Airworthiness Directives; Boeing Model 737 Series Airplanes

DEPARTMENT OF TRANSPORTATION

14 CFR Part 39

[Docket No. 93-NM-79-AD; Amendment 39-8789; AD 94-01-07]

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 certain Boeing Model 737 series airplanes, that requires

repetitive tests of the main rudder power control unit (PCU) to detect

excessive internal leakage of hydraulic fluid, stalling, or reversal,

and the eventual replacement of the main rudder PCU with an improved

model. This amendment is prompted by results of an investigation which

revealed that the secondary slide in the servo valve of certain PCU's

can go past the intended maximum-travel position. The actions specified

by this AD are intended to prevent secondary slide overtravel from

occurring, which could cause the rudder actuator piston and the rudder

to operate with reduced force capability or to move in a direction

opposite to the intended direction; this could result in reduced

controllability of the airplane.

DATES: Effective March 3, 1994.

The incorporation by reference of certain publications listed in

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

of March 3, 1994.

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: Kenneth W. Frey, Aerospace Engineer,

Seattle Aircraft Certification Office, Systems & Equipment Branch, ANM-

130S, FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW.,

Renton, Washington 98055-4056; telephone (206) 227-2673; fax (206) 227-

1181.

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

Aviation Regulations to include an airworthiness directive (AD) that is

applicable to certain Boeing Model 737 series airplanes was published

in the Federal Register on August 16, 1993 (58 FR 43301). That action

proposed to require periodic tests of the main rudder PCU to detect

excessive internal leakage of hydraulic fluid, stalling, or reversal,

and the correction of discrepancies. That action also proposed to

require the eventual replacement of the main rudder PCU with an

improved model; such replacement would constitute terminating action

for the periodic tests.

Interested persons have been afforded an opportunity to participate

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

the comments received.

Several commenters request that the proposed rule be revised to

require a one-time internal leakage test of the PCU, instead of the

proposed repetitive tests every 750 flight hours. These commenters

consider that the existing pre-flight controls check is adequate to

detect rudder control anomalies that would be due to dual servo valve

secondary slide overtravel; since this check is performed prior to each

flight, it will verify the function of the rudder control system on a

regular basis. These commenters believe that the proposed repetitive

test is not appropriate or reliable, since it is sensitive to variables

such as temperature and the inspecting technician's individual

technique and judgment. In addition, some of these commenters believe

that the secondary slide overtravel problems are attributable to either

adverse tolerance build-up or improper assembly of the PCU during

manufacture, and are not due to wear or deterioration of the valve over

time; therefore, these commenters consider that repetitive tests are

unnecessary. One commenter states that the proposed test has not proved

to be reliable in identifying rudder PCU anomalies, and that this test

could cause a rise in the number of removals of PCU's that would

typically be serviceable; this would have a severe economic impact on

affected operators.

The FAA does not concur with the commenters' request. First, the

FAA notes that rapid rudder inputs are a factor in uncovering rudder

control anomalies. During a pre-flight check, the flight crew may not

cycle the rudder PCU at a rate fast enough to uncover secondary slide

overtravel. For this reason, the FAA does not agree that all rudder

control anomalies due to secondary slide overtravel can be detected

during pre-flight checks.

Second, although the FAA agrees that the required repetitive test

is not completely repeatable and could be subject to the judgment of

the technician, the FAA recognizes it as a written procedure for

maintenance crews to follow, which emphasizes cycling of the rudder

pedals at a maximum rate. While this test cannot ensure that the

secondary slide overtravel anomaly does not exist, it can detect

certain conditions that contribute to secondary slide overtravel. For

example, the overtravel anomaly, combined with either (1) A secondary

summing arm missing its stop or (2) a primary slide jamming to the

secondary slide, would be detected during the repetitive test. During

bench testing on one PCU, the secondary summing arm was observed

hitting its stop on some occasions and missing it on others; this

intermittent condition, if combined with the secondary overtravel

anomaly and rapid rudder inputs, could cause rudder control problems.

