Retrofitting Cost Modeling in Aircraft Design
2. Retrofitting Solutions: Benefits and Risks
3. Retrofitting Costs Methodology
- Recurring costs. This category includes the costs of all the activities, items and materials that are directly related to the number of aircraft produced. According to Beltramo et al. , this includes the costs accrued for all labor and raw materials related to the production of major components and subassemblies by the aircraft manufacturer, for the acquisition of those components that are not produced by the aircraft manufacturer, and for all the labor required to integrate the major components and subassemblies into a finished aircraft.
- Non-recurring costs. This category is mainly linked to the development costs. According to Markish , it includes all the operations required to bring an aircraft concept to production. As a consequence, this cost is not dependent on the number of aircraft produced. Non-recurring costs include costs related to the preliminary design, detail design, tooling, testing and certification. All these operations must be considered for each of the parts comprising the aircraft.
- Development costs. This coincides with the non-recurring costs, the initial investment to support the retrofit project.
- Conversion costs. This corresponds mainly to the recurring costs. This cost item is associated with the practical actions needed to modify the aircraft.
- Equipment costs. Expenditures on the purchase of every kind of equipment (i.e., engine or OBS), materials and ground equipment correspond to a recurring cost.
3.1. Development Costs
3.2. Conversion Costs
3.3. Equipment Costs
- The minimum number of aircraft retrofitted to obtain a discount on equipment costs (NDISC).
- The minimum discount on equipment costs (DISC).
3.4. Capital Costs
4. Case Study: Methodology Applied to a Regional Jet Aircraft
- High BPR-geared turbofan engine installation, leading to improvements in fuel consumption, noise, emissions and maintenance costs.
- OBS architecture electrification, offering more electric and all-electric architectures (MEA/AEA), improving fuel efficiency, maintenance and costs.
- More Electric 1 (MEA1). The hydraulic system is completely removed, along with its distribution system. All actuators are electric.
- More Electric 2 (MEA2). The peculiarity of this architecture is represented by the electrification of the wing ice protection system (WIPS) and the environmental control system (ECS). This is a bleedless configuration with electrically driven compressors and hydraulic pumps that are powered by electric motors.
- All-Electric (AEA). An all-electric architecture adopts the innovative features of MEA1 and MEA2; thus, neither the hydraulic nor the pneumatic system are present. No bleed air is required and the pneumatic power is produced by dedicated compressors.
4.1. Development Activities
4.2. Conversion Activities
4.3. Retrofitting Equipment
4.4. Regional Jet Aircraft Retrofitting Costs
5. Impact of Retrofitting Activities
5.1. Parametric Analysis and Results
5.2. Retrofitting Savings
Conflicts of Interest
- New engine attachment points. New engines may be installed on different wing attachment points compared to the previous ones. A higher bypass ratio means that the fan size is increased; as a result, mounting these engines under a wing could be a challenging task that requires great engineering effort.
- Wing stress analysis. Due to the different geometries and characteristics of the new engines, the inertia, force and thrust generated will certainly change. The static aeroelastic deformation of the wing structure and load distributions, bending moment and torque need to be studied. For this purpose, a new structural finite-element model of the wing/engine system must be established.
- Wing reinforcement design. A possible conclusion of the wing stress analysis may be the realization that a wing reinforcement is needed, due to the issues described in the previous points.
- Flutter analysis. The engine module position modification along the wing in both spanwise and chordwise directions can influence the flutter characteristics. The natural vibration modes of the structure may also change with the adoption of new actuators. The structure should be capable of supporting this at the critical loads present on the maneuvering diagram.
- Panel removal and installation. The hydraulic and pneumatic circuits run across the wings and the fuselage, to connect the energy sources to the various users. If the onboard systems are modified, it is necessary to remove the fuselage panels and reinstall them after the replacement. The engineering effort will be focused on planning the operations of the fuselage panel disassembly and assembly.
- Aerodynamics. A computational fluid dynamics (CFD) analysis must be carried out to predict the drag, lift, noise, performance, structural and thermal loads for the updated aircraft systems.
- Performance. The aircraft mass distribution is an important parameter to be considered during the design process, due to its significant influence on performance and inertia. If the new engines are located at a greater distance from the fuselage, they will make a greater contribution to the rolling moment of inertia of the aircraft. In addition, their weight and efficiency changes will all influence the aircraft’s mass distribution.
