October 1, 2004
By Barry Schiff
Barry Schiff is the author of the book Test Pilot: 1,001 Things You Thought You Knew About Aviation.
There are very few pilots who have not heard the warning, "When flying from high to low, look out below." This ditty is intended to remind us that when flying from relatively high to relatively low atmospheric pressure, actual altitude decreases even while maintaining a constant indicated altitude. In other words, the aircraft is slowly descending without the pilot realizing it. It also reminds us that the same phenomenon occurs when flying into lowering ambient temperatures.
Virtually every pilot knows to apply this knowledge when queried during written examinations, yet only a few actually apply it — especially as it relates to temperature — in the real world, and this leads to an almost predictable number of controlled-flight-into-terrain accidents every winter. A CFIT accident is particularly sad because it means that a pilot has flown a perfectly good airplane into the ground. Such an accident most frequently occurs during actual instrument approaches, a time when pilots cannot see obstructing terrain in time to avoid it.
Such accidents rarely occur as the result of lowering atmospheric pressure. Pilots use the current altimeter setting to prevent such an altimeter error. The insidious culprit is substandard temperature because pilots do not have an "automatic" way to compensate for cold air (as they do for low pressure).
Canadian pilots, who know something about flying in frigid climes, have used an internationally adopted correction chart published in their Aeronautical Information Publication (the Canadian equivalent of our Aeronautical Information Manual) to compensate for altimeter errors that occur when operating in substandard temperatures. The chart shows that when 1,500 feet above the airport at an ambient temperature of minus-30 degrees C (minus-22 degrees F), the altimeter indicates 280 feet higher than actual. Consider that when on a nonprecision final approach an aircraft is provided with 300 feet of obstacle clearance, at a minimum. So if you maintain indicated altitude perfectly, you might miss the offending obstacle by 20 feet (unless your landing gear is down). Otherwise — or if the temperature is slightly colder — you would compete with the obstacle for space no matter how accurately you or the autopilot flew the airplane. Using a final segment that begins at 2,000 feet above the airport? The error at an ambient temperature of minus-30 degrees C is 380 feet.
The greater your altitude above the airport (the station providing the altimeter setting) and the colder the temperature, the greater the error.
Because the minimum obstacle clearance provided on initial and intermediate approach segments is 1,000 and 500 feet agl, respectively, you may believe that these segments are safer during cold weather. The opposite is true. Pilots navigating these segments are more vulnerable to altimeter error because they are flying at altitudes higher above the altimeter-reporting source, enlarging the error.
For example, if the minimum altitude for an approach segment is 5,000 feet above the airport and the temperature is the same (minus-30 degrees C), the altimeter error will be 950 feet. In the extreme of an ambient temperature of minus-50 degrees C, the error will be 1,500 feet.
U.S. airlines and military services asked the FAA for years to provide temperature-correction charts to use during cold-weather operations. These requests fell on deaf ears until a year or so ago. This is when the FAA finally acquiesced by adding without fanfare the ICAO Cold Temperature Error Table to the AIM (Chapter 7, Section 2-3).
Despite its availability, however, few pilots are aware of this chart. Those who have seen it probably do not use it. I have yet to find one who does, but this might be because so few appreciate the dangers inherent in making certain instrument approaches in winter weather.
On the other hand, perhaps we can get away without the chart. To do so, however, requires that pilots first accept the reality that such altimeter errors really exist and threaten safety when executing nonprecision instrument approaches in substandard temperatures in the vicinity of rising terrain. Once this is accepted absolutely, then all we need to do is adopt the philosophy of not being in such a hurry to descend to the minimum segment altitudes shown on approach plates. Many of us tend to dive as rapidly as possible to these minimum altitudes when there usually is no need to do so. As a matter of fact, if you were to level off a few or several hundred feet above minimum segment altitudes, your flights would be that much safer irrespective of ambient temperature. Minimum altitudes are just that, minimum altitudes. Unless there is a specific need to cross a fix at some specific altitude (such as when mandatory crossing altitudes are designated), I typically cross approach fixes at some higher altitude, a technique that makes the approach more relaxed and elevates my personal comfort index.
The coldest ambient surface temperature ever recorded on Earth was minus-127 degrees F (Vostok, Antarctica, 1960), and this was exclusive of any wind-chill effect. I couldn't guess how much in error an altimeter might be above that place; correction charts do not go that low. In contrast, the hottest temperature recorded was 136 degrees F (in the shade) at El Azizia, Libya, in 1922 and would cause actual altitude to be much higher than indicated.
Visit the author's Web site ( www.barryschiff.com).
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