63718 Federal Register / Vol. 71, No. 210 / Tuesday, October 31, 2006 / Proposed Rules 

DEPARTMENT OF TRANSPORTATION 

Federal Aviation Administration 

14 CFR Part 25 

[Docket No. NM357; Notice No. 25–06–11–

SC] 

Special Conditions: Boeing Model 737–

900ER Series Airplanes; Interaction of 

Systems and Structures 

AGENCY: Federal Aviation 

Administration (FAA), DOT. 

ACTION: Notice of proposed special 

conditions. 

SUMMARY: This action proposes a special 

condition for the Boeing Model 737–

900ER airplane. This airplane will have 

a novel or unusual design feature(s) 

when compared to the state of 

technology envisioned in the 

airworthiness standards for transport 

category airplanes. These design 

features include interaction of systems 

and structures. The applicable 

airworthiness regulations do not contain 

adequate or appropriate safety standards 

for this design feature. The proposed 

special condition contains 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: We must receive your comments 

by November 20, 2006. 

ADDRESSES: You must mail two copies 

of your comments to: Federal Aviation 

Administration, Transport Airplane 

Directorate, Attn: Rules Docket (ANM–

113), Docket No. NM357, 1601 Lind 

Avenue, SW., Renton, Washington, 

98057–3356. You may deliver two 

copies to the Transport Airplane 

Directorate at the above address. You 

must mark your comments: Docket No. 

NM357. You can inspect comments in 

the Rules Docket weekdays, except 

Federal holidays, between 7:30 a.m. and 

4 p.m. 

FOR FURTHER INFORMATION CONTACT: 

Todd Martin, Aerospace Engineer, 

Airframe/Cabin Safety Branch, ANM–

115, Transport Airplane Directorate, 

Aircraft Certification Service, 1601 Lind 

Avenue, SW., Renton, Washington 

98057–3356; telephone (425) 227–1178; 

facsimile (425) 227–1323; electronic 

mail Todd.Martin@faa.gov. 

SUPPLEMENTARY INFORMATION:

Comments Invited 

We invite interested people to take 

part in this rulemaking by sending 

written comments, data, or views. The 

most helpful comments reference a 

specific portion of the special condition, 

explain the reason for any 

recommended change, and include 

supporting data. We ask that you send 

us two copies of written comments. 

We will file in the docket all 

comments we receive, as well as a 

report summarizing each substantive 

public contact with FAA personnel 

concerning this special condition. You 

can inspect the docket before and after 

the comment closing date. If you wish 

to review the docket in person, go to the 

address in the ADDRESSES section of this 

preamble between 7:30 a.m. and 4 p.m., 

Monday through Friday, except Federal 

holidays. 

We will consider all comments we 

receive on or before the closing date for 

comments. We will consider comments 

filed late if it is possible to do so 

without incurring expense or delay. We 

may change this special condition based 

on the comments we receive. 

If you want the FAA to acknowledge 

receipt of your comments on this 

proposal, include with your comments 

a pre-addressed, stamped postcard on 

which the docket number appears. We 

will stamp the date on the postcard and 

mail it back to you. 

Background 

On June 5, 2002, The Boeing 

Company, P.O. Box 3707, Seattle, 

Washington 98124, applied for an 

amendment to Type Certificate No. 

A16WE to include the new Model 737–

900ER. The Model 737–900ER, which is 

a derivative of the Model 737–900 

currently approved under A16WE, is a 

large transport airplane with two flight 

crew and the capacity to carry 215 

passengers. The airplane is powered by 

two CFM 56–7B or CFM–567B/2 series 

turbofan engines. 

Type Certification Basis 

Under the provisions of § 21.101, 

Boeing must show that the Model 737–

900ER meets the applicable provisions 

of 14 CFR part 25, as amended by 

Amendments 25–1 through 25–108, 

except for earlier amendments as agreed 

upon by the FAA. These regulations 

will be incorporated into the Type 

Certificate No. A16WE after type 

certification approval of the 737–900ER. 

In addition, the certification basis 

includes other regulations, special 

conditions and exemptions that are not 

relevant to this proposed special 

condition. Refer to Type Certificate No. 

A16WE for a complete description of 

the certification basis for this model 

airplane. 

If the Administrator finds that the 

applicable airworthiness regulations 

(i.e., 14 CFR part 25) do not contain 

adequate or appropriate safety standards 

for the Model 737–900ER because of a 

novel or unusual design feature, special 

conditions are prescribed under the 

provisions of § 21.16. 

