Special Condition: Sikorsky Model S76C Helicopter, Electronic Flight Instrument System

DEPARTMENT OF TRANSPORTATION

14 CFR Part 29

[Docket No. 94-ASW-1; Special Condition 29-ASW-13]

Special Condition: Sikorsky Model S76C Helicopter, Electronic

Flight Instrument System

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final special condition; request for comments.

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SUMMARY: This special condition is issued for the Sikorsky Model S76C

helicopter modified by Sikorsky Aircraft, a Division of United

Technologies Corporation. This helicopter will have a novel or unusual

design feature associated with the Electronic Flight Instrument System.

The applicable airworthiness regulations do not contain adequate or

appropriate safety standards for the protection of these critical

function systems from the effects of external high intensity radiated

fields (HIRF). This special condition contains additional safety

standards that the Administrator considers necessary to establish a

level of safety equivalent to that provided by the applicable

airworthiness standards.

DATES: The effective date of this special condition is May 2, 1994.

Comments must be received on or before June 1, 1994.

ADDRESSES: Comments may be mailed in duplicate to: Federal Aviation

Administration (FAA), Office of the Assistant Chief Counsel, Attn:

Rules Docket No. 94-ASW-1, Fort Worth, Texas 76193-0007, or delivered

in duplicate to the Office of the Assistant Chief Counsel, 2601 Meacham

Blvd., room 663, Fort Worth, Texas 76137.

Comments must be marked Docket No. 94-ASW-1. Comments may be

inspected in the Rules Docket weekdays, except Federal holidays,

between 9 a.m. and 3 p.m.

FOR FURTHER INFORMATION CONTACT:

Mr. Robert McCallister, FAA, Rotorcraft Directorate, Policy and

Procedures Group, Fort Worth, Texas 76193-0112; telephone (817) 222-

5121.

SUPPLEMENTARY INFORMATION: The FAA has determined that notice and

opportunity for prior public comment hereon are impracticable because

these procedures would significantly delay issuance of the approval

design and thus delay delivery of the affected helicopter. These notice

and comment procedures are also considered unnecessary since the public

has been previously provided with a substantial number of opportunities

to comment on substantially identical special conditions and their

comments have been fully considered. Therefore, good cause exists for

making this special condition effective upon issuance.

Comments Invited

Although this final special condition was not subject to notice and

opportunity for prior public comment, comments are invited on this

final special condition. Interested persons are invited to comment on

this final special condition by submitting such written data, views, or

arguments as they may desire. Communications should identify the

regulatory docket number and be submitted in duplicate to the address

specified under the caption ADDRESSES. All communications received on

or before the closing date for comments will be considered. This

special condition may be changed in light of 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

submitted in response to this notice must submit with those comments a

self-addressed, stamped postcard on which the following statement is

made: ``Comments to Docket No. 94-ASW-1.'' The postcard will be date

and time stamped and returned to the commenter.

Background

On October 12, 1993, Sikorsky Aircraft, West Palm Beach, Florida,

applied for a Supplemental Type Certificate for installation of an

Electronic Flight Instrument System in a Sikorsky Aircraft Model S76C

helicopter. This is a 13 passenger, twin engine, 11,400 pound transport

category helicopter.

Type Certification Basis

The certification basis established for the Sikorsky Model S76C

helicopter includes: 14 CFR part 29 (part 29) effective February 1,

1965, Amendments 29-1 through 29-11; in addition, portions of

Amendments 29-12, specifically, Secs. 29.67, 29.71, 29.75, 29.141,

29.173, 29.175, 29.931, 29.1189(a)(2), 29.1555(c)(2), 29.1557(c);

portions of Amendment 29-13, specifically Sec. 29.965; Sec. 29.1325 of

Amendment 29-24; Sec. 29.811 of Amendment 29-30; Amendment 36-14 of

part 36, appendix H; instrument flight criteria for S-76 (interim)

dated February 10, 1977; Special Conditions 29-82-NE-3 (Docket No.

17721) dated March 27, 1978; Equivalent Safety Finding for

Sec. 29.173(b), National Environmental Act of 1969; Noise Control Act

of 1972; Sec. 29.865 including Sec. 29.25 of Amendment 29-12, when

cargo hook system, P/N 76255-02000, is installed; and for external load

operations, part 133, including Amendments 1-4. Compliance with the

following optional requirements has been established: Ditching

provisions Sec. 29.563 including Secs. 29.801 and 29.807(d) and

excluding Secs. 29.1411, 29.1415, and 29.1561 of Amendment 29-12, when

emergency flotation gear, P/N 76076-02002, is installed. For over-water

operations, compliance with the operating rules and Secs. 29.1411,

29.1415, and 29.1561 must be shown.

If the Administrator finds that the applicable airworthiness

regulations do not contain adequate or appropriate safety standards for

this helicopter because of a novel or unusual design feature, special

conditions are prescribed under the provisions of Sec. 21.16 to

establish a level of safety equivalent to that established in the

regulations.

