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as quickly as possible, and use
maximum TAS.
Jet engines are more efficient at
higher altitudes (and lower
temperatures) because less fuel is
needed for a given thrust. On top of
that, lift and drag are reduced, which
means less thrust is required anyway.
Jets also work best at the top end of
the RPM range.
So, jet engines produce a lot of
power, but they use a lot of fuel
doing it, especially at low level. A
jet’s efficiency comes from range,
not endurance, so you must plan to
get as far as possible in any flight
(sometimes called gross fuel flow, but
see Specific Ground Range, below). It
follows that long range flights need
huge fuel reserves, another good
reason for planning, because the
Commercial Department would
rather you carried passengers than
fuel. You are always walking a fine
line with complex aircraft.
There are one or two ways of
increasing fuel efficiency in the
cruise. The cruise climb, in an ideal
350 Canadian Professional Pilot Studies
world, would mean that you climb
steadily as your fuel is used up. This
is not convenient for ATC, so, in
practice, a stepped climb is used
instead, where you go up a couple of
thousand feet at a time.
When finally at a level, another
method is to set a constant power and
accept increasing airspeed, assuming
maximum speeds are not exceeded,
but it makes ETAs harder to
calculate. The technique mostly
adopted is to set a constant airspeed
and reduce power as fuel is used.
Long Range Cruise means choosing a
reasonable TAS which reduces as
fuel burns off and weight decreases.
Not surprisingly, there are charts for
this, which will give you cruising air
miles per lb of fuel. Because fuel
flow varies due to weight at the start
and end of each leg, when using fuel
flow charts, etc., you should estimate
the mid-weight for the leg in
question, because it would be
artificially high at the start. It need
not be exact, as you only need an
average fuel flow. Thus, you need to
learn to interpolate between columns
and rows.
Fuel Management
For most trips, this revolves around
getting the maximum range for a
given amount of fuel or, looked at
another way, how little you can get
away with on a fixed distance.
However, occasionally you are asked
to hold, or patrol, and the question
of how long you can stay airborne
arises, namely endurance.
The optimum cruise level for a
turbojet is where you can obtain the
best angle of attack at max
continuous power, which will
increase as fuel is burned off. A
constant Mach cruise is held near the
optimum level, and power is reduced
to maintain it, since the speed to
maintain optimum level also
decreases, and constantly changing
speed and altitude is inconvenient.
Maximum Range Speed comes out of
the Lift/Drag curve. For propdriven
planes, this coincides with
L/DMAX but, as with best ROC,
turbojets achieve this at higher
speeds. For a headwind, speed up to
minimise the time affected. With a
tailwind, slow down and let the wind
do the work.
When looking into the average fuel
table, you will see that fuel usage
decreases with height, but so does
the TAS, which offsets most of the
advantage gained, so altitude is not
always the answer, especially if the
winds change. Thus, fuel calculations
are done with a view to counting
units of fuel per unit of distance
over the ground.
Specific Ground Range
Older hands might know this as
gross fuel flow. It's useful when
IFR and planning on a
particular fuel consumption, but
actually burning something
entirely different. It also gives
you an idea of the most efficient
cruising altitude, allowing for
winds. Thus, the best altitude
for range will be that which
gives you the most SGR, and
you need the groundspeed first.
Divide fuel flow by
groundspeed to get lbs per nm,
or the other way to get nm per
lb of fuel, if those units are
used, e.g. 600lb/hr at 120 kts is
5lb/nm or 0.2 nm/1lb fuel.
Flight Planning 351
For example, if an aircraft flying
at FL 320 has a TAS of 494 kts,
a headwind component of 50
kts and a fuel flow of 7900 kg
per hour, its fuel burn would be
17.8 kg/nm. At FL 350, with
TAS 486 kts, head wind 55 kts
and fuel flow of 7500 kg/hr, it
might be 17.4 kg/nm. Other
things being equal, the best FL
for range would therefore be FL
　
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