[Federal Register Volume 64, Number 57 (Thursday, March 25, 1999)]
[Proposed Rules]
[Pages 14401-14408]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-7276]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 23

[Docket No. CE149; Notice No. 23-98-05-SC]


Special Conditions: Soloy Corporation Model Pathfinder 21 
Airplane; Airframe

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed special conditions.

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SUMMARY: This notice proposes special conditions for the Soloy 
Corporation Model Pathfinder 21 airplane. The Model Pathfinder 21 
airplane is a Cessna Model 208B airplane as modified by Soloy 
Corporation to be considered as a multiengine, part 23, normal category 
airplane. The Model Pathfinder 21 airplane will have a novel or unusual 
design feature associated with installation of the Soloy Dual Pac 
propulsion system, which consists of two Pratt & Whitney Canada Model 
PT6D-114A turboprop engines driving a single, Hartzell, five-blade 
propeller. The applicable airworthiness regulations do not contain 
adequate or appropriate safety standards for this design feature. These 
proposed special conditions contain the additional safety standards for 
this design feature. These proposed 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 for multiengine airplanes.

DATES: Comments must be received on or before April 26, 1999.

ADDRESSES: Comments on this proposal may be mailed in duplicate to: 
Federal Aviation Administration, Regional Counsel, ACE-7, Attention: 
Rules Docket, Docket No. CE149,601 East 12th Street, Kansas City, 
Missouri 64106, or delivered in duplicate to the Regional Counsel at 
the above address. Comments must be marked: Docket No. CE149. Comments 
may be inspected in the Rules Docket weekdays, except Federal holidays, 
between 7:30 a.m. and 4:00 p.m.

FOR FURTHER INFORMATION CONTACT: David Keenan, Federal Aviation 
Administration, Aircraft Certification Service, Small Airplane 
Directorate, ACE-112, 601 East 12th Street, Kansas City, Missouri, 816-
426-5688, fax 816-426-2169.

SUPPLEMENTARY INFORMATION:

Comments Invited

    Interested persons are invited to participate in the making of 
these proposed special conditions by submitting such written data, 
views, or arguments as they may desire. Communications should identify 
the regulatory docket or notice number and be submitted in duplicate to 
the address specified above. All communications received on or before 
the closing date for comments will be considered by the Administrator. 
The proposals described in this notice may be changed in light of the 
comments received. All comments received will be available in the Rules 
Docket for examination by interested persons, both before and after the 
closing date for comments. A report summarizing each substantive public 
contact with FAA personnel concerning this rulemaking will be filed in 
the docket. Persons wishing the FAA to acknowledge receipt of their 
comments

[[Page 14402]]

submitted in response to this notice must include with those comments a 
self-addressed, stamped postcard on which the following statement is 
made: ``Comments to Docket No. CE149.'' The postcard will be date 
stamped and returned to the commenter.

Background

    On February 6, 1992, Soloy Corporation applied for a supplemental 
type certificate (STC) for the Model Pathfinder 21 airplane, which 
would notify the Cessna Model 208B airplane by installing the Soloy 
Dual Pac propulsion system. This propulsion system consists of two 
Pratt & Whitney Canada (PWC) Model PT6D-114A turboprop engines driving 
a single, Hartzell, five-blade propeller through a combining gearbox. 
Soloy Corporation is seeking approval for this airplane, equipped with 
a Soloy Dual Pac propulsion system, as a multiengine airplane Title 14 
CFR part 23 is not adequate to address a multiengine airplane with a 
single propeller. Hence, the requirement for these proposed special 
conditions, which will be applied in addition to the applicable 
sections of part 23.
    The Soloy Dual Pac population system is mounted in the nose of the 
Model Pathfinder 21 airplane. With this arrangement, an engine failure 
does not cause an asymmetric thrust condition that would exist with a 
conventional twin turboprop airplane. This asymmetric thrust compounds 
the flightcrew workload following an engine failure. The Model 
Pathfinder 21 airplane configuration has the potential to substantially 
reduce this workload.
    Since the Model Pathfinder 21 airplane produces only centerline 
thrust, the only direct airplane control implications of an engine 
failure are the change in torque reaction and propeller slip stream 
effect. These transient characteristics require substantially less crew 
action to correct than an asymmetric thrust condition and do not 
require constant effort by the flightcrew to maintain control of the 
airplane for the remainder of the flight.

Safety Analysis

    The FAA has conducted a safety analysis that recognizes both the 
advantages and disadvantages of the proposed Model Pathfinder 21 
airplane. The scope of this safety analysis was limited to the areas 
affected by the unique propulsion system installation and assumes 
compliance with the design-related requirements of these proposed 
special conditions. The FAA examined the accident and incident history 
of small twin turboprop operations for the years of 1983 to 1994 in the 
United States and United Kingdom. The FAA evaluated each event and 
determined if the outcome, given the same pilot, weather, and airplane 
except with centerline thrust and one propeller, would have been more 
favorable, less favorable, or unchanged. Examination of the incident 
data revealed a number of failure modes that, if not addressed as part 
of the Model Pathfinder 21 airplane design, could result in a potential 
increase in the number of accidents for the Model Pathfinder 21 
airplane compared to the current fleet. Examples of such failure modes 
include loss of propeller blade tip or failure of the propeller control 
system. Although these proposed special conditions contain provisions 
to prevent catastrophic failures of the remaining non-fail-safe 
components of the Model Pathfinder 21 airplane after compliance with 
the design related requirements, the analysis assumes that these 
components will fail in a similar manner to the failures contained in 
the incident data. Given these assumptions, the FAA determined that the 
projected accident rate of the Model Pathfinder 21 airplane would be 
equal to or lower than the current small twin turboprop airplane fleet. 
Considering that analysis, the FAA has determined that the advantages 
of centerline thrust compensate for the disadvantages of the non-fail-
safe-design features. Once that determination was made, these proposed 
special conditions were formulated with the objective of substantially 
reducing or eliminating risks associated with the non-redundant systems 
and components of the Model Pathfinder 21 airplane design that have 
been identified and providing a level of safety equivalent to that of 
conventional multiengine airplanes.
    The FAA data review conducted to prepare these proposed special 
conditions is applicable only to the Model Pathfinder 21 airplane. For 
the concept of a single-propeller, multiengine airplane to be extended 
to other projects, a separate analysis of the accident and incident 
data for similarly sized airplanes would be required. If the advantages 
of centerline thrust compensated for the disadvantages of the non-fail-
safe components, based on the service history of similarly sized 
airplanes, development of separate special conditions would be 
required.

