Airworthiness Directives; Boeing Model 737 Series Airplanes

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 98-NM-383-AD; Amendment 39-11175; AD 99-11-05]

RIN 2120-AA64

Airworthiness Directives; Boeing Model 737 Series Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

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SUMMARY: This amendment adopts a new airworthiness directive (AD),

applicable to all Boeing Model 737 series airplanes, that requires

repetitive displacement tests of the secondary slide in the dual

concentric servo valve of the power control unit (PCU) for the rudder,

and replacement of the valve assembly with a modified valve assembly,

if necessary. This amendment is prompted by reports of cracking found

in PCU secondary servo valve slides. The actions specified by this AD

are intended to prevent failure of the secondary slide and consequent

rudder hardover and reduced controllability of the airplane.

DATES: Effective June 28, 1999.

The incorporation by reference of certain publications listed in

the regulations is approved by the Director of the Federal Register as

of June 28, 1999.

ADDRESSES: The service information referenced in this AD may be

obtained from Boeing Commercial Airplane Group, P. O. Box 3707,

Seattle, Washington 98124-2207. This information may be examined at the

Federal Aviation Administration (FAA), Transport Airplane Directorate,

Rules Docket, 1601 Lind Avenue, SW., Renton, Washington; or at the

Office of the Federal Register, 800 North Capitol Street, NW., suite

700, Washington, DC.

FOR FURTHER INFORMATION CONTACT: R.C. Jones, Aerospace Engineer,

Systems and Equipment Branch, ANM-130S, FAA, Transport Airplane

Directorate, Seattle Aircraft Certification Office, 1601 Lind Avenue,

SW., Renton, Washington 98055-4056; telephone (425) 227-1118; fax (425)

227-1181.

SUPPLEMENTARY INFORMATION: A proposal to amend part 39 of the Federal

Aviation Regulations (14 CFR part 39) to include an airworthiness

directive (AD) that is applicable to all Boeing Model 737 series

airplanes was published in the Federal Register on January 13, 1999 (64

FR 2161). That action proposed to require repetitive displacement tests

of the secondary slide in the dual concentric servo valve of the power

control unit (PCU) for the rudder, and replacement of the valve

assembly with a modified valve assembly, if necessary.

Interim Action

This is considered interim action until final action is identified,

at which time the FAA may consider further rulemaking.

Opportunity To Comment

Interested persons have been afforded an opportunity to participate

in the making of this amendment. Due consideration has been given to

the comments received.

Support for the Proposed Rule

Several commenters express support for the proposed rule.

1. Requests To Extend the Initial Compliance Time

Several commenters request that the initial compliance time be

extended for the displacement test. While the proposed rule specifies

an initial compliance time of 4 months for certain airplanes, the

commenters suggest extensions of the initial compliance time ranging

from an initial compliance time of 8 months to an initial compliance

time of 2 years. The following identifies justifications provided by

the commenters for increasing the compliance time:

Some of the commenters state that testing and analysis to

date indicate that the servo valve of the PCU can sustain the highest

loads expected to occur in the normal service life of the Model 737

fleet of airplanes. The testing and analysis also indicate that a

single valve leg crack still permits the PCU to function normally for

periods of time much greater than the proposed 4-month compliance time.

Other commenters assert that an inadequate number of

qualified repair facilities exist, and that the number of PCU's in the

fleet are inadequate to permit compliance with the proposed AD. To meet

the compliance time for the 3,000 and more PCU's that would require

testing would likely ground a significant number of airplanes.

Two commenters state that the financial implications of

meeting the proposed compliance time could result in bankruptcy of one

or more small airlines.

One commenter states that the shipping time alone, without

consideration of any other factors, would prevent operators from

completing the displacement tests within the compliance time specified

in the proposal.

Several commenters state that all spares facilities are at

maximum use and spare PCU's are all being used in order to comply with

the requirements of AD 97-14-04, amendment 39-10061 (62 FR 35068, June

30, 1997).

Another commenter states that the turnaround time for

replacing units not modified in accordance with AD 97-14-04 is

approximately 30 to 45 days. Such turnaround time for those units would

prevent some operators from complying within the proposed compliance

time.

One other commenter expresses a serious concern that

accomplishment of all the testing done in the limited time proposed (4

months) could result in the introduction of various maintenance errors

that would possibly introduce a new unsafe condition.

