Do both altimeters have to be of the same type?

[peter-h]

I have the KEA130A on the LHS and a similar looking "cheap" one on the RHS (United Instruments).

LHS:

RHS:

Both are as fitted in the factory, year 2002, and the aircraft was built to the US Type Certificate. It is currently N-reg.

I would like to replace the RHS one with the single pointer type e.g.

Is there any regulation prohibiting this?

My understanding is that you can install "anything" in the RHS panel (of a single pilot aircraft, of course) provided that

- the item is TSOd or PMAd
- the aircraft TCDS doesn't say anything to the contrary
- you aren't removing anything which is in the TCDS
- load analysis is OK, etc
- it is not EFIS (that needs an STC)
- there is no autopilot connection
- no existing AFMS needs altering as a result

What concerns me is whether there is a legal requirement for both altimeter types to match visually.

AIUI this would be a Minor Alteration because it is not a "basic change" to the static system and the function is equivalent.

 

[flyingron]

Why would both altimeters have to match? One might argue there are good reasons for them to NOT match.

TSO and PMA means they are legal to install in aircraft, it may not mean they are legal to install in YOUR aircraft. You don't indicate what AIRCRAFT you're talking about, but most of the small fry, your observations are correct provided you are not a commercial operator.

 

[James331]

Are you getting into the flight levels so often you need the other one.

Sounds like you're spending money and effort for the sake of spending money and effort.

 

[flyingron]

Well his first one is an encoding altimeter, so at the minimum he probably can get by with a non-encoding one (unless he also has two transponders or a way to otherwise use the second encoding unit).

Again, since he won't disclose the aircraft or the operations he's talking about it's impossible to make any statements about legality.

 

[Anticept]

Replacing instruments with a different part requires a Supplemental Type Certificate if it is not already approved for installation in the aircraft (as in, covered by the manufacturer's equipment list or prior STC for this aircraft), and doesn't have a PMA with an applicable direct replacement section (this information is contained in the PMA document).

Type Certificate Data Sheets (TCDS) won't list instrumentation. Long ago there were documents called "Aircraft Specifications", which DID list every single piece of equipment approved for an aircraft. It got cumbersome very quickly, so the TCDS system came about in the 1960's. TCDS only lists the major items, while things like specific instrumentation are covered by the equipment lists. TCDS can be altered using the Supplemental Type Certificate system.

As for altimeters: they don't have to be the same type unless an ops spec requires it, or there is some sort of installation limitation (such as encoder compatibility).

 

[peter-h]

It is a Socata TB20, Part 91.

Replacing instruments with a different part requires a Supplemental Type Certificate if it is not already approved for installation in the aircraft

Do you have a reference?

This is for the RHS.

 

[Anticept]

Doesn't matter if it's part 91. You can't just install any instrument on a standard airworthiness aircraft. It has to be approved for that aircraft and configuration, or have a PMA with the approvals in it. This isn't my burden to prove to you, it's you who has to prove to the FAA that you can make this ALTERATION in accordance with an approval. That's the key. The word, APPROVAL. Parts have approvals (PMA), the installers have approvals (certificates), and the installation has approvals (type certificates, STCs, PMA direct replacement, etc). You're still changing equipment in the aircraft, even if it were an optional secondary. (removal of a optional secondary would not require the major alteration procedures, since it's optional equipment, what is important is INSTALLATION of a different part). Optional is dictated by the equipment list in your AFM.

Now, if you are changing the RHS altimeter to match the make and model of the LHS, I'm 90% confident you can do a drop in replacement as long as the LHS doesn't place any limits on what the RHS has to be. This is because the new instrument is already approved for installation by the same paperwork that allows the LHS. A lot of instrumentation also has huge blanket approvals in the PMAs as well, or come with a blanket STC, making the 337 easy.

I will admit, I do have this little bug in the back of my mind itching saying there might be a hitch with this being a secondary instrument, and likely the secondary is not required for this type of aircraft. So I followed up with the FAA and will hear from them next week. The tech ops on the phone was in strong agreement, but said he wants to consult with one other person just in case.

EDIT: Just saw you said the RHS is equipped by the factory and made some edits. You better check your airplane equipment list in your airplane flight manual. Make sure that the RHS is labeled "optional equipment" or you will have to have an STC or PMA direct replacement part.

 

[peter-h]

Don't worry - I will forget this proposition.

 

[Anticept]

Don't worry - I will forget this proposition.

I make sure not to mention who or where to the FAA about my questions. I don't throw people under the bus unless I really think what you are doing is really unsafe (you'll get a warning from me first), and nothing you have said makes me think so

I'm still following up, because I like to CYA! Plus, I'm sure you would want to know!

 

[Anticept]

Don't worry - I will forget this proposition.

Following up, I made a couple calls to the FSDO and to an avionics shop.

Installation of avionics requires the same approval process as everything else, but once you have an approval for one item, you can put in as many as you want as long as there is nothing else throwing a wrench in the works.

So, here's what I would recommend:

1. Consult an avionics shop if you just want this mod done and don't care how. If you want to do your own research, proceed.
2. Consult your AFM equipment list to see if that right side altimeter is required or optional (specifically, that MODEL). If optional, proceed to next step. If it's required, it makes things complicated, consult an avionics shop and let them deal with it.
3. Consult the aircraft TCDS and make sure it's not required to have two differing altimeters. If it doesn't require two different alts, proceed to next step. If it does require two different alts, you've hit a wall and would need an STC at this point and things get really complicated. (I REALLY doubt a small airplane would require two altimeters, let alone two different models)
4. Make sure that altimeter on the right isn't required for, say, an autopilot system. It should only have static lines going to it, and maybe a couple wires for the light. This one you'll have to research on your own because there's enough AP systems to make your head spin.
5. Call the manufacturer to see if there was anything else that was missed, or just let an avionics shop deal with it .

I'm sure I missed something, someone else can chime in if I did . Really, just let an avionics shop put their name on it!