In Praise of the Altimeter

My Warrior’s altimeter is probably the simplest instrument on the panel, really nothing more than a calibrated barometer. It doesn’t even tell me my real altitude — on a very warm or very cold day it can be off by a thousand feet or more. Still, when I sat down to think about it, I surprised myself by coming up with four uses for this piece of low-tech steam gadgetry beyond finding my current altitude, starting with weather forecasting (note: I make no guarantee that any of these is practical or reliable).

Weather Forecasting

Because the altimeter is a barometer, I can use it to forecast the weather in a clumsy way. With any barometer, when the mercury is falling, the weather is probably getting worse, since a low-pressure system is likely moving in; when the mercury is rising, the weather is probably getting better, since a high-pressure system is likely moving in.

So, let’s say that I land at a small, unattended airport for a leisurely lunch with a friend. Before I tie down the plane, I set the altimeter so that it shows the field elevation of 550 feet. Three hours later, I come back and the altimeter reads 750 feet — that means that the pressure has fallen by 1/5 of an inch of mercury in three hours (one inch of mercury is 1,000 feet), and the weather is going down. Where I live, in central Canada, the prevailing winds are from the west, so that’s probably where the low-pressure system is moving in from. That means that I can look up at the sky at the current weather, and then make a reasonable guess that if I fly west, towards the low, the weather will get worse than what I see right now, and if I fly east, away from the low, the weather will get better; north and south, of course, are anybody’s guess. If I came back after three hours and the altimeter read 1,550 feet, then the pressure is falling fast (one inch of mercury in three hours), and the weather change will probably be more severe.

The same thing works in the air — if the altimeter settings from airports along my route are getting lower, there’s a good chance that I’m flying towards worsening weather; if they’re getting higher, there’s a good chance I’m flying towards improving weather. In the air, that works moving north and south as well.

Detecting Inversions

The thermometer tells me the outside air temperature at my current altitude, but not the temperature of the air below me. However, I can use the altimeter to at least get a clue about what’s going on underneath if I compare its reading with the elevation displayed on a GPS (preferably one with WAAS). The altimeter is calibrated to show my altitude +/- 50 feet only when it has the correct altimeter setting and the air temperature is exactly 15 degrees Celsius at sea level and the temperature follows a standard lapse rate of 1.98 degC for every 1,000 ft up to my altitude.

On a cold winter day, the altimeter should show me much higher than I really am: for example, the altimeter might read 10,000 feet while the GPS shows me at 9,100 feet. On a hot summer day, the opposite is true: the altimeter might read 10,000 feet while the GPS shows me at 10,500 feet. When I don’t see the error I expect, that tells me that something unusual is going on beneath the plane — for example, if I’m at 10,000 feet indicated with an outside air temperature of -30 degC and the GPS shows that I actually am near 10,000 feet, I can take a fairly reasonable guess that I’m in cold air overrunning an inversion, and that things will get much warmer if I descend a bit (which could be bad in the winter, since I might end up in the ideal icing zone of 0 degC to -10 degC). Likewise, if my actual altitude isn’t above my indicated altitude on a hotter than average day, it’s a sign that there might be cooler air below me.

This is something I’d like to work on a bit. I’m still not sure how practical it would be in the cockpit.

Finding Vertical Air Currents

Glider pilots know all about using thermals and other vertical air currents to gain altitude. Unfortunately, those of us who fly underpowered planes also end up having to learn about these currents, since sometimes they’re the only way we can climb past 9,000 or 10,000 feet on a hot summer day in a fully-loaded plane. The vertical speed indicator lags a fair bit, so it’s not all that useful for finding rising air — by the time it shows a climb, I might already be past the air current. The altimeter, on the other hand, reacts almost instantly, and used together with the airspeed indicator, it tells me when I’m in rising air during a climb:

  • If the altimeter needle starts moving faster and the indicated airspeed stays the same or increases, I’m in a column of rising air; often, I’ll slow down the plane below Vy to spend more time in the column. If the indicated airspeed is decreasing, then turbulence has probably just pitched the nose up.
  • If the altimeter needle starts moving slower (or even falling) and the indicated airspeed stays the same or decreases, I’m in a column of falling air; often, I’ll speed up the plane well above Vy to spend less time in the column, even if it means levelling out or descending a bit. If the indicated airspeed is increasing, the turbulence has probably just pitched the nose down.

These will sound funny to people who fly behind big, powerful engines, but they matter a lot to someone dragging four people behind a 160 hp O-320. In level cruise, finding the currents is a bit simpler: assuming a constant power setting, if I have to push the yoke forward to keep the altimeter needle still, I’m in rising air, and if I have to pull the yoke backward, I’m in falling air.

Maintaining Attitude in IMC

Finally, the altimeter can help me maintain my attitude in IMC, especially in stratus cloud (where vertical currents are rare). Along with a rising airspeed, a falling altimeter needle is one of the first signs that the wings are not level — if the attitude indicator shows that I’m level and the engine is still producing the same power but the altimeter shows me descending from cruise or descending faster than expected in a planned descent, it’s time to cross-check the turn coordinator, vacuum gauge and compass to see if the attitude indicator is malfunctioning. This is especially useful during instrument approaches, when the altimeter is near the centre of the scan: it’s one of the first places I should be able to detect the beginning of a spiral.

In fact, in a severe partial panel situation (losing all gyros), the altimeter could be essential for maintaining attitude, though that kind of situation is very rare and very difficult to manage if it does happen.

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About David Megginson

Scholar, tech guy, Canuck, open-source/data/information zealot, urban pedestrian, language geek, tea drinker, pater familias, red tory, amateur musician, private pilot.
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