Major Structural Modifications on a Boeing 777-200LR Fuselage

Tiago Francisco Oliveira Espırito Santo Macieiratiago.macieira@tecnico.ulisboa.pt

Instituto Superior Tecnico, Universidade de Lisboa, Portugal

January 2021

Abstract

The exponential evolution in the aeronautical industry in the last century opened the market fornew business opportunities, such as aircraft modifications. Whether it is a repair or an alteration,these modifications must be certified, by the company that is doing them or, in case they do nothave the privileges to do so, by a regulatory agency such as the European Union Aviation SafetyAgency (EASA) or the Federal Aviation Administration (FAA), through the use of a SupplementalType Certificate (STC). Two major modifications are presented : i) a Ka-band antenna, used forwireless internet access and ii) a quad camera, used for passenger entertainment. In both of them,the installation location investigation and the detailed aircraft environment definition are done, sinceit is considered to be one of the keys for design success. All structural provisions are produced byJet Aviation, in-house, for both installations, although in the Ka-band antenna installation there isthe additional guidance from the industry standard Aeronautical Radio Incorporated (ARINC) 791.Three quad camera design variations have been studied by using the finite element method (FEM), inorder to achieve a weight and cost reduction, while maintaining the structural integrity. Finally theimportant documents needed, in order to apply for the STC are presented.Keywords: Aircraft modifications, Ka-band antenna, Quad camera, Static analysis, ModificationCertification

1. Introduction

The aviation industry is one of the most broad in-dustries that exists, that has been growing expo-nentially since the 1940’s due to the increasing de-mand for passenger and freight services, the techno-logical progress and the associated investment [1].This growth created new market opportunities inthe field of aircraft modifications, where companieslike Jet Aviation have a large presence nowadays.This company, which operates within the GeneralDynamics Aerospace Group, is a global recognizedleader in the business aviation industry. Their ser-vices range from completions and refurbishments toaircraft modifications.

These modifications can be divided into repairsand alterations, which can be major or minor. Bothof the modifications depicted in this work (Ka-bandSatellite Communication (SATCOM) system andquad camera) are major alterations, which meansthey ”...might appreciably affect weight, balance,structural strength, performance, powerplant oper-ation, flight characteristics, or other qualities af-fecting airworthiness...”[2].

In order to certify these modifications an STCneeds to be created, since a product is being al-tered by introducing a major change to it [3]. The

agency that will certify this works modifications willbe EASA, since the country of origin of the aircraft,Azerbaijan, belongs to the community of countriesthat cooperate with EASA [4].

The design of both the Ka-band SATCOM andquad camera systems, as well as the static analysisof the latter, will be discussed in the next sections,since the two main objectives of this work were topresent two different fuselage major alterations, aswell as trying to improve one of the designs withthe tools used in an industry environment.

2. Aircraft Telecommunication Systems andDesign Standards

The aeronautical communication market, whichwas limited to cockpit and control towers communi-cations, has shifted focus to wireless-in-cabin com-munications, due to the market’s demand to makeair travel more pleasant and productive for the pas-sengers [5, 6]. The use of this type of technologyis still limited however, due to the high investmentneeded. That is why it is usually not requested bycommercial airlines and it is more of a private cus-tomer request. There are two ways of obtaining awireless internet connection on board, which are :Air-to-Ground (ATG) and SATCOM systems.

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2.1. Airplane WI-FI Connection

The ATG is a ground based communication systemwhich uses broadband towers that transmit, routeand receive data to and from the aircraft [7]. Thissystem has some drawbacks like service disruptionif passing over large bodies of water or remote ter-rain (since there are no towers in these locations)as well as lower internet speed (3-10 MegaBytes(MBps) per aircraft). The advantages, when com-pared to the SATCOM system, is the lower overallcosts (equipment, maintenance, etc) and lower sys-tem latency [6].

