Special Conditions: Learjet Inc., Model 45 Airplane, Lightning and High-Intensity Radiated Fields

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM-95; Notice No. SC-94-2-NM]

Special Conditions: Learjet Inc., Model 45 Airplane, Lightning

and High-Intensity Radiated Fields

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of proposed special conditions.

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SUMMARY: This document proposes special conditions for the Learjet Inc.

(Lear) Model 45 airplane. This new airplane will utilize new avionics/

electronic systems that provide critical data to the flightcrew. The

applicable regulations do not contain adequate or appropriate safety

standards for the protection of these systems from the effects of

lightning and high-intensity radiated fields. 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.

DATES: Comments must be received on or before June 17, 1994.

ADDRESSES: Comments on this proposal may be mailed in duplicate to:

Federal Aviation Administration, Office of the Assistant Chief Counsel,

Attn: Rules Docket (ANM-7), Docket No. NM-95, 1601 Lind Avenue SW.,

Renton, Washington, 98055-4056; or delivered in duplicate to the Office

of the Assistant Chief Counsel at the above address. Comments must be

marked: Docket No. NM-95. Comments may be inspected in the Rules Docket

weekdays, except Federal holidays, between 7:30 a.m. and 4 p.m.

FOR FURTHER INFORMATION CONTACT: Mark Quam, FAA, Standardization

Branch, ANM-113, Transport Airplane Directorate, Aircraft Certification

Service, 1601 Lind Avenue SW., Renton, Washington, 98055-4056, or

telephone (206) 227-2145.

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

before further rulemaking action is taken on these proposals. The

proposals contained in this notice may be changed in light of 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 notice must submit with those

comments a self-addressed, stamped postcard on which the following

statement is made: ``Comments to Docket No. NM-95.'' The postcard will

be date stamped and returned to the commenter.

Background

On January 27, 1992, Learjet Inc. (Lear), 8220 West Harry Street,

Wichita, KS 67209-2942, P.O. Box 7707, Wichita, KS 67277-7707, applied

for a new type certificate in the transport airplane category for the

Model 45 airplane. The Learjet Model 45 is a T-tail, low wing, medium

sized business jet powered by two Garrett TFE 731-20 turbofan engines

mounted on pylons extending from the aft fuselage. Each engine will be

capable of delivering 3,500 lbs. thrust (3,650 lbs. thrust with auto

performance reserve). The airplane will be capable of operating with

two flight crewmembers and a maximum of ten passengers (standard is

eight passengers).

Type Certification Basis

Under the provisions of Sec. 21.17 of the FAR, Learjet must show,

except as provided in Sec. 25.2, that the Model 45 meets the applicable

provisions of part 25, effective February 1, 1965, as amended by

Amendments 25-1 through 25-75. In addition, the proposed certification

basis for the Model 45 includes part 34, effective September 10, 1990,

plus any amendments in effect at the time of certification; and part

36, effective December 1, 1969, as amended by Amendments 36-1 through

the amendment in effect at the time of certification. The special

conditions that may be developed as a result of this notice will form

an additional part of the type certification basis. In addition, the

certification basis may include other special conditions that are not

relevant to these proposed special conditions.

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 45 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 of the FAR after public notice, as required by Secs. 11.28

and 11.29, and become part of the type certification basis in

accordance with Sec. 21.17(a)(2).

Special conditions are initially applicable to the model for which

they are issued. Should the type certificate for that model be amended

later to include any other model that incorporates the same novel or

unusual design feature, or should any other model already included on

the same type certificate be modified 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).

Novel or Unusual Design Features

The Model 45 incorporates new avionic/electronic installations,

including primary flight displays, an electronically controlled braking

system, digital electronic engine controls, an engine indication and

crew alerting system (EICAS), multifunction display, and a digital

autopilot/flight director system. These systems may be vulnerable to

lightning and high-intensity radiated fields external to the airplane.