Special conditions, as appropriate, are issued in accordance with

Sec. 11.49 and become part of the type certification basis in

accordance with Sec. 21.101(b)(2). Provision is made for the public

comment period in Sec. 11.28.

Discussion

The Sikorsky Model S76C helicopter, at the time of the application

for modification by Sikorsky Aircraft, was identified as having

modifications that incorporate one and possibly more electrical,

electronic, or combination of electrical and electronic (electrical/

electronic) systems that will perform functions critical to the

continued safe flight and landing of the helicopters. The electronic

flight instrument system performs the attitude display function. The

display of attitude, altitude, and airspeed is critical to the

continued safe flight and landing of the helicopters for IFR operations

in instrument meteorological conditions. After the design is finalized,

Sikorsky Aircraft will provide the FAA with a preliminary hazard

analysis that will identify any other critical functions performed by

the electrical/electronic systems that are critical to the continued

safe flight and landing of the helicopters.

Recent advances in technology have prompted the design of aircraft

that include advanced electrical and electronic systems that perform

functions required for continued safe flight and landing. However,

these advanced systems respond to the transient effects of induced

electrical current and voltage caused by the high intensity radiated

fields (HIRF) incident on the external surface of the helicopters.

These induced transient currents and voltages can degrade the

performance of the electrical/electronic systems by damaging the

components or by upsetting the systems' functions.

Furthermore, the electromagnetic environment has undergone a

transformation not envisioned by the current application of

Sec. 29.1309(a). Higher energy levels radiate from operational

transmitters currently used for radar, radio, and television; the

number of transmitters has increased significantly.

Existing aircraft certification requirements are inappropriate in

view of these technological advances. In addition, the FAA has received

reports of some significant safety incidents and accidents involving

military aircraft equipped with advanced electrical/electronic systems

when they were exposed to electromagnetic radiation.

The combined effects of technological advances in helicopter design

and the changing environment have resulted in an increased level of

vulnerability of the electrical and electronic systems required for the

continued safe flight and landing of the helicopters. Effective

measures to protect these helicopters against the adverse effects of

exposure to HIRF will be provided by the design and installation of

these systems. The following primary factors contributed to the current

conditions:

(1) Increased use of sensitive electronics that perform critical

functions, (2) reduced electromagnetic shielding afforded helicopter

systems by advanced technology airframe materials, (3) adverse service

experience of military aircraft using these technologies, and (4) an

increase in the number and power of radio frequency emitters and the

expected increase in the future.

The FAA recognizes the need for aircraft certification standards to

keep pace with technological developments and a changing environment

and in 1986 initiated a high priority program to (1) determine and

define electromagnetic energy levels; (2) develop guidance material for

design, test, and analysis; and (3) prescribe and promulgate regulatory

standards

The FAA participated with industry and airworthiness authorities of

other countries to develop internationally recognized standards for

certification.

The FAA and airworthiness authorities of other countries have

identified a level of HIRF environment that a helicopter could be

exposed to during IFR operations. While the HIRF requirements are being

finalized, the FAA is adopting a special condition for the

certification of aircraft that employ electrical/electronic systems

that perform critical functions. The accepted maximum energy levels

that civilian helicopter system installations must withstand for safe

operation are based on surveys and analysis of existing radio frequency

emitters. This special condition will require the helicopters'

electrical/electronic systems and associated wiring to be protected

from these energy levels. These external threat levels are believed to

represent the worst-case exposure for a helicopter operating under IFR.

The HIRF environment specified in this special condition is based

on many critical assumptions. With the exception of takeoff and landing

at an airport, one of these assumptions is that the aircraft would be

not less than 500 feet above ground level (AGL). Helicopters operating

under visual flight rules (VFR) routinely operate at less than 500 feet

AGL and perform takeoffs and landings at locations other than

controlled airports. Therefore, it would be expected that the HIRF

environment experienced by a helicopter operating VFR may exceed the

defined environment by 100 percent or more.

This special condition will require the systems that perform

critical functions, as installed in the aircraft, to meet certain

standards based on either a defined HIRF environment or a fixed value

using laboratory tests.

The applicant may demonstrate that the operation and operational

capabilities of the installed electrical/electronic systems that

perform critical functions are not adversely affected when the aircraft

is exposed to the defined HIRF environment. The FAA has determined that

the environment defined in Table 1 is acceptable for critical functions

in helicopters operating at or above 500 feet AGL. For critical

functions of helicopters operating at less than 500 feet AGL,

additional factors must be considered.

