[Federal Register Volume 64, Number 59 (Monday, March 29, 1999)]
[Rules and Regulations]
[Pages 14818-14820]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-7626]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR part 25

[Docket No. NM153, Special Conditions No. 25-143-SC]


Special Conditions: Learjet Model 35, 35A, 36, 36A Airplanes; 
High Intensity Radiated Fields (HIRF)

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final special conditions; request for comments.

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SUMMARY: These special conditions are issued for Learjet Model 35, 35A, 
36, 36A airplanes modified by Learjet, Inc. These airplanes will have 
novel and unusual design features when compared to the state of 
technology envisioned in the airworthiness standards for transport 
category airplanes. These special conditions contain the additional 
safety standards that the Administrator considers necessary to 
establish a level of safety equivalent to that provided by the existing 
airworthiness standards.

DATES: The effective date of these special conditions is March 19, 
1999; Comments must be received on or before April 28, 1999.

ADDRESSES: Comments on these special conditions may be mailed in 
duplicate to: Federal Aviation Administration, Office of the Regional 
Counsel, Attn: Rules Docket (ANM-7), Docket No. NM153, 1601 Lind Avenue 
SW., Renton, Washington, 98055-4056; or delivered in duplicate to the 
Office of the Regional Counsel at the above address. Comments must be 
marked: Docket No. NM153. 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: Ross Landes, FAA, Standardization 
Branch, ANM-113, Transport Airplane Directorate, Aircraft Certification 
Service, 1601 Lind Avenue SW., Renton, Washington, 98055-4056; 
telephone (425) 227-1071; facsimile (425) 227-1149.

SUPPLEMENTARY INFORMATION:

Comments Invited

    The FAA has determined that good cause exists for making these 
special conditions effective upon issuance; however, interested persons 
are invited to submit such written data, views, or arguments as they 
may desire. Communications should identify the regulatory docket and 
special conditions

[[Page 14819]]

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. These special conditions may 
be changed in light of the comments received. All comments submitted 
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 request must submit with those comments a self-addressed, 
stamped postcard on which the following statement is made: ``Comments 
to Docket No. NM153.'' The postcard will be date stamped and returned 
to the commenter.

Background

    On January 7, 1999, Learjet, Inc. applied for a supplemental type 
certificate (STC) to modify Learjet Model 35, 35A, 36, and 36A 
airplanes listed on Type Certificate A10CE. The modification 
incorporates the installation of dual LITEF LCR-92, Attitude and 
Heading Reference Systems (AHRS) that provide air data input to both 
pilot and copilot flight instruments displaying critical flight 
parameters (attitude) to the flightcrew. The AHRS can be susceptible to 
disruption to both command/response signals as a result of electrical 
and magnetic interference. This disruption of signals could result in 
loss of all critical flight displays and annunciations or present 
misleading information to the pilot.

Type Certification Basis

    Under the provisions of 14 CFR 21.101, Learjet, Inc. must show that 
the Learjet Model 35, 35A, 36, and 36A airplanes, as changed, continue 
to meet the applicable provisions of the regulations incorporated by 
reference in Type Certificate No. A10CE, or the applicable regulations 
in effect on the date of application for the change. The regulations 
incorporated by reference in the type certificate are commonly referred 
to as the ``original type certification basis.'' The certification 
basis for the modified Learjet Model 35, 35A, 36, and 36A airplanes 
includes 14 CFR part 25, dated February 1, 1965, with Amendments 25-1 
through 25-18, as amended by Type Certificate Data Sheet (TCDS) A10CE.
    If the Administrator finds that the applicable airworthiness 
regulations (i.e., part 25, as amended) do not contain adequate or 
appropriate safety standards for the Learjet Model 35, 35A, 36, and 36A 
airplanes because of novel or unusual design features, 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 after public notice, as required by Secs. 11.28 and 11.29, 
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 Learjet, Inc. apply at a later date for a 
design change approval to modify any other model already included on 
the same type certificate to incorporate the same novel or unusual 
design feature, these special conditions would also apply to the other 
model under the provisions of Sec. 21.101(a)(1).

Novel or Unusual Design Features

    The modified Learjet Model 35, 35A, 36, and 36A airplanes will 
incorporate a new dual Attitude and Heading Reference System (AHRS) 
that performs critical functions and was not available at the time of 
certification of these airplanes. This system may be vulnerable to high 
intensity radiated fields (HIRF) external to the airplane.

Discussion

    There is no specific regulation that addresses protection 
requirements for electrical and electronic systems from HIRF. Increased 
power levels from ground-based radio transmitters and the growing use 
of sensitive electrical and electronic systems to command and control 
airplanes have made it necessary to provide adequate protection.
    To ensure that a level of safety is achieved equivalent to that 
intended by the regulations incorporated by reference, special 
conditions are needed for the Learjet Model 35, 35A, 36, and 36A which 
require that new electrical and electronic systems, such as the AHRS, 
that perform critical functions be designed and installed to preclude 
component damage and interruption of function due to both the direct 
and indirect effects of HIRF.

