It’s a smooth flight on a beautiful evening over the Washington cascades as your passenger peers out of the window of the family Cessna 172. “How high are we?” she asks curiously. You respond simply with 8,000 feet, seemingly satisfying her interest. But there’s actually more to the story…
The 5 types of Altitude that pilots reference:
Giving your passenger the simple answer was probably the best option. Nevertheless, pilot’s actually need to be aware of five different types of altitude. Yes, five! Knowing how to calculate and reference each is a critical pilot skill that every aviator must know. It is often a question that comes up on the Private Pilot checkride!
The five types of altitude are:
Indicated Altitude
This is the altitude read directly from the aircraft’s altimeter when it is calibrated to the current barometric pressure setting. This makes it possible for pilots to obtain a make-shift reference to an aircraft’s True Altitude. Careful though: you must be sure to regularly update the setting to the current barometric pressure. Failure to do so will result in erroneous readings.
True Altitude
This is the altitude as measured from Mean Sea Level (MSL). Imagine it as if you could make the earth beneath you disappear, revealing the level of the ocean below. The height created between you and the water is your True Altitude. If you are standing on earth, your true altitude would actually be the same as your elevation. This is the altitude used by air traffic controllers to instruct aircraft.
Absolute Altitude
Altitude measured from the terrain directly below the aircraft. This is also refered to as altitude Above Ground Level (AGL). This is important to know especially during flight planning. We definitely don’t want to hit any mountains!
Pressure Altitude
Altitude measured from a standard datuum plain of 29.92″ Hg You can find this by dialing this barometric pressure setting into the aircraft’s altimeter. Pressure Altitude is used when calculating aircraft performance as well as when operating in class A airspace (above 18,000 feet MSL).
Density Altitude
Referred to as the altitude the airplane “thinks” it’s at, Density Altitude is “Pressure Altitude corrected for non-standard Temperature”. Temperature and pressure have a huge effect on aircraft performance. Knowing this figure helps pilots determine take-off distance, landing distance, as well as climb performance.
How does an Altimeter work?
So how, exactly, do pilots know their indicated altitude? They reference an Altimeter! This device, essentially a barometer, is able to measure ambient air pressure and translate that information into an altitude value of use to the pilot. Each pressure level in the atmosphere has reasonably predictable pressure characteristics. This allows the instrument to reference a corresponding pressure value and then aggregate that into flight level values on the face of the device.
It does this by measuring the natural compressing and expanding characteristics of the air by way of pressure. As pressure increases, air molecules congregate together, taking up less space or volume. While in lower pressure, molecules space farther apart–using greater volume. This means that the space needed to occupy the same number of air molecules is greater. In a confined space, such as a balloon, this expanding and contracting of air can be measured. Measuring the circumference of a balloon, ostensibly with a tap measure, for instance, could then be correlated with the predicted altitude values (since altitudes have predictable pressure). This is much like how an Altimeter works it magic!
Anaroid wafers, as they are called, are similar to the balloon used in the example. As air expands at higher altitudes, air pressure decreases, causing the wafers to become farther spaced apart. This is measured through a series of linkages and gears which then move “hands”, similar to a clock, on the face of the instrument.
Inversely, as air contracts at lower altitudes, the wafers become less spaced apart. This generates a lower altitude reading.
How do you Set the Altimeter?
Before an altimeter can work correctly it must be calibrated. To “set” the altimeter, a pilot must know the current ambient air pressure. This is usually found by referencing the airports current ATIS information or by obtaining the airfields current METAR. Pilot’s may also receive updated altimeter settings in-flight from air traffic control.
Once the pilot has this value, they can adjust the altitude by toggling the barometric scale adjustment knob until the correct value is selected in the altimeter setting window, also known as the Kollsman window.
What if you don’t know the airports current barometric pressure? No Problem! Simply set the altimeter to the current field elevation found on a sectional or chart supplement. This will have the effect of rendering the correct indicated altitude. Just be sure to update the setting with ATC as you fly along through varying pressure environments while enroute!
What happens if you don’t regularly update the pressure setting? Well, it reads incorrectly…
Just remember the old pilot adage:
"High to Low - Look out Below!"
This means, that if you are going from an area of high pressure to an area of lower pressure the altimeter will read lower than you actually are.
"Low to High - Look to the Sky!"
If going from an area of low pressure to an area of high pressure the altimeter will read higher than you actually are.
