4/11/2008 AC 20-161

CHAPTER 1. INTRODUCTION 

1-1. PURPOSE. 

a. This advisory circular (AC) will show you how to gain Federal Aviation Administration 

(FAA) approval of aircraft onboard weight and balance systems (OBWBS). We at the FAA 

recommend you use additional documents, referenced throughout this AC, to help you show 

compliance with the regulatory requirements of your type or supplemental type certification 

program. The documents supplement the engineering and operational judgment used to form the 

basis of any compliance findings on OBWBS. 

b. This AC is not mandatory and does not constitute a regulation. It describes an 

acceptable means, but is not the only means, to show you how to gain certification for OBWBS. 

However, if you use the means described in this AC, you must follow it entirely. Because the 

method of compliance presented is not mandatory, the term “must” applies only to an applicant 

who chooses to follow this particular method in its entirety. 

1-2. WHO DOES THIS AC AFFECT. 

a. Chapter 2 provides guidance for type certificate holders who are installing an OBWBS 

under Title 14 of the Code of Federal Regulations (14 CFR) parts 23, 25, 27, or 29. 

b. Chapter 3 provides guidance for operational certificate holders who are required to have 

an approved weight and balance control program under 14 CFR parts 91 subpart K, 121, 125, or 

135. 

1-3. WHAT TYPES OF CERTIFICATION GUIDANCE IS COVERED IN THIS AC. 

This guidance is for applicants seeking certification through: 

• Type certification (TC), 

• Supplemental type certification (STC), 

• Amended type certificate (ATC), 

• Amended supplemental type certificate (ASTC), or 

• Parts Manufacturer Approval (PMA). 

1-4. WHY USE AN OBWBS VS. STANDARD PRACTICE. 

a. For many years, international best practice has allowed the use of standardized 

passenger weights to calculate the aircraft’s take-off weight and center of gravity (CG) as part of 

an approved weight and balance control program. Using standardized weights assumed that the 

individual weights of passengers averaged out to a mean weight. When the aircraft’s passenger 

load is calculated this way it would not cause the aircraft weight limitation to be exceeded. 

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However, differences between the actual passenger load weight and the calculated passenger 

load weight can occur when using these standard passenger weights due to variation in actual 

passenger weights. To help eliminate these differences, an aircraft OBWBS can calculate the 

actual weight and CG of the aircraft (within accuracy tolerances of the OBWBS), and therefore 

remove the need to use standard passenger weights.

b. Calculating an aircraft’s weight and CG accurately before flight is essential to comply 

with the certification limits established for the aircraft. These limits include various weight and 

CG limits, such as zero fuel, taxi, takeoff, flight, and landing limits. Aircraft manufacturers 

specify a maximum certificated take-off weight as well as CG limitations, and provide data on 

the performance capabilities of the aircraft. The most restrictive condition for a given take-off 

determines the allowable take-off weight and CG limitations. 

c. An OBWBS certified to weigh an aircraft and compute CG from equipment onboard the 

aircraft can be the primary means of providing weight and balance information if it receives 

certification and operational approval. 

d. The design of the OBWBS, and the OBWBS operating procedures and system 

limitations must all be consistent with how the OBWBS determines the weight and CG location 

used to dispatch the aircraft. 

 

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CHAPTER 2. CERTIFICATION PROCESS FOR INSTALLING AN OBWBS 

2-1. OBWBS ACCEPTABLE MEANS OF COMPLIANCE. To install an OBWBS you 

must meet the certification guidelines in this AC. Coordinate the certification program with the 

responsible FAA Aircraft Certification Office as described in FAA Order 8110.4C, Type 

Certification, through the certification program notification (CPN) process, Order 8110.48, How 

to Establish the Certification Basis for Changed Aeronautical Products, or in Order 8110-42, 

Parts Manufacturer Approval Procedure. The OBWBS must also be approved for operation by 

the FAA’s Flight Standards Operations inspectors. See chapter 3 of this AC for guidance on 

how to obtain approval as a primary dispatch system for operations. 

a. Certification Plan. Prepare a certification plan that covers the installation of the 

OBWBS equipment. We recommend that you submit it early in the certification process. 

Include the following items: 

(1) Project description and schedule, 

(2) System description, including the aircraft system interfaces and any aircraft system 

modifications made to accommodate the OBWBS installation, 

(3) Certification basis and means of compliance, 

(4) Communication and coordination, 

(5) Human factors plan, 

(6) Test plan, 

(7) Conformity plan, 

(8) Continued airworthiness plan, and 

(9) Compliance documentation. 

b. Means of Compliance. An acceptable means of compliance with the applicable 

airworthiness regulations must address the following regulations as a minimum: 

Instructions for Continued Airworthiness 

and Maintenance Manuals 14 CFR § 21.50(b) 

Equipment Function and Installation 14 CFR §§ 23, 25, 27, 29.1301 

Equipment, Systems, and Installations 14 CFR §§ 23, 25, 27, 29.1309 

Pilot Compartment 14 CFR §§ 23, 25, 27, 29.771(a) 

High-intensity Radiated Fields (HIRF) 

Protection 14 CFR §§ 23.1308, 25, 27.1317, 29.1317 

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Cockpit Controls 14 CFR §§ 23, 25, 27, 29.777(a) 

Instrument Arrangement and Visibility 14 CFR §§ 23, 25, 27, 29.1321 

Warning, Caution, and Advisory Lights 14 CFR §§ 23, 25, 27, 29.1322 

Electrical Systems and Equipment – 

General 14 CFR §§ 23, 25, 27, 29.1351 

Circuit Protective Devices 14 CFR §§ 23, 25, 27, 29.1357 

Hydraulic Systems 14 CFR §§ 23, 25, 27, 29.1435 

Pressurization and Pneumatic Systems 14 CFR §§ 23, 25, 27, 29.1438 

Weight & Center of Gravity 14 CFR §§ 23, 25, 27, 29.1519 

Instructions for Continued Airworthiness 14 CFR §§ 23, 25, 27, 29.1529 

Aircraft Flight Manual – General 14 CFR §§ 23, 25, 27, 29.1581 

Operating Limitations 14 CFR §§ 23, 25, 27, 29.1583 

Lightning and Static Electricity 14 CFR §§ 23.867 and 25.1316 

Fire Protection 14 CFR §§ 23.1359, 25.869, and 29.1359 

Loading Information 14 CFR §§ 23.1589, 27.1589, and 29.1589 

Electronic Equipment 14 CFR §§ 23.1431 and 25.1431 

Electric Cables 14 CFR §§ 23.1365 and 27.1365 

Switches 14 CFR §§ 23.1367 and 27.1367 

Instrument Systems 14 CFR §§ 25.1333 and 29.1333 

(1) Demonstrate the system accuracy, operational accuracy, and repeatability of the 

installed OBWBS. See paragraphs 2-2 and 2-3 in the AC for guidance. 

(2) Demonstrate system accuracy for the maximum time period allowed between 

OBWBS calibration on the installed OBWBS. 

(3) Include limitations and systems operating instructions in the AFM/AFMS and the 

instructions for continued airworthiness. 

(4) If software is used, 

(a) You must meet the requirements in: 

• Radio Technical Commission for Aeronautics (RTCA), Inc. document 

RTCA DO 178B, Software Conditions in Airborne systems and Equipment Certification, dated 

December 1, 1992, or the most current revision, and 

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• Society of Automotive Engineers (SAE) International Aerospace 

Recommended Practice (ARP) 4754, Certification Considerations for Highly Integrated or 

Complex Aircraft Systems, dated November 1, 1996, or the most current revision. 

