54-12-01 MARTIN: Applies to All Model 404 Aircraft.
Compliance required as indicated.
An analysis of the design and the service experience of Model 404 main landing gear oleo drag struts, P/N E100008, has resulted in the establishment of a 6,500-hour service life figure for these assemblies.
It will be permissible to continue these assemblies in service not to exceed 8,500 hours total service time, provided the drag strut piston rods and forked end fittings are subjected to the following inspections when they attain 6,500 hours of service and every 250 service hours thereafter until replacement is made with new drag strut assemblies, P/N E100050, in accordance with item 5. (See item 6 for increasing inspection interval to 500 hours.)
NOTE: A 10X to 30X enlarging lens must be used when accomplishing the visual examination portions of these inspections.
1. Perform dye penetrant inspection for cracks on the entire threaded portion of the piston rod, P/N SE997512. (The location at which a crack would be most likely to occur is through the root of a thread which lies in or adjacent to the faying plane between the piston terminal, P/N SE997514, and the locknut, P/N SE997515, when these parts are in their installed position on the drag strut piston rod.)
2. Perform a dye penetrant or a Zyglo inspection for cracks on the end bosses of the forked end fitting, P/N SE997590. In order to inspect these areas satisfactorily, it is necessary to remove the bronze bushings, P/N SE997517, from the end bosses. (Laboratory analysis of drag strut forked end fittings which failed during landing gear repeat load tests determined the origin of the crack, in each instance, to be located at the inner surface of the 5/16-inch bolt hole adjacent to the bronze bushing. It is essential, therefore, that these and all other surfaces lying in the plane of the least cross-sectional area be scrutinized most critically.)
3. If the above inspections revealminute indications of a crack not clearly definable, the area in question should be subjected to a macro etch inspection.
(a) The steel piston rod may be etched with a saturated aqueous solution of ammonium persulphate.
(b) The forged aluminum forked end fittings may be etched with a 30- percent aqueous solution of sodium hydroxide.
4. A piston rod or forked end fitting found to contain a crack must be scrapped.
5. Reinspection of parts found to be satisfactory shall be made every 250 service hours (or 500 hours per item 6), until replacement struts of the new configuration, P/N E100050, become available. Original struts, P/N E200008, must be replaced at 8,500 hours service life, at which time the inspection program can be discontinued. (When the new design drag struts become available, no attempts should be made to interchange assembly components, but a complete replacement of old design struts with new design struts should be effected.)
6. The 250-hour inspection interval may be increased to 500 hours provided the following is accomplished:
(a) Chamfer all edges of the cross bolt hole in the P/N SE99759C oleo drag strut fork fitting.
(b) Maintain a minimum torque of 900 inch-pounds on the piston rod jam nut.
(c) Instruct the pilots to use the brakes in such a manner as to minimize the strut bottomings.
(Martin Service Instruction Letter No. 404-65 covers this same subject.)