CALCULATING IN-FLIGHT WINDS

Transcription

1 CALCULATING IN-FLIGHT WINDS

2 SITUATION: The TH of your aircraft is 350 with the TAS of 150 kts. GS has been determined to be 160 kts, and the Track is 355. What is the wind?

3 ESTIMATE! First compare TAS to GS to determine HW/TW component: GS<TAS =HW. Now, compare TH to TK: TH<TK means a Right Drift which is caused by a Left Crosswind. We know that we are experiencing a Left Tailwind. The 10% Rule states that at a TAS of 150 kts, 15 kts of crosswind will give us about 6º of drift. A drift angle of 5º would only require about 13 knots of crosswind. The in-flight wind would be closer to the wing line at about 225º/18 knots

4 Heading The word heading refers to the direction in which an aircraft is pointing. Heading may be measured from true north, magnetic north, or compass north, and is always expressed in degrees as a three figure notation. Thus, an aircraft heading true west is said to be heading 270(T). This could be 265(M) or 262(C), depending upon the amount of variation at the aircraft's position and deviation concerned. Similarly, an aircraft heading in a true direction of north east is said to be heading 045(T).

5 Track The track of an aircraft is the general term used to indicate the path followed by the aircraft over the ground. It will usually differ from the heading due to the wind velocity. If the aircraft is flying in conditions of zero wind then the track and the heading will be the same. Also, if the aircraft is heading directly into wind, or flying directly downwind, the track and the heading will be the same. In all other cases there will be an angular difference between track and heading, called 'drift', which is governed by the wind velocity. The angular difference occurs because the aircraft now has two forces acting on it simultaneously, and will therefore move in the resultant direction

6 Direction of the route between two wishes places is called the required track or course. The path which the aircraft subsequently follows is called the actual track and if the calculations made by the pilot are accurate, and the forecast wind velocities are correct, then the required track and the track made good will be the same. In these circumstances the aircraft should remain on track. However, if the actual track made good is not the same as the required track the aircraft is said to be off track and the angular difference which then exists between the actual track and the required track is called the track error. Track error is expressed in degrees port (P) or starboard (S) of the required track.

7 Velocity A velocity comprises two elements, a direction and a speed. In navigation the direction is measured clockwise from a given datum which is usually true north. Speed is the rate of movement per unit of time. The speeds used in navigation are nautical miles per hour which are named knots or abbreviated kt.

8 True Airspeed (TAS) and Groundspeed (GS) The speed of movement of an aeroplane relative to the undisturbed air is referred to as the true airspeed. If the speed is measured relative to the path over the surface of the Earth, it is referred as the groundspeed of the aircraft (G/S).

9 Heading. Wind Velocity. A heading of an aircraft is the direction in which the nose is pointing. Wind velocity is the direction from which the wind is blowing and its speed of movement over the Earth.

10 EXAMPLE An aircraft has a TAS of 300 kt and heading 290 (T). The drift is 17 port and the groundspeed as 345 kt. Determine by vector construction the wind velocity affecting the aircraft at this time.

11 SOLUTION Remember that drift is the angle between the aircraft's heading and track measured left or right (port or starboard) of the heading. Construct true north datum. Select a suitable scale ic. 1cm - 25nm Plot the true heading 290(T). Mark with one arrow. Measure the distance to scale for 1 hour at TAS (300nm = 12cm). This point is the air position. 5. Calculate the true track (290-17) = 273 (T) Plot the track 273(T). Mark with two arrows. Along the track to the same scale, measure I hour at groundspeed (345nm = 13.8cm). This point is the ground position. Join the ends of the two vectors. Remember that wind blows from air position to ground position. Mark with three arrows. Measure the length of the wind vector (approx. 4.2cm).

12 Solution TN Scale 1cm : 25kt TAS 300kt WS/WD 105kt/210 0 Hdg (T) Ground speed 345kt; Track (T) Drift 17 0 Port

13 EXAMPLES An aircraft heading 070 (T) at TAS 250 kts is making actual track of 090 (T) at a ground speed of 270 kts. Find the wind velocity An aircraft is heading 270 (T) at TAS 240 kts when the Wind Velocity is 200/60. Find the track and ground speed An aircraft is required to fly between two positions such that the required track between them is 045(T). The forecast W/V is 090/70. The aircraft's TAS is 260 kts. Find the heading to steer and expected ground speed for the flight Aircraft Hdg 090(T), TAS 450kts, W/V 025/50. What are TA and GS