Turn coordinator -v- Attitude Indicator

[CharlieDeltaUK]

Apart from the balance ball, does the run coordinator show anything which the attitude indicator doesn't show? I know the AI shows pitch, but does the turn coordinator have anything not apparent on the AI?

As a separate question, I'm also trying to understand the difference between a turn indicator and a turn coordinator.

Thanks folks

[Tinstaafl]

A turn indicator is a rate based gyros ie the faster the heading change the more it reacts. It doesn't react to angle of bank except indirectly when AoB results in a change of heading. Roll the aircraft while maintaining a constant heading (sideslip) shouldn't show a reaction.

An attitude indicator reacts to show angle of bank and pitch attitude. It doesn't react to heading change eg keep the wings level & yaw the aircraft shouldn't show a reaction.

A turn co-ordinator is like a turn indicator but with the gyro's axis inclined w.r.t. the longitudinal axis. This makes it able to sense aircraft roll a little bit. The idea being that AoB usually preceeds a turn so you have an earlier - or perhaps, more obvious - opportunity to recognise a need to make a control input to stop or prevent the heading change sooner.

[piperarcher]

The turn co-ordinator is a useful backup in case the vacuum pump fails, and the vacuum / gyro driven instruments (i.e. the DI and AI) all stop working. Combined with the VSI, and the compass, you can cross reference them to confirm you are straight and level.

The turn co-ordinator also indicates the angle of bank for a rate 1 turn (although if you know its e.g. 17 degrees in your plane), then you can also use the AI to do a rate 1 turn as well. I think partial panel training is still part of the PPL (it certainly is for the IMC training), so it gets used there quite a bit too.

[Tinstaafl]

Turn indicators also show R1 turns. Some can even show higher rates.

Not really correct to say a TC shows *AoB* for a rate 1 turn. It shows that the a/c is *doing* a R1 turn but not how that rate of heading change is achieved. If balanced then the AoB must be that needed to achieve R1 - but the TC isn't showing AoB directly. You have to derive that information yourself. If not balanced the a/c could still be doing a R1 turn - but it won't be at the AoB used for a balanced R1 turn.

As an experiment, next time you fly set what would normally be a balanced R1 AoB but use rudder to prevent any heading change. Watch what happens to the TC.

A disadvantage to a TC is during inverted spins. AoB and turn direction don't match so the TC can give erroneous turn information, unlike a 'standard' Turn Indicator.

[rasti121]

As a separate question, I'm also trying to understand the difference between a turn indicator and a turn coordinator.

I think the turn indicator was an instrument with a needle indicating the rate of turn (not the "wings"). It might not have been equipped with a ball (slip indicator is the correct name?). I would say the turn coordinator has always to be equipped with a ball since in order to achieve a coordinated turn the ball must be in the middle (no slip/skid).

In theory you don't have to keep the angle of bank (as opposed to rate of turn which should be kept) constant and achieve a coordinate turn (speed change is required), the ball should always be middle (ish).

I see why these two instruments are a bit confusing, in normal controlled flight their indications might be similar, maybe you can ask an instructor to demonstrate a fully developed spin (in an approved aircraft) and observe the instruments (a lot of braveness required!) - this is the point where the direction of turn is most important (and correctly always shown only on turn indicator/coordinator) in order to regain controlled flight. Maybe the instructor could also demonstrate some less drastic ways of opposite indication for these 2 instruments - some suggested here previously.

[bravobravo74]

It might not have been equipped with a ball (slip indicator is the correct name?).

Inclinometer

[Piper.Classique]

You can cause a horizon to topple. Also, usually, either the turn co-ordinator or turn indicator will have a different power source to the artificial horizon. Not that I have an artifical horizon, just a turn indicator. I don't have an aircraft cleared for instrument flying and use outside references, and a good vario (for towing gliders). I have never felt the need for a DI either.

[BillieBob]

I think the turn indicator was an instrument with a needle indicating the rate of turn (not the "wings").

