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
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 V1 or 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 V 1 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. Th is 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. Th is 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 corr esponds 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.
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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.
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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 V1 or 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 cha nge 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 cha nge 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.
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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