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I've seen a few panel shots where the traditional turn coordinator has been replaced with a second artificial horizon, as shown below.
Panel with second horizon gyro

The benefits seem obvious - if my artificial horizon failed I'd certainly like to have a fully functional back-up rather than a turn coordinator with no pitch information.

Under what circumstances/conditions are these second horizon gyros acceptable as replacements?
Are there any drawbacks that I should be aware of if I'm considering installing one.

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    $\begingroup$ In an inverted flat spin the turn coordinator is the only one that will give you a good idea which way the a/c is spinning so you can use the correct rudder to stop. Artificial horizon or DI will not give you correct readings. Stay out of inverted flat spins and you should be fine. $\endgroup$ – Radu094 Feb 10 '14 at 9:23
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    $\begingroup$ @StallSpin: Are you sure you don't mix up artificial horizon and rate of turn? It's the artificial horizon that is often wrong in a spin, because it can suffer gimbal lock and because it's calibration mechanism actually decalibrates it in extreme manoeuvres. Rate of turn has neither gimbals nor calibration and should not suffer any problems. $\endgroup$ – Jan Hudec Feb 10 '14 at 19:36
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    $\begingroup$ @Radu094 Your BALL is subject to centripetal forces when you enter the spin. Its location in the aircraft relative to the center of mass will almost always determine which direction it deflects, and it almost always deflects to the outside of the spin (a ball on the left will deflect left, a ball on the right will deflect right). I don't know if all or any gyro RoT indicators will function correctly in a spin, I've always been under the impression that they will also be subject to side forces depending on where they are mounted. $\endgroup$ – StallSpin Feb 11 '14 at 3:26
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    $\begingroup$ @StallSpin: The slip-skid ball will be useless, definitely. But that is not the instrument we are discussing. Gyros however don't respond to centripetal forces, but to rotation. Reading of rate-of-turn gyro will be always correct (as long as it works at all), reading of free gyro (attitude indicator) will only be correct if gimal lock does not occur. For simple gimball that means there is pitch and bank limit beyond which it won't show correctly. It's more likely to be exceeded in spiral dive where the bank is higher. $\endgroup$ – Jan Hudec Feb 11 '14 at 5:51
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    $\begingroup$ @StallSpin I think most pilots are guilty of using the shorthand "Turn Coordinator" to refer to either of the components when in reality one rate-of-turn indicator + one slip-skid (balance) indicator = one "Turn Coordinator". Generally not a problem since other pilots can usually figure out what part of the instrument you intend them to be looking at :) $\endgroup$ – voretaq7 Feb 11 '14 at 18:02
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Check out AC 91-75.

I believe it has everything you're looking for. It seems the FAA really doesn't like turn coordinators these days:

“[T]he FAA believes, and all other commenters apparently agree … the rate-of-turn indicator is no longer as useful as an instrument which gives both horizontal and vertical attitude information.”

Which is completely true...

I don't see a drawback, really. You'll also have to buy a slip-skid indicator (it seems that most replacement attitude indicators can be ordered bundled with one.), so you're really not losing anything.

Edit: The other posts and comments raise some good discussion, I suggest reading those as well.

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    $\begingroup$ Interesting that in all the FAA's talk about how awesome it is to have a second horizon they never bring up the gimbal lock issue. $\endgroup$ – voretaq7 Feb 11 '14 at 17:05
  • $\begingroup$ If you get in a spin in IMC, you should rely primarily on the airspeed indicator and the rate-of-turn gyro. The inclinometer ball cannot be trusted; it is likely to be centrifuged away from the center of the airplane — giving an indication that depends on where the instrument is installed, telling you nothing about the direction of spin. The artificial horizon (attitude indicator) cannot be trusted since it may have tumbled due to gimbal lock. It is better to trust the rate-of-turn, which cannot suffer from gimbal lock, it has no gimbals. (Remember, it is a rate gyro not a free gyro) $\endgroup$ – p1l0t Feb 11 '14 at 19:18
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The How It Flies in section on Spiral Dive Recovery mentions:

If you don’t have good outside references, you should not rely on the attitude indicator (artificial horizon). The attitude indicator contains a gyro mounted on ordinary mortal gimbals, which can only accommodate a limited range of pitch and bank angles. A steep spiral can easily cause the gyro to tumble, whereupon it will need several minutes of relatively straight and level flying before it can re-erect itself. Military aircraft have non-tumbling attitude indicators, but you’re not likely to find such things in a rented Skyhawk. Therefore, you should roll the wings level by reference to the rate-of-turn gyro.8 Being a rate gyro (as opposed to a free gyro) it has no gimbals, and therefore can’t possibly suffer from gimbal lock.