The SATCOM system is also a communicationsystem which contains three components : AircraftEarth Station (AES), Ground Earth Station andthe satellite. The focus will be on the AES sinceit is directly installed on the aircraft. This systemtransmits, receives and processes signals through amounted antenna on top of the fuselage or tail [8].Overall, the SATCOM system has a wider coveragesince it does not have disruptions over the ocean anddeserted land because it automatically connects tothe closest satellite in orbit. Furthermore, the inter-net speeds have a significant improvement (as muchas 70 MBps per aircraft). The disadvantages residein the higher latency, since the signal needs to travelup to space and back, and on the higher overall costs[6, 9]. In figure 1 it is possible to observe these twoways of WI-FI connection in an aircraft.

Figure 1: WI-FI connection in an aircraft [10].

The SATCOM system uses high frequencies tocommunicate with the satellite, such as Ku/Ka-bands which have frequencies of 12-18 GHZ and 26-40 GHZ, respectively [11]. The K-band, which hasfrequencies in between the two behind mentionedbands, is not used for long range communicationssince it is highly absorbed by water vapor on the at-mosphere [12]. With these higher frequencies comesthe higher throughput (more data flow) and higherbandwidths, although it also means more suscepti-

bility to rain interference [13].

2.2. ARINC 791 StandardThis standard sets the desired characteristics ofthe Ka/Ku-band SATCOM system intended tobe installed on the B777-200LR. This system iscomposed by the Outside Antenna Equipment(OAE), the Ka/Ku Band Radio Frequency Unit(KRFU), Ka/Ku Band Aircraft Network Data Unit(KANDU), Modem and Manager (MODMAN) andAircraft Personality Module (APM) [8].

The most important components of the OAE arethe antenna aperture, which is a high gain radiatingstructure that receives and transmits the Ka/Ku-band radio signals; the radome, which is used toprotect the OAE from environmental agents; theadapter plate, which provides the mechanical inter-face between the antenna subsystems and the air-craft through seven fittings (produced in Jet Avia-tion) and the skirt which provides an interface be-tween the radome and the aircraft fuselage. TheARINC 791 standard was used due to its severalinterchangeability advantages and guidance on theinitial design concepts [8].

2.3. Fuselage Cut-OutsWhen a fuselage cut-out is made it should be rein-forced with a doubler, which is a sheet metal part,in order to carry the load that would have beencarried in the cut-out panel. It will also help toredistribute the load [14].

Furthermore, these cut-outs are usually donewith a round shape since it leads to lower stressconcentrations (localized high stress that occur ata geometric discontinuity) [15]. It is also importantto note that the reason why open holes are morecritical to the structure is because the stress concen-trations on them are higher, than when comparedto a hole with a rivet, since the rivet will create acompression field and reduce the stress concentra-tions on the hole [16]. This is why special attentionneeds to be given to skin cut-outs used for cablepassing or other similar functions.

The design of a new structural modification in-cludes, inevitably, the use of rivets and its distri-bution. In order to have a safe design there arecertain rules that should be followed, such as theinter-rivet pitch, edge margin and radius clearance.The inter-rivet pitch should be between 4D-6D (Dis the diameter of the rivet) in order to avoid thehigh stress concentrations from consecutive rivetsand the inter rivet buckling, which is more likely tohappen if the rivets are farther away [17]. The edgemargin is set to 2D+1mm according to static andfatigue requirements and in order to take into con-sideration the oversize of the rivets, when a repairis being made [17]. Finally the radius clearance,which is the distance that a rivet should have from

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a specific radial component, is set to D+1mm sincethe head of a rivet has a dimension of approximatelybetween 1.25D-1.66D, so by granting this clearancethe clash to the radial component is avoided.

3. KA-Band Antenna Structural System De-sign

The Ka-band antenna installation is a major mod-ification that requires a thorough investigation ofthe B777-200LR primary structure and engineeringknowledge in order to be able to design the adequatestructural attachment of the SATCOM system.