For these reasons, the FAA has determined that the required repetitive

tests will contribute to flight safety because they exercise the rudder

pedals at a rapid rate and they can detect high internal leakage within

the servo valve.

Several commenters request that the compliance time for replacing

the main rudder PCU be extended from the proposed 5 years to at least 7

years. These commenters state that such an extension is necessary to

avoid the economic burden to operators that would be caused by a

campaign of scheduled removals. A 7-year compliance time would

correspond to a typical overhaul period for the main rudder PCU, and

would avoid unscheduled removal of PCU's that exhibit no operational

anomalies. Some commenters note that the design feature that allows

secondary slide overtravel has existed for over 25 years and, over that

time period, has accumulated 55 million flight hours in the affected

fleet with no known in-flight control problems. These commenters state

that it would be extremely rare for the problem condition to exist in

combination with other factors so as to result in a reduction of rudder

control capability. For these reasons, these commenters consider that

extending the compliance time for completion of the replacement would

not appreciably affect the safety of the fleet.

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

compliance time for replacement of the PCU. Although the FAA

acknowledges that no known in-flight control problems have been

documented on Model 737 airplanes with regard to the subject condition,

the FAA does recognize that two PCU's have been removed from Model 737

series airplanes after the pilot identified rudder control problems

during the pre-flight check. In developing an appropriate compliance

time for this action, the FAA considered the safety implications of

incidents such as these, the availability of required parts, and the

normal maintenance schedules for timely accomplishment of the

replacement. Further, only the original equipment manufacturer

possesses approved data allowing it to accomplish the rework of the

affected PCU's; therefore, the compliance time also was based on the

capability of this manufacturer to schedule and modify all of the

affected servo valves. In light of these items, the FAA has determined

that 5 years for compliance is appropriate. The FAA considers that a 5-

year compliance time allows ample time for all affected PCU's to be

modified by the manufacturer, without creating a burden on either the

operators or the manufacturer. Five years also allows ample time for

the replacement to be accomplished coincidentally with scheduled major

airplane inspection and maintenance activities, thereby minimizing the

costs associated with special airplane scheduling. However, paragraph

(c) of the final rule does provide affected operators the opportunity

to apply for an adjustment of the compliance time if data is presented

to justify such an adjustment.

One commenter strongly supports the proposed rule, but considers

that the 5-year compliance time for replacement of the PCU's is too

long. This commenter requests that replacement of all affected PCU's

should begin immediately as replacement parts become available, instead

of allowing operators to operate airplanes equipped with potentially

defective PCU's for up to 5 years. This commenter notes that the

addressed anomaly was first discovered in July 1992 and, by the time

the proposed rule becomes effective and the end of the 5-year

compliance period is reached, approximately 7 years will have elapsed

since then. This amount of time is too long, considering the

significance of the addressed unsafe condition.

The FAA does not concur with the commenter's request to shorten the

compliance time for replacement of the PCU's. For the same reasons

explained previously, the FAA has determined that a 5-year compliance

time is appropriate. The repetitive tests of the PCU required by this

AD will provide an acceptable level of safety in the interim.

Additionally, the commenter should not assume that all operators will

wait until the end of the compliance time before replacement the PCU's.

In fact, the FAA has been advised that the PCU manufacturer has already

begun rework activity and some airlines have already begun replacement

procedures.

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

be revised to provide operators with the option of installing a

serviceable, non-modified PCU, rather than only a modified PCU, if a

discrepancy is detected during any test. Operators should be allowed to

use this non-modified PCU, provided that it is repetitively tested

until replacement of all PCU's is required in 5 years. Such a revision

to the rule would prevent unnecessary grounding of an airplane if a

discrepancy is detected and a modified PCU is not available. The FAA

concurs and has revised the final rule accordingly.

