This article explains how an altimeter works and how variations of barometric pressure affect the altimeter. The methods used to set the proper pressure into the Kollsman window are described along with the effects of barometric pressure variations on a long flight.
The basic concepts of pressure, barometric pressure, and barometers are described in another article, "Pressure, Barometers, and Barometric Pressure" which appears in this same group of articles.
Topics:
- 1. Altimeter, Principle of Operation
2. Reported barometric pressures are corrected to sea level.
3. Sea-level Pressure Varies from Day to Day.
4. How to Read Actual Barometric Pressure from an Altimeter
5. Ways to Set the Altimeter
6. Flying to and from Airports at Different Elevations
7. Flying to where the Barometric Pressure is Different
8. Altitudes reported by Mode C Transponders Acknowledgments
1. Altimeter, Principle of Operation
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An altimeter is actually an aneroid barometer calibrated to indicate altitude instead of barometric pressure. An adjustment knob is provided on the front to allow a pilot to set the current sea-level pressure in the Kollsman window. What the altimeter does is subtract the actual barometric pressure at its location from the current sea-level pressure and express this difference as an altitude. Here's a formula that shows the calculation embodied in the mechanical mechanism of the altimeter: |
- Altitude indicated = ( 1,000 feet/in Hg ) x (Kollsman -- Actual Barometric )
This formula is not super accurate because the constant multiplier of 1,000 feet per inch of mercury is only approximate. It comes from the pressure lapse rate of the standard atmosphere, as explained in the article on pressure and barometers. And, this multiplier changes with altitude. Nevertheless, the formula is helpful in our efforts to understand the principles underlying the operation of the altimeter.
In order to shorten the formula, let's write it like this:
- A = 1,000 x ( K -- B )
where
- A is the altitude indicated on the face of the altimeter, in feet
K is the pressure setting in the Kollsman window, in inches of mercury
B is the actual barometric pressure at the location of the altimeter.
Now, before we get a bad case of hives or nervous twitters from math-a-phobia, let's do some examples to see how this works. We'll calculate the indicated altitude for various combinations of actual barometric pressure and Kollsman settings. |
 Kollsman, setting 30.31 |
Example 1:
Suppose the sea-level pressure is 30.00 in/Hg. Actual barometric pressure is 28.00 in/Hg. What's the altitude?
First of all, we set the sea-level pressure in the Kollsman window. Therefore, K = 30.00. Then,
- A = 1,000 x ( K -- B )
A = 1,000 x ( 30.00 -- 28.00 )
A = 1,000 x 2.00 = 2,000 feet ... indicated altitude on the altimeter.
Example 2:
Sea-level pressure is 29.80 in/Hg. The actual pressure at the location of the altimeter is 27.50. What's the altitude?
- A = 1,000 x ( K -- B )
A = 1,000 x ( 29.80 -- 27.50 )
A = 1,000 x 2.30 = 2,300 feet ... indicated altitude on the altimeter.
Question: What pressure do we always set in the Kollsman window?
Answer: Sea level pressure. (That is, unless you're flying at or above 18,000 feet, in which case instrument flight rules mandate that you set it to 29.92.)
Example 3:
The weather service reports the barometric pressure as 30.15 at Davidson County Airport near Lexington, NC. Our plane is sitting on the ramp, gassed up, and ready to go. Joe Tumbleweed has a barometer in his hangar that reads an actual pressure of 29.45.
When we get into the plane and set the reported 30.15 in/Hg into the Kollsman window, what altitude will the altimeter on our plane indicate?
- A = 1,000 x ( K -- B )
A = 1,000 x ( 30.15 -- 29.45 )
= 1,000 x 0.70 = 700 feet ... indicated altitude on the altimeter.
This is the field elevation of the airport.
2. Reported barometric pressures are corrected to sea level.
Barometric pressures reported by the tower and by automatic weather observation stations (AWOS) as "altimeter settings" are corrected to sea level. So are the pressures reported by the weather person on the evening news.
If this were not the case, barometric pressures reported at stations having high elevations, such as Denver, Colorado, would always be low. Locations near sea level would always be high. By correcting all the readings to sea level, a meteorologist can get a true picture of the state of the atmosphere as far as the "mountains and valleys" of air over the surface of the earth are concerned.
The normal, typical range of variation of barometric pressure is about one inch of mercury, centered on the average at any location. Remembering the pressure lapse rate, this corresponds to a change in elevation of about 1,000 feet. Which is to say, indicated barometric pressures are affected more by the change in elevation than by the daily change.
