Altimeter Anomaly

Shawn

Shawn

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Shawn
I will open this question with the fact that I already have a trip to the avionics shop scheduled and gonna have it looked into...but had an altimeter issue today that I am not quite sure what the issue is.

Cursing along at 7500'. 1973 Cessna 182P with a steam gauge altimeter and also and Aspen partial glass panel with a digital altimeter tape. ATC comes on and asks "verify that you are at 7500...local altimeter setting at 29.97"

OK, I was being lazy and a bit high by 100'...Look at altimeter...yup, 7600, look as Aspen, yup 7600, look at setting, yup, 29.97.

"ATC: I show you at 7900"...huh?

Look over at transponder and it is showing a pressure altitude of 78. Pull up GPS altitude and it reading 7900.

Realize essentially that there is a 300' difference between what ATC sees and what my altimeter and Aspen read even when I got reestablished at 7500' according to my altimeter. I got this call once before cruising at 6500' through Bravo airspace...off by about 300' but reading correctly.

Had a CFI with me and we talk through possible issues. I do check field elevation as part of my preflight check once the local altimeter setting is entered and sure enough when we got back, it was spot on on the ground. Both of us couldn't some up with anything obvious.

...and it is not consistent...have had ATC call out our altitude as traffic advisories to other aircraft and it is spot on at all different altitudes.

Other than it is just outta whack at altitude...any glaringly obvious reasons for the discrepancy?

On another note how does ATC read your altitude in their system? As I understood it, your transponder sends out your pressure altitude and then it is adjusted for local altimeter settings on their end...is that correct?
 
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The mode C altitude reported is based on 29.92, the correction is made for local altimeter in the ground computers before displaying to the controller.

Why some report 300 ft errors and others do not? Hard to say.
Is it after a long flight? Could be the alt encoder is getting errors if it overheats?
Bench test for a duration in a heated environment by an avionics shop might be in order.
 
BillTIZ said:
Is it after a long flight? Could be the alt encoder is getting errors if it overheats?

Bench test for a duration in a heated environment by an avionics shop might be in order.
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First time it was right after a climb out from departure airport, but today was after a few hours in the air. Both days were pretty mild temps bit good idea to test.
 
Tell avionics shop to ditch the encoder and use the Aspen for encoding - should have been done during the instal anyway (my shop skipped the step also).
 
One day we will look back at pressure altimeters in the same light as doctors who thought blood letting was a good idea. GPS altitude reporting will become the norm for all aircraft, and the sooner the better IMHO.

Take a look at your GPS altitude (usually accurate within 10') and your stone age altimeter. Why do we use the more inaccurate system? :rolleyes:
 
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Altimeters do fail. I was coming home one day after a trip under the Atlanta airspace. Visually descended to pattern altitude and the altimeter was showing 1,000 higher than my eyeballs said. Landed and it was showing 2,000' higher than the ground elevation.

A week and a couple of hundred dollars later, a repair shop in San Antonio returned the repaired altimeter, complete with current bench test.

I was just glad that the stuck altimeter didn't create an airspace bust.
 
Geico266 said:
One day we will look back at pressure altimeters in the same light as doctors who thought blood letting was a good idea. GPS altitude reporting will become the norm for all aircraft, and the sooner the better IMHO.

Take a look at your GPS altitude (usually accurate within 10') and your stone age altimeter. Why do we use the more inaccurate system? :rolleyes:
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Are you serious?
 
GPS is **NOT** accurate to 10' in the vertical. The FAA won't even let you use just the GPS as vertical guidance on an approach by itself.
 
WAAS GPS is very accurate for true altitude. Unfortunately, that's not how traffic is separated in the National Airspace System. Variations in temperature or pressure from the standard lapse rates are not accounted for by our pressure altimeters, so when we see 7500 on the altimeter with the local altimeter setting, odds are we're not really at 7500 MSL. That's OK for traffic separation, since we're all using the same local altimeter setting so we're all off by the same lapse rate error, but if you set your altimeter to match GPS altitude in cruise, you won't be where everyone else expects you to be.

Further, those errors disappear as your height above the location where the local altimeter setting was taken goes down, which is why it's OK for us to use LPV approaches. Note that this is also the reason you have to use an altitude correction factor from the AIM Table 7-2-3 for MDA/DA when the temperature at your destination is well below freezing. In fact, Transport Canada, requires that you have that table in the cockpit for winter ops in the northern regions.
 
flyingron said:
GPS is **NOT** accurate to 10' in the vertical. The FAA won't even let you use just the GPS as vertical guidance on an approach by itself.
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You never heard of GPS approaches? :dunno:

Even my hand held is accurate to within 3-7 feet. I've used it for preliminary land survey work. The instruments in our airplanes are from the stone age.

My AP will fly the GPS RNAV approach to the runway. Try that with an altimeter sensitive to airpressure. :eek:
 
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Ron Levy said:
WAAS GPS is very accurate for true altitude. Unfortunately, that's not W traffic is separated in the National Airspace System. Variations in temperature or pressure from the standard lapse rates are not accounted for by our pressure altimeters, so when we see 7500 on the altimeter with the local altimeter setting, odds are we're not really at 7500 MSL. That's OK for traffic separation, since we're all using the same local altimeter setting so we're all off by the same lapse rate error, but if you set your altimeter to match GPS altitude in cruise, you won't be where everyone else expects you to be.