Type Certification Basis

    Under the provisions of 14 CFR part 21, Sec. 21.101, Soloy 
Corporation must show that the Model Pathfinder 21 airplane continues 
to meet the applicable provisions of the regulations incorporated by 
reference in Type Certificate (TC) Data Sheet A37CE or the applicable 
regulations in effect on the date of application for change. The 
regulations incorporated by reference are commonly referred to as the 
``original type certification basis.'' The regulations incorporated by 
reference in TC No. A37CE are as follows:
    The type certification basis for Cessna Model 208B airplanes shown 
on TC Data Sheet A37CE for parts not changed or not affected by the 
changes proposed by Soloy Corporation is part 23 of the Federal 
Aviation Regulations dated February 1, 1965, as amended by Amendments 
23-1 through 23-28; part 36 dated December 1, 1969, as amended by 
Amendments 36-1 through 36-18; Special Federal Aviation Regulations 
(SFAR) 27 dated February 1, 1974, as amended by Amendments 27-1 through 
27-4. Soloy Corporation must show that the Model Pathfinder 21 airplane 
meets the applicable provisions of part 23, including multiengine 
designated sections, as amended by Amendment 23-42 (the Pathfinder 21 
type certification basis is based on the date of STC application: 
February 6, 1992) for parts changed or affected by the change. Soloy 
Corporation has also elected to comply with Sec. 23.561, Emergency 
Landing Conditions--General (Amendment 23-48); Sec. 23.731, Wheels 
(Amendment 23-45); Sec. 23.733, Tires (Amendment 23-45); Sec. 23.783, 
Doors (Amendment 23-49); Sec. 23.807, Emergency Exits (Amendment 23-
49); Sec. 23.811, Emergency Exit Marking (Amendment 23-46); 
Sec. 23.901, Installation (Amendment 23-51); Sec. 23.955, Fuel Flow 
(Amendment 23-51); Sec. 23.1041, Cooling--General (Amendment 23-51); 
Sec. 23.1091, Air Induction System (Amendment 23-51); Sec. 23.1181, 
Designated Fire Zones; Regions Included (Amendment 23-51); 
Sec. 23.1189, Shutoff Means (Amendment 23-43); Sec. 23.1305, Powerplant 
Instruments (Amendment 23-52); and Sec. 23.1351, Electrical Systems and 
Equipment--General (Amendment 23-49). The type certification basis for 
the Model Pathfinder 21 airplane also includes parts 34 and 36, each as 
amended at the time of certification. Soloy Corporation may also elect 
to comply with subsequent part 23 requirements to facilitate operators' 
compliance with corresponding part 135 requirements. The type 
certification basis for this airplane will include exemptions, if any; 
equivalent level of safety findings, if any; and the special conditions 
adopted by this rulemaking action.
    If the Administrator finds that the applicable airworthiness 
regulations

[[Page 14403]]

(part 23, as amended) do not contain adequate or appropriate safety 
standards for the Model Pathfinder 21 airplane because of a novel or 
unusual design feature, special conditions are prescribed under the 
provisions of Sec. 21.16.
    Special conditions, as appropriate, are issued in accordance with 
Sec. 11.49 after public notice, as required by Sec. 11.28 and 
Sec. 11.29(b), and become part of the type certification basis in 
accordance with Sec. 21.101(b)(2).
    Special conditions are initially applicable to the model for which 
they are issued. Should the applicant apply for an STC to modify any 
other model included on the same TC to incorporate the same novel or 
unusual design feature, the special conditions would also apply to the 
other model under the provisions of Sec. 21.101(a)(1).
    The Soloy Dual Pac was certified as a propulsion system under part 
33 and special conditions in Docket No. 93-ANE-14; No. 33-ANE-01 (62 FR 
7335, February 19, 1997) under STC No. SE00482SE to the PWC Model PT6 
engine TC E4EA. Those special conditions were created in recognition of 
the novel and unusual features of the proposal, specifically the 
combining gearbox.

Novel or Unusual Design Features

    The Model Pathfinder 21 will incorporate a noval or unusual design 
feature by installing the Soloy Dual Pac propulsion system, which 
consists of two PWC Model PT6D-114A engines driving a single, Hartzell, 
five-blade propeller through a Soloy-designed combining gearbox. The 
combining gearbox incorporates redundant freewheeling drive, governing, 
and lubricating systems. A system of one-way clutches both prevents the 
propeller shaft from driving the engine input shafts and allows either 
engine to drive the propeller should the other engine fail.

Airplane Design Features

    The Model Pathfinder 21 airplane is a modified Cessna Model 208B 
airplane converted to a multiengine, normal category, combination nine-
passenger/freight airplane. The proposed modification includes the 
installation of the Soloy Dual Pac engine, installation of a different 
propeller, addition of a six-foot fuselage extension and integral 
belly-mounted cargo compartment, alterations to the langing gear, and 
an increase of the maximum gross weight to 12,500 pounds. The proposed 
changes to the Cessna Model 208B airplane are discussed below.