Another commenter requests that the proposed rule be revised to

address third-party repaired parts. This commenter states that at least

one repair facility is producing FAA-approved replacement slide and

sleeve assemblies to overhaul the Parker-Hannifin main rudder PCU dual

servo valves. The commenter believes that it is the FAA's

responsibility to ensure that third-party replacement parts are

reworked in a controlled manner to the same level of safety as that

provided by the retrofit plan developed by the original equipment

manufacturer (OEM), since the replacement parts were approved solely by

the FAA without support from the OEM.

The FAA does not concur that such a revision to the rule is

necessary. Paragraph (c) of the final rule contains a provision that

allows third-party replacement part manufacturers to submit proposed

design changes to the FAA for approval as alternative methods of

compliance with the AD.

One commenter suggests that the discussion of the requirements of

the proposed rule be revised to clarify that the test of the main

rudder PCU is necessary not only to detect excessive internal leakage

of hydraulic fluid, but to detect and correct possible stalling and

reversal as well. The FAA concurs. Although the test is accomplished to

detect excessive internal leakage, the AD requires that any

discrepancy, such as stalling and reversal, that is detected during the

test must be corrected prior to further flight. The AD references

Boeing Service Letter 737-SL-27-82-B for a description of the possible

discrepancies that could be found during the required test. The wording

to the preamble of this final rule has been revised to include

reference to the possible discrepancies of stalling and reversal;

however, the wording of the AD itself needs no revision.

This same commenter suggests that the description of the unsafe

condition on which this AD action is based could be worded more

accurately. This commenter states that the actions specified by the AD

are intended to prevent secondary slide overtravel from occurring,

which could cause the rudder actuator piston and the rudder to

``operate with reduced force capability or to move in a direction

opposite to the intended direction.'' The FAA concurs with the

commenter's suggestion and has revised the appropriate wording

accordingly.

Two commenters suggest that the cost impact information in the

preamble to the notice was not totally accurate in showing the complete

costs to operators, especially those costs related to the required

repetitive tests. One commenter states that each test would require

approximately 8 work hours to accomplish, and that, based on the 750-

flight hour repetitive test interval, most airplanes would be required

to be tested between 6 and 10 times over the next 5 years (until

replacement of the PCU is completed). Another commenter states that the

total time required for removing and installing the PCU is 20 work

hours. The FAA concurs that the economic information should be

clarified. The cost impact figures, as stated in the preamble to the

notice, were based on the latest data that the FAA had at that time.

The cost impact information, below, has been revised to delineate the

costs of the required actions, based on this latest information

provided by the commenters.

Paragraphs (a)(1) and (a)(2) of the final rule have been revised to

correctly reference paragraph ``3.B.'' of Boeing Service Letter 737-SL-

27-82-B for the description of possible discrepancies that could be

found during the required test. The notice had incorrectly referenced

paragraph ``B.'' rather than ``3.B.''

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 determined that these changes will neither

significantly increase the economic burden on any operator nor increase

the scope of the AD.

There are approximately 2,448 Model 737 series airplanes of the

affected design in the worldwide fleet. The FAA estimates that 729

airplanes of U.S. registry will be affected by this AD.

It will take approximately 8 work hours per airplane to accomplish

the required test actions, at an average labor rate of $55 per work

hour. Based on these figures, the total cost impact of the tests

required by this AD on U.S. operators is estimated to be $320,760, or

$440 per airplane, per test.

According to information provided by commenters to the notice that

preceded this rule, some airplanes may require to be tested between 6

and 10 times prior to the required replacement of the PCU (within 5

years). In the case of those airplanes requiring 6 tests, the total

cost impact of the required repetitive tests would be $2,640 per

airplane over 5 years; in the case of those airplanes requiring 10

tests, the total cost impact of the required repetitive tests would be

$4,400 per airplane over 5 years.

The number of required work hours, as indicated above, is presented

as if the tests required by this AD were to be conducted as ``stand

alone'' actions. However, in actual practice, these tests could be

accomplished coincidentally or in combination with normally scheduled

airplane inspections and other maintenance program tasks. Therefore,

the actual number of necessary ``additional'' work hours would be

minimal in many instances. Additionally, any costs associated with

special airplane scheduling should be minimal.

