Posts Tagged altimeter error
Sometimes reading a textbook on how something works doesn’t really do really good. For example, take the altimeter. How does it really measure pressure altitude and how does it translate that to the gears and linkages?
I was able to take a trip to a pretty fascinating museum in Chicago called the Museum of Science and Industry. In this museum was many things on many different topics but in particular they had a section devoted to aviation. Inside this section included many things including a full size replica of the 1903 Wright Flyer, a real Boeing 727 airplane.and displays on many of the scientific principles of how the instruments work.
One of these scientific principles is of the sealed aneroid wafer. The aneroid wafer is the device that makes the altimeter work. The principle behind the aneroid wafer is its ability to expand and contract with a difference in pressure. Many people believe the aneroid wafer pressure that is sealed inside is at sea level pressure but it really doesn’t matter. One thing I do know is that the pressure selected will aide in maximum resolution of the aneroid wafer. The animated GIF on the right shows what happens to the aneroid wafer as the pressure of the air is reduced.
Through gears and linkages the motion of the aneroid wafer is transferred to the face of the altimeter. Let’s take a look on the inside of the altimeter. From the picture below, we can see the sealed aneroid wafer with the ability to move up and down. This action is translated into an arm that is connected to a rotatable shaft. This shaft is connected to a rocker arm that in turn is connected to gears. These gears are designed in such a way that it moves the three levers on the face of the altimeter.
Now is a good time to discuss the Kollsman window. The window allows the pilot to select a different reference pressure datum. For example, if the current altimeter setting for an airport is 29.72” Hg we can set that pressure datum into the altimeter through the small dial in the picture above. It will rotate the big dial so the needles read within 75’ of the airport elevation on takeoff. The difference between a pressure altimeter required in §91.205 VFR day instruments and the sensitive altimeter required by §91.205 IFR instruments is the addition of the Kollsman window.
I’ve covered this fairly well in previous blog posts but it becomes absolutely clear in the picture above. Take a look again at the gears and linkages. You will notice the gears are made quite well and are designed to move the dials accurately. These gears and linkages cannot predict that the atmosphere will behave in a manner other than what it was described. This manner is roughly 1” of pressure loss equates to 1000’ gain in elevation. If the atmosphere has a non standard lapse rate (due to weather systems or non-standard temperature) the altimeter will not be accurate enough to show your true altitude above the sea.
GOING FROM HIGH TO LOW… LOOK OUT BELOW applies to both pressure and temperature changes. The most common use for this phrase deals with travelling cross country where you enter a different pressure system. This, however, can be corrected by following FAR §91.121 stating the pilot must set the altimeter to the current altimeter setting every 100NM. There are errors associated with non-standard pressure and non-standard temperature and a discussion of these errors are in my previous post.
Typically an altimeter that displays lower than true altitude is a good thing as the aircraft is actually higher than the indicated altimeter. This happens when there is a high pressure system or a higher than standard temperature. Since the aircraft is higher than indicated they are in no danger of hitting a mountain. Since each aircraft has the same error there is no danger of a mid-air collision due to it.
The altimeter will display higher than true altitude when there is a low pressure system or a lower than standard temperature. Applying the same reasoning from above, we can see that terrain becomes a factor but a midair does not. It is this inherent danger with terrain on cold days that pilots need to worry about.
I once heard a story that went something like this. I was flying a Falcon jet over the mountains at the MEA on a cold wintery night and I look down to see the trees and notice they are much closer than they usually are. Figuring I was at a lower altitude than I should be I cross check the altimeter and the low enroute chart to confirm I was at the right altitude. Sure enough, I was right at the MEA but the mountain trees looked so close as if I could reach out of the airplane and touch them. Later, I did some research and found the altimeters had some inherent errors in them I didn’t know before.
The FAA ATC system in the United States understands the effects of cold temperature on the altimeter but they will not adjust the minimum vectoring altitude to correct for them. Our friends in Canada, understand this limitation and will provide corrections to minimum vectoring altitudes. All is not lost though, the United States Air Force has a PowerPoint presentation available discussing this very altimeter error and is probably seeking the ATC system to change this practice.
Since Canada is ahead of the U.S. in this regard, I am going to reference the Aeronautical Information Manual from Canada. Their book is a lot like ours and is separated into sections. We are going to look into the “RAC” section or Rules of the Air and Air Traffic Services. On page 10 of the PDF document you will notice a previously posted picture of the cold weather correction chart. I personally think the FAA version is easier to look at though. In fact, the version the FAA has in their handbooks comes from the Canadians.
OK. Here is the formula as stated from Transport Canada.
The “Altitude Correction Chart” was calculated assuming a linear variation of temperature with height. It is based on the following equation, which may be used with the appropriate value of to, H, Lo and Hss to calculate temperature corrections for specific conditions. This equation produces results that are within five percent of the accurate correction for altimeter setting sources up to 10 000 ft and with minimum heights up to 5 000 ft above that source.