- Flight quality. A certain amount of engineering effort is involved in the study and in the evaluation of the longitudinal and lateral-directional stability and control characteristics of the retrofitted aircraft.
- Weight and center-of-gravity analysis. The proper distribution of weight plays a large and important role in an aircraft’s overall performance. Both performance and stability depend on the location of the center of gravity. Therefore, all flight tests must be conducted with an accurate knowledge of the location of the center of gravity at any one point in time.
- Structural loads. An analysis that is performed on all the aircraft in terms of the new structural loads is required for certification purposes and to understand if reinforcing element installation is required.
- Electrical generation/distribution. Power must be provided by an additional electrical generator and distribution system. These components must be sized appropriately and relocated along the aircraft.
- ECS, IPS, air conditioning, FCS. All the components that connect to the new electrified system must be redesigned.
- Load and failure analysis and new installation drawings. For the overall OBS architecture installation, failure analysis must be performed, and new component drawings must be provided.
- OBS design, engine installation, engine FADEC, and autopilot. The simultaneous engine and OBS upgrades imply the installation of a new FADEC (full-authority digital engine control) system and new autopilot software.
- Wind tunnel tests. Once the new engine has been chosen, the combination airframe and new engine must be tested. A wind-tunnel test campaign must be organized and carried out to predict the aerodynamic performance of individual aircraft components, as well as the new overall configuration. The engineering effort required to process the test data and to obtain the new drag polar curves is also considered.
- Flight tests. After the retrofit updates, a flight test campaign is carried out to determine the new aircraft characteristics (previously estimated via wind tunnel tests), to assist the engineering design and developmental process and to verify the attainment of technical performance specifications and objectives, to establish the system’s operational effectiveness and operational suitability.
- System tests on a complete A/C and RIG test. Several test systems must be assessed to analyze the behavior of the new onboard systems, starting from the standalone component up to its integration into the aircraft. Four different RIG tests must be performed: tests of the electrical, propulsion, avionic and flight control systems. In addition, an avionics software development process is required by law. This cost item must be considered since the engine’s FADEC and the autopilot are changed.
- Ground vibration-resonance test. The modifications to the structure and mass distribution could bring the necessity of new ground vibration tests, performed to meet certification requirements.Technical documentation. After such an innovation, it is essential to make an engineering effort to update the various aircraft manuals: the repair manual, the aircraft flight manual (AFM), the flight crew operating manual (FCOM), and the weight and balance manual (WBM).
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|Wing Area||81.40 m2|
|Design Mission||1890 nm + 100 nm + 5% reserve|
|Typical Mission||720 nm|
|Maximum Take-off Weight||39,500.00 kg|
|Payload Mass||9180.00 kg|
|Fuel Mass||6450.00 kg (design mission)|
|T0 ISA Sea Level (Single-engine)||78,200 N|
|OBS Architecture||Main Characteristics|
|Retrofit Package||Reference OBS||Electrified OBS|
|New Engine||Engine Upgrade||Engine Up. + MEA1||Engine Up. + MEA2||Engine Up. + AEA|
|Development Cost Assumption|
|Engineering cost||EUR 80/h|
|Flight test cost||EUR 7000/h|
|Wind tunnel test cost||EUR 5000/h|
|RIG test cost||EUR 15 million per test|
|Working time per year||1760 h|
|Field||Type||People||Years||Costs [Million EUR]|
