Friday, April 26, 2013


Having understood direction and distance, it is now important to understand speed and velocity, as relevant to air navigation. This is primarily because in basic air navigation there is no way of measuring distance in the air. Distance can only be estimated by the formula that we learnt in school, viz. S = ut, or distance is equal to velocity multiplied by time flown. It would be simple if the medium through which we fly remained stationary.

However, the medium of air through which we fly is moving too, displacing our aircraft along with it. You will study the ‘what’ and ‘why’of winds in your meteorology classes. You must have flown kites, and should thus be pretty familiar with how they drift with the wind. Aircraft, even the largest ones, are affected in a similar manner by these same winds. In this chapter we will try and understand the constituent parts of the velocity triangle that forms the basis of air navigation.

Speed and Velocity: Speed is a linear quantity, whereas velocity is a vector quantity. Speed is distance covered in a unit of time, whereas velocity is speed in a given direction. Both are expressed in knots or nautical miles/ hour. In air navigation we are concerned with the following velocities: -

  • Aircraft velocity through the air – given by True Air Speed (TAS) and heading (direction in which the fore and aft axis of the aircraft is pointing) (Single arrow)
  • Aircraft velocity on the ground – given by Ground speed and Track (Twin arrow)
  • Wind velocity – Speed and direction of the wind. (Three arrows)

Vectorial Addition: Since all of the above are vector quantities, they can all be represented graphically by straight lines in the given direction. The length of the lines would be proportionate to the speed at a given scale. We can vectorially add two vectors and get the resultant third vector. An aircraft going from place A to B is subjected to its own velocity (TAS and heading) through the air, and the wind velocity. (Wind velocity is the speed of the wind and direction from which it is blowing). Both these vectors combine to produce a resultant, the third vector comprising of the speed and the direction that the aircraft follows on the ground, known in aviation as ground speed and track.

The Velocity Triangle: The velocity triangle comprises of six variables, as shown above – TAS, G/S, Heading, track, wind direction and wind speed. In case we know any four it would be possible to find out the other two by solving the velocity triangle. In air navigation, we can measure the desired track to go from A to B from the map or chart; we can find the best TAS to fly for the desired altitude from the POH; and the wind velocity can be obtained from the meteorological briefing. With these four variables known, it is possible to find the other two, viz. the heading and the ground speed, by solving the velocity triangle with the help of the Dalton computer or electronic flight computer.

Drift: Drift is a direct consequence of the wind velocity – if the wind is from the left, the aircraft would drift to the right (starboard drift), and vice versa. In a pilot’s language, with a wind from the left, the pilot would need to offset the aircraft nose into the wind or to the left in order to maintain the desired track on ground, or heading would be left of track for a starboard drift. 
  • It is important to understand drift and the consequent offset of the aircraft nose required to counter it. 
  • Drift is the angular difference between the heading and track of an aircraft, and is port or starboard depending on whether the track is port or starboard of heading.
Solve: In nil wind conditions what is the relationship between TAS and GS.
Solve: In nil wind conditions what is the drift.
Solve: An aircraft is preparing to land on an airfield with two runways; 09/ 27 and 12/ 30. The reported surface winds are 120/ 30 kts. After landing the pilot applies the same braking pressure in all cases.
  • Which is the best runway to land on? Why?
  • On which approach would the aircraft appear moving the slowest to a ground observor? Why?
  • On which approach would the aircraft have the highest rate of descent? Why?
  • On which runway would the aircraft stop in the shortest distance? Why?
  • In case the aircraft were to plan landing on all four runways - list out the brake temperatures at the end of the landing roll in each case, from hottest (4) to coolest (1).

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