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.