In addition to the applicable 

airworthiness regulations and special 

conditions, the Model 737–900ER must 

comply with the fuel vent and exhaust 

emission requirements of 14 CFR part 

34 and the noise certification 

requirements of 14 CFR part 36. 

The FAA issues special conditions, as 

defined in § 11.19, under § 11.38, and 

they become part of the type 

certification basis under § 21.101. 

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 or similar novel 

or unusual design feature, or should any 

other model already included on the 

same type certificate be modified to 

incorporate the same or similar novel or 

unusual design feature, the special 

conditions would also apply to the other 

model under § 21.101. 

Novel or Unusual Design Features 

Model 737–900ER airplane will 

incorporate novel or unusual design 

features. This proposed special 

condition addresses equipment which 

may affect the airplane’s structural 

performance, either directly or as a 

result of failure or malfunction. 

This proposed special condition is 

identical or nearly identical to those 

previously required for type 

certification of other Boeing airplane 

models. The proposed special condition 

was derived initially from standardized 

requirements developed by the Aviation 

Rulemaking Advisory Committee 

(ARAC), comprised of representatives of 

the FAA, Europe’s Joint Aviation 

Authorities (now replaced by the 

European Aviation Safety Agency), and 

industry. 

Discussion 

In addition to the requirements of part 

25, subparts C and D, the following 

special condition applies. 

Interaction of Systems and Structures 

The Boeing Model 737–900ER is 

equipped with systems that may affect 

the airplane’s structural performance 

either directly or as a result of failure or 

malfunction. The effects of these 

systems on structural performance must 

be considered in the certification 

analysis. This analysis must include 

consideration of normal operation and 

of failure conditions with required 


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structural strength levels related to the 

probability of occurrence. 

Applicability 

As discussed above, this special 

condition is applicable to the Boeing 

Model 737–900ER. Should Boeing apply 

at a later date for a change to the type 

certificate to include another model 

incorporating the same novel or unusual 

design feature, this special condition 

would apply to that model as well. 

Certification of the Model 737–900ER 

is currently scheduled for February 

2007. Because a delay would 

significantly affect the applicant’s 

installation of the systems and 

certification of the airplane we are 

shortening the public comment period 

to 20 days. 

Conclusion 

This action affects only certain novel 

or unusual design features on one model 

of airplane. It is not a rule of general 

applicability. 

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 Proposed Special Condition 

Accordingly, the Federal Aviation 

Administration (FAA) proposes the 

following special condition as part of 

the type certification basis for Boeing 

Model 737–900ER airplanes. 

Interaction of Systems and Structures 

In addition to the requirements of part 

25, subparts C and D, the following 

proposed special condition would 

apply: 

a. For airplanes equipped with 

systems that affect structural 

performance—either directly or as a 

result of a failure or malfunction—the 

influence of these systems and their 

failure conditions must be taken into 

account when showing compliance with 

the requirements of part 25, subparts C 

and D. Paragraph b, below, must be used 

to evaluate the structural performance of 

airplanes equipped with these systems. 

b. Interaction of Systems and 

Structures. 

(1) General: The following criteria 

must be used for showing compliance 

with this special condition for 

interaction of systems and structures 

and with § 25.629 for airplanes 

equipped with flight control systems, 

autopilots, stability augmentation 

systems, load alleviation systems, flutter 

control systems, and fuel management 

systems. 

(a) The criteria defined herein address 

only the direct structural consequences 

of the system responses and 

performances. They cannot be 

considered in isolation but should be 

included in the overall safety evaluation 

of the airplane. These criteria may, in 

some instances, duplicate standards 

already established for this evaluation. 

These criteria are applicable only to 

structures whose failure could prevent 

continued safe flight and landing. 

Specific criteria that define acceptable 

limits on handling characteristics or 

stability requirements when operating 

in the system degraded or inoperative 

modes are not provided in this special 

condition. 

(b) Depending upon the specific 

characteristics of the airplane, 

additional studies may be required that 

go beyond the criteria provided in this 

special condition in order to 

demonstrate the capability of the 

airplane to meet other realistic 

conditions, such as alternative gust or 

maneuver descriptions for an airplane 

equipped with a load alleviation system. 

(c) The following definitions are 

applicable to this paragraph. 

Structural performance: Capability of 

the airplane to meet the structural 

requirements of part 25. 

