Monday, February 11, 2013

Weight and Balance - A Personal Delima - Safety

Center of gravity and Weight Balance of an Aircraft

The point at which the entire weight of an airplane would balance perfectly. The center of gravity affects the stability of the plane and, for safe flight, the plane must remain within specified limits established by aircraft manufacturers while in flight.

When the weight of a plane is at or below the allowable limits for its type and configuration and its center of gravity is within the allowable range and both will remain so or the duration of the flight, the plane is said to be within weight and balance.

The configuration refers to whether the plane is parked, moving on the ground, take-off, landing for example.
You may define maximum weights for different situations. Maximum weight may be higher, for large aircraft, in take-off configuration than for maximum landing weight. (weight is lost from fuel burned.)

Fuel burned may affect the center of gravity. In a Beechcraft Bonanza, the center-of-gravity may move toward the rear of the plane as fuel is used since the fuel tanks slope downward from fore to aft in the wings. Depending on other load considerations you have to make sure your center-of-gravity stays within the allowable range for the entire flight.

Since safety is the theme it does matter when weight and balance are incorrect in fixed wing aircraft. 

Placing the center of gravity of a plane outside the allowed range can lead to a unavoidable crash. If the plane is outside of its acceptable fore-aft axis the plane may pitch uncontrollably down or up. This excessive pitch change could occur in all phases of flight or certain phases where it doesn't occur on takeoff but could move outside of its fore-aft axis before you land. Calculations of center of gravity must take those possibilities into account. One example is the rear-most baggage compartment of the Beech Craft V-Tail Bonanza. It is placarded for 275 pounds. If you carry three adult passengers and your-self the fore-aft center of gravity may be close to the aft limit of the envelope. If your flying to reach a vacation spot you may pile golfing equipment into that placarded 275 pound baggage compartment. 

That was a fatal mistake. On take-off the plane would nose up with severe control problems. If you were successful in combating control of your aircraft during takeoff the  control of the plane in flight would  become worse as fuel moves the center of gravity further aft. This is a big killer of pilots that are not careful about weight and balance.

If the weight of the aircraft is out of range (heavier than the maximum weight) the plane may not achieve or sustain controlled, level flight. Excessive take-off weight may make it impossible to take off within available runway lengths, or it may completely prevent take-off. If you do manage to take-off, without incident, it may slow your rate of climb and/or make climbing beyond a certain altitude difficult or impossible. Refer to prior article dealing with Mountain Flying for the dangers with an under-powered plane and/or over gross aircraft in mountainous territory.

With you always in flight safety,