Barry Schiff retired from TWA as a captain in 1998. He has been flying for 49 years.
This is the time of year when pilots are deluged with articles and material reminding them of the hazards associated with winter operations. Most of these time-honored warnings deal with familiar topics such as structural and induction icing, operating on contaminated runways, and starting cold-soaked engines.
There is one critical winter topic, however, that usually is given short shrift or ignored altogether. It deals with dangerous altimeter errors that occur during flight in significantly substandard temperatures. Perhaps this is because the hazard is subtle and not fully understood.
Although altimeter errors can affect the safety of VFR flights, the topic should be profoundly important to instrument pilots, who often have as little as 300 feet of obstacle protection when on final approach or while circling at the MDA (minimum descent altitude). Any erosion of such a skimpy safety margin can hardly be tolerated, yet this is exactly what happens when operating in substandard temperatures. In these conditions, true (or actual) altitude is less than that shown on the altimeter, not a particularly pleasant thought when passing only 300 feet above obstacles that cannot be seen. Can such errors lead a pilot to fly unwittingly into an obstacle? Absolutely. Paraphrasing Tennessee Ernie Ford, this should make pilots shooting instrument approaches in the winter as nervous as long-tailed cats in a room full of rocking chairs.
The Canadians have been more aware of such lurking dangers than we have been. Included in that country's Aeronautical Information Publication (Canada's equivalent of our Aeronautical Information Manual) for many years has been a chart that provides altitude corrections to be added to minimum approach altitudes to preserve obstacle protection.
I suggested to the FAA on three occasions that a similar chart be included in the AIM, but the idea fell on deaf ears. Last year, however, the FAA decided that the hazards caused by cold-weather altimeter indications should finally be acknowledged. A table similar to the one published by the Canadian government was unceremoniously added to Chapter 7 of the AIM. (I do not take credit for this addition. It has been 13 years since my last attempt to get the FAA to address this safety issue.)
Start with the reported ground temperature and the HAA (height above airport) of any published minimum approach altitude. If the reported temperature is minus 30 degrees Celsius and the HAA (of the circling altitude, for example) is 600 feet, the altimeter error is 120 feet. Under these conditions, the altimeter would be off by 120 feet, a figure that should be added to the circling altitude (in this case) to preserve the 300-foot obstacle clearance. In reality, this 120-foot figure should be added to all published approach altitudes.
Many pilots make the mistake of thinking that higher minimum approach altitudes reflect increased safety margins. Not so. High MDAs simply indicate the presence of higher obstacles in the vicinity of the airport. The safety margin usually is the same: 300 feet.
Unfortunately, higher MDAs result in greater altimeter errors. Altimeter errors resulting from nonstandard temperatures increase with altitude above the station from which an altimeter setting is obtained. This is reflected in the "Cold Temperature Error Table" in the AIM. When the reported temperature is minus 50 degrees C and the HAA is 1,000 feet, the altimeter error is 300 feet. Unless this error is added to all approach altitudes, the aircraft will be in danger of rolling its wheels along the tops of obstacles in the approach path. Similarly, HAAs of 2,000, 3,000, and 4,000 feet under the same conditions should be elevated by corrections of 590, 890, and 1,190 feet, respectively.
Altimeter errors in the winter should also be of concern to VFR pilots operating over mountainous terrain at night. Cruising altitudes can be sufficiently high above reporting stations to produce altimeter errors in excess of 1,000 feet, and this is exclusive of errors caused by aberrational pressure changes that occur over mountainous terrain when the wind is strong (because of venturi effect). This partially explains why minimum instrument cruising altitudes over mountainous terrain are 2,000 feet above the highest en route obstacle instead of the 1,000 feet used elsewhere. (Under extreme conditions, altitude errors in the winter can exceed 2,000 feet.)
Other altimeter errors include those caused by missetting an altimeter, being provided an incorrect setting (it happens), and using an unreliable altimeter.
According to the FAA, a pilot noting an altitude discrepancy of 75 feet while on the ground with a properly set altimeter should refer the problem to a repair station. Although a maximum-allowable altimeter error is not designated for either. VFR or IFR flight, common sense dictates what is acceptable. (Maximum-allowable errors stipulated for passing a biennial static system test are applicable only at the time of the test.)
Also, if the altimeter is slightly in error, the FAA recommends that the pilot use the altimeter setting and not airport elevation to set the altimeter before departure. The error noted may not be consistent throughout a flight.
The question "How high is up?" may be of philosophical interest to some people, but it is a subject that pilots can ill afford not to ponder.
Visit the author's Web site ( www.barryschiff.com).
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