Flight limitations: Limitations that 

can be applied to the airplane flight 

conditions following an in-flight 

occurrence and that are included in the 

flight manual (e.g., speed limitations 

and avoidance of severe weather 

conditions). 

Operational limitations: Limitations, 

including flight limitations, that can be 

applied to the airplane operating 

conditions before dispatch (e.g., fuel, 

payload, and Master Minimum 

Equipment List limitations). 

Probabilistic terms: The probabilistic 

terms (probable, improbable, and 

extremely improbable) used in this 

special conditions are the same as those 

used in § 25.1309. 

Failure condition: The term failure 

condition is the same as that used in 

§ 25.1309. However, this special 

condition applies only to system failure 

conditions that affect the structural 

performance of the airplane (e.g., system 

failure conditions that include loads, 

change the response of the airplane to 

inputs as gusts or pilot actions, or lower 

flutter margins). 

(2) Effects of Systems on Structures. 

(a) General. The following criteria 

will be used in determining the 

influence of a system and its failure 

conditions on the airplane structure. 

(b) System fully operative. With the 

system fully operative, the following 

apply: 

(1) Limit loads must be derived in all 

normal operating configurations of 

system from all the limit conditions 

specified in subpart C ( or used in lieu 

of those specified in subpart C), taking 

into account any special behavior of 

such a system or associated functions or 

any effect on the structural performance 

of the airplane that may occur up to the 

limit loads. In particular, any significant 

non-linearity (rate of displacement of 

control surface, thresholds or any other 

system non-linearities) must be 

accounted for in a realistic or 

conservative way when deriving limit 

loads from limit conditions. 

(2) The airplane must meet the 

strength requirements of part 25 (static 

strength, residual strength), using the 

specified factors to derive ultimate loads 

from the limit loads defined above. The 

effect of non-linearities must be 

investigated beyond limit conditions to 

ensure that the behavior of the system 

presents no anomaly compared to the 

behavior below limit conditions. 

However, conditions beyond limit 

conditions need not be considered, 

when it can be shown that the airplane 

has design features that will not allow 

it to exceed those limit conditions. 

(3) The airplane must meet the 

aeroelastic stability requirements of 

§ 25.629. 

(c) System in the failure condition. 

For any system failure condition not 

shown to be extremely improbable, the 

following apply: 

(1) At the time of occurrence. Starting 

from 1g level flight conditions, a 

realistic scenario, including pilot 

corrective actions, must be established 

to determine the loads occurring at the 

time of failure and immediately after 

failure. 

(i) For static strength substantiation, 

those loads multiplied by an 

appropriate factor of safety that is 

related to the probability of occurrence 

of the failure are ultimate loads to be 

considered for design. The factor of 

safety (FS) is defined in Figure 1. 


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(ii) For residential strength 

substantiation, the airplane must be able 

to withstand two thirds of the ultimate 

loads defined in paragraph (c)(1)(i) of 

this section. For pressurized cabins, 

these loads must be combined with the 

normal operating differential pressure. 

(iii) Freedom from aeroelastic 

instability must be shown up to the 

speeds defined in § 25.629(b)(2). For 

failure conditions that result in speed 

increases beyond Vc/Mc, freedom from 

aeroelastic instability must be shown to 

those increased speeds, so that the 

margins intended by § 25.629(b)(2) are 

maintained. 

(vi) Failures of the system that result 

in forced structural vibrations 

(oscillatory failures) must not produce 

loads that could result in detrimental 

deformation of primary structure. 

(2) For the continuation of the flight. 

For the airplane in the system failed 

state and considering any appropriate 

reconfiguration and flight limitations, 

the following apply: 

(i) The loads derived from the 

following conditions (or used in lieu of 

the following conditions) at speeds up 

to Vc/Mc or the speed limitation 

prescribed for the remainder of the 

flight must be determined: 

(A) The limit symmetrical 

maneuvering conditions specified in 

§§ 25.331 and 25.345. 

(B) The limit gust and turbulence 

conditions specified in §§ 25.341 and in 

25.345. 

(C) The limit rolling conditions 

specified in § 25.349 and limit 

unsymmetrical conditions specified in 

§§ 25.367 and 25.427(b) and (c). 

(D) The limit yaw maneuvering 

conditions specified in § 25.351. 

(E) The limit ground loading 

conditions specified in §§ 25.473 and 

25.491. 

(ii) For static strength substantiation, 

each part of the structure must be able 

to withstand the loads in paragraph 

(c)(2)(i) of this special condition 

multiplied by a factor of safety, 

depending on the probability of being in 

this failure state. The factor of safety is 

defined in Figure 2. 


