Advisory U.S. Department of Transportation Federal Aviation Circular Administration Subject: AIRCRAFT ONBOARD Date: 4/11/2008 AC No: 20-161
WEIGHT AND BALANCE SYSTEMS Initiated by: AIR-130
FOREWORD This advisory circular (AC) gives manufacturers and installers an acceptable means of compliance to meet the installation, operation, and airworthiness requirements for aircraft onboard weight and balance systems (OBWBS).
Susann Caller Susan J. M. Cabler Thomas K. Toula Acting Manager Manager Aircraft Engineering Division, AIR-100 Air Transportation Division, AFS-200 Aircraft Certification Service Flight Standards Service Page 1
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 opera tional 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 GU IDANCE 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. ST ANDARD 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.
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.
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 Page 4
Cockpit Controls Instrument Arrangement and Visibility Warning, Caution, and Advisory Lights Electrical Systems and Equipment – General Circuit Protective Devices Hydraulic Systems Pressurization and Pneumatic Systems Weight & Center of Gravity Instructions for Continued Airworthiness Aircraft Flight Manual – General Operating Limitations Lightning and Static Electricity Fire Protection Loading Information Electronic Equipment Electric Cables Switches Instrument Systems 14 CFR §§ 23, 25, 27, 29.777(a) 14 CFR §§ 23, 25, 27, 29.1321 14 CFR §§ 23, 25, 27, 29.1322 14 CFR §§ 23, 25, 27, 29.1351 14 CFR §§ 23, 25, 27, 29.1357 14 CFR §§ 23, 25, 27, 29.1435 14 CFR §§ 23, 25, 27, 29.1438 14 CFR §§ 23, 25, 27, 29.1519 14 CFR §§ 23, 25, 27, 29.1529 14 CFR §§ 23, 25, 27, 29.1581 14 CFR §§ 23, 25, 27, 29.1583 14 CFR §§ 23.867 and 25.1316 14 CFR §§ 23.1359, 25.869, and 29.1359 14 CFR §§ 23.1589, 27.1589, and 29.1589 14 CFR §§ 23.1431 and 25.1431 14 CFR §§ 23.1365 and 27.1365 14 CFR §§ 23.1367 and 27.1367 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 Page 5
• 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 aircra ft 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 Page 6
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 Page 7
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 Page 8
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 V 1 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.
• 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 Page 10
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 Fuel- Quantity, 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 – ½ of one percent of the mean aerodynamic chord (MAC) and ½ 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.
(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 diffe rences 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.