Light Sport Aircraft Weight and Balance Control
LSA Definition of Term
LSA is a category of simple, very basic, small, light-weight, low-performance aircraft, other than a helicopter or powered-lift, and a classification of aircraft specific to the United States. The Federal Aviation Administration (FAA) defines LSA as an aircraft with a maximum gross takeoff weight of not more than 1,320 pounds (600 kg) for aircraft not intended for operation over water, or 1,430 pounds (650 kg) for aircraft intended for operation over water; a maximum airspeed in level flight of 120 knots (220 kilometers per hour (km/h); 140 miles per hour (mph)); a maximum stall speed of 45 knots (83 km/h; 52 mph); a maximum seating capacity of no more than two persons (including the pilot); fixed undercarriage and fixed-pitch or ground-adjustable propeller; and a single reciprocating engine (if powered).
An aircraft that qualifies as LSA may be operated by the holder of a sport pilot certificate,whether registered as LSA or not. Pilots with a private, recreational, or higher pilot certificate may also fly LSA, even if their medical certificate have expired, as long as they have a valid driver’s license to prove that they are in good enough health to fly. LSA also have less restrictive maintenance requirements and may be maintained and inspected by traditionally certificated aircraft maintenance technicians (AMTs) or by individuals holding a Repairman: Light Sport certificate,and (in some cases) by their pilots and/or owners.
Weight and Balance
Aircraft such as balloons, powered parachutes, and WSC do not require weight and balance computations because the load is suspended below the lifting mechanism. The CG range in these types of aircraft is such that it is difficult to exceed CG limits. For example, the rear seat position and fuel of a WSC aircraft are as close as possible to the hang point with the aircraft in a suspended attitude. Thus, load variations have little effect on the CG. This also holds true for lighter-than-air aircraft, such as a balloon basket or gondola. While it is difficult to exceed CG limits in these aircraft, pilots should never overload an aircraft, as doing so may cause structural damage and/or failures.
Weight affects performance; therefore, pilots should calculate weight and remain within the manufacturer’s established limits at all times.
WSC Aircraft
WSC aircraft are one- and two-place aircraft that exceed the criteria of an ultra-light vehicle but do meet the criteria of an LSA. The definition for WSC can be found in 14 CFR part 1. A WSC aircraft used for sport and private pilot flying must be registered with an FAA N-number, have an airworthiness certificate, a pilot’s operating handbook (POH), and/or limitations with a weight and loading document aboard.
As mentioned earlier, WSC aircraft are commonly called trikes. These aircraft have few options for loading because they lack places to put useful load items. One-place trikes have only one seat and a fuel tank, which means the only variables for a flight are amount of fuel and weight of the pilot. Two-place trikes can accommodate a pilot and a passenger. This version may have a small storage bin in addition to the fuel tank.
The most significant factor affecting the weight and balance of a trike is the weight of the pilot and, if the aircraft has two seats, the weight of the passenger. The trike acts somewhat like a single, main-rotor helicopter because the weight of the aircraft hangs like a pendulum under the wing. Figure 4-2 shows a two-place trike, in which the mast and the nose strut come together slightly below the wing attach point.
When the trike is in flight,the weight of the aircraft hangs from the wing attach point. The weight of the engine and fuel is behind this point, the passenger is almost directly below this point, and the pilot is forward of this point. The balance of the aircraft is determined by how all these weights compare. The wing attach point, with respect to the wing keel, is an adjustable location. The attach point is moved slightly forward or slightly aft, depending on the weight of the occupants. For example, if the aircraft is flown by a heavy person, the attach point can be moved farther aft, bringing the wing forward to compensate for the change in CG. Figure 4-3 shows a close-up of the wing attach point and the small amount of forward and aft movement that is available.
Figure 4-3. Wing attach point.
