by Gerard van Es, National Aerospace Laboratory (NLR), Amsterdam, The Netherlands
Many pilots (both commercial and private) 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 centre of gravity (CG). Unfortunately, each year there are a number of accidents related to weight and balance issues. Many of these occurrences could have been avoided if more attention was given to the weight and balance.
Aircraft are designed and certified to operate within certain weight and balance limits (see Canadian Aviation Regulations (CARs) Standard 527.27-Centre of Gravity Limits). Exceeding these limits can be dangerous. The regulations provide the stability, controllability, and strength requirements at all allowable CG positions and corresponding weights. The extreme forward and aft CGs must be established for all certified weight limits. The condition that typically determines the forward CG limit is that the aircraft shall be controllable on landing. This means that the aircraft shall be able to be trimmed at the high lift values required for the desired landing speeds (including abuse cases). Other flight control cases that can influence the forward limit of the CG are the capability to make a prompt avoidance pitch-up manoeuvre, the capability to make a prompt nose-down recovery at low speed, and adequate pitch control in abnormal configurations (failure cases). The above-mentioned conditions all apply to free air. On the ground, the forward CG limit is basically determined by the maximum loads on the nose landing gear for an aircraft with a tricycle gear configuration.
Static longitudinal stability is the most important factor in determining the aft CG limit. At the aft CG limit position, the aircraft should demonstrate that a positive natural stability exists, that the aircraft is capable of pitch control at low speeds and high thrust (e.g. during a go-around), and that an adequate control is possible in abnormal configurations. On the ground, the aft CG limit is determined by the minimum loads on the nose landing gear required for good nose wheel steering, the maximum loads on the main landing gears, the tipping tendency of the aircraft, and adequate directional control during the take-off run after an engine failure. These last conditions apply to aircraft with a tricycle gear, and not to those with a taildragger configuration.
What happens if the certified limits as defined in the CG envelope are exceeded? From design, the aircraft flight characteristics will be adversely affected whenever the certified limits are exceeded. For instance, as the CG moves aft, the aircraft will become less stable as the CG approaches the neutral point. If the CG lies aft of the neutral point, the coordination and control motions required to maintain a stable flight condition will exceed the capability of the pilot, and the aircraft will become uncontrollable. On the ground, CG aft of the aft limit can result in a tail strike due to the pitch-up of an aircraft with a tricycle gear configuration (even at low speeds during the take-off roll when power is applied to the engines). The effect of a CG position forward of the forward limit is evidenced by a decrease in elevator control capability. Because of excessive stability, the elevator control required to manoeuvre the aircraft is increased. At some point, elevator control might become insufficient to perform required manoeuvres, such as the flare during landing and a go-around. During takeoff, the CG position can be moved forward until it reaches the point where the aircraft is very stable but cannot be rotated, or can only be rotated with great difficulty because the elevator has reached its maximum deflection. An adverse CG position can also have significant effects on the loads imposed on the aircraft’s structural components and could cause structural failure. Exceeding the maximum weights as specified in the aircraft flight manual (AFM) does not necessarily adversely affect the flight characteristics. For instance, exceeding the maximum landing weight could result in a landing gear collapse. However, the landing gear structure is designed with a standard safety margin assuming a higher load than obtained during a normal landing at maximum landing weight. With this, it could be possible to land the aircraft somewhat beyond the maximum landing weight. Overweight landings are often made during emergency or precautionary landings. Exceeding the maximum takeoff weight (MTOW) will affect the flight performance characteristics. The take-off ground-roll distance increases and the climb performance decreases. As long as the aircraft is not significantly overweight, it should be able to take off safely. However, the margins reduce rapidly when an engine failure occurs during an overweight takeoff, if the runway is short for the aircraft, or if there are high obstacles along the take-off flight path that the aircraft has to clear.
Aircraft that have wing-mounted propellers can be faced with a unique problem when flying with a CG close to the aft limit. Control can be lost during the approach after selecting landing flaps followed by the initiation of power increase and/or a go-around. Lowering the flap will move the neutral point forward and change the pitching moment (this effect is not limited to propeller aircraft). The pilot feels this as a tendency for the aircraft to pitch-up, and needs to push forward on the control column to hold a steady flight path by lowering the elevator. The pilot will re-trim the aircraft by winding the trim wheel forward, which moves the trim tab to keep the elevator in the new position without the pilot having to maintain a push force on the control column. One feature of aircraft with wing-mounted propellers is that when the engines accelerate from idle power to full power, the neutral point moves forward (up to 10percent of the mean aerodynamic chord!). When the actual CG position is close to or slightly aft of the certified aft limit due to incorrect loading, the aircraft may be just stable during takeoff and cruise. However, this situation can change during landing, in which case the aircraft may become unstable after lowering the flaps to landing position, and may show a very strong pitch-up tendency. The normal reaction to increase power to recover from the pitch-up or to make a go-around will make things even worse as the neutral point moves forward significantly with the increase in power on aircraft with wing-mounted propellers.
More information on weight and balance issues can be found in a safety study conducted by the author, "Analysis of aircraft weight and balance related safety occurrences," (Report No. NLR TP-2007-153).-Ed.