Powerplant

    The original PWC Model PT6A-114 engine is replaced with the Soloy 
Dual Pac propulsion system, consisting of two Model PT6D-114A engines 
and a Soloy Corporation-designed propulsion drive system. The FAA has 
issued STC No. SE00482SE approving the Soloy Dual Pac propulsion 
system. A Hartzell propeller part number HC-B5MA-3H1/M11296NK-5, which 
is a steel-hubbed, five-blade, aluminum, constant-speed, single-acting, 
reversible-pitch propeller, is replacing the original three-blade 
Hartzell or McCauley propeller. The propulsion installation and 
associated systems, mounts, instrumentation, firewall, exhaust stacks, 
and cowling are all impacted by this modification.

Fuselage Extension

    The most significant structural modification is a 72-inch extension 
in the fuselage aft of the wing trailing edge. The new fuselage section 
is designed and manufactured using the same conventional formed sheet 
metal bulkhead, stringer, and skin methods used by Cessna in the basic 
airplane. The section has a constant cross section and is positioned in 
the widest and tallest portion of the rear fuselage. Also, the control 
cables are extended due to the fuselage extension.

Airframe Structure

    Structural reinforcements are added to the basic fuselage structure 
to accommodate the higher increased takeoff gross weight. Reinforcement 
of wing structure is also required to accommodate the higher wing 
loading. The empennage structure is unchanged from the basic airplane.

Cargo Pod

    A cargo pod is added to the underside of the fuselage. New lower 
fuselage reinforcement angles serve as the attachment means for the 
cargo pod that runs the full width of the fuselage. The fuselage/engine 
compartment bulkhead is extended to form the forward end of the cargo 
pod.

Cabin

    The cabin arrangement places the nine passengers directly behind 
the flightcrew. Cargo is secured in the aft portion of the cabin. The 
floor of the fuselage extension is equipped with the Brownline seat 
tracks and cargo attachment fittings that are used in the Cessna Model 
208B airplane. Features to satisfy current crashworthiness regulations 
are being added to the cabin, including cargo retention barriers and 
relocation of the passenger door. The cargo door is unchanged.

Landing Gear

    The original main landing gear is placed with larger land gear, 
wheels, and brakes. The nose gear support structure is replaced and the 
nose gear strut is pressurized for shock absorption.

Instrumentation

    The flight deck is being modified to include an additional set of 
engine instruments, propulsion drive system instrumentation, and other 
flight deck indications required for multiengine airplanes.

Engine Controls

    The flight deck modifications include an additional power lever and 
condition lever to accommodate the second engine.

Fuel System

    The fuel system is being modified to provide independent fuel feed 
capability to each engine. In addition, pilot selectable crossfeed 
function is available. The two fuel wing tanks remain unchanged.

Electrical System

    A dual redundant electrical system with independent batteries is 
being added as part of this modification. All components are located in 
the cargo pod immediately aft of the engine firewall, except for the 
generators that are installed on the engines.

Maximum Takeoff Weight

    The maximum gross takeoff weight of the aircraft is increased from 
8,750 pounds to 12,500 pounds.

Discussion

    Elements of these proposed special conditions have been developed 
to replace part 23 standards for which the Model Pathfinder 21 airplane 
design, because of the single propeller system, cannot comply using the 
criteria usually applied to multiengine airplanes, namely 
Sec. 23.903(c), Engines. Other elements of these proposed special 
conditions have also been developed to supplement part 23 standards 
that are considered inadequate to address the Model Pathfinder 21 
airplane design, namely Secs. 23.53, 23.67, 23.75, 23.903(b), 23.1191, 
23.1305, 23.1545, 23.1585, and 23.1587.
    The part 23 requirement that is most affected by the multiengine, 
single propeller Model Pathfinder 21 airplane arrangement is 
Sec. 23.903(c). Section 23.903(c) states, ``The powerplants must be 
arranged and isolated from each other to allow operation, in at least 
one configuration, so that the failure or

[[Page 14404]]

