[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