Reading height with the altimeter
Around 1900, the French Louis Paul Cailletet invented the first altimeter, which also found its way into early airplanes. However it was in 1928, when the German emigrant Paul Kollsman modified the then used altimeters and thus invented the altimeter that is still in use today. He founded his company Kollsman Instruments Co the same year with just $500 to sell his invention. Ten years later his company was worth an estimated $4 million.
Let's have a closer look at a Kollsman style altimeter...
An altimeter looks like an analogue clock
A typical altimeter in a general aviation aircraft looks a bit like an analogue clock. It has a circular scale from 0 to 9 and three pointers show the current altitude in 100 ft, 1,000 ft, and 10,000 ft. If the 10,000 ft pointer points to 0, the 1,000 ft pointer to 5, and the 100 ft pointer to 2, the aircraft is at an altitude of 5200 ft. Most altimeters also have a striped sector; when the stripes are visible the current altitude is below 10,000 ft.
The altimeter is only able to show an altitude to a relative outside air pressure. This is why altimeters also have a Kollsman knob and window that lets pilots change the air pressure reference so they can read a meaningful altitude. We'll explain how this works in a bit. First, let's have a look how an altimeter works inside and why air pressure is so important...
Air pressure makes an altimeter 'tick'
An altimeter is nothing more than a barometer which is calibrated to read atmospheric pressure as an altitude. The prevailing air pressure at any point in the atmosphere is called static pressure. Why do we have static air pressure? Although air has little weight it is enough to create pressure at the earth's surface.
An altimeter is nothing more than a barometer which is calibrated to read atmospheric pressure as an altitude.
The closer to the ground, the more air above and hence the greater the air pressure. The principle of decreasing pressure at higher altitudes is exactly what makes an altimeter work. We know, for example, that for each 30 ft altitude, the air pressure reduces approximately by 1 hPa on average (at least within the first 5000 ft), therefore the pressure lapse rate directly relates to height above the earth's surface.
The altimeter is calibrated against the pressure lapse rate
To measure static pressure an airplane has static vents that are connected to the altimeter via tubes. The altimeter is calibrated against the pressure lapse rate in the so called International Standard Atmosphere (ISA). This is how the altimeter knows what altitude corresponds to what static pressure measured at the static vents. So, we know now that an increase in static pressure means that the altitude drops (the aircraft is closer to the ground) and vice versa. Let's have a closer look at what the Kollsman window does...
Adjusting for highs and lows
We know that pressure relates to altitude, but we really need to know the static air pressure on the ground level to get our altitude in feet. Unfortunately, the ground level air pressure fluctuates with the prevailing weather systems, depending if we are currently located within a high or low pressure system. So we really need to be able to set the current ground pressure, and this is what the Kollsman knob and window allow us to do. First, we need to know what the local pressure is, then we need to set it until the correct value is indicated in the Kollsman window, and then we know our true elevation in feet. Let's have a look at different Kollsman settings...
Which reference pressure to set?
In aviation there are three different standards to set the pressure in the Kollsman window. Any of these settings give us a different altitude reading.
The most commonly used setting in general aviation is QNH, which is the current pressure at mean sea level (MSL). If the correct QNH is set, the altimeter reads the height above mean sea level. This is also called altitude. When someone refers to altitude, they are referring to height above mean sea level.
With the correct QNH setting the altimeter reads the height above mean sea level.
Another not so common setting is the QFE. This is the pressure at the elevation of an airfield and indicates the height above the landing level. When someone refers to height, they are referring to height above ground. At higher altitudes the QNE is often used, which is the standard altimeter setting of 1013 hPa. This is used for high level flight, where altitude is given as a flight level (e.g., an altitude of 15000 ft is a flight level of 15).
The altitude at which QNH and QNE are used differs from country to country. Why the change? At lower altitudes it is important that pilots know their true altitude to avoid terrain and obstructions. With the correct QNH set, pilots can refer to charts and avoid obstructions. At higher altitudes the danger if aircraft collision is more imminent. While the QNE setting does not give you a true height above ground reading, all aircraft in the same area read the same altitude. If every aircraft would use the correct QNH this wouldn't be a problem. However, the QNH needs to be manually updated quite frequently, so it's very likely that different aircraft have different settings. Hence, the QNE standard makes flying safer at high altitudes.
When pressure introduces errors
One problem with pneumatic altimeters is the so called barometric error. As the height reading depends on the Kollsman value, the pilot obviously has to frequently update the pressure setting to match the QNH. For example if the airplane would fly towards an area of low pressure, and the pilot wouldn't update the pressure setting but keep the altitude indicated on the altimeter, the aircraft would actually descent! It is important to bear in mind that the altimeter only indicates an altitude relative to the current pressure setting.
