Question regarding error in altitude

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MikeLima

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MikeLima
I was going to transition class b airspace this morning at 7500 feet. I was on flight following. As I approached the airspace, approach cleared me in at 7500 feet. A few moments later approach contacted me and said (not an exact quote), " 182 I have you at 7800. Correct altimeter is 3042." I checked and had 3042 in already and was reading 7500. I replied as much and recieved nothing back.

I am low time and rural and don't fly in radar controlled airspace very often and this was the fist time this has occurred. Is this an atmospheric oddity or do I have an equipment problem and is there anything else I should have done?

Thanks.

ML
 
You may have an altimeter or an encoder issue. When on the ground at your home drome, set the altimeter and check to see if it is reading very close to the published field elevation. That could rule out or identify an altimeter problem. When flying, query ATC. "Ferndock Approach, I've had a report of an altimeter error. I am showing 7500, can you verify?" Or something like that. If your altimeter checked on the ground, an in-flight error might point to the encoder. This is all assuming your pitot/static check is up to date. If not, get it done.
 
If it was Memphis, I've noticed a fair amount of trainees used for flight following. Could be he was looking at your Mode C pressure altitude and didn't extrapolate/convert to MSL before speaking, and when he noticed his error, there was no need to respond.
 
Memphis TRACON encourages everybody in the area to use FF. Probably not a bad idea, actually. My take on the OP is that there may be something wrong with either his altimeter or encoder. I had a "call this number" incident (made me a little nervous!), and when I talked to the controller, he said I was in his airspace and he realized I probably had an encoder problem when he was reading 7,500 msl and I was in the pattern (1,200msl) at West Memphis. He gave me a few hints to try and said use FF and they would help me diagnose the problem. I did what he suggested, went up and talked to them, and wound up replacing the encoder (about $300 as I recall).
 
A mode C reading of 200' in error that's different than what your altimeter indicates is within limits for ATC, I've had it happen several times. If it happens one time and with only a certain ATC sector, than its probably just the controlling facility. If its a consistent issue with multiple ATC facilities and your getting the same error, than it's probably you.

Also, try cleaning off your antennas as a dirty antenna can mess with the mode C.
 
Work with the controller,then go see a good avionics shop.
 
What model transponder do you have? Some of the newer ones will display the pressure altitude from the encoder. At any altitude that should be close to (i.e within 100 ft) what your altimeter says if you set 29.92 in the Kollsman window.
 
weirdjim said:
Do you really believe that? It is an interesting heresy, I'll admit.

Jim
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Ever heard of the rusty bolt effect or passive intermodulation? Can't hurt I suppose, but it may just be a bunch of hogwash.
 
MikeLima said:
I was going to transition class b airspace this morning at 7500 feet. I was on flight following. As I approached the airspace, approach cleared me in at 7500 feet. A few moments later approach contacted me and said (not an exact quote), " 182 I have you at 7800. Correct altimeter is 3042." I checked and had 3042 in already and was reading 7500. I replied as much and recieved nothing back.

I am low time and rural and don't fly in radar controlled airspace very often and this was the fist time this has occurred. Is this an atmospheric oddity or do I have an equipment problem and is there anything else I should have done?

Thanks.

ML
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The controller should have had you stop mode c with an error of 300 feet.

There are 8 points of possible failure between your altimeter and the controller's mode c read out in the displayed data block readout. That is why we verify mode c.

tex
 
JohnAJohnson said:
If it was Memphis, I've noticed a fair amount of trainees used for flight following. Could be he was looking at your Mode C pressure altitude and didn't extrapolate/convert to MSL before speaking, and when he noticed his error, there was no need to respond.
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There is no such thing as using trainees for flight following. One controller (or a trainee and a qualified controller) works all of the traffic in the assigned airspace. The radar automation system applies the local altimeter setting to the pressure altitude transmitted by the transponder and displays the resulting altitude.
 
I had the SAME exact issue that was brought to my attention during Bravo transitions. 300' feet off at 6500'

Ended up being a simple static line leak. Find a good avionics shop and it should be an easy diagnosis.
 
