Altimeter accuracy

[bonal]

Lately I have seen that per ATC my altimeter is off by about 200 ft when asked by control what I am showing. What is possible cause is this a calibration issue and how many AMUs to correct.

[M016576]

Water in your static lines, a clogged port, etc. did you try pulling the alt static source to see if the altitude corrects itself?

I'd go in and get an IFR certification (~$250.00). They are good for 2 years and they test your static system as well as transponder up to the service ceiling of your aircraft. Cheap diagnostics and you get a certification out of it.

[bonal]

Forgive my lack of knowledge what is the alt static source and how do I pull it?

[M016576]

Not sure about the D, but in the J, it's a big knob located next to the pilots yoke.

[Marauder]

Lately I have seen that per ATC my altimeter is off by about 200 ft when asked by control what I am showing. What is possible cause is this a calibration issue and how many AMUs to correct.

 

Dude! You're telling my story! I posted a thread about being 200' off as well. I don't remember what equipment you have on board, but in my case I have 3 altimeters and the blind encoder. My two Aspens were always within 20' of each other and the stand-by altimeter was always within 40' of them. What I couldn't see was what my blind encoder was reporting. It turns out that my blind encoder was off and my altimeters were also all off a bit. Between the 4 sources, they led to the 200' sometimes 300' discrepancy.

 

I switched over to a 327 transponder and if I change the Kollsman on my altimeters to 29.92, I can see if they all match the pressure altitude that the transponder is reporting out. There can be several things that cause the error. Does your plane have an IFR cert on it? If you are flying around controlled airspace, it might be worthwhile to have it done and let them find the problems.

[Marauder]

Forgive my lack of knowledge what is the alt static source and how do I pull it?

On my F, it is located under the panel and on the left side of the panel. I think on some planes, it is on the panel itself. Should be labelled ALT STATIC SOURCE or something like that

[N601RX]

The older planes did not come with alternate static unless it was added latter. Assuming your transponder doesn't display the altitude from the encoder, it can be displayed on older most panel mount gps if the encoder wired into the gps. It is often found on the maintaince screen. Some of the portable traffic watch systems such as the Monroy also display the transmitted altitude.

[bonal]

I only have the one altimeter and from what ATC is telling me they are receiving what the instrument is showing and it's off by 200ft. When it goes in for annual I will get the IFR cert done. When I go to the hangar today I will search for the source knob. I know it's not on the panel though so I'll look underneath hopefully it will be labeled "this knob dummy"

[mikesalman]

Marauder, what year is your F? mine is a 68 and I do not have a alt static source.

[Marauder]

Marauder, what year is your F? mine is a 68 and I do not have a alt static source.

Mine is a 1975.

[Marauder]

I only have the one altimeter and from what ATC is telling me they are receiving what the instrument is showing and it's off by 200ft. When it goes in for annual I will get the IFR cert done. When I go to the hangar today I will search for the source knob. I know it's not on the panel though so I'll look underneath hopefully it will be labeled "this knob dummy"

What transponder do,you have?

[bonal]

It's a Narco AT165R digital

[Marauder]

Great! Then it is easy to know what your plane is reporting. When you are in flight, set your altimeter to 29.92 and it should match the pressure altitude that shows in the transponder window for pressure altitude.

[bonal]

I will check that out.

[carusoam]

Some older planes used the VSI(?) for alternate static source...

In case of emergency, just break glass...?

From my C POH...

"An alternate static pressure source valve is installed under the flight panel. flight panel."

Yep, just like that....

IIRC, my 65C had a small square knob, pull to activate alt static source.

Best regards,

-a-

[bonal]

If I get current pressure for my field and set my Alt it should agree with my known field elevation right?

[drapo]

Well in my ATC world, Montréal Terminal, when there is a discrepancy of more than 200 feet, the mode C is considered as unservicable and we tell the pilot to shut the mode C down ( on instead of alt) and in some cases, they are refused clearance to enter class C airspace. That's valid for all of Canada.

[carusoam]

Bonal,

A calibrated barometer will do that for you...

If your field has an official calibrated source for barometric pressure, set the pressure in kohlsman window.

Check if the altimeter matches the field elevation.

That is important, but doesn't have much to do with the blind encoder knowing and broadcasting the right altitude.

Calibration is an interesting science. Some people earn a living doing just that.

Best regards,

-a-

[Guest]

Some older planes used the VSI(?) for alternate static source...

In case of emergency, just break glass...?

From my C POH...

"An alternate static pressure source valve is installed under the flight panel. flight panel."

Yep, just like that....

IIRC, my 65C had a small square knob, pull to activate alt static source.

Best regards,

-a-

Some airplanes also had the alternate staic valve plumbed into the fittings on the back of the VSI.

