AD 78-23-12

78-23-12 PRATT & WHITNEY AIRCRAFT: Amendment 39-3315. Applies to Pratt & Whitney Aircraft JT8D-1, -1A, -1B, -7, -7A, -7B, -9 and -9A turbofan engine models not incorporating third stage turbine blade retention rivets, P/N 759351 or P/N 618749.

Compliance required as indicated unless already accomplished.

To prevent third stage turbine blade rivet failure which could result in failure of the low turbine shaft and/or noncontainment of turbine blade and vane debris, accomplish the following:

1. Inspect engines for proper position of the third stage turbine blade in the disk blade slot in accordance with the procedures in the Pratt & Whitney Aircraft JT8D Maintenance Manual, P/N 481671, Section 72-00, Borescope or Radioisotope Inspection of Third Stage Turbine Blade, or equivalent means approved by the Chief, Engineering and Manufacturing Branch, New England Region, prior to the accumulation of 3,000 hours time in service since installed in disk or within the next 600 hours time in service after the effective date of this AD, whichever is later. Engines with no measurable third stage turbine blade mismatch or displacement must be re-inspected every 1,000 hours time in service thereafter. Engines with third stage turbine blade roots displaced axially more than .032 inch relative to the disk rear surface or engines with third stage turbine blade root platform rear surface displaced axially more than .032 inch relative to an adjacent third stage turbine blade root platform rear surface must be removed prior to further flight. Engines with third stage turbine blade mismatch or axial displacement .032 inch or less shall be subject to 300 hour repetitive displacement inspections. Engines with third stage turbine blade mismatch confirmed by an initial displacement inspection and two 300 hour repetitive displacement inspections during which there is no change in blade position, indicating blades were mismatched at last assembly, may then revert to the 1,000 hour repetitive inspection interval.

NOTE: a. Mismatch of the blade relative to the disk or the blade root platform rear surface relative to an adjacent blade root platform rear surface is the result of manufacturing tolerance build-up.

b. Displacement of the blade is axial movement of the blade relative to its position when originally installed.

c. A piece of .032 inch safety wire may be used with the borescope technique as a guide to determine the position of the blade relative to the disk rear face or the blade root platform rear surface relative to an adjacent blade root platform rear surface.

d. The rigid borescope and radioisotope inspection methods provide blade displacement information by comparing one blade root platform rear surface to an adjacent blade root platform rear surface. The flexible borescope inspection method provides blade displacement information by comparing the blade root with the disk rear surface.

2. Install by June 30, 1980, improved third stage turbine blade retention rivets, P/N 759351, in accordance with Pratt & Whitney Aircraft Service Bulletin No. 4592, Revision 1, dated August 20, 1976, or later revision approved by the Chief, Engineering and Manufacturing Branch, FAA, New England Region.

Upon request of the operator, an FAA maintenance inspector, subject to prior approval of the Chief, Engineering and Manufacturing Branch, FAA, New England Region, may adjust the inspection intervals specified in this AD to permit compliance at an established inspection period of the operator if the request contains substantiating data to justify the increase for that operator.

The manufacturer's alert service bulletin identified and described in this directive is incorporated herein and made a part hereof pursuant to 5 U.S.C. 552(a)(1). All persons affected by this directive who have not already received this document from the manufacturer may obtain copies upon request to Pratt & Whitney Aircraft, Division of United Technologies Corporation, 400 Main Street, East Hartford, Connecticut 06108. This document may also be examined at the Federal Aviation Administration, New England Region, 12 New England Executive Park, Burlington, Massachusetts 01803, and at FAA Headquarters, 800 Independence Avenue, S.W., Washington, D.C. 20591. A historical file on this AD which includes the incorporated material in full is maintained by the FAA at its Headquarters in Washington, D.C., and at New England Region.

This amendment becomes effective December 20, 1978.