Exemption No. I 1087 UNITED STATES OF AMERICA DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION KANSAS CITY , MO 64106

In th e matter of the petition of CUB CRAFTERS, INCORPORATED for an exemption from§ 23.629 of Title 14 , Code of Federal Regulations Regulatory Docket No. FAA 2014-0297

DENIAL OF EXEMPTION B y letter dat ed April 28, 2014, Mr. Daniel Glaser, Certification Manager, Cub Crafters , Incorporated (Cub Crafters , 1918 South 16th A venue , Yakima, W A 98903 petitioned the Federal Aviation Administrat ion (FAA) on behalf of Cub Crafters for an exemption from § 23.629 ofTitle 14 , Code of Federal Regulations ( 14 CFR). T he proposed exe mption , if granted , wou ld show compliance to 14 CFR 23.629 without the performance of flight flutter testing. The petitioner requests relief from the following regulation: Section 23.629 prescribes , in pertinent part , that in order to demonstrate th at the a irplan e is free from flutter, di vergence , and control reve r sal , the applic ant must either perform a rational flutter analysis or comply with a simplified prevention criteria. Additionally, regardless of the method cho s en , the app li cant must spec ifi cally perform flight flutter testing utilizing proper and adequate attempts to exc ite flutter modes. The petitioner supports its request with the following information:

" C ub Crafters propo s es the following as mitigating factors regarding safety to support their exemption request: 1. Design cruise s peed (V c ) is 1 40 knots , never exceed speed (V N E ) is 15 5 knots , and the dem o n strated dive speed (VoF) is 180 knots. Therefore, it is a slow airplane.

ACE- 1 4-276

- E2 2. Th e design has hi g h drag. Th e aircraft was designed to haul large volumes of cargo. lt h as a l arge square sect ion fuselage. The landing gear i s fixed and the wings are s upp o rted by exte rn a l struts. 3. T he fu se la ge u ses proven and predictable construct ion of a s tee l tube frame w ith aluminum ski n . The w in gs a nd tails b o th u se thick airfoils (12% thickness to c hord ratio). The w in gs u se a NACA 4412 airfo il and th e vert ica l and hori zo nt al tails use a NACA 0012 ai rfo il. Th e wings and tails are co n structed of aluminu m with two spars, rib s, stringers and skins. Th e s tru cture i s very s tiff. 4. In it s ori g in a l ce rtificati o n , compliance was d e mons trated by p e rforming a gro und vi bration s urv ey a nd by a nal ysis. This was carri e d ou t by J o hn Bake r , a Flutter D E R w h o had over 35 years of experie nce w h e n he did thi s ana lysis. This conse rvative analysis h ad ample margins and it was accepted for type certifi cati on. 5. The EL- l i s of a gro up of airp l anes for which comp li ance b y ana lyses for freedom from flutter i s applicable due to it s methods of co n stru c ti on, high drag , and s l ow speed. The exampl es used in the preamble for amendment 4 8 to just ify the requirem e nt for fli g ht te s t s di ffer s ignifi ca n tly fro m thi s case. 6. 23 .629(i) (am en dm ent 48) stat es (emphasis add e d) : "For modificati o n s to the type d es ig n that could affect th e flutter c harac t e ri stic s, compliance w ith paragraph (a) of this sect ion mu s t b e shown , 

### exce pt that ana l ysis based on previous ly approved data may be used alon e to show .freedom from flu tter, control reve r sal and divergence,fo r all speeds up to the speed specifiedfor !h e selected m e thod''. T hi s app li catio n for type ce rtifi cation of the E L-l i s uniqu e in the sense that it is a Type Ce rtificate that h ad been i ss ued pr ev io u s ly and then revok e d for the reason s m e nti o n e d above. lt is, strictly speaking , n ot a modifi cat ion of an existing design , but rather a n ew application based on a design w hi c h h as not b een modified since i t was issued a Type Ce rtific a te. There fo re , whi le not applicable accor din g to th e letter o f the law , Cu b Crafters posits that th e in tent of the law i s met and 23.629( i) pro v id es tec hnical justification for not having to conduct a fli g ht t es t. 7. The airplane was design ed a nd built for a maximum gross we ig ht of 4 , 800 lb s and all certificati on ana lyses and ground t es t s were performed using this we ig ht. 

### In fligh t test , wi th th e exception of the Part 36 n oise certificat ion , the airplane was flown at a maximum weight of 4 , 500 l bs. The nois e certification was done at 4 , 200 lb s and hence th e limitation on the t y p e certific ate. 8. 

