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Special Conditions: Cirrus Design Corporation, Model SF50; Fire Extinguishing for Upper Aft Fuselage Mounted Engine

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Federal Aviation Administration (FAA), DOT.


Final special conditions; request for comments.


These special conditions are issued for the Cirrus Design Corporation, model SF50 airplane. This single turbofan engine airplane will have a novel or unusual design feature(s) associated with mounting the engine in the aft fuselage. The applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature. These special conditions contain the additional safety standards that the Administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards.


The effective date of these special conditions is April 12, 2010.

We must receive your comments by May 20, 2010.


Mail two copies of your comments to: Federal Aviation Administration, Regional Counsel, ACE-7, Attn: Rules Docket No. CE305, 901 Locust, Kansas City, MO 64106. You may deliver two copies to the Regional Counsel at the above address. Mark your comments: Docket No. CE305. You may inspect comments in the Rules Docket weekdays, except Federal holidays, between 7:30 a.m. and 4 p.m.

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Leslie B. Taylor, Federal Aviation Administration, Small Airplane Directorate, Aircraft Certification Service, 901 Locust, Room 301, Kansas City, MO 64106; telephone (816) 329-4134; facsimile (816) 329-4090, email

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The FAA has determined that notice and opportunity for prior public comment hereon are impracticable because these procedures would significantly delay issuance of the design approval and thus delivery of the affected aircraft. In addition, the substance of these special conditions has been subject to the public comment process in several prior instances with no substantive comments received. The FAA therefore finds that good cause exists for making these special conditions effective upon issuance.

Comments Invited

We invite interested persons to submit such written data, views, or arguments as they desire. The most helpful comments reference a specific portion of the special conditions, explain the reason for any recommended change, and include supporting data. We ask that you send us two copies of written comments.

We will file in the docket all comments we receive, as well as a report summarizing each substantive public contact with FAA personnel about these special conditions. You may inspect the docket before and after the comment closing date. If you wish to review the docket in person, go to the address in the ADDRESSES section of this preamble between 7:30 a.m. and 4 p.m., Monday through Friday, except Federal holidays.

We will consider all comments we receive by the closing date for comments. We will consider comments filed late if it is possible to do so without incurring expense or delay. We may change these special conditions based on the comments we receive.

If you want us to let you know we received your comments on these special conditions, send us a pre-addressed, stamped postcard on which the docket number appears. We will stamp the date on the postcard and mail it back to you.


On September 9, 2008, Cirrus Design Corporation applied for a type certificate for their new model SF50. The model SF50 is a 7 seat (5 adults and 2 children), pressurized, retractable gear, carbon composite, airplane with one turbofan engine mounted partially in the upper aft fuselage.

The single turbofan engine is mounted on the upper aft fuselage, not in the pilot's line of site. Upper aft fuselage mounted engine installations, along with the need to protect such installed engines from fires, were not envisioned in the development of the part 23 normal category regulations.Start Printed Page 20517

Type Certification Basis

Under the provisions of 14 CFR 21.17, Cirrus Design Corporation must show that the model SF50 meets the applicable provisions of part 23, as amended by Amendment 23-1 through Amendment 23-59 thereto.

If the Administrator finds that the applicable airworthiness regulations, part 23, do not contain adequate or appropriate safety standards for the model SF50 because of a novel or unusual design feature, special conditions are prescribed under the provisions of § 21.16.

In addition to the applicable airworthiness regulations and special conditions, the model SF50 must comply with the fuel vent and exhaust emission requirements of 14 CFR part 34 and the noise certification requirements of 14 CFR part 36; and the FAA must issue a finding of regulatory adequacy under section 611 of Public Law 92-574, the “Noise Control Act of 1972.”

The FAA issues special conditions, as defined in § 11.19, under § 11.38 and they become part of the type certification basis under § 21.17(a)(2).

