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ship – knots were tied in it at regular
intervals, hence the name). 1 nautical
mile (nm) is taken as 6080 feet,
slightly more than a statute mile,
which is 5280 feet.
A kilometre is 1000 metres, and is a
1/10,000 of the distance between
the Equator and either Pole. 8 km
equals 5 statute miles.
Triangle of Velocities
In flying between point A and point
B, you will only get there by just
pointing the nose in the right
direction if there is no wind, or if it
is exactly on the nose or tail. This is
very rarely the case, so your aircraft
would drift off course, according to
the wind's direction, if you did
nothing to correct it. In other words,
you would end up a certain distance
left or right of the original target (in
the early days of the North Sea,
when navaids weren't around, pilots
would build in a slight error to their
calculations, so that they would
know which side of the rig they were
just in case it all went wrong).
The smart thing to do would be to
make a heading correction towards
the wind's direction to maintain a
straight track. This, unfortunately
does two things. Firstly, the body of
the aircraft is inclined more sideways
to the track and, secondly,
groundspeed is reduced, because
some of the energy from the engine
is used in keeping it there.
You can see from the above that the
speed of the aircraft through the air
is not necessarily the same as its
speed over ground - if you are flying
into wind, you will go slower relative
to the surface, and faster if the wind
is behind you. An aircraft in flight is
affected by the wind both along its
axis and from the side, or from a
head/tail or beam component.
You work out what the wind's effect
on your trip will be by getting the
forecast winds from the flight
planning office, and working out a
combination of three sides of a
triangle, called the triangle of velocities,
because a velocity expresses a
combination of speed and direction,
and we are concerned with those of
your aircraft, the wind and the
difference between them.
First of all, a few definitions:
· Track. The path the aircraft
intends to follow over the
ground, represented by the line
on a map from one point to
another (Track Made Good is the
actual path – the difference
between them is Track Error).
· Heading. The direction in
which the aircraft is pointed,
according to its compass, with
reference to True North,
because Track (above) and
W/V (below) are.
· Wind Velocity. The speed and
direction of the wind, based on
True North. The faster your
aircraft, the less its effect.
· True Air Speed (TAS). The speed
of the aircraft relative to the
148 JAR Private Pilot Studies
atmosphere, not necessarily the
same as that indicated, and not
necessarily the same as…..
· Ground Speed, or the speed of the
aircraft over the ground.
· Drift. The difference between
track and heading due to wind.
· Air Position. The position the
aircraft would have reached
without allowing for the wind.
· DR Position. The calculated
position of the aircraft.
· ETA. Estimated Time of
Arrival.
· Fix. Definite confirmation of
position by ground observation,
radio aids or astro nav.
The velocity of an aircraft in flight
will therefore consist of its heading
and airspeed, the former usually
expressed with reference to True
North:
In the diagram above, the heading is
270°(T) - the single arrow is the
symbol for the heading vector,
pointing the right way, of course.
When plotting, a scale is used, so if
the heading vector were 3 inches
long, at 50 kts to an inch it would
equal an airspeed of 150 kts, or the
air position after one hour of flight. If
we added the wind speed and
direction, the resultant between
them would represent track and
groundspeed, also to scale:
In this case, the wind vector is half
an inch long, meaning 25 kts,
coming from the North. Joining the
ends would therefore show your
ground position after one hour, and
your track and groundspeed, after
measurement (you will have deduced
already that two arrows are used for
the track and three for the wind –
the track arrows always go in the
opposite direction to the other two).
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