AD 95-25-11 ACTIVE

This amendment adopts a new airworthiness directive (AD) that is applicable to all EMBRAER Model EMB-120 series airplanes. This action requires revising the FAA-approved Airplane Flight Manual (AFM) to limit the revolutions per minute (RPM) of the propeller during ground operation, and removing and installing a new placard. This AD also requires revising the FAA-approved maintenance program to limit the maximum RPM of the propeller during ground operations. This amendment is prompted by reports of failures of in-service propellers and subsequent testing, which revealed that operating the propeller at or near certain nominal propeller RPM produces high vibration stress. The actions specified in this AD are intended to limit exposure to high vibration stresses during ground operations under certain weather conditions; this situation could accelerate fatigue cracking if corrosion is present in the propeller, which could lead to the failure of the propeller and subsequent reduced controllability of the airplane.

Final rule; request for comments

95-25-11 EMBRAER: Amendment 39-9457. Docket 95-NM-236-AD.
Applicability: All Model EMB-120 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 use the authority provided in paragraph (e) of this AD to request approval from the FAA. This approval may address either no action, if the current configuration eliminates the unsafe condition; or different actions necessary to address the unsafe condition described in this AD. Such a request should include an assessment of the effect of the changed configuration on the unsafe condition addressed by this AD. In no case does the presence of any modification, alteration, or repair remove any airplane from the applicability of this AD.

Compliance: Required as indicated, unless accomplished previously.

To prevent failure of the propeller and subsequent reduced controllability of the airplane due to high stresses on the propeller at certain nominal propeller revolutions per minute (RPM), accomplish the following:

(a) Within 14 days after the effective date of this AD, accomplish the requirements of paragraphs (a)(1) and (a)(2) of this AD.

(1) Revise the Limitations Section (under "Propeller") of the FAA-approved Airplane Flight Manual (AFM) (in the basic AFM and in AFM Supplement 4) to include the following. This may be accomplished by inserting a copy of this AD in the AFM.

"Condition Levers must be in the MIN RPM position during all ground operations, except when cleared for takeoff or during landing roll.

"Power Levers must remain at or below Flight Idle during all ground operations, except for brief (approximately 5 seconds) excursions as needed to maneuver the airplane.

"CAUTION: Ground operation above Flight Idle significantly increases propeller stress under certain adverse wind conditions (e.g., tailwinds or rear crosswinds). Operation in this RPM range must be avoided to the maximum extent practicable."

(2) Revise the Normal Procedures Section (under "CLEARED INTO POSITION") of the FAA-approved AFM to separate the current procedures listed under the "CLEARED INTO POSITION" heading into two separate headings, as follows, to delay movement of condition levers until cleared for takeoff. This may be accomplished by inserting a copy of this AD into the AFM.

"CLEARED INTO POSITION
Landing Lights Switches ...................................................... ON
STROBE Light Switch ......................................................... ON
Transponder ....................................................................... ON

"CLEARED FOR TAKEOFF
Condition Levers ........................................................ MAX RPM
Multiple Alarm Panel Lights .................. CHECK EXTINGUISHED"

(b) Within 14 days after the effective date of this AD, remove the placard having part number (P/N) 120-30915-001 on the instrument panel of the cockpit.

(c) Within 30 days after the effective date of this AD, install a new placard having P/N 120- 61757-001 on the instrument panel of the cockpit.

(d) Within 14 days after the effective date of this AD, revise the FAA-approved maintenance program to incorporate the following into Chapter 61-00-00, "Propeller System Operating Limitations," and Chapter 71-00-00, "Propeller Operating Limitations," of the airplane maintenance manual:

"To prevent excessive propeller stress, do not operate above 60% Np UNLESS:
The wind is less than 10 knots, OR
The airplane is headed into the wind +/-45 degrees.
Wind direction must be monitored locally at the run up site."

(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, Atlanta Aircraft Certification Office (ACO), FAA, Small 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, Atlanta ACO.

NOTE 2: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Atlanta ACO.

(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.

(g) This amendment becomes effective on December 27, 1995.

The FAA has received reports of failures of in-service Hamilton Standard 14RF, 14SF, and 6/5500/F series propellers. Result of inspections have revealed internal corrosion in the taper bore and external damage to the shank section on these propellers. Such internal corrosion or external damage reduces the design allowable stress levels of the propeller material. Further inspections revealed that the corrosion and damage is evenly distributed amongst the subject propellers. However, fracturing has occurred predominately on the Hamilton Standard 14RF series propellers installed on EMBRAER Model EMB-120 series airplanes. This disparity has prompted an investigation into operational differences between the airplanes utilizing these propellers.