Special conditions are initially applicable to the model for which they are issued. Should the type certificate for that model be amended later to include any other model that incorporates the same novel or unusual design feature, the special conditions would also apply to the other model.

Novel or Unusual Design Features

The model SF50 will incorporate the following novel or unusual design features: An aft fuselage mounted engine is not in the pilot's line of sight. This type of configuration was not envisioned in the development of part 23 normal category airplanes. Therefore, a special condition for the fire extinguishing system for the engine on the model SF50 is required.

Regulations requiring and defining engine compartment fire extinguishing systems already exist for part 23 commuter category airplanes. These regulations will provide an adequate level of safety for the normal category model SF50 with the aft mounted engine except SC 23.1195 will require a two shot system.

As the extinguishing agent is subject to change during the service life of the airplane, the certification basis must include 14 CFR 23.1197, 23.1199, 23.1201 in their entirety.


Part 23 has historically addressed fire protection through prevention, identification, and containment. Prevention has been accomplished by minimizing the potential for ignition of flammable fluids and vapors. Identification has traditionally been achieved by the location of the engines within the pilot's primary field of view and/or with the incorporation of fire detection systems. This philosophy has provided for both the rapid detection of a fire and confirmation when it has been extinguished. Containment has been provided through the isolation of designated fire zones through flammable fluid shutoff valves and firewalls. The containment philosophy also ensures that components of the engine control system will function effectively to permit a safe shutdown of the engine. However, containment has only been required to be demonstrated for 15 minutes. In the event of a fire in a traditional part 23 airplane, the corrective action is to land as soon as possible. For a small, simple aircraft originally envisioned by part 23, it is possible to descend the aircraft to a suitable landing site within 15 minutes. Thus, if the isolation means do not extinguish the fire, the occupants can safely exit the aircraft prior to the firewall being breached. These simple and traditional aircraft normally have the engine located away from critical flight control systems and primary structure. This has ensured that throughout the fire event the pilot can maintain control and continue safe flight. It has also made predicting the effects of a fire relatively easy. Other design features of these simple and traditional aircraft, such as low stall speeds and short landing distances, ensure that even in the event of an off field landing the potential for a catastrophic outcome has been minimized.

Excluding commuter category, normal category airplanes incorporating one or more engines on the aft fuselage were not envisioned in part 23. Engine(s) located on the aft fuselage offer minimal opportunity to visually detect a fire. The ability to extinguish an engine fire becomes extremely critical due to this location. In a traditional pylon engine there is a standoff distance from the fuselage where there is no possible impingement of fluid or flame on the fuselage. Thus after 5 minutes if the fluid lines succumb to the fire any liberated fluid would not come into contact with any other critical structure or the fuselage. In essence the engine could burn off of the pylon and not adversely compromise the fuselage. The Cirrus design configuration does not benefit from this consideration and thus there is a greater risk due to fire. Also, if there was a fire due to a buildup of fuel in the exhaust nozzle a low velocity flame could impinge upon the fuselage or empennage.

Airplanes of the classic configuration with twin aft pylon mounted engines have fire extinguishing “one-shot” systems. A two shot system is necessary for fuselage embedded engines since the metallic components in the fire zone can be hot enough to re-ignite flammable fumes after the first fire has been extinguished. The consequences of a fire in these locations can be more varied, adverse, and difficult to predict than the engine fire envisioned for a typical part 23 airplane. The Cirrus aft engine installation is more indicative of an embedded engine rather than a pylon mounted engine.


As discussed above, these special conditions are applicable to the model SF50. Should Cirrus Design Corporation apply at a later date for a change to the type certificate to include another model incorporating the same novel or unusual design feature, the special conditions would apply to that model as well.


This action affects only certain novel or unusual design features on one model of airplanes. It is not a rule of general applicability and affects only the applicant who applied to the FAA for approval of these features on the airplane.

