AD 99-15-14 Airworthiness Directives; Sikorsky Aircraft-Manufactured Model CH-54B Helicopters

CH-54B helicopters with main gearbox second stage lower planetary plate (plate), part number (P/N) 6435-20516-101, installed, certificated in any category. Note 1: This AD applies to each helicopter identified in the preceding applicability provision, regardless of whether it has been otherwise modified, altered, or repaired in the area subject to the requirements of this AD. For helicopters 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 (c) 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.

Required as indicated, unless accomplished previously. To prevent failure of the plate due to fatigue cracking, which could lead to failure of the main gearbox and subsequent loss of control of the helicopter, accomplish the following: (a) For main gearbox assemblies containing plate, part number (P/N) 6435-20516-101, with 1,600 or more hours time-in-service (TIS): Note 2: If the TIS hours of the plate is not known, use the main gearbox assembly's total operating time. (1) Prior to the first flight of each day, inspect the main oil filter for magnesium contamination. If magnesium contamination is discovered, replace the main gearbox assembly. (2) Within the next 100 hours TIS after the effective date of this AD, and thereafter at intervals not to exceed 200 hours TIS, conduct a borescope inspection of the plate for cracks in the area of the nine lightening holes (see Figure 1). If a crack is found, replace the plate with an airworthy plate. The plate, P/N 6435- 20516-101, is part of the main gearbox second stage planetary set (P/N 6435-20514-041), which is a serialized matched set, and must be replaced as a set. BILLING CODE 4910-13-P [[Page 39400]] [GRAPHIC] [TIFF OMITTED] TR22JY99.017 BILLING CODE 4910-13-C [[Page 39401]] (b) At the next overhaul of the main gearbox assembly, inspect and rework the plate, P/N 6435-20516-101, as follows: (1) Fluorescent magnetic particle inspect the plate per ASTM E1444 in circumferential and longitudinal directions using a wet continuous method. Pay particular attention to the area around the nine 1.750-inch diameter lightening holes. (2) If a crack is found, the plate is unairworthy. Replace it with an airworthy plate. (3) If no crack is found, rework the plate as follows, ensuring that all plate surfaces are free of any crack, scratch, dent, or corrosion. (i) Measuring from the center of each 1.750-inch diameter lightening hole, machine 0.015/0.020 inch from the radius of the hole (see Figure 2). Machined surface roughness shall not exceed 63 microinches AA rating. BILLING CODE 4910-13-P [[Page 39402]] [GRAPHIC] [TIFF OMITTED] TR22JY99.018 BILLING CODE 4910-13-C [[Page 39403]] (ii) Apply a 0.030/0.050-inch radius on the top and bottom edge of each hole. (4) Fluorescent magnetic particle inspect the reworked areas per ASTM E1444 in circumferential and longitudinal directions using a wet continuous method. (5) If a crack is found, the plate is unairworthy. Replace it with an airworthy plate. (6) If no crack is found, rework the plate as follows: (i) Remove the protective finish from the specified areas on the top and bottom of the plate as follows: (A) Mask the top and bottom of the plate leaving exposed a 3.20- inch minimum circumferential band centered on 13.75-inch diameter of plate (see Figure 2). Mask the area to protect the thrust washer and the surrounding areas from vapor blast. (B) Using a vapor blast machine, remove the protective finish from the exposed circumferential band on the top and bottom of the plate. Use No. 220 aluminum oxide grit at a pressure of 80-90 pounds per square inch. (ii) Shot peen the specified areas on the plate by remasking the top and bottom of the plate leaving exposed the 3.20-inch minimum circumferential band centered on 13.75-inch diameter of the plate. Mask the area to protect the thrust washer and the surrounding areas from the shot peening process. (iii) Shot peen the inside diameter of the lightening holes and the upper and lower surfaces of the plate in the 3.20-inch minimum circumferential band to 0.008 to 0.012A intensity, ensuring 200% coverage per MIL-S-13165C or latest revision. Use cast steel shot, size 170. Use a tracer dye inspection method. Note 3: Overspray is permitted to allow a feathering application during the peening process from the peened surface to the non-peened surface. (iv) Finish the reworked surfaces as follows: (A) Clean the surfaces thoroughly with acetone (Fed. Spec O-A- 51, or equivalent). (B) Apply Presto black or blueing touchup solution to the reworked surfaces with cotton swabs. The solution temperature must be between 21 deg. C and 49 deg. C (70 deg. F to 120 deg. F). Keep the surfaces wet for about three minutes to get a uniform dark color. (C) Rinse the surface in cold running water and dry with forced air. Note 4: A hot water rinse may be used after the cold water rinse to speed up drying time. (D) Using steel wool, Grade 00 or finer, rub the surfaces lightly. Polish with a soft cloth and then coat with a preservative oil (MIL-C-15074). (v) Identify the reworked plate by stamping the number of this AD after the part number. Use a low-stress depth-controlled impression-stamp with full fillet depth of no more than 0.003 inch (see Figure 2). Marking must be such that it cannot be construed as part of the part number. (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, Rotorcraft Certification Office, FAA, Rotorcraft Directorate. Operators shall submit their requests through an FAA Principal Maintenance Inspector, who may concur or comment and then send it to the Manager, Rotorcraft Certification Office. Note 5: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Rotorcraft Certification Office. (d) 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 helicopter to a location where the requirements of this AD can be accomplished. (e) This amendment becomes effective on August 26, 1999. Issued in Fort Worth, Texas, on July 15, 1999. Henry A. Armstrong, Manager, Rotorcraft Directorate, Aircraft Certification Service. [FR Doc. 99-18683 Filed 7-21-99; 8:45 am] BILLING CODE 4910-13-P