Actually, the turn indicator is an instrument with a needle indicating the rate of turn but the important thing is that it responds only to yaw. There are certain modes of flight in which the turn co-ordinator, which responds to both yaw and roll, cannot be relied upon to give a correct reading and a turn indicator is essential.

[Flamin_Squirrel]

Actually, the turn indicator is an instrument with a needle indicating the rate of turn but the important thing is that it responds only to yaw. There are certain modes of flight in which the turn co-ordinator, which responds to both yaw and roll, cannot be relied upon to give a correct reading and a turn indicator is essential.

It is not really correct to say that either turn indicator or co-ordinator respond to yaw. Their purpose is to indicate the rate of rotation about the normal axis i.e. how quickly you're turning (changing heading). While this can momentarily result from yawing, that's not it's primary purpose.

The difference in function between turn indicator and co-ordinator is a subtle one (there is a slight difference in how the gyro gimbal is arranged) but essentially the turn co-ordinator will respond to a limited extent to roll (a turn indicator will not). This means a turn co-ordinator will begin to show a turn as you roll into it, before your heading starts to change, making it more responsive.

[The Terminator]

@ Flamin_Squirrel

Yaw is rotation about the normal axis.

Edit to answer thread:

And to answer the original question, Nether turn co-ordinator nor turn and slip measure angle of bank, obviously, as the angle of bank that gives a rate one turn when in a co-ordinated turn changes depending on the aircraft speed, meanwhile the markings on the instrument do not.

[peterh337]

does the run coordinator show anything which the attitude indicator doesn't show?

In practical terms, the answer is No because if you know the simple formula you can get a Rate 1 turn from the AI just as well.

In fact doing so is quite desirable because it saves scanning a second instrument.

The internal construction of the two is very different.

[Intercepted]

In fact doing so is quite desirable because it saves scanning a second instrument.

Another reason for using the AI for a Rate 1 turn is the bad precision of many turn indicators, especially in some club aircrafts.

[Pull what]

The turn indicator has nothing directly to do with bank. The TC shows roll rate response as well as turn due to the gyro being mounted at 30 degs from the perpendicular. TheTC indication represents the sum of the roll rate and the yaw rate so it responds more quickly at the beginning and end of a turn than the TI.
The handicap of the TC is that it will show roll rate in turbulence and in this situation can be very difficult to follow.


Both instruments always have a slip ball as part of the display

[mrmum]

Both instruments always have a slip ball as part of the display

No ball, but to be fair, there is a slip needle.

A bit more up-to-date and no ball.

[mrmum]

Well that didn't work well did it, the pictures are here (but the wrong way round)
http://http://www.google.co.uk/imgres?q=aircraft+turn+indicators&hl=en&gbv=2&biw=1639&bih=7 71&tbm=isch&tbnid=pXzQbIPMDfn6JM:&imgrefurl=http://www.aircraftspruce.tv/catalog/inpages/belite_tri.php&docid=ipIHftPOhwvYKM&itg=1&imgurl=http://www.aircraftspruce.tv/catalog/graphics/10-04386x.jpg&w=300&h=299&ei=LiDtTpC7GYeT8gOroqDrCQ&zoom=1
and here
http://http://www.google.co.uk/imgres?q=aircraft+turn+indicators&hl=en&gbv=2&biw=1639&bih=7 71&tbm=isch&tbnid=52TQ4jJHnF2YnM:&imgrefurl=http://www.specialistauctions.com/auctiondetails.php%3Fid%3D1516479&docid=_ymE2y-2qR_8WM&imgurl=http://www.specialistauctions.com/makethumb.php%253Fpic%253Duplimg/mb134282_6871eed902619d362f6dfeb4bc72ef12_mainpic.jpg%2526w% 253D500%2526sq%253DY&w=500&h=500&ei=LiDtTpC7GYeT8gOroqDrCQ&z oom=1

[Pull what]

MOST instruments always have a slip ball as part of the display-ok?