by Jonathan M. Stern
Overall Weight Limitations
The overall weight of the loaded airplane must be within design tolerances. The lift produced by the airplane must be sufficient to overcome weight by a margin that allows safe climb characteristics. The capability of an airplane to climb is a function of the amount of power available in excess of that necessary to sustain level flight. For this reason, an airplane with two engines may lose all of its climb capability when one engine fails.
An overloaded airplane is unsafe for numerous reasons. An overloaded airplane has, to varying degrees, each of the following characteristics:
- Increased Takeoff Speed—because more lift is necessary to counter the additional weight, higher speed is necessary to create sufficient lift to attain flight.
- Longer Takeoff Roll—the increase in necessary speed for takeoff and slower acceleration due to increased weight translates to more runway required to accelerate the airplane to takeoff speed. It is possible to overload an airplane to a point where no amount of runway is sufficient to reach takeoff speed. Were it not for forces such as aerodynamic drag and friction of the landing gear against the runway, this would not be true, but these forces are present and limit the performance of the airplane.
- Reduced Climb Angle—increases in weight must be countered by additional lift. Lift that is otherwise available for climb performance now must support the additional weight. The airplane's capability to out climb obstructions near the airport may be compromised.
- Reduced Rate of Climb—for the same reason that the angle at which the airplane can climb is reduced, the rate at which it can climb is also reduced. This means more problems if an engine fails during or shortly after takeoff.
- Lower Ceilings—because air density normally decreases as you go up in the atmosphere, there is an altitude at which an airplane climbs no more. This is known as the absolute ceiling of the airplane, and it occurs where the maximum indicated airspeed in level flight is just above stall speed. As the weight of the airplane is increased, the stall speed increases. Accordingly, an increase in weight results in a reduction in absolute ceiling and, in severe situations where there is high terrain, it may be impossible for the airplane to climb above the terrain.
- Lower Cruising Speeds—production of additional lift to counteract greater weight results in an increase in drag. This increased drag reduces the speed at which the airplane travels, thereby exacerbating the problem of the increased stall speed.
- Shorter Range—because cruising speeds are reduced by overloading the airplane, the range of the airplane is also reduced. On a trip that calls for most of the airplane's normal range, the destination may prove to be unreachable.
- Less Maneuverability—the heavier the airplane is, the less maneuverable it becomes. This is so because the force necessary to change the speed or direction of an object in motion increases with the mass of the object.
- Higher Landing Speeds—because stall speed is higher when the airplane is overloaded, higher approach and landing speeds are necessary.
- Greater Landing Distance—increases in touchdown speed increase roll out distance exponentially. Therefore, an increase in weight that requires touchdown at a speed that is only five percent greater than normal may have a significant impact on the required runway length.
- Aircraft Structure Overload—although the primary concern of an overloaded airplane is its effect on aerodynamic performance, a secondary concern is its effect on structural components, such as landing gears.
A pilot has relatively few choices on how to respond to a potential overload situation. First, it is clear that the load must be restricted so that the airplane is at or below its maximum gross weight. Second, except in rare circumstances where equipment in the airplane, such as radios or seats, are removed, the only way to limit the total load is to reduce the number of passengers, the amount of baggage or cargo, or the fuel load.
If the planned fuel load is greater than that necessary to get the airplane to its destination, on to its alternate (on certain instrument flights), and leave the required reserve (45 minutes at normal cruise speed for instrument flights and nighttime visual flights; 30 minutes for daytime visual flights), then a reduced fuel load should be the first consideration. Of course, this assumes that fuel is available at the destination or alternate and, in some cases, such an assumption is invalid. At six pounds per gallon, reducing the fuel load can result in substantial weight savings.