(b) For additional guidance see FAA advisory circulars: 

• AC 23.1309-1, Equipment Systems and Installations in Part 23 Airplanes, 

• AC 25.1309-1, System Design and Analysis, 

• AC 27-1, Certification of Normal Category Rotorcraft, and 

• AC 29-2, Certification of Transport Category Rotorcraft. 

(6) If the hardware contains complex electrical devices performing functions that 

cannot be evaluated by tests or analysis, meet the requirements in: 

• RTCA DO-254, Design Assurance Guidance for Airborne Electronic 

Hardware, or the most current revision, and 

• AC 20-152, RTCA/DO-254 Design Assurance Guidance for Airborne 

Electronic Hardware. 

(7) You must also follow the environmental requirements in RTCA/DO-160F, 

Environmental Conditions and Test Procedures for Airborne Equipment, dated December 6, 

2007, or the most current revision. 

2-2. CERTIFYING THE OBWBS ACCURACY UNDER ENVIRONMENTAL 

INFLUENCES. As part of compliance with the airworthiness requirements, you must 

demonstrate the accuracy within the specified tolerances and the reliability of the installed 

OBWBS. Certification of an OBWBS installation on a 14 CFR part 23, 25, 27, or 29 aircraft 

should include demonstrations of the OBWBS system accuracy and the OBWBS operational 

accuracy. Work with your aircraft certification office (ACO) to determine which method to use 

to demonstrate the OBWBS system and operational accuracy requirements. See paragraph 2-3

of this AC for a description of methods. 

a. Weight and Balance Curtailment. If a method in this AC determines that a weight 

curtailment is required, apply the curtailment to both structural and performance limited weights. 

Examples of performance-limited weights include weights limited by takeoff distance, 

accelerate-stop distance, takeoff climb, obstacle clearance, en route climb, approach climb, 

landing climb, and landing distance. This weight accuracy is used to determine CG accuracy to 

ensure that the calculated CG remains within the certificated CG limits. 

b. Ground Tests. When ground tests are required you can reduce the total number of 

aircraft ground tests by using a validated simulation of the aircraft and the OBWBS installation. 

The fidelity and accuracy of the simulation model should be substantiated with actual aircraft 

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ground tests. Simulation validation is necessary in order to establish that the simulation matches 

the actual aircraft behavior. The results from further simulated ground tests carried out with the 

validated simulator may then be used in lieu of further physical ground tests. 

c. Terms. 

(1) OBWBS System Accuracy. An OBWBS achieves its most accurate weight and 

balance measurements on a motionless aircraft in a controlled (temperature regulated, no winds, 

fixed aircraft configuration, no payload movement) environment. This accuracy is the OBWBS 

system accuracy. 

(2) OBWBS Operational Accuracy. The accuracy of the OBWBS weight and 

balance measurements may decrease when the aircraft is disturbed by temperature changes, 

winds, aircraft configuration changes (flaps, slat motion) or payload shifting (passenger 

movement). The OBWBS is expected to operate routinely in this disturbed environment. This 

accuracy is the OBWBS operational accuracy. 

d. OBWBS Display of Weight and Balance Measurements. An OBWBS can display 

weight and balance measurements that take into account the established operational accuracy of 

the system and any necessary curtailments. If so, the displayed OBWBS output can be compared 

directly to the applicable operational weight and balance limits. On the other hand, the OBWBS 

system may display weight and balance measurements that are affected by disturbances, that is, 

the displayed measurements do not take the operational accuracy into account. For this type of 

OBWBS, displayed values must be adjusted to account for operational accuracy before being 

compared to the applicable operational weight and balance limits. 

e. Basic Principles of OBWBS Accuracy Demonstration Methods. The following basic 

principles apply to three gear tricycle configurations. Adapt these principles appropriately for 

other configurations. 

(1) Determining CG. 

(a) The CG position (system or operational) accuracy derives from weight 

accuracy through the geometry of the airplane landing gear and the load measured on each 

landing gear. The OBWBS separately measures the load on the nose gear and the load on the 

main gear. The sum of the measured loads on all landing gear, and the weight of the landing 

gear itself, is the total airplane weight. The CG position calculated using the measured weight on 

the main landing gear and nose gear is the measured CG position. 

(b) The (system or operational) CG position accuracies can be determined from 

the actual main gear and nose gear load accuracies for the OBWBS installation. 

(2) Calculating Under Operational and Environmental Conditions. The 

cumulative operational accuracy of two or more operational or environmental conditions can be 

calculated by taking the square root of the sum of the squares of the accuracies (addition in 

quadrature), or you can demonstrate through analysis or test. You choose which of these 

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methods to employ. Provide charts, graphical look-up tables, numerical tables or step-by-step 

instructions in the AFM/AFMS to determine the operational accuracy due to two or more 

operational or environmental conditions for a specific departure. The OBWBS may calculate 

and display that operational accuracy using operational or environmental condition data provided 

to the OBWBS, or you can refer to the AFM/AFMS for limitations. 

(3) Controlled Environment Measurements. For all the following accuracy 

determination methods (see paragraph 2-3 of this AC), you must first obtain weight and CG 

measurements from the OBWBS with the aircraft in a controlled environment. To establish the 

OBWBS system accuracy in a controlled environment the aircraft should be in a hangar sitting 

on precision aircraft scales. AC 120-27E, Aircraft Weight and Balance Control, provides 

instructions on how to accomplish precision airplane weight measurements. 

(4) Additional Curtailments. Consider airframe manufacturer-provided guidance 

regarding application of additional curtailments in adverse environmental conditions (such as 

wind gusts greater than 35 knots) or unique environments (such as airports with elevations above 

6,000 feet) that exceed the parameters assumed in the AFM/AFMS specific limitations and 

conditions (see paragraph 2-9 of this AC). Determine any additional curtailments by analysis 

and verify by testing that duplicates or simulates the operational conditions. The AFM/AFMS 

limitations can state that in operational use, such additional curtailments need only be applied 

when applicable conditions exist.

2-3. OBWBS ACCURACY DETERMINATION METHODS. 

a. Takeoff Performance Based Method. 

(1) This method examines the influence of OBWBS system and operational weight and 

balance accuracies on aircraft takeoff performance. OBWBS operational accuracies that result in 

at most a ± 1.5 knot error change in either V1 or V2 speed, or a 100 foot increase in takeoff or 

accelerate-stop distance, whichever is greater, are accepted without weight curtailments for 

OBWBS operational accuracy. OBWBS operational accuracies that result in greater errors than 

these require appropriate curtailment. 

(2) From the minimum value of V1 to the maximum value of V2 speed, determine the 

size of the weight error that results in at most a ± 1.5 knot change in V1 or V2 speed, or a 100 

foot increase in the takeoff or accelerate-stop distances, whichever is greater, for the full range of 

takeoff weights. 

(3) These errors may be used to determine a corresponding operational weight accuracy 

for a given takeoff weight. This operational weight accuracy is used to determine CG accuracy. 

(4) The operational accuracies derived from the impact on takeoff performance help 

determine the range of the allowable operational and environmental conditions for the OBWBS 

without curtailment for OBWBS operational accuracies. For example, assume the OBWBS 

operational accuracy results in at most a ± 1.5 knot change in V1 or V2 speed, or a 100 foot 

increase in the takeoff or accelerate-stop distances, whichever is greater, over a specific range of 

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wind gust or wind velocities for a given temperature day and given takeoff weight. The 

AFM/AFMS limitations section for a given temperature day may then list this wind gust range as 

a limitation on the use of the OBWBS results without curtailment for operational accuracy. 