Further, those errors disappear as your height above the location where the local altimeter setting was taken goes down, which is why it's OK for us to use LPV approaches. Note that this is also the reason you have to use an altitude correction factor from the AIM Table 7-2-3 for MDA/DA when the temperature at your destination is well below freezing. In fact, Transport Canada, requires that you have that table in the cockpit for winter ops in the northern regions.
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This.

Which is why, someday, pilots will look back at the instruments we now use for altitude and wonder..... "How did they ever survive?"
 
Geico266 said:
One day we will look back at pressure altimeters in the same light as doctors who thought blood letting was a good idea. GPS altitude reporting will become the norm for all aircraft, and the sooner the better IMHO.

Take a look at your GPS altitude (usually accurate within 10') and your stone age altimeter. Why do we use the more inaccurate system? :rolleyes:
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All fine and great until your GPS ****s out, which could be your equipment, could be interference, could be a failure of the entire system...If this occurs on a 200 overcast day with a quarter mile of visibility you're going to be really ****ed off that you pulled your static altimeter which is incredibly reliable and dependent on nothing.
 
Also I would hardly call a pressure altimeter unreliable or not precise.
 
jesse said:
All fine and great until your GPS ****s out, which could be your equipment, could be interference, could be a failure of the entire system...If this occurs on a 200 overcast day with a quarter mile of visibility you're going to be really ****ed off that you pulled your static altimeter which is incredibly reliable and dependent on nothing.
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Multiple satellite failures and device failures are highly unlikely. I carry 3 of them. The chances of that happening are as close to zero as you can get.

You are too young to be tied to and rely on old school, outdated technology when better and more accurate instrumentation is available. The day will come when we will be required to have more accurate vertical reporting. The old altimeter will go the way of the buggy whip as aviation progresses.

I remember when CFIs would not allow GPS in the cock pit. :lol:
 
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Geico266 said:
Multiple satellite failures and device failures are highly unlikely. I carry 3 of them. The chances of that happening are as close to zero as you can get.

You are too young to be tied to and rely on old school, outdated technology when better and more accurate instrumentation is available. The day will come when we will be required to have more accurate vertical reporting. The old altimeter will go the way of the buggy whip as aviation progresses.

I remember when CFIs would not allow GPS in the cock pit. :lol:
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I've lost gps coverage and integrity on more than occasion. The technology isn't something you bet your life on yet. You'll pry my pressure altimeter out of my cold dead hands :)
 
jesse said:
I've lost gps coverage and integrity on more than occasion. The technology isn't something you bet your life on yet. You'll pry my pressure altimeter out of my cold dead hands :)
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Might have to after you run into someone because you were 500' off. ;)

The future demands more accurate altitude reporting. The voodoo we use now just won't cut it in the future.
 
Pressure altitude is voodoo?? I'm beginning to question if this is just Geico having fun with our reactions.
 
What kind of interface do you have between the encoder and the transponder? If it's the Gillham grey code, a bad pin on either end of the harness will give you an error in the transponder output that could be on the order of hundreds of feet (depending on the pin) sometimes and no error at others.
 
Geico266 said:
Might have to after you run into someone because you were 500' off. ;)

The future demands more accurate altitude reporting. The voodoo we use now just won't cut it in the future.
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I don't think the amount of error you're claiming is the reality of pressure altimeters. When everyone is playing on the same page, the local altimeter setting, it's plenty reliable enough for traffic separation. Yes it won't say precisely what your GPS altitude says but it's precise enough since the other airplanes experience the same error. It's also plenty reliable enough for terrain separation on approaches that are rather close to the ground. Temperature extremes can tweak that a bit but isn't a factor for the vast majority of operations.

I think your opinion of pressure altimeters would change if we started your instrument training :)
 
Pedals2Paddles said:
Pressure altitude is voodoo?? I'm beginning to question if this is just Geico having fun with our reactions.
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Take your GPS and check it against your altimeter. Go from ground level to 12,500' msl. Do it over a couple months and see how tempersture and pressure changes the readings (even when adjusted for) on your old school altimeter -v- the GPS.

Get back to me. ;)

The error rate should be alarming to everyone. 2-3 hundred feet! Old school altimeters is like living in fantasy land. Like Ron and Jesse said as long as everyone is living in the same "fantasy land" (my words not theirs) life is good. Accurate? No. :no:
 
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Geico266 said:
Take your GPS and check it against your altimeter. Do it over a couple months and see how pressure changes the readings (even when adjusted for) on your old school altimeter -v- the GPS.

The error rate should be alarming to everyone. Old school altimeters is like living in fantasy land.
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So how much difference do you expect there to be between a baro altimeter with the correct setting and the GPS when you are at the airport on the surface? Ever notice the GPS altitude changing up and down while it is sitting on the ground, totally motionless, and with a clear view of the sky? I would be alarmed if an altimeter did the same as the GPS. :yikes:
 
Please cite where a pressure altimeter anomaly has caused a pattern of accidents that would not have happened if using GPS altitude. Actually, I'll make it easier. You can include near accidents and close calls too. Please enlighten us, the FAA, and the aviation industry about this problem that you and only you have stumbled upon.
 