Discussion

The existing lightning protection airworthiness certification

requirements are insufficient to provide an acceptable level of safety

with new technology avionic and electronic systems. There are two

regulations that specifically pertain to lightning protection: one for

the airframe in general (Sec. 25.581), and the other for fuel system

protection (Sec. 25.954). There are, however, no regulations that deal

specifically with protection of electrical and electronic systems from

lightning. The loss of a critical function of these systems due to

lightning would prevent continued safe flight and landing of the

airplane. Although the loss of an essential function would not prevent

continued safe flight and landing, it could significantly impact the

safety level of the airplane.

There is also 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 proposed for the Lear Model 45, which would require that

new technology electrical and electronic systems be designed and

installed to preclude component damage and interruption of function due

to both the direct and indirect effects of lightning and HIRF.

Lightning

To provide a means of compliance with these proposed special

conditions, clarification of the threat definition of lightning is

needed. The following ``threat definition,'' based on FAA Advisory

Circular 20-136, Protection of Aircraft Electrical/Electronics Systems

Against the Indirect Effects of Lightning, dated March 5, 1990, is

proposed as a basis to use in demonstrating compliance with the

lightning protection special condition, with the exception of the

multiple burst environment, which has been changed to agree with the

latest recommendation from the Society of Automotive Engineers (SAE)

AE4L lightning committee.

The lightning current waveforms (Components A, D, and H) defined

below, along with the voltage waveforms in AC 20-53A, will provide a

consistent and reasonable standard that is acceptable for use in

evaluating the effects of lightning on the airplane. These waveforms

depict threats that are external to the airplane. The effect of these

threats on the airplane and its systems depends upon several factors,

including installation configuration, materials, shielding, airplane

geometry, etc. Therefore, test (including tests on the completed

airplane or an adequate simulation) and/or verified analyses need to be

conducted in order to obtain the resultant internal threat to the

installed systems. The electronic systems may then be evaluated with

this internal threat in order to determine their susceptibility to

upset and/or malfunction.

To evaluate the induced effects to these systems, three

considerations are required:

1. First Return Stroke: (Severe Strike--Component A, or Restrike-

Component D). This external threat needs to be evaluated to obtain the

resultant internal threat and to verify that the level of the induced

currents and voltages is sufficiently below the equipment ``hardness''

level.

2. Multiple Stroke Flash: (\1/2\ Component D). A lightning strike

is often composed of a number of successive strokes, referred to as

multiple strokes. Although multiple strokes are not necessarily a

salient factor in a damage assessment, they can be the primary factor

in a system upset analysis. Multiple strokes can induce a sequence of

transients over an extended period of time. While a single event upset

of input/output signals may not affect system performance, multiple

signal upsets over an extended period of time (2 seconds) may affect

the systems under consideration. Repetitive pulse testing and/or

analysis needs to be carried out in response to the multiple stroke

environment to demonstrate that the system response meets the safety

objective. This external multiple stroke environment consists of 24

pulses and is described as a single Component A followed by 23 randomly

spaced restrikes of \1/2\ magnitude of Component D (peak amplitude of

50,000 amps). The 23 restrikes are distributed over a period of up to 2

seconds according to the following constraints: (1) The minimum time

between subsequent strokes is 10ms, and (2) the maximum time between

subsequent strokes is 200ms. An analysis or test needs to be

accomplished in order to obtain the resultant internal threat

environment for the system under evaluation.

3. Multiple Burst: (Component H). In-flight data gathering projects

have shown bursts of multiple, low amplitude, fast rates of rise, short

duration pulses accompanying the airplane lightning strike process.

While insufficient energy exists in these pulses to cause physical

damage, it is possible that transients resulting from this environment

may cause upset to some digital processing systems.

The representation of this interference environment is a repetition

of short duration, low amplitude, high peak rate of rise, double

exponential pulses that represent the multiple bursts of current pulses

observed in these flight data gathering projects. This component is

intended for an analytical (or test) assessment of functional upset of

the system. Again, it is necessary that this component be translated

into an internal environmental threat in order to be used. This

``Multiple Burst'' consists of repetitive Component H waveforms in 3

sets of 20 pulses each. The minimum time between individual Component H

pulses within a burst is 50 microseconds, the maximum is 1,000

microseconds. The 3 bursts are distributed according to the following

constraints: (1) The minimum period between bursts is 30ms, and (2) the

maximum period between bursts is 300ms. The individual ``Multiple

Burst'' Component H waveform is defined below.

