| The speed of an aircraft relative to its | | | | of sound. |
| surrounding air mass is called airspeed | | | | The speed of sound is variable. |
| and is normally measured in Knots. | | | | Depending on air temperature, relative |
| As you know already from school, the | | | | humidity and pressure, it is |
| term "speed" relates to distance | | | | approximately 1 117 feet or 340.48 |
| traveled per unit time. | | | | meters per second at 59 degrees |
| One Knot refers to one nautical mile per | | | | Fahrenheit. |
| hour. One nautical mile is equal to | | | | By the way, Concorde crosses the |
| 1.151 statute miles or 1.852 kilometers. | | | | Atlantic ocean with Mach 2.0 at 55 000 |
| The airspeed indicator displays the | | | | feet altitude. |
| speed of your airplane or helicopter | | | | Altitude seems like a simple term, it |
| through the air by comparing ram air | | | | means height. But in aviation, it can |
| pressure with static air pressure. The | | | | have many meanings. In this article I |
| greater the differential, the greater | | | | will guide you through the different |
| the speed. | | | | terms and definitions. |
| Now let us take a closer look at the | | | | 1. TRUE ALTITUDE |
| various definitions and meanings of the | | | | This is the actual or exact altitude |
| term "airspeed". | | | | above mean sea levelnormally measured in |
| 1. INDICATED AIRSPEED - IAS | | | | feet. |
| This is the speed shown on the aircraft | | | | 2. ABSOLUTE ALTITUDE |
| airspeed indicator and is used in pilot | | | | This is the actual or exact altitude |
| controller communications. | | | | above the surface. We also refer to |
| 2. CALIBRATED AIRSPEED - CAS | | | | height above ground level. If you fly |
| Calibrated airspeed is the indicated | | | | over open water, absolute and true |
| airspeed corrected for installation and | | | | altitude are the same. |
| instrument errors. Although these errors | | | | 3. INDICATED ALTITUDE |
| are minimized by the manufacturer, they | | | | It is the altitude above mean sea level |
| cannot be totally eliminated throughout | | | | indicated on the altimeter when set to |
| the full range of operating speeds. | | | | the current local altimeter setting. |
| 3. EQUIVALENT AIRSPEED - EAS | | | | Altimeter setting is the value to which |
| Equivalent airspeed is calibrated | | | | the pressure scale of the altimeter is |
| airspeed corrected for the compression | | | | set so the altimeter indicates true |
| of the air at a particular altitude. In | | | | altitude at field elevation. |
| the higher speed ranges, an aircraft | | | | 4. PRESSURE ALTITUDE |
| passes through the atmosphere so rapidly | | | | Pressure altitude is the altitude in the |
| that the air is compressed in front of | | | | standard atmosphere where pressure is |
| it. | | | | the same as where you are. It is |
| This error, often called | | | | indicated by the altimeter when the |
| compressibility, is generally considered | | | | barometric scale is set to 29.92 inches |
| insignificant when operating below 200 | | | | of mercury or 1013.25 millibars. |
| Knots. | | | | 5. DENSITY ALTITUDE |
| 4. TRUE AIRSPEED - TAS | | | | It is simply the pressure altitude |
| True airspeed is the calibrated airspeed | | | | corrected for non-standard temperature, |
| or EAS as appropriate corrected for | | | | pressure and humidity. |
| pressure altitude and temperature. | | | | Density altitude is not a height |
| It is the airspeed of an aircraft | | | | reference. It is primarily used as an |
| relative to undisturbed air and is used | | | | index to aircraft performance. |
| primarily in flight planning. On a calm | | | | On a hot day, the air becomes thinner |
| day, your true airspeed is the same as | | | | and density altitude increases and vice |
| ground speed assuming level flight. | | | | versa on a cold day. High density |
| 5. MACH | | | | altitude is a real hazard since it |
| Mach is theratio of the aircraft's true | | | | reduces aircraft performance in three |
| airspeed to the speed of sound. For | | | | ways. It reduces engine power, thrust |
| example, a speed of Mach 0.7 means the | | | | and lift. |
| aircraft is flying at 70% of the speed | | | | |