3.D : Performance and Limitations

Aircraft have a designed allowed Center of Gravity range within which the aircraft can be loaded. This is a range where the aircraft exhibits behaviors that are within the desired certification constraints. Loading the aircraft outside this range risks compromising the safe operation of the aircraft.

The formula above is used to calculate the CG for the aircraft. Therefore to calculate the CG for a given loading in the aircraft first start with the empty weight. From there list everything that will be loaded into the aircraft, including people, fuel, baggage, and everything else to be loaded. Add up all the weights and first determine if the loading is within the aircraft’s certified max gross weight. If it is over the max gross weight, something has to be left behind.

With that done next identify where all the items will be loaded, and calculate the moment of each item. This is most commonly done by adding up all the weights at a given station (where the station is a load point at a given distance from the aircraft datum), then adding up all the moments for all the stations.

Once the total weight has been determined along with the total moment, the CG can be calculated based on the formula above. Then using the weight and balance information in the POH determine if the CG falls within the allowed range. If it does not, change the loading until it is within the designed CG limits. Never fly with the aircraft loaded outside the manufacturer’s designed CG range!

Even though air is light, and seems insubstantial, it has mass and is affected by gravity and therefore it can exert force. Air pressure, under standard conditions at sea level, averages about 14.7 pounds per square inch. Since air is a gas it can be both compressed and expanded, therefore it’s density can change.

The density of the air has significant effects on the aircraft’s performance. As the density of the air increases (i.e. lower air density) airplane performance increases, and vice versa. There are a number of things that impacts air density. The key items that can alter density are listed here.

Regarding humidity, in simple terms the reason for this relationship is that Oxygen molecules (O₂) are being replaced with water molecules (H₂O). Hydrogen is considerably lighter than oxygen, and by replacing oxygen with hydrogen the density of the air decreases.

Lower air density affects performance in various ways. It causes the engine to produce less power because the engine will be taking in a smaller mass of air. The propeller is less effective in thinner air because there is less mass to be accelerated backward. And lift is less in thin air because the efficiency of the wing is decreased simply due to fewer molecules against which to work.

At power settings less than 75% and at density altitudes greater than 5,000 feet, the engine must be leaned for maximum takeoff power. Otherwise the mixture will be excessively rich which will deter engine performance. A higher elevations hi temperatures may have such an effect on density altitude that safe operations may not be possible. Even at lower temperatures with excessive humidity performance can be marginal and weight may need to be reduced.

Airplane performance charts are normally found in Section 5 of the POH/AFM. These charts are used to determine expected aircraft performance under a variety of conditions. They can be used to help calculate cruise performance, stall speeds based on aircraft configuration, wind components (crosswind and headwind), takeoff and landing distances, climb performance, true airspeed, and maximum flight duration.

In order to make use of these charts we need to know the pressure altitude (PA). This altitude is the indicated altitude when the altimeter setting window is set to 29.92. From this pressure altitude the density altitude (DA) can be calculated. The DA is the PA corrected for non-standard temperature, and the DA value directly relates to aircraft performance.

The normal way the chart is used is to start at the bottom of the chart with the temperature. Move up from that point to the position on the chart corresponding to the PA, and then from there follow the trend line to take the reading at the edge of the chart which will correspond to the DA.

Once the performance expectations for the current aircraft and conditions are determined, these are related to the airport/runway environment in which the aircraft will be operated. HOWEVER, it is critical to remember that these numbers are optimal/maximum performance numbers. These charts do not make any allowance for pilot proficiency or mechanical imperfections. It is also always possible that changes in weather conditions can make previously calculated performance numbers invalid. Therefore, plan ahead and be ready to confirm that the numbers being use are still valid.

The limitations published for an aircraft delineate the boundaries within which the aircraft can be safely operated. Exceeding any of these limits risk an adverse outcome. These include, but are not limited to the following scenarios.