AD 90-17-03

90-17-03 ENSTROM HELICOPTER CORPORATION: Amendment 39-6686. Docket No. 83-ASW-39. Applicability: All Enstrom Model F-28A, F-28C, F-28C-2, 280, and 280C series helicopters; to Model F-28F series helicopters with serial numbers (S/N's) 506, 507, 509, 510, 511, 512, 513, 514, 515, 517, 527, 700, 701, 702, and 704; and to Enstrom Model 280F series helicopter with S/N's 1212 and 1500.

Compliance: Required as indicated, unless already accomplished.

To prevent tail rotor drive shaft coupling failure that could result in loss of directional control and possible loss of the helicopter, accomplish the following:

(a) Within 5 hours' time in service after the effective date of this AD, determine if splined tail rotor drive shaft couplings, P/N 28-13009-1, are installed. Enter the part number of the tail rotor drive shaft couplings that are installed, the number of hours time in service, and the date in the log book.

NOTE: There are two tail rotor drive shaft couplings for each aircraft and there are two coupling designs approved for use on Enstrom helicopters: (1) the splined coupling, P/N 28-13609-1, and, (2) the 7-plate flex pack coupling (Dana Corp. Element No. A005-1992).

(b) For splined couplings found to have P/N 28-13609-1--

(1) Before further flight, remove from service and replace with an airworthy coupling any splined coupling which has 1200 or more hours' time in service;

(2) Before further flight, disassemble the tail rotor drive shaft couplings with less than 1200 hours' time in service and accomplish the following:

(i) Visually and dimensionally inspect for wear and proper tooth contact in accordance with Figure 1. Measure the height of the spline crown shown in Figure 2 at the center with a steel scale (having graduations of 1/100 inch) and a 10 power glass. Replace with airworthy parts any couplings that have a center crown height of less than 0.015 inch.

(ii) Test both the male and female portions of the coupling for material hardness. Test the male portion on the inner circular face as shown in Figure 3. Test the end of the stud of the female portion as shown in Figure 4. Use three readings and average the readings. Replace with airworthy parts any couplings which have average readings below 25 on the Rockwell "C" scale.

(iii) Magnetic particle inspect both portions of those couplings that have been installed on aircraft having a history of crash damage. Replace any couplings found to be cracked with airworthy parts.

(iv) Lubricate and reassemble couplings which meet the requirements of this paragraph before return to service.

NOTE: Enstrom Service Directive Bulletin 0065, Revision A, dated June 1, 1984, and the Maintenance Manual/Maintenance Manual Supplement for the respective models pertain to these procedures.

(3) At intervals not to exceed 100 hours' time in service after the initial inspection of paragraph (b)(2), partially disassemble the forward and aft tail rotor drive shaft couplings, P/N 28-13609-1. Repack the couplings with LE3752, Andok-B, Shell-14, Shell-16, or any grease meeting MIL-G-18709, prior to return to service;

NOTE: Enstrom Maintenance Manual, pages MM 3-5, MM 3-6, and MM 3-7 pertain to this procedure.

(4) Within 600 hours' time in service or at the next annual inspection, whichever occurs first after the initial inspections of paragraph (b)(2), and thereafter at each annual inspection, inspect and lubricate the forward and aft tail rotor drive shaft couplings in accordance with paragraphs (b)(2)(i) and (b)(2)(iv) of this AD; and

(5) Before reaching 1200 hours' time in service, replace with airworthy parts all couplings with P/N 28-13609-1.

(c) Rotorcraft that have Enstrom 7-plate flex pack couplings, P/N 28-01041-1, are exempt from the requirements of paragraph (b) of this AD.

(d) An alternate method of compliance, which provides an equivalent level of safety, may be usedwhen approved by the Manager, Chicago Aircraft Certification Office, FAA, 2300 East Devon Avenue, Room 232, Des Plaines, Illinois 60018.

(e) In accordance with Sections 21.197 and 21.199, flight is permitted to a base where the maintenance required by this AD may be accomplished.

This amendment supersedes AD 83-18-04, Amendment 39-4721.

This amendment (39-6686, AD 90-17-03) becomes effective on September 7, 1990.

AD 90-17-03

Figure 1. Checking inner spline wear on coupling.

NOTE: Tooth wear is measured by placing a 6 inch steel rule parallel to the crown at the top edge of the driven side. A piece of .125 x .010 inch shim stock is then placed between the tooth and the rule, and pressing the rule against the tooth, check if the shim can be removed. If the shim slips out, the coupling is to be rejected and replaced with an airworthy component.