3.1. Antenna Installation PositionIn order to position the antenna on the aircraftthere are several aspects that need to be taken intoconsideration such as :

� Aerodynamic Considerations

In general an aircraft can be divided into two sec-tions, when it comes to the aerodynamic domain.The first section is called the critical aerodynamicarea and goes from the nose of the aircraft until themiddle of the wings. This area is more sensitive toflow perturbations since they can change the flowconditions reaching the wings. The second sectionis the non critical aerodynamic area and includesthe region from the middle of the wings until theend of the fuselage [18]. It is then concluded thatthe most suitable location to install the antennais the non critical aerodynamic area. Aditionally,it should be installed in the vertical plane of sym-metry of the aircraft, to avoid anti-symmetric flowperturbations.

� Fuselage Bending Moment

The bending moment increases along the fuselagehaving its maximum at the wingbox and then de-creasing until the tail. It is therefore structurallypreferable not to perform modifications on the cen-tral fuselage since it would decrease the lifespan ofthe installation and reduce the inspection intervalsof the antenna, due to the higher bending loads.

� Aircraft External and Internal Equipment

The original aircraft already has different equip-ment in place (antennas, lights, etc) when it is pro-duced in order to allow its correct functioning. Inorder to avoid electromagnetic, structural and aero-dynamic interferences it is important to have a lookat the B777-200LR layout and analyze the availableinstallation options. The same happens with theinternal aircraft structure, which should be checkedin order to see if the installation would be in clashwith an already existing internal component.

� Illumination Obstruction

The final aspect to take into consideration is theobstruction of the beacon light, which has an allow-able limit according to EASA’s Certification Spec-ifications (CS) 25 Amendment 22 Section 25.1401.By doing a mathematical analysis it was concludedthat the antenna was fully contained inside the al-lowable obstruction zone thus being in agreementwith the limit set by CS 25.

By taking all the previous items into considera-tion it was concluded that the installation would beplaced between frame Station (STA) 1455 and STA1546.5 since it fulfills all the necessary requirements.The term STA refers to frame stations, which arenumbered in order to allow a faster identification.

3.2. Aircraft’s Environment ModellingIn order to have a precise design it is important touse several environment modeling techniques, thusassuring that the aircraft’s primary environmentis reproduced with the utmost reliability. One ofthese tools is the Three Dimensional (3D) scan-ning, which replicates the installation area by scan-ning it physically through the use of a hand scan.Furthermore it is also useful to use certain draw-ings supplied by Boeing, such as the frame and skindrawings. To complement this the Structural Re-pair Manual (SRM) should be used, since it con-tains relevant information regarding the fuselage’sradius, typical pre-approved repairs, among others.

With these three tools it was possible to build aCATIA [19] model that included the most impor-tant primary structure components of the aircraftsuch as :

� Skin

The aircraft’s skin covers all fuselage and car-ries cabin pressure (tension) and shear loads. Itis made of several panels which are optimized interms of weight by being machined with differentthicknesses. This means that there needs to be spe-cial attention when installing the provisions, sincethey should be installed onto a single skin thickness.Additionally the skin curvature is given in the SRMas being 122 inches.

� Stringers

The stringers are made to carry the longitudi-nal tension and compression loads. In the B777-200LR they have a Z cross-section, although theirdimensions cannot be perfectly determined due tothe inaccessibility to the stringer’s shape drawing.Nonetheless the important factor in this design wasthe stringer holes position since it will influence thedoubler shapes.

� Frames

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The frames help to maintain the fuselage’s crosssection shape and redistribute loads in the skin.They are also optimized in terms of weight, rea-son why they are machined with pockets of differ-ent thicknesses. It is also important to take thesethickness changes into consideration when attach-ing a component onto the frames.

� Shear Ties and Stabilizers

The shear ties make the connection between theskin and the frames whereas the stabilizers connectthe stringers and the frames, both with the objec-tive of helping to distribute the loads between com-ponents.

