AOPA Turbine Pilot Magazine - July 2021 - 92

P&E PERFORMANCE
Takeoff distance (ft)
Power: Full throttle, 2,700 rpm
Mixture: Lean (appropriate fuel flow)
Flaps: Up
Gear: Retract after positive climb rate
Cowl flaps: Open
Not to be
used for
intermediate
obstacle
heights
3,000
2,500
12,000
2,000
10,000
8,000
1,500
10,000
8.000
6,000
4,000
2,000
SL
500
-40 -30 -20 -10 0 10 20 30 40 50
Outside air temperature (deg C)
3,200
3,000
2,800
Weight (lb)
2,600
2,400 0
Wind component (kt)
10 20 30
50
Obstacle height (ft)
THIS TAKEOFF PERFORMANCE CHART uses atmospheric conditions, aircraft weight, winds, and obstacle heights to estimate the distance
needed in the ground roll as well as that to clear a 50 foot obstacle. The highlighted example refers to departure from Laramie, Wyoming,
on a day with a density altitude of almost 9,000 feet. Operating below maximum gross weight and taking off into the wind help minimize
takeoff distance. The yellow line in the third panel emphasizes the penalty a tailwind exacts on takeoff performance.
Figure 1
and I superimposed the corresponding density
altitudes to the left of the first reference
line. It assumes that you've already computed
the pressure altitude for the field which is
1,000 × (29.92 - altimeter setting) + field elevation.
For example, the day we departed, the
altimeter setting was 30.22 in Hg so the pressure
altitude was 1,000(29.92 - 30.22) + 7,284
= 6,984 feet msl. Given that engines produce
more power at lower altitudes, we got a little
help from pressure since the pressure altitude
was 300 feet lower than field elevation.
In my own flying, I've seen altimeter
settings as low as 29.70 and as high as 30.70
and that corresponds to adding 220 feet or
subtracting 780 feet from the field elevation
respectively to obtain pressure altitude.
While lower pressure can hurt performance,
those effects pale relative to other ingredients
in takeoff performance.
Density altitude is pressure altitude
adjusted for nonstandard temperature,
which exacts a greater impact on takeoff performance
than variations in pressure. The
temperature that day was 17 degrees Celsius,
about 15 degrees C above standard conditions
for the field elevation. That meant our density
altitude was almost 9,000 feet msl and
92 | AOPA PILOT July 2021
the nonstandard temperature added about
300 feet to the takeoff roll.
The panel structure implies that any
combination of field elevation, altimeter setting,
and temperature that produces the same
density altitude will result in the same takeoff
performance, other factors being equal.
Weight. The second panel incorporates
weight into the calculation of takeoff distance.
It makes sense that the line segments
slope downward since weight is directly
related to takeoff distance. But note that
those lines become steeper at higher density
altitudes. This means that a given weight
addition has a greater impact on takeoff performance
at higher density altitudes than at
lower altitudes. In other words, if my other
son, Pete, had come on the trip with us, the
increase in takeoff roll at Laramie would have
been greater than at our home airport with a
much lower density altitude that day.
Winds. The slopes of the line segments
in the third panel highlight the profound
effect of winds on takeoff performance. The
15-knot headwind component we enjoyed
shaved 250 feet from our takeoff roll-a
Obstacles. Using the final panel, we
expect a ground roll of 1,050 feet and 1,800
feet down the runway, our aircraft should be
50 feet above the runway. Now, this panel
20-percent improvement. Remember to
calculate the headwind component and not
merely the wind speed. For example, if the
winds were 15 knots from a direction 45
degrees off runway heading, then the headwind
component would be less than 11 knots.
When I am flight planning, I usually plan on
calm winds and figure that taking off into the
wind will only improve my performance.
The performance charts include information
for taking off with a tailwind and
I extended those with dashed lines only
to emphasize their slopes. The line segments
for a tailwind are much steeper in the
upward direction than those for the headwind
sloping downward. Had we taken off
with a 10-knot tailwind, it would have added
500 feet to the takeoff roll alone compared
with a no-wind scenario and continued to
exact a severe penalty as we climbed away
from the airport. The moral of the story is
that taking off with a tailwind hurts performance
far more than a headwind helps it.
6,000
4,000
2,000
1,000
Density altitude (ft)
Reference line
Reference line
Reference line
Pressure altitude (ft)
CATHERINE CAVAGNARO
Distance (ft)
Headwind
Tailwind

AOPA Turbine Pilot Magazine - July 2021

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