A vibration/loads survey and analysis was conducted by Hamilton Standard on the Hamilton Standard 14RF series propellers installed on EMBRAER Model EMB-120 series airplanes. The survey and analysis results verified that high vibration stresses could occur on all Hamilton Standard propeller installations during ground operation in tail and cross winds (adverse winds) when the propeller is operated at or near 73% of the nominal propeller revolutions per minute (RPM). For EMBRAER Model EMB-120 series airplanes equipped with Hamilton Standard 14RF series propellers, operation at 73% RPM is coincident with a propeller natural frequency at exactly twice the RPM (denoted as 2P), which produces high vibration stresses. For all other airplanes equipped with this propeller, the 2P natural frequency is below the normal propeller operating speed and, therefore, a resonant vibration could not occur.

Subsequent propeller vibration survey testing on a Model EMB-120 series airplane revealed a possible small downward shift in the 2P frequency RPM on these airplanes relative to the original 1983 certification test data. Such a shift could increase cyclic stress on the propeller blade during normal ground operations. The possible cause of such a shift could be attributed to wear and normal changes in the propeller mass properties while in-service.

Operation of Hamilton Standard 14RF series propellers installed on EMBRAER Model EMB-120 series airplanes at or near 73% of the nominal propeller RPM, if not corrected, could result in high stresses on the propeller. Such high stresses subsequently could accelerate fatigue cracking at existing internal corrosion pits in the propeller, which could lead to the failure of the propeller and subsequent reduced controllability of the airplane.

This airplane model is manufactured in Brazil and is type certificated for operation in the United States under the provisions of section 21.29 of the Federal Aviation Regulations (14 CFR 21.29) and the applicable bilateral airworthiness agreement.

Since an unsafe condition has been identified that is likely to exist or develop on other airplanes of the same type design registered in the United States, this AD is being issued to limit ground operation of Hamilton Standard 14RF series propellers installed on EMBRAER Model EMB-120 series airplanes at or near 73% of the nominal propeller RPM, which could result in high stresses on the propeller; this condition could accelerate fatigue cracking at existing internal corrosion pits in the propeller, which could lead to the failure of the propeller and subsequent reduced controllability of the airplane. This AD requires revising the Limitations and Normal Procedures Sections of the AFM to reduce ground operating RPM to MIN RPM (approximately 50%), except for brief excursions as needed to maneuver the airplane, which will limit exposure to high cyclic stresses on the propeller. This AD also requires removal of a placard and installation of a new placard on the instrument panel of the cockpit. Additionally, this AD requires revising the FAA-approved maintenance program to limit the maximum RPM for ground operations.

This is considered to be interim action until final action is identified, at which time the FAA may consider further rulemaking.

Since a situation exists that requires the immediate adoption of this regulation, it is found that notice and opportunity for prior public comment hereon are impracticable, and that good cause exists for making this amendment effective in less than 30 days.

Comments Invited
Although this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, comments are invited on this rule. Interested persons are invited to comment on this rule by submitting such written data, views, or arguments as they may desire. Communications shall identify the Rules Docket number and be submitted in triplicate to the address specified under the caption "ADDRESSES." All communications received on or before the closing date for comments will be considered, and this rule may be amended in light of the comments received. Factual information that supports the commenter's ideas and suggestions is extremely helpful in evaluating the effectiveness of the AD action and determining whether additional rulemaking action would be needed.

Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need to modify the rule. All comments submitted will be available, both before and after the closing date for comments, in the Rules Docket for examination by interested persons. A report that summarizes each FAA-public contact concerned with the substance of this AD will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this rule must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket Number 95-NM-236-AD." The postcard will be date stamped and returned to the commenter.

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.

The FAA has determined that this regulation is an emergency regulation that must be issued immediately to correct an unsafe condition in aircraft, and that it is not a "significant regulatory action" under Executive Order 12866. It has been determined further that this action involves an emergency regulation under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). If it is determined that this emergency regulation otherwise would be significant under DOT Regulatory Policies and Procedures, a final regulatoryevaluation will be prepared and placed in the Rules Docket. A copy of it, if filed, 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, 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 USC 106(g), 40101, 40113, 44701.

39.13 - [Amended]
2. Section 39.13 is amended by adding the following new airworthiness directive:

Submit comments in triplicate to the Federal Aviation Administration (FAA), Transport Airplane Directorate, ANM-103, Attention: Rules Docket No. 95-NM-236-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.
Information concerning this amendment may be obtained from or examined at the FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington; or at the FAA, Atlanta Aircraft Certification Office, Small Airplane Directorate, Campus Building, 1701 Columbia Avenue, Suite 2-160, College Park, Georgia.

Carla J.W. Worthey, Aerospace Engineer, Flight Test Branch, ACE-116A, FAA, Atlanta Aircraft Certification Office, Small Airplane Directorate, Campus Building, 1701 Columbia Avenue, Suite 2-160, College Park, Georgia 30337-2748; telephone (404) 305-7364; fax (404) 305-7348.