The FAA concurs that the initial compliance time for accomplishment

of

[[Page 27906]]

the displacement test can be extended. Further, the FAA has reviewed

certain testing that indicates that valve slides with cracks can meet

the control valve limit and ultimate load requirements as well as

survive numerous life cycles. Additionally, the FAA has reviewed the

results of analyses that indicate valves with single cracks can

withstand an interval greater than the proposed 4-month interval. While

the testing and analyses results are not definitive proof that a second

crack will not develop, the results are evidence that valves with

single cracks are safe in-service for a limited interval. The FAA also

acknowledges that the number of PCU's in the fleet and the number of

qualified repair facilities may not be adequate to permit compliance

for the fleet within 4 months. In light of this information, the FAA

has determined that the compliance time of paragraphs (a)(1), (a)(2),

and (a)(3) of this AD can be extended to 16 months. The final rule

reflects this change.

2. Requests To Extend the Repetitive Displacement Testing Intervals

One commenter requests that the repetitive testing intervals be

extended from 12,000 to 12,800 flight hours to coincide with a major

check in the Boeing Maintenance Planning Document. Another commenter,

the airplane manufacturer, requests that the interval be extended from

12,000 to 24,000 flight hours. Both commenters state that the

displacement testing interval should coincide with scheduled heavy

maintenance to preclude an undue burden on operators and to reduce any

potential maintenance errors.

The FAA concurs that the repetitive testing interval may be

extended from 12,000 to 24,000 flight hours. The FAA finds that, based

on results of testing and analysis (referred to in comment 1. of this

AD), extending the testing interval will not adversely affect the

safety of the fleet. Additionally, the FAA concurs that, in this case,

less chance of maintenance errors will occur if the testing is

accomplished during scheduled heavy maintenance. Paragraph (b) of this

AD has been revised to specify an interval of 24,000 flight hours for

the repetitive displacement testing.

3. Requests To Withdraw the Proposed Rule

One commenter, the airplane manufacturer, states that there is no

technical data to support the position that ``an unsafe condition is

likely to exist or develop'' as stated in the proposed rule. The

commenter states that, on the other hand, the FAA's concern of a

possible condition developing into a ``thru-crack'' condition on both

the 1st leg and the 2nd leg of the secondary slide clevis is based on a

hypothetical and unsubstantiated extrapolation of failures. Further,

the commenter states that, although it agrees that ``thru-cracking'' of

both the 1st leg and 2nd leg of the secondary slide clevis would be an

unsafe condition, the commenter strongly disagrees that such a

condition exists in service or that it is likely to develop while the

slide is installed in a rudder PCU. The commenter also asserts that,

based on the results of testing and analyses by both the airplane

manufacturer and the PCU manufacturer, the secondary slide is not

susceptible to cracking after installation in the PCU. The commenter

concludes that the only plausible cause of cracking of the secondary

slides is slide mishandling or in-process damage.

Another commenter states that testing and analyses performed by

both the airplane manufacturer and PCU manufacturer indicate that

sufficient redundancy is provided in the rudder system to operate

almost 18 lifetimes with one broken leg of the secondary slide.

Additionally, the commenter asserts that, even in a worst-case scenario

of both legs of the secondary slide having a thru-fracture, and a piece

of material causing both primary and secondary slides to jam that

results in full rudder deflection, sufficient controllability of the

airplane would be ensured by the rudder pressure limiting device

required by AD 97-14-03.

The FAA infers that the commenters are requesting that the proposed

rule be withdrawn. The FAA does not concur. The root cause of the

cracked servo control valves of the PCU has not been determined. The

only way these cracks have been duplicated so far is by dropping or

hammering the secondary valve slide. The FAA considers it unlikely that

all of the 12 control valves had been dropped or hammered, which

suggests that there may be additional factors that make the secondary

valve slide susceptible to cracking. This may indicate that lower load

phenomena (and possibly normal handling) may be responsible for the

cracking. The lack of complete information makes it essential that the

valve slides be removed from the fleet. Additionally, a single crack in

the secondary control valve slide clevis reduces the load bearing

redundancy of the valve to a single load path. Loss of the remaining

load path could result in uncommanded rudder motion to a hardover

position.

The bases of the design of the control system on Boeing Model 737

series airplanes is that no single failure shall result in an unsafe

condition, and that either loss of a single redundant mechanism will be

detectable or the remaining redundant mechanism will survive for the

life of the airplane. It is generally accepted by the affected

operators and the FAA that some valves, possibly up to 50 valves, in

the fleet are cracked. Some airplanes may be reduced to single-thread

systems. Additionally, the rudder pressure-limiting device does not

reduce pressure on the Model 737 ``classic'' airplanes at altitudes

below 1,000 feet on takeoff or below 750 feet during landing. During

those particular conditions, uncommanded rudder motion to a hardover

condition may be catastrophic. In light of these findings, the FAA has

determined that the requirements of this AD are appropriate and

necessary.

