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.
(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