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