Similar to airplanes, sailplanes, and powered parachutes, increasing weight creates increases in speed and descent rate. However, the WSC aircraft has a unique characteristic. Adding weight to a WSC aircraft creates more twist in the wing because the outboard leading edges flex more. With less lift at the tips, a nose-up effect is created and the trim speed lowers. Therefore, adding weight can increase speed similar to other aircraft, but reduce the trim speed because of the increased twist unique to the WSC aircraft. Each manufacturer’s make/model has different effects depending on the specific design. For detailed weight and balance information, characteristics, and operating limitations, always reference the specific manufacturer’s manual or POH for the make and model. Figure 4-4 shows an example of a weight and loading sheet that would be issued with a WSC aircraft. Every aircraft has its own weight and loading data that should come from the manufacturer. The example in Figure 4-4 comes from Airborne, an Australian company, named Airborne XT WSC aircraft. For additional information, refer to the Weight-Shift Control Aircraft Flying Handbook (FAA-H-8083-5).
Powered parachutes have many of the same characteristics as WSC aircraft when it comes to weight and balance. They have the same limited loading, with only one or two seats, and a fuel tank. A powered parachute acts like a pendulum with the weight of the aircraft hanging beneath the inflated wing (parachute).
The point at which the inflated wing attaches to the structure of the aircraft is adjustable to compensate for pilots and passengers of varying weights. With a very heavy pilot, the wing attach point would be moved forward to prevent the aircraft from being too nose heavy. Figure 4-8 illustrates the structure of a powered parachute and the location of the wing attachment.
A powered parachute used for sport and private flying must be registered with an FAA N-number, have an airworthiness certificate,a POH, and/or limitations with a weight and balance document aboard. The aircraft must be maintained properly by the aircraft owner, or other qualified personnel, and have the aircraft logbooks available for inspection. Always refer to the POH for weight and balance information specific to the powered parachute being flown. For additional information, refer to the Powered Parachute Flying Handbook (FAA-H-8083-29).
Weight and Balance Computations (Amateur-Built Aircraft)
A good weight and balance calculation is the keystone of flight testing an amateur-built aircraft. Accurately determining the aircraft’s takeoff weight and ensuring that the CG is within the aircraft’s design for each flight is critical to conducting a safe flight test.
The aircraft should be level when weighed, span wise and fore and aft in accordance with the kit manufacturer’s instructions, and should be in the level flight position. It is highly recommended that the aircraft be weighed in an enclosed area using three calibrated scales. Bathroom scales are not recommended because they are not always accurate.
Single-Engine Aircraft Weight and Balance Computations
Determining the Loaded Weight and CG
An important part of preflight planning is determining that the aircraft is loaded so its weight and CG location are within the allowable limits. The methods of accomplishing this are the manual computational method using weights, arms, and moments; the chart method using weight and moment indexes [Figure 5-2]; and the loading graph method, which eliminates the need for some mathematical calculations.
Figure 5-2. Airplane loading diagram.
Manual Computational Method
The manual computational method uses weights, arms, and moments. It relates the total weight and CG location to a CG limits chart similar to those included in the Type Certificate Data Sheet (TCDS) and the Pilot’s Operating Handbook/Aircraft Flight Manual (POH/AFM).
A worksheet, such as the one shown in Figure 5-3, provides a means to record and compute pertinent weights, arms, and moments for all onboard fuel, personnel, equipment, cargo, and baggage that is not included in the aircraft’s basic empty weight (BEW). Figure 5-4 is a sample of a typical equipment list where many of the pertinent weights and moment values can be found.
As part of preflight planning, fill in the blanks in the worksheet with the specific data for the flight.The following weights were used to complete the sample weight and balance worksheet in Figure 5-3.
| Pilot | 120 lb |
| Front seat passenger | 180 lb |
| Rear seat passenger | 175 lb |
| Fuel (88 gal) | 528 lb |
| Baggage A | 100 lb |
| Baggage B | 50 lb |
Multiply each item’s weight by its arm to determine the moment. Then, determine the total weight and the sum of the moments. Divide the total moment by the total weight to determine the CG in inches from the datum. For this example, the total weight is 3,027 pounds and the CG is 43.54 inches aft of the datum (a negative result would have indicated a CG forward of the datum).
