2-1 CHAPTER 2. AIRCRAFT WEIGHTS AND LOADING SCHEDULES
2.1 Establishing Aircraft Weight.
2.1.1 How Does an Operator Establish the Initial Weight of an Aircraft? Prior to being placed into service, each aircraft is weighed and the empty weight and center of gravity (CG) location are established. New aircraft are normally weighed at the factory and are eligible to be placed into operation without reweighing as long as the W&B records were adjusted for any alterations and modifications to the aircraft. Some modifications to aircraft, however, warrant reweighing. See paragraph 2.1.4.3 for a discussion of when it might be unsafe to fail to reweigh an aircraft after it has undergone modification(s). Aircraft that transfer from one operator to another that has an approved program do not need to be reweighed prior to use by the receiving operator unless: (1) more than 36 calendar-months have elapsed since last individual or fleet weighing or (2) some other modification to the aircraft warrants that the aircraft be weighed (e.g., paragraph 2.1.4.3).
Aircraft transferred, purchased, or leased from an operator without an approved W&B program and that have been unmodified or only minimally modified can be placed into service without being reweighed if the last weighing was accomplished by an acceptable method within the last 12 calendar-months and a W&B change record was maintained by the operator. Examples of such an acceptable method are methods consistent with the manufacturer’s instructions or that adhere to the current edition of Advisory Circular (AC) 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair.
2.1.2 How Does an Operator Document Changes to an Aircraft’s W&B? The W&B system should include methods, such as a log, ledger, or other equivalent electronic means, by which the operator will maintain a complete, current, and continuous record of the Basic Empty Weight (BEW) and CG of each aircraft. Alterations and changes affecting the W&B of the aircraft should be recorded in this log. Changes in the amount of weight or in the location of weight in or on the aircraft should be recorded whenever the weight change is at or exceeds the weights listed in Table 2-1, Incremental Weight Changes That Should Be Recorded in a Weight and Balance Change Record.
Table 2-1. Incremental Weight Changes That Should Be Recorded in a Weight and Balance Change Record In the weight change record of a— An operator should record any weight changes of— Large cabin aircraft +/- 10 lb or greater Medium cabin aircraft +/- 5 lb or greater Small cabin aircraft +/- 1 lb or greater
2.1.3 How Does the Operator Maintain the BEW? The loading schedule may utilize the individual weight of the aircraft in computing operational W&B or the operator may choose to establish fleet empty weights for a fleet or group of aircraft.
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2.1.3.1 Reestablishment of BEW. The BEW and CG position of each aircraft should be reestablished at the reweighing periods discussed in paragraph 2.1.4. In addition, it should be reestablished through calculation whenever the cumulative change to the W&B log is more than plus or minus one-half of 1 percent (0.5 percent) of the maximum landing weight or whenever the cumulative change in the CG position exceeds one-half of 1 percent
(0.5 percent) of the mean aerodynamic chord (MAC). In the case of helicopters and airplanes that do not have a MAC-based CG envelope
(e.g., canard-equipped airplane), whenever the cumulative change in the CG position exceeds one-half of 1 percent (0.5 percent) of the total CG range, the W&B should be reestablished.
Note: When reestablishing the aircraft BEW between reweighing periods, the weight changes may be computed provided the weight and CG location of the modifications are known; otherwise the aircraft must be reweighed.
2.1.3.2 Fleet Operating Empty Weights (FOEW). An operator may choose to use one weight for a fleet or group of aircraft if the weight and CG of each aircraft is within the limits stated above in paragraph 2.1.3.1 for establishment of BEW. When the cumulative changes to an aircraft W&B log exceed the weight or CG limits for the established fleet weight, the empty weight for that aircraft should be reestablished. This may be done by moving the aircraft to another group or by reestablishing new FOEWs.
2.1.4 How Often are Aircraft Weighed?
2.1.4.1 Individual Aircraft Weighing Program. Aircraft are normally weighed at intervals of 36 calendar-months. An operator may extend this weighing period for a particular model aircraft when pertinent records of actual routine weighing during the preceding period of operation show that W&B records accurately reflect aircraft weights and CG positions are within the cumulative limits specified for establishment of BEW (see paragraph 2.1.3.1). Under an individual aircraft weighing program, an increase should not be granted that would permit any aircraft to exceed 48 calendar-months since its last weighing, including when an aircraft is transferred from one operator to another. In the case of helicopters, increases should not exceed the time that is equivalent to the aircraft overhaul period.