(5) The AFM/AFMS calls out operational accuracies as a function of the range of the 

operational and environmental conditions (see paragraph 2-9 of this AC). The OBWBS weight 

and balance results may be used for takeoff without curtailment for operational accuracy only 

when the takeoff speed accuracy constraints specified in paragraph 2-3.a(1) of this AC are met 

and the OBWBS measured CG position ± the CG position operational accuracy remains within 

the takeoff limits of the CG envelope. 

• The OBWBS measured weight ± weight operational accuracy results in no more 

than a ± 1.5 knot change in V1 or V2 speed, or a 100 foot increase in the takeoff or acceleratestop distances, whichever is greater, and 

• The OBWBS measured CG position ± CG position operational accuracy remains 

within the takeoff limits of the CG envelope. 

 

(6) If the cumulative OBWBS operational accuracy does not meet the criteria of 

paragraph 2-3.a(5) of this AC, the OBWBS results must be curtailed for operational accuracy 

before using them for takeoff. 

(7) The operator should use an actual takeoff weight corresponding to the measured 

weight plus weight operational accuracy, and the most adverse CG position corresponding to the 

measured CG position plus or minus the operational CG accuracy. The operator may need to 

remove or shift payload to keep these values within the weight and balance envelope. See 

appendix 4 of this AC for an example of this method applied to a B767-300ER. 

b. Specific Operations Method. This method uses OBWBS weight and balance 

measurements taken while the aircraft is subjected to a pre-defined range of environmental 

disturbances in order to establish operational accuracy and procedural adjustments (i.e., weight 

and/or CG envelope curtailments). Operational accuracy levels, expressed as maximum weight, 

and CG errors experienced are then applied as curtailments to the original manufacturer's 

envelope, as published in the type certificate data sheet, AFM/AFMS, or weight and balance 

manual. This method offers the smallest hurdle to determination of an envelope for OBWBS 

operation, but will typically result in the largest curtailments when compared to the other 

methods offered. 

(1) To demonstrate the OBWBS operational accuracy, you should: 

• Establish the proposed range of operational and environmental conditions in 

which the OBWBS will operate. These conditions determine the content of the limitations and 

conditions as described in paragraph 2-9 of this AC. 

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• Ground test the OBWBS under each of the conditions identified in the 

limitations and conditions as described in paragraph 2-9.a of this AC. You can use analysis as 

an alternative to testing if you can substantiate the analysis approach. 

• For example, suppose the aircraft manufacturer provides environmental or 

operational limitations, or procedures, that ensure the airplane always operates within the 

certified weight and CG envelope. Your OBWBS installation may operate within those same 

restrictions without demonstration. 

(2) Use the OBWBS operational accuracy to identify all operational limitations (such 

as weight and/or CG envelope curtailments when operating at specific points in the range of 

limitation values) determined through this process in the AFM/AFMS. 

c. Weight and Balance Procedures Method (Load Buildup Method). The OBWBS 

operational accuracy may be compared to existing OEM and FAA recommended procedures for 

computing weight and balance values for a given aircraft configuration. These procedures, also 

known as load buildup methods, have acceptable accuracies as proven in past service experience. 

The load buildup method weight and CG accuracies as derived from analysis of these procedures 

help determine the range of the allowable operational and environmental conditions for the 

OBWBS without curtailment for OBWBS operational accuracies. The curtailed envelope 

applicable when using the load buildup method also applies to the OBWBS weight and CG 

measurements as long as OBWBS operational accuracy remains equal to or better than the 

accuracy determined for the load buildup method. Curtail the limits of the CG envelope for any 

OBWBS operational accuracy that is worse than the accuracy of the load buildup method. 

(1) Assess the load buildup method’s operational accuracy by considering a variety of 

factors recommended by the aircraft manufacturer and AC 120-27E, Aircraft Weight and 

Balance Control, as guidance material. See figure 1 in this AC. 

FIGURE 1. FACTORS TO CONSIDER FOR LOAD BUILDUP METHOD. 

Factor Effect on 

Weight 

Effect on 

CG 

Scale accuracy during reweigh X X 

Fuel quantity indicating accuracy X X 

Allowable weight/CG variation prior to 

“reestablishment” of operational empty weight 

(or allowance for potential variation due to use of 

fleet weights) 

X X 

Variation in catering/provisioning X X 

Variation in male/female ratio X X 

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Passenger weight variation X X 

Crew weight variation X X 

Baggage weight variation X X 

Cargo weight variation X X 

(2) Tolerable errors associated with each factor are as follows: 

(a) Scale accuracy for reweigh – Typically 0.2% of basic empty weight (BEW), 

based upon reported scale accuracy for analog scale systems or 0.1% of BEW for digital scale 

systems. Analog accuracy value may be used for aircraft for which the aircraft OEM guidance 

does not recommend or require the use of digital scales. 

(b) Fuel quantity indicating accuracy – Typically 3% of full capacity. See: 

• SAE AS-405C, Fuel and Oil Quantity Instruments, dated July 2001, or the 

most current revision; 

• Military document MIL-G-8798, General Specifications for FuelQuantity, Capacitor-Type Gage System, dated September 30, 1992, or the most current revision, 

or 

• MIL-G-26988C, General Specification for Liquid Quantity Gage, dated 

Sep 28, 1995, or the most current revision. 

(c) Operational Empty Weight Variation – 1⁄2 of one percent of the mean 

aerodynamic chord (MAC) and 1⁄2 of one percent of maximum landing weight, as outlined in AC 

120-27E. 

(d) Variation in catering/provisioning – Amount of variation that could be 

expected to exist, undetected, in galley provisioning. Typically 20% of the standard galley stock 

or galley cart weights. 

(e) Variation in male/female ratio – Amount of variation in ratio of male to female 

passengers permitted without adjusting passenger weights in use, typically 10% deviation from 

ratio upon which passenger weights were based, as outlined in AC 120-27E. 

(f) Passenger weight variation – Typically a 1% variation in passenger weight plus 

a 2% variation in carry-on baggage and personal item weights, defined as tolerable error by 

AC 120-27E. 

(g) Crew weight variation – Typically a 25% variation in the standard 

crewmember weights, or that amount found to be justified based upon sampling or survey of 

crewmember weights. 

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(h) Baggage weight variation – Typically a 2% variation in baggage weight 

(including both checked and designed heavy weight bags), as defined as tolerable error by 

AC 120-27E. 

(i) Cargo weight variation – Typically a 1% variation in actual cargo weight 

verses reported cargo weight, based upon typical scale accuracy. 

 (3) The items eligible for consideration do not include those which originate from 

human error factors, such as baggage miscount, incorrect passenger count, omitted cargo weights 

or omitted jump seat occupants. The cumulative accuracy of the load buildup method may be 

calculated by taking the square root of the sum of the squares of the factor accuracies (addition in 

quadrature), or demonstrate through analysis or test. You choose which of these methods to 

employ. 

(4) In demonstrating the OBWBS operational accuracy, you should: 

• Establish the proposed range of operational and environmental conditions in 

which the OBWBS will operate. These conditions determine the content of the limitations and 

conditions as described in paragraph 2-9.a of this AC. 