3.3. Installation BreakdownThe complete antenna’s methodology is presentedin figure 2 and shows all the components used inthe installation.

Figure 2: Exploded view of the Ka-band antennainstallation [20].

The installation was divided into two parts : ex-ternal and internal. The external part is composedof seven fittings, doublers and gussets (they areinside the aircraft but are projected at the sametime as the other external components) and twofeedthroughs. The internal part is composed of sev-eral intercostals, T-brackets and shims.

3.4. External Structural InstallationThe fittings are an assembly of a machined part anda bearing which connects to the adapter plate. TheARINC 791 provides the coordinates for the attach-ment points of the fittings which was the start of thedesign. The adapter plate allows some flexibility inthe X attachment coordinates (aircraft’s flight di-rection) due to the frame spacing, which varies fromaircraft to aircraft. From the coordinates it waspossible to design the fittings, by extruding them

until the skin was reached and by using some of theARINC 791 manual’s additional guidance.

The gussets are machined parts that are used inorder to transfer the loads from the fittings to theintercostals. The positions of the fitting’s fastenerswill influence the design of the gussets, hence whythey are designed at this phase. Fitting seven (fig-ure 3) had a different gusset concept, since only oneintercostal was used at this location.

The doublers are thin sheet metal parts that areused to strengthen the skin due to the drilling ofthe fitting’s attachment holes, and in order to redis-tribute the shear loads that would be concentratedon the fitting’s fasteners. The skin has a thicknessof 0.09 inches (at the panels where the fittings are)so, according to the SRM, the doubler would needto be 0.1 inches thick which is what was chosen.The design of the rivet patterns was also done atthis stage, by following the design rules already pre-sented. Finally, all the doublers were made withlobes at the stringer areas, in order to allow adjust-ment margin due to the uncertainty on the stringerrivets positions.

The feedthroughs were positioned by using theguidelines from the ARINC 791 manual. Theseparts are similar to a doubler, the only differenceis that they have a larger central hole to allow ca-ble passing and a cable seal in the interior of theaircraft in order to secure the cables.

3.5. Internal Structural InstallationIn order to transfer the loads from the antenna’sstructure to the frames, an internal installation partwas done which consisted of connecting the inter-costals to the gussets and frames. The connectionto the gussets was made by using eight fasteners(four on each intercostal), and the connection tothe frames was made by using two T-brackets perintercostal. Since the frames do not have a constantthickness then it is necessary to use shims (smallmachined parts) that make the connection betweenthe frames and T-brackets thus compensating thedifferent frame thicknesses.

Furthermore, fitting seven only attaches to oneintercostal (this was a new approach when com-pared to some past company projects) since it isonly loaded in the Z axis, which is not the mostcritical loading direction for an intercostal (that isY). This concept can be tested as well on fitting’stwo and six since the loading conditions are similar,though a detailed analysis would be required. Thisdesign came with the advantage of weight savingwhich was 2 %, reason to why it seems interestingto, in the future, try and reduce more intercostals.

The final external and internal installation de-sign, with the proper aircraft environment can beseen in figure 3. After the design was made, thepart drawings (doubler’s, gusset’s,...) and installa-

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tion drawings were completed. At the end severaldocuments needed to be prepared, in order to cer-tify the installation.

Figure 3: Ka-band SATCOM system structural in-stallation with the fitting’s numbers shown.

4. Quad Camera Structural Installation andStatic Analysis

The quad camera technology can be used for a va-riety of applications such as ground maneuveringduring taxi, in-flight entertainment and external se-curity. Moreover, this system provides a 360º fieldof view by splitting the fields in four views : For-ward, Aft, Left and Right. It is usually installedon the lower fuselage of the aircraft, in order to ful-fill the applications depicted above. The structuralinstallation and the static analysis were done, as away of better understanding important engineeringconcepts such as the margins of safety of a compo-nent.