To emphasize and reinforce these ideas, here are a couple of questions. Remember, Joe Tumbleweed has a barometer in his hangar, and he likes to read it, stare it, and reflect upon it in order to impress visitors. The elevation of his hangar is 700 feet above sea level.
Question: Is the reported barometric pressure the same as what Joe Tumbleweed's barometer indicates?
Answer: No. Barometric pressures reported by the weather service are corrected to sea level. Joe's barometer reads the actual pressure there in the hangar.
Question: Joe wonders if his new barometer is working like it should. So, one evening after a hard day's hangar flying, he reads his barometer. It says, 29.10 in/Hg. Joe then rushes home, turns on the TV, and hears them report, "Barometric pressure is 29.80 inches and steady." Should Joe call technical support in regard to his barometer?
Answer: No, he would only show his ignorance if he did. His barometer, at 700 feet elevation above sea level reads the actual pressure in the hangar at the airport. The weather on TV reports barometric pressures corrected to sea level.
Now, an elevation of 700 feet above sea level corresponds to 0.70 inches of mercury. If Joe subtracts this from the pressure given on TV, he gets 29.8 - 0.70 which is 29.10. This is what his barometer indicated.
Question: Does Joe's barometer do him any good, really?
No, not unless he corrects the reading to sea level pressure by adding about 0.7 in/Hg to the indicated pressure. The actual barometric pressure is not something a pilot has to know. In fact, this is what is measured by the altimeter but indicated as altitude.
3. Sea-level Pressure Varies from Day to Day.
With all the talk about standard pressures and sea level and 29.92 in/Hg, it's easy to come to believe that the pressure at sea level is always 29.92. But this is not the case.
It is just as natural for the barometric pressure to vary at an airport located near the ocean as it is anywhere else. Take Wilmington, NC for example. The elevation of the big airport there is a mere 32 feet above sea level. Does their barometric pressure always read very nearly 29.92?
Heck No! It varies there just like anywhere else, and it really plunges when a hurricane or tropical depression comes ashore.
Because sea-level pressure varies, and because pilots must set the sea-level pressure in the Kollsman window in order for the altimeter to indicate correctly, it is necessary for a pilot to set his altimeter before each flight, and sometimes during the flight if a long distance is being covered. Setting the altimeter is a standard item on a pilots checklist.
4. How to Read Actual Barometric Pressure from an Altimeter
There may not be a pressing reason for wanting to do this because the actual barometric pressure at a particular location is not something a pilot has to be aware of in normal, average, run-of-the-mill flying. But, just as a curiosity, here's how it can be done.
Let's do the math. The altimeter equation from above is
- A = 1000 x ( K -- B )
Suppose we adjust the altimeter until the indicated altitude is zero. Putting this into the equation gives us
- 0 = 1000 x ( K -- B )
Now, the only way 1000 times something can equal zero is for the "something" to be zero. In this case, (K -- B) must be zero, and the only way this can happen is for K to be the same as B.
So what do we do? Adjust the altimeter until the indicated altitude is zero. The Kollsman window will then indicate the actual barometric pressure at the location of the altimeter.
If Joe Tumbleweed had known this, he could've checked his barometer against an altimeter in a plane right there in the hangar, without having to write it down and then hurry home.
5. Ways to Set the Altimeter
It is important, of course, for a pilot to set the altimeter before taking off. There are two ways this can be done:
A. Set the Kollsman window to the reported "altimeter setting" or barometric pressure obtained from the tower or from an automated weather observation station (AWOS).If you're operating from a towered field, the tower will routinely give you the altimeter setting along with the winds and other pertinent information, like permission to taxi, take off, etc. You will not have to ask for it specifically.
B. Simply adjust the altimeter until the indicated altitude matches that of the field elevation.When you do this, the Kollsman window will automatically show the sea-level barometric pressure. However, this is not a particularly important detail. Some pilots fly happily (and safely) for years without paying one bit of attention to the Kollsman window, and they probably couldn't read it if they did pay attention to it.
However, these pilots always set the altimeter to the field elevation prior to taking off. The field elevation of an airport is always posted prominently, and it is given on sectional charts as well.
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A related method involves setting the altimeter to the altitude indicated by a GPS receiver on board the aircraft. This accomplishes the same thing as setting it to field elevation prior to takeoff. However, because a GPS can indicate the elevation above sea level of the plane while it is in flight, the pilot can set his altimeter without reference to the Kollsman window, even in flight.
