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mile out and back together. Then
divide the result into the endurance.
Otherwise, to get the (normal) PNR
time in minutes:
E x H
O + H
where E = endurance (in minutes),
O = gspd out (kts) and H = gspd
home (kts). All it does is find the
ratio of the groundspeeds and divide
them into the endurance, assuming
normal TAS. For example, with 3
hours' endurance, and a 90 kt
groundspeed outbound, with 150 kts
home, the equation is:
180 x 150 27000
90 + 150 240
The answer is 112.5 minutes, based
on a ratio of 9:15. Just apply the
groundspeed to get the distance if
you want to mark it on the map, or
work it out with this:
E x O x H
O + H
The PNR moves into wind from the
mid point (exam question), which
will be half the endurance in nil wind
conditions (this is a useful check for
gross error). The exam does not
necessarily take account of the TAS
changing as weight decreases,
328 Canadian Professional Pilot Studies
multiple tracks or climb and descent
figures, but they are something you
might want to consider in real life.
Critical Point
Where it's just as quick to go on or
go back, useful if you have an
emergency and want to land as soon
as possible. It has nothing to do with
fuel, so just because you have the CP, you
don't necessarily have the range. The
formula is:
D x H
O + H
where D = total distance. Others as
above, but the speeds could be
reduced ones after the CP if an engine
fails. If you have multiple legs (say,
on an airway route), just add them all
up and treat them as one distance.
The CP also moves into wind from
the mid-point (exam question).
You can do this (and PNR) on the
slide rule by placing the groundspeed
home on the inner scale against
groundspeed out on the outer scale.
Then look for two numbers which,
when added together, comprise the
distance or Time To Turn, as
appropriate (187.5 and 112.5 for the
above example).
The CP can differ depending on the
type of emergency. If it's a medical
one, for instance, your aircraft will
have no change in performance
characteristics and you won't lose
any airspeed. However, if you lose
an engine, it will all change
drastically, as you'll also find out if
you lose pressurisation and have to
descend. As most aircraft keep
similar fuel flows with less airspeed
with one engine out, this means
effective range will decrease by
whatever percentage.
This is where proper reserves
become important, because that
slower speed will eat into whatever
you planned to have left over.
So, to summarise, used reduced
airspeeds after an engine failure, but
use full airspeed to calculate an ETA
at the CP.
Oil
Just check before flight that the
engine has been topped up
according to the manufacturer’s
recommendations, and that no
excess oil consumption has taken
place between flights.
Maps
When planning a flight, the first
thing to do is to draw a line on the
map to represent the track you wish
to fly. Find the mid-point and mark
it with a cross. Then mark the line at
10 nm intervals (or split the two
halves into quarters), and draw
dotted lines branching out at 10°
from the origin. These may be left
out once you know what you're
doing, but when learning they are
very useful when calculating drift
once you find the wind is different
from that forecast and you need to
recalculate on the run. The less you
have to do in the air, the better, as
your first priority is to fly the
aircraft, and you don’t want to start
getting rulers out and spreading your
map around. In fact, you will find
that a lot of your job lies in the
planning – your time in the air is a
relatively small part of the process. If
it isn't, there's something wrong.
Flight Planning 329
When starting out, circle over the
departure point and depart exactly
from it (when you're a flying ace, just
depart from the circuit), accurately
fly your planned heading for six
minutes, note your position, then
compare it to the map, and the 10
nm intervals you drew on it.
Whatever distance you have flown in
six minutes is your groundspeed
multiplied by ten, so 10 miles in 6
minutes is 100 kts. You now have
either confirmation of planned
groundspeed or a new one, so you
can create a new ETA. On your
commercial test flight, after about
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