To determine whether or not the airplane is properly loaded for this flight,use the CG limits chart. [Figure 5-5] Draw a line vertically upward from the CG of 43.54 inches and one horizontally to the right from the loaded weight of 3,027 pounds. These lines cross inside the envelope, which shows the airplane is properly loaded for takeoff, but 77 pounds overweight for landing. Note that for this sample chart, the envelope is defined by the solid black line that indicates CG limits at or below the maximum weight for takeoff and landing. There is an additional region identified by a segmented black line that includes weights suitable only for takeoff. It is important to note these subtle differences as they may or may not be found in every POH/AFM.
Figure 5-5. CG limits chart from a typical POH.
Loading Graph Method
The charts and graphs found in the POH/AFM can help simplify and expedite the preflight weight and balance computation process. Some use a loading graph and moment indexes rather than the arms and moments. These charts eliminate the need for calculating moments and make computations quicker and easier. [Figure 5-6]
Moments determined by multiplying the weight of each component by its arm result in large numbers that are awkward to calculate and can become a source of mathematical error. To eliminate these large numbers, moment indexes are sometimes used. The moment is divided by a reduction factor, such as 100 or 1,000, to get the moment index. The loading graph provides the moment index for each component to avoid mathematical calculations. The CG envelope uses moment indexes rather than arms and moments.
The CG limits envelope is the enclosed area on a graph of the airplane loaded weight and the CG location. If lines drawn from the weight and CG cross within this envelope, the airplane is properly loaded.
Loading Graph
Figure 5-6 is a typical loading graph taken from the POH of a modern four-place airplane. It is a graph of load weight and load moment indexes. Diagonal lines for each item relate the weight to the moment index without having to use mathematical calculations.
Compute Weight and Balance Using the Loading Graph
To compute the weight and balance using the loading graph in Figure 5-6, make a loading schedule chart like the one in Figure 5-7. In Figure 5-6, follow the horizontal line for 300 pounds load weight to the right until it intersects the diagonal line for pilot and front passenger. From this point, drop a line vertically to the load moment index along the bottom to determine the load moment for the front seat occupants. This is 11.1 lb-in divided by 1,000. Record it in the loading schedule chart. Determine the load moment for the 175 pounds of rear seat occupants along the diagonal for second row passengers or cargo. This is 12.9; record it in the loading schedule chart.
Figure 5-7. Loading schedule chart.
Determine the load moment for the fuel and the baggage in areas A and B in the same way and enter them all in the loading schedule chart. The maximum fuel is marked on the diagonal line for fuel in terms of gallons or liters. The maximum is 88 gallons of usable fuel. The total capacity is 92 gallons, but in our example, 4 gallons are unusable and have already been included in the empty weight of the aircraft. The weight of 88 gallons of fuel is 528 pounds and its moment index is 24.6. The 100 pounds of baggage in area A has a moment index of 9.7 and the 50 pounds in area B has an index of 5.8. Enter all of these weights and moment indexes in the loading schedule chart and add all of the weights and moment indexes to determine the totals.
Transfer totals to the CG moment envelope in Figure 5-8. The CG moment envelope is an enclosed area on a graph of the airplane loaded weight and loaded moment. If lines drawn from the weight and loaded moment cross within this envelope, the airplane is properly loaded. The loading schedule from the example in Figure 5-7 shows that the total weight of the loaded aircraft is 3,027 pounds, and the loaded airplane moment divided by 1,000 is 131.8.
Figure 5-8. CG moment envelope.
Referring to Figure 5-8, draw a line vertically upward from 131.8 on the horizontal index at the bottom of the chart and a horizontal line from 3,027 pounds in the left-vertical index. These lines intersect within the dashed area, which shows that the aircraft is loaded properly for takeoff, but it is too heavy for landing (similar to the previous example). Because of this, if the aircraft had to return for landing immediately after takeoff, it would need to fly long enough to burn 77 pounds (slightly less than 13 gallons) of fuel to reduce its weight for landing.
Multiengine Aircraft Weight and Balance Computations
Weight and balance computations for small multiengine airplanes are similar to those discussed for single-engine airplanes. See Figure 6-1 for an example of weight and balance data for a typical light twin-engine airplane.
Figure 6-1. Typical weight and balance data for a light twin-engine airplane.
