July 12, 2013

Mr. Earl Lawrence

Manager

Small Airplane Directorate

Federal Aviation Administration

901 Locust Street

Kansas City, Missouri 64106

FAA Project No.: TC01422AK-A

Re: Request for Exemption from 14 CFR §23.562.

Dear Mr. Lawrence:

Sherpa Aircraft, Inc. hereby petitions to the FAA for an exemption from 14 CFR §23.562 Amendment 

23-62, for Sherpa Aircraft, Inc. Model SH550 and SH650.

The SH550 aircraft is a 6 occupant aircraft with a maximum gross weight of 4750 lbs. and a power off 

flaps down stall speed of 40 knots. The SH650 aircraft is an 8 occupant aircraft with a maximum gross 

weight of 7,750 lbs. and a power off flaps down stall speed of 40 knots. Sherpa Aircraft, Inc. will 

perform static test on the seat and harnesses and equip both model aircraft with a 4-point harness with 

a TSO. Justification for this exemption is based on low stall speed of the aircraft and the fact that head 

impact is unlikely.

The purpose of this petition for exemption is to permit type certification of the SH550 and SH650 with 

seats that have not shown compliance to §23.562, Emergency Landing Dynamic conditions, effective 

at Amendment level 23-62. Sherpa Aircraft, Inc. will demonstrate mitigating factors to ensure that 

safety will not be adversely affected.

Sherpa Aircraft, Inc. is petitioning for this exemption in accordance with paragraph 14 CFR §11.81 

under Petitions for Rulemaking.

Section of CFR affected

Section §23.562 specifies dynamic test conditions for qualification of crew/passenger seat, as well as 

the restraint system.

Rationale

Sherpa Aircraft, Inc. proposes the following as mitigating factors regarding safety in support of the 

exemption request:

1. Sherpa Aircraft, Inc. will perform a static test on the SH550 and SH650 seat and harness 

system to an ultimate load of 3,060 lbs. The ultimate load will represent 170 lbs. (simulated 

occupant weight) multiplied by 18 G, which represents the required longitudinal design load 

factor for items of mass. This load factor was introduced at the same time that the §23.562 

was adopted, and viewed as a suitable static alternative to dynamically testing items of mass. 

The strength of the occupant seating system (consisting of the seat, seat attachment 

hardware, the harness, and the airframe attachment points of the harness) will thus be 

demonstrated to be equivalent in strength to an item of mass restraint. In addition, the 

proposed test requirements far exceeds the current requirement of Part 23 for static seat 

system testing, which is 9 G in the forward longitudinal direction.

2. The SH550 and SH650 will have a flaps-down, power-off stall speed that is much lower than 

aircraft with a stall speed of 61 knots for which this regulation was envisioned. The 34% 

slower stall speed of the SH550 and SH650 will result in a at least 57% reduction in impact 

of the seat and occupant, greatly reducing the structural load on the seat and giving the 
SH550 and SH650 aircraft a wider margin of safety. In addition, the FAA has previously 

granted similar exemptions to other aircraft types with stall speeds of 45 knots or less.

3. The level of compliance required for this type certificate category far exceeds that of an 

experimental/homebuilt category aircraft, a Light Sport Category (LSA) or Part 21 primary 

category aircraft. This higher level of certification will result in an increased level of safety for 

the SH550 and SH650 over the CAR 3, Experimental or LSA class aircraft against which the 

SH550 and SH650 will compete.

4. The Pilot and Copilot seats will be equipped with a 4-point inflatable TSO harness, the 

passenger seats will be equipped with a 3-piont TSO harness. The seating system will be 

tested as mentioned above in Item 1.

5. The head flail envelope will be determined analytically using data from the following 

publication: “Aircraft Crash Survival Design Guide, Volume 1 – Design Criteria and 

Checklists,” report number USAAVSCOM TR 89-D-22A. Other applicants for similar 

exemption have successfully argued that the head flail envelope can be derived analytically 

using the report. The seat will be equipped with a 4-point harness with a TSO.

Public Interest

The public interest will be served if this exemption is granted because it will enable the SH550 and 

SH650 to:

1. Ensure that the SH550 and SH650 are competitive with aircraft of similar configuration, weight 

and speed that have recently been granted exemptions for reasons that are very similar to the 

ones given in this petition.

2. It will offer to the market an aircraft with Part 23 normal category type certification that will have 

an enhanced level of safety over equivalent CAR 3, LSA and homebuilt type aircraft that compete 

directly with it.

3. The local, state, and national economies will experience tangible income benefits due to the 

resulting number of employment positions required for the production of these aircraft. Further, it 

is reasonable to assume that job security for Airframe and Powerplant mechanics will benefit as a 

result of the routine maintenance and continued airworthiness requirements of these aircraft.

Exercise of Exemption Outside of the United States

Regarding the desire of Sherpa Aircraft, Inc. to exercise the privileges of the exemption outside the 

United States, we plan to market and export the SH550 and SH650 to customers in several countries 

outside of the United States. We understand that the exemption to a Part 23 regulation is a certification 

issue, and not an operational issue, and that it will be listed on the Type Certificate Data Sheet 

(TCDS), which will be reviewed by other Civil Aviation Authorities in their validation of the SH550 and 

SH650 certification basis.

Sincerely,

_____________________

Alan Pendergrass

Structures DER

Sherpa Aircraft, Inc.