Exemption No. I 1087 

UNITED STATES OF AMERICA 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

KANSAS CITY, MO 64106 

In the 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 

By letter dated 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 Administration (FAA) on behalf of Cub Crafters for an exemption from 

§ 23.629 ofTitle 14, Code of Federal Regulations (14 CFR). The proposed exemption, if 

granted, would 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 that the 

airplane is free from flutter, divergence, and control reversal, the applicant must either 

perform a rational flutter analysis or comply with a simplified prevention criteria. 

Additionally, regardless of the method chosen, the applicant must specifically perform 

flight flutter testing utilizing proper and adequate attempts to excite flutter modes. 

The petitioner supports its request with the following information: 

"Cub Crafters proposes the following as mitigating factors regarding safety to support 

their exemption request: 

1. Design cruise speed (V c) is 140 knots, never exceed speed (V NE) is 15 5 knots, 

and the demonstrated dive speed (VoF) is 180 knots. Therefore, it is a slow 

airplane. 

ACE- 14-276-E 
2 

2. The design has high drag. The aircraft was designed to haul large volumes of 

cargo. lt has a large square section fuselage. The landing gear is fixed and the 

wings are supported by external struts. 

3. The fuselage uses proven and predictable construction of a steel tube frame with 

aluminum skin. The wings and tails both use thick airfoils (12% thickness to 

chord ratio). The wings use a NACA 4412 airfoil and the vertical and horizontal 

tails use a NACA 0012 airfoil. The wings and tails are constructed of aluminum 

with two spars, ribs, stringers and skins. The structure is very stiff. 

4. In its original certification, compliance was demonstrated by performing a 

ground vibration survey and by analysis. This was carried out by John Baker, a 

Flutter DER who had over 35 years of experience when he did this analysis. 

This conservative analysis had ample margins and it was accepted for type 

certification. 

5. The EL-l is of a group of airplanes for which compliance by analyses for 

freedom from flutter is applicable due to its methods of construction, high drag, 

and slow speed. The examples used in the preamble for amendment 48 to justify 

the requirement for flight tests differ significantly from this case. 

6. 23.629(i) (amendment 48) states (emphasis added): "For modifications to the 

type design that could affect the flutter characteristics, compliance with 

paragraph (a) of this section must be shown, except that analysis based on 

previously approved data may be used alone to show .freedom from flutter, 

control reversal and divergence, for all speeds up to the speed specifiedfor !he 

selected method''. This application for type certification of the EL-l is unique in 

the sense that it is a Type Certificate that had been issued previously and then 

revoked for the reasons mentioned above. lt is, strictly speaking, not a 

modification of an existing design, but rather a new application based on a 

design which has not been modified since it was issued a Type Certificate. 

Therefore, while not applicable according to the letter of the law, Cub Crafters 

posits that the intent of the law is met and 23.629(i) provides technical 

justification for not having to conduct a flight test. 

7. The airplane was designed and built for a maximum gross weight of 4,800 lbs 

and all certification analyses and ground tests were performed using this weight. 

In flight test, with the exception of the Part 36 noise certification, the airplane 

was flown at a maximum weight of 4,500 lbs. The noise certification was done 

at 4,200 lbs and hence the limitation on the type certificate. 

8. In flight tests, the airplane was safely flown at speeds up to VDF = 180 kts. At 

this speed, the pilot performed controllability and maneuverability checks. 

There was no trend towards control reversal and no tendency of divergent 

behavior. Furthermore, while flutter exciters were not employed, the pilot did 
3 

perform "stick raps" - sharply striking the elevator, aileron, and rudder pilot 

controls - in an attempt to excite flutter. ln all three axes the airplane 

demonstrated adequate structural damping and no flutter tendency. 

9. Cub Crafters intends to recertify the airplane to the same speed, altitude, and 

weight envelope as the original Type Certificate. 

l 0. The test article was manufactured over l 5 years ago for the original 

certification. It was not outfitted to accommodate flutter exciters and 

retrofitting it now would be a significant (and prohibitive) burden on Cub 

Crafters. As discussed in 6, above, this burden would not be placed on Cub 

Crafters (or other manufacturer) if this were a modification. For commercial 

reasons, such a retrofit may not make sense for Cub Crafters and could 

terminate the program. 

II. The Type Certificate was revoked per the letter dated March 7, 2008 (Case 

number 2007EAS80002). The reason for the action was that the holder 

demonstrated that it lacked the qualifications necessary to hold the Type 

Certificate. It was not due to an airworthiness issue. Had that not occurred, this 

airplane would be grandfathered and still eligible for production and 

commercial sale today. 

Granting this exemption for the Gavilan EL-l will serve the public interest for the 

following reasons: 

l. It will provide a path for safely establishing compliance with 14 CFR 23 at a 

cost which makes the project viable. 

2. As a single-engine, non-turbine cargo airplane, this airplane serves an 

important niche that is not well represented in the market. It will have lower 

acquisition and operating costs than a turbo-prop cargo plane. Some 

communities in our country (and other parts of the world) depend upon 

airplanes as a sole or primary means to transport goods and supplies. This 

airplane will enable operators to cost-effectively serve those communities. 

3. This airplane has a sizeable payload and fuel capacity. As such, it is an 

excellent platform for missions that may require long durations with some 

specialized equipment, such as search & rescue missions and law enforcement 

patrol missions. It is clearly in the public's interest that these missions can be 

conducted cost effectively. 

