Artificial Horizon versus Turn Coordinator in very light VFR aircraft.

[Tee Emm]

One thing has always puzzled me in many VFR only light aircraft. And that is the airworthiness policy (if any) behind the installation of the turn coordinator. This instrument is a gyro operated, electrically powered, flight instrument and as far as I know was designed as an alternate source of flight instrument power should the suction driven artificial horizon fail for whatever reason. It takes considerable skill to fly the turn coordinator in IMC.

Of course, aircraft with only a turn coordinator for pitch and roll reference are VFR only. Many modern ultra light types have a turn coordinators only. If an electrical failure occurs, you have no attitude indicator left to fall back on. But as presumably you will be in clear air it doesn't really matter as long as you have a visual horizon. In that case what is the point of having any attitude flight instrument unless one day you might need it for real.

So, in these types, why not have an electrically powered artificial horizon instead of a electrically powered turn coordinator? For very light homebuilt aircraft there is no airworthiness requirement to have two attitude indicators (AH and TC) with separate power supplies, since these aircraft are VFR only.

An AH is far easier to use if the aircraft happens to get in IMC and less chance of the pilot losing control while struggling to fly on instruments on a TC only. Is there a significant cost difference between an electrically powered AH and a TC? Give me an AH anytime rather than wrestling with a TC which requires serious instrument flying skills.
....

[Genghis the Engineer]

This has nothing to do with airworthiness, purely because they are VFR only aeroplanes, and thus these instruments aren't required - only a compass, ASI, Altimeter and basic engine instruments.

The instruments fitted are entirely market driven, presumably with an emphasis from flying instructors who like them for teaching purposes, and to a lesser extent PPLs who want a "Get out of Gaol Card".

Also with micro/ultra light aeroplanes, there's always a weight penalty, and an AI is a heavy instrument, which is often the main reason for not fitting one. Most AIs also rely upon vacuum, and Rotax engines don't take a vacuum pump so far as I know.

G

[sycamore]

Look at the Transair catalogue for approved and non-approved instruments,electrically driven as well..
Personally, I`d always go for the AI/AH,approved or not...

[Dan Winterland]

The turn co-ordinator is a very poor instrument which is very hard to use for any sort of instrument flying. I was developed in the US when aircraft manuafacturers were doing their best to make their aircraft feel like automobiles of the time. The TC was an aid to making flying more smooth.

The turn and slip is far superior. All my training (military aircraft) was done using the T and S and the club I later instructed at had all the club aircraft refitted with them.

My usual GA mount is an aerobatic VFR only aircraft. A AI/AH will get damaged with constsnt aerobatiing so a T and S has been fitted as an emergency ''get you down'' aid.

[Denti]

We used turn and slip indicators only during out abinitio training, official line was that it is a much superior IFR instrument. Anyway, was kinda used to it since i started out in glider flying and turn slip indicators together with a clock where the usual instrumentation for cloud flying (glider IFR). Although in the gliders it displayed a one minute standard turn.

[rigpiggy]

The US now allows replacement of the TC with a second AH, presumably with a slip ball somewhere. I can't find the reg, someone else will i am sure

[wing_and_a_prayer]

spins...?

For determining spin direction recognition the AH/DG/Ball are all pretty useless but a TC could help, although a TI is superior since it only senses yaw and I suppose the TC could give erroneous indications as it also senses roll...

[Shawn Coyle]

My two cents worth. From a lot of basic IFR training demonstrations in a helicopter simulator (actually FTD) that had two different models - one had a turn and slip and the other a second artificial horizon.
When the main artificial horizon was failed in IMC conditions, the survival rate for trained, experienced IFR pilots who had only the turn and slip indicator was pretty low. Everyone who had the second artificial horizon did OK.
Given that you can make an iPhone an attitude indicator, why are we sticking with the turn and slip (or turn coordinator)?

[janrein]

TI TC AH considerations

Tee Emm

Consider if you like some trade-offs among the attitude sensing resp. turning sensing and indicating devices.

TI vs TC, "pure" vs "easy"

The TI indicates pure turn rate about the vertical axis, pure yaw rate,
banking in in a well coordinated turn initiation (keeping ball centered)
it gradually will move out and stabilise when the bank angle and therefore the turn rate are stabilised themselves. An "honest" or "pure" instrument so to speak.