### In fli g ht te s t s, the airp lane was sa f e ly fl own at spee d s up to VDF 

## = 180 kts. A t this speed, th e pilot performed co ntroll abilit y and maneu verabili t y c heck s. There was no trend toward s co ntro l reve rsal and no tendency of di ve rgent behavior. Furth e rmore , w hil e flutter exc iters were not employed, the pilot did3 perform "stick rap s"

- s harpl y s triking th e e levator , aileron , and rudder pilot control s - in an attempt to exci te flutter. ln all three axes the a irpl ane demon s trated a dequate structura l damping and no flutte r te nd e n cy . 9. C ub Craft e r s inte nd s to recerti fy the airplane to the same s p ee d , a ltitud e, and weight enve lope as th e ori g inal Type Certificate. l 0

. The t est article was manufa ctured over l 5 year s ago f or th e original certification. It was not outfitted to accommodate flutter exc it e r s and retrofitting it now would b e a significant (and prohibitive) burden on Cub Crafters. As di sc u sse d 

### in 6, above, thi s burden would not b e placed on Cub Crafte r s (or other manufa c ture r) if this were a modification

. Fo r commercial reaso n s , such a retrofit m ay not make sense f o r Cub C raft e r s a nd could termin ate the program. II. The Typ e Ce rtificate wa s revo ked per the lette r dated Marc h 7 , 2 008 (Cas e numb er 2007EAS80 00 2). T h e reason for th e acti o n was that th e h o lder demon s trated th at it lacked th e qualification s n ecessary to hold the Type Certificate. It was not du e to an a irworthi n ess i ss ue. Had that n o t occurred , thi s airp lane would be grand fa th ered and s till e li g ibl e for produ ctio n and commercial sale today . G ra ntin g thi s exe mption for the Gavilan E L-l will se rve the public inte res t for the f o ll owing reason s: 

### l. It w ill provide a path f o r safe ly establishing co mpli a nce w ith 14 CFR 23 at a cost w hic h makes the project v iable. 2. As a si n g l e-e n g ine , non-turbine cargo a irpl a n e, this airplane se rve s an import ant niche that i s not we ll represented in the market. It wi ll hav e lower acqu i s iti o n and operating costs than a turbo-prop cargo plan e. Some communiti es in our country (and other part s of the w orld ) dep e nd upon airp lanes a s a so le or primary means to tran s port goods and s uppli es. Thi s airplane will e nable operators to cost-effectively serve tho se communitie s. 3. This a irpl ane has a sizeable payload and fuel capacity

. As s u c h , it i s an excellent pl a tform for missions that may require long dura tion s w ith some specialized eq uipment , s u c h as se arch 

### & resc u e mission s and law enforcement patrol missio n s. It is clearl y in the public' s int e r

est that the se mis s ions can be conducte d cost effectiv e ly. 4. The manufacture , sales , and operation of thi s airplane will prov ide jobs for man y people. Skilled labor ers w ill manuf actur e the airplan es . Salespeople will brok er deal s to buy and se ll th ese airplan es. The EL -l is intended for small commerc i al operators in rural comm uniti es w h ere ground tran s port is difficult ,4 such as A laska. T h erefore , it has the potential to bring significant eco nomic benefit to these areas. Pilots and A&P m ec hani cs wi ll be needed to ope rate the a ircraft. And owners will n o doubt create additional jobs in the s upport and conduct of their business activities. Th ese jobs and i ncome w ill e nhance the local , state , and national econo mi es. " A s umm ary of the petition was publi s hed in the Federal Regi s ter o n June 6 , 20 14 (79 FR 32807). No comments were received. 

### The FAA's ana lysis is as follows: In consideration of the forego in g proposed mitigating factors regarding safety to support the applicant 's exemption request , the FAA find s th e fo ll owing: 1. Stating th e d esign c rui se speed i s " slow" on l y indi cates it s top speed r e lative to other airplanes. A "s l ower" airplane does not indi cate that flutter wi ll not occur, or even that it i s less likely. 2. T h e hi gh drag i s a description of the airp lane design. The sali ent point being tha t it h as " hi g h " dra g. " High" i s a re lative t erm and does not indi cate that flutt er will not occ ur , or even t h at it i s less lik e ly. 3. The fuselage and stee l tube frame wit h aluminum sk in is also a description of the airp la ne design. T h e ap pli cant states that proven and pr ed icta ble co n s truction i s used w ith th e sali ent point being t h at the airplan e is "v ery st i ff '

. Very stiff i s relative terminology , and st iffness is on ly a portion of the equatio n concerning flutter. An object may be "v ery stiff' ' (and " s low " as in the previou s item), and yet under the right cond iti ons, may still ex p erience flu tte r. T he T aco m a Narrows B ri dge ("very stiff'