malfunction of any engine, or the failure or malfunction (including 
destruction by fire in the engine compartment) of any system that can 
affect an engine (other than a fuel tank if only one fuel tank is 
installed), will not: (1) prevent the continued safe operation of the 
remaining engines; or (2) require immediate action by any crewmember 
for continued safe operation of the remaining engines.'' This is a 
fail-safe requirement since it takes advantage of the redundancy 
provided by having multiple engines that are isolated from each other, 
which is intended to ensure that no single failure affecting one engine 
will result in the loss of the airplane. In conventional twin turboprop 
airplanes, this isolation is, in part, provided by the inherent 
separation of having each engine mounted on opposite sides of the 
airplane driving its own propeller. Installation of the engines on 
either side of the airplane automatically provides a degree of 
separation of critical systems, such as the electrical and fuel 
systems, and minimizes the effect of high vibration, rotor burst 
failures, and engine case burn-through from the opposite engine. This 
separation aids in preventing any single failure from jeopardizing 
continued safe operation of the airplane. In contrast, the nearness of 
the engines to each other driving a combining gearbox with a single 
propeller in the Model Pathfinder 21 airplane arrangement is inherently 
less isolated from certain types of failure modes. As a result, many 
failure modes that do not pose a significant hazard on conventional 
multiengine airplanes could threaten continued safe operation of the 
Model Pathfinder 21 airplane unless specific additional precautions are 
taken to prevent hazardous secondary effects.
    To ensure a level of safety equivalent to that provided by 
conventionally arranged twin turboprop airplanes, the FAA evaluated the 
relative advantages and disadvantages of each arrangement while 
striving to maintain, as much as possible, the fail-safe and isolation 
design requirement of Sec. 23.903(c). Only for those areas of the 
design where the fail-safe and isolation design philosophy could not be 
maintained did the FAA consider other options, such as requiring 
components with a proven reliability, an enhanced maintenance program, 
and additional testing. The FAA's analysis and derivation of each of 
the special condition requirements is discussed in the Description of 
Proposed Requirements section below.
    Soloy Corporation Soloy Dual Pac Engine Special Conditions (Docket 
No. 93-ANE-14; No. 33-ANE-01) were developed for the propulsion system 
to maintain the fail-safe and isolation design philosophy up to the 
propeller shaft. They include the design requirement that the 
propulsion system must be able to provide controllable power, which is 
at least fifty percent of rated power, for any probable engine failure. 
This includes failures in the propulsion drive system.
    Even after complying with the part 33 special conditions, Soloy 
Corporation's design still contains several single failure modes of 
non-redundant components that could cause a total loss of thrust. These 
components include the single propeller hub and blade assembly, 
propeller shaft, and propeller control system. Common propeller system 
failure modes are eliminated or the hazard significantly reduced by the 
design and maintenance requirements contained in these proposed special 
conditions, which are intended to reduce the risk of these failures to 
an acceptable level. Rotorcraft techniques, including development of a 
critical parts plan, are used to mitigate the risks associated with the 
non-fail-safe components because Soloy Corporation's propulsion system 
concept is similar to twin engine, single rotor propulsion systems of 
twin engine rotorcraft in certain aspects.
    The propulsion system installation design of the Model Pathfinder 
21 airplane is potentially more critical when assessing the rotorburst 
and engine case burn-through design requirements set forth in 
Sec. 23.903(b)(1). Section 23.903(b)(1) states, ``Turbine engine 
installations. For turbine engine installations--(1) Design precautions 
must be taken to minimize the hazards to the airplane in the event of 
an engine rotor failure or of a fire originating inside the engine 
which burns through the engine case.'' For conventional twin turboprop 
airplanes, compliance with this regulation has involved a degree of 
inherent protection by having the engines installed some distance apart 
from one another and on opposite sides of the airplane fuselage. This 
level of inherent protection is not provided as part of the Pathfinder 
21 configuration.
    In addressing propeller assembly structural failures, uncontained 
engine failures, and engine case burn-through, these proposed special 
conditions allow Soloy Corporation to select components with excellent 
service histories. While compliance to part 23 establishes adequate 
safety standards, in-service operations identify long term durability 
problems and problems associated with operations that the condensed 
evaluation of the critical conditions during a certification program 
cannot. Propeller assembly structural failures, uncontained engine 
failures, and engine case burn-through will most likely be catastrophic 
for the Model Pathfinder 21 airplane, but are only occasionally 
catastrophic for conventional twin turboprop airplanes. The probability 
of each of these events occurring on conventional small twin turboprop 
airplanes is on the order of one in ten million hours based on the 
service history as discussed in the Safety Analysis section. Therefore, 
for the purposes of these special conditions, it is reasonable and 
appropriate to require ten million hours free of specific failure modes 
as an acceptable level of proven reliability needed to establish a 
level of safety equivalent to that of conventional multiengine 
airplanes.

Description of Proposed Requirements

    The FAA has reviewed the part 23 standards and identified that 
Sec. 23.53, Takeoff Speeds, Sec. 23,67(c) and (d), Climb: one engine 
inoperative, Sec. 23.69, Enroute Climb/Descent, and Sec. 23.75(g), 
Landing Distance, are inadequate to address the effects of propeller 
control system failure modes in a manner consistent with how these 
sections address specific engine failure conditions. Sections 
23.1191(a) and 23.1191(b), Firewalls, do not adequately define the 
locations of firewalls needed to isolate the engines and propulsion 
drive system of the Soloy Dual Pac propulsion system. Additionally, the 
FAA has identified that Sec. 23.1305(c), Powerplant Instruments, is 
inadequate because it does not recognized a propulsion system 
installation with a combining gearbox whose oil system is separate from 
either engine. Furthermore, the FAA has identified that 
Sec. 23.1545(b)(5), Airspeed Indicator, Sec. 23.1585(c), Operating 
Procedures and Sec. 23.1587(a), Performance Information; do not 
recognize a propeller system installation independent from either 
engine. Elements of these proposed special conditions have been 
developed to ensure that these unique aspects of the Model Pathfinder 
21 airplane are addressed in a manner equivalent to that established by 
part 23 standards.

Propulsion System

    The propulsion drive system includes all parts necessary to 
transmit power from the engines to the propeller shaft. This includes 
couplings, universal joints, drive shafts, supporting bearings for 
shafts, brake assemblies, clutches, gearboxes, transmissions, any 
attached accessory pads or drives, and any cooling fans that are 
attached to, or mounted on, the propulsion drive system. The propulsion 
drive system for

[[Page 14405]]