|Structure||New engine attachment points |
Wing stress analysis
Wing reinforcements design
Panels removal and installation
Weight and barycenter analysis
|OBS Design||Load and failure analysis, new installation drawings |
ECS electrical pack
Thermal IPS design
Air conditioning distribution
FCS electrical actuation
OBS design, engine installation
Engine FADEC, autopilot
|Testing||Wind-tunnel test support |
Flight test support
System tests on the complete A/C
Ground vibration-resonance test
Wing static test and support
Wind tunnel test
RIG test (4×)
|Retrofitting Activity||Non-Recurring Development Costs (Million EUR)|
|Design and Analysis||Testing||Data, Travels and Documentation||Total|
|Engine Upgrade||77.1 (48%)||43.4 (27%)||40.4 (25%)||160.9|
|MEA1||86.7 (48%)||55.2 (30%)||40.4 (22%)||182.3|
|MEA2||89.9 (53%)||39.2 (23%)||40.4(24%)||169.5|
|AEA||103.2 (51%)||57.6 (29%)||40.4 (20%)||201.2|
|Engine Up. + MEA1||103.7 (38%)||90.8 (34%)||74.4 (28%)||268.9|
|Engine Up. + MEA2||106.7 (42%)||74.8 (29%)||74.4 (29%)||255.9|
|Engine Up. + AEA||120 (42%)||93.2 (32%)||74.4 (26 %)||287.6|
|Conversion Cost Data|
|Engine replacement operators||50|
|OBS replacement operators||60|
|Operation cost||EUR 80/h|
|Working time per month||160 h|
|Field||Type||Months||Costs (Million EUR)|
|Pylon, engine, nacelle |
Wing skin panel
|New engine attachments points |
Spar, ribs, skin reinforcement
|Pylon, engine, nacelle |
Wing skin panel
|Fuselage skin panel |
Seats, interiors, floors
|Electrical distribution and generation |
ECS, IPS, APU, TPs
Fuselage skin panels
Reception, painting and delivery
|Retrofitting Activity||Recurring Conversion Costs (Million EUR)|
|Material Travels and Management||Total|
|Engine Upgrade||3.84 (61%)||0.46 (7%)||2.05 (32%)||6.35|
|MEA1||0 (0%)||3.26 (57%)||2.45 (43%)||5.71|
|MEA2||0 (0%)||2.46 (50%)||2.45 (50%)||4.91|
|AEA||0 (0%)||5.68 (70%)||2.45 (30%)||8.13|
|Engine Up. + MEA1||3.84 (32%)||3.72 (31%)||4.50 (37%)||12.06|
|Engine Up. + MEA2||3.84 (34%)||2.92 (26%)||4.50 (40%)||11.26|
|Engine Up. + AEA||3.84 (27%)||6.14 (42%)||4.50 (31%)||14.48|
|Equipment||Unit Price (Million EUR)|
|Engine BPR 15||9.5|
|Development Cost (Million EUR)||Conversion Cost (Million EUR)||Equipment Cost (Million EUR)|
|Engine Up. + MEA1||268.9||12.06||26.5|
|Engine Up. + MEA2||255.9||11.26||27.4|
|Engine Up. + AEA||287.6||14.48||27.3|
|Costs vs. Savings Analysis Hypothesis|
|Flights per day||7|
|Operative days per year||358 (a–b check included)|
|Flights per year||2506|
|Flight hours per year||3579 (block time = 1.5 h)|
|Years of utilization||12|
|Aircraft residual value||10%|
|Manufacturer profit margin||7%|
|Learning curve rate||0.95|
|Fuel price||EUR 0.65/kg of kerosene/EUR 88 per barrel|
|Noise taxes||Frankfurt airport taxes|
|Fleet||Retrofit Type||Profit Margin||Develop.||Conv.||Equip.||CAPITAL|
|300||Engine Upgrade |
Engine Up. + AEA
|500||Engine Upgrade |
Engine Up. + AEA
|700||Engine Upgrade |
Engine Up. + AEA
|Retrofit Type||Engine Upgrade||AEA||Engine Up. + AEA|
|∆ Fuel (Million EUR)||0.80||0.04||1.26|
|∆ Emissions Charges (Million EUR)||18.5 × 10−4||3.9 × 10−4||3.0 × 10−4|
|∆ Noise Charges (Million EUR)||0.14||1.4 × 10−4||0.14|
|∆ Maintenance (Million EUR)||0.12||0.19||0.25|
|SAVINGS (Million EUR)||1.06||0.23||1.65|
|Capital−Savings (Million EUR Per Year)|
|Number of Aircraft|
|Retrofit Type||Engine Upgrade||0.13||−0.13||−0.14|
|Engine Up. + AEA||0.43||0.02||−0.013|
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Della Vecchia, P.; Mandorino, M.; Cusati, V.; Nicolosi, F. Retrofitting Cost Modeling in Aircraft Design. Aerospace 2022, 9, 349. https://doi.org/10.3390/aerospace9070349
Della Vecchia P, Mandorino M, Cusati V, Nicolosi F. Retrofitting Cost Modeling in Aircraft Design. Aerospace. 2022; 9(7):349. https://doi.org/10.3390/aerospace9070349Chicago/Turabian Style
Della Vecchia, Pierluigi, Massimo Mandorino, Vincenzo Cusati, and Fabrizio Nicolosi. 2022. "Retrofitting Cost Modeling in Aircraft Design" Aerospace 9, no. 7: 349. https://doi.org/10.3390/aerospace9070349