Note: Per § 125.91(b), no person may operate an airplane in a part 125 operation unless the current empty weight and CG are calculated from the values established by an actual weighing of the airplane within the preceding 36 calendar-months.
2.1.4.2 Fleet Weighing. An operator may choose to weigh only a portion of the fleet every 36 months and apply the weight and moment change determined by these sample weighings to the remainder of the fleet. For each aircraft
2-3 weighed, the new aircraft empty weight and moment is determined by the weighing and entered in the aircraft weight log. The difference between this new aircraft weight and moment and the previous aircraft weight and moment shown in the log is the weight and moment change. The average of the weight and moment changes for the aircraft weighed as part of this fleet weighing is then entered as an adjustment to the aircraft weight logs for each of the aircraft in the fleet that were not weighed.
2.1.4.2.1 A fleet is composed of a number of aircraft of the same type. (For example, B747-200s in a passenger configuration and B747-200 freighters should be considered different fleets. Likewise, B757-200s and B757-300s should be considered different fleets.) The primary purpose of defining a fleet is to determine how many aircraft should be weighed in each weighing cycle.
A fleet may be further divided into groups to establish FOEWs.
Table 2-2. Number of Aircraft to Weigh in a Fleet For fleets of— An operator must weigh (at minimum)— 1 to 3 aircraft All aircraft 4 to 9 aircraft 3 aircraft, plus at least 50 percent of the number of aircraft greater than 3 More than 9 aircraft 6 aircraft, plus at least 10 percent of the number of aircraft greater than 9
2.1.4.2.2 In choosing the aircraft to be weighed, the aircraft in the fleet having the most hours flown since last weighing should be selected.
2.1.4.2.3 An operator should establish a time limit such that all aircraft in a fleet are eventually weighed. Based on the length of time that a fleet of aircraft typically remains in service with an operator, the time limit should not exceed 18 years (six 3-year weighing cycles). It is not intended that an operator be required to weigh any remaining aircraft in the event that business conditions result in retirement of a fleet before all aircraft have been weighed.
2.1.4.3 Weighing Aircraft—Modifications. For most aircraft modifications, a mathematical calculation of the W&B change is practical. For some modifications, such as interior reconfigurations, the large number of parts removed, replaced, and installed may make an accurate determination of the W&B change by computation impractical.
2.1.4.3.1 When the accuracy of the calculation is questionable, the weight and moment change estimate should be verified by reweighing the aircraft. The operator should weigh two or more aircraft to confirm the computed weight change estimate. The operator may choose to weigh the aircraft before and after the modification, or just after the modification. If the weighings are inconsistent with the computed weight change estimate, then additional aircraft should be weighed as prescribed in Table 2-2, based on the size of the fleet.
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2.1.4.3.2 The operator may choose not to calculate the weight change but to reestablish the aircraft W&B by reweighing the aircraft prior to subsequent revenue operation. An operator using an individual aircraft weighing program would weigh each aircraft modified, and an operator using a fleet weighing program would weigh the number of aircraft as prescribed in Table 2-2, based on the size of the fleet.
2.1.5 What Procedures Should Be Used to Weigh an Aircraft?
2.1.5.1 An operator should take precautions to ensure that it weighs an aircraft as accurately as possible. These precautions include checking to ensure that all required items are aboard the aircraft and the quantity of all fluids aboard the aircraft is considered. An operator should weigh the aircraft in still air.
2.1.5.2 An operator should establish and follow instructions for weighing the aircraft that are consistent with the recommendations of the aircraft manufacturer and scale manufacturer. The operator should ensure that all scales are certified and calibrated by the manufacturer or a certified laboratory, such as a civil department of weights and measures, or the operator may calibrate the scale under an approved calibration program. The operator should also ensure that the scale is calibrated within the manufacturer’s recommended time, or time periods, as specified in the operator’s approved calibration program.
Note: If manufacturer’s data is not available, the operator is responsible for developing appropriate weighing instructions for its particular aircraft.
2.2 Aircraft Loading Schedules.
2.2.1 What is a Loading Schedule?
2.2.1.1 The loading schedule is used to document compliance with the certificated W&B limitations contained in the manufacturer’s Airplane Flight Manual (AFM) or Rotorcraft Flight Manual (RFM), Type Certificate Data Sheet (TCDS), and Weight and Balance Manual (WBM).