4.1. Structural Installation

The first step was to determine the camera’s posi-tion, for which the horizontal and vertical fields ofview were considered. The location with the leastamount of view obstruction was found to be forwardto the wings, since both on the center of the fuselageand closer to the tail, there would be the problem ofobstruction due to the wings and engines. Further-more on the central fuselage the bending momentsare higher, which would worsen the static analysisresults. With this in consideration, the camera wasinstalled between STA 718 and STA 739 at the Xsymmetry axis of the aircraft (Y = 0).

The environment design and installation weredone afterwards. It was seen that in this region ofthe aircraft the skin had a thickness of 0.1 inches,with no pockets, which provides more space for ad-ditional fastener installation. The stringers werealso modeled in order to check the position of itsrivets, which will be important for the doubler de-sign.

The installation design process started by analyz-ing the camera itself and its interface requirements.It has four holes that serve as attachments to theskin and a central connector. In order to perform

the wire connection to the antenna, a fuselage pen-etration is required and thus a local reinforcementas well. Since the camera attachment holes are veryclose to one each other (closer than the general ruleof minimum pitch of 4D), then this would resultin a high stress concentration on that area whichwould reduce drastically the number of cycles forcrack propagation. For this reason an adapter platewas also created, with six fasteners that serve asattachments to the skin. Finally the rivet patternwas developed, having a circular shape around thecut-out and a more rectangular shape near the dou-bler’s borders. The adapter plate’s screws were at-tached to nutplates which were used to keep themin place. Four extra sheet metal plates were used toinstall the nutplates on, thus preventing drilling theaircraft’s skin with the non structural holes of thenutplates, that would be very close to each other.

Just like in the Ka-band system, the part and in-stallation drawings were done in order to help in thecertification process and to ease the installation ofthe camera. The design of the camera is presentedin figure 4, as done in CATIA V5.

Figure 4: Quad camera structural installation.

4.2. Finite Element Method Model Defini-tion

An important aspect when doing an installation ischecking if it fulfills all stress requirements by per-forming different analyses such as the static analy-sis and the Damage Tolerance Analysis (DTA). Theprogram FEMAP [21] was used in order to conductthe static analysis, which consists in calculating themargins of safety of the components that are beinginstalled or already in place, like the aircraft’s skin,in order to check if they are statically adequate [22],which is verified if the margin of safety is biggerthan zero.

In order to to build the FEM model the materials,geometries, properties, meshes, loads and boundaryconditions were all defined and will be presented

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next.

� Geometry

The skin was represented without any curvaturesince it would not influence the results that much,due to the fact that the fuselage radius is very largeand the installation is located at Y = 0. The dou-bler was also modeled as a non curved plate and thestringers as two straight lines below the skin. Fi-nally the rivet pattern and cut-out were designed,with an extra circle around them in order to assure agood quality mesh at those locations, where the re-sults should be as accurate as possible. The cameraand the adapter plate, as well as their screws werenot modeled due to the low camera weight (1kg),from which it was concluded that the inertial loadcases would not have much influence on the staticanalysis.

� Materials

The doubler is made of Aluminum (AL) alloy2024 T3, the skin of AL alloy 2523 T3 and thestringer of AL alloy 7150 T77511. Their Young’smodulus, Poisson ratio and density were taken fromthe Metallic Materials Properties Development andStandardization (MMPDS) [23].

� Properties and Mesh

The skin and doubler were modeled with plateelements, both squares and triangles, being definedby their thickness tskin = 0.1 in and tdoubler =0.11 in. The stringer was modeled as a rod element,having thus a section area Astringer = 0.1801 in2.The rivets were modeled with spring elements whichsimulate the shear and tensile stiffness and rigid el-ements which simulate the rivet on the skin anddoubler. With these properties it was possible tomesh the model, taking into consideration that aconvergence analysis was done beforehand in orderto select the most appropriate mesh size.