4. Request To Eliminate Paragraph (a)(2) of the Proposed Rule

Another commenter states that requiring performance of the

displacement test [as specified in paragraph (a)(2) of the proposal]

prior to installation of the PCU required by AD 97-14-04 will have a

negative effect in the ability of operators to accomplish compliance

with that AD. The commenter points out that the resources and units

from the spares pool will be consumed in the effort to comply with the

proposed rule. In addition, the commenter states that the wording of

the proposal could be interpreted to mean that those PCU's installed

prior to the effective date need to be removed and re-tested, even

though they could have already been tested. The FAA infers that the

commenter is requesting that paragraphs (a)(2) and (c) of the proposal

be removed.

The FAA concurs that paragraph (a)(2) of the proposal should be

deleted. The FAA considers that it would be more efficient for an

operator to install a PCU that is in compliance with AD 97-14-04 and

this final rule, but acknowledges that it could prohibit an operator

from installing a serviceable unit that complies with AD 97-14-04

simply because the displacement test required by this final rule had

not been accomplished. Therefore, the FAA has deleted paragraph (a)(2)

of this AD and revised paragraph (a)(1) to remove the phrase ``prior to

the effective date of this AD.'' Paragraph (a)(1) of this AD now

applies to all Model 737-100, -200, -300, -400, and -500 series

airplanes regardless of whether AD 97-14-04 is incorporated before or

after the effective date of this AD. Additionally, the FAA

[[Page 27907]]

has renumbered the sub-paragraphs of paragraph (a) of this AD to

reflect the deletion of paragraph (a)(2) of this AD. The FAA also has

revised paragraph (c) of this AD to specify that only PCU's that have

completed a successful displacement test, as signified by the letter

``C'' after the serial number, may be installed as of 16 months after

the effective date of this AD. See Item 5. of this AD for further

discussion of the revision of paragraph (c) of this AD.

5. Request To Revise Paragraph (c) of the Proposed Rule

One commenter, the airplane manufacturer, requests that the wording

of paragraph (c) of the proposal be revised. The commenter states that

the current wording stating that no person shall install a main rudder

PCU on any airplane unless that PCU's nameplate has been vibro-engraved

with the letter ``C'' following the serial number of the PCU should be

revised to specify ``with the letter C' or greater.'' The commenter

states that by adding the words ``or greater,'' it allows for the

possibility of future revisions to the PCU. Additionally, the commenter

points out that if ``or greater'' is not added, it would mean that

installing a newer version PCU would not comply with the requirements

of the proposal.

The FAA concurs for the reasons submitted by the commenter and has

revised paragraph (c) of the final rule accordingly. In addition, the

FAA has extended the compliance time requirement for this paragraph to

``as of 16 months after the effective date of this AD.'' The FAA has

determined that, in light of the data supporting the increase of the

initial and repetitive compliance times required for the displacement

testing and the fact that there could be a shortage of available

spares, extending the compliance time of paragraph (c) to correspond

with the initial compliance time for the displacement testing is

appropriate.

6. Requests To Revise the Reporting Requirements

Two commenters request that the reporting requirements of paragraph

(d) of the proposed AD be revised. One of these commenters requests

that the reporting requirement for the initial displacement testing

should be revised to 10 days for those failed control valves that fail

the initial displacement test and 30 days for those control valves that

pass the initial displacement testing. No justification for that

request was provided. The other commenter requests deletion of the

requirement to report results for control valves that pass the

repetitive displacement tests. The commenter states that limiting the

reporting data to those control valves that fail any repetitive

displacement testing will provide all the necessary data for analysis.

The commenter points out that eliminating the requirement to report

control valves that pass the displacement testing of the repetitive

inspections would reduce the burden to operators, as well as to the

FAA.

The FAA concurs that reporting only PCU's that fail repetitive

displacement testing will provide adequate information to determine the

secondary valve slide condition after extended in-service time. The FAA

considers that, in the interest of relieving some burden on the

operators, the reporting times for all displacement testing may be

extended from 10 days to 30 days. The FAA finds that extending the

reporting time will not adversely affect safety. The FAA has revised

paragraph (d) of this AD to reflect these changes.

7. Request To Revise Corrective Action

One commenter requests that paragraph (b)(2) of the proposed rule,

which requires accomplishment of corrective action in accordance with a

method approved by the Manager, Seattle Aircraft Certification Office,

be revised. The commenter states that the only corrective action

available to operators is to replace the dual servo valve with a valve

that passes the displacement test. Therefore, the commenter asserts

that it is unnecessary to require approval of corrective actions from

the FAA.

The FAA concurs for the reason given by the commenter. Paragraph

(b)(2) of the final rule has been revised to specify that the

corrective action (replacement of the dual servo valve with a valve

that passes the displacement test) shall be accomplished in accordance

with the applicable alert service bulletin.