Clarence

[bonal]

If I find there is a selection for alt source and it corrects the problem don't I still have a problem with my normal source.

[Marauder]

If I find there is a selection for alt source and it corrects the problem don't I still have a problem with my normal source.

 

Sort of. If the line were really plugged, the other two instruments that use static pressure would also be impacted. Here is what you would see from all three:

 

Altimeter

We can start with the altimeter: It senses altitude based on pressure changes coming from the static port. if the static port is blocked, then the pressure in the line isn't changing. It will continue to show whatever altitude you had at the time of blockage, since that correlates to the internal pressure of the air currently trapped in the line.

Vertical Speed Indicator

The vertical speed indicator works by means of a calibrated leak. That allows it to compare the ambient pressure reading now with what it read a moment ago. Since that pressure reading from the static port is now a constant, the VSI will sense no change, and indicate a constant zero feet per minute.

Airspeed Indicator

Those two are very straight-forward. The reading on the airspeed indicator takes a little more thinking though, since both the static and pitot connect to it. The static port is reading calm-air pressure, while the pitot tube senses ram air. The air entering the pitot tube is forced in as a result of facing into the wind, so higher speed through the air translates to higher pressure in the tube.

 

​What you are describing sounds more like a calibration issue. Not sure where you Narco is getting it's digital input, but like was mentioned above, I would do the IFR cert if it is close to being due.

[bonal]

I understand the static system and AI and VS as well as the alt all seem to be operating normally will do some checks on today's flight. Will do an IFR cert this annual it has not been done for at least 2 plus years

[Vance Harral]

The good news is that all aircraft flying around are on the same calibration standard as each other.  Therefore, the error seen will be the same error of all other aircraft flying around in the same atmospheric conditions.

 

In a mode-C environment (most aircraft still do not have ADS-B out), the altitude ATC sees on their scopes is the pressure altitude broadcast by your encoder, adjusted for nonstandard pressure, i.e. the altimeter setting.  Encoder altitude differs from true altitude due to (a) altimeter setting ( altimeter accuracy, © encoder accuracy, and (d) encoder resolution being limited to 100 foot intervals.  Only (a) is common to all aircraft flying int he same atmospheric conditions.  The others depend on the individual aircraft.

 

To put some numbers on it... Altimeter tolerance is +/- 80' at 10,000 MSL, and encoder error is +/- 125' of the altimeter.  Now assume you have a couple of aircraft using the same Kollsman window settings (call it 29.92 to simplify things). The first one is flying at an altimeter-indicated altitude of 9869', with an altimeter that reads 80' lower than true altitude, and an encoder that reads 120' lower than the altimeter.  Its true altitude is 9869+80=9989'; but the encoder will broadcast a pressure altitude of 9869-120=9749=9700 with rounding; an error of 289' due solely to the state of the aircraft.  Say the second one is flying at an altimeter-indicated altitude of 10080', with an altimeter that reads 80' higher than pressure altitude, and an encoder that reads 120' higher than the altimeter.  The second aircraft's true altitude is 10080-80=10000', and its encoder will broadcast a pressure altitude of 10080+120=10200.  In this case, ATC will "see" a vertical separation of 500', but the actual separation is only 20'.  Even if the two aircraft talk to each other on the radio and say their (indicated) altitude, they'll think they are further apart than they actually are.

 

Food for thought, especially for those of you who might see a mode-C-derived target on your TIS-B or ADS-B-in box, and decide it's no factor (or even worse, initiate an avoidance maneuver), without first visually identifying the target.

 

FYI, we just got done with our biennial pitot/static/altimeter/transponder check, and I talked to the tech about this.  It's tempting to think most systems are not right at the edge of the error tolerances, but the opinion of the tech was that edge settings are actually pretty common.  One reason for this is the most common (i.e. cheapest) blind encoder has a very sensitive, single-turn pot for adjusting calibration.  Once it's tweaked to anywhere within the 125' tolerance, techs tend to leave it alone and not try for better numbers, due to the likelihood of pushing it out of tolerance in the other direction.  He also said the encoder does drift over time, and that there are plenty of scofflaws who let their biennial checks slide for months or years.  Again, food for thought.

[N201MKTurbo]

You keep talking about altimeter error, but you have no way of knowing weather it is the altimeter or encoder that has the error. All you know is that they are different. All that ATC knows is what your encoder is reporting and what you say your altitude is.

The only way to know is to have both compared to a calibrated altimeter, which is what an IFR cert is. Even if you don't fly IFR it is a good idea to have it done every few years. They almost always find something to fix.

[bonal]

So now let's factor in a Garmin 296 indicating altitude to the PIC with the altimeter set to current pressure should the two show the same altitude. I will have the IFR cert done no matter what.