[Kyle Boatright]

When using Flight Following, ATC usually sees my altitude as up to 200' below what my altimeter is reading when adusted to the local altimeter setting. What would cause that and how do I correct it?

Some facts:

1) The transponder/encoder has always passed its biannual certification checks.

2) My pitot/static system is built per Van's plans.

 

[Kevin Horton]

Has the altimeter accuracy been checked? It is possible that the transponder and encoder are telling the truth, and that the altimeter has some instrument error. It may be a simple fix, as many (all?) sensitive altimeters have an adjustment that can be made to change the relationship between the altimeter setting indication and the mechanism inside the altimeter.

Maybe it is time to have an instrument shop hook their tester up to the altimeter and encoder at the same time and test them both.

 

[Kyle Boatright]

Has the altimeter accuracy been checked? It is possible that the transponder and encoder are telling the truth, and that the altimeter has some instrument error. It may be a simple fix, as many (all?) sensitive altimeters have an adjustment that can be made to change the relationship between the altimeter setting indication and the mechanism inside the altimeter.

Maybe it is time to have an instrument shop hook their tester up to the altimeter and encoder at the same time and test them both.

It just occurred to me that the altimeter was reworked a couple of years ago after it became "sticky". It was repaired by a certified shop and came back with calibration tests showing it to be within the accuracy limits specified in the "regs". In addition, the altimeter shows field elevation (plus or minus a few feet) when set to field elevation at the airport.

Since the altitude discrepancy existed both before and after the altimeter rebuild, I'm inclined to believe that the problem is somewhere else in the system. Maybe a leak in the pitot/static system...

Anyone got a DIY procedure for testing the integrity of the pitot/static system?

 

[WiseGuy]

Set altimeter to 29.92 and they should match.
Your transponder gives Pressure altitude!

 

[robertahegy]

Wiseguy is exactly right. If your altimeter is not set at 29.92, it should not agree with the encoder. The encoder is set at 29.92 and should always transmit pressure altitude.

Roberta

 

[Kyle Boatright]

Wiseguy is exactly right. If your altimeter is not set at 29.92, it should not agree with the encoder. The encoder is set at 29.92 and should always transmit pressure altitude.

Roberta

Yes, but...

ATC corrects the transponder reading by whatever correction factor is necessary to account for the local atmospheric pressure:

http://bathursted.ccnb.nb.ca/vatcan/fir/moncton/WeeklyTopics/Archives/20040620/CurrentTopic.html

So, if everything (my altimeter, transponder, encoder, pitot static sytem, etc) is working properly, ATC's altitude readout should correspond to what I see on the altimeter.

 

[aadamson]

When using Flight Following, ATC usually sees my altitude as up to 200' below what my altimeter is reading when adusted to the local altimeter setting. What would cause that and how do I correct it?

Some facts:

1) The transponder/encoder has always passed its biannual certification checks.

2) My pitot/static system is built per Van's plans.

Kyle,

There is some nice tongue in cheek comments below. So in order to help you sort this out, I thought I'd translate.

a) yes, your transponder will *display - if you have a Garmin* the altitude that is coming from the Encoder, which is hard set to 29.92. This means that the only time that your altimeter, set with something other than 29.92, will match your transponder, is when you have your altimeter also set to 29.92.

b) However, with the above said. In real life, ATC will correct your radar return and mode C/S data with the current Baro for your location and if you are up flying and ask them for a Mode C readout on you, the altitude that they give you back, *should* match (pretty close), what your altimeter says. (leave your baro set to whatever you took off with, or what is the current atis baro for where you are flying).

So, in summary. If you want to make sure that your Altimeter and Transponder are accurate. Go up on a VFR flight, outside of the Atlanta Class B area, and make sure you have no other traffic around you, then adjust your altimeter to 29.92 and press the button on your garmin transponder so that it read pressure altitude (I'm assuming that you have a Garmin and it seems most do now a days - applicable to the 327/330, 320 doesn't have a display). They should read the same. Set your baro back to whatever on the altimeter and go have fun flying. Or you can calso ask for a Mode C readout from ATC. If you do this, *do not* adjust your altimeter to 29.92, leave it at whatever is the local setting. For example at LZU, which you and I are both out of, the tower can give you a Mode C report. This is helpful, dial up the atis, set the baro on the altimeter. Then ask the tower for your Mode C readout. If they are off by a bunch, something is the matter, if they are the same, see the last step above and have fun! /

Hope this helps.