8. Request To Add Precautionary Language

Two commenters request that the FAA add wording to the proposed

rule to specify that only properly trained maintenance personnel and

appropriate repair facilities are used to accomplish the displacement

testing and replacement of the valve assemblies specified in the

proposal. The commenters state that, in the past, it appears that some

repair stations did not have proper facilities or properly trained

personnel, and maintenance errors were made. The commenters assert that

the disassembly and testing are complex and require special maintenance

knowledge and special equipment. The commenters request that

precautionary language specifying that only appropriately trained

personnel and appropriate maintenance facilities may be used to

accomplish the requirements of this AD be added to the proposal to

preclude the risk of maintenance errors.

The FAA does not concur with the commenters' request to include the

requested precautionary language in the final rule. The FAA

acknowledges that displacement testing and replacement of the valve

assemblies specified in this final rule may be complex and may require

special maintenance knowledge and special equipment. However, existing

maintenance regulations and guidance should ensure that appropriate

personnel perform maintenance and that appropriate equipment and repair

facilities are used.

9. Request To Clarify Compliance With Testing Requirements

One commenter requests that the proposal be revised to clarify that

vibro-engraving the letter ``C'' on the serial number constitutes

compliance that the unit has met the requirements of the AD and that no

further testing is required. The commenter states that the

clarification is necessary because the proposal applies to airplanes by

line number and does not account for the possibility that a tested PCU

may be installed on an affected airplane.

The FAA acknowledges that some clarification is necessary. First,

this AD applies to all Boeing Model 737 series airplanes as stated in

the applicability of this AD, not just to airplanes that are specified

by certain line numbers. Second, the application of the letter ``C''

(or greater letters, see Item 5.) to the serial number of the PCU does

not mean that the PCU is in compliance with the full requirements of

this AD. Such application of the letter ``C'' or greater letters

constitutes only compliance with the requirements of the initial

displacement test. Third, the application of the letter ``C'' or

greater letters does not mean that no further testing is required. The

specific reasons for the repetitive testing requirements of this AD and

the consideration of these requirements as interim action is discussed

elsewhere in Item 12. of this AD. The FAA, however, concurs that

clarification may be necessary in paragraph (c) of this AD to ensure

that accomplishment of the application of the letter ``C'' or greater

letters does constitute compliance with the requirement to accomplish

the initial displacement test. Paragraph (c) of this AD has been

revised to clarify this point.

[[Page 27908]]

10. Request To Add New Service Information

One commenter, the airplane manufacturer, requests that both

Revisions 1 of Boeing Alert Service Bulletins 737-27A1221 and 737-

27A1222, both dated January 28, 1999, be added to the proposed rule as

appropriate sources of service information. The commenter states that

minor changes were made in the new alert service bulletins.

The FAA has reviewed and approved both Revisions 1 of the alert

service bulletins. The FAA has determined that the revised alert

service bulletins contain not only minor changes, but changes that

contain descriptive material that is clarifying in nature. Since those

revisions to not add any burden to operators, the FAA has revised

paragraphs (a) and (b) of the final rule to reflect both Revisions 1 of

the service bulletins as the applicable sources of service information

for this AD. The FAA also has revised the final rule by adding a new

NOTE 2 that specifies that accomplishment of the initial displacement

testing in accordance with earlier editions of the service bulletins is

acceptable for the initial displacement testing required by this AD.

11. Requests To Revise Cost Impact

Several commenters request that the cost impact information provide

more realistic estimates of the costs for affected airplanes. These

commenters request that the proposal include estimates of cost for such

items as: scheduling and administrating; removing and replacing of the

PCU, shipping of the PCU's; performing the displacement testing and the

full Acceptance Test Procedure (ATP) if completed by a third party; and

the estimated costs of performing the displacement testing

repetitively.

The FAA does not concur that the cost impact information should be

revised. The cost estimates provided in this AD represent the time

necessary to perform only the actions actually required by this AD. The

FAA recognizes that, in accomplishing the requirements of any AD,

operators may incur ``incidental'' costs in addition to the ``direct''

costs. The cost analysis in AD rulemaking actions, however, typically

does not include incidental costs, such as the time required to gain

access and close up; planning time; or time necessitated by other

administrative actions. Because incidental costs may vary significantly

from operator to operator, they are almost impossible to calculate.

The replacement of the valve assembly that the commenters refer to

are actions that must be accomplished in the event that the results of

the displacement testing are outside the limits specified in the

service bulletin. Typically, the economic analysis of an AD is limited

to the cost of actions actually required by the rule. It does not

consider the costs of ``on condition'' actions (that is, actions taken

to correct an unsafe condition if found), since those actions would be

required to be accomplished, regardless of AD direction, in order to

correct an unsafe condition identified in an airplane and to ensure

operation of that airplane in an airworthy condition, as required by

the Federal Aviation Regulations.

