Altimeter accuracy

[Marauder]

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.

 

Nice explanation. 

[N601RX]

I decided to go with the Transcal encoder a couple of years ago. It's about half the sized of the others and has both serial and parallel output. It also has a feature where the shop can connect it to a laptop computer and type in correction values for each 1000 ft increment which are stored in EPROM.

[Marauder]

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.

 

Rarely. Keep in mind GPS altitude is absolute altitude above sea level which is calculated from the satellite network. Our barometric altimeters display pressure altitude and will vary based on atmospheric conditions for the parcel of air we fly in.  

[Bob - S50]

When you set the local altimeter, does the altimeter show field elevation or is off by 200'?

 

If it is correct, I'd have the encoder checked.  If it is off by 200' I'd have the altimeter and pitot static checked.

 

Bob

[rbridges]

I had to have my altimeter overhauled. Minor differences down low but out of spec at higher simulated altitudes during ifr certification. I believe it cost around 250.

[cujet]

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

Yes, that's generally correct. However, it's possible the ATIS has not been updated and a pressure front is moving through. Therefore, it's possible to have a small difference. It's also possible the ATIS, AWOS or other broadcast is not dead-on accurate. (I've seen this occasionally)

 

Also, many of the above posts are correct, but a bit misleading. The transponders encoder is ALWAYS equivalent to a setting of 29.92. They DO NOT compensate/adjust or change output via input from the "Kollsman window" or other baro input. 

 

For example, your current baro setting is 30.15 and you are flying at exactly 6500 feet and you know this is exactly right, because the 6499 foot mountain top is right under your belly. Q) What is your transponder reporting? A) 6300. 

 

Let's answer this question by saying that one inch of Hg is equal to about 1000 feet. So, the difference between 29.92 and 30.15 is 0.22 in/HG or about 220 feet. The answer is that your transponder will be reporting, and ATC will be receiving 6300 feet. To complicate matters a touch, due to system hysteresis, inaccuracy and a slight delay in response, I suspect you could be reporting 6200 feet.  As transponders generally "toggle" every 100 feet and are designed not to go back and forth constantly between 100 foot increments. 

[RobertGary1]

Are you draining your static and pitot systems before flight?

[bonal]

Are you draining your static and pitot systems before flight?

To be honest I don't know how that is done. Lay it on me I'm ready to learn a valuable lesson about my airplane.

[carusoam]

I have two push button drains on the O... Pilot side, fuselage behind the wing and front underside of the wing...

I don't think my 65 C had one.

It had a dead leg that would be emptied at annual...

The POH has a copy of the tubing layout and where everything is attached.

Best regards,

-a-

[Hank]

Anthony, my 1970 C has those same two drains. Guess it depends on the year.

[bonal]

I have two push button drains on the O... Pilot side, fuselage behind the wing and front underside of the wing...

I don't think the C had one.

It had a dead leg that would be emptied at annual...

The POH has a copy of the tubing layout and where everything is attached.

Best regards,

-a-

Yep that's what I have well at least I wasn't missing some thing

[cliffy]

I think I remember reading that the Datum used by our GPS's does not coincide with "sea level" It's an electronic datum (close but not exact) Altitude displyed may be close to actual MSL but it nay also drift at times with the signal received from the sats.

The altimeter has its own adjustment in the Kolsman window to match the Kolsman indication to the correct altitude. We used to be able to adjust that but no longer. Only done with test equipment now and it has legal limits on how far it can be adjusted before the altimeter has to be pulled and overhauled..

The encoder also could be replying incorrectly and can only be tested by a shop.

3rd could be a leak somewhere in the static system. Quit possible with the old plastic lines we have in Mooneys.

The only way to really correct it is to have a Pitot/static/encoder IFR check done by a reputable shop.

[ryoder]

To be honest I don't know how that is done. Lay it on me I'm ready to learn a valuable lesson about my airplane.

I thought you were pulling his leg. I also don't know how to do this.

[RobertGary1]

Each system has a button you just push during your walk around. On my F the static drain button is under the battery box, the pitot is just below the leading edge of the left wing, near the root.

 

-Robert

[Marauder]

Here is what they look like outside of the plane:

[Vance Harral]

Each system has a button you just push during your walk around. On my F the static drain button is under the battery box, the pitot is just below the leading edge of the left wing, near the root.

 

I discourage pushing them on every walk around, as I think this tends to wear out the springs and seals (the only time our airplane ever failed a pitot/static check was due to a worn-out static drain).  Certainly good to push them any time you think moisture might have gotten in the system - after rain, washing, etc. - and/or every few preflights just to make sure they haven't frozen up.

 

If you've never fiddled with yours before, be sure to clean them before operating, as you don't want to push oil and dirt up into the mechanism.  Provided they're clean, it's OK if they're a little hard to push in, as this usually just indicates the spring holding them in the sealed position is in good shape.  Our static drain was very easy to push in when we first got the airplane.  But a failed pitot/static test was traced to that drain being leaky.  The replacement drain is much harder to push in, but no failed checks in the ensuing 8 years.

[bonal]

So I have the style that collects in a container inside the fuselage that gets drained at annual which for me was about 8 months ago the plane has been hangared and we have been in a drought that makes me think I moved to Arizona so I wonder if there could be a problem with obstruction after I confirm my readings for both the altimeter and the transponder I will open it up,and take a look see