• Ground test the OBWBS under each of the conditions identified in the 

limitations and conditions as described in paragraph 2-9.a of this AC. 

• Use the OBWBS operational accuracy to identify any operational limitations 

(such as weight and/or CG envelope curtailments when operating at specific points in the range 

of limitation values) determined through this process in the AFM or AFMS. 

d. OBWBS Operational Demonstration Method. This method may be used to evaluate 

OBWBS operational accuracy during revenue service for a trial period, or in non-revenue 

operation as part of a planned ground test program. 

(1) Continue to use the original weight and balance program, as applicable, during the 

demonstrations with the OBWBS installed for evaluation purposes only. 

(2) During a trial period in revenue service, the current load buildup system is used as 

the primary means of performing the weight and balance functions for the airplane, and records 

all OBWBS measurements. 

(3) Ground tests can demonstrate equivalent safety between an OBWBS implementation 

and the current load buildup system by comparing to precision aircraft scale weighing. Note that 

differences between the scale and load buildup CG’s may exist that would not be considered as 

part of the load buildup CG error. These differences include load buildup curtailments as 

described in AC 120-27E, Aircraft Weight and Balance Control, as well as differences in the 

weights being compared (for example, taxi weight vs. takeoff weight). 

(4) In determining the OBWBS operational accuracy, use the OBWBS system accuracy 

that is at least as accurate as, or better than, the current load buildup system. 

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(5) The limits of the OBWBS weight and CG envelope must be curtailed for any 

OBWBS operational accuracy that exceeds the accuracy of the load buildup method. 

 

(6) Design the demonstration plan to allow multiple tests at multiple airplane weight and 

CG configurations. Include sufficient trials in the demonstration plan to show to a confidence 

interval of at least 95% that the statistics of the OBWBS weight and balance measurements are at 

least as accurate as the original program being used. Conduct a minimum of twenty five 

weighing trials. 

(7) At least 30% of the tests should be at weights within 10% of maximum airplane 

takeoff weight; at least 10% of the tests should be at weights within 10% of the minimum 

airplane takeoff weight; and the tests should cover the aircraft weight and CG envelopes. 

 

(8) Also, test applicable environmental considerations throughout the operating 

envelope. During a revenue service demonstration, the actual airplane weight can be obtained 

with a precision scale weighing of the loaded airplane prior to departure. 

 

2-4. SAFETY ASSESSMENT PROCESS GUIDANCE. 

a. System Safety Assessments and Analyses. You must identify the levels of hazard 

associated with installation and use of the system. Determine the maximum acceptable levels of 

“probability of failure” for the system as installed on the aircraft. Show that the system complies 

with safety objectives. You can find references for additional guidance on conducting safety 

assessments and analyses in appendix 1 of this AC. 

b. Dispatching an aircraft with calculated aircraft weight or CG within its certificated 

weight or CG limits when the aircraft is actually outside its weight or CG limits is 

hazardous/severe-major or catastrophic level of hazard depending on the magnitude of the 

weight or CG error. Hazard severity can be mitigated by manufacturers’ recommended risk 

reduction strategies to protect against severely overloading the aircraft, or severely loading the 

aircraft out of balance, such as recommended operating and training procedures for loading the 

aircraft, or other independent reasonableness checks on the OBWBS measured weight and CG 

position. 

c. Dispatching an aircraft with calculated aircraft weight or CG within its certificated 

weight or CG limits when the calculated aircraft weight or CG is incorrect, but within the 

certified limits, is a minor, major, hazardous/severe-major, or catastrophic level of hazard 

depending on the magnitude of the weight or CG error. This error can have an effect on aircraft 

configuration or operational performance factors such as trim setting. Hazard severity can be 

mitigated by manufacturers’ recommended risk reduction strategies to protect against severely 

overloading the aircraft, or severely loading the aircraft out of balance, such as recommended 

operating and training procedures for loading the aircraft, or other independent reasonableness 

checks on the OBWBS measured weight and CG position. 

d. Failure conditions. Any failure condition of the OBWBS must not affect the 

functioning of other systems nor reduce the integrity of other systems if the OBWBS interfaces 

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with other aircraft systems. 

2-5. SYSTEM DESIGN CONSIDERATIONS. Design the OBWBS on objectives and 

principles of the fail-safe concept, which considers the effects of failures and combinations of 

failures in defining a safe design. The fail-safe design concept uses the following design 

principles in order to ensure a safe design. The use of only one of these principles is seldom 

adequate. A combination of two or more is usually needed to provide a fail-safe design, that is, 

to ensure that major failure conditions are remote, hazardous failure conditions are extremely 

remote, and catastrophic failure conditions are extremely improbable: 

a. Designed integrity and quality, including life limits to ensure intended function and 

prevent failures. 

b. Redundancy or backup systems to enable continued function after any single or any 

number of failures. 

c. Isolation of systems, components, and elements so the failure of one does not cause 

the failure of another. 

d. Proven reliability so multiple independent failures are unlikely to occur at one time. 

e. Failure warning or indication. 

f. Flight crew procedures specifying corrective action for use after failure detection. 

g. The capability to check a component’s condition. 

h. Designed failure effect limits, to sustain damage, to limit the effects of a failure. 

i. Designed failure path to control and direct the effects of a failure in a way that limits 

its safety impact. 

j. Margins or Factors of Safety to allow for any undefined or unforeseeable adverse 

conditions. 

k. Error Tolerance that considers adverse effects of foreseeable errors during the 

aircraft’s design, test, manufacture, operation, and maintenance. 

l. Other design considerations include: 

(1) Use of other aircraft systems (electrical, hydraulic, pneumatic). 

(2) Connections to other aircraft systems. 

(3) Environmental hazards identification and qualification testing. 

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2-6. DISPLAY SYSTEM. The OBWBS must display the collected data (weight and CG) in a 

form easily understood by the flight crew. The display hardware can be stand-alone or interfaced 

with existing equipment, such as a multi-function display, flight management system, printer, or 

other compatible display systems. The actual display format used will depend on the onboard 

display hardware, the options made available by OBWBS, and the features desired by the user. 

The indicated weight and CG position must as a minimum account for the system accuracy of the 

OBWBS. Regardless of what format is used, the display presentation must show the crew data 

that’s organized and easily used. Find additional guidance on applicable display systems in: 

• Section 2-1.b, Means of Compliance, in this AC, 

• AC 25-11, Transport Category Airplane Electronic Display Systems, and 

• AC 23.1311-1B, Installation of Electronic Displays in Part 23 Airplanes.

2-7. BACKUP SYSTEM. If the OBWBS fails to function properly, use an alternative method 

of determining aircraft weight and CG location for dispatch. This alternative method must be 

approved for operations requiring an operating certificate. 

2-8. TEST CONSIDERATIONS. Show that the OBWBS meets appropriate industry-adopted 

environmental qualification standards, including radiated emissions, for equipment operating in 

an airborne environment. If using an OBWBS in aircraft flight operations show that it has no 

adverse impact or interferes with other aircraft systems. A manufacturer, installer, or operator 

can test and validate to ensure proper operation and non-interference with other installed 

systems. Determine the accuracy of the OBWBS by test or by analyses that are verified by 

testing. 

a. Ground Test. Conduct a certification ground test for each OBWBS installation. The 

level of testing required will be determined by the scope of the installation. Items to consider for 

ground test include: 

(1) Self test functions (built in test equipment). 

(2) Evaluation of all OBWBS aircraft interfaces. 

