Steve writes:

<<Does your e-mail mean that the following document (supposedly from the EAA) is all wrong (see attached full document)?:>>
<snip>
<<Where is the linkage in the reg's or other pronouncements which link 'altimeter' to TSO for a Part 91 operator?>>

The document Steve attached says basically the same thing as the point I was trying to make. I'll try again.

There is a difference between "meeting the requirements of a TSO" and being "TSOed". In the first case I could build an altimeter, test it in an appropriate manner and demonstrate that it meets or exceeds the minimum performance standards set out in the TSO and then go on to install it in my airplane and be IFR legal with regards to the "Altimeter" requirement of 91.205. In the latter case I can go and buy an altimeter that has a TSO "sticker" and has, therefore, been tested by the manufacturer and been demonstrated to the FAA that it meets the minimum standard of performance to earn the title "Altimeter".

You do NOT have to use TSOed (instruments that have a TSO sticker) instruments in your experimental airplane. You DO have to use instruments that meet or exceed the minimum performance standards as defined by the FAA (in the TSOs). In any case, at some point the instrument that you put in your airplane that you want to use as an altimeter to meet the requirements of 91.205 must have demonstrated that it deserves to be called an "Altimeter".

Suppose I take a barometer and mark the face crayon so it now reads in feet. If I climb up a mountain the barometer will tell me my approximate altitude. Is this modified barometer an altimeter? According to Webster it is but according to the FAA it is NOT because it does not meet the minimum performance requirements of an "Altimeter". Anyone can call anything an "altimeter" but when the FAA says "altimeter" a little bubble forms over their head with TSO-C10b floating inside. They do not need to say "altimeter that conforms to TSO-C10b" because in the FAA's world there is no such thing as an altimeter that does not conform to TSO-C10b because if it does not conform it is not an "Altimeter".

The link between part 91 to TSOs is in the definition of the names of the instruments listed in 91.205. The TSO is where the word "Altimeter" is defined in terms of function and performance.

Let us say that you just greased a three pointer after zooming out of the clag from minimums. As you tie down your bird the local FAA dude comes walking up with 91.205 tucked under his arm and "ramp check" written on his forehead. "Show me your altimeter" comes the decree.  You can:
1) point to the TSOed  standby altimeter. QED
2) point at a dark piece of glass and try to explain that it "really is" an "altimeter" and have the documentation on hand to prove it.
3) dissolve into the puddle of urine that is mysteriously forming at your feet.

If you plan to use answer # 2 be advised that the biannual Pitot Static inspection will likely NOT satisfy the inspector.

Are TSOed instruments required? NO.

Is it a good idea to have enough TSOed instruments in the panel  to satisfy 91.205 if you plan to fly IFR? Absolutely. It is the simplest, easiest, cheapest and safest thing to do.

Regards
Brent Regan