8 That is, the turn needle or turn coordinator, whichever you happen to have.

And in section on Spin Recovery it adds:

If you get into a spin in instrument conditions, you should rely primarily on the airspeed indicator and the rate-of-turn gyro. The inclinometer ball cannot be trusted; it is likely to be centrifuged away from the center of the airplane — giving an indication that depends on where the instrument is installed, telling you nothing about the direction of spin. The artificial horizon (attitude indicator) cannot be trusted since it may have tumbled due to gimbal lock. It is better to trust the rate-of-turn, which cannot possibly suffer from gimbal lock, since it has no gimbals. Remember, it is a rate gyro (not a free gyro), so it doesn’t need gimbals.

If you have good enough artificial horizon designed to avoid gimbal-lock, the you don't need turn coordinator. But with ordinary artificial horizon the turn coordinator combined with vertical speed indicator (despite it's problems like rather slow reaction time) are useful backup for recovering from in-flight upsets.

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  • $\begingroup$ Interesting point about driving the mechanical artificial horizon into gimbal lock with aggressive maneuvering. I wonder if they've eliminated this issue with solid-state (MEMS gyroscope) horizons (since those are essentially three single-axis gyros similar to the rate-of-turn gyro). A subject for a future question :-) $\endgroup$ – voretaq7 Feb 11 '14 at 7:02
  • $\begingroup$ @voretaq7: If the system is made of solid state rate gyros with integration by computer, it can't suffer gimbal lock, because it does not have any gimbal. Plus the solid state gyroscopes with good integrator are actually more precise than mechanical ones. I suppose it's part of a reason why glass cockpit suites generally don't feature turn coordinator any more. $\endgroup$ – Jan Hudec Feb 11 '14 at 9:11
  • $\begingroup$ @JanHudec; you'd be amazed how often software is poorly written. Gimbal lock in software is quite possible. :) Although I'd have to hope that a developer of actual aircraft software would know what Gimbal lock is, and not end up in it. Didn't a space shuttle have an issue with Gimbal lock in software once..? $\endgroup$ – falstro Feb 11 '14 at 9:23
  • $\begingroup$ @roe: I know well how much software out there is utter crap. But the integration code is not that big and if it does not correctly combine the angles, it should manifest progressively rather than only beyond certain angle as for gimbals, so it should be easier to discover in testing. $\endgroup$ – Jan Hudec Feb 11 '14 at 9:38
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    $\begingroup$ @JanHudec Mathematically it depends on if they're using Euler angles internally, or deriving them later (e.g. from quaternations) - the former uses cheaper hardware and is easier to code, but suffers from gimbal lock issues when axes cross their 90-degree points ; the latter requires "extra" gyros (more expensive hardware) and some math (more time writing software), but is virtually impossible to gimbal lock. $\endgroup$ – voretaq7 Feb 11 '14 at 16:26
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I can think of a couple of things, the most prominent is the power source. Turn indicators are usually electrically powered, whereas attitude indicators are vacuum powered, and the idea is to provide redundancy should either power source fail. This is not always the case though, and if your second attitude indicator is powered electrically, that's obviously a non-issue.

The second thing is the absence of a ROT indication, forcing you to calculate the correct bank angle for a standard rate turn should you ever really need one. The only situation I can think of where it is imperative that you can do a standard rate/half rate turn is a no-gyro approach in which case you've probably also lost your vacuum source, and you don't want to start playing mind games, but it's not as crucial as the power source redundancy.

As for whether it's acceptable, StallSpin seems to have that covered. The pictured second attitude indicator is fitted with a slip indicator, so that should cover that aspect.

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    $\begingroup$ I agree with this, the ROT is much better to trust (as far as instruments) in a spin. Also there are times in IFR especially in a no-gyro situation that it becomes useful. I have had a gyro failure in actual IMC and knowing standard rate was crucial for the no-gyro turns ATC was giving me. The plane I was flying actually had an electric AI with an inclinometer (castleberry), but we kept the TC on the co-pilot side. Best of both worlds it seems. $\endgroup$ – p1l0t Feb 11 '14 at 19:22
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The turn coordinator or turn and slip indicator (they are different) usually are electric powered. The artificial horizon is usually vacuum driven. If you replace the turn coordinator with an artificial horizon, it should be electric, assuming the existing one is vacuum.

Rate gyros are not prone to gimbal lock, or "toppling", as are most attitude indicators. If an attitude indicator topples, it can take several minutes to correct.

Also, read 91.205. Generally, a gyroscopic rate-of-turn indicator (needle) and a slip-skid indicator (ball) are required for IFR flight. Two attitude indicators won't do.

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    $\begingroup$ Be careful with blanket pronouncements: two attitude indicators in lieu of a gyroscopic rate-of-turn indicator (needle) is perfectly acceptable for IFR flight in a pretty large chunk of the GA fleet (per the previously-referenced AC two attitude indicators can be an Administrator-approved method of compliance with the requirement to have a gyroscopic rate-of-turn indicator specified by section 91.205(d)(3) if certain requirements are met). $\endgroup$ – voretaq7 Jul 2 '15 at 6:34

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