The History of the Turn & Bank and Turn Coordinator, as written by Old Bob, Beechcraft Maven and Ancient Aviator.

Old Bob wants us all to remember this: "If we stop the turn, we will survive."

My editorial Comment: No matter what turn indicating electric gyro you use in your plane, have your partial panel/instrument scan game in good shape. IMHO, Carnahan and Kennedy did not and they and their passengers paid a terrible price for this lack of proficiency. Here are the Kennedy NTSB Report & Carnahan NTSB Report

Here is Old Bob's history lesson:

The T&B dates from World War I days. It is a gyroscope mounted level to the aircraft's longitudinal axis so that any time the aircraft turns, a needle will be displaced. Some of the very early turn needles were hinged at the top. Others were hinged at the bottom, but in either case, the gyro was only sensitive to left or right motion. It was not affected by pitch or by roll. Sometime between the two World Wars, the inclinometer was added to the instrument and most manufacturers hinged the needle at the bottom. With the addition of the inclinometer, it became commonly known as the Turn and Bank. It didn't show the bank directly, but the ball (inclinometer) would indicate a slip or skid as appropriate.

During and just after WWII most T&Bs were made more sensitive and little dog houses were added on each side of the previous single indicator to show a standard rate turn of three degrees per second. To get a standard rate turn on an instrument that only had the single indicator at the top, the aircraft was flown so that the side of the needle just touched the one and only "dog house". The new style was called a double needle width T&B. It was noted that if the instrument was not mounted square to the longitudinal axis of the aircraft, inaccuracies were induced which could make the needle wiggle when it should not. As an example, if the instrument panel was angled such that the top of the panel was forward of the bottom, the instrument would show a yaw in the wrong direction each time that the aircraft wing was displaced up or down. If the instrument panel was angled the other way, (I cannot imagine that ever happened) it would show a yaw in the direction the airplane was rolling.

In either case, it was considered important that the instrument be mounted so that it's gyroscope unit was as parallel to the longitudinal axis of the aircraft's motion as was possible. Some very smart engineer realized that if the gyroscope was mounted so that the front end was higher than the rear of the instrument, it would indicate a yaw any time the aircraft was rolled and that once it was in a stable turn it would then indicate the actual yaw being developed by the turn.

He decided to use that effect as the basis for an economical autopilot. Several other autopilot engineers jumped on the idea and a standard T&B instrument mounted with the front end thirty to forty-five degrees up in the air was used in several early light plane auto pilots to provide sensing. There was no indication given to the pilot. It seemed like a very good idea because if you roll, it is a good chance that the airplane will soon be turning. After the use of a canted gyro for a low cost autopilot had become firmly established, somebody came up with the idea that a human pilot could benefit from that early indication of a roll just as did the autopilot. That was when the Turn Coordinator style of indication was born.

It consists of a gyroscope mounted with the front end higher than the rear and it will show roll as soon as it develops and will also show yaw. What that means is that you cannot tell just by looking at the instrument whether it is showing roll or yaw, you have to integrate with other instrumentation to tell what is happening.

At first, it seemed like a very good idea and most of us embraced it's use. For the reasons I have mentioned so very often, many of us became disenchanted with the presentation. I still think it is a good sensor for a cheap autopilot. A true attitude based autopilot will work better, but a well damped canted gyro that is fed through a good electronic balancing device to direct the servos will work adequately. That is what S-Tec uses. They have been very successful in that regard. Not as good as an attitude based unit, but more than adequate for the purpose.

More than you ever wanted to know, but that is the history. What it means to you is that anytime the little wing on the TC drops, you are either yawing or rolling. You cannot tell which just by looking at the instrument. Once the aircraft is in a steady, non-rolling turn, the instrument will be showing only yaw. If you fly a knife edge and look at the TC it will show a wings level attitude. Obviously that is wrong, but it is doing exactly what it should do. You are neither rolling nor yawing during a knife edge. It is showing that you are neither rolling nor yawing. I think that is what confuses people. The instrument cannot tell the difference between a roll and a yaw.

For an autopilot that works just fine. It never has to think. For we human beings, it takes more evaluation and interpretation than some of us care to use. I think the TC should be removed from the pilots primary scan and used only for what it does best. Drive a low cost autopilot. If that is more confusing than informative, I apologize, but I would be happy to answer further questions.

Here is Old Bob's discussion of the Turn & Bank (T&B) instrument:

BobsV35B@aol.com

  History of the Jeppesen Approach Plates

The History of the Jeppesen approach plate, as written by Old Bob, Beechcraft Maven and Ancient Aviator on 11 January, 2010:

BobsV35B@aol.com