multiple altimeters in the plane, do all require certification?

[FORANE]

When there are multiple altimeters in the panel, is each and every one required to be certified when doing the biennial static check for IFR operations?
What if one of the altimeters is strictly backup and intentionally not connected to the static system in order to serve as a backup for frozen static altimeter failure?
What if it's plackarded vfr only or inop?
What is it's in an experimental?

 

[Witmo]

Assuming it's not in an aircraft requiring two pilots and the second altimeter is not plumbed into the primary's static plumbing, then no, it wouldn't need IFR certification if it's placarded VFR only.

 

[FORANE]

Assuming it's not in an aircraft requiring two pilots and the second altimeter is not plumbed into the primary's static plumbing, then no, it wouldn't need IFR certification if it's placarded VFR only.

Yes, assumptions are correct. Any chance of getting a reference where this may be found?
What about another altimeter that is plumbed into the static but plackarded vfr only? Would it too be required to pass certification?

 

[Bell206]

the second altimeter is not plumbed into the primary's static plumbing, then no, it wouldn't need IFR certification if it's placarded VFR only.

I know of guidance where separate altitude encoders don't need certification, but don't know of any guidance giving multiple altimeters a pass on IFR certs especially with 91.411 stating "each" altimeter. Would be interested in the reference as well.

 

[rotormatic1]

From AC 43.13-1B chapter 12:


12-19. ALTIMETERS. Aircraft conducting operations in controlled airspace under instru
ment flight rule (IFR) are required to have their static system(s) and each altimeter in- strument inspected and tested within the previ- ous 24 calendar months.

 

[FORANE]

So you guys are thinking it is against regs to have one of the altimeters as strictly backup and intentionally not connected to the static system in order to serve as a backup for frozen static altimeter failure even if it's plackarded vfr only or inop? How else would that altimeter be tested?

 

[flyer770]

The only references I’m aware of is if you’re intending to use it for IFR flight. Placard it “VFR only” and be done with it.

If you’re that concerned about altimeter failure to want a second one though, I’d rather have a heated pitot/static probe and connect it to the static system. Just having it open to cabin pressure will introduce an error as cabin pressure will be different than actual altitude depending on what vents you have open and how the air flows around the airframe.

 

[FORANE]

Just having it open to cabin pressure will introduce an error as cabin pressure will be different than actual altitude .

Yes, and the indicated airspeed will be slightly elevated also. But, the inaccuracies will not be too significant in my experience given they are known errors and the rationale for using cabin pressure in lieu of static pressure.
I like the idea of plackarding vfr only. Just trying to stay within the intent of regulations doing it.

 

[flyer770]

Placard, not plackard.

Just curious, why the push for having a second altimeter that isn’t connected to the regular static system?

 

[Bell206]

So you guys are thinking it is against regs to have one of the altimeters as strictly backup and intentionally not connected to the static system in order to serve as a backup for frozen static altimeter failure even if it's plackarded vfr only or inop?

The way 91.411 is written it states "each" altimeter requires the test for IFR. And there is no provision for an "VFR only" altimeter placarded or otherwise as all altimeters are considered "VFR" to begin with. As to "frozen" static issues, most aircraft (not all) that need this use a dedicated alternate static source valve plus there is guidance on how to perform the tests for this scenario whether the aircraft is pressurized or not. I believe its in AC 43-xx.

How else would that altimeter be tested?

The altimeter 43 App E tests can be performed on the bench or in the aircraft. If you read 91.411 it separates out each requirement altimeter, static system, ALT report system. Each has its own test requirement with the static system side a mechanic function is so desired. The AC number above I can't remember gives an explanation of this.

 

[FORANE]

Just curious, why the push for having a second altimeter that isn’t connected to the regular static system?

Have you ever experienced a frozen static system?
Often our actions are guided by past experiences.

 

[flyer770]

Have you ever experienced a frozen static system?
Often our actions are guided by past experiences.

Sure. Switch to alternate static source (usually the cabin), compensate for the minor pressure difference, no problem. Certainly no worries about placarding second altimeters. I’d rather go with the well accepted path of alternate static air than go into the minefield of worrying about what to label a second altimeter. My own experience is to keep things simple and well proven.

 

[MauleSkinner]

Sure. Switch to alternate static source (usually the cabin), compensate for the minor pressure difference, no problem. Certainly no worries about placarding second altimeters. I’d rather go with the well accepted path of alternate static air than go into the minefield of worrying about what to label a second altimeter. My own experience is to keep things simple and well proven.

Plus, an alternate static system will normally have a calibration chart to increase its accuracy.

If you don’t have alternate static plumbing, there’s also the old standby of breaking the glass on the VSI, but that does have limited effectiveness in an EFIS airplane.

 

[FORANE]

I have an alternate static port that opens to the cabin. No need to break glass...
It just seems much easier to recognize and respond to the failure when there's another EFIS already working.
Seems like a no brainer to me for one of the 3 to be plumbed this way, and given that I am the only pilot for this aircraft I wouldn't be too worried about what to label the non-primary backup EFIS.

 

[Jim Rosenow]

We've got 2 altimeters in the R182. When the co-pilot side went outta spec the avionics guys just placarded it 'vfr only'. Now I'm gonna have go crawl under there and see if it's plumbed in or open. Hadn't wondered before.

Logic says plumbed if it was certified?

Jim

 

[FORANE]

We've got 2 altimeters in the R182. When the co-pilot side went outta spec the avionics guys just placarded it 'vfr only'. Now I'm gonna have go crawl under there and see if it's plumbed in or open. Hadn't wondered before.

Logic says plumbed if it was certified?

Jim

I'm going to bet you are correct in your logic. Nothing wrong with that either in my opinion. I believe the spec is 25 foot error max. Also, depending on your avionics guy, he may or may not have added the typical vibration an altimeter sees in flight when doing the test. If an altimeter is tested in the plane it may not be shaken enough to have the needle settle to its most accurate reading.