Bill,

Your statement " Your statement that TSO is required for legal flight is simply untrue." is simply untrue. What I did say was "You MUST have at least one of each of the required instruments and they MUST meet the TSO."

You are confusing a "TSO" with a "TSO Authorization" and a "TSO Design Approval". See FAR 21.601.

If you want to install an Altimeter in your airplane, and you intend for it to satisfy the altimeter part of the "Required Equipment" then to be considered an "Altimeter" it must meet the TSO for "Altimeter" because that is how the FAA defines "Altimeter". It does not matter if the altimeter was manufactured at a facility that has TSO Authorization or the altimeter has TSO Design Approval. It does not need a TSO sticker on the back (Experimental) but it still must meet the minimum functional and performance requirements specified in the TSO.

Show the picture below to your avionics guy. Tell him you intend to install this instrument in your aircraft, plumb it into the static system and use it to satisfy the "Altimeter" minimum equipment requirement. It is, after all, an Altimeter that tells you your altitude at least as well as a GPS. Being an expert, he will tell you that this instrument does not satisfy the requirement  because it has insufficient resolution, it does not have barometric compensation and it does not have the required accuracy. Ask him to show you where it says that an "Altimeter" must have these features and characteristics and he will show you the TSO-C10b for "Altimeter". QED

Not willing to take the advice of an expert, you proceed to install the altimeter in the experimental aircraft you are building. You call for a FAA inspection to issue the Airworthyness Certificate. He refuses because you do not have an altimeter that meets the minimum performance requirements. Where are those minimum performance requirements found? In TSO-C10b.

Now you are really pissed off and want to prove your point so you purchase a sensitive altimeter with a baro adjustment that does NOT have a TSO sticker. You install same, perform a Pitot Static ramp check per FAR 91.411 and call the FAA inspector back. He now looks at the new altimeter and sees that it has sufficient resolution and a baro adjustment capability and you show him the Pitot Static test results and he issues the Airworthyness Certificate. Why? Because you have demonstrated that the Altimeter meets the requirements set forth in TSO-C10b.

You MUST have at least one of each of the required instruments and they MUST meet the TSO, regardless of whether the manufacturer has TSO Authorization or the altimeter has TSO Design Approval. You could build the altimeter yourself out of scrap metal  and it would likely be legal to use in an experimental aircraft IF you could demonstrate that it meets TSO-C10b.

To further confuse the issue, TSO compliance has nothing to do with reliability. If you have TSO Authorization it only means you are repeatable in your methods and practices and keep sufficient records. You could be doing it wrong but the FAA is satisfied you do it wrong every time.

The first step in demonstrating reliability is DO160 environmental testing. The next is proper systems design and integration. After that is proper operation and maintenance.

With regard to vacuum driven gyros (a misnomer as they are actually driven by a pressure differential), they may indeed be less reliable than a battery backed up electric spinning mass gyro, however because it is and entirely separate system operating on different physics it is highly unlikely that you will get simultaneous EFIS and vacuum gyro failures.

In my aircraft I have an electric spinning mass AH with its own battery. In a friend's IV-P, he wanted a vacuum standby so we designed it so that in the event of a vacuum pump failure the differential cabin pressure would run the gyro.

Bill and I agree that replacing the vacuum pump for a standby alternator is a fair trade, but replacing a spinning mass AH with an experimental EFIS? No way. Depending on the panel, it may not even be legal for IFR flight.

Regards
Brent Regan