I was looking for an answer to my question in context of a failed VSI or maybe a backup altimeter. Suppose my VSI is showing a defect, say, reading +100 ft/sec while on the ground. I would normally note this, maybe tape a piece of paper over it to avoid confusion, and that's it. But now you're telling me that I should deactivate this instrument, which would require me to dive behind the panel and start messing about with the pitot/static system (actually the static only, but you get the point) - which I'm not allowed to do (and rightfully so, I'm not arguing that).
So even though a VSI is not required for VFR flight, if it shows a defect it has to be disconnected, not just placarded U/S. And I'm not allowed to disconnect it myself so in effect it has just turned into a no-go item, right?
Backpacker, first of all, I did NOT tell you to "dive behind the panel and start messing about." Nothing of the kind.
You need to determine if indeed you have a problem. It's very common for the VSI in light airplanes to be indicating a slight climb or descent on the ground. This doesn't necessarily indicate a "defect." In fact, any instrument training handbook I've ever seen states that one should note the difference between the reading on the ground and zero, and apply it in flight. Thus, if the VSI is indicating -100 fpm, then in flight if one intends to establish a 500 fpm descent, one uses the -600 fpm marking on the gauge. Simple enough.
If indeed you do have a static problem, then you need to determine if it's affecting the rest of your system. You're quite correct that you can't fly with inoperative instruments or equipment unless it's in a condition that makes the aircraft "properly altered." You can accomplish this in four ways: a "ferry permit" (special flight permit); applying a Minimum Equipment List (MEL); Deactivating or placarding as required per 91.213; the use of an STC or other supporting paperwork which changes the certification and airworthiness documentation for the aircraft. In each of these conditions, a legal means is made to alter the airplane and it's type certification.
If your VSI has a problem, your altimeter may have a problem. As I said before, you need to look carefully at what's going on and determine if it presents a hazard to flight. You may not be qualified to make that determination; you may need the help of a mechanic to make the determination. Ultimately it's up to you to take the responsibility for the airworthiness every time you get in an aircraft and put it in service, but part of your responsibility is also to defer to qualified individuals in determining the status of a problem or condition.
A day VFR flight, you may be able to go fly without instruments that aren't required for day VFR flight...so long as they're in a condition which meets the requirements of the regulation.
I need to again include the caveat that I don't work with the CAA for certification or maintenance, and can't speak to regulation in the UK, or to JAA maintenance regulation, specifically. Therefore, you may see some difference with local regulation and policy. That part, you'll have to ferret out for yourself. We've discussed 91.213 of the US Federal Code of Regulations, however, and that part I can address.
When you placard a part of a system, you may be unwittingly placarding the entire system. The static system is interconnected; a problem with the system affects multiple instruments. In some aircraft, it can affect dozens of other components and systems. In the airplane I'm flying at the moment, for example, a pitot blockage can affect nearly 30 other items. Static problems affect fewer items, but still have big impacts on what I can and can't do with the airplane, legally (and safely). Even big airplanes with redundant systems have crashed as a result of pitot and static blockages. Clearly it can be serious.
Back to day, VFR: if indeed there is a static system problem (how do you tell it's just the VSI, or that the VSI is actually failed?), then if you're making a determination about something being inoperative, it's actually the static system which is inoperative. This affects more than the VSI, and now you're having to consider instruments that are required for day, VFR flight. Now the airplane can't fly day VFR. You see the point?
All me an example, if you will. As a new pilot in the PB4Y-2 (a version of the B-24), a WWII bomber with dirt-simple instrumentation, very much like a 172, I was sent on a fire dispatch in Florida. The airplane had been sitting in the rain. Heavy rain, in fact. As we taxied out, during execution of the checklist, we noted a discrepancy in the altimeters. Not a big deal, we thought, as we'll not be climbing above 500' anyway, during the entire flight. In fact, the entire flight would be conducted as much by "feel" as by instrument work, due to the low altitude, contact nature of the flying.
Bear in mind that these kind of flights were very quick; five minutes from notification to airborne, in many cases. Often quick enough that one didn't always have the coordinates of the destination when departing; the dispatch might be simply "takeoff and fly south." We might get the destination once in the air; it was firefighting, and time was often a very important consideration. This isn't something that one has to worry about with a weekend rental or pleasure flight, or a condition one shouldn't impose on one's self during a typical private flight, anyway.
On the takeoff roll, I was head down making adjustments to the propellers through a series of switches on the center pedestle. The nose of the airplane was too long to see over, especially once it had been raised, so to see out one had to sit up, stick one's head in a bubble window to the side, and look forward, with a very limited view. Being very new, I was trying to get everything done, perhaps a little behind the curve, so to speak, and was surprised to hear the captain yell "Rotate!" I thought I'd missed my call, and glanced at my airspeed indicator.
Rotation speed was 95 knots, but when I glanced at my airspeed indicator, I saw barely 70 knots. I felt the wheels skipping slightly, just the same as one might in a light airplane when it's ready to fly. The 4Y was flown very much like an oversize light airplane. I felt the captain rotate, and again felt the airplane skip slightly like it was trying to fly, and shortly thereafter the nose slammed back down. That got my attention.