The airplane in this example was weighed to determine its basic empty weight (BEW) and empty weight center of gravity (EWCG). The weighing conditions and results are:
Weight with fuel drained and oil full:
| Right wheel scales | 1,084 lb, tare 8 lb |
| Left wheel scales | 1,148 lb, tare 8 lb |
| Nose wheel scales | 1,202 lb, tare 14 lb |
Determine the Loaded CG
First, add the weights indicated by the individual scales and then subtract the tare weights to determine the BEW. Next, using the BEW and EWCG, the loaded weight and CG of the aircraft can be determined with data from Figure 6-2, using a chart such as the one in Figure 6-3.
| Fuel (140 gal) | 840 lb |
| Front seats | 320 lb |
| Row 2 seats | 310 lb |
| Forward baggage | 100 lb |
| Aft baggage | 90 lb |
Chart Method Using Weight, Arm, and Moments
Make a chart showing the weight, arm, and moments of the airplane and its load.
The loaded weight for this flght is 5,064 pounds, and the CG is located at 42.47 inches aft of the datum.
To determine that the weight and CG are within the allowable range, refer to the CG range chart in Figure 6-4. Draw a line vertically upward from 42.47 inches from the datum and one horizontally from 5,064 pounds. These lines cross inside the envelope, showing that the airplane is properly loaded.
Figure 6-4. Sample CG range chart.
Determining the CG in Percentage of Mean Aerodynamic Chord (MAC)
Refer again to Figures 6-2 and 6-3.
The loaded CG is 42.47 inches aft of the datum.
The MAC is 61.6 inches long.
The LEMAC is located at station 20.1.
The CG is 42.47 – 20.1 = 22.37 inches aft of LEMAC.
Use the formula in Figure 6-5 to findthe CG in percent MAC.
Figure 6-5. Finding CG in percent MAC.
The loaded CG is located at 36.3 percent MAC.
The Chart Method Using Weight and Moment Indexes
As mentioned in the previous category, anything that can be done to make careful preflight planning easier makes flying safer. Many manufacturers furnish charts in the Pilot’s Operating Handbook/Aircraft Flight Manual (POH/AFM) that use weight and moment indexes rather than weight, arm, and moments. The charts also help reduce errors by including tables of moment indexes for the various weights.
Consider the loading for this particular flight
Cruise fuel flow = 16 gallons per hour
Estimated time en route = 2 hours, 10 minutes
Reserve fuel = 45 minutes = 12 gallons
Total required fuel = 47 gallons
The pilot completes a chart like the one in Figure 6-6 using moment indexes from tables in Figures 6-7 and 6-8.
Figure 6-7. Sample weight and moment index for occupants.
Figure 6-8. Sample weight and moment index for baggage.
The moments divided by 100 in the index column are found in the charts in Figures 6-7 through 6-9. If the exact weight is not in the chart, interpolate between the weights that are included. When a weight is greater than any of those shown in the charts, add the moment indexes for a combination of weights to get that which is desired. For example, to get the moments divided by 100 for the 320 pounds in the front seats, add the moment index for 100 pounds (105) to that for 220 pounds (231). This gives the moment index of 336 for 320 pounds in the front seats.
Figure 6-9. Sample weight and moment index for fuel.
Use the moment limits versus weight envelope in Figure 6-10 to determine if the weight and balance conditions are within allowable limits for both takeoff and landing at the destination. The moment limits versus weight envelope is an enclosed area on a graph of three parameters. The diagonal line representing the moment divided by 100 crosses the horizontal line representing the weight at the vertical line representing the CG location in inches aft of the datum. When the lines cross inside the envelope, the aircraft is loaded within its weight and CG limits.
Takeoff: – 3,781 lb and 4,296 moment divided by 100
Landing: – 3,571 lb and 4,050 moment divided by 100
Locate the moment divided by 100 diagonal line for 4,296 and follow it down until it crosses the horizontal line for 3,781 pounds. These lines cross inside the envelope at the vertical line for a CG location of 114 (113.6) inches aft of the datum.
The maximum allowable takeoff weight is 3,900 pounds, and this airplane weighs 3,781 pounds. The CG limits for 3,781 pounds are 109.8 to 117.5. The CG of 114 (113.6) inches falls within these allowable limits.