Dan Winterland made me see where the TC may have come from. Possibly the TI was found too honest, bluntly revealing aeroplane steering errors, like pushing insufficient or no rudder when initiating a turn the TI will "stick" initially in such a situation or even swing to the opposite direction, however this is a true(!) indication and should actually be appreciated as a help for refining steering skills, together with proper use of the ball. The Turn Indicator roughly indicates one-third roll rate and two thirds turn rate, so initially banking-in is sensed and indicated and thereafter when bank is held constant the resulting turn is sensed and indicated, a smoother indication, and actually camouflageing steering errors as mentioned. The ball will still indicate such errors as well, so when the ball is being kept centered also with a TC a properly coordinated turn can be made and confirmed by your instruments. The problem is that the ball is all too often ignored. In a well coordinated turn (i.e. maintaining ball centered) the TC will give an immediated indication (vs TI moving out gradually during turn entry) due to the roll rate when turn rate is yet to be established.

TI (or TC) vs AH, "robust" vs "easy"

Since the TI is a one-axis gyro it is very robust and will generally survive all attitude changes including (un-)intentional extreme maneuvers and it will keep indicating rate of turn around the top-axis of the aeroplane during and after such maneuvers. Maintaining proper attitude in IMC with a TI without an AH however requires more skill and involves interpreting TI together with ball and airspeed, typical "partial panel" flying, a skill required for Instrument Rated pilots. With just TI ("stick"), ball and airspeed recovering from unusual attitudes is even more challenging.

The AH is more intuitive, simulating the external horizon and maintaining attitude is easier, and so is restoring from an unusual attitude. However the instrument is significantly more prone to failure in more extreme loss of attitude or prolonged turning.

Gliders certified for cloud flying can be required to have either a TI, an AH or both, dependant on the manufacturer's POH. If I had only room for one, I would definitely choose the TI.

The point in VFR-only aeroplanes like Ultralights is that any of the TI/TC AH instruments is redundant to the external horizon. In unexpected loss of VMC the instrument is a back-up, so it would require a double failure (loss of VMC and loss of instrument) to be at a total loss. In IFR-rated aeroplanes the AH is considered primary instrument and the TI/TC secondary instrument and a back-up if the first one would fail. Again a double failure would be required to be at a total loss. Cloudflying certified gliders would mostly climb into and up through cumulus cloud, extendedly spiralling upward which might upset an AH but not a TI. Flying in other type of cloud and with low cloud basis would almost never occur and in the rare occurance of a loss of control, it would normally result in a spin, if not recoverable when in the cloud then usually recoverable once again below cloud with external reference restored and significant height over ground remaining.

In the end, knowing the various aspects, it is all about your own preferences as neither TI/TC or AH are required for VFR-only aeroplanes.

Happy landings,

jr

[Pilot DAR]

Electric artificial horizons tend to be considerably more expensive than electric turn co-ordinators or turn and slips. As stated they are easier to damage in unusual attitudes. If it is an either or situation, the slip information is probably more useful to a newer pilot for improving his piloting precision. It is certainly a lot more useful for spin avoidance, if you let yourself get that far...

In my opinion, if gyros are not installed, the pilot is much less likely to extend himself into IMC conditions. A compotent IMC pilot should have no problem flying decent stable flight on needle ball and airspeed. VSI and altimeter are a bonus.

Having delivered a helicopter not equipped with any gyros whatever, 1900 miles, it was easy to remind myself to not fly without reference to the ground even for a moment temptign as it might have been. I expect that generally, pilots of very light aircraft would be wise to also fly only with reference to the ground. If limited instrumentation reminds them of this, so much the better! Aircraft don't happen to get into IMC, pilots fail to plan and maintain flight so as to assure entry into IMC does not occur. It was a new and very worthwhile self disciplne exercise for me in the helicopter to remain effectively VMC at all times.

[Piltdown Man]

I think janrein's case is spot-on but I'd like to add a small rider. If you only have one instrument, it is important to know the one you have is not telling porkies. An A/H running on a flat(ish) battery is as much use at tits on a frog. However, a T&S will still give relative information when run on a low voltage. It won't be accurate but you will know if you are turning or not. And remember the reason it is fitted? It is the back-up for getting back to VFR not the key to going IMC.

PM

[Shawn Coyle]

There is no requirement in the VFR certification standards for any attitude device for Part 23 'Normal' category airplanes or Part 27 'Normal' category helicopters, even for night flight.

[green granite]

Don't know what's wrong with a bit of string and a plumb bob myself.

[Loose rivets]

This is something I'm passionate about. When they took the tied gyro out of jet transport aircraft, I let out a long howl of protest. As a new Ppruner, I got flamed when I talked of reverting to a tied gyro with all the horizons TU.