2.2.1.2 The loading schedule is developed by the operator based on its specific loading calculation procedures and provides the operational limits for use with the operator’s W&B program approved under this AC. These approved operational limits are typically more restrictive and may not exceed the manufacturer’s certificated limits. This is because the loading schedule is generally designed to check only specific conditions (e.g., takeoff and zero fuel) known prior to takeoff, and must account for variations in W&B in flight. Loading the aircraft so that the calculated W&B is within the approved operational limits will maintain the actual W&B within the certificated limits throughout the flight.
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2.2.1.3 Development of a loading schedule represents a trade-off between ease of use and loading flexibility. A schedule can provide more loading flexibility by requiring more detailed inputs, or it can be made easier to use by adjusting the operational limits to account for the uncertainty caused by the less detailed inputs.
2.2.1.4 Several types of loading schedules are commonly used, including computer programs as well as “paper” schedules, which can be graphical, such as an alignment (“chase around chart”) system, slide rule, or numerical, such as an adjusted weight or index system.
2.2.1.5 It is often more convenient to compute the balance effects of combined loads and to display the results by using “balance units” or “index units.” This is done by adding the respective moments (weight times arm) of each item.
Graphing the moments’ results in a “fan grid” where lines of constant balance arms (BA) or percent MAC are closer together at lower weights and further apart at higher weights. Direct graphical or numerical addition of the balance effects are possible using these moment values.
2.2.1.6 To make the magnitude of the numbers more manageable, moments can be converted to an index unit. For example:
N ote: Where datum is the reference BA that will plot as a vertical line on the fan grid, M and K are constants that are selected by the operator.
M is used to scale the index values, and K is used to set the index value of the reference BA.
2.2.2 How Should an Operator Determine the Weight of Each Fluid Used Aboard the Aircraft?
An operator should use one of the following:
1. The actual weight of each fluid,
2. A standard volume conversion for each fluid, or
3. A volume conversion that includes a correction factor for temperature.
2.3 Constructing a Loading Envelope.
2.3.1 What Should an Operator Consider when Constructing a Loading Envelope? Each operator complying with this AC must construct a loading envelope applicable to each aircraft being operated. The envelope will include all relevant W&B limitations. It will be used to ensure that the aircraft is always operated within appropriate W&B limitations, and will include provisions to account for the loading of passengers, fuel, and cargo; the in-flight movement of passengers, aircraft components, and other loaded items; and the usage or transfer of fuel and other consumables. The operator must be able to
2-6 demonstrate that the aircraft is being operated within its certificated W&B limitations using reasonable assumptions that are clearly stated.
2.3.2 What Information from the Aircraft Manufacturer Should an Operator Use? The construction of the loading envelope will begin with the W&B limitations provided by the aircraft manufacturer in the WBM, TCDS, or similar approved document as required by part 91, § 91.9 and §§ 121.141, 125.75, 125.91, and 135.23(b). These limitations will include, at minimum, the following items, as applicable:
1. Maximum zero fuel weight.
2. Maximum takeoff weight.
3. Maximum taxi weight.
4. Takeoff and landing CG limitations.
5. In-flight CG limitations.
6. Maximum floor loadings—including both running and per square foot limitations.
7. Maximum compartment weights.
8. Fuselage shear limitations.
9. Any other limitations provided by the manufacturer.
2.3.3 What Should the Operator Consider When Curtailing the Manufacturer’s Loading Envelope?
2.3.3.1 The operator should curtail the manufacturer’s loading limitations to account for loading variations and in-flight movement that are encountered in normal operations. For example, if passengers are expected to move about the cabin in flight, the operator must curtail the manufacturer’s CG envelope by an amount necessary to ensure that movement of passengers does not take the aircraft outside its certified envelope. If the aircraft is loaded within the new, curtailed envelope, it will always be operated within the manufacturer’s envelope, even though some of the loading parameters, such as passenger seating location, are not precisely known.
2.3.3.2 In some cases, an aircraft may have more than one loading envelope for preflight planning and loading. Each envelope must have the appropriate curtailments applied for those variables that are expected to be relevant for that envelope. For example, an aircraft might have separate takeoff, in-flight, and landing envelopes. Passengers are expected to remain seated in the cabin during takeoff or landing. Therefore, the takeoff and landing envelope does not need to be curtailed for passenger movement.