JohnAJohnson said:
If it was Memphis, I've noticed a fair amount of trainees used for flight following. Could be he was looking at your Mode C pressure altitude and didn't extrapolate/convert to MSL before speaking, and when he noticed his error, there was no need to respond.
Click to expand...
Controllers don't "do the math" on that. The Altimeter setting is entered into Radar system and it "does the math." Many, if not most Radars nowdays have the setting entered automatically. On some of the older ones the controller does it manually. That the controller didn't tell him to "stop altitude squawk" tells me that the "bad readout" they were getting got resolved on their end. They are required to have you stop altitude squawk if it differs by more than 200 feet from the altitude you say you are at. Verifying that your Mode C readout is correct is the primary reason we check in with our altititude when getting transferred to a new controller.
 
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That's why controllers have one of those metal calculators while on position. ;)
 
Shawn said:
Ended up being a simple static line leak. Find a good avionics shop and it should be an easy diagnosis.
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That would be my guess and they can be a real pain to track down (in large aircraft), a slightly loose fitting, cracked hose, pinhole in a tube, seen it all. Just need a pitot static tester and some caps and plugs. Start isolating.
 
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Do you have the deviation sheet from the last static certification? It should show how far off the altimeter reads at different pressure altitudes.
Also, when was your last static (and xpdr) cert?
I'd first talk to the shop that did the certification.
 
GlennAB1 said:
I have never seen an altimeter deviation sheet.
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I assume you are joking, but can't see the humor. Every biennial pitot-static transponder check produces one of those sheets, which is attached to your aircraft log. If it's out of tolerance or missing, you can't fly IFR.
The "deviation" measured is the difference between your readout vs. the one on the calibrated altimeter owned by the avionics shop (typically).
 
GlennAB1 said:
I work for an airline. We do the required checks and call it good or fix it, no sheet!
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What exactly is the testing session recorded on? Don't you record the resulting error of each test data point?


I've never seen a certification of a pilot's altimeter, encoder, ADC or compass without it or similar record.

 
GlennAB1 said:
Please show me the FAA records requirement for altimeter and transponder test and inspection.
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So what you're saying is you just look at the numbers on the air data test set and if good, move to the next point. Nothing in writing other than a stamp at the end. No checklist or anything?
 
There is no need/requirement to record deviation that is within tolerance. Don't confuse your repair station manual requirement with FAR. We are allowed to exceed FAR requirements.
 
Well can't speak for airline maintenance procedures but filling out the deviation sheet seems pretty standard for avionics shops so my guess is there's something directing them to do it. In any event, as an aircraft owner I appreciate getting the record of how my system performed whether there's a requirement to record it if not even though I've been present both times the 91.411 and 413 checks were performed on my plane.
 
weirdjim said:
Do you really believe that? It is an interesting heresy, I'll admit.

Jim
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higher frequencies are more affected by stuff in the way of the signal, than the lower frequencies. Given the length of a transponder antenna, I would suspect it to transmit on a fairly high frequency, in the cm range. And as a result stuff on the antenna can cause problems. mostly weaker signal though.
 
RotorDude said:
I assume you are joking, but can't see the humor. Every biennial pitot-static transponder check produces one of those sheets, which is attached to your aircraft log.
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Nope.

Of the three places that I've had this done, exactly zero of them gave me a deviation sheet.

If it's out of tolerance or missing, you can't fly IFR.
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I Follow Roads all the time.
 
GlennAB1 said:
There is no need/requirement to record deviation that is within tolerance. Don't confuse your repair station manual requirement with FAR. We are allowed to exceed FAR requirements.
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That's just it, who is doing these inspections on these small personal aircraft? 145 certificate holders.
 