� Loads

In order to run the model it is necessary to ap-ply loads to it. Four load cases were considered,which take into account the fuselage loads, whichare a combination of the cabin pressure loads andthe fuselage bending, as well as the ultimate loadfactors that affect the aircraft. The aerodynamicloads are not being considered because they are verysmall when compared to the rest of the consideredloads, due to the size of the camera.

For the fuselage loads, a detailed investigationof the aircraft was needed, in order to determinefactors such as its fuselage weight, fuselage area,among others. In regards to the load factors, the

positive and negative limit maneuvering load fac-tors where compared with the positive and negativelimit gust load factors, as to determine which onewas the most conditioning for the flight envelope.With this it was determined that the ultimate loadfactors down and up were 5.1g and 1.5g respectively.The emergency landing load factors were also taken,since there is a load case that depends of this pa-rameter, from CS25.561 as being 6g down and 3gup.

With all this information it was possible to de-termine the total circumferential and longitudinalstresses of the four load cases, whose descriptionswill be presented next. The first load case (LC101)is the combination of the longitudinal and circum-ferential pressure loads only (ultimate) multipliedby 1.33. The second load case (LC102A) is the com-bination of the longitudinal load due to the flightenvelope up and the ultimate pressure. In the bot-tom part of the fuselage the internal pressure andup flight loads create tension, which means thatboth loads are added in order to calculate the totallongitudinal load. The third load case (LC102B)is the combination of the longitudinal load due tothe flight envelope down and the ultimate pressure.Since now the flight loads are in the down directionthen a compression will be created in the bottompart of the fuselage. By this reason, the bendingcontribution is subtracted. The fourth and last loadcase (LC103) represents the longitudinal load dueto the emergency landing (ultimate) [24].

Finally these stresses are converted into loads andapplied on the skin and stringers.

� Constraints

The applied constraints were used in order to tryand better approximate this model to reality, beingthen applied in the four corners of the skin andedges of the stringers.

4.3. Static AnalysisIn general, the margin of safety for the skin, doublerand other components in tension or compression is[24] :

MS =σTu

σVMmax

− 1 , (1)

where σTu is the ultimate tensile stress andσVMmax is the maximum Von Mises stress on thesurface. The Von Mises stress is being used to de-termine the margin of safety because this parametertakes into account different stress components suchas σX , σY and τXY [25], thus predicting the stresson the model in a more accurate way.

Furthermore, in order to obtain the fastenersmargins of safety, equation 2 was used, where themaximum shear force applied to a specific fastenertype was taken from FEMAP and replaced in it [24].

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MS =σSu ×Af

Smax × 1.5− 1 , (2)

where σSu is the ultimate shear stress of the fas-teners material, Af is the fastener’s cross sectionarea and Smax is the maximum shear force appliedto the fastener. The factor 1.5 is included as a safetyfactor.

Finally another type of analysis that is usuallydone is the bearing’s margin of safety calculation.The bearing forces exist due to the presence of arivet in its hole which can cause deformation if theultimate bearing load is surpassed. In order to cal-culate this margin of safety, the formula from equa-tion 3 applies [24]:

MS =tplate ×D × Fbu

Smax × 1.5− 1 , (3)

where tplate is the thickness of the plate in whichthe fastener is going through, D is the fastener’sdiameter and Fbu is the bearing’s ultimate load.

By running the model the stress distributionswere obtained, for the doubler and skin as well asthe shear load distribution on the fasteners. In fig-ure 5 and 6 the skin Von Mises stress distributionand the fasteners shear loads are presented.

Figure 5: Von Mises stress (psi) distribution on theSkin.

Figure 6: Shear loads (lbf) on the fasteners of thedoubler.

The value of the maximum Von Mises stress, infigure 5, was expected to happen at the cut-outand in reality it does, however, in the model, thestresses around the rivets are unrealistic due to therigid elements, which explains the obtained results.This is not a problem and only means the modelis more conservative. By using the values obtainedfrom FEMAP, the margins of safety were calculatedstarting with the values presented in table 1.

Table 1: Skin, doubler and rivets margins of safety.