If an airplane flies towards an area of low pressure, and the pilot doesn't update the pressure setting, the aircraft would actually descent!
As explained earlier, the altimeter is calibrated in the International Standard Atmosphere. The ISA assumes a standard temperature. However, temperature varies, just as air pressure does, and to make matters worse, temperature is related to air pressure. Warmer air relates to a lower pressure lapse rate and vice versa. This means if our aircraft is in warmer air and the pressure lapse rate is lower, our aircraft will be at a higher altitude than indicated on the altimeter.
No altimeter is perfect
Disturbances and changes of airflow past the static vents affects the air pressure at the static vents. Therefore, large angles of attack or slip and skid can affect the reading on the altimeter.
It also takes a small amount of time for any pressure changes to travel down the tube connecting the static vents and the altimeter. This leads to a small lag in the altimeter readings, especially during rapid climbs and descents
Sometimes the static vents can become blocked, for example, if they freeze over. In that case the air becomes trapped and the altimeter will continue to indicate the same altitude - it'll appear locked. Some aircraft have an alternative static vent inside the cabin. If an aircraft doesn't, the pilot can break the glass of the vertical speed indicator (VSI) - the instrument we discuss in a minute. Since the altimeter and the VSI are connected, this will allow the static pressure in the cabin to find its way to the altimeter. One thing to be aware of is that, due to the airflow over the cabin, the static pressure inside the cabin is actually lower than outside. Therefore the aircraft will be lower than indicated.
That finishes our look at the altimeter. Let's have a closer look at another pressure instrument - the vertical speed indicator.
Vertical Speed Indicator
In the early days of balloons, balloonists did not only want to know how high they are, but also how fast they were going up or down. The vertical speed indicator gives this information and is, like the altimeter, linked to air pressure. With the building of their airship fleet, the Germans built more sophisticated instruments that also found their way into airplanes.
Rather than measuring static pressure directly, the VSI measures the rate of change of static pressure, which is calibrated into rate of change of altitude.
In powered aircraft, the VSI supplies the pilot with crucial information during starting and especially landing, since the pilot can directly control the rate of decent. The VSI is also very important in air gliders as it indicates rising air which will take the glider into higher air. After World War One, the VSI was introduced into gliding by the Austrian Robert Kronfeld, which revolutionised the gliding sport as glider pilots now could actively seek areas of rising air. In gliders the VSI is commonly called variometer after the German word for the VSI (gliders were the only aircraft allowed in Germany after the first world war, hence the gliding sport was especially developed in Germany).
Let's have a look at a VSI...
A pointer either indicates up or down
The VSI has a circular scale with zero being to the left middle. The upper scale indicates rate of climb, the lower half indicates rate of descent. There is only one pointer which points to either rate of climb or rate of decent, depending on what the aircraft does. The scale on the VSI is usually hundreds of feet per minute. The variometer in gliders often indicates meter per second.
Lets have a look how the VSI works...
The VSI measures change of static pressure
Like the altimeter, the VSI is driven by air pressure. However, rather than measuring static pressure directly, it measures the rate of change of static pressure, which is calibrated into a rate of change of altitude. The VSI, in princple, compares stored static pressure against the current static pressure. The VSI stores the pressure in a container and as the pressure changes, the VSI shows the difference in pressure. The container is connected to the static port by a small valve which allows the stored pressure to slowly adjust to the current pressure. However, the stored pressure always lags behind, hence the VSI always shows a difference.
No instrument is perfect
As discussed above the stored static pressure in the VSI always lags behind. Naturally, this introduces a noticable lag in the VSI reading. Therefore, pilots should not "chase the needle", i.e., constantly adjust rate of altitude change, but rather wait until the needle has settled. While the VSI can be used as a secondary indicator for level flight (i.e., when the VSI indicates zero, the flight is level), the altimeter should remain the primary instrument for height information.
As discussed above, pressure fluctuations at the static vents can lead to errors in the indicated change of altitude. It's worth noting, that small errors occur due to this effect during the takeoff roll of small airplanes.
When the static port becomes blocked (e.g., iced over), the pressure in the VSI becomes trapped and it will indicate zero change.
This concludes our look at the two instruments that use pressure to convey height information.
Christian Stock
Normally seen and heard scooting about in light GA craft of various sorts, I surmised the Messerschmitt Bf 109 would be like swapping the Family sedan for a Porsche 911. This aircraft certainly has a lot more power compared to what I'm normally used to, but flying in a WW2 fighter certainly made for an interesting change.