 

[w1curtis]

In addition to you transponder/encoder check during 91.411 (IFR) certification your static system and altimeter should also be checked (and certified) during this certification. If you only performed a 91.413 (VFR) certification, it may not have been checked. Also, if your aircraft is certified for IFR (91.411) you static system is required to be re-certified any time there is worked performed on it. But as mentioned above, set you altimeter to 29.92 and it should match what is being reported by the transponder. I don't see what type of transponder you have. If your transponder does not display its pressure altitude (PA), a GPS connected to it would also show the reported PA.

 

[Mike F]

Part of the initial VFR and recurring IFR certification process is to check the correlation between the altimeter and encoder. The maximum allowable difference is +/- 125' at all altitudes up to the service ceiling. Sounds like your encoder has wandered a bit and probably needs a slight adjustment. The best way to do this is to pull the altimeter and encoder and calibrate them on the bench using the proper test equipment.

 

[Kyle Boatright]

Kyle,

There is some nice tongue in cheek comments below. So in order to help you sort this out, I thought I'd translate.

<snippage>

So, in summary. If you want to make sure that your Altimeter and Transponder are accurate. Go up on a VFR flight, outside of the Atlanta Class B area, and make sure you have no other traffic around you, then adjust your altimeter to 29.92 and press the button on your garmin transponder so that it read pressure altitude (I'm assuming that you have a Garmin and it seems most do now a days - applicable to the 327/330, 320 doesn't have a display). They should read the same. Set your baro back to whatever on the altimeter and go have fun flying. Or you can calso ask for a Mode C readout from ATC. If you do this, *do not* adjust your altimeter to 29.92, leave it at whatever is the local setting. For example at LZU, which you and I are both out of, the tower can give you a Mode C report. This is helpful, dial up the atis, set the baro on the altimeter. Then ask the tower for your Mode C readout. If they are off by a bunch, something is the matter, if they are the same, see the last step above and have fun! /

Hope this helps.

I was in the Class B yesterday when I was reminded of the discrepancy. I got the ol' "Your transponder shows 5,300', is your altimeter set to 30.4, and if so, what does your altimeter show?" as I was transitioning right across the top of Hartsfield.

The *most* interesting thing about yesterday's flight was the course and altitude change right as the batteries in my handheld went tango uniform. Of course, I hadn't been paying attention to my compass heading, so I was a motivated battery changer. Unfortunately, the new batteries were in a Kyle proof package (one of those bubble packs that would survive a trip through an industrial shredder), so it was an interesting couple of minutes. I think I invented a couple of new swear words...

 

[w1curtis]

Part of the initial VFR and recurring IFR certification process is to check the correlation between the altimeter and encoder. The maximum allowable difference is +/- 125' at all altitudes up to the service ceiling. Sounds like your encoder has wandered a bit and probably needs a slight adjustment. The best way to do this is to pull the altimeter and encoder and calibrate them on the bench using the proper test equipment.

Mike,

As noted above, there is no requirement to check the altimeter in 91.413, the VFR certification. Only the encoder and transponder. The requirement to check the altimeter, is only in 91.411, the IFR certification. That is one of the reason why a VFR certification is cheaper than and IFR certification. If your aircraft is VFR only, there is usually no need to pay more for the 91.411 certification.

 

[Kevin Horton]

Since the altitude discrepancy existed both before and after the altimeter rebuild, I'm inclined to believe that the problem is somewhere else in the system. Maybe a leak in the pitot/static system...

Anyone got a DIY procedure for testing the integrity of the pitot/static system?

The standard static system leak test is described in FAR 23.1325(2)(i):

(i) Unpressurized airplanes. Evacuate the static pressure system to a pressure differential of approximately 1 inch of mercury or to a reading on the altimeter, 1,000 feet above the aircraft elevation at the time of the test. Without additional pumping for a period of 1 minute, the loss of indicated altitude must not exceed 100 feet on the altimeter.

I've done this by using a piece of tape to cover one static port. I pushed a piece of soft rubber hose over the other static port, and sucked on it until the altimeter increased more than 1000 ft. Pinch the tube shut, and keep it pushed tightly over the static port. Watch the altimeter, and it shouldn't drop more than 100 ft in one minute.

Please confirm that the encoder is plumbed into the static system, rather than being vented to the cockpit.

If your transponder will show the pressure altitude, I agree with the other posters who suggest a test where you compare that reading against the altimeter indication with 29.92.

 

[Mike F]

As noted above, there is no requirement to check the altimeter in 91.413, the VFR certification.