The following current waveforms constitute the ``Severe Strike''

(Component A), ``Restrike'' (Component D), ``Multiple Stroke'' (\1/2\

Component D), and the ``Multiple Burst'' (Component H).

These components are defined by the following double exponential

equation:

i(t)=Io(e-at-e-bt)

where:

t = time in seconds,

i = current in amperes, and

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Severe strike Multiple stroke (\1/2\ Multiple burst

(component A) Restrike (component D) component D) (component H)

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Io, amp......................................... = 218,810 109,405 54,703 10,572

a, sec-1........................................ = 11,354 22,708 22,708 187,191

b, sec-1........................................ = 647,265 1,294,530 1,294,530 19,105,100

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This equation produces the following characteristics:

ipeak = 200 KA 100 KA 50 KA 10 KA

and,

(di/dt)max(amp/sec) = 1.4 x 1011 1.4 x 1011 0.7 x 1011 2.0 x 1011

= @t=0+sec @t=0+sec @t=0+sec @t=0+sec

(di/dt), (amp/sec) = 1.0 x 1011 1.0 x 1011 0.5 x 1011 ................

= @t=.5s @t=.25s @t=.25s ................

Action Integral (amp2 sec) = 2.0 x 106 0.25 x 106 0.0625 x 106 ................

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.

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Average (V/

Frequency Peak (V/M) M)

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

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

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

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

30 MHz-70 MHz................................. 30 30

70 MHz-100 MHz................................ 30 30

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

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

400 MHz-700 MHz............................... 4,020 935

700 MHz-1000 MHz.............................. 1,700 170

1 GHz-2 GHz................................... 5,000 990

2 GHz-4 GHz................................... 6,680 840

4 GHz-6 GHz................................... 6,850 310

6 GHz-8 GHz................................... 3,600 670

8 GHz-12 GHz.................................. 3,500 1,270

12 GHz-18 GHz................................. 3,500 360

18 GHz-40 GHz................................. 2,100 750

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The envelope given in paragraph 2 above is a revision to the

envelope used in previously issued special conditions in other

certification projects. It is based on new data and SAE AE4R

subcommittee recommendations. This revised envelope includes data from

Western Europe and the U.S.

As discussed above, the proposed special conditions would be

applicable initially to the Learjet Model 45. Should Learjet apply at a

later date for a change to the type certificate to include another

model incorporating 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 the Learject

Model 45 airplane. It is not a rule of general applicability and

affects only the manufacturer who applied to the FAA for approval of

these features on the airplane.

List of Subjects in 14 CFR Part 25

Aircraft, Aviation safety, Federal Aviation Administration,

Reporting and recordkeeping requirements.

The authority citation for these proposed special conditions is as

follows:

Authority: 49 U.S.C. app. 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; and 49 U.S.C. 106(g).

The Proposed Special Conditions

Accordingly, the Federal Aviation Administration (FAA) proposes the

following special conditions as part of the type certification basis

for the Learjet Model 45 series airplanes.

1. Lightning Protection: (a) 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 lightning.

(b) Each essential function of electrical or electronic systems or

installations must be protected to ensure that the function can be

recovered in a timely manner after the airplane has been exposed to

lightning.

2. 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.

3. For the purpose of these special conditions, the following

definitions apply:

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.

Essential Functions. Functions whose failure could contribute to or

cause a failure condition that would significantly impact the safety of

the airplane or the ability of the flightcrew to cope with adverse

operating conditions.

Issued in Renton, Washington, on April 12, 1994.

Darrell M. Pederson,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service, ANM-100.

[FR Doc. 94-10381 Filed 5-2-94; 8:45 am]

BILLING CODE 4910-13-M