this multiengine installation must be designed with a ``continue to 
run'' philosophy. This means that it must be able to power the 
propeller after failure of one engine or failure in one side of the 
drive system, including any gear, bearing, or element expected to fail. 
Common failures, such as oil pressure loss or gear tooth failure, in 
the propulsion drive system must not prevent the propulsion system from 
providing adequate thrust. These design requirements, and other 
propulsion drive system requirements, are included in the part 33 
special conditions, and, therefore, are required as part of these 
proposed special conditions.
    Special 23.903(b)(1) states, in part, ``Design precautions must be 
taken to minimize the hazards to the airplane in the event of a rotor 
failure.'' Part 33 containment requirements address blade failures but 
do not require containment or failed rotor disks; therefore, 
Sec. 23.903(b)(1) requires that airplane manufacturers minimize the 
hazards in the event of a rotor failure. This is done by locating 
critical systems and components out of impact areas as much as 
possible. The separation inherent in conventional twin engine 
arrangements by locating the engines on opposite sides of the fuselage 
provides good protection from engine-to-engine damage. Although most 
multiengine installations have the potential for an uncontained failure 
of one engine damaging the other engine, service history has shown that 
the risk of striking the opposite engine is extremely low.
    The Model Pathfinder 21 airplane propulsion system installation 
does not have the inherent engine-to-engine isolation of a conventional 
twin turboprop airplane. For the Model Pathfinder 21 airplane to obtain 
a level of safety equivalent to that of a conventional multiengine 
airplane, the effects of rotor failure must be addressed. Soloy 
Corporation must demonstrate that the engine type in relevant 
installations has at least ten million hours of service time without a 
high energy rotor failure (for example, disks, hubs, compressor wheels, 
and so forth). Additionally, for any lower energy fragments released 
during this extensive service life of the engine (for example, blades), 
a barrier must be placed between the engines to contain these low 
energy fragments. Even after installation of a barrier, engine-to-
engine isolation following failure of either engine could be 
compromised through the common mount system or shared system interfaces 
such as firewalls, electrical busses, or cowlings. Soloy Corporation 
must, therefore, demonstrate any loads transmitted through the common 
mount system as a result of an engine failure do not prevent continued 
safe flight and landing with the operating engine.
    Section 23.903(b)(1) also addresses damage caused by engine burn-
through. Engine case burn-through results in a concentrated flame that 
has the capacity to burn through the firewall mandated by Sec. 23.1191; 
therefor, Sec. 23.903(b)(1) requires that design precautions must be 
taken to minimize the hazards to the airplane in the event of a fire 
originating in the engine that burns through the engine case. Similar 
to uncontained engine failures, the conventional multiengine airplane 
arrangement provides inherent protection from engine-to-engine damage 
associated with engine case burn-through by placing the engines on 
opposite sides of the fuselage. The Model Pathfinder 21 airplane 
propulsion system does not have this inherent isolation; therefore, the 
FAA is requiring that engine type in a relevant installation to have at 
least ten million hours of service time without an engine case burn-
through, or that a firewall able to protect the operating engine from 
engine case burn-through be installed between the engines.
    Soloy Corporation is not required to show compliance to Sec. 21.35, 
per Sec. 21.115 because the Model Pathfinder 21 airplane certification 
is being conducted under an STC project. Section 21.35(f)(1), Flight 
Tests, requires aircraft incorporating turbine engines of a type not 
previously used in a type certificated aircraft operate for at least 
300 hours with a full complement of engines that conform to a type 
certificate as part of the certification flight test. The propulsion 
system installation is, however, different from any other airplane 
previously certified; therefore, the FAA is requiring as part of these 
proposed special conditions that Soloy Corporation show compliance with 
Sec. 21.35(f)(1).

Propeller Installation

    As demonstrated by the data discussed in the Safety Analysis 
section, propeller blade failures near the hub result in substantial 
airplane damage on a conventional twin turboprop airplane. One of the 
eight events was catastrophic. Blade debris has damaged critical 
components and structure of the airplane, and large unbalanced loads in 
the propeller have led to engine, mount, and wing structural failure. 
In contrast, service history has demonstrated that blade tip failures 
are not necessarily catastrophic on a conventional multiengine airplane 
because the flightcrew is able to secure the engine with the failed 
propeller and safely land the airplane. However, if the Model 
Pathfinder 21 airplane's single propeller failed near the tip, the 
failure would be likely to result in a catastrophic accident caused by 
the total loss of thrust capability and severe vibration. Other 
propeller system structural failures would be equally catastrophic; 
therefore, steps must be taken to reduce the potential for propeller 
system structural failures.
    As discussed earlier, the FAA has determined additional testing is 
required for non-redundant components to ensure that equivalency to the 
fail-safe and isolation requirements of Sec. 23.903(c) is met. The 
Model Pathfinder 21 airplane's single propeller system must be 
installed and maintained in such a manner as to substantially reduce or 
eliminate the occurrence of failures that would preclude continued safe 
flight and landing. To ensure the propeller installation and production 
and maintenance programs are sufficient to achieve the fail-safe 
equivalency requirement, these proposed special conditions include a 
2,500 cycle validation test. This corresponds to the FAA's estimated 
annual usage for a turboprop airplane operating in scheduled service. 
An airplane cycle includes idle, takeoff, climb, cruise, descent, and 
reverse. The test must utilize production parts installed on the engine 
and should include a wide range of ambient and wind conditions, several 
full stops, and validation of scheduled and unscheduled maintenance 
practices.
    Furthermore, these proposed special conditions require 
identification of the critical parts of the propeller assembly, which 
are components whose failure during ground or flight operation could 
cause a catastrophic effect on the airplane, including loss of the 
ability to produce controllable thrust. The FAA is proposing to require 
that a critical parts plan, modeled after plans required by Joint 
Aviation Requirements 27 and 29 for critical rotorcraft components, be 
established and implemented for the critical components of the 
propeller assembly. This plan draws the attention of the personnel 
involved in the design, manufacture, maintenance, and overhaul of a 
critical part to the special nature of the part. The plan should define 
the details of relevant special instructions to be included in the 
Instructions for Continued Airworthiness. The Instructions for 
Continued Airworthiness, required by Sec. 23.1529, should contain life 
limits, mandatory overhaul intervals, and conservative damage limits 
for return to