12. Request To Delete the ``Interim Action'' Section

One commenter, the airplane manufacturer, requests that the

``Interim Action'' section of the proposal be deleted. The commenter

states that there is no data to indicate that there continues to be any

diminished level of safety once the rudder PCU has successfully

completed a displacement test. The commenter concludes that there is no

known safety concern that will require a ``final action.'' The

commenter also requests that reference in the preamble of the proposal

to a final action not being identified yet be deleted. The commenter

asserts that satisfactory results of displacement testing is adequate

proof that cracking does not exist in the PCU.

The FAA does not concur; a final solution to terminate the required

repetitive displacement tests may be necessary. As stated previously,

12 cracked control valves have been reported to date. However, the root

cause for the cracking has not been positively determined. The only way

these cracks have been duplicated so far is by dropping or hammering

the secondary valve slide. The FAA considers it unlikely that all 12

control valves had been dropped or hammered. The FAA considers it more

likely that lower load phenomena (and possibly normal handling) may be

responsible for the cracking, which indicates that there may be

additional factors that make the secondary valve slide susceptible to

cracking. The lack of a root cause, varying sensitivity of different

control valves to cracking, and uncertainties associated with damage

tolerance analyses on the valve material indicates to the FAA that

valve design may not be adequate and that cracking may occur in the

future. The FAA does not consider that the results of a single

displacement test is proof that cracking will not eventually occur in

the PCU valve.

For the reasons stated above, the FAA does not concur that it is

unnecessary to specify that the FAA may consider further rulemaking. No

change to the final rule in this regard is necessary.

13. Requests To Allow Dye Penetrant Inspections

Several commenters request that dye penetrant inspections be

required instead of displacement testing. One commenter asserts that,

if a cracked valve has been dye penetrant inspected and found to be

free of cracking, no further displacement testing should be required.

This same commenter states that, since there are no delayed cracking

mechanisms involved, a previous dye penetrant inspection to detect any

cracking is sufficient. Another commenter states that dye penetrant

inspection actually detects cracking better than the displacement test.

That commenter states that the dye penetrant inspection is better

because it can detect all cracking and that accomplishment of a

displacement test could leave a valve installed that contains small

cracks. The commenter further asserts that completion of a dye

penetrant inspection should suffice as a terminating action for the

proposed actions.

The FAA does not concur that dye penetrant inspection should be

required in lieu of displacement testing. The FAA considers that dye

penetrant inspection techniques have varied levels of crack detection

capability. Some dye penetrant inspection techniques may not have the

capability to detect some cracking that can propagate to failure of a

single leg. Additionally, the displacement test is performed on an

assembled PCU. This ensures that secondary control valve is in its most

protected configuration, and that the secondary valve slide is not

subjected to further handling. Therefore, it is unnecessary to revise

the final rule in this regard.

14. Requests To Credit Dye Penetrant Inspections

Two commenters request that PCU's that have been inspected

previously with dye penetrant be exempt from the proposed requirement

to accomplish displacement testing. The commenters also request that,

at a minimum, the FAA increase the initial compliance time and

repetitive intervals of the proposed AD for those PCU valves that have

had a dye penetrant inspection. The commenters assert that, since the

cause of the valve cracking is due to handling prior to the valve

assembly,

[[Page 27909]]

accomplishment of a dye penetrant inspection and careful assembly

provide acceptable assurance that the control valve is not cracked and

does not need to be displacement tested.

The FAA does not concur with the commenters' request. The FAA finds

that PCU valves that have had a dye penetrant inspection must undergo

the displacement testing at the same initial and repetitive intervals

as the other valves. As discussed previously (Item 13.), dye penetrant

inspection techniques have varied levels of crack detection capability.

Some dye penetrant inspection techniques may not have the capability to

detect partial cracks that can propagate into a leg failure. The

displacement testing will detect partial cracking by causing the crack

to propagate to failure of a single leg. Additionally, after a dye

penetrant inspection is accomplished, the secondary control valve slide

is again subject to handling because the valve must be cleaned and

reassembled. The FAA considers that the increase in initial compliance

time (as discussed in Item 1.) and the intervals for the repetitive

displacement testing provided in this final rule should provide some

additional time for completing the displacement testing. No change is

necessary to the final rule in regard to dye penetrant inspection.

15. Request To Eliminate the Installation Requirements of AD 97-14-04

One commenter requests that the FAA suspend the requirement to

install PCU valves required by AD 97-14-04. The commenter states that

until the root cause of the secondary slide cracking is identified, the

PCU's required by AD 97-14-04 should not be installed.

