The weight shift formula is an important part of understanding your weight and balance and is crucial for pilots flying with passengers and baggage. This equation must be known by all commercial pilots when studying for the CPAER Exam, so let’s look into how to use the weight shift formula.
The weight shift formula can be used to shift weight or add/remove weight to get the airplane within the weight and balance limitations by solving 1 of the 4 variables. It is important to note that while using the weight shift formula concerning centre of gravity, positive values indicate further aft and negative values indicate towards the firewall.
Example 1
You are flying a 2500 pound aircraft loaded with 2 passengers and their baggage. Your boss wants you to load the aircraft at the most aft centre of gravity to save money on fuel during cruise. You complete the weight and balance with all baggage loaded in baggage area 1 and determine the C of G is 1.75″ forward from the most rearward position. Given baggage area 1 is 110″ and baggage area 2 is 150″, how much baggage should be transferred to baggage area 2?
Using the weight shift formula we determined that you will need to move approx. 109 pounds to baggage area 2 to shift the C of G 1.75″ aft.
Example 2
You are conducting a sightseeing flight of 2 passengers + yourself. You have your current fuel and weight of the passengers so you calculate your weight and balance to see if you are within the limit. You determine that your total weight is 2500lbs and current C of G is 44″ Aft of the datum and the aft limit is 47″. Your passengers show up with their kid who is 100lbs and asks if they would be able to come as well. Determine if your C of G will remain within limits given an aft cabin moment arm of 60″?
Using the weight shift formula you can calculate the C of G will shift back .62 making it 44.62 and still within your limit of 47″.
The weight shift formula can be used to solve many different scenarios by plugging in your knowns and solving for 1 unknown. This is also quicker than guessing on moving weight and calculating an entire new weight and balance.