Attitude indicator vs Turn Coordinator

[tathepilot]

Is it possible for your A/I to show a bank in one direction and your T/C to show a turn in the opposite direction?
If so under what conditions would this be possible.

Today as I was climbing out in Imc my student was flying the airplane. I reached down to grab my chart and as I looked up at the A/I it showed a climbing right turn (we were turning). My student did pitch up excessive above 10 deg, when I looked at the T/C it showed a turn to the left. That got my attention right away. I tooked the A/C and scaned the panel to try and detect a system or instrument failure, to no avail everything was fine.

For a second I thought that my eyes were playing tricks on me because I could swear this is what I saw.

Any thoughts??

 

[Mmmmmm Burritos]

It's possible the turn coordinator gyro is getting worn. I've flown a plane or two where if you sneeze the TC will show a big deflection. Basically if you move the ailerons, the TC will move very quickly to full deflection. The gyro needs to be refurbished in this case. Other than the extreme sensitivity, it will show normal indications. No flag, no noise, etc.

Your student may have been in a right banked turn but had a slight amount of left aileron and was returning to level (maybe intentionally, maybe not!). An oversensitive TC would have shown a left bank in this case while the AI still showed a right bank. Remember the TC works on precession, not rigidity in space like the AI.

 

[pilotman2105]

Would an uncoordinated turn have anything to do with it?

I'm not sure. Just trying to throw out options.

 

[dmspilot00]

The Turn Coordinator is designed to show rate of turn and rate or roll. Therefore, if the miniature aircraft in a TC in banked left, it could indiciate either a) the aircraft is turning left, or, b) the aircraft is rolling to the left. This is the difference between the "turn coordinator" and the "turn needle." Turn needle shows only rate of turn.

I was trying to fly partial panel during IFR training on a "worn" TC that Mmmm Burritos mentioned. Actually, I even had to do a PAR approach. Every time we hit a little bump or I would move the ailerons, the TC would jump wildly. I was ready to take a hammer and smash it.

 

[Mmmmmm Burritos]

The Turn Coordinator is designed to show rate of turn and rate or roll. Therefore, if the miniature aircraft in a TC in banked left, it could indiciate either a) the aircraft is turning left, or, b) the aircraft is rolling to the left. This is the difference between the "turn coordinator" and the "turn needle." Turn needle shows only rate of turn.

Yeah I was gonna mention this- the turn indicator (or needle) only shows rate of turn or change of heading. If you are good with the rudder, you can actually do a complete aileron roll and the turn indicator will not show any deflection.

This is why they came out with the newer TC, which has the gyro canted so it shows precession in both roll and yaw (change of heading actually).

 

[tathepilot]

Your student may have been in a right banked turn but had a slight amount of left aileron and was returning to level (maybe intentionally, maybe not!). An oversensitive TC would have shown a left bank in this case while the AI still showed a right bank.

This makes sense.

When it initally happend I was like oh *hit here we go. You know, I have never realized this before.

 

[labbats]

I had the exact same thing happen to me on one of my first actual IMC lessons after I became a CFII. We were in and out of towering cumulus and getting bumped around pretty good. Naturally, I freaked out in a similar fashion and got out of the clouds and on the ground. With a little research and humility around the old CFIs I found the reason.

The attitude indicator should be used solely for reference when in clouds during bumps, because your turn coordinator is very susceptible to moving around and showing the wrong thing. Next time you fly, do dutch rolls or some quick manuevers in VMC and you'll notice the turn coordinator showing all kinds of misinformation while the attitude indicator is right on.

 

[tathepilot]

The attitude indicator should be used solely for reference when in clouds during bumps, because your turn coordinator is very susceptible to moving around and showing the wrong thing. Next time you fly, do dutch rolls or some quick manuevers in VMC and you'll notice the turn coordinator showing all kinds of misinformation while the attitude indicator is right on.

This is true, lately I think I've been putting too much emphasis on the T/C as a redundency for the A/I. What has been mentioned in this post makes perfect sense. When push comes to shove in an actual emergency I see how initially it may be difficult to identify the failed system or instrument.

 

[labbats]

I just teach people to always use the attitude indicator for an initial movement.
Take the last digit off your airspeed and add 5, then roll that amount of bank into your turn with the attitude indicator. Then, if the air is smooth, fine tune it with the T/C.

 

[Tinstaafl]

Bear in mind a Turn Coordinator, unlike a Turn & Bank needle, is sensitive to roll as well as yaw. Its gyro is deliberately mounted at a slight angle to cause this.

It's normal to have an AoB in one direction but during a roll towards upright have the TC indicate the (left) roll eg AI indicates some amount of bank to the right but the a/c to be rolling to the left. The TC indicates to the left (although not necessarily an accurate rate of turn).