Ask the Mechanic – Weight and Balance

 Ask the Mechanic – Weight and Balance

A subject that pilots study in training is Weight and Balance. It is obvious too much weight will make it difficult for a plane to lift off the ground, but what is the ‘balance’ part of this topic? How does it affect the pilot as he/she loads the plane, or when the aircraft gets in the air?
 
The four forces acting on an aircraft in flight are; Weight, Lift, Thrust and Drag. The forces of flight are interconnected, and a change in one affects the others. We are interested in Weight, and its inseparable companion, Balance. Children on a teeter-totter learn that the heavier child can take control from a lighter sibling even though they are the same distance from the pivot point.
 
It was found there was a point where the weight of the aircraft was concentrated – known as the Centre of Gravity (CG). If the CG was slightly forward or aft of this point, then the controls could compensate and the aircraft could be flown safely. If this range was exceeded, the pilot would be unable to control the angle that the wings made with the air making the aircraft unstable and unpredictable.
 
To control this potentially lethal factor, the aircraft designer had to arrange the weights of individual components so the balance of the aircraft was in harmony. The weight of the engine chosen had to be counterbalanced by the weight of the structure, passengers, fuel and freight. Could the plane take full fuel and a full passenger load at the same time, or would one have to be limited? Could it take the baggage and freight for a commercial flight?
 
The aircraft is weighed in a level attitude with scales under each landing gear. The other critical factor is the distance each weight is located from a reference point. The weights and distances would be recorded and calculations made. From this calculation a graph would show the range of weights and moments that would keep the CG within limits. This information was given in the pilot’s handbook so that a quick determination could be made before every flight to ensure the safety of the flight.
 
But what if the pilot disregarded this information, or the load was incorrect? The consequences could be serious. If the weight is excessive, the takeoff run would be longer because the takeoff speed would be higher, the rate and angle of climb would be reduced and maneuverability would be decreased.
 
The aircraft’s balance and where its CG is located, is even more critical to the safety of flight because the CG’s location affects the aircraft’s stability. The more aft the CG, the more unstable the aircraft. Forward pressure on the elevator control may be necessary to keep the aircraft from pitching up and stalling. Conversely, a forward CG needs backward pressure on the elevator control and nose-up trim. A forward CG makes it difficult to rotate for takeoff and flare for landing. Incorrect loading can cause the aircraft to be outside the CG limits – even when it is within its gross weight limit.
 
 
What are the weight and balance concerns for a pilot when taking his father-in-law and grand-children for a flight on a sunny, summer afternoon? Firstly, the pilot must estimate the weight of the load – the passengers and fuel. For a short flight, perhaps half-full fuel tanks will be sufficient as this will reduce the overall weight. Now where are the passengers going to be seated? The father-in-law would like to sit in the front seat so he can see out better. This leaves the two small children in the rear seats. Will this make the plane too nose heavy? Is it better to place one child in the front seat to even the load? What if they decide to go a little further afield and deliver the package of books to Aunt Betty. This will take more fuel and the books in the rear baggage com-partment will put the CG further back. Will this be a problem? Or can they go with partial fuel and refuel at the island? Maybe they should take more fuel and put the books in the cabin so that the CG is not so far back. The pilot can use the aircraft handbook to run through the various scenarios, or, more likely these days, plug the information into a digital assistant to get the answer.
 
But get an answer he must. The consequences of poor pre-flight planning are serious.