[[Page 14406]]

service and repair, as appropriate, for the critical parts identified 
in accordance with these proposed special conditions.
    On a conventional multiengine airplane, the flightcrew will secure 
an engine to minimize effects of propeller imbalance. Most of these 
airplanes also incorporate quick acting manual or automatic propeller 
feathering systems that further reduce the time the airplane is exposed 
to the effects of propeller imbalance. In addition to the propeller 
blade failures discussed earlier, the unbalanced condition could be 
caused by a propeller system failure such as loss of a de-icing boot, 
malfunction of a de-icing boot in icing conditions, an oil leak into a 
blade butt, asymmetric blade pitch, or a failure in counterweight 
attachment. The Model Pathfinder 21 airplane design does not provide 
any means to reduce the vibration produced by an unbalanced propeller; 
therefore, these proposed special conditions require that the engines, 
propulsion drive system, engine mounts, primary airframe structure, and 
critical systems must be designed to function safely in the high 
vibration environment generated by those less severe propeller 
failures. In addition, the degree of flight deck vibration must not 
jeopardize the crew's ability to continue to operate the airplane in a 
safe manner. Component failures that generate vibrations beyond the 
capability of the airplane must be addressed as a critical part in the 
same manner as required for propeller blade failures.

Propeller Control System

    Propeller control system failures on a conventional twin engine 
airplane may result in a one-engine-inoperative configuration. To 
ensure an equivalent level of safety in the event of a propeller 
control system failure, these proposed special conditions require that 
the Model Pathfinder 21 airplane propulsion system be designed such 
that the airplane meets the one-engine-inoperative requirements of 
Sec. 23.53 and Sec. 23.67 after the most critical propeller control 
system failure.
    There are several means to accomplish these proposed special 
condition elements. Soloy Corporation plans to address them by 
providing a mechanical high-pitch stop, which would be set to a ``get 
home'' pitch position, thereby preventing the propeller blades from 
rotating to a feather-pitch position when oil pressure is lost in the 
propeller control system. This would allow the propeller to continue to 
produce a minimum amount of thrust as a fixed-pitch propeller. These 
proposed special conditions provide design requirements that the FAA 
has determined are critical to a default fixed-pitch position feature. 
These include maintaining engine and propeller limits following an 
automatic or manual pitch change, the ability to manually select and 
deselect the default fixed-pitch position in flight in the event of a 
propeller control system failure that does not result in a loss of oil 
pressure, and the means to indicate to the flightcrew when the 
propeller is at the default fixed-pitch position.

Propulsion Instrumentation

    On a conventional multiengine airplane, the pilot has positive 
indication of an inoperative engine created by the asymmetric thrust 
condition. The airplane will not yaw when an engine or a portion of the 
propulsion drive system fails because of the centerline thrust of the 
Model Pathfinder 21 airplane propulsion system installation. The 
flightcrew will have to rely on other means to determine which engine 
or propulsion drive system element has failed in order that the correct 
engine is secured; therefore, these proposed special conditions require 
that a positive indication of an inoperative engine or a failed portion 
of the propulsion drive system must be provided.
    Section 23.1305 requires instruments for the fuel system, engine 
oil system, fire protection system, and propeller control system. This 
rule is intended for powerplants consisting of a single-engine, 
gearbox, and propeller. To protect the portions of the propulsion drive 
system that are independent of the engines, additional instrumentation, 
which includes oil pressure, oil quantity, oil temperature, propeller 
speed, gearbox torque, and chip detection, is required.

Fire Protection System

    On a conventional twin engine airplane, the engines are 
sufficiently separated to eliminate the possibility of a fire spreading 
from one engine to another. Since the Soloy Dual Pac propulsion system 
is installed in the nose of the airplane, the engines are separated 
only by a firewall. The fire protection system of the Model Pathfinder 
21 airplane must include features to isolate each fire zone from any 
other zone and the airplane in order to maintain isolation of the 
engines during a fire; therefore, these proposed special conditions 
mandate that the firewall required per Sec. 23.1191 be extended to 
provide firewall isolation between either engine and the propulsion 
drive system. These proposed special conditions require that heat 
radiating from a fire originating in any fire zone must not affect 
components in adjacent compartments in such a way as to endanger the 
airplane.

Airplane Performance

    Section 23.67, and paragraphs in Sec. 23.53, Sec. 23.69 and 
Sec. 23.75, provide performance requirements for multiengine airplanes 
with one engine inoperative. These rules are not adequate for 
multiengine, single propeller airplanes. In these proposed special 
conditions, the airplane configuration requirements specified in 
Sec. 23.53(b)(1), Sec. 23.67(c)(1), Sec. 23.69(b), and Sec. 23.75(g) 
have been adapted to accommodate the propeller system of the Model 
Pathfinder 21 airplane to ensure a level of safety equivalent to that 
of conventional multiengine airplanes.

Airspeed Indicator

    Section 23.1545(b)(5) provides one-engine-inoperative marking 
requirements for the airspeed indicator. This rule is not adequate to 
address critical propeller control system failures on the Model 
Pathfinder 21 airplane. As a result, these proposed special conditions 
require that the airspeed markings required by Sec. 23.1545(b)(5) be 
based on the most critical flight condition between one engine 
inoperative or a failed propeller control system in order to ensure a 
level of safety equivalent to that of conventional multiengine 
airplanes.

Airplane Flight Manual

    Sections 23.1585 and 23.1587 require pertinent information to be 
included in the Airplane Flight Manual (AFM). These rules are not 
adequate to address critical propeller control system failures on the 
Model Pathfinder 21 airplane. As a result, these proposed special 
conditions require that the critical procedures and information 
required by Sec. 23.1585, paragraph (c), and Sec. 23.1587, paragraphs 
(c)(2) and (c)(4), include consideration of these critical propeller 
control system failures in order to ensure a level of safety equivalent 
to that of conventional multiengine airplanes.

Applicability

    As discussed above, these special conditions are applicable to the 
Model Pathfinder 21 airplane. Should Soloy Corporation apply at a later 
date for an STC to modify any other model included on TC No. A37CE to 
incorporate the same novel or unusual design feature, the special 
conditions

[[Page 14407]]

would apply to that model as well under the provisions of 
Sec. 21.101(a)(1).