The FAA does not concur. Although the root cause of the PCU valve

cracking has not been identified, testing and analysis indicate that a

cracked valve will perform its intended function for a certain period

of time. The repetitive displacement testing will identify any cracked

valves and facilitate their removal. The valves installed in accordance

with AD 97-14-04 eliminate design ``features'' that could lead to

potentially unsafe flight conditions (e.g., reversal, overstroke, and

high residual pressures). Therefore, the FAA considers the benefits of

continued incorporation of AD 97-14-04 to outweigh the risks of

secondary valve cracking. The FAA has determined that unless new

information develops that reveals evidence contrary to the need for the

implementation of the requirements of AD 97-14-04, those requirements

are still valid and necessary to ensure the operational safety of the

fleet. No change is necessary to the requirements of this final rule in

this regard.

16. Request To Add an Inspection to the Requirements of the Proposed

Rule

One commenter requests that the proposed rule be revised to add an

inspection for chipping in the area of the clevis. The commenter states

that a control valve that was removed from a kit had a particle missing

from the clevis end that appeared to be chipped off. Therefore, the

commenter states that it would be prudent to inspect for chipping to

ensure that other chipped valves are in the fleet.

The FAA does not concur that an inspection for chipping should be

added to the final rule. Although the FAA agrees that the valves in the

fleet should not be chipped, only one control valve that was chipped

has been detected. The FAA considers that the mechanism causing the

chip is independent of the cause of the cracking of control valves. If

an inspection requirement to the final rule increases the burden of the

operator, it would necessitate issuing a supplemental notice of

proposed rulemaking (NPRM) to permit public comment in accordance with

the Administrative Procedures Act (APA). The FAA has determined that

delay of the final rule is not warranted based on the identified unsafe

condition addressed in this rule. However, the FAA may consider

separate rulemaking to address the concern of possible chipped

secondary control valves.

17. Request To Remove Requirement To Test Slides Already in Service

The commenter states that slides currently installed on PCU's do

not need to be displacement tested. The commenter asserts that, once

control valves are installed, they are protected from damage. The

commenter concludes that the requirement to test slides already in

service should be deleted from the proposal.

The FAA does not concur. Even though slides installed on PCU's are

substantially protected, two issues exist that indicate that PCU's in

service need to be tested. One, a cracked valve was detected on a PCU

removed from service. Two, the root cause of the cracking and

sensitivity to cracking has not been established. Since the FAA finds

that all cracked control valves must be removed from the fleet, all

valves that are installed must be tested. No change to the final rule

is necessary in this regard.

18. Request To Postpone Requirements Until a Terminating Action is

Provided

One commenter, an airline operator, requests that the FAA define a

terminating action for the repetitive displacement tests required by

the proposed AD. The commenter states that time should be allotted to

find a terminating action in order to reduce the risk of errors

occurring from repeating displacement testing. The FAA infers that the

commenter is requesting that the requirements specified in the proposal

be postponed until a terminating action is provided.

The FAA does not concur that additional time to develop and approve

a terminating action is warranted to delay issuance of this final rule.

The FAA has determined that the identified unsafe condition must be

addressed even though the terminating action has not been developed and

approved yet. The FAA, however, has been advised that a design

improvement of the clevis of the secondary control valve slide that is

not susceptible to cracking may be currently in development. Once a

design is reviewed and approved by the FAA, further rulemaking may be

considered as specified in the ``Interim Action'' section of this AD.

19. Request To Revise the Initial Compliance Time

One commenter requests that the FAA revise the initial compliance

time for the displacement testing from the proposed 4 months to 120

days. The commenter states that correction of the identified unsafe

condition addressed by the proposal is critical to flight safety.

The FAA does not concur that the compliance time should be revised

for the reason suggested by the commenter. However, the FAA has revised

the initial compliance time of the final rule to 16 months for the

reasons specified in Item 1. The FAA considers that the extension of

compliance time is justified and will not adversely effect the safety

of the fleet.

Editorial Change to the Proposal

The FAA inadvertently included Boeing Model 737-900 series

airplanes in the applicability of the NPRM. Since that model has not

yet been certificated, the FAA has removed it from the final rule.

Conclusion

After careful review of the available data, including the comments

noted above, the FAA has determined that air safety and the public

interest require the adoption of the rule with the changes previously

described. The FAA has

[[Page 27910]]

determined that these changes will neither increase the economic burden

on any operator nor increase the scope of the AD.

Cost Impact

There are approximately 3,059 Boeing Model 737 series airplanes of

the affected design in the worldwide fleet. The FAA estimates that

1,334 airplanes of U.S. registry will be affected by this AD, that it

will take approximately 1 work hour per airplane to accomplish the

required actions, and that the average labor rate is $60 per work hour.

Based on these figures, the cost impact of the AD on U.S. operators is

estimated to be $80,040, or $60 per airplane, per cycle.

It will take 9 work hours to remove and reinstall or replace the

PCU. For Model 737-100, -200, -300, -400, and -500 series airplanes,

however, concurrent accomplishment of this AD and AD 97-14-04 will

preclude the necessity to accomplish this replacement action twice,

thereby offsetting the cost impact on operators.

