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