Conclusion

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

List of Subjects in 14 CFR Part 23

    Aircraft, Aviation safety, Signs and symbols.

Citation

    The authority citation for these special conditions is as follows:

    Authority: 49 U.S.C. 106(g), 40113 and 44701; 14 CFR 21.16 and 
21.101 and 14 CFR 11.28 and 11.29(b).

The Proposed Special Conditions

    Accordingly, the Federal Aviation Administration (FAA) proposes the 
following special conditions as part of the type certification basis 
for the Soloy Corporation Model Pathfinder 21 airplane modified by 
Soloy Corporation.
    (a) Propulsion System.
    (1) Engine Requirements. The propulsion system must comply with the 
Soloy Corporation Soloy Dual Pac Engine Special Conditions (Docket No. 
93-ANE-14; No. 33-ANE-01), published in Federal Register, Volume 62, 
Number 33, dated February 19, 1997.
    (2) Engine Rotor Failure. In addition to showing compliance with 
Sec. 23.903(b)(1) (Amendment 23-40), compliance must be shown with the 
following:
    (i) The engine type to be installed must be shown to have 
demonstrated a minimum of ten million hours of actual service 
experience in installations of equivalent or higher disk rotation 
loading without an uncontained high energy rotor failure; and, a shield 
capable of preventing all fragments of an energy level that have been 
released during uncontained engine failures experienced in service from 
impacting the adjacent engine must be installed; and,
    (ii) It must be shown that the adjacent engine is not affected 
following any expected engine failure.
    (3) Engine Case Burn-Through. In addition to showing compliance 
with Sec. 23.903(b)(1) (Amendment 23-40), the engine type to be 
installed must be shown to have demonstrated a minimum of ten million 
hours of actual service experience in installations of equivalent or 
higher combustor pressures and temperatures without an engine case 
burn-through event; or a firewall capable of containing a fire 
originating in the engine that burns through the engine case must be 
installed between the engines.
    (4) Propulsion System Function and Reliability Testing. The 
applicant must complete the testing required by Sec. 21.35(f)(1) 
(Amendment 21-51).
    (b) Propeller Installation.
    (1) The applicant must complete a 2,500 airplane cycle evaluation 
of the propeller installation. This evaluation may be accomplished on 
the airplane in a combination of ground and flight cycles or on a 
ground test facility. If the testing is accomplished on a ground test 
facility, the test configuration must include sufficient interfacing 
system hardware to simulate the actual airplane installation, including 
the engines, propulsion drive system, and mount system.
    (2) Critical Parts.
    (i) The applicant must define the critical parts of the propeller 
assembly. Critical parts are those parts whose failure during ground or 
flight operation could cause a catastrophic effect to the airplane, 
including loss of the ability to produce controllable thrust. In 
addition, parts, of which failure or probably combinations of failures 
would result in a propeller unbalance greater than that defined under 
paragraph (b)(3), are classified as critical parts.
    (ii) The applicant must develop and implement a plan to ensure that 
the critical parts identified in paragraph (b)(2)(i) are controlled 
during design, manufacture, and throughout their service life so that 
the risk of failure in service is minimized.
    (3) Propeller Unbalance. The applicant must define the maximum 
allowable propeller unbalance that will not cause damage to the 
engines, propulsion drive system, engine mounts, primary airframe 
structure, or to critical equipment that would jeopardize the continued 
safe flight and landing of the airplane. Furthermore, the degree of 
flight deck vibration caused by this unbalance condition must not 
jeopardize the crew's ability to continue to operate the airplane in a 
safe manner.
    (c) Propeller Control System.
    (1) The propeller control system must be independent of the turbine 
engines such that a failure in either turbine engine or an engine 
control system will not result in loss of propeller control.
    (2) The propeller control system must be designed so that the 
occurrence of any single failure or probable combination of failures in 
the system which would prevent the propulsion system from producing 
thrust at a level required to meet Sec. 23.53(b)(1)(ii) (Amendment 23-
34) and Sec. 23.67(c) (Amendment 23-42) is extremely improbable.
    (3) The propeller control system must be designed to implement a 
default fixed-propeller pitch position in the event of a propeller 
control system failure.
    (i) An automatic or manual pitch change to the default fixed-pitch 
position must not exceed any limitation established as part of the 
engine and propeller type certificates;
    (ii) A means, independent of the primary propeller control system, 
to manually select and deselect this position in flight must be 
provided and designed to prevent inadvertent operation; and
    (iii) A means to indicate to the flightcrew when the propeller is 
at the default fixed-pitch position must be provided.
    (d) Propulsion Instrumentation.
    (1) Engine Failure Indication. A positive means must be provided to 
indicate when an engine is no longer able to provide torque to the 
propeller. This means may consist of instrumentation required by other 
sections of part 23 or these special conditions if it is determined 
that those instruments will readily alert the flightcrew when an engine 
is no longer able to provide torque to the propeller.
    (2) Propulsion Drive System Instrumentation. In addition to the 
requirements of Sec. 23.1305 (Amendment 23-52), the following 
instruments must be provided for any power gearbox or transmission:
    (i) An oil pressure warning means and indicator for each pressure-
lubricated gearbox;
    (ii) A low oil quantity indicator for each gearbox, if lubricant is 
self-contained;
    (iii) An oil temperature indicator;
    (iv) A tachometer for the propeller;
    (v) A torquemeter for the transmission driving a propeller shaft if 
the sum of the maximum torque that each engine is capable of producing 
exceeds the maximum torque for which the propulsion system has been 
certified under 14 CFR part 33; and
    (vi) A chip detecting and indicating system for each gearbox.
    (e) Fire Protection System.
    (1) In addition to Sec. 23.1191(a) and (b) (not amended),
    (i) Each engine must be isolated from the other engine and the 
propulsion drive system by firewalls, shrouds, or equivalent means; and
    (ii) Each firewall or shroud, including applicable portions of the 
engine cowling, must be constructed so that no

