Hi guys,
I have a question about which I have been looking for an answer since a long time. As I fly around the world now in my (t)rusty DC-3 I have three devices that tell me about my speed. First there is the IAS, it reads about 170 knots. Second is my GPS, which reads 211 knots. Third is DME for NAV 2, which reads 199 knots. So three devices, three different speeds. But what is my actual speed?
Crash
Your isntruments are showing two of your airspeeds. The one on the DME is explained above but the other two are your Indicated Airspeed, shown on your Airspeed Indicator and your Ground Speed, shown on the GPS. There's another airpseed you need to be concerned with but that one you will have to calculate. It's your True Airspeed.
I copied this from the Learning Center in the sim.
Indicated airspeed (IAS)
The speed shown on the airspeed indicator uncorrected for variations in atmospheric density, installation error, or instrument error. Except at sea level under standard atmospheric conditions, IAS does not correspond to the aircraft's actual speed through the surrounding air (that is, its true airspeed, or TAS).
True Airspeed (TAS)
An aircraft's actual speed through the surrounding air. As an aircraft climbs, the surrounding air becomes less dense. Therefore, indicated airspeed tends to decrease as altitude increases. To determine how fast the airplane is really moving through the air, the pilot calculates TAS based on the aircraft's current pressure altitude and the outside air temperature. A pilot must know TAS to solve navigation problems and file flight plans. As a rule of thumb, at a given indicated airspeed, true airspeed increases about 2 percent for each 1,000 feet (305 meters) of altitude. Therefore, an aircraft flying at an indicated airspeed of 100 knots at 10,000 feet (3,050 meters) is actually flying at about 120 knots through the air.
Ground Speed
An aircraft's speed relative to the ground; an aircraft's true airspeed corrected for the effects of a headwind or tailwind. For example, if an aircraft is flying level at 120 knots with a 15-knot headwind, its ground speed is 105 knots.
Posted: Sat Mar 05, 2005 4:09 pm