than it is now and the directional gyro showed a card that, when you turned right, moved to the left, but at least it told you what the heading was if properly set with reference to the compass. The only advantage I can remember for it was on instrument takeoffs. It had one-degree marks and could be used to accurately track the runway. It was at about the same time the TC and smaller gyros came along that manufacturers started arranging the instruments in the panel as they are now. Before, they were slapped into the panel willy-nilly, in no particular order. Screw it in where it would fit. The better arrangement of instruments made instrument flying easier. When it was adopted it was called the "basic T." So a lot on our instrument panels changed at about the same time. That truly muddies the waters if we are to look at any possible effect of the TC on loss of control accidents. Let's look anyway.
In 1965 the turn and bank was still, by far, in all instrument panels. Most panels were not well arranged, most had bigger artificial horizons and most had those old heading indicators. It's really getting to apple/orange time, but the total flight hours in 1965 were likely about the same as they are today, with far less instrument flying done. How much less, nobody knows. When comparing the more modern but pre-glass airplanes with those flown in 1965, the difference in loss of control accidents is nothing short of spectacular. In 1965, there were 70 fatal accidents involving VFR pilots losing control of airplanes in clouds followed by an uncontrolled collision with the ground. There were six involving IFR pilots losing control in clouds. That compares with more recent times when there were 15 VFR and 22 IFR losses of control in clouds. I did not count losses of control where alcohol was a factor. There were a lot of those in 1965.
I don't think there is any way those numbers can definitely be tied to instrumentation. When a VFR pilot loses control in clouds, he's not doing anything that he's not expected to do. Maybe what the high 1965 number tells us is that there was more scud-running done then and pilots were not good at it.
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