(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 suffici ent 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 demonstrati on, 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 referenc es 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 Page 13
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.
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, th e display presentation must show the crew data that’s organized and easily used. Find additi onal 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 Page 15
a flight test for each installation, and we will include operational and environmental considerations.
2-9. AIRCRAFT FLIGHT MANUAL/AIRC RAFT 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 depa rt 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 conf iguration for all operational aircraft weighing:
(a) Wing flap and stabilizer positions.
(b) Oleo strut extension.
(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 CONT INUED 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.
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 consideri ng 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, justificati on, 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 e quipment 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.
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. Operat ors 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.
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 Page A1-1
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
Appendix 2 Page A2-1 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.
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 = W a + W e = (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 = CG a + CG e = (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 V 1 at weight W1 to takeoff velocity V2 at weight W2 is given by:
(V2 / V1) = (W 2 / W1) ½ So, the ratio of the actual weight/measured weight verses the actual takeoff velocity to assumed takeoff velocity is given by:
(Vactual /Vassumed) = (W a / (1 + α) × Wa) ½ = (1 / (1 + α)) ½ or rearranging the equation, the actual velocity needed for takeoff in terms of the velocity assumed to be needed for takeoff will be:
Vactual = V assumed / (1 + α) ½ If the measured airplane weight is 10% higher than the actual weight, for example, then:
Vactual = V assumed / (1 + 0.1) ½ = 0.95 × V assumed Page A3-1
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 = V assumed / (1 - 0.1) ½ = 1.05 × V assumed 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) = (W 2 / 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 Page A3-2