AD 90-17-02

90-17-02 ENSTROM HELICOPTER CORPORATION: Amendment 39-6688. Docket No. 90-ASW-28. Applicability: Enstrom Model F-28, F-28A, F-28C, F-28C-2, F-28F, 280, 280C, 280F, and 280FX series helicopters certificated in any category.

Compliance: Required as indicated, unless already accomplished.

To prevent the loss of tail rotor thrust and directional control that could result in substantial damage or loss of the helicopter, accomplish the following:

(a) Within the next 5 hours' time in service after the effective date of this AD, inspect the tail rotor drive shaft couplings. Enter the part number of the two drive shaft couplings installed, the number of hours' time in service on each coupling, and the date in the log book.

NOTE: Enstrom helicopters use one of the following three coupling designs:

(1) splined coupling P/N 28-13609-1, (2) five-plate flex pack coupling (Dana Corp. Element No. A005-1991); or (3) seven-plate flex pack coupling, (Dana Corp. Element No. A005-1992).

(b) Before further flight, remove any five-plate flex-pack couplings, Dana Corp. P/N A005-1991, found during the inspection. After removal:

(1) Inspect the condition of the coupling hub flanges, Enstrom P/N 28-13613-1 and 28-13614-1, to which the couplings mount. The flanges must be flat within 0.010-inch, and the bolt holes must not have any evidence of elongation. Replace any flange deformed beyond these limits with an airworthy part before further flight; and

(2) Install the seven-plate flex-pack coupling, Enstrom Kit No. 28-01041-1.

NOTE: Enstrom Kit No. 28-01041-1 includes all of the necessary parts to replace a five-plate coupling with a seven-plate coupling. Two kits per rotorcraft are required. The replacement beveled washers for the seven-plate flex-pack coupling are 0.010-inch thinner than beveled washers for the five-plate flex-pack coupling. Consequently, a direct exchange should not require any additional shimming.

(c) Within the next 100 hours' time in service and at 100-hour intervals thereafter, inspect each seven-plate flex-pack coupling, Dana Corp. P/N A005-1992 or Enstrom Kit No. 28- 01041-1, for compliance with deformation and airworthiness limits as follows:

(1) Locate the tension and compression sides by comparing the couplings to Figure 1.

(2) Inspect the couplings for flex pack distortion in the shape of a bow. Compare any distortion to the acceptable limits shown in Figure 2.

(3) Inspect the couplings for flex pack distortion in the