Flight with an aft centre of gravity

From a very early stage in their training, pilots are taught the importance of loading an aircraft correctly so that it will be within its centre of gravity limitations for flight.

The further forward an aircraft is progressively loaded, the more longitudinally stable it becomes, right up to the point that it can no longer be properly controlled. With the aircraft’s nose tending to pitch in the direction of the landing gear, the required elevator or stabiliser trim settings in all regimes of flight will be further up than normal. Stick forces required to counteract the nose down tendency will also be higher than they usual.

Conversely, the further aft an aircraft is progressively loaded, the more longitudinally unstable it becomes. The nose will tend to pitch towards the top of the cockpit and the control column needs to be pushed forwards. If the aircraft is loaded beyond the aft centre of gravity limit, the pitch-up tendency of the nose cannot be contained and control of the aircraft will be lost.

Every aircraft has a different c of g envelope

There are thousands of different types of aircraft in circulation around the world, all of which vary in size, shape, weight and performance. Each aircraft has a unique specific centre of gravity envelope within which it must be operated.

Mistakes have been made

From time to time, mistakes have been made in the loading of aircraft right across the spectrum − from small executive aircraft, to light, medium and heavy airliners and freighters. Erroneous forward centres of gravity have resulted in an inability to get aircraft airborne timeously, usually resulting in an overrun of the runway. Centres of gravity that were aft of the limit have resulted in accelerated rotation rates and a pitch-up into extreme nose-up attitudes that have been followed by loss of control due to a stall.

An aft centre of gravity is deliberately planned

Notwithstanding all of the above, with the multitude of electronic and automated systems and applications that exist in this day and age, airliners are deliberately loaded to achieve a centre of gravity position that is as close to the aft limit of the envelope as is possible. This practice, used by airlines all over the world, offers certain advantages that translate into savings in operating costs.

How an aft centre of gravity reduces costs

The tail-plane of an aircraft contributes to the total amount of lift generated by its aerofoils. If it happens to be generating an upward lifting force, the main wings will not need to “work” that hard, because the contribution from the tail-plane will be helping to balance the weight of the aircraft.

Consider two similar airliners both weighing exactly 60 000kg and flying at the same speed and height. For argument’s sake, let’s make a hypothetical, but nevertheless reasonable, assumption that each aircraft’s tailplane, with its elevators in the neutral position, is contributing 3 000kg of lift. This equates to 5% of the overall lift of the aircraft. The main wings, therefore, need to produce only 57 000kg of lift in order for the total lift that is generated to exactly equal the weight of the aircraft.

A forward c of g is undesirable

Now, the first aircraft has a full forward cargo compartment and a centre of gravity that is on the forward limit. The aircraft is nose-heavy and the pilot needs to pull back on the control column and fly with nose-up trim in order to maintain height. The elevators on the aft side of the stabiliser would have moved upwards,thus generating a downward or negative, tail-down force on the tailplane.

If we were to assume that this downward force amounted to 3 000kg, the main wing would need to produce the full 60 000kg of lift on its own as there would be no contribution to the overall lift from the tailplane. This would be done by flying the wing at an increased angle of attack. The adverse reaction to the increased angle of attack would be an associated increase in drag that would require additional thrust. The rate of fuel consumption would then increase. Also, the stall margin of the aircraft would be reduced as the wing’s angle of attack would be closer to the critical stalling angle of attack than it would be ordinarily.

An aft c of g results in cost savings

The second aircraft has an empty forward cargo compartment. The cabin and aft cargo compartment has been loaded to achieve a centre of gravity that’s on the aft limit. The aircraft is tail-heavy and the pilot would need to push forward on the control column and fly with nose-down trim. The elevators would have moved downwards and would be generating an upward force in order to balance the pitch-up tendency of the nose. If we were to assume that this upward force amounted to a further 3 000kg on top of the 3000kg that already existed, then the main wing would only need to produce 54 000kg of lift as a result of the extra contribution from the tailplane. The aircraft’s wing would be able to fly with an angle of attack that was meaningfully reduced. The rate of fuel consumption would be lower and the stall margin of the aircraft would be increased because the wing’s angle of attack would be further from the critical stalling angle of attack than it would usually be.

So, flying with an aft centre of gravity gives the best economy, but be sure to check that your aircraft is within the weight and balance limits!