The applicant may also demonstrate by a laboratory test that the

electrical/electronic systems that perform critical functions can

withstand a peak electromagnetic field strength in a frequency range of

10 KHz to 18 GHz. If a laboratory test is used to show

compliance with the defined HIRF environment, no credit will be given

for signal attenuation due to installation. A level of 100 v/m and

other considerations, such as an alternate technology backup that is

immune to HIRF, are appropriate for critical functions during IFR

operations. A level of 200 v/m and further considerations, such as an

alternate technology backup that is immune to HIRF, are more

appropriate for critical functions during VFR operations. Applicants

must perform a preliminary hazard analysis to identify electrical/

electronic systems that perform critical functions. The term

``critical'' means those functions whose failure would contribute to or

cause a failure condition that would prevent the continued safe flight

and landing of the helicopters. The systems identified by the hazard

analysis as performing critical functions are required to have HIRF

protection.

A system may perform both critical and noncritical functions.

Primary electronic flight display systems and their associated

components perform critical functions such as attitude, altitude, and

airspeed indications. HIRF requirements would apply only to the systems

that perform critical functions.

Compliance with HIRF requirements will be demonstrated by tests,

analysis, models, similarity with existing systems, or a combination of

these methods. The two basic options of either testing the rotorcraft

to the defined environment or laboratory testing may not be combined.

The laboratory test allows some frequency areas to be under tested and

requires other areas to have some safety margin when compared to the

defined environment. The areas required to have some safety margin are

those shown, by past testing, to exhibit greater susceptibility to

adverse effects from HIRF; and laboratory tests, in general, do not

accurately represent the aircraft installation. Service experience

alone will not be acceptable since such experience in normal flight

operations may not include an exposure to HIRF. Reliance on a system

with similar design features for redundancy, as a means of protection

against the effects of external HIRF, is generally insufficient because

all elements of a redundant system are likely to be concurrently

exposed to the radiated fields.

The modulation that represents the signal most likely to disrupt

the operation of the system under test, based on its design

characteristics, should be selected. For example, flight control

systems may be susceptible to 3 Hz square wave modulation while

the video signals for electronic display systems may be susceptible to

400 Hz sinusoidal modulation. If the worst-case modulation is

unknown or cannot be determined, default modulations may be used.

Suggested default values are a 1 KHz sine wave with 80 percent

depth of modulation in the frequency range from 10 KHz to 400

MHz and 1 KHz square wave with greater than 90 percent depth

of modulation from 400 MHz to 18 GHz. For frequencies where

the unmodulated signal would cause deviations from normal operation,

several different modulating signals with various waveforms and

frequencies should be applied.

Acceptable system performance would be attained by demonstrating

that the critical function components of the system under consideration

continue to perform their intended function during and after exposure

to required electromagnetic fields. Deviations from system

specifications may be acceptable but must be independently assessed by

the FAA on a case-by-case basis.

Table 1.--Field Strength Volts/Meter

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Frequency Peak Average

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10-100 KHz.......................................... 50 50

100-500............................................. 60 60

500-2000............................................ 70 70

2-30 MHz............................................ 200 200

30-100.............................................. 30 30

100-200............................................. 150 33

200-400............................................. 70 70

400-700............................................. 4020 935

700-1000............................................ 1700 170

1-2 GHz............................................. 5000 990

2-4................................................. 6680 840

4-6................................................. 6850 310

6-8................................................. 3600 670

8-12................................................ 3500 1270

12-18............................................... 3500 360

18-40............................................... 2100 750

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Conclusion

This action affects only certain unusual or novel design features

on one model of helicopter. It is not a rule of general applicability

and affects only the applicant who applied to the FAA for approval of

these features on the affected helicopter.

The substance of this special condition for similar installations

in a variety of helicopters has been subjected to the notice and

comment procedure and has finalized without substantive change. It is

unlikely that prior public comment would result in a significant change

from the substance contained herein. For this reason, and because a

delay would significantly affect the certification of the helicopter,

which is imminent, the FAA has determined that prior public notice and

comment are unnecessary and impractical, and good cause exists for

adopting this special condition immediately. Therefore, this special

condition is being made effective upon issuance. The FAA is requesting

comments to allow interested persons to submit views that may not have

been submitted in response to prior opportunities for comment.

List of Subjects in 14 CFR Part 29

Aircraft, Air transportation, Aviation safety, Rotorcraft, Safety.

The authority citations for this special condition are as follows:

Authority: 49 U.S.C. 1344, 1348(c), 1352, 1354(a), 1355, 1421

through 1431, 1502, 1651(b)(2); 42 U.S.C. 1857f-10, 4321 et seq.;

E.O. 11514; 49 U.S.C. 106(g).

The Special Condition

Accordingly, pursuant to the authority delegated to me by the

Administrator, the following special condition is issued as part of the

type certification basis for the Sikorsky Model S76C helicopter:

Protection for Electrical and Electronic Systems From High Intensity

Radiated Fields

Each system that performs critical functions must be designed and

installed to ensure that the operation and operational capabilities of

these critical functions are not adversely affected when the helicopter

is exposed to high intensity radiated fields external to the

helicopter.

Issued in Fort Worth, Texas, on April 14, 1994.

Mark R. Schilling,

Acting Manager, Rotorcraft Directorate Aircraft Certification Service.

[FR Doc. 94-10288 Filed 4-29-94; 8:45 am]

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