2.3.3.3 Upon determination of the curtailed version of each envelope, the most restrictive points (for each condition the operator’s program will check) generated by an “overlay” of the envelopes will form the aircraft “operational
2-7 envelopes.” Operators must adhere to the limitations relevant to such envelopes. By restricting operation to these operational envelopes, compliance with the manufacturer’s certified envelope will be ensured in all phases of flight, based upon the assumptions within the curtailment process. An operator may choose to not combine the envelopes but observe each envelope independently. However, due to calculation complexity, this is typically only possible through automation of the W&B calculation.
2.3.3.4 An operator could make an allowance for curtailment of the aircraft loading envelope to account for minor errors in bag counts and documentation, provided the operator provides valid survey data to ensure the accuracy of the baggage weight. The survey data should show the average variance an operator experiences in bag counts and include calculations for these variances to demonstrate that no limitations were exceeded. The operator should include the average variance in their loading schedule developed in paragraph 2.3.3.3.
Note: For acceptable methods used in surveying weights, see paragraph 3.3.
2.3.3.5 The specific amount of weight added to the Basic Operating Weight (BOW) would be based on the operator conducting a survey of their bag count and documentation errors and providing the appropriate amount of curtailment to ensure the aircraft remains within the CG envelope. This could be in the form of a minor tolerance specific to their approved program, rather than a broadly applicable allowance for all operators.
2.3.4 What are Some Examples of Common Curtailments to the Manufacturer’s Loading Envelope? The following paragraphs provide examples of common loading curtailments.
Appendix C, Sample Operational Loading Envelope, also provides an example of how operators may calculate these curtailments. Operators must include curtailments appropriate to the operations being conducted. Each of the items mentioned below is a single curtailment factor. The total curtailment of the manufacturer’s envelope is computed by combining the curtailments resulting from each of these factors.
2.3.4.1 Passenger Distribution. The operator must account for the seating of passengers in the cabin. The loading envelope does not need to be curtailed if the actual seating location of each passenger is known. If assigned seating is used to determine passenger location, the operator must implement procedures to ensure that the assignment of passenger seating is incorporated into the loading procedure. Operator procedures should take into account the possibility that passengers may not sit in their assigned seat and how this may affect CG during takeoff and landing.
2.3.4.1.1 If the actual seating location of each passenger is not known, the operator may assume that all passengers are seated uniformly throughout the cabin or a specified subsection of the cabin. If this assumption is made, the operator
2-8 must curtail the loading envelope to account for the fact that the passenger loading may not be uniform. The curtailment may make reasonable assumptions about the manner in which people distribute themselves throughout the cabin. For example, the operator may assume that window seats are occupied first, followed by aisle seats, followed by the remaining seats (window-aisle-remaining seating). Both forward and rear loading conditions should be considered. That is, the passengers may fill up the window, aisle, and remaining seats from the front of the aircraft to the back, or the back to the front.
2.3.4.1.2 If necessary, the operator may divide the passenger cabin into subsections or “zones” and manage the loading of each zone individually. It can be assumed that passengers will be sitting uniformly throughout each zone, as long as the curtailments described in the previous paragraph are put in place.
2.3.4.1.3 All such assumptions should be adequately documented.
2.3.4.2 Fuel. The operator’s curtailed loading envelope must account for the effects of fuel. The following are examples of several types of fuel-related curtailments:
2.3.4.2.1 Fuel Density. A certain fuel density may be assumed and a curtailment included accounting for the possibility of different fuel density values. Fuel density curtailments only pertain to differences in fuel moment caused by varying fuel volumes, not to differences in total fuel weight. The fuel gauges in most transport category aircraft measure weight, not volume. Therefore, the indicated weight of the fuel load can be assumed accurate.
2.3.4.2.2 Fuel Movement. The movement or transfer of fuel in flight.
2.3.4.2.3 Fuel Usage In Flight. The burning of fuel may cause the CG of the fuel load to change. The effect of fuel burning down to the required reserve fuel or to an acceptable fuel amount established by the operator should be accounted for.
A curtailment may be included to ensure that this change does not cause the CG of the aircraft to move outside of the acceptable envelope.