I looked up, put my head in the bubble and saw nothing but trees. We were nearing the end of the runway. I broke the safety wire on the emergency jettison switches, and prepared to blow the tank doors. The captain rotated hard and we were airborne, but went through the trees at the end. We found ourselves in a real fight; a struggle to stay airborne. The captain called for flaps up; this was normally accomplished at 130 knots, but when I checked my airspeed, it was just coming through 95...we were just at rotation speed. I yelled "unable!" Raising the flaps on that airplane made a 50 knot difference in stall speed, and we were still at rooftop level. You can imagine, if you've ever retracted flaps just after takeoff and started sinking, the possible outcome. We were also heavy, at 80,000 lbs.
At that point we were both heads outside, in the bubbles, calling out powerlines, structures, obstacles, because we were going around them, not over. Very, very gradually we climbed. The captain yelled "I don't understand it. I'm over 130 knots; we should be flying!" I yelled back (it was a very loud airplane inside; most communication was done with hand signals) that I was showing 95. We looked at each other and the lightbulb finally went on. "Alternate Static!" The captain opened his, I opened mine, and his sytems quickly dropped back to match mine. He had followed errant indications, had attempted to rotate early, and the nose-high, early rotation delayed our lift off, as well as killed climb performance as we ended up airborne behind the power curve, low, and slow.
There's more to the story, but the point is, drawn out though it may be, one shouldn't make assumptions about instrument error,and what else may be affected by the instrument error. It may not be a simple busted VSI; you may have a static problem, or additional problems. I've seen a number of cases of static blockage that included pitot blockage, too. In fact, I flew in one location where leaf-cutter bees could plug up or block any port in the airplane, including fuel drains, in an amazingly short period of time; where there's smoke, there's often also fire...you may find more than one blockage, or blockage at more than one point in the system.
In the case of the Turn and Bank mentioned earlier, it's well to remember that the problem may be a short in the wiring, not just the instrument. One could placard the instrument, and then still end up with a fire. One needs to make sure the problem is fixed. I saw a Cessna 150 catch fire once because the cigarette-lighter adapter (DC adapter receptacle) in the airplane that was placarded inoperative was a little more than simply inoperatve.
Point is, there's more to the picture. Or could be.
No, not entirely.
As with most regulation, looking in one place usually won't cut it. There's nearly always more to the big picture.
Let's say you elect to fly with the inoperative VSI. You summon a mechanic, but without opening the static system and blowing out the lines, and putting the VSI on a test bench, the mechanic can't do too much. We've already noted some potential implications of a static or pitot (or both) failure; I'm sure you can think of more. The mechanic glances over the system, and says he doesn't see anything wrong (In the scenario involving the PB4Y, both pilots were A&P mechanics, as well). You put an inoperative sticker on the instrument and go fly. Sounds simple enough. If the aircraft has a squawk sheet or flight maintenance record it should reflect the change.
The mechanic can't do too much with the system, and it isn't self-sealing from one static instrument to the other, because the one instrument can affect another (remember that the alternate static relief in some aircraft is to break the glass on the VSI). You may be operating an unairworthy airplane, and to take the US regulation a bit farther...if the paperwork isn't complete and the aircraft isn't in a properly altered condition, then the airworthiness certificate is invalidated; you're now flying an airplane without a legal airworthiness certificate.
The regulation states that either the pilot, or "a person appropriately rated to perform maintenance" must determine that the aircraft is airworthy. Remember that you're held accountable for the determination. "It looks good," isn't really a determination, nor is "I think it's okay." How, as PIC, will you know the extent of the problem, or whether you have a VSI issue or a static system issue? If you have a static system issue, you'll definitely be flying an unairworthy airplane.
This also applies to the original scenario. The TC or TBI is placarded inoperative. A mechanic is summoned, and he gets behind the panel to disconnect the cannon plug on the instrument. This done, you apply the placard, and off you go. To be legal, reference needs to be made to Part 43, covering maintenance, which will address, among other things, maintenance records. Again, if the maintenance records aren't ammended to show these changes, then the airworthiness certificate is invalidated, the airplane is made unairworthy, and one can be violated (the subject of enforcement action) for flying both an unairworthy aircraft, and one without a valid airworthiness certificate. Simply slapping a piece of tape on there with an "INOP" sticker is often not enough.
Not to belabor a long post any more, but the standard for airworthiness should be briefly addressed. Airworthiness is a two-part concept; the aircraft must be legal, and it must be safe. We mostly understand the safe part; it's what we're looking for on a preflight, and inflight. The safe part may involve things we can't see, however, and we need to be careful to consider that what we think is the problem, may simply be the tip of the iceberg; it may be something else entirely. That slight engine roughness might simply be carburetor ice, but may be more; it may be oil fouling a spark plug, it may be a valve problem, it may be a timing problem. When determining what's safe, it's important not to overlook something and allow an unsafe condition to exist.
I once dealt with a Piper Seneca which pilots reported made a popping noise with the second "notch" of flaps in the traffic pattern. It didn't do this on the ground, but in flight, and most who flew it guessed that it had a bad pulley in the cable system for the flaps. It turned out to be a spar that was cracked in three places, grinding and popping against itself as it shifted position along the break, when the flaps were applied under a load. Given more time, I've little doubt that it would have failed in flight and shed a wing. Be careful not to make the wrong determination.
The other part, or second part of airworthiness is the legality. Unless the airplane is in full compliance with it's type certification, then it's not airworthy. An airplane can be properly altered to conform with additional documentation and be legal, such as conformance with a supplemental type certificate. This supplements the original type certificate, modifying it. Likewise, compliance with 91.213 meets the requirement to be "properly altered in a manner acceptable to the administrator" (FAA considerations here, again)...but one has to be careful that one is actually performing according to 91.213...which by default also includes Part 43, and in some cases additional regulation.