Richard Collins did NOT advocate removing the turn and slip, what he did advocate was replacing the traditional instrument with a second AI AND installing a 2 inch turn and bank somewhere else in the panel. Without a turn and slip the aircraft would NOT be IMC legal (IFR in IMC), but might be legal under VFR (see posts above).
Actually, Richard Collins did very champion replacing the turn and bank (or turn coordinator, as the case may be) with a second attitude gyro. His wasn't simply a recommendation to install another attitude gyro, but he strongly felt that the 2nd attitude gyro presented better information than the TBI. Especially in the case of a failure involving loss of of the primary attitude indicator.
The designers obviously believed the location of the turn and slip was adequate for partial panel.
Not so much that it was adequate, but that it was part of the basic instrument package from the earliest days of instrument flying. This goes back to the days of compass, ball, and airspeed, with the turn and bank indicator being an additional means of setting establishing a heading.
At the risk of some having a fit because I invoke an example from a more complex aircraft, it's worth noting that in more complex aircraft we seldom if ever teach, train, or use timed turns. Instead, we handle instrument failures with backup instruments. In a light airplane, this becomes the partial panel world, and it makes sense to resort to timed turns if one has no heading information and is down to the TC or TBI. Fair enough. Having a backup reference powered by a different source, however, is an even better idea.
The problem with the attitude indicator is that while it does provide better overall awareness of one's attitude (try flying partial panel off a turn coordinator in turbulence, while IMC), it does't give actual turn information or turn rate information to enhance one's ability to stay on a heading.
One can still do a very credible job with an attitude indicator, however. For timed turns, using a specific bank angle is closer to the way it's done in complex or advanced aircraft, anyway.
A really handy technique that works in a light airplane is to use the attitude indicator to make corrections. I used to do a job that had an observer requiring slight corrections to heading, and the observer might say "give me 20 left." Snap to a 20 degree left bank, hold for three seconds, and go wings level. It's very close. Same for a 30 degree course correction, or a 10 degree course correction.
I'm sure there are those out there that can fly a 1 degree course of the magnetic compass in turbulence at night in a cloud...but I'm definitely not one of them. I'm not one who has found the magic of being able to fly an accurate heading on the TBI alone in turbulent, IMC, partial panel conditions. I find that use of the attitude indicator enhances awareness substantially.
This may all be a mute point (and I mean mute, as in silent, rather than moot) in a Cessna 172, where one tends to do not a lot of hard IMC work (or shouldn't, really). It's a worthy discussion of instrument flying in general, however, which is a good byproduct of thread drift.
I understand that the proposal was to replace the t&B/TC with a second AI, in that position, but doing so would not conform to the design requirement stated.
This is the whole point of field approvals for major repairs or alterations, as well as supplemental type certificates.
So which would you prefer IF you could only have an AI or a T&S?
I'd rather have the attitude indicator. I spend more time in large aircraft lately than small aircraft, but I'm more interested in attitude awareness,and most turns I do are at 25 degrees of bank (same as what the flight director provides).
The second AI would not meet the intent of the design requirement 23.1321:
The instrument that most effectively indicates the attitude must be on the panel in the top centre position; [/font]
in the position of the TC.
A second attitude indicator does indeed conform to those requirements. However, those requirements are for certification, not continued airworthiness thereafter, and installation of a second attitude indicator is not restricted or changed by the requirements of 14 CFR 23.1321. Especially not when an existing attitude indicator is already in place.
I have, on a number of occasions, taken off with an U/S instrument that was not required for that type of flight (day VFR), and simply taped a piece of paper over said instrument - more to prevent confusion than anything else. Was I legal?
No. The inoperative instruments or equipment must not only be placarded, but deactivated in an acceptable manner. One needs to know the system in use and check with the current maintenance publications to know if and how an item can be removed or deactivated. In the even the item is removed, which is advisable in some cases but not all, the weight and balance documentation must also be ammended.
And as an aside, if you disconnect a pitot/static instrument from the pitot/static tubes, do these connectors auto-seal or do you have to put some kind of plug in them, to prevent false readings on other pitot/static instruments?
Pilot DAR already addressed this, but no, pitot-static systems aren't self sealing. If you do open the system, you also invalidate the IFR certification of the system, and a complete test and certification of the pitot-static system must be performed again before the aircraft can be certified once more for IFR flight.
To open these systems up requires mechanic certification (or "engineer" certification, if you will), and should be left only to those trained to work on the airframe. Improper handling of the components can introduce all kinds of problems. I've experienced a complete instrument loss on two occasions in airplanes in which someone had opened the pitot-static system and not capped the lines while work was being done. Insects entered the system, and died. In IMC, the insects apparently swelled and plugged the system off, causing all kinds of havoc. They were in various parts of the system, too, causing unusual failures that didn't conform to what one might normally expect from a blocked pitot tube, or blocked static source.
The information you get from an AI doesnt change. The purpose of having a second is to take over from the first, when the first fails. The only extra useful information is the ability to check one against the other BUT you still need to be sure you know which one to believe so there is an additional inherent danger in having two of which you need be aware.
I think you may have misunderstood my point. When I said that the second attitude indicator provides more useful information, I wasn't referring to it augmenting the primary attitude indicator. You're correct that it doesn't provide anything extra over the existing attitude indicator. It does, however, provide a lot more useful data if the primary attitude indicator fails, than what the turn coordinator can do for maintaining control of the airplane.