Flight Training - July 2013 - 31

Center of gravity

Center of pressure

bag in the baggage compartment, behind
the rear seat. The second passenger then
walked around the airplane and sat in the
back seat with the smaller bag. During the
takeoff, witnesses stated that the airplane
appeared slow, with a nose-high pitch
attitude, and an immediate dip of the right
wing. They recalled the initial climbout
was also low and slow. The tower controller asked the pilot if he was experiencing
any difficulty, and the pilot responded
that he was, and was going to turn back
to the airport. A witness noted that the
airplane appeared to be having difficulty
gaining altitude, that the wings were moving up and down, and that the propeller
was spinning. The airplane then made a
sudden, sharp left turn and descended to
the ground. A post-accident examination
of the wreckage revealed no evidence of
preimpact mechanical anomalies. Damage
was consistent with a left-turning stall/
spin at impact. The airplane’s observed
nose-high attitude, and pitch trim found in
the full nose-down
position, indicated the likelihood that
the airplane was loaded with an aft
center of gravity.”
Calculating balance requires knowing the moment, or torque, that a given
weight is putting on the longitudinal axis.
To find moment, multiply the weight
by the arm, or distance from some fixed
reference point, which is found in the
airplane manual.

IF YOU LOAD your aircraft too far aft, the
aircraft becomes unstable. The nose will want
to rise because the tail is too heavy—if you have
more weight in the back than the elevator can
overcome, a spin may become unrecoverable.

For example, a 170-pound pilot sits
in a Cessna 172 with the seat all the way
forward (34 inches behind the reference
point, or datum). By simply sitting there,
that pilot is twisting the longitudinal axis
with 5,780 “pound inches” of moment
(170 pounds x 34 inches = 5,780 lb-in.)
with respect to the reference datum.
Add up the moments of the airplane
and all the items (including fuel) that are
in the airplane. Once you have the total
weight and moment for your proposed
flight, use the charts in the manual to
determine whether you are within limits.
A few airplanes (and certainly the
private pilot knowledge test) may require
that you also find the center of gravity
mathematically. For that number, divide
total moment by total weight. If you were
measuring moment in pound-inches, that
last equation will give you center of gravity in inches behind the reference datum.

Your flight instructor can and should go
over these calculations with you a few
times. In some airplanes the center of
gravity will shift as you burn fuel on a
long trip, requiring you to determine the
balance at the beginning of the trip—and
again for the conclusion of the trip, to
make sure you’re still within CG limits
when it’s time to land.
For some pilots, all this math can
be a drag. Today there are many good
programs and apps available for PCs,
smartphones, and glass cockpits that
make these calculations easier. There’s
no good excuse not to know where your
weight and balance situation stands.
One day, with the burdens of training
and testing behind you, you may even
find yourself thinking, I just want to go
fly! I’ve already done a million things to
prepare, and I’m tired of it. I don’t need to
calculate weight and balance every time.
We’ll be fine!
The really dangerous part is that you
very likely will be correct. The flight will,
in fact, be fine. So will the next flight.
Maybe even the next. Eventually, however, it won’t. There’s a popular quote
attributed to Capt. A.G. Lamplugh in the
1930s: “Aviation in itself is not inherently
dangerous. But to an even greater degree
than the sea, it is terribly unforgiving of
any carelessness, incapacity, or neglect.”
Unfortunately, time has shown that
aviation can be just forgiving enough to
let some lazy habits develop. Try not to
confuse good luck with good piloting.
Natalie Bingham Hoover, ATP/CFII/MEI, has given
more than 3,000 hours of instruction. She lives in
Germantown, Tennessee.

WEIGHT-AND-BALANCE RULES FOR REAL-WORLD FLYING
+ Practice calculating weight and balance until you are comfortable with it.
+

Check to make sure any program you are using is set correctly, and that the charts
you are using are the correct ones.

+

Whatever method you use for calculating, make sure the weight inputs are correct.

+

Even if you only verify that the airplane is loaded in a way you have already checked
before, or if you simply plug some numbers into an app, do it! Anything at all is better
than nothing.
JULY 2013 FLIGHT TRAINING

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Flight Training - July 2013

Table of Contents for the Digital Edition of Flight Training - July 2013