Pedals2Paddles said:
Please cite where a pressure altimeter anomaly has caused a pattern of accidents that would not have happened if using GPS altitude. Actually, I'll make it easier. You can include near accidents and close calls too. Please enlighten us, the FAA, and the aviation industry about this problem that you and only you have stumbled upon.
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He is just tweaking us. How else can he build up so many posts.
 
Pedals2Paddles said:
Also I would hardly call a pressure altimeter unreliable or not precise.
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Geico266 said:
Set it and check it against a GPS. Once you know the amount of error you'll be surprised. ;)
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Perhaps someone is confusing accuracy and precision, and mixing it in with time? ;)

At a given location and time, a pressure altitude is quite precise. Precision is what is needed for traffic separation, not accuracy.

kajqra.jpg
 
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ElPaso Pilot said:
Perhaps someone is confusing accuracy and precision, and mixing it in with time? ;)

At a given location and time, a pressure altitude is quite precise. Precision is what is needed for traffic separation, not accuracy.

accuracy_vs_precision_556.jpg
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I vote this as best response.

Now I understand!
 
I had a similar issue last week
Left 9D9 with altimeter set to airfield elevation.
As we approached GRR he gave us altimeter correction asked what our cruise altitude would be I report 6,500 ft, He reported radar contact at 6,300 ft. exactly where we were in the climb.
GRR handed us off to MKG he gave us altimeter correction 30.04 we set it a few moments later N ----T please fly at the correct VFR altitude you show 6,100.
It was really bumpy and I was at 6,400 at one point according to my Alt..
I was concerned so called MKG and said we are now showing 6,500 at 30.04 what correction would you like me to do?
All he said was I want you to fly at the correct VFR altitude for your heading,my heading was 335. I then flew at 6,600 to" be safe" and heard no more out of him till he handed us off to Minn Ctr.
My transponder was checked but is not IFR cert.two months ago, it was correct 5 minutes earlier in GRR off in MKG and again off but acceptable with Minn Ctr. I think the very pleasant gal at Minn. Ctr said VFR tolerance was 300ft.
My equipment issue or theirs?
 
My guess it is your equipment. You mentioned that your transponder was checked but is not IFR cert two months ago. The 91.413 check is a use requirement and does not specify IFR, so if you had the 91.413 check performed within the last 24 calendar months, you may use it, otherwise you can't. An IFR check is related to 91.411 and includes an altimeter and static check as well as a correspondence check between your altimeter and your encoder if they are not the same piece of equipment. Point being, your altimeter and encoder can drift apart over time and if they are detected to be 300 feet or more apart, you should turn the mode C off. Regardless, you should fly a correct altitude and not be 100 feet high to compensate for an altimeter that is more than 200 feet different than the encoder.
 
Pedals2Paddles said:
Also I would hardly call a pressure altimeter unreliable or not precise.
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I agree -- it measures what it's designed to measure very precisely. However, it is not designed to measure true height above Mean Sea Level anywhere other than where the altimeter setting was taken, and with a standard temperature lapse rate. As I said, it is very accurate for keeping airplanes separated vertically by set amounts, and determining (indirectly) a safe height above ground when relatively near the ground where the altimeter setting reading was taken (except when it's really cold), but that's not the same as measuring true height above Mean Sea Level at cruise altitudes, at which it is not particularly good.

OTOH WAAS GPS is very good at measuring with stunning accuracy your actual height above Mean Sea Level -- except that's not what we use to separate airplanes from each other vertically (at least, not for now).
 
Pedals2Paddles said:
Please cite where a pressure altimeter anomaly has caused a pattern of accidents that would not have happened if using GPS altitude. Actually, I'll make it easier. You can include near accidents and close calls too. Please enlighten us, the FAA, and the aviation industry about this problem that you and only you have stumbled upon.
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One off the top of my head is AA 1572. And I think the FAA and the aviation industry are well aware of this one.
 
"Contributing factors were the failure of the BDL approach controller to furnish the flight crew with a current altimeter setting, and the flight crew’s failure to ask for a more current setting"

How can you fault the altimeter for that?
 
Norman said:
"Contributing factors were the failure of the BDL approach controller to furnish the flight crew with a current altimeter setting, and the flight crew’s failure to ask for a more current setting"

How can you fault the altimeter for that?
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Did you read the whole report, including the discussion of the effect of the strong wind blowing over the ridge, and the temperature?
 
At the time of that accident, I lived under the extended final approach course at about mile 12. The winds were howling that night, and the gust front that caused them blacked out many hundred thousand homes from Virginia to Massachusetts. It was without a doubt a good night to be on the ground. I didn't read the whole report posted, so ... did it mention that the BDL tower was evacuated because the wind had flexed the tower windows enough to pull them out of the metal frames? The windows were in imminent danger of being blown in.

-Skip
 
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