2.3.4.3 Fluids. The operator’s curtailed CG envelope must account for the effects of galley and lavatory fluids. These factors include such things as:
1. Use of potable water in flight.
2. Movement of water or lavatory fluids.
2.3.4.4 In-Flight Movement of Passenger and Crew. The operational envelope must account for the in-flight movement of passengers, crew, and equipment.
This may be done by including a curtailment equal to the moment change caused by the motion being considered. It may be assumed that all passengers, crew, and equipment are secured when the aircraft is in the takeoff or landing
2-9 configuration. Standard operational procedures may be taken into account.
Examples of items that can move during flight are:
2.3.4.4.1 Flight Deck Crewmembers Moving to the Lavatory. Flight deck crewmembers may move to the most forward lavatory in accordance with the security procedures prescribed for crews leaving the cockpit. An offsetting credit may be taken if another crewmember moves to the flight deck during a lavatory trip.
2.3.4.4.2 Flight Attendants (F/A) Moving Throughout the Cabin. Operators should take their standard operating procedures into account. If procedures do not dictate otherwise, it should be assumed that the F/As can travel anywhere within the compartment to which they are assigned.
2.3.4.4.3 Service Carts Moving Throughout the Cabin. Operators should consider their standard operating procedures. If procedures do not dictate otherwise, it should be assumed that the service carts can travel anywhere within the compartment to which they are assigned. If multiple carts are in a given compartment and no restrictions are placed on their movement, then the maximum number of carts, moving the maximum distance, must be considered. The weight of the number of F/As assigned to each cart must also be considered. The assumed weight of each cart may be the maximum anticipated cartload or the maximum design load, as appropriate for the operator’s procedures.
2.3.4.4.4 Passengers Moving Throughout the Cabin. Allowances should be made for the possibility that passengers may move about the cabin in flight. The most common would be movement to the lavatory, described below. If a lounge or other passenger gathering area is provided, the operator should assume that passengers move there from the centroid of the passenger cabin(s). The maximum capacity of the lounge should be taken into account.
2.3.4.4.5 Passengers Moving to the Lavatory. Operators should account for the CG change caused by passengers moving to the lavatory. Operators should develop reasonable scenarios for the movement of passengers in their cabins and consider the CG shifts that can be expected to occur. Generally, operators may assume that passengers move to the lavatories closest to their seats. In aircraft with a single lavatory, operators should consider movement from the “most adverse” seat. Operators may make assumptions that reflect operator lavatory and seating policies. For example, operators may assume that coach passengers may only use the lavatories in the coach cabin, if that is the operator’s normal policy.
2.3.4.5 Movement of Flaps and Landing Gear. If the manufacturer has not already done so, the operator must account for the movement of landing gear, flaps, wing leading edge devices, or any other moveable components of the
2-10 aircraft. Devices deployed only while in contact with the ground, such as ground spoilers or thrust reversers, may be excluded from such curtailments.
2.3.4.6 Baggage and Freight. It can be assumed that baggage and freight may be loaded at the centroid of each baggage compartment. Operators do not need to include a curtailment if procedures are used to ensure the cargo is loaded uniformly and physically restrained (secured) to prevent the contents from becoming a hazard by shifting between zones or compartments.
2.4 Onboard W&B Systems.
2.4.1 How Does an Onboard W&B System Compare to a Conventional Weight Buildup Method?
2.4.1.1 An operator may use an onboard W&B system to measure an aircraft’s W&B as a primary means to dispatch an aircraft, provided the FAA has certified the system and approved the system for use in an operator’s W&B control program. This paragraph discusses the differences an operator should consider when using an onboard W&B system compared to a conventional weight buildup method. This paragraph addresses only the operational considerations related to the use of an FAA-approved onboard W&B system.
2.4.1.2 Like operators using a conventional weight buildup method to calculate W&B, an operator using an onboard W&B system as a primary W&B control system should curtail the manufacturer’s loading envelope to ensure the aircraft does not exceed the manufacturer’s certificated weight and CG limits.
However, an operator using an onboard W&B system would not need to curtail the loading envelope for assumptions about passenger and bag weight or distribution.
2.4.1.3 Because an onboard W&B system measures the actual weight and CG location of an aircraft, an operator may not need to include certain curtailments to the loading envelope to account for variables such as passenger seating variation or variation in passenger weight. However, an operator should curtail the loading envelope for any system tolerances that may result in CG errors. Using an onboard W&B system does not relieve an operator from the requirement to complete and maintain a load manifest.