That fact was that I'd spent hours and hours and hours, flying with (when there was a good bloke in the right) bits of cardboard positioned so that he could see at least two A-Hs and I could see none. Dirty weather, right down to minimums, time and time again. It's one hell of an instrument, and it doesn't often fail. Heh, Not often.

One night in a DAK, I repeatedly adjusted the rudder and waited for the horizons to settle to the T&S. Something was horribly wrong, and I beat a quick path to the stars. The T&S had gone kind of stogy, and wall well off. There was enough starlight to settle the horizons for the very dark ADF to minimas. A lesson learned.

After the BE Viscount, I often thought I might carry a battery powered T&S. It was to be some years before the tiny glider ones were developed, and by then I was behind glass, with a reasonably conventional standby. Another Heh! The first time I flew an ATR, the vertical info disappeared in anger. One 'dark and stormy' I lost all the glass, and really did not feel good about that one little instrument. I thought of the guy that lost it, while calling to ATC "Am I turning? I need to know if I'm turning." Some of his last words.

Such a simple instrument would have saved him...if he'd practiced with it of course.

[Genghis the Engineer]

Such a simple instrument would have saved him...if he'd practiced with it of course

And that of-course is the real issue, especially in light aeroplanes. The instrumentation on just about anything will get you safely out of IMC - if the pilot is adequately trained in its use.

G

[WeekendFlyer]

Eeek!

Slightly concerned about the comment above that an iPhone can be used as an attitude indicator. Erm, yes it can, but only in a stationary situation where you can guarantee the acceleration vector is due to gravity alone, i.e. on the ground while not moving! The iPhone and other similar smart phones use a 3-axis accelerometer to detect the gravity acceleration vector and thus determine which way is up.

Of course, in an aircraft during flight the acceleration vector can move and change due to all sorts of factors, which is why the TC and AI are GYRO INSTUMENTS, not accelerometer instruments. Attempting to use an iPhone as an attitude data source in an aircraft in IMC is almost certainly going to kill you, so please don't try it!

[Another St Ivian]

Being a shameless fanboi here...

The iPhone 4 has gyro's now, as well as accelerometers

(This being said, you can't tell which source an app is using...so still highly inadvisable!)

[goldfish85]

Replacing the T&B by a second approved attitude indicator is allowed as an "FAA-approved equivalent" in Advisory Circular AC-91-75. This is not a new regulation since the reg (91.205) allows for "FAA-approved equivalents." This interpretation only applies to Part 23 airplanes under 12500 lb. A power source separate from the primary attitude reference is required.

The T&B is normally required as a relatively-cheap back-up to the attitude indicator. Because of its construction it is more rugged than an attitude indicator as is not succeptable to gimbal lock or tumbling. In a slow, straight-wing airplane it lends itself to instrument flight. In a fast airplane, it is far too insensitive.

The turn-coordinator is a T&B with the gyro axis tilted so that it is responsive to both yaw rate and a little bit of roll rate, making partial panel turns a little easier. I understand from some old-timers that the moving airplane format was chosen so that pilots could tell the turn-coordinator from the attitude indicator.

Back in the old days, my instrument IP (no it wasn't Orville Wright) didn't believe on those new-fangled attitude indicators and I hardly had a chance to use one until I had my instrument rating.

Goldfish

[flyer101flyer]

Have to disagree with post that turn coordinator is inferior to turn rate indicator. By canting gyro axis to incorporate roll sensing as well as yaw sensing, you get a much better response to small roll inputs. You are also much less likely to overcorrect when rolling to wings-level after an accidental steep bank. Everythings works better and more intuitively. Also, precise turn coordination (rudder use) becomes less important as the instrument becomes much less sensitive to adverse yaw.

There's a reason that pilots used to be taught to fly the needle primarily with the rudder, and the ball with the stick. It's a clunky and imperfect system, (and completely dependent on some degree of yaw-roll coupling via dihedral etc), but at least you don't see a temporary wrong-way swing of the needle due to making a strong roll input with the stick and failing to give enough rudder to eliminate adverse yaw. Not nearly so much of an issue with the "turn coordinator".

Of course most modern airplanes don't have much adverse yaw, but the principal still remains...

There is some danger of an un-practiced pilot reading the turn coordinator backwards, like an artificial horizon. This could be fixed by going back to the old style of display (needle an ball) while keeping the innards of the modern instrument. Or, by practice. Or, by painting a horozon depiction on the background disk and making it roll left and right, while keeping the airplane in front stationary. Like an artificial horizon.

I think pilots should get more chances to practice partial-panel flying in actual IMC...

[Shawn Coyle]

partial panel in a simulator is much better and safer than in actual IMC.