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