Understanding Pressure Altitude
Pressure changes as altitude increases. Most people know the adage that “air becomes thinner as you go higher”. This means that air molecules become farther spaced apart–taking up a greater volume as you venture higher. (See figure below) So how does this relate to Pressure Altitude? As mentioned before, pressure altitude is measured from a Standard Datuum Plane of 29.92″ Hg. But atmospheric pressure is not always 29.92? So what is the purpose for a “Standard Atmosphere” and thereby, Pressure Altitude?
Pressure Altitude is important for two reasons:
To help predict aircraft performance
The Standard Atmosphere was adopted as a way to give pilots a reference point for aircraft performance planning purposes as well as relative means to measure the atmosphere in Meteorology.
When an aircraft takes off, it will almost never encounter the same temperature and pressure environment. Since this atmospheric phenomenon has such a great effect on aircraft performance pilots need to be able to precisely calculate this effect. Specifically, pilots need to be able to calculate take-off performance, landing distance, engine performance, and climb rate. Pressure Altitude is the root figure needed to derive Density Altitude, the principle figure used to calculate aircraft performance.
To standardize altitudes when operating above Flight Level 18,000
As aircraft fly high above, they fly much faster. This means they cover a greater area more quickly–traversing various pressure areas. Since indicated altitude depends on an accurate pressure setting, it would be near impossible for all pilots to have the correct setting at all times. More importantly, If two aircraft were to have the wrong setting, they could possibly collide! The use of setting the same Pressure Altitude reference of 29.92 about 18,000 feet MSL was adopted as a way to simplify operations and keep aircraft safe from colision.
We measure Pressure Altitude by using the atmospheres predictable changes in pressure characteristics. Using a Barometer, pressure changes by approximately 1″ Hg per 1,000 feet up to 10,000 ft.
When manufactures test fly aircraft, they characterize the operating specifications referencing Pressure Altitudes. When operating at higher altitude, as shown in the figure, the molecules are farther apart. Aircraft performance will predictably deteriorate as altitude increases. When at lower altitudes, the opposite is true. Air molecules are closer together; they are more dense. This has the effect of allowing the aircraft to perform more favorably.
The importance of Density Altitude
I mentioned that Pressure Altitude is used to help determine performance. This isn’t the end of the story. The problem is, Pressure Altitude only references pressure. There is another factor of the equation when discussing the aircraft’s performance–Temperature!
As temperature increases, air molecules become more excited. (See figure). This causes them to become more spaced apart decreasing their pressure and thus, their density. Since the molecules are more spaced apart, aircraft performance deteriorates.
Since the wings, engine, and propeller have less molecules to interact with, the wing generates less lift and the engine, less thrust. Pilot’s actually calculate this effect before every departure. By referencing Pressure Altitude, pilots apply a correction factor for temperature rendering Density Altitude.
Many pilots think of Density Altitude as “the altitude the airplane thinks it’s at” since it is possible to be at a very low physical altitude, but due to temperature and height above sea level, have a density altitude that is such of a very high altitudes performance characteristics.
High Density Altitude deteriorates aircraft performance in three ways:
Reduced lift generated by the wings
Since air is less dense at higher altitude, there are less air molecules for aerodynamic surfaces to interact with. Based on Newton’s Third Law of Motion, every action has an equal and opposite reaction, less molecules equals less force. The wings can’t produce as much lift.
Reduced thrust generated by the propeller
Aircraft propellers are airfoilds just like the wings. In the same way that wings produce less lift, the propeller also generates less thrust.
Reduced engine performance
Because air is less dense, less air molecules enter the engine cylinders to be combusted with fuel. Because both fuel and air are required to generate combusion, less air means less combusion and therefore, less engnergy output from the aircraft’s engine.
You may hear of pilots “leaning” the engine or adjusting the “mixture”. This is neccessary to help mitigate the effects of lower density at higher altitudes. If the mixture isn’t correctly set, the engine can begin to run rough or even fail if this type of neglegent operation is prolonged.
Since aircraft have specific density altitudes they operate best at, it is possible that after a certain density altitude, all performance is lost. This has actually be the result of many aircraft accidents as they fail to take off when conditions are at such a high density altitude that the aircraft cannot achieve the required performance for liftoff. It is critical, and required, that pilots be familiar with their aircraft performance and current Density Altitude before every departure.
So, what’s our Altitude?
Well, it really just depends on what type of altitude you are interested to know!
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