L5-Speeds25-11-2005 | |
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L5-Speeds25-11-2005 | |
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Here are short explanations about the speeds commonly used in aeronautics. 1 - INDICATED AIRSPEED (IAS) : The IAS
is the speed the pilot has onboard on the airspeed indicator. When
configuring your flight simulator, e.g. FS2002 or FS9, there are two choices,
IAS or TAS. For flying on IVAO, it must be set to IAS to be consistent
with the speed instructions that may be issued by a Controller. The IAS is a safety speed for handling the aircraft. Particularly, stall speeds (Vs, Vso) or gear and flaps limitation speeds (Vle, Vlo, Vfe) are indicated airspeeds. In FS, make sure to select SHOW IAS in the AIRCRAFT / REALISM menu. When expressed in knots (general case), the IAS is often written KIAS. This is the speed used in pilot/controller communications under the general term "airspeed". 2 - TRUE AIRSPEED (TAS) : The TAS is the speed of an aircraft into the air mass it is in, whatever the density of this air mass is. It is often shown on FMS or GPS or can be calculated from IAS (see below). The TAS is a flight planning and navigation speed which enables calculations of deviation or ETA (see GS). When expressed in knots (general case), the TAS is often written KTAS. Relation between TAS and IAS : - for IAS between 240 and 400 kt and FL between 50 and 250, a good approximation is given by TAS = IAS + FL/2 Imagine an aircraft steady at FL 120 and 320 KIAS, KTAS = 320 + 120/2 = 380 - for IAS lower than 240 kt, TAS = IAS + (1.5% IAS x altitude) (alt expressed in thousands of feet). Imagine an aircraft steady at FL 150 and 220 KIAS, KTAS = 220 + ((1.5% x 220) x 15) = 220 + (3.3x15) = 270 However, remember the lower and slower the aircraft is, the closer to the IAS is the TAS. 3 - GROUND SPEED (GS) : The GS is the TAS corrected with the wind speed and represents the speed of the aircraft relative to the ground. It is shown on FMS and GPS or can be calculated from TAS when wind direction and speed are known (see below). This is the speed you need to calculate your estimated time of arrival (ETA) over a point. Imagine your TAS is 260 kt and you have a 20 kt headwind. Your GS is 260-20 = 240 kt. This means you fly 4 NM a minute (240/60). If you read on your DME you are at 30 NM from a VOR/DME and the controller asks your estimate over this navaid, the answer is 30/4 = 7.5 mins. Another way is to calculate your base factor : bf = 60/GS. Here, you have 0.25. Your estimated time of arrival over the navaid is ETA = d x bf. In our example, you find 30 x 0.25 = 7.5 mins. The GS is also the one you can see on Pro Controller's screens (this is normal, a radar calculates the moving speed of a mobile according to the ground). Remember the speed you see on the screen is usually different from the one the pilot sees onboard. You can have a pilot flying at 220 KIAS (indicated airspeed onboard) that will appear at 270 kt (GS) on your scope. If below 10,000 ft, do not ask the pilot to reduce speed to 250 kt, he/she will answer he/she cannot comply with your instruction. 4 - MACH : M = TAS / a where "a" is the sound speed. Approximatively,we can consider that M.10 = 60 KTAS. For Mach 0.8, KTAS is about 8 x 60 = 480 kt, Mach 1 is 600 KTAS, Mach 1.2 is 720 KTAS. 5 - OTHER SPEEDS : They are all indicated airspeeds, shown on the airspeed indicator onboard. a) TAKE-OFF : V1 = take-off decision speed (or critical speed). Before V1, the pilot can abort take-off. After V1, the pilot MUST take off. VR = take-off rotation speed at which the pilot pulls the flight controls to rise the nose and take off. V2 = take-off safety speed to be reached before passing 35 ft above runway altitude. b) CRUISE : Va = Maneuvering speed, max speed at which flight controls can be fully maneuvered. Vno = Normal Operation speed, max cruise speed. Vne = Never Exceed speed. Vmo = Maximum Operating speed (KIAS). Mmo = Maximum Operatinf Speed (Mach). c) APPROACH AND LANDING : Vfe = Maximum speed with Flaps Extended. Vlo = Maximum speed for Landing gear Operations (retraction and extension). Vle = Maximum speed when Landing gear Extended. Vs = Stalling speed (at max weight). Vso = Stalling speed with gear and flaps Out (at max weight). Vref = Reference speed (or landing speed) = 1.3 x Vso (Vref also known as Vat).
d) APPROACH REGULATION : Minimum clean speed = minimum speed with gear/flaps/slats up and airbrakes in, usually about 1.5 x Vso. Minimum approach speed = Vref (see above), 1.3 x Vso. |
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