It will take approximately 20 work hours per airplane to accomplish

the required replacement of the PCU (removal and installation), at an

average labor rate of $55 per work hour. Required parts will be

supplied by the manufacturer at no cost to operators. Based on these

figures, the total cost impact of the replacement required by this AD

is estimated to be $801,900, or $1,100 per airplane.

The 5-year compliance time for the replacement of the PCU, as

specified in paragraph (b) of this AD, will allow ample time for the

replacement to be accomplished coincidentally with scheduled major

airplane inspection and maintenance activities, thereby minimizing the

costs associated with special airplane scheduling.

The total cost impact figures described above are 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. However, the

FAA has been advised that several operators already have accomplished

at least the initial required test of the PCU's on their affected

fleets, and certain operators already have replaced some PCU's.

Therefore, the future economic cost impact of this rule on U.S.

operators is less than that indicated above.

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 14 CFR part

39 of the Federal Aviation Regulations as follows:

PART 39--AIRWORTHINESS DIRECTIVES

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

Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C.

106(g); and 14 CFR 11.89.

Sec. 39.13 [Amended]

2. Section 39.13 is amended by adding the following new

airworthiness directive:

94-01-07 Boeing: Amendment 39-8789. Docket 93-NM-79-AD.

Applicability: Model 737 series airplanes; line positions 1

through 2453, inclusive; certificated in any category.

Compliance: Required as indicated, unless accomplished

previously.

To prevent the rudder actuator piston and the rudder to operate

with reduced force capability or to move in a direction opposite to

the intended direction, which could result in reduced

controllability of the airplane, accomplish the following:

(a) Within 750 flight hours after the effective date of this AD,

perform a test of the main rudder power control unit (PCU), part

number 65-44861-2/-3/-4/-5/-6/-7/-8/-9, to detect internal leakage

of hydraulic fluid, in accordance with Boeing Service Letter 737-SL-

27-82-B, dated July 13, 1993.

(1) If no discrepancy, as described in paragraph 3.B. of the

Service Letter, is detected, repeat the test at intervals not to

exceed 750 flight hours.

(2) If any discrepancy, as described in paragraph 3.B. of the

Service Letter, is detected during any check, prior to further

flight, accomplish either paragraph (a)(2)(i) or (a)(2)(ii) of this

AD:

(i) Replace the main rudder PCU with a serviceable PCU in

accordance with the Model 737 Overhaul Manual. After such

replacement, repeat the test at intervals not to exceed 750 flight

hours.

(ii) Replace the main rudder PCU with a new main rudder PCU

having part number 65-44861-11 or 65C37052-2/-3/-4/-5/-6/-7/-8/-9,

in accordance with Boeing Service Bulletin 737-27-1185, dated April

15, 1993. Such replacement constitutes terminating action for the

tests required by this AD.

(b) Within 5 years after the effective date of this AD, replace

the main rudder PCU, part number 65-44861-( ), with a new main

rudder PCU having part number 65-44861-11 or 65C37052-2/-3/-4/-5/-6/

-7/-8/-9, in accordance with Boeing Service Bulletin 737-27-1185,

dated April 15, 1993. Such replacement constitutes terminating

action for the tests required by this AD.

(c) 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 Aircraft Certification

Office (ACO), FAA, Transport Airplane Directorate. 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: Information concerning the existence of approved

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

obtained from the Seattle ACO.

(d) Special flight permits may be issued in accordance with FAR

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

requirements of this AD can be accomplished, provided that the

airplane has not failed the internal leakage test required by this

AD.

(e) The tests shall be done in accordance with Boeing Service

Letter 737-SL-27-82-B, dated July 13, 1993. The replacement shall be

done in accordance with Boeing Service Bulletin 737-27-1185, dated

April 15, 1993. 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.

(f) This amendment becomes effective on March 3, 1994.

Issued in Renton, Washington, on January 3, 1994.

Darrell M. Pederson,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 94-347 Filed 1-31-94; 8:45 am]

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