6. Flying to and from Airports at Different Elevations
Suppose we plan a flight from EXX in Lexington, NC, where the field elevation is 733 feet, to an airport up in the mountains near Spruce Pine, NC. The elevation of Morrison Field at Spruce Pine is 2750 feet.
The photo at right is a portion of a sectional chart showing Davidson County Airport near Lexington, NC. The field elevation of 733 feet is underlined in green. Also note the AWOS-3 radio frequency of 119.825. This can be tuned in to get the altimeter setting and other weather information.
The photo also shows field elevations for three other grass strips, Tara, South River, and Flying M, with field elevations of 756, 650, and 845 feet. Incidentally, the Flying M is the grass strip pictured first in the article on "Getting Lined Up on Final" in the piloting section.
Click to Enlarge
Now, back to planning our trip. We assume, here, for simplicity, that the barometric pressure (corrected to sea level) is the same at Spruce Pine as it is in Lexington. This is likely to be the case, for practical purposes, because the distance is only about 90 miles. And today, a dome of high pressure is in place over the entire southeast, so any significant change in barometric pressure is unlikely. And, the visibility is easily 50 miles.
We set the altimeter in our plane to the field elevation at Lexington, namely 700 feet. We don't even look at the Kollsman window. After taking off and heading west, we climb to our cruising altitude of 4,500 feet, sit back, and enjoy the view.
As we approach the mountains, the terrain rises significantly, and as we get near our destination, we start planning the details of our approach and landing. Haven't touched the altimeter since we left Lexington.
Click to Enlarge This photo shows Morrison field at Spruce Pine. Note the field elevation. The red line shows our approximate flight path to the airport.
Pattern altitudes are typically 800 feet above the level of the runway. So, adding 800 feet to the field elevation of 2,750 gives us 3,550 for the pattern at Morrison. This means we must descend about 1,000 feet from our cruising altitude of 4,500. However, there is high terrain to the north, east, and west of the field. Peaks of the mountains exceed 4,000 feet.
We elect to approach the field from the southwest, flying up a little valley. A call on the radio yields nothing. There is nobody home. We continue, descending to our pattern altitude of 3,550 feet. We now are below the peaks of the mountains to the front and sides of us. Press on. There's the field, with no indication of any activity of any kind.
We circle the field and then enter a left downwind for Runway 34. While flying a normal pattern, we note that it carries us fairly close to the high terrain to the east of us, and we fly parallel to and almost level with the top of a little ridge on final approach. No problem. That's why we have eyes. We land, uneventfully.
No, throughout this long story, how much did we have to adjust the altimeter? Not at all. That's the point. We set it to the field elevation before we departed Lexington and did not touch it after that.
However, we read the elevation of Morrison Field from the sectional chart, and added 800 feet to get the pattern altitude. And then we flew the pattern at that altitude. It is advisable to do this as opposed to just "eyeballing the field," because the proximity of the high terrain on three sides makes it a bit difficult for flat-land pilots to judge their altitude at an unfamiliar airport. In this circumstance, the tendency is to wind up too high.
And guess what the altimeter indicates after we were on the ground at Morrison Field. Answer: 2,780 feet. That's close to the field elevation of 2,750, but not exactly. Apparently the barometric pressure is just slightly less at Morrison as compared to Lexington. (The pressure difference to account for this amounts to only 0.03 in/Hg. That's only one and a half of the smallest divisions on the Kollsman window.)
7. Flying to where the Barometric Pressure is Different
The assumption here is that we set our altimeter to field elevation where we take off. This automatically sets sea-level pressure to the proper value in the Kollsman window. After taking off, we climb to our cruising altitude and hold this altitude during the flight by referring to the altimeter.
Let's have another look at the altimeter equation:
- A = 1,000 x ( K -- B )
During our flight, the Kollsman setting K is a fixed value corresponding to what we dialed in by setting the altitude at the field elevation before taking off. We fly at a constant altitude by keeping (K -- B) equal to the same value.
In slightly different terms, the quantity (K -- B) corresponds to a certain density of the air, or equivalently, to how much the air is compressed. As we fly along over the surface of the earth maintaining our altitude by reference to our altimeter, we will follow a path in the vertical dimension that always has the same air density. (This is the same as saying we will follow a path of equal pressure.)