2.4.2 What Measures Should an Operator Take to Obtain Operational Approval for an Onboard W&B System?
2.4.2.1 System Calibration. An operator should develop procedures to calibrate its onboard W&B system equipment periodically in accordance with the manufacturer’s instructions. An operator may calibrate its system with operational items or fuel aboard the aircraft to test the system at a representative operational weight. However, an operator may not use an onboard W&B system in place of procedures described in paragraph 2.1 for weighing the aircraft to establish BEW or CG location.
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2.4.2.2 Demonstration of System Accuracy. As part of the approval process, an operator should demonstrate that the onboard W&B system maintains its certificated accuracy. An operator should only have to conduct this demonstration once for each type aircraft with a similarly installed onboard W&B system. For the demonstration, the operator should use the accuracy demonstration test provided in the maintenance manual portion of the Supplemental Type Certificate (STC) or type certificate (TC) of the onboard W&B system.
2.4.3 What Operational Considerations Should an Operator Take into Account when Using an Onboard W&B System?
2.4.3.1 Certification Limits. An operator using an onboard W&B system as its primary means of calculating W&B should have procedures in place to ensure that the system is operated within the limits established during the system’s certification process.
2.4.3.2 Environmental Considerations. An operator using an onboard W&B system should ensure that it uses the system within the environmental limits established by the manufacturer. Environmental conditions that may affect the performance of an onboard W&B system include temperature, barometric pressure, wind, ramp slope, rain, snow, ice, frost, dew, and deicing fluid.
2.4.3.3 Aircraft Considerations. An operator using an onboard W&B system should ensure the weight and CG measured by the system are not affected by the aircraft configuration, such as the movement of flaps, stabilizers, doors, stairways or jetways, or any connections to ground service equipment. Other factors that an operator should consider include engine thrust, oleo strut extension, and aircraft taxi movement.
2.4.3.4 Takeoff Trim Settings. If the aircraft manufacturer provides trim settings for takeoff based on the aircraft’s CG location, an operator using an onboard W&B system should ensure that the onboard W&B system provides flightcrew members with adequate information to determine the appropriate trim setting.
2.4.3.5 Operational Envelope. The operational envelope for onboard W&B systems should be developed using the same procedures described in other parts of this AC, with the exception that the operational envelope does not need to be curtailed for passenger random seating and passenger weight variance. Also, note that the fuel load is subtracted from the measured takeoff weight to determine the zero fuel weight and CG, instead of being added to the zero fuel weight as part of the load buildup. In addition, an operator should curtail the CG envelope for any system CG tolerance.
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2.4.3.6 Complying with Compartment or Unit Load Device (ULD) Load Limits.
When using an onboard W&B system, an operator should develop in its W&B control program a method to ensure that it does not exceed the floor, linear or running loading limits specified for a compartment or ULD. If an operator develops appropriate procedures, an operator may request approval to exclude bag counts from its load manifest. The following are two examples of acceptable means to demonstrate compliance with compartment load limits.
2.4.3.6.1 An operator may assign a standard average weight to bags. Operators should see paragraphs 3.3 and 3.4 for baggage. Based on that standard average weight, the operator may place a placard in each compartment stating the maximum number of bags permitted. An operator may also create a table that lists the total weight associated with a given number of bags to ensure the operator does not exceed the load limit of a compartment or ULD.
2.4.3.6.2 By conducting sample loadings, an operator may demonstrate that the average density of the bags it places in a compartment or ULD would not allow it to exceed the compartment or ULD load limits inadvertently.
2.4.4 May an Operator Use the Information in this AC to Develop a Backup System? An operator using an onboard W&B system as its primary means of measuring W&B may use the guidance in this AC to develop a backup system based on a conventional weight buildup provided that the backup system has been approved by the Certificate Management Office (CMO)/Flight Standards District Office (FSDO). Should the primary onboard W&B system become inoperative, the operator should have provisions for deferring the inoperative equipment until repairs can be made prior to further operations.
The FAA may grant relief for an onboard W&B system through the minimum equipment list (MEL). An operator using an onboard W&B system should not use the backup system unless:
1. The onboard system is inoperative;
2. The onboard system has been deferred in accordance with the aircraft MEL; and
3. The operator has been approved to use average weights or conventional weight buildup.