I can understand repair stations retaining that record, even providing a copy to the owner as a courtesy. Just not an FAA requirement, unless it's in their RSM to do so.
 
bluerooster said:
higher frequencies are more affected by stuff in the way of the signal, than the lower frequencies. Given the length of a transponder antenna, I would suspect it to transmit on a fairly high frequency, in the cm range. And as a result stuff on the antenna can cause problems. mostly weaker signal though.
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Sometimes I just log into this group for laughs like this.

The ground radar interrogates the aircraft transponder on 1030 MHz. and the transponder replies on 1090 MHz. Splitting the difference and converting to wavelength we can say that this is a 30 cm (28.3 to be precise) signal.

When the transponder replies, it does so with two framing pulses at the beginning and end of this digital string: a series of 12 pulses each 0.45usec wide separated by 1.45usec. Since 2^12=4096, we say that there are 4096 separate squawk codes available. The ident pulse, if it replies, does so 4.35usec after the last discrete code pulse. Another interrogation, another burst of pulses from the transponder (which also flashes the little blinkie green light on the transponder).

If there is an altitude encoder, the transponder immediately replies with another two framing pulses with the same exact information coding system but leaves out the D1 pulse to identify the response as altitude as opposed to discrete code.

Now, rather than take wild a$$ guesses as to how oil on the transponder antenna will magically let the discrete code through yet scramble the altitude code that comes through microseconds later, why not tell me from your technical background how you think this might happen? I've only been designing aircraft electronics for 50+ years, so I sure don't know everything. Tell me, how do you see this happening?

Jim
 
GlennAB1 said:
I can understand repair stations retaining that record, even providing a copy to the owner as a courtesy. Just not an FAA requirement, unless it's in their RSM to do so.
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I can't imagine any (sane) pilot who flies single pilot single engine IFR in IMC (including low approaches) not having in his/her aircraft logbook the latest deviation sheet along with the pitot-static leak, transponder check, etc. When you fly IMC, you trust your life (and that of your pax) to the viability of the aircraft, including the altimeter.
 
If it's that important, shouldn't you have an altitude correction card next to your altimeter, like the stby compass. I don't think rooting around in your logbook in IMC is a great idea.

And, did you just insinuate Capt. Geoff is insane?
 
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Ryanb said:
Ever heard of the rusty bolt effect or passive intermodulation? Can't hurt I suppose, but it may just be a bunch of hogwash.
Click to expand...

Of course. Rusty bolt, if rusted in JUST the right proportions with JUST the right amount of pressure makes a ferric oxide half-a$$ed semiconductor and if whapped with enough power can turn into a varactor parametric mixer which will produce passive intermod (or crossmod, depending on a lot of factors beyond the scope of today's lesson).

The problem, of course, is that loosening the bolt to investigate destroys the PN junction, as does moderate vibration. The effect HAS been reported, although I'd more expect to see it in the technical periodical, "Journal of Irreproduceable Results".

Last I looked oily antennas don't cause rust.

Jim
 
GlennAB1 said:
If it's that important, shouldn't you have an altitude correction card next to your altimeter, like the stby compass. I don't think rooting around in your logbook in IMC is a great idea.
And, did you just insinuate Capt. Geoff is insane?
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No, nobody (sane) routinely carries the aircraft logbooks in the plane (they are worth too much to risk loss or damage), and this is not a "correction card" like the compass, it's just a deviation report, which shows that your altimeter is reasonably calibrated and doesn't leak or stick excessively (all within a legally specified tolerance). You want it because that's proof that a certificated professional diligently performed this work. After all, it's your own backside that will be strapped to a machine that's hurtling towards the ground in low IMC where the only thing stopping you from crashing might be the altimeter, whose validity is attested to in that report.
 
JohnAJohnson said:
If it was Memphis, I've noticed a fair amount of trainees used for flight following. Could be he was looking at your Mode C pressure altitude and didn't extrapolate/convert to MSL before speaking, and when he noticed his error, there was no need to respond.
Click to expand...
Displayed mode c is automatically corrected for local pressure before being displayed, the controller does not have to do anything except make sure the correct local altimeter setting is entered into his system. Which he does not do, it's automated and can be corrected.
 
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