(3) Evaluation of identified failure modes. 

(4) Evaluation of location of OBWBS controls, and the presentation of pertinent 

weight and CG data to the flight crew. 

b. Flight Test. Base the level of flight test required to validate a particular OBWBS 

installation on the type of aircraft, and aircraft system architecture. Conduct a certification flight 

test for the first OBWBS installation to verify the installation integrity and demonstrate that the 

installed OBWBS has no adverse impact on other aircraft systems. We give credit for previously 

certified installations, simulation and ground testing. We will evaluate the actual requirement for 

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a flight test for each installation, and we will include operational and environmental 

considerations. 

2-9. AIRCRAFT FLIGHT MANUAL/AIRCRAFT FLIGHT MANUAL SUPPLEMENT. 

You, the applicant, must determine if there are any limitations of the system and, if so, how they 

will affect aircraft operations. Include any limitations affecting operations in the AFM/AFMS. 

As a minimum, provide instructions in the limitations section of the AFM/AFMS that include the 

following: 

a. Limitations and Conditions. Total system accuracy, system accuracy, and the effects 

upon accuracy due to environmental and operational limitations, must determine the limitations 

of the OBWBS and how those limitations are accounted for by the OBWBS and the procedures 

described within the AFM/AFMS. All limitations and conditions must be identified to ensure 

the aircraft does not operate outside the certified CG envelope. Include all limitations affecting 

flight operations in the following items, as applicable: 

(1) Environmental:

(a) Wind velocity and direction. Conduct ground tests, or analysis verified by 

tests, for an aircraft of known weight and CG to determine the effects of winds. You must test 

the effects of headwinds, tailwinds, and cross winds. 

(b) De-icing fluids. De-icing fluids depart the aircraft during roll-out. Conduct 

ground tests, or analysis verified by tests, to determine a reasonable weight and then subtract 

from the aircraft weight as displayed on the OBWBS with de-icing fluid. The effect of de-icing 

fluids on aircraft CG calculations must also be addressed. 

(c) Rain, snow, ice, frost, dew. If ground tests or analysis provide data 

supporting a reduction in aircraft weight with these factors present, as compared to the OBWBS 

displayed weight, then you can reduce aircraft weight by a standard weight used for the factor. 

The effect of rain, snow, ice, frost, and dew on aircraft CG calculations must also be addressed. 

(d) Ground slope, both lateral and longitudinal. The OBWBS must have a 

means to account for ground slope, or provide guidance to compensate for the effects of ground 

slope. 

(e) Environmental temperature, barometric pressure. Demonstrate the effects 

on accuracy through tests or analysis over the temperature and pressure range that will be used. 

(2) Configuration of aircraft to conduct a weighing. Establish aircraft weighing 

configuration during the ground test program. State the established aircraft weighing 

configuration in the AFM/AFMS. Use this configuration for all operational aircraft weighing: 

(a) Wing flap and stabilizer positions. 

(b) Oleo strut extension. 

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4/11/2008 AC 20-161

(c) Engine and auxiliary power unit thrust.

(d) Fuel load and its location. 

(e) Entrance and cargo door position. 

(f) Passenger, serving carts, and crew positioning. 

(g) Wing flap, stabilizer, entrance door(s), and cargo door(s) positioning. 

(h) Stairways, jetway, or ground service connections. 

b. Restricted areas of operation. As applicable. 

c. Restricted times of operation. As applicable. 

d. Operational considerations for normal/abnormal procedures.

e. Recommended method of zero fuel weight and CG computation. 

2-10. INSTRUCTIONS FOR CONTINUED AIRWORTHINESS (ICA). 

a. During the certification process of the aircraft onboard weight and balance system 

installation, complete the ICA in accordance with: 

• 14 CFR § 21.50(b). 

• 14 CFR §§ 23, 25, 27, 29.1529. 

• Appendix G of 14 CFR part 23, 

• Appendix H of 14 CFR part 25, 

• Appendix A of 14 CFR parts 27, 29. 

• FAA Order 8110.54, Instructions for Continued Airworthiness, Responsibilities, 

Requirements, and Content. 

b. Format the ICA so it is compatible with other maintenance instructions for the aircraft. 

c. The ICA is required for the OBWBS, and any required information relating to the 

interface of the system with the aircraft. Address all applicable OBWBS installed under a 

particular modification within the ICA. Address the integrity of system, maintenance 

instructions of the OBWBS and include calibration instructions and intervals. Calibration must 

consider the effect of unsprung hardware weight, including any changes to that weight. You 

must specify periodic checks in the ICA to ensure the OBWBS reliability and accuracy remain 

acceptable. 

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4/11/2008 AC 20-161

d. Each ICA must contain an “Airworthiness Limitations” section. 

2-11. MASTER MINIMUM EQUIPMENT LIST (MMEL). Develop a proposed MMEL 

with appropriate justification. Develop procedures for safely dispatching the aircraft using the 

MMEL. Determine any MMEL allowance considering criticality of the OBWBS functionality. 

MMEL allowances must be substantiated based on the OBWBS functional hazard assessment. 

The MMEL must include an alternate method based on an acceptable conventional weight 

buildup to determine aircraft weight and CG location to be used if the OBWBS fails to function 

properly. Submit the proposed MMEL, justification, and procedures to the flight operations 

evaluation board chairman in the aircraft evaluation group for FAA evaluation and approval. If 

you have modified the OBWBS, revise the MMEL to address the OBWBS hardware or software 

changes. Submit the MMEL with the OBWBS equipment changes to the FAA for approval. 

The FAA approving office (for example, flight standards district office) will coordinate with 

ACO engineering when evaluating the revised MMEL. 

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CHAPTER 3. OPERATIONAL APPROVAL PROCESS FOR ONBOARD WEIGHT 

AND BALANCE SYSTEMS 

3-1. OPERATIONAL APPROVAL GUIDANCE. This chapter is for certificate holders 

required to have an approved weight and balance control program under 14 CFR parts 91 

Subpart K, and 14 CFR parts 121, 125, or 135. Compliance with these guidelines allows you to 

use a certified OBWBS for primary dispatch. See AC 120-27E, Aircraft Weight and Balance 

Control, chapter 2, paragraphs 211, 212, 213, and 214. 

3-2. BACKUP SYSTEM. If you are an operator, use the guidance in the AC 120-27E to 

develop a backup system based on a conventional weight buildup. If you develop and receive 

approval for a backup system, the FAA may grant you relief to include an OBWBS in the 

operator’s minimum equipment list (MEL).

3-3. OPERATIONS SPECIFICATIONS. If you obtain operational approval for use of an 

OBWBS, operations specifications (OpSpecs) will be issued by the FAA. OpsSpecs paragraphs 

E096 and A096 will be issued to certificate holders and program managers who are authorized to 

use only actual weights and an OBWBS. Operators that wish to use average weights, and a 

conventional load build-up system, as a back-up weight and balance system, will be issued 

OpSpec paragraphs A097-A099, as applicable, A011, and E096. Operators authorized to use 

average weights as a back-up system will not be issued A096. For 14 CFR part 91 operators, a 

letter of authorization (LOA) number A096 or A097-A099 and A011 as applicable, can be 

issued. 

