(Image from http://www.mountainflying.com/)

By Captain Shem Malmquist

Barometric altimeters are prone to various errors. Most pilots understand the effects of non-standard pressure. We correct for this below transition altitude by setting the local altimeter setting as provided by ATC. We are not concerned with it above transition as the altitude is high enough that any errors will not result in loss of terrain separation. To ensure traffic separation, the U.S. has established a chart of “lowest useable flight level”. In other countries this is handled either the same way, or by providing a buffer area, or transition area, between the boundary of transition altitude and transition level.

Altimetry can also be affected by temperature. The old adage “high to low, look out below” applies to temperature as well as pressure. As the temperature increases above ISA, the altimeter will indicate that you are lower than you actually are, and when it is colder than ISA it will indicate that you are higher. It is the latter that gives us concern.

This issue is not addressed in FAA procedure or airway design, although the USAF and Transport Canada do provide guidance, as does FAA in the AIM. Essentially, cold air leads to a denser airmass.  As a result, the pressure as you move higher in the airmass changes faster than under ISA. Instead of dropping one inch per thousand feet, the rate of change is somewhat more. The effect is that the actual pressure at a given altitude is less than it would be otherwise, and, so when you are at that altitude the altimeter will read too high.

The altimeter setting at the field will be accurate, but the further away you get from the reporting station, the more inaccurate it becomes.

The effect on the crew is that they are still aiming to be at a designated indicated altitude when flying a SIAP, a SID or a STAR. However, that altitude does not take into account the effects of temperature. As always, it comes down to the crew to determine the minimum safe altitude for the particular operation. Naturally, we should never rely on ATC for terrain clearance (it is not their job unless you are on a radar vector), and in this case ATC has no way to determine your true altitude.

How do you determine what the correct altitude should be? Well, many of the old “wiz wheel” E6B and similar flight computers have the capability of making the correction from calibrated to true altitude, so that is one way. Another would be to use the chart found in the AIM TBL 7−2−3.

How serious an issue is this? Well, that depends on how cold it is or how far above the reporting station you are flying. Essentially, the colder it is or the higher you are, the worse it is. For an airport ambient temperature of minus 40c will put you almost 200’ low at 1000’ AGL. If you are crossing a ridge line 10,000 feet above the station and it is minus 18c (not that cold for 10,000’), you are actually 500 feet lower than your altimeter is reading. Now, in some locations with a very high airport, imagine the error crossing an adjacent ridgeline. For example, crossing the Rocky Mountains going into DEN, you might be 10,000 feet above the airport at 15,000 feet. A wintertime temperature of minus 30c at 15,000 is not unreasonable.  This scenario will result in you being almost 1,000 feet lower than you think you are – and that is ignoring any local venturi effects of the winds over the ridgeline.

Remember, the formula is only dependent the aerodrome temperature and how high you are above the reporting station. The higher the airport elevation the worse the effect will be due to the lower temperature associated with the airport elevation.

Altimeters can also be affected by strong winds across terrain. This factor should be considered when an approach procedure is designed, and it is possible that the lack of such consideration was a factor contributing to the AA accident at BDL a few years ago.  Altimeters are affected by the venturi effect of winds blowing across terrain.  The winds create a localized “low pressure area”, and as we all learned, “high to low, look out below”.  This is obviously only an issue when there is terrain present to create the effect, as there was on runway 15 at BDL.  Consider backing up your barometric indications with radar altitude, if possible, and be generally aware of altimeter errors.  The following chart provides some insight to the extent of wind errors:

       Windspeed* (kts)                     Altimeter Error (ft)

*Windspeed values were measured 100 ft above airport elevation

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