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Qj = (Tj) (Pj) where: 

Tj = Average time spent in failure condition 

j (in hours) 

Pj = Probability of occurrence of failure mode 

j (per hour) 

Note: If Pj is greater than 10¥3 per flight 

hour, then a 1.5 factor of safety must be 

applied to all limit load conditions specified 

in subpart C. 

(iii) For residual strength 

substantiation, the airplane must be able 

to withstand two thirds of the ultimate 

loads defined in paragraph (c)(2)(ii). For 

pressurized cabins, these loads must be 

defined combined with the normal 

operating differential pressure. 

(iv) If the loads induced by the failure 

condition have a significant effect on 

fatigue or damage tolerance, then their 

effects must be taken into account. 

(v) Freedom from aeroelastic 

instability must be shown up to a speed 

determined from Figure 3. Flutter 

clearance speeds V′ and V′′ may be 

based on the speed limitation specified 

for the remainder of the flight, using the 

margins defined by § 25.629(b). 


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V′= Clearance speed as defined by 

§ 25.629(b)(2). 

V′′= Clearance speed as defined by 

§ 25.629(b)(1). 

Qj= (Tj)(Pj) where: 

Tj= Average time spent in failure condition 

j (in hours) 

Pj= Probability of occurrence of failure mode 

j (per hour) 

Note: If Pj is greater than 10¥3 per flight 

hour, then the flutter clearance speed must 

not be less than V′′. 

(vi) Freedom from aeroelastic 

instability must also be shown up to V′ 

in Figure 3 above for any probable 

system failure condition combined with 

any damage required or selected for 

investigation by § 25.571(b). 

(3) Consideration of certain failure 

conditions may be required by other 

sections of this Part, regardless of 

calculated system reliability. Where 

analysis shows the probability of these 

failure conditions to be less than 10¥9, 

criteria other than those specified in this 

paragraph may be used for structural 

substantiation to show continued safe 

flight and landing. 

(d) Warning considerations. For 

system failure detection and warning, 

the following apply: 

(1) The system must be checked for 

failure conditions, not extremely 

improbably, that degrade the structural 

capability below the level required by 

part 25 or significantly reduce the 

reliability of the remaining system. As 

far as reasonably practicable, the 

flightcrew must be made aware of these 

failures before flight. Certain elements 

of the control system, such as 

mechanical and hydraulic components, 

may use special periodic inspections, 

and electronic components may use 

daily checks in lieu of warning systems 

to achieve the objective of this 

requirement. These certification 

maintenance requirements must be 

limited to components the failures of 

which are not readily detectable by 

normal warning systems and where 

service history shows that inspections 

will provide an adequate level of safety. 

(2) The existence of any failure 

condition, not extremely improbable, 

during flight that could significantly 

affect the structural capability of the 

airplane and for which the associated 

reduction in airworthiness can be 

minimized by suitable flight limitations 

must be signaled to the flightcrew. For 

example, failure conditions that result 

in a factor of safety between the airplane 

strength and the loads of part 25, 

subpart C, below 1.25 or flutter margins 

below V′′ must be signaled to the crew 

during flight. 

(e) Dispatch with known failure 

conditions. If the airplane is to be 

dispatched in a known system failure 

condition that affects structural 

performance or affects the reliability of 

the remaining system to maintain 

structural performance, then the 

provisions of this Special Condition 

must be met, including the provisions of 

paragraph (b), for the dispatched 

condition and paragraph (c) for 

subsequent failures. Expected 

operational limitations may be taken 

into account in establishing P)j as the 

probability of failure occurrence for 

determining the safety margin in Figure 

1. Flight limitations and expected 

operational limitations may be taken 

into account in establishing Qj as the 

combined probability of being in the 

dispatched failure condition and the 

subsequent failure condition for the 

safety margins in Figures 2 and 3. These 

limitations must be such that the 

probability of being in this combined 

failure state and then subsequently 

encountering limit load conditions is 

extremely improbable. No reduction in 

these safety margins is allowed, if the 

subsequent system failure rate is greater 

than 1E–3 per flight hour. 

Issued in Renton, Washington, on October 

19, 2006. 

Jeffrey E. Duven, 

Acting Manager, Transport Airplane 

Directorate, Aircraft Certification Service. 

[FR Doc. 06–8974 Filed 10–30–06; 8:45 am] 

BILLING CODE 4910–13–M 


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