- s tee l and reinforced concrete) being one exampl e wh e re flutter occ urred with a stationary object in ap pro x i mate ly 40 mph ("very s low " ) wind s. This was a slightly differen t form of flutter than occurs w ith a ircraft , but s till flutte r. Thi s example is used to illu strate why relativistic arguments regardin g " slow" and " rigid " are nonp ers u as ive relative t o thi s phenomenon. 4. Ground vibration s urvey and analysis is not a mitigating fac tor that suppo rt s exemption from the current regu latory requ irement for flight flutter tests in additio n to th e ana lysis and Ground Vibration Test (GVT) accomp li shed pr ev io u s ly. 5. The airplan e examples discus se d in the preamble of amendment 48 may not h ave been si milar to thi s airp l ane , but the reason fo r the rul e change was not specific t o a particu lar type of airp lane. The rule was c h anged b ecause the re is sti ll a risk due to uncertainty w ith only ana lysis. The refo re, it mu st b e valid ated with flight testing. 6. Section 23.629( i) does not pro vi de technical justification for not perfo rmin g flight fl utt e r testing. Thi s regulation on l y applies w hen an airp lane h as already s h own5 

### compliance with the appl icabl e reg ulation s for its certification basis. The EL-l airplane has not. lt was determin ed for thi s project that th e applicant mu st apply for a new TC and therefore has not yet comp li ed with a ll the applicable flutt e r requirements of§ 23.629. 7. Designed maximum g ross weight i s not a miti gating factor that supports exemptio n from the current regu latory requirement for flight flutt e r tests in addition to the analysis and G rou nd Vibration Test (GVT) accompli shed previously. 8. The description of the flight-testing provided by the applicant is anecdotal information to s how that flight flutter testin g was performed in an informal manner. T h e flight-testing described was not a dedicated flight flutter test regimen; utili z ing proper and adequate attempts to induce flutter as required by the regulations. No eng in ee rin g data in the form of test reports exist to subs tant iate the claims that testing occurred in that manner, or even in the manner described by the applicant. lf proper flight flutter test data does exist, there is no need for an exemption. 9. Certifying the EL-l to the same speed, altitude, and weight e nvelo pe as the original Type Certificate is not a mitigatin g factor that supports exemp tion from the current regulatory requirement for fli ght flutter tests in addition to the analysis and Ground Vibration Test (GVT) accomp li shed previous l y. 10. It 

### is true, th at this economic burden may not be placed on a modifier ( i f certain criteria are met) ; however, Cub Crafters i s not modifyin g an existing Type Certificated airplane. 11. If the TC had not been revoked , the n this a irplane wou ld be grandfathered and still e ligibl e for production and commercia l sale today. Also, its design , certification basis and compliance showing would be based on the standards and regulations in force when it received its origi n al certification. However , the applicant seeks a new TC and the expectation of the public is that a new Type Certificated airplane wi ll meet the current airworthiness standards that ex ist toda y. The current a irworthin ess s tandards require a more rigorou s and thorough flutt e r in ves tigation u s ing analysis and formal flight-testing. In consideration of the petitioner 's reasons that granting this exemption serves the public interest, the FAA finds the following: 1

. Keeping the projects costs low to make a project v iabl e does not serve the public interest as a w h ole. Low co s t products are of interest to those consumers who buy the product; however, the FAA must consider the safety risk to passengers and the individual 's on the ground. 2. P rov iding a product to a niche market at a lower cost without complying w ith the req uired flutter analysis and testing does not serve the public int erest as a whole

### 6 because it may lowe r the le ve l of safety. T hi s i s true for any airpl an e and any market. 3. W e agree sea rch and rescu e and law e nforceme nt patrol mi s s i o n hav e a public inter est. Ho weve r , providing a product to a niche market at a lower cost w ith o ut complying w ith th e required flutter ana lys is and testing do e s not se r ve th e publi c intere s t as a whol e b ecau se it may lower the le ve l of safety. 4. We und ers tand , as with any new airplane , job s wi ll be c reated which is in the public inte rest. However, lowe rin g the level of saf ety through nonc o mpliance wi th the regulatory require ment to conduct flutt e r analys is and test in g to creat e jobs i s not in the public inte rest as a who le. 

### The FAA's Decision: In co n s id e rati o n of th e forego in g , 1 find that a grant of exemption wou ld not be in the publi c in te r est. Therefore , purs uant to the authority contained in 49 U .S.C

. 

### §§ 1 06(f), 40113 and 4470 l , del ega te d to me by the Administrator , the petition of Cub C rafters , In corporated for an exe mpti on from 14 CF R 23.629 is hereby de nied

. Iss ued in Kan sas C ity , MO, o n October 20 , 2014

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E arl Lawrence Man ager , Small Airplane Dir ec torate Aircraft Certifi cati on Service