The margins of safety around the larger cut-out,on both the skin and doubler were also checked, inorder to allow the comparison between the differentload cases, to see which one lead to the most criticalresults. These MS values may or may not be thesame as the skin and doubler’s MS from table 1, dueto the already mentioned unrealistic stresses due tothe rigid elements. These values are presented intable 2.

Table 2: Skin and doubler margins of safety at thecut-out.

The bearing’s MS were also calculated and arepresented in table 3.

Table 3: Skin and doubler bearing’s MS for bothrivet types.

By looking at the results it is clear that LC101was the most critical load case for the skin anddoubler, while LC102B was the most critical loadcase for the rivets. The bearing’s MS are very high,which is why they are usually not calculated dueto the already existing knowledge of its guaranteed

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safety. To conclude, this installation which is goingto be mentioned as configuration 1, passed on thestatic analysis.

5. Design Improvements

After doing the quad camera design, an improve-ment strategy with the objective of achieving alighter, cheaper and equally secure design wasmade. The design concepts were varied and thestatic results, weight and installation cost were an-alyzed.

5.1. Configuration 2

The first noticeable aspect when designing the quadcamera installation is the large and thick doubler,which has a high impact in terms of weight of theinstallation. Therefore, this design improvement in-vestigation started by trying to reduce the doubler.This was also motivated by the fact that the stringerrivets were being used, in the doubler, which shouldbe avoided because when a rivet is removed there isa high probability of needing an oversize rivet at theinstallation, in order to use that same hole. A largerfastener may concentrate more loads and thus leadto a crack initiation and propagation earlier thanexpected. To sum up, the doubler of configuration2 is sitting between both stringers, being then muchsmaller than the doubler from configuration 1.

The weight reduction aspect was achieved, havingreduced it by 19.1%. The overall cost was also re-duced since less holes need to be drilled on the skin,which means less installation time required. If theinstallation takes less two hours then the companywould save around 388 euros. The static analysisresponse will be presented in the final comparisonamong all the design configuration proposals, how-ever the skin was now more loaded, which is whatmotivated the next configuration. This is a good ex-ample of design to weight and design to cost, whichis crucial in the aerospace industry.

5.2. Configuration 3

A different approach was followed in order to tryand reduce the stresses on the skin of configuration2, by using internal L-brackets and stringer attach-ments which connected the doubler to the stringer,thus trying to lower the stresses on the skin. Unlikeconfiguration 2, the weight of the installation hasnow increased, when compared to configuration 1,by 3.49%. The overall cost is assumed to be thesame as in the first configuration.

The results of the static analysis are not verysatisfactory, even though the worst case scenariofor the skin has now a higher MS, when comparedto configuration 2, which means that this strategyachieved its purposes to some extent.

5.3. Configuration 4Another design variation strategy was followedwhich resided in changing the doubler’s thicknessand not its length. Instead of using the SRM ruleof thicknesses, which resulted in a doubler with athickness of 0.11 inches, the rule from [22] was fol-lowed which states that :

1 ≤ StiffnessRatio =(E × t)doubler(E × t)skin

≤ 1.5 , (4)

where (E× t)doubler represents the Young’s mod-ulus and thickness of the doubler, and (E × t)skinrepresents the Young’s modulus and thickness of theskin. The structural installation is adequate if thestiffness ratio is between 1 and 1.5, being consideredtoo stiff when the value is greater than 1.5 and notstiff enough when it is less than 1.0. From this, andby taking a look at the existing sheet metal thick-nesses, it was concluded that the minimum dou-bler thickness that could be used was 0.1 incheswhich means a 10% installation weight reduction,from configuration 1. The overall cost was main-tained, and the stresses on the skin and doubler gotincreased.

5.4. Configurations ComparisonIn order to compare the results, the lowest valuesof the margins of safety, for each comparison pa-rameter were taken for all four configurations, sincethey represent the critical cases. The results werepresented by using a spider chart (figure 7), whichrepresents the results from configuration 2, 3 and 4in regards to 1. An additional line was made in red,that represents MS = 0 which is the limit case.