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4/11/2008 AC 20-161 

Appendix 1 

APPENDIX 1. RELATED REGULATIONS AND DOCUMENTS

Regulations Title 

14 CFR Part 21 Certification Procedures for Products and Parts 

14 CFR Part 23 Airworthiness Standards: Normal, Utility, Acrobatic, and 

Commuter Category Airplanes 

14 CFR Part 25 Airworthiness Standards: Transport Category Airplanes 

14 CFR Part 27 Airworthiness Standards: Normal Category Rotorcraft 

14 CFR Part 29 Airworthiness Standards: Transport Category Rotorcraft 

14 CFR Part 43 Maintenance, Preventive Maintenance, Rebuilding, and 

Alteration 

14 CFR Part 119.49 Contents of Operations Specifications 

14 CFR Part 121.153 Aircraft Requirements: General 

14 CFR Part 121.373 Continuing Analysis and Surveillance 

14 CFR Part 121.419 Pilots and Flight Engineers: Initial, Transition, and 

Upgrade Ground Training 

14 CFR Part 121.422 Aircraft Dispatchers: Initial and Transition Ground 

Training 

14 CFR Part 121.693 Load Manifest: All Certificate Holders 

14 CFR Part 91 General Operating and Flight Rules 

14 CFR Part 135 Operating Requirements: Commuter and On-Demand 

Operations 

14 CFR Part 125 Certification and Operations: Airplanes Having a Seating 

Capacity of 20 or More Passengers or a Maximum 

Payload Capacity of 6000 Pounds or More 

Advisory Circulars Title 

AC 23.1309-1C Equipment, Systems, and Installations in Part 23 

Airplanes 

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Appendix 1 

AC 23.1311-1B Installation of Electronic Displays in Part 23 Airplanes 

AC 25.1309-1A System Design and Analysis 

AC 25-11 Transport Category Airplane Electronic Display 

Systems 

AC 25.19 Certification Maintenance Requirements 

AC 27-1B, Change 1 Certification of Normal Category Rotorcraft 

AC 29-2C, Change 1 Certification of Transport Category Rotorcraft 

AC 120-16 Air Carrier Maintenance Programs 

AC 120-27E Aircraft Weight and Balance Control 

FAA Orders Title 

Order 8110.4C Type Certification 

Order 8110.48 How to Establish the Certification Basis for Changed 

Aeronautical Products 

Order 8110-42 Parts Manufacturer Approval Procedure 

Order 8110.54 Instructions for Continued Airworthiness, 

Responsibilities, Requirements, and Content 

Industry Documents Title 

SAE ARP 4754 Certification Considerations for Highly-Integrated or 

Complex Aircraft Systems 

SAE ARP 4761 Guidelines and Methods for Conducting the Safety 

Assessment Process on Civil Airborne Systems and 

Equipment 

RTCA DO-178B Software Considerations in Airborne Systems and 

Equipment Certification 

RTCA DO-160F Environmental Conditions and Test Procedures for 

Airborne Equipment 

RTCA DO-254 Design Assurance Guidance for Airborne Electronic 

Hardware 

Page A1-2
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Appendix 2 

APPENDIX 2. TERMS AND DEFINITIONS 

Backup System Backup means of computing aircraft weight and balance for 

dispatching the aircraft instead of the primary one. 

Failure An occurrence that affects the operation of a component, part, or 

element so it can no longer function as intended (Includes both loss of 

function and malfunction). Note: Errors may cause failures, but are 

not considered to be failures. 

Load Buildup System Method of establishing an aircraft’s weight and center of gravity with 

the assumptions established in AC 120-27E, Aircraft Weight and 

Balance Control, includes, average passenger weight, average bag 

weight, male to female passenger ratio, fuel weight. 

Maximum Zero Fuel 

Weight 

The maximum permissible weight of an aircraft with no disposable 

fuel and oil. 

Onboard Weight and 

Balance System 

Weighs the aircraft and payload, and computes the aircraft center of 

gravity from equipment onboard the aircraft. The system displays the 

actual weight, and balance information for use in primary dispatch of 

the aircraft. 

Primary Dispatch 

System 

A system that generates aircraft weight and balance data used to 

dispatch an aircraft for flight. 

Redundancy The presence of more than one independent means for accomplishing 

a given function or flight operation.

System A combination of components, parts, and elements, which are 

interconnected to perform one or more functions. 

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Appendix 3 

APPENDIX 3. EFFECTS OF WEIGHT AND CG ON AIRCRAFT PERFORMANCE 

As referenced in section 2-3.b(5) of this AC, use the following formulas to determine how 

aircraft performance is affected by different weights and CG. 

Let the measured weight Wm, the sum of the actual weight Wa, and the weight error We, 

where the weight error is due to inaccuracy inherent in the system and to operational 

disturbances, be expressed as a percentage of Wa as: 

Wm = Wa + We = (1 + α) × Wa 

and let 0 ≤ α < 1. 

 Let the measured CG location be CGm be defined as the sum of the actual CG location CGa and 

the CG location error CGe, where the CG location error is due to inaccuracy inherent in the 

system and to operational disturbances: 

CGm = CGa + CGe = (1 + α) × CGa 

and let 0 ≤ α < 1. 

 

The ratio of the actual weight/erroneous weight and the actual CG position/erroneous CG 

position will establish a non-dimensional percentage accuracy bound on the acceptable, 

allowable inaccuracies in the OBWBS measurements as a function of flight phase and functional 

hazard severity. 

For example, in takeoff, with all other factors held constant (i.e. airplane configuration and 

runway conditions,) the airplane takeoff velocity is proportional to the square root of the airplane 

weight. The ratio of the takeoff velocity V1 at weight W1 to takeoff velocity V2 at weight W2 is 

given by: 

(V2 / V1) = (W2 / W1) 1⁄2 

So, the ratio of the actual weight/measured weight verses the actual takeoff velocity to assumed 

takeoff velocity is given by: 

(Vactual /Vassumed) = (Wa / (1 + α) × Wa) 1⁄2 

= (1 / (1 + α)) 1⁄2 

or rearranging the equation, the actual velocity needed for takeoff in terms of the velocity 

assumed to be needed for takeoff will be: 

 Vactual = Vassumed / (1 + α) 1⁄2 

If the measured airplane weight is 10% higher than the actual weight, for example, then: 

Vactual = Vassumed / (1 + 0.1) 1⁄2 = 0.95 × Vassumed 

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Appendix 3 

APPENDIX 3. EFFECTS OF WEIGHT AND CG ON AIRCRAFT PERFORMANCE, 

continued

the crew might find the airplane can takeoff at a shorter distance down the runway than expected, 

and at a slower velocity than expected based on what the OBWBS measured weight indicated. 

If the measured airplane weight is 10% lower than the actual weight, for example then: 

Vactual = Vassumed / (1 - 0.1) 1⁄2 = 1.05 × Vassumed 

and the crew might find the airplane takes off longer distance down the runway than expected, 

and at a higher velocity than expected based on what the OBWBS measured weight indicated. 

The percentage error in weight measurement (and thus accuracy) can be related to a takeoff 

performance impact and its hazards. 

In the same manner the effect of weight errors (and thus accuracies) on takeoff distance can be 

assessed. With all other factors held constant (i.e. airplane configuration and runway conditions) 

the airplane takeoff distance is proportional to the square of the airplane weight. The ratio of the 

takeoff distance S1 at weight W1 to takeoff distance S2 at weight W2 is given by: 

(S2 / S1) = (W2 / W1) 2 

So, the ratio of the actual weight/measured weight verses the actual takeoff distance to assumed 

takeoff distance is given by: 

(S actual /S assumed) = (W a / (1 + α) × W a) 2 = (1 / (1 + α)) 2 

or rearranging the equation, the actual distance needed for takeoff in terms of the assumed 

distance needed for takeoff will be: 

 S actual = S assumed / (1 + α) 2 

If the measured airplane weight is 10% higher than the actual weight, for example, then: 

S actual = S assumed / (1 + 0.1) 2 = 0.83 × S assumed 

and the crew finds the airplane can takeoff a shorter distance down the runway than expected 

based on what the OBWBS measured weight indicated. 