High-Intensity Radiated Fields (HIRF)

    With the trend toward increased power levels from ground-based 
transmitters, plus the advent of space and satellite communications, 
coupled with electronic command and control of the airplane, the 
immunity of critical digital avionics systems to HIRF must be 
established.
    It is not possible to precisely define the HIRF to which the 
airplane will be exposed in service. There is also uncertainty 
concerning the effectiveness of airframe shielding for HIRF. 
Furthermore, coupling of electromagnetic energy to cockpit-installed 
equipment through the cockpit window apertures is undefined. Based on 
surveys and analysis of existing HIRF emitters, an adequate level of 
protection exists when compliance with the HIRF protection special 
condition is shown with either paragraphs 1, or 2 below:
    1. A minimum threat of 100 volts per meter peak electric field 
strength from 10 KHz to 18 GHz.
    a. The threat must be applied to the system elements and their 
associated wiring harnesses without the benefit of airframe shielding.
    b. Demonstration of this level of protection is established through 
system tests and analysis.
    2. A threat external to the airframe of the following field 
strengths for the frequency ranges indicated.

------------------------------------------------------------------------
                                                     Frequency
    Field strength (volts per meter)     -------------------------------
                                               Peak           Average
------------------------------------------------------------------------
10 kHz-100 kHz..........................              50              50
100 kHz-500 kHz.........................              50              50
500 kHz-2 MHz...........................              50              50
2 MHz-30 MHz............................             100             100
30 MHz-70 MHz...........................              50              50
70 MHz-100 MHz..........................              50              50
100 MHz-200 MHz.........................             100             100
200 MHz-400 MHz.........................             100             100
400 MHz-700 MHz.........................             700              50
700 MHz-1 GHz...........................             700             100

[[Page 14820]]

 
1 GHz-2 GHz.............................            2000             200
2 GHz-4 GHz.............................            3000             200
4 GHz-6 GHz.............................            3000             200
6 GHz-8 GHz.............................            1000             200
8 GHz-12 GHz............................            3000             300
12 GHz-18 GHz...........................            2000             200
18 GHz-40 GHz...........................             600             200
------------------------------------------------------------------------
The field strengths are expressed in terms of peak root-mean-square
  (rms) values.

    The threat levels identified above differ from those used in 
earlier special conditions. They are the result of an FAA review of 
existing studies on the subject of HIRF, in light of the ongoing work 
of the Electromagnetic Effects Harmonization Working Group of the 
Aviation Rulemaking Advisory Committee. In general, these standards are 
less critical than the threat level that was previously used as the 
basis for earlier special conditions.

Applicability

    As discussed above, these special conditions are applicable to 
Learjet Model 35, 35A, 36, and 36A airplanes modified by Learjet, Inc. 
Learjet Inc. may apply at a later date for design change approval to 
modify any other model included on the same type certificate to 
incorporate the same novel or unusual design feature, these special 
conditions would apply to that model as well under the provisions of 
Sec. 21.101(a)(1).

Conclusion

    This action affects only certain design features on Learjet Model 
35, 35A, 36, and 36A airplanes modified by Learjet, Inc. It is not a 
rule of general applicability and affects only the applicant who 
applied to the FAA for approval of these features on the airplane.
    The substance of the special conditions for this airplane has been 
subjected to the notice and comment procedure in several prior 
instances and has been derived without substantive change from those 
previously issued. It is unlikely that prior public comment would 
result in a significant change from the substance contained herein. For 
this reason, and because a delay would significantly affect the 
certification of the airplane, which is imminent, the FAA has 
determined that prior public notice and comment are unnecessary and 
impracticable, and good cause exists for adopting these special 
conditions immediately. Therefore, these special conditions are 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 the prior opportunities for comment described above.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

    The authority citation for these special conditions is as follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.

The Special Conditions

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the following special conditions are issued as part of 
the type certification basis for Learjet Model 35, 35A, 36, and 36A 
airplanes modified by Learjet, Inc.
    Protection from Unwanted Effects of High-Intensity Radiated Fields 
(HIRF). Each electrical and electronic system that performs critical 
functions must be designed and installed to ensure that the operation 
and operational capability of these systems to perform critical 
functions are not adversely affected when the airplane is exposed to 
high intensity radiated fields.
    For the purpose of these special conditions, the following 
definition applies:
    Critical Functions. Functions whose failure would contribute to or 
cause a failure condition that would prevent the continued safe flight 
and landing of the airplane.

    Issued in Renton, Washington, on March 19, 1999.
John J. Hickey,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service, ANM-100.
[FR Doc. 99-7626 Filed 3-26-99; 8:45 am]
BILLING CODE 4910-13-P