Figure 7: Comparison between configurations 1, 2,3 and 4.

The first thing that is clear when analyzing fig-ure 7 it that configuration 3 is never the best one interms of static analysis. This combined with the in-crease in weight make it a non-viable option for this

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type of installation. Additionally, configuration 4has a very similar profile of margins of safety to con-figuration 1, and a weight reduction of 10%, whileconfiguration 2 has a noticeable reduced margin ofsafety on the skin (although it is still far from thered boundary), but other advantages which shouldnot be undermined such as the 19.1% weight andcost reductions.

Based on the margins of safety and by taking intoconsideration the weight and cost saving, then it isconcluded that the design of configuration 2 wouldbe very interesting to pursue, which means, doingthe fatigue and DTA analysis and then comparingit with configuration 1. If those extra analyses ledto similar results when compared to configuration1, then it would be beneficial for the company tochoose the improved design. In case the results donot prove that configuration 2 is better, then thefatigue and DTA of configuration 4 should be done,since it also shows promising static stress results.

6. Certification and Compliance DocumentsA modification project is not over once the designand stress analyses are finished. There are severaldocuments that need to be prepared in order to ap-ply for the STC and successfully certify the modi-fications.

This documentation is often divided into two cat-egories which are the certification documents andthe compliance documents. The purpose of the lat-ter is to prove compliance to the different certifi-cation chapters while the certification documentssummarize all the certification aspects of the mod-ification, and guarantee that all certification chap-ters impacted by the modification are addressed.

A list of these documents and their descriptionsis presented in [26].

Once all the documents are released, then theSTC application can go forward to the competentauthority, which in this project, as already stated,was EASA, which will approve the modification ifall requirements are fulfilled.

7. ConclusionsThe primary objective of this work was to presentaircraft modifications as done in Jet Aviation, whileshowing two different installations : a Ka-band an-tenna and a quad camera. Even though they aredifferent systems it was possible to see that their de-sign goes through similar processes such as choosingthe installation location and defining the aircraft’senvironment on that area.

Furthermore, the industry standards such as theARINC 791 are used in order to ease the design ofthe structural modification, through the use of al-ready existing instructions. The design guidelines,such as the inter-rivet pitch, also have a scientificbackground which means that by following them,

the analysis and calculation effort is considerablyreduced, which benefits the project’s budget. Asseen, the Ka-band design was successful, havingbeen in accordance with the stress department aswell. The stress analyses still need to be approvedand the documentation prepared, in order to fullyfinish the project.

Regarding the quad camera, the design was alsoapproved and the static analysis results were sat-isfactory. This modification brought the secondbiggest objective of this work which was doing animprovement to the released design, that resultedin a weight and cost reduction without compromis-ing the structural integrity of the structure. In or-der to test this, three new configurations were cre-ated, which introduced variations to the releaseddesign such as reducing the doubler’s size, thicknessor adding more parts to the installation.

In terms of results it was seen that configuration3 did not achieve what was supposed to, being thenconfiguration 2 and 4 more promising. In regardsto the original design, configuration 2 achieved a19.1% weight reduction and a cost reduction ofabout 388 euros, and configuration 4 a 10% weightreduction. The fatigue and DTA analysis still needto be done in order to determine the inspection in-tervals of the installation and therefore conclude thebest installation option.

At last it was seen that the documentation partis very important since it will help certify the mod-ification.

To conclude, several future work opportunitieswere analyzed. The first aspect is regarding theKa-band antenna, since fitting’s 2 and 6 can proba-bly also have one intercostal, since they are loadedin the same direction as fitting 7. One other inter-esting continuation to the work would be to finishthe analysis of configuration 2 and 4 of the quadcamera, by doing the fatigue and DTA, thus de-termining if either one of them is better than theoriginal configuration or not.

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