From reading the AIM, it shows this formula to be accurate up to 12,000 feet AGL. Another more accurate formula is presented in the manual that is accurate up to 36,000. I think the formula presented is good enough to calculate this chart. An analysis of the formula takes the minimum MSL height (MEA) and multiplies it by the difference between the current temperature and standard temperature) and divides that result by absolute temperature in Kelvin.
Why is it that the FAA has deemed a minimum 2000’ MEA on IFR low enroute charts in mountainous terrain instead of the standard 1000’ minimum for non-mountainous terrain. Is it possible that our altimeter is less accurate in the higher terrain of the mountains? Maybe, but the altimeter is calibrated at least once every 2 years to be able to be used under IFR flight. So what is actually happening? The answer lies within how the altimeter determines the current pressure altitude and the assumptions that are made with respect to how the atmosphere works.
A quick review of the atmosphere. We know the standard atmosphere at sea level is 15° C and 29.92” Hg and weighs 14.7 pounds per square inch. We are taught that the standard pressure decreases at a rate of 1” Hg per 1000’ and at 18,000 feet the atmosphere is half of the atmosphere at sea level. We are also taught the standard temperature decreases at a rate of 2° C per 1000’. The pressure of the air at sea level is determined by the weight of the air above it. Weight is affected by the temperature of the air as well as the pressure.
A standard altimeter uses an aneroid wafer that senses the pressure of the air and translates that pressure into an altitude. Inside the aneroid wafer is a sealed chamber with a certain pressure (not important to know but it is speculated to be at standard pressure 29.92” Hg). As the pressure of the air outside the aneroid wafer changes either due to an increase in pressure or weather conditions (i.e. low or high pressure system), the aneroid wafer will expand or contract and through gears and linkages will adjust the altitude on the altimeter.
A couple things to notice about this picture. The only thing that moves the gears and linkages is the pressure changes on the aneroid wafer. There is a way to set the altimeter setting which adjusts the gears as well but doesn’t have an effect on this discussion just yet. For the altimeter to work correctly, and to pass the required altimeter checks of §91.411 it must be calibrated to the standard pressure and rates of change. This means the gears and linkages are calibrated to read 1000’ MSL when the pressure is 28.92” Hg and the temperature reads 13° C. The altimeter is constructed to assume standard conditions all the way up through the atmosphere.
How often is the atmosphere standard? A recent friend of mine made a poignant statement and said the atmosphere has never been standard in the 40+ years of his aviation career. Interesting.
The altimeter setting for a field is the barometric pressure that field would be at if it was at sea level. Put another way, the altimeter setting is a correction for non standard atmospheric conditions to ensure the altimeter is accurate at the field.
I always assumed that the early pilots came up with the slogan “From High to Low look out below” dealt with the altimeter of the day, a pressure altimeter. An altimeter without a Kollsman window. In fact, a sensitive altimeter (one with a Kollsman window) is subject to the same errors though they are corrected somewhat. In the graphic below, we can see that a higher temperature (or a higher pressure) will cause our true altitude (above MSL) to increase above our altimeter referenced altitude. If the air pressure and/or temperature are above standard the true altitude will be higher than the altimeter referenced altitude and if the pressure and/or temperature are lower, the altimeter referenced altitude will be lower. Non-standard pressure has a greater effect on altimeter errors than non-standard temperature.
Unfortunately, it is quite difficult to determine with any reliance non-standard pressure trends and relay them to the altimeter so the instrument reads correctly. With that said, we might start to understand why there is a higher MEA for mountainous terrain than non-mountainous terrain. Let me explain, the altimeter setting we use is from a station and the altimeter is calibrated to read true altitude (MSL) at that station. The farther away from the station the less accurate the altimeter becomes. Since, we cannot quantify non standard pressure, in reference to the altimeter, a large error can happen. Unfortunately, the mountain tops don’t adjust themselves for non-standard pressure so a larger safety margin is necessary due to the distance the plane is from the altimeter setting source. To prove this point, I have pulled up some METAR reports from the Rocky mountains with weather stations on top of the passes versus METAR reports from the airports below. If you take a look at this graphic to the right, you can see that Monarch Pass and Cottonwood Pass both have altimeter setting higher than the lower airports. Monarch Pass is currently at 30.75” Hg and Cottonwood Pass is at 30.48” Hg. The lower airports are at 30.37” Hg and 30.36” Hg respectively. The surface analysis chart shows a high pressure system over the area during these METAR readings.
There is a way to calculate the effects non standard temperature has on true altitude. Our Canadian friends to the north of us are required to compensate for cold temperature when they shoot an instrument approach. It will be a discussion for another blog on how the chart is calculated but a decent formula to us is for every degree below standard and altitude above altimeter station (in 1000s feet) subtract four feet. We can see that a non standard temperature has a greater effect the higher above the station than at a lower station. In essence, the temperature error decreases to zero as the pilot completes the instrument approach. It is those intermediate segments that can cause issues for the non acquainted pilot to cold weather.
In the end, we know the altimeter is affected by both non-standard pressure and non-standard temperature. The FAA has designed safety into the airway system to minimize the effects pressure and temperature have on the altimeter and although the FAA is not yet there with requiring cold weather corrections to instrument approaches I will imagine they will soon follow suit like our Canadian friends to the north.