If the measured airplane weight is 10% lower than the actual weight, for example, then: 

S actual = S assumed / (1 - 0.1) 2 = 1.23 × S assumed 

And the crew find the airplane takes off at a longer distance down the runway than expected as 

based on what the OBWBS measured weight indicated. A large enough error or inaccuracy, in 

percent of actual airplane weight, can result in a takeoff distance that exceeds available runway 

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Appendix 3 

APPENDIX 3. EFFECTS OF WEIGHT AND CG ON AIRCRAFT PERFORMANCE, 

continued

length or doesn’t allow for enough stopping distance if the crew elects to abort the takeoff. The 

percentage error in weight measurement (and thus accuracy) can be related to a takeoff 

performance impact and its hazards. 

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Appendix 4 

APPENDIX 4. TAKEOFF PERFORMANCE BASED METHOD EXAMPLE 

4-1. TAKEOFF PERFORMANCE BASED METHOD GUIDANCE. As referenced in 

section 2-3.c.(7) of this AC, this guidance will help an OBWBS applicant determine the 

operational accuracy for an OBWBS installed on a B767-300ER fleet using the takeoff 

performance based method. 

a. OBWBS operational accuracies that result in at most a ± 1.5-knot error or change in 

V or 1 V2, or a 100 foot increase in takeoff or accelerate-stop distance, whichever is greater, are 

accepted without curtailments or procedural adjustments for OBWBS operational accuracy. 

b. OBWBS operational accuracies that result in greater than a ± 1.5 knot error or change 

in V1 or V2, or more than a 100 foot increase in takeoff or accelerate-stop distance, whichever is 

greater, require appropriate curtailment for operational accuracy. 

c. The applicant uses this guidance to map out an analysis approach that determines the 

need to curtail for the accuracy of the OBWBS-reported weight and CG position when using the 

OBWBS in operational conditions. 

d. Review information published in the B767-300ER Airplane Flight Manual (AFM). 

The AFM may present the information used to determine airplane takeoff performance speeds 

and required takeoff runway length as graphs. Interpolation may be required for weights not 

exactly shown on these charts. An applicant may use a computer program or parametric 

equations that can be shown to accurately represent the approved AFM data set. 

4-2. THE EFFECT OF WEIGHT CHANGE ON V1 SPEED. 

a. The applicant uses the AFM data to first determine the effect of airplane weight 

variation on V1 to establish the relationship between incremental weight change and resulting 

incremental V1 speed change. Use this relationship to determine the magnitude of weight error 

that results in a ± 1.5- knot change in V1.

b. By definition the V1 speed is at or below VR. For many airplanes the ratio of V1 to VR

varies from 1.0 (per regulation) to some lower limit (e.g., 0.7 for the B767-300ER per AFM 

operationally defined limits). In general, a change in VR will not result in a change in V1 of 

greater magnitude than the change in VR . Therefore, the applicant may evaluate the effect of 

weight changes on VR and then relate this change to V1 using the appropriate ratio for the AFM 

operationally defined limits for the airplane type. 

c. In this example, the applicant determines that across takeoff flap settings of 5, 15 and 20 

degrees, the effect of airplane weight on VR as a function of the airplane’s weight can be 

represented as follows: 

(1) In the weight range of 200,000 pounds to 250,000 pounds, the applicant sees that a 

change in weight of 10,000 pounds results in a 3.5 knot change in VR, and that this relationship 

holds across the airplane weight range. This range is dubbed the low-weight “band.” 

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Appendix 4 

APPENDIX 4. TAKEOFF PERFORMANCE BASED METHOD EXAMPLE, continued

(2) In the weight range of 250,000 pounds to 350,000 pounds, the applicant sees that a 

change in weight of 10,000 pounds results in a 3 knot change in VR, and that this relationship 

holds across the airplane weight range. This range is dubbed the mid-weight “band.” 

(3) In the weight range of 350,000 to 430,000 pounds, the applicant sees that a change 

in weight of 10,000 pounds results in a 2.5 knot change in VR, and that this relationship holds 

across the airplane weight range. This range is dubbed the high-weight “band.” 

d. The relationships in these bands can scale by the ratio of V1/VR for those performance 

scenarios using a V1/VR ratio less than 1.0. 

e. The OBWBS accuracies resulting in at most a ± 1.5-knot change in V1 in each band 

when using a V1/VR ratio of 1.0 are therefore: 

• ± 1.5 knot change × (10,000 pounds/ 3.5 knot change) = ± 4,286 pounds in the lowweight band, or ± 4300 pounds with rounding. This weight offset corresponds to an OBWBS 

weight accuracy of 2.15% at 200,000 pounds to 1.72% accuracy at 250,000 pounds over the lowweight band. 

• ± 1.5 knot change × (10,000 pounds/ 3 knot change) = ± 5,000 pounds in the midweight band. This weight offset corresponds to an OBWBS weight accuracy of 2% above 

250,000 pounds to 1.43 % accuracy at 350,000 pounds over the mid-weight band. 

• ± 1.5 knot change × (10,000 pounds/ 2.5 knot change) = ± 6,000 pounds in the 

high- weight band. This weight offset corresponds to an OBWBS weight accuracy of 1.71% 

above 350,000 pounds to 1.4 % accuracy at 430,000 pounds over the high-weight band. 

f. Alternatively, the applicant can fit a curve across these bands in order to smooth out the 

discontinuities or jumps in weights at the band edges. 

4-3. THE EFFECT OF WEIGHT CHANGE ON V2 SPEED. 

a. The analysis of the effect of airplane weight change on V2 is similar to the analysis of 

the effect of airplane weight change on V1, except that V2 AFM data are readily available. There 

is no need to relate the analysis to VR data. 

b. With respect to V2, the applicant determines from the AFM data that across takeoff 

flap settings of 5, 15 and 20 degrees of flap. 

(1) In the weight range of 200,000 pounds to 250,000 pounds, the applicant sees that 

a change in weight of 10,000 pounds results in a 2.75 knot change in V2, and that this 

relationship holds across the low-weight band. 

Page A4-2 
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Appendix 4 

APPENDIX 4. TAKEOFF PERFORMANCE BASED METHOD EXAMPLE, continued

(2) In the weight range of 250,000 pounds to 350,000 pounds, the applicant sees that 

a change in weight of 10,000 pounds results in a 2.5 knot change in V2 and that this relationship 

holds across the mid-weight band. 

(3) In the weight range of 350,000 to 430,000 pounds, the applicant sees that a change 

in weight of 10,000 pounds results in a 2 knot change in V2 and that this relationship holds 

across the high-weight band. 

c. The OBWBS accuracies that result in at most a ± 1.5 -knot change in V2 in each band 

are therefore: 

• ± 1.5 knot change × (10,000 pounds/ 2.75 knot change) = ± 5,454 pounds in the 

low- weight band, or ± 5400 pounds with rounding. This weight offset corresponds to an 

OBWBS weight accuracy of 2.7% at 200,000 pounds to 2.16% accuracy at 250,000 pounds over 

the low-weight band. 