The cost impact figure discussed above is based on assumptions that

no operator has yet accomplished any of the requirements of this AD

action, and that no operator would accomplish those actions in the

future if this AD were not adopted.

Regulatory Impact

The regulations adopted herein will not have substantial direct

effects on the States, on the relationship between the national

government and the States, or on the distribution of power and

responsibilities among the various levels of government. Therefore, in

accordance with Executive Order 12612, it is determined that this final

rule does not have sufficient federalism implications to warrant the

preparation of a Federalism Assessment.

For the reasons discussed above, I certify that this action (1) is

not a ``significant regulatory action'' under Executive Order 12866;

(2) is not a ``significant rule'' under DOT Regulatory Policies and

Procedures (44 FR 11034, February 26, 1979); and (3) will not have a

significant economic impact, positive or negative, on a substantial

number of small entities under the criteria of the Regulatory

Flexibility Act. A final evaluation has been prepared for this action

and it is contained in the Rules Docket. A copy of it may be obtained

from the Rules Docket at the location provided under the caption

ADDRESSES.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Incorporation by

reference, Safety.

Adoption of the Amendment

Accordingly, pursuant to the authority delegated to me by the

Administrator, the Federal Aviation Administration amends part 39 of

the Federal Aviation Regulations (14 CFR part 39) as follows:

PART 39--AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as

follows:

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

Sec. 39.13 [Amended]

2. Section 39.13 is amended by adding the following new

airworthiness directive:

99-11-05 Boeing: Amendment 39-11175. Docket 98-NM-383-AD.

Applicability: All Model 737 series airplanes, certificated in

any category.

Note 1: This AD applies to each airplane identified in the

preceding applicability provision, regardless of whether it has been

modified, altered, or repaired in the area subject to the

requirements of this AD. For airplanes that have been modified,

altered, or repaired so that the performance of the requirements of

this AD is affected, the owner/operator must request approval for an

alternative method of compliance in accordance with paragraph (e) of

this AD. The request should include an assessment of the effect of

the modification, alteration, or repair on the unsafe condition

addressed by this AD; and, if the unsafe condition has not been

eliminated, the request should include specific proposed actions to

address it.

Compliance: Required as indicated, unless accomplished

previously.

To prevent failure of the secondary servo valve slide in the

rudder power control unit (PCU) due to cracking of the slide, and

consequent rudder hardover and reduced controllability of the

airplane, accomplish the following:

Displacement Testing

(a) Perform a displacement test of the secondary slide in the

dual servo valve in the rudder PCU, in accordance with Boeing Alert

Service Bulletin 737-27A1221, Revision 1, dated January 28, 1999

(for Model 737-100, -200, -300, -400, and -500 series airplanes); or

737-27A1222, Revision 1, dated January 28, 1999 (for Model 737-600,

-700, and -800 series airplanes); at the applicable time specified

by paragraph (a)(1), (a)(2), (a)(3), or (a)(4) of this AD. Repeat

the displacement test on that PCU thereafter at intervals not to

exceed 24,000 flight hours.

Note 2: Accomplishment of the initial displacement testing

required by paragraph (a) of this AD in accordance with Boeing Alert

Service Bulletin 737-27A1221, dated January 14, 1999 (for Model 737-

100, -200, -300, -400, and -500 series airplanes); or 737-27A1222,

dated January 14, 1999 (for Model 737-600, -700, and -800, series

airplanes) is acceptable only for the initial compliance

requirements of this AD.

(1) For Model 737-100, -200, -300, -400, and 500 series

airplanes: Conduct the displacement test within 16 months after the

effective date of this AD.

(2) For airplanes equipped with a PCU having part number 65-

44861-12 and having serial number (S/N) 3509A or lower: Conduct the

displacement test within 16 months after the effective date of this

AD.

(3) For Model 737-600, -700, and -800 series airplanes having

line numbers 1 through 222 inclusive: Conduct the displacement test

within 16 months after the effective date of this AD.

(4) For all other airplanes: Conduct the displacement test prior

to the accumulation of 24,000 total flight hours on the PCU, or

within 30 days after the effective date of this AD, whichever occurs

later.

Corrective Actions

(b) If the results of the displacement test required by

paragraph (a) of this AD are outside the limits specified by Boeing

Alert Service Bulletin 737-27A1221, Revision 1, dated January 28,

1999 (for Model 737-100, -200, -300, -400, and -500 series

airplanes), or 737-27A1222, Revision 1, dated January 28, 1999 (for

Model 737-600, -700, and -800 series airplanes): Prior to further

flight, accomplish the actions specified in paragraphs (b)(1) and

(b)(2) of this AD.

