Often a pilot regards the airplane's weight and balance data as information of interest only to engineers, dispatchers, and operators of scheduled and nonscheduled air carriers. Along with this idea, the reasoning is that the airplane was weighed during the certification process and that this data is valid indefinitely, regardless of equipment changes or modifications. Further, this information is mistakenly reduced to a workable routine or "rule of thumb" such as: "If I have three passengers, I can load only 100 gallons of fuel; four passengers - 70 gallons." Admittedly, this rule of thumb is adequate in many cases, but as the subject "Weight and Balance" suggests, we are concerned not only with the weight of the airplane but also the location of its center of gravity. The importance of the C.G. should have become apparent in the discussion of stability, controllability, and performance. If all pilots understood and respected the effect of C.G. on an airplane, then one type of accident would be eliminated from our records: "PRIMARY CAUSE OF ACCIDENT - AIRPLANE CENTER OF GRAVITY OUT OF REARWARD LIMITS AND UNEQUAL LOAD DISTRIBUTION RESULTING IN AN UNSTABLE AIRPLANE. PILOT LOST CONTROL OF AIRPLANE ON TAKEOFF AND CRASHED."
The reasons airplanes are so certificated are obvious when one gives it a little thought. For instance, it is of added value to the pilot to be able to carry extra fuel for extended flights when the full complement of passengers is not to be carried. Further, it is unreasonable to forbid the carriage of baggage when it is only during spins that its weight will adversely affect the airplane's flight characteristics. Weight and balance limits are placed on airplanes for two principal reasons:
1. Because of the effect of the weight
on the airplane's primary structure and its performance characteristics;
2. Because of the effect the location of this weight has on flight characteristics, particularly in stall and spin recovery and stability.