[[Page 14408]]

hazardous quantity of liquid, gas, or flame can pass from the isolated 
compartment to the other engine and the propulsion drive system and so 
that firewall temperatures under all normal or failure conditions would 
not result in auto-ignition of flammable fluids and vapors present in 
the other engine and the propulsion drive system.
    (2) Components, lines, and fittings located in the engine and 
propulsion drive system compartments must be constructed of such 
materials and located at such distances from the firewall that they 
will not suffer damage sufficient to endanger the airplane if a fire is 
present in an adjacent engine compartment.
    (f) Airplane Performance.
    (1) In addition to Sec. 23.53(b)(1) (Amendment 23-34), the 
airplane, upon reaching a height of 50 feet above the takeoff surface 
level, must have reached a speed of not less than 1.3 VS1, 
or any lesser speed, not less than VX plus 4 knots, that is 
shown to be safe under all conditions, including turbulence and the 
propeller control system failed in any configuration that is not 
extremely improbable.
    (2) In lieu of Sec. 23.67(c)(1) (Amendment 23-42), the steady climb 
gradient must be determined at each weight, altitude, and ambient 
temperature within the operational limits established by the applicant, 
with the airplane in the following configurations:
    (i) Critical engine inoperative, remaining engine at not more than 
maximum continuous power or thrust, wing flaps in the most favorable 
position, and means for controlling the engine cooling air supply in 
the position used in the engine cooling tests required by Sec. 23.1041 
(Amendment 23-7) through Sec. 23.1045 (Amendment 23-7);
    (ii) Both engine operating normally and the propeller control 
system failed in any configuration that is not extremely improbable, 
the engines at more than maximum continuous power or thrust, wing flaps 
in the most favorable position, and means for controlling the engine 
cooling air supply in the position used in the engine cooling tests 
required by Sec. 23.1041 (Amendment 23-7) through Sec. 23.1045 
(Amendment 23-7).
    (3) Enroute climb/descent.
    (i) Compliance to Sec. 23.69(a) (Amendment 23-50) must be shown.
    (ii) The steady gradient and rate of climb/descent must be 
determined at each weight, altitude, and ambient temperature within the 
operational limits established by the applicant with--
    (A) The critical engine inoperative, the engines at not more than 
maximum continuous power, the wing flaps retracted, and a climb speed 
not less than 1.2 VS1.
    (B) Both engines operating normally and the propeller control 
system failed in any configuration that is not extremely improbable, 
the engines at not more than maximum continuous power, the wing flaps 
retracted, and a climb speed not less than 1.2 VS1.
    (4) In addition to Sec. 23.75 (Amendment 23-42), the horizontal 
distance necessary to land and come to a complete stop from a point 50 
feet above the landing surface must be determined as required in 
Sec. 23.75 (Amendment 23-42) with both engines operating normally and 
the propeller control system failed in any configuration that is not 
extremely improbable.
    (g) Airspeed Indicator. In lieu of the requirements of 
Sec. 23.1545(b)(5) (Amendment 23-23), for one-engine inoperative or the 
propeller control system failed in any configuration that is not 
extremely improbable, whichever is most critical, the best rate of 
climb speed VY, must be identified with a blue sector 
extending from the VY speed at sea level to the 
VY speed at an altitude of 5,000 feet, if VV is 
less than 100 feet per minute, or the highest 1,000-foot altitude (at 
or above 5,000 feet) at which the VY is 100 feet per minute 
or more. Each side of the sector must be labeled to show the altitude 
for the corresponding VY.
    (h) Airplane Flight Manual.
    (1) In addition to the requirements of Sec. 23.1585(c) (Amendment 
23-34), the following information must be included in the Airplane 
Flight Manual (AFM):
    (i) Procedures for maintaining or recovering control of the 
airplane at speeds above and below VS1 with the propeller 
control system failed in any configuration that is not extremely 
improbable.
    (ii) Procedures for making a landing with the propeller control 
system failed in any configuration that is not extremely improbable and 
procedures for making a go-around with the propeller control system 
failed in any configuration that is not extremely improbable, if this 
latter maneuver can be performed safely; otherwise, a warning against 
attempting the maneuver.
    (iii) Procedures for obtaining the best performance with the 
propeller control system failed in any configuration that is not 
extremely improbable, including the effects of the airplane 
configuration.
    (2) In lieu of the requirements of Sec. 23.1587(c)(2) and (c)(4) 
(Amendment 23-39), the following information must be furnished in the 
Airplane Flight Manual:
    (i) The best rate-of-climb speed or the minimum rate-of-descent 
speed with one engine inoperative or the propeller control system 
failed in any configuration that is not extremely improbable, whichever 
is more critical.
    (ii) The steady rate or gradient of climb determined in paragraph 
(f)(2)(i) or paragraph (f)(2)(ii) of these special conditions, 
whichever is more critical, and the airspeed, power, and airplane 
configuration.
    (3) The steady rate and gradient of climb determined in paragraph 
(f)(3) of these special conditions must be furnished in the Airplane 
Flight Manual.
    (4) The landing distance determined under Sec. 23.75 (Amendment 23-
42) or in paragraph (f)(4) of these proposed special conditions 
whichever is more critical.

    Issued in Kansas City, Missouri on
March 9, 1999.
Marvin Nuss,
Acting Manager, Small Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 99-7276 Filed 3-24-99; 8:45 am]
BILLING CODE 4910-13-M