4. The manufacture, sales, and operation of this airplane will provide jobs for 

many people. Skilled laborers will manufacture the airplanes. Salespeople will 

broker deals to buy and sell these airplanes. The EL-l is intended for small 

commercial operators in rural communities where ground transport is difficult, 
4 

such as Alaska. Therefore, it has the potential to bring significant economic 

benefit to these areas. Pilots and A&P mechanics will be needed to operate the 

aircraft. And owners will no doubt create additional jobs in the support and 

conduct of their business activities. These jobs and income will enhance the 

local, state, and national economies." 

A summary of the petition was published in the Federal Register on June 6, 2014 

(79 FR 32807). No comments were received. 

The FAA's analysis is as follows: 

In consideration of the foregoing proposed mitigating factors regarding safety to support 

the applicant's exemption request, the FAA finds the following: 

1. Stating the design cruise speed is " slow" only indicates its top speed relative to other 

airplanes. A "slower" airplane does not indicate that flutter will not occur, or even 

that it is less likely. 

2. The high drag is a description of the airplane design. The salient point being that it 

has "high" drag. "High" is a relative term and does not indicate that flutter will not 

occur, or even that it is less likely. 

3. The fuselage and steel tube frame with aluminum skin is also a description of the 

airplane design. The applicant states that proven and predictable construction is used 

with the salient point being that the airplane is "very stiff'. Very stiff is relative 

terminology, and stiffness is only a portion of the equation concerning flutter. An 

object may be "very stiff'' (and " slow" as in the previous item), and yet under the 

right conditions, may still experience flutter. The Tacoma Narrows Bridge ("very 

stiff' - steel and reinforced concrete) being one example where flutter occurred with 

a stationary object in approximately 40 mph ("very slow") winds. This was a slightly 

different form of flutter than occurs with aircraft, but still flutter. This example is 

used to illustrate why relativistic arguments regarding " slow" and "rigid" are nonpersuasive relative to this phenomenon. 

4. Ground vibration survey and analysis is not a mitigating factor that supports 

exemption from the current regulatory requirement for flight flutter tests in addition 

to the analysis and Ground Vibration Test (GVT) accomplished previously. 

5. The airplane examples discussed in the preamble of amendment 48 may not have 

been similar to this airplane, but the reason for the rule change was not specific to a 

particular type of airplane. The rule was changed because there is still a risk due to 

uncertainty with only analysis. Therefore, it must be validated with flight testing. 

6. Section 23.629(i) does not provide technical justification for not performing flight 

flutter testing. This regulation only applies when an airplane has already shown 
5 

compliance with the applicable regulations for its certification basis. The EL-l 

airplane has not. lt was determined for this project that the applicant must apply for a 

new TC and therefore has not yet complied with all the applicable flutter 

requirements of§ 23.629. 

7. Designed maximum gross weight is not a mitigating factor that supports exemption 

from the current regulatory requirement for flight flutter tests in addition to the 

analysis and Ground Vibration Test (GVT) accomplished previously. 

8. The description of the flight-testing provided by the applicant is anecdotal 

information to show that flight flutter testing was performed in an informal manner. 

The flight-testing described was not a dedicated flight flutter test regimen; utilizing 

proper and adequate attempts to induce flutter as required by the regulations. No 

engineering data in the form of test reports exist to substantiate 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 envelope as the original 

Type Certificate is not a mitigating factor that supports exemption from the current 

regulatory requirement for flight flutter tests in addition to the analysis and Ground 

Vibration Test (GVT) accomplished previously. 

10. It is true, that this economic burden may not be placed on a modifier (if certain 

criteria are met); however, Cub Crafters is not modifying an existing Type 

Certificated airplane. 

11. If the TC had not been revoked, then this airplane would be grandfathered and still 

eligible for production and commercial sale today. Also, its design, certification 

basis and compliance showing would be based on the standards and regulations in 

force when it received its original certification. However, the applicant seeks a new 

TC and the expectation of the public is that a new Type Certificated airplane will 

meet the current airworthiness standards that exist today. The current airworthiness 

standards require a more rigorous and thorough flutter investigation using 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 viable does not serve the public 

interest as a whole. Low cost 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. Providing a product to a niche market at a lower cost without complying with the 

required flutter analysis and testing does not serve the public interest as a whole 
6 

because it may lower the level of safety. This is true for any airplane and any 

market. 

3. We agree search and rescue and law enforcement patrol mission have a public 

interest. However, providing a product to a niche market at a lower cost without 

complying with the required flutter analysis and testing does not serve the public 

interest as a whole because it may lower the level of safety. 

4. We understand, as with any new airplane, jobs will be created which is in the public 

interest. However, lowering the level of safety through noncompliance with the 

regulatory requirement to conduct flutter analysis and testing to create jobs is not in 

the public interest as a whole. 

The FAA's Decision: 

In consideration of the foregoing, 1 find that a grant of exemption would not be in the 

public interest. Therefore, pursuant to the authority contained in 49 U .S.C. §§ 1 06(f), 40113 

and 44 70 l , delegated to me by the Administrator, the petition of Cub Crafters, Incorporated 

for an exemption from 14 CFR 23.629 is hereby denied. 

Issued in Kansas City, MO, on October 20 , 2014 . 

&!~ Earl Lawrence 

Manager, Small Airplane Directorate 

Aircraft Certification Service