[Federal Register Volume 59, Number 175 (Monday, September 12, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-22018]


[[Page Unknown]]

[Federal Register: September 12, 1994]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM-95; Special Conditions No. 25-ANM-88]

 

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

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions are issued for the Learjet Inc. 
(Lear) Model 45 airplane. This new airplane utilizes 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 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.

EFFECTIVE DATE: October 12, 1994.

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, (206) 227-2145.

SUPPLEMENTARY INFORMATION: 

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 is 
capable of delivering 3,500 lbs. thrust (3,650 lbs. thrust with auto 
performance reserve). The airplane is 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. These special 
conditions 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 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 (HIRF) 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 protecton 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 needed for the Lear Model 45, which 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 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/Electronic 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, tests (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 Stroke'' (\1/2\ Component D), and the ``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 in 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

----------------------------------------------------------------------------------------------------------------
                                                                               Multiple                         
                                   Severe          Restrike(component D)      stroke(\1/           Multiple     
                            strike(component A)                             2\component D)    burst(component H)
----------------------------------------------------------------------------------------------------------------
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         
This equation                                                                                                   
 produces the                                                                                                   
 following                                                                                                      
 characteristics:                                                                                               
    ipeak...........   =   200 KA                 100 KA                  50 KA              10 KA              
and,                                                                                                            
    (di/dt)max(amp/s   =   1.4 x 1011             1.4 x 1011              0.7 x 1011         2.0 x 1011         
     ec).                                                                                                       
                       =   @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    =   2.0 x 106              0.25 x 106              0.0625 x 106                          
     (amp2 sec).                                                                                                
----------------------------------------------------------------------------------------------------------------

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. 

------------------------------------------------------------------------
                                                       Peak(V/   Average
                     Frequency                           M)       (V/M) 
------------------------------------------------------------------------
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       900
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
------------------------------------------------------------------------

    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, these special conditions are applicable 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).

Discussion of Comments

    Notice of proposed special conditions No. SC-94-2-NM for the 
Learjet Model 45 airplane was published in the Federal Register on May 
3, 1994 (59 FR 22766). No comments were received, and the special 
conditions are adopted as proposed.

Conclusion

    This action affects only certain design features on the Learjet 
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, Reporting and recordkeeping 
requirements.

    The authority citation for these 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 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 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 August 23, 1994.
Norman B. Martenson,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service, ANM-100.
[FR Doc. 94-22018 Filed 9-9-94; 8:45 am]
BILLING CODE 4910-13-M