William,

Read 91.413(b). I perform 91.411 and 91.413 inspections and always check the data correspondence on new VFR installations. If you change out the encoder or altimeter, it should be checked again. If nothing is changed, the recurring VFR inspections do not require retesting of the altimeter/encoder correlation but I usually spot check it to make sure it's still within limits.

91.413(b): Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), Appendix E of part 43 of this chapter.

43 E(c): Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Integration Test. The test must be conducted by an appropriately rated person under the conditions specified in paragraph (a). Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C at a sufficient number of test points to ensure that the altitude reporting equipment, altimeters, and ATC transponders perform their intended functions as installed in the aircraft. The difference between the automatic reporting output and the altitude displayed at the altimeter shall not exceed 125 feet.


​

 

[w1curtis]

William,
Read 91.413(b). I perform 91.411 and 91.413 inspections and always check the data correspondance on new VFR installations. If you change out the encoder or altimeter, it should be checked again. If nothing is changed, the recurring VFR inspections do not require retesting of the altimeter/encoder correlation but I usually spot check it to make sure it's still within limits.

91.413(b): Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), Appendix E of part 43 of this chapter.


43 E(c): Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Integration Test. The test must be conducted by an appropriately rated person under the conditions specified in paragraph (a). Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C at a sufficient number of test points to ensure that the altitude reporting equipment, altimeters, and ATC transponders perform their intended functions as installed in the aircraft. The difference between the automatic reporting output and the altitude displayed at the altimeter shall not exceed 125 feet.

​

Non-sequitur! OK, where does it say Altimeter? It only talks about the Transponder/encoder. The integrated system it is talking about is the encoder/transponder system. While you may perform beyond the regulation, the requirements of the regulation is quite clear. The VFR certification, 91.413, only seek to ensure that your encoder and transponder meets a certain spec. It does not say anything about the altimeter, unlike 91.411.

 

[Mike F]

91.413(b): Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), Appendix E of part 43 of this chapter.​

The "integrated system" is, indeed, the transponder, encoder and wiring.
"Data correspondence" is the relationship between the altimeter and encoder.

If you "install" a new integrated system or you perform maintenance that might affect data correspondence, you must comply with 43, App "E", paragraph c, to make sure the data correspondence error between the altimeter and encoder is within limits (+/- 125').

 

[Bryan Wood]

When you set the altimeter to the field elevation does the barometric pressure match what the atis is giving you? There is a user adjustment on the front of your altimeter that can make up 200' if you determine it is the altimeter. To accomplish this you will see a small flush screw on the front lower left area on the altimeter. Back this screw out, but don't take it out all the way. You will need the threads to be engaged. At some point you will be able to slide the screw to the left which pulls the piece that it is attached to dis-engating the baro from the altimeter. You can now make changes with the knob and then re-engage it. It will make sense once you start to do it. Good luck.

 

[w1curtis]

91.413(b): Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), Appendix E of part 43 of this chapter.



​

The "integrated system" is, indeed, the transponder, encoder and wiring.

"Data correspondance" is the relationship between the altimeter and encoder.

If you "install" a new integrated system or you perform maintenance that might affect data correspondance, you must comply with 43, App "E", paragraph c, to make sure the data correspondance error between the altimeter and encoder is within limits (+/- 125').

This may be splitting hairs and I still contend that a recurrent 91.413 certification does NOT include a check of the altimeter but the reason I said it was non-sequitur was if you check FAR 43.1(b) it specifically excludes aircraft with experimental airworthiness certificates. While part 91 definitely applies to experimentals, part 43 specifically excludes it.

We will have to agree to disagree on the meaning of "integrated system" and "data correspondence." My contention is, since the altimeter is specifically spelled out in 91.411 they could just as easily have spelled it out in 91.413, but they did not. I guess that's why we have lawyers--to work out differences in interpretation.

 

[Mike F]

Please, no lawyers...! The regs can be confusing, that's for sure. Let me try to convince you one more time...

Part 91 applies to experimental aircraft. Part 43 does not. However, if a Part 91 regulation requires you to do something according to a procedure contained in Part 43, you must comply.

91.411 requires you to use the procedure outlined in Part 43, Appendix E.

91.413 requires you to use the procedure outlined in Part 43, Appendix F, and paragraph c of Appendix E.

There is no requirement to check the data correspondence between the altimeter and encoder on a recurring basis for VFR aircraft. It only needs to be done after a new installation or after a modification that could affect data correspondence.

 

[w1curtis]

OK, I see what's happening. I did not get you emphasis on *initial* certification and I was thinking *recurrent* certification. Got it now!