If the sea-level barometric pressure is the same everywhere, our flight path will then remain at the same true altitude above sea level for the entire flight. But if the barometric pressure changes, our path will either climb or descend, in terms of true altitude. This assumes we fly the plane so as to keep the altimeter always "nailed" on the cruising altitude, and we don't change the altimeter setting.
There are two cases to consider:
Flying from a Low to a High:
In this case, the barometric pressure is higher at our destination than it is at the airport we just left. Now, as we fly along, the actual barometric pressure corresponding to our cruising altitude is going to slowly increase. And, the density of the air at our (true) cruising altitude will slowly increase. But we will not know that the air density has changed due to increased barometric pressure. We will assume that the increase in density is due to a loss of altitude. After all, the indicated altitude is slowly decreasing.
So, what will we do? Adjust trim, engine power, or whatever so that we keep the altimeter indication from going down. The result is that we will climb to a true altitude that is greater than our initial cruising altitude. We will climb in search of the same air density we had been tracking all along. And all the while, our altimeter will read very nearly the same thing.
In the equation, if (K -- B) is constant and B increases because we're flying into an area with greater barometric pressure, the indicated altitude will decrease. This is saying the same thing. We will climb to maintain the same altimeter indication.
An old saying: "Low to high, toward the sky."
Flying from a High to a Low:
This is just the opposite of the situation described above. Flying into a low pressure causes the indicated altitude to slowly increase. If we maintain the same indicated altitude by descending, our true altitude will gradually decrease.
A person flying on instruments or at night in a region where there is high terrain or obstacles must be aware of this and be sure to update the altimeter setting frequently.
Another old saying: "High to low, look out below."
Practically speaking ...
A VFR pilot flying a plane that cruises at 100 mph or less doesn't have to worry a lot about rapidly changing barometric pressures. Even on a long trip, it is unlikely that a single leg of the flight will cover more than 200 miles, and the barometric pressure will not change a lot in this distance. (If it does, you will probably have other problems to worry about ... weather and winds!)
If we simply reset the altimeter at each fuel/comfort stop to the field elevation at that point, a long flight gets broken down into a series of short ones. Altimeter settings will not be a problem.
Now, if you are even remotely involved with the ATC system, the controllers issue altimeter settings routinely. In this case, you simply update the setting in the Kollsman window. And that's it. Additionally, you can get the altimeter setting from tuning in an ATIS or AWOS or by contacting a flight service station.
8. Altitudes reported by Mode C Transponders
Most ultralight aircraft do not have transponders. Typically, only those that are operated in or near the airspace around major airports have them.
A transponder is an electronic unit on an aircraft that actively communicates with the radar used by ATC. Whereas ordinary radar functions by receiving reflected waves from the target, a transponder actually transmits a signal back to the radar transmitter. This improves the sensitivity of the system tremendously and allows the use of much lower radar transmitter power.
The transponder has a provision for setting a four-digit code that serves to identify the plane. The generic code for VFR flights is 1200. However, an IFR flight or a VFR flight that is using flight following or is operating under ATC control in a radar service area will be assigned an individual code.
The code is called the "squawk." The controller might say something like "Altimeter 30.14, winds 340 at 15 knots, squawk 1234." This means we are to set 1234 as the code in the transponder. That is our number, and we are 1234.
Mode A transponders were the first developed and only communicate position (along with the squawk). Mode C transponders are capable of reporting the altitude of the plane as well as its position. This enables the controllers to form a detailed picture of the planes showing on the screen.
Altitude reporting transponders are calibrated to report "pressure altitude." That is, the "altimeter" associated with the transponder is permanently set for 29.92 in/Hg, the sea-level standard. Once the reported altitude is received on the ground, the reported altitude is then adjusted for the local barometric pressure, corrected to sea level, of course. This way, the pilot does not have to worry about setting the barometric pressure in the transponder, and at the same time, the system is not subject to errors that a pilot might make in setting the pressure.
The bottom line is that a pilot doesn't have to set a barometric pressure on a transponder.
Acknowledgments: I would like to express my appreciation to Bud Connolly for his many explanations and for steering me toward information relating to barometers, altimeters, and so forth; for confirming and validating things I think I may have finally figured out, and also for letting me know when I had an item "not exactly the way it is" here and there. Thanks, Bud!
And to the members of the FlyChallenger group who posted many descriptions and explanations, and who raised many questions worthy of investigation, I owe you a big "Thank You" as well.