The substance of these special conditions has been subjected to the notice and comment period in several prior instances and has been derived without substantive change from those previously issued. It is unlikely that prior public comment would result in a significant change from the substance contained herein. Therefore, because a delay would significantly affect the certification of the airplane, which is imminent, the FAA has determined that prior public notice and comment are unnecessary and impracticable, and good cause exists for adopting these special conditions upon issuance. The FAA is requesting comments to allow interested persons to submit views that may not have been submitted in response to the prior opportunities for comment described above.

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List of Subjects in 14 CFR Part 23

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The authority citation for these special conditions is as follows:

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Authority: 49 U.S.C. 106(g), 40113 and 44701; 14 CFR 21.16 and 21.17; and 14 CFR 11.38 and 11.19.

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The Special Conditions

Accordingly, pursuant to the authority delegated to me by the Administrator, the following special conditions are issued as part of the type certification basis for Cirrus Design Corporation model SF50 airplanes.

Fire Extinguishing for Upper Aft Fuselage Mounted Engine

SC 23.1195 Fire Extinguishing Systems

Fire extinguishing systems must be installed and compliance shown with the following:

(a) Except for combustor, turbine, and tailpipe sections of turbine-engine installations that contain lines or components carrying flammable fluids or gases for which a fire originating in these sections is shown to be controllable, a fire extinguisher system must serve each engine compartment.

(b) The fire extinguishing system, the quantity of the extinguishing agent, the rate of discharge, and the discharge distribution must be adequate to extinguish fires. An individual “two shot” system must be used.

(c) The fire extinguishing system for a nacelle must be able to simultaneously protect each compartment of the nacelle for which protection is provided.

SC 23.1197 Fire Extinguishing Agents

The following applies:

(a) Fire extinguishing agents must—

(1) Be capable of extinguishing flames emanating from any burning of fluids or other combustible materials in the area protected by the fire extinguishing system; and

(2) Have thermal stability over the temperature range likely to be experienced in the compartment in which they are stored.

(b) If any toxic extinguishing agent is used, provisions must be made to prevent harmful concentrations of fluid or fluid vapors (from leakage during normal operation of the airplane or as a result of discharging the fire extinguisher on the ground or in flight) from entering any personnel compartment, even though a defect may exist in the extinguishing system. This must be shown by test except for built-in carbon dioxide fuselage compartment fire extinguishing systems for which—

(1) Five pounds or less of carbon dioxide will be discharged, under established fire control procedures, into any fuselage compartment; or

(2) Protective breathing equipment is available for each flight member on flight deck duty.

SC 23.1199 Extinguishing Agent Containers

The following applies:

(a) Each extinguishing agent container must have a pressure relief valve to prevent bursting of the container by excessive internal pressures.

(b) The discharge end of each discharge line from a pressure relief connection must be located so that discharge of the fire extinguishing agent would not damage the airplane. The line must also be located or protected to prevent clogging caused by ice or other foreign matter.

(c) A means must be provided for each fire extinguishing agent container to indicate that the container has discharged or that the charging pressure is below the established minimum necessary for proper functioning.

(d) The temperature of each container must be maintained under intended operating conditions to prevent the pressure in the container from —

(1) Falling below that necessary to provide an adequate rate of discharge; or

(2) Rising high enough to cause premature discharge.

(e) If a pyrotechnic capsule is used to discharge the extinguishing agent, each container must be installed so that temperature conditions will not cause hazardous deterioration of the pyrotechnic capsule.

SC 23.1201 Fire Extinguishing System Materials

The following apply:

(a) No material in any fire extinguishing system may react chemically with any extinguishing agent so as to create a hazard.

(b) Each system component in an engine compartment must be fireproof.

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Issued in Kansas City, Missouri on April 12, 2010.

Steve Thompson,

Acting Manager, Small Airplane Directorate, Aircraft Certification Service.

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[FR Doc. 2010-9026 Filed 4-19-10; 8:45 am]