Many pilots, from sport pilot to commercial pilot, tend to underestimate the importance of proper weight and balance of their aircraft. Load sheets are taken for granted and hasty calculations are made of the aircraft’s CG. Unfortunately, each year there are a number of accidents related to weight and balance issues. Many of these occurrences could have been avoided had more attention been given to weight and balance.
When a manufacturer designs an aircraft and the Local Aviation Administration certifies it, the specifications identify the aircraft’s maximum weight and the limits within which it must balance. The weight and balance system commonly employed among aircraft consists of three equally important elements: the weighing of the aircraft, the maintaining of the weight and balance records, and the proper loading of the aircraft.
The maximum weight of an aircraft is based on the amount of lift the wings or rotors can provide under the operating conditions for which the aircraft is designed. For example, if a small general aviation aircraft required a take-off speed of 200 mph to generate enough lift to support its weight, that would not be safe. Taking off and landing at lower airspeeds is certainly safer than doing so at higher speeds.
Aircraft balance is also a significant factor in determining if the aircraft is safe to operate. An aircraft that does not have good balance can exhibit poor manoeuvrability and controllability, making it difficult or impossible to fly. This could result in an accident, causing damage to the aircraft and injury to the people on board. Safety is the primary reason for concern about an aircraft’s weight and balance.
Another important reason for concern about weight and balance is the efficiency of the aircraft. Improper loading reduces the efficiency of an aircraft from the standpoint of ceiling, manoeuvrability, rate of climb, speed, and fuel consumption. If an aircraft is loaded in such a way that it is extremely nose-heavy, higher than normal forces are exerted at the tail to keep the aircraft in level flight. The higher than normal forces at the tail create additional drag, which requires additional engine power and therefore additional fuel flow to maintain airspeed.
The most efficient condition for an aircraft is to have the point where it balances fall close to, or exactly at, the aircraft’s centre of lift. If this were the case, little or no flight control force would be needed to keep the aircraft flying straight and level. In terms of stability and safety, however, this perfectly balanced condition might not be desirable.
The effects of overloading include:
· reduced acceleration/greater take-off speed, take-off run, & distance to clear a 50 ft. obstacle
· decreased angle of climb/reduced obstacle clearance capability after take-off
· excessive loads on landing gear, especially if the runway is rough
· reduced ceiling, rate of climb, & range
· impaired manoeuvrability & controllability
· increased stall speeds
· increased landing speeds, requiring a longer runway
· reduced braking effectiveness & structural strength margins
· with twin-engine aircraft, failure to climb/maintain height on one engine
Forward and aft limits on the centre of gravity (CG) are established during type certification; they are the extreme CG positions at which longitudinal stability requirements can be met.
Exceeding the forward CG limit usually results in:
· difficulty in rotating to take-off attitude
· increased stall or minimum flying speed against full up elevator
· extra tail down-force requiring more lift from wing, resulting in greater induced drag. This means higher fuel consumption and a reduced range
· inadequate nose up trim in landing configuration, necessitating a pull force throughout the approach making it more difficult to fly a stable approach
· difficulty in flaring/holding the nose wheel off after touch down. Inability to hold the nose up during a bounce can result in damaged nose landing gear and propeller
· increased loads on the nose landing gear
Exceeding the aft CG limit usually results in:
· pitch-up at low speed & high power, leading to premature rotation on take-off, or inadvertent stall in the climb or during a go-around
· on a tailwheel type, difficulty in raising the tail/maintaining directional control on the ground
· difficulty trimming, especially at high power
· longitudinal instability, particularly in turbulence, with possible reversal of control forces
· degraded stall qualities to an unknown degree
· more difficult spin recovery, unexplored spin behaviour, delay in/inability to recover