• ± 1.5 knot change × (10,000 pounds/ 2.5 knot change) = ± 6,000 pounds in the 

mid-weight band. This weight offset corresponds to an OBWBS weight accuracy of 2.4% above 

250,000 pounds to 1.71% accuracy at 350,000 pounds over the mid-weight band. 

• ± 1.5 knot change × (10,000 pounds/ 2 knot change) = ± 7,500 pounds in the 

high- weight band. This weight offset corresponds to an OBWBS weight accuracy of 2.14% 

above 350,000 pounds to 1.74 % accuracy at 430,000 pounds over the high-weight band. 

d. Once again, the applicant can fit a curve across these bands in order to smooth out the 

discontinuities or jumps in weights at the band edges. 

 

4-4. THE EFFECT OF WEIGHT CHANGE ON TAKEOFF/ACCELERATE-STOP 

DISTANCE. 

a. For an uncorrected (i.e., no runway slope, no wind) takeoff at flaps 5, the applicant 

determines from AFM data that at the most extreme all-engines-operating takeoff distance 

situation, a change in 5000 pounds of weight in the high-weight band means a 500 foot 

difference in takeoff distance, and for less extreme conditions, a change of 5,000 pounds in 

weight means a change of up to 400 feet in the all-engines-operating takeoff distance across the 

low-weight to mid-weight bands. 

b. The OBWBS accuracies that result in at most a 100-foot increase in the all-engine 

takeoff distance are therefore on the order of: 

• ± 100 foot change × (5,000 pounds / 400 foot change) = ± 1,250 pounds or ± 1,300 

pounds after rounding. This weight offset corresponds to an OBWBS weight accuracy of 0.52 % 

above 250,000 pounds to 0.37% accuracy at 350,000 pounds over the low to mid-weight band. 

Page A4-3 
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Appendix 4 

APPENDIX 4. TAKEOFF PERFORMANCE BASED METHOD EXAMPLE, continued

• ± 100 foot change × (5,000 pounds / 500 foot change) = ± 1,000 pounds in the 

most extreme takeoff distance situation. This weight offset corresponds to an OBWBS weight 

accuracy of 0.29% above 350,000 pounds to 0.23% accuracy at 430,000 pounds over the highweight band. 

4-5. OBWBS OPERATIONAL ACCURACY AND OBWBS-MEASURED WEIGHT 

CURTAILMENT. 

a. OBWBS operational accuracies that result in at most a ± 1.5-knot error or change in 

V or 1 V2, or a 100 foot increase in takeoff or accelerate-stop distance, whichever is greater, are 

accepted without curtailments or procedural adjustments for OBWBS accuracy. 

b. The applicant notes that the weight changes in paragraph 4-4 of this appendix that 

result in more than a 100 foot change in takeoff/accelerate-stop distance all result in less than a 

± 1.5-knot change in V1 or V2 (see paragraphs 4-2 and 4-3 of this appendix, respectively). 

Therefore the weight change that results in a 100 foot change to the takeoff/accelerate-stop 

distances does not set the maximum operational accuracy for OBWBS use without curtailment. 

c. The applicant notes that the weight changes in paragraph 4-3 of this appendix that 

result in a 1.5-knot change in V2 will change V1 by more than 1.5 knots (see paragraph 4-2 of 

this appendix). Therefore, the weight change that results in a 1.5-knot change in V2 does not set 

the maximum operational accuracy for OBWBS use without curtailment. 

d. The applicant notes that the weight changes in paragraph 4-2 of this appendix that 

result in a 1.5-knot change in V1 will not change V2 by more than 1.5 knots (see paragraph 4-3 of 

this appendix). Therefore, the weight change that results in a 1.5-knot change in V1 does set the 

maximum operational accuracy for OBWBS use without curtailment. 

e. Therefore, the applicant concludes that the reported weights from the OBWBS can be 

used without curtailment for OBWBS operational accuracy as long as the OBWBS-reported 

weight is within: 

• ± 4,300 pounds of the actual airplane weight in the low-weight band. 

• ± 5,000 pounds of the actual airplane weight in the mid-weight band. 

• ± 6,000 pounds of the actual airplane weight in the high-weight band. 

f. In general, the required OBWBS weight accuracy can be expressed in terms of the 

ratio of these weight uncertainties per band to the actual airplane weight in that band. 

g. For example, at an actual weight of 200,000 pounds the OBWBS operational accuracy 

needs to be ± 2.15 % to use the reported weight without operational curtailment, and for an 

actual weight of 430,000 pounds the OBWBS operational accuracy needs to be ± 1.4 % to use 

the reported weight without operational curtailment for OBWBS accuracy. 

Page A4-4 
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Appendix 4 

APPENDIX 4. TAKEOFF PERFORMANCE BASED METHOD EXAMPLE, continued

h. Now, per the guidance in paragraph 2-2.d(3) of this AC, the applicant determines the 

OBWBS system accuracy by obtaining weight and CG measurements from the aircraft while it 

sits on precision aircraft scales within a controlled environment such as hanger. 

i. Suppose for example the OBWBS-reported weight during the system accuracy testing 

is within: 

• ± 1,000 pounds of the actual airplane weight in the low-weight band. 

• ± 2,000 pounds of the actual airplane weight in the mid-weight band. 

• ± 2,500 pounds of the actual airplane weight in the high-weight band. 

j. Then the applicant knows that operational conditions such as configuration changes, 

wind gusts and winds, temperature changes, passenger movement, etc., can result in up to the 

following weight errors before the need to curtail the weight and CG envelope for the operational 

accuracy of the OBWBS: 

• ± 3,300 pounds of additional uncertainty about the actual airplane weight in the 

low-weight band, 

• ± 3,000 pounds of additional uncertainty about the actual airplane weight in the 

mid-weight band, and 

• ± 3,500 pounds of additional uncertainty about the actual airplane weight in the 

high-weight band. 

k. Alternatively, if the measured OBWBS system accuracies in these weight bands 

exceed the weight accuracies resulting in no more than 1.5-knot change in V1, the OBWBS 

cannot be used without curtailment for OBWBS accuracy at any time. 

l. This example illustrates the importance of the OBWBS system accuracy. The better 

the system accuracy, in general, the larger the operational environment within which the 

applicant may use the OBWBS system weight measurements without curtailment for OBWBS 

accuracy. 

4-6. CONCLUSIONS. 

a. This example shows how an applicant can map out an analysis approach to 

systematically consider the effect of weight changes on V1, V2 and takeoff/accelerate-stop 

distances. This analysis determines the operational accuracies that result in at most a ± 1.5 -knot 

error or change in V1 or V2, or a 100 foot increase in takeoff or accelerate-stop distance, 

Page A4-5 
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Appendix 4 

APPENDIX 4. TAKEOFF PERFORMANCE BASED METHOD EXAMPLE, continued

whichever is greater, which can be accepted without curtailments or procedural adjustments for 

OBWBS accuracy. OBWBS operational accuracies that result in greater than ± 1.5 knot error or 

change in V1 or V2, or more than a 100 feet increase in takeoff or accelerate-stop distance, 

whichever is greater, require appropriate curtailment for OBWBS accuracy. 

b. The analysis method shown in this example can be readily modified to consider the 

effects of pressure altitude, outside air temperature, winds, and runway slope, as appropriate, 

when determining the relationship of change in weight to change in takeoff/accelerate-stop 

distance. 

Page A4-6