(1) Replace the valve assembly, in accordance with the

applicable alert service bulletin, with a serviceable valve

assembly. And

(2) Following installation of the replacement valve assembly in

accordance with paragraph (b)(1) of this AD, perform the

displacement test required by paragraph (a) of this AD on that

assembly, in accordance with the applicable alert service bulletin.

If the test results are outside the limits specified by the

applicable alert service bulletin, prior to further flight, replace

the valve assembly with a serviceable valve assembly in accordance

with the applicable alert service bulletin, and repeat the

displacement test required by paragraph (a) of this AD on that

assembly.

Note 3: Boeing Alert Service Bulletin 737-27A1222, Revision 1,

dated January 28, 1999, refers to Parker Service Bulletin 381500-27-

01, dated December 22, 1998, as an additional source of service

information for accomplishment of the displacement test for Model

737-600, -700, and -800 series airplanes.

(c) As of 16 months after the effective date of this AD, no

person shall install on any airplane a main rudder PCU having serial

number (S/N) 3509A or lower (for Model 737-100, -200, -300, -400,

and -500 series airplanes) or S/N 0299 or lower (for Model 737-600,

-700, and -800 series airplanes)

[[Page 27911]]

unless that PCU's nameplate has been vibro-engraved with the letter

``C'' or letters greater than ``C'' following the serial number. PCU

nameplates that have been vibro-engraved with the letter ``C'' or

letters greater than ``C'' following the serial number are

considered to be in compliance with the requirements for the initial

inspection of this AD.

(d)(1) Within 30 days after accomplishing the initial

displacement test required by paragraph (a) of this AD: Submit a

report of the testing to the Manager, Seattle Aircraft Certification

Office (ACO), FAA, Transport Airplane Directorate, 1601 Lind Avenue,

SW., Renton, Washington 98055-4056; fax (425) 227-1181. The report

must include the displacement testing results (both positive and

negative findings), test data for any failed valve assemblies, a

description of any discrepancies if found, the part number and

serial number of each rudder PCU tested, and the airplane serial

number.

(d)(2) Within 30 days after accomplishing any repetitive

displacement testing required by paragraph (a) of this AD: Submit a

report of any failed valve assembly to the Manager, Seattle Aircraft

Certification Office (ACO), FAA, Transport Airplane Directorate,

1601 Lind Avenue, SW., Renton, Washington 98055-4056; fax (425) 227-

1181. The report must include the displacement testing results of

any failed valve assembly, test data for any failed valve

assemblies, a description of any discrepancies found, the part

number and serial number of each rudder PCU with a failed valve

assembly, and the airplane serial number.

(d)(3) Within 30 days after accomplishing the initial

displacement test required by paragraph (a) of this AD: Submit

failed valve assemblies for analysis to Parker Hannifin Corporation,

Chief Engineer, Customer Support Operations, 16666 Von Karman

Avenue, Irvine, California 92606.

(d)(4) Information collection requirements contained in this

regulation have been approved by the Office of Management and Budget

(OMB) under the provisions of the Paperwork Reduction Act of 1980

(44 U.S.C. 3501 et seq.) and have been assigned OMB Control Number

2120-0056.

Alternative Methods of Compliance

(e) An alternative method of compliance or adjustment of the

compliance time that provides an acceptable level of safety may be

used if approved by the Manager, Seattle ACO. Operators shall submit

their requests through an appropriate FAA Principal Maintenance

Inspector, who may add comments and then send it to the Manager,

Seattle ACO.

Note 4: Information concerning the existence of approved

alternative methods of compliance with this AD, if any, may be

obtained from the Seattle ACO.

Special Flight Permits

(f) Special flight permits may be issued in accordance with

sections 21.197 and 21.199 of the Federal Aviation Regulations (14

CFR 21.197 and 21.199) to operate the airplane to a location where

the requirements of this AD can be accomplished.

Incorporation by Reference

(g) The actions shall be done in accordance with Boeing Alert

Service Bulletin 737-27A1221, Revision 1, dated January 28, 1999, or

Boeing Alert Service Bulletin 737-27A1222, Revision 1, dated January

28, 1999. This incorporation by reference was approved by the

Director of the Federal Register in accordance with 5 U.S.C. 552(a)

and 1 CFR part 51. Copies may be obtained from Boeing Commercial

Airplane Group, P. O. Box 3707, Seattle, Washington 98124-2207.

Copies may be inspected at the FAA, Transport Airplane Directorate,

1601 Lind Avenue, SW., Renton, Washington; or at the Office of the

Federal Register, 800 North Capitol Street, NW., suite 700,

Washington, DC.

(h) This amendment becomes effective on June 28, 1999.

Issued in Renton, Washington, on May 13, 1999.

D. L. Riggin,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 99-12690 Filed 5-21-99; 8:45 am]

BILLING CODE 4910-13-P