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Barometric Altimeter Accuracy

Posted by on February 6, 2011 18 Comments Category : Flight Instructor Blog Tags :

The barometric (baro) altimeter, measures altitude based on a model of the atmosphere, in particular, how the pressure and temperature of the air changes with altitude, the higher the altitude, the lower the pressure and the lower the temperture. The standard pressure at sea level is 29.92 inches of mercury and the standard temperature is 15 degrees C or 59 degrees F. If the pressure is other than the standard, the altimeter provides a barometer setting to adjust to the non standard value. But, there is no capability to adjust for a non standard temperature.

According to FAR 91.121, when operating below 18,000 feet, the altimeter must set to a current ground station within 100 NM of the aircraft position. Usually this is accomplished by adjusting the altimeter to the departing airport barometer and periodically to airports along the route, at least every 100 nautical miles, and finally to the destination airport setting before landing.

So, how accurate is the altimeter for determining the actual altitude of the aircraft. First, nobody bothered to inform the atmoshpere that it had to comply with the model and it rarely matches the exact model. Second, as you get further from the airport location where the altimeter setting was determined, both in altitude and in distance, the less accurate the altimeter becomes in displaying the actual altitude. So the altimeter indicates the most accurately when departing and arriving, assuming you are close to the location where the altimeter setting was measured. This is good, since it is nice to have an accurate altimeter display, particularly when you are landing or making an instrument approach. Third, the model for the temperature verses altitude is rarely ever correct, and there isn’t a means of correcting the displayed altitude for temperature.

When enroute, it is important for VFR traffic to adhere to the hemispheric rule and for IFR traffic to maintain assigned altitudes. This allows for vertical separation of opposite direction VFR traffic as well as for vertical separation of IFR and VFR traffic. If we all play by the rules and set out altimeters to a local altimeter setting, vertical separation is possible as all aircraft will display the same altitude on their altimeter for the same height. We might not be at the actual altitude that the altimeter is indicating, but all aircraft will have the same error. A conforming altimeter will be able to reproduce the same altitude indication for the same atmoshperic pressure, even if it doesn’t match the actual altitude.

Undoubtedly, you have heard the refrain “from hight to low, lookout below”. It applies to going from a high pressure area to a low pressure area and equally going from a higher temperature to a lower one. In both cases, the altimeter will indicate higher than you actually are. If you are on an IFR flight in IMC conditions, this can ruin your day when you are close to the ground. In extremely cold conditions, there are adjustments on instrument approaches that are required to be made for minimum altitudes and DA/MDA. Vertical guidance systems such as Baro-VNAV will have temperature limits that prohibit the systems use on approaches if temperature limits are exceeded.

What about using the baro altimeter in terrain avoidance systems? On cold and hot days, the actual altitude can be off by hundreds of feet and in more extreme cases more than a 1000 feet. So, if you have a choice in the terrain equipment to use a GPS altitude source or a baro altitude source, I would recommend you set it to GPS altitude. In general, GPS altitude will be more accurate than baro altitude. GPS altitude is affected by the geometry of the satellites overhead, but is unaffected by temperature or pressure and does not need a barometer setting. Standard GPS, non WAAS corrected, vertical accuracy is plus or minus 9 meters, or about +/- 30 feet. GPS altitude is a calculation based on the height above a surface defined by an ellipsoid that models the earth’s shape at sea level. WG84 is the name of the model and is close to a sphere that bulges slightly at the equator. In some places the model is off by small amounts up to 100 feet, but this is corrected for in the GPS by using a correction database.

If you own a GPS (usually a portable) that displays GPS altitude, you will see that it seems to agree fairly closley with the altimeter when you are on the ground, although it bounces around more; Whereas at altitude, the GPS altitude can disagree with the altimeter by hundreds of feet and occasionally over 500 feet. For the most part, you are witnessing the temerature error in the baro altimeter. During the summer, the GPS altitude will be above the baro altimeter and in the winter it will be below. So why don’t we all use the GPS altitude? That wouldn’t be a very good idea unless every aircraft used it, the plain old baro altimeter found in virtually every aircraft will aid us from bumping into each other.

Temperatures that are warmer than standard will make the altimeter read lower than the actual altitude. At first glance, this doesn’t seem to be much of a problem, because we will be higher than it indicates and less likely to hit something on the ground. But if you are shooting an instrument approach and want to descend below the ceiling, you may be stuck in the clouds, even though you are still well above the minimum altitude. A more likely and insidious effect can also be found on an ILS or LPV instrument approach with several step down minimums along the final approach course, for example at CLT, which has a note on the ILS 36L approach chart that reads “When assigned by ATC, intercept glidepath at 3000, 4000, 5000, 6000, or 7000″. Apparently this does not relieve the pilot of complying with the step down minimum altitudes. Since the glideslope is fixed in space, on a hot day, the altimeter will read lower than the actual altitude, and the pilot may get a pilot deviation from ATC for indicating below the step down altitude when following the glidepath.

18 Comments



  1. skyboyCFI on Feb 06, 2011

    Is this a lesson, or a question? Lol Sorry. :-P



  2. Paul Tocknell on Feb 06, 2011

    It’s in the Flight Instructor Blog section which qualifies it as a lesson. If you have content that you would like to contribute, please let me know.

    Paul



  3. John D. Collins on Feb 06, 2011

    I did not write it as a question, but as a lesson or starting point for discussion, that is why I posted it on the Instructor blog and not as a question. For those that might be interested there is a good website that has an altimeter simulator that I recommend. See http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx .



  4. Wesley Beard on Feb 07, 2011

    John, Excellent post! You mentioned that we are required to adjust for non standard temperature on an approach. The GPS approach plates will have a statement “Uncompensated Baro-VNAV not authorized below -20°c or above +45°c.” The uncompensated phrase allows us to shoot these approaches outside the temperature range as long as we compensate for the altitude temperature error. Unfortunately our U.S. ATC system doesn’t automatically authorize these altitude deviatons and we must ask them to use the compensated system.

    The other thing to consider is while flying at the MEA when it is cold outside will put you sometimes dangerously close to mountains. In mountainous terrain we are afforded a 2000′ AGL buffer but as John pointed out, in cold temperatures we are much lower than our indicated altimeter. The U.S. ATC system will not automatically give us a higher altitude based on the cold temperature like our Canadian friends. I personally think our ATC system should compensate our altitude whenever the temperature is below freezing.



  5. John D. Collins on Feb 07, 2011

    Wesley,

    Good comments on temperature compensation, although I would recommend that ATC base any altitude compensation on how much a given altitude is below the ISA temperature for that altitude, as at 7500 feet and above, the ISA temperature is normally below freezing, so you would not want to base compensation on just the value of the temperature. I would expect that ATC if it did compensate for low temperatures, would just assign higher altitudes.



  6. skyboyCFI on Feb 07, 2011

    So our instructor blogs hit the question forum? Okay. :-P



  7. MaggotCFII on Feb 17, 2011

    Looking at your posting, last paragraph, “Temperatures that are warmer than standard will make the altimeter read lower than the actual altitude.”

    Got the following from the Pilot’s Handbook of Aeronautical Knowledge: Ch 7, pg 7-4;

    “Adjustments to compensate for nonstandard pressure do not compensate for nonstandard temperature. Since cold air is denser than warm air, when operating in temperatures that are colder than standard, the altitude is lower than the altimeter indication. It is the magnitude of this “difference” that determines the magnitude of the error. It is the difference due to the cold temperatures that concerns the pilot. When flying into a cooler air mass while maintaining a constant indicated altitude the true altitude is lower.”

    Should your “warmer” word be “cold”?



  8. John D. Collins on Feb 17, 2011

    Both my statement and the statement you quoted from the Pilot’s Handbook are correct. It all has to do with the choice of your reference. When conditions are colder than standard, you could say that the actual altitude is lower than the indicated altitude or the indicated altitude is higher than the actual altitude. The same is true for warmer than standard temperatures. You could say that the actual altitude is higher than the indicated altitude or the indicated altitude is lower than the actual altitude. It is real easy to stumble over the words.

    Does this make sense?



  9. MaggotCFII on Feb 17, 2011

    Gotcha!



  10. Brian on Mar 17, 2011

    Here is a video on Cold Weather Altimetry that I think will fit well in this discussion:

    http://www.robinmaiden.com/2008/01/cold-weather-altimetry/



  11. Larry Culver on Jun 10, 2011

    Good lesson. Thanks. FYI, I recently experienced a 400′ altimeter error when setting the altimeter prior to flight; the OAT was 65 F at 1300 MSL with 29.96 from ATIS. I flew the same aircraft only 2 weeks prior with only a 30′ error. This is the largest baro altimeter error I have seen in 45 years of active flying. It makes me question the reliability of baro altimeters in flight with much more seriosity. I am now more serious about having a GPS altimeter display onboard to backup the baro if no radar altimeter is installed.



  12. Emmett on Dec 29, 2012

    Am I the first engineer or student who finds the terminology more than a bit confusing?

    Before my question let me get in kudos for the discussion of temperature compensation! Very clear and concise.

    And while I feel I have a good understanding of the underlying principles of non-temperature compensated altimetry, I did want to confirm my understanding and perhaps illustrate the source of my confusion having now been through Wiki and the AIM and the FAA Pilot’s Handbook.

    If I understand correctly
    Pressure altitude = Indicated Altitude when the Kohlsman is set to 29.92
    But the term Indicated Altitude is rarely used and has some definition issues – from the AIM:
    Indicated Altitude− The altitude as shown by an
    altimeter. On a pressure or barometric altimeter it is
    altitude as shown uncorrected for instrument error
    and uncompensated for variation from standard
    atmospheric conditions.

    So Indicated sounds like Pressure

    So what is the proper term for the output of a pressure or barometric altimeter that has been adjusted for non-standard barometric conditions not related to temperature?

    i.e. What is the name for the data that comes from setting the Kohlsman?
    Can’t call it Baro Alt. can we?



  13. John D. Collins on Dec 29, 2012

    I would think indicated altitude would be the altitude displayed on the altimeter, regardless if it was a pressure altitude or a Baro corrected altitude. Most of the time altitude is used by itself to mean Baro corrected altitude, although Baro corrected altitude is a better term and Baro altitude is a common short hand version. Baro altitude is only valid at the location where it is established and the further one gets away from the location, both in terms of distance and height, the greater the possible error. As you indicated, the Baro altitude is based on a model of the atmosphere and assumes a standard pressure gradient and temperature lapse rate.



  14. Emmett on Dec 29, 2012

    Pretty much the conclusion I came to also. But once we step out of the cockpit into complex systems, I have to sling this data around on buses and in requirements. This gap in rigid terminology breeds confusion.

    Not unusual, I find the older a system I work on, the more loose-goosey the terminology.
    I’ve settled on the following – correct me if you have better:

    Pressure altitude is sensed and uncorrected in any way i.e. static pressure
    This vehicle doesn’t require any static source error correction
    Baro Corrected altitude is adjusted with current pressure differential from station
    Density altitude is Baro Corrected that is temperature corrected

    Impact is raw sensed
    Calibrated Airspeed is sensed corrected for aircraft errors
    TAS is then adjusted for altitude and temperature

    How am I doing?
    Emmett



  15. John D. Collins on Dec 29, 2012

    I am not sure what your usage of the terms is for. Pressure altitude is altitude measured with constant pressure reference, such as 29.92. For example, a blind encoder has the pressure adjustment set at the factory and adjusted when installed in an aircraft to meet the tollerances required by 91.411. The output is electrical and is measured in hundreds of feet assuming a reference pressure of 29.92. You can also read pressure altitude on a barometric altimeter by adjusting the Kollsman window setting to 29.92.

    Density altitude is Pressure Altitude corrected for temperature and dew point. On the E6B, the dew point is not considered. It is not strictly an altitude conversion, it is used to determine the effective density of the air and therefore the corresponding performance equivalent for the airframe and engine. Density altitude has no correlation to Baro Altitude.



  16. Emmett on Dec 29, 2012

    I was doing OK up to this point: “Density altitude has no correlation to Baro Altitude.”

    Am I confusing Density altitude calculations with temperature compensation for VNAV on approach?

    And I can’t thank you enough for helping
    Emmett



  17. John D. Collins on Dec 29, 2012

    I hope not. Density altitude is meant to help estimate the effect of temperature and altitude have on performance.

    With a Baro VNAV system, it is able to calculate a vertical glide path using the barometric pressure in conjunction with a position source, usually a GPS, In most instances, the glidepath and minimums are not compensated for temperature, so there are limits placed on temperature for the procedure when flown with an uncompensated Baro VNAV system. A WAAS GPS is authorized to fly the same LNAV/VNAV type procedures, but since its path is not a function of temperature, the limits are not applicable for this type of avionics.

    You would not want to use a Baro VNAV system with temperature compensation for descents to charted altitudes along airways, assigned altitudes, feeder routes, or approach procedures with step down fixes prior to intercepting a LNAV/VNAV glidepath, as you would place the aircraft at a different altitude than all the other aircraft using the airspace and if your altitude varied by more than 200 feet, you could be subject to an altitude violation and a pilot deviation by the FAA. The only portion of the approach that you would need or want compensation for temperature is from the glidepath intercept altitude to the DA. In most cases, this provides no operational benefit as long as the temperatures remain within the approach limits specified on the approach procedure.



  18. Jose.L Jr on Oct 23, 2013

    John, thank you for sharing.
    I’ve posted some calculation Scilab file, they can be used for pressure altitude calculation and density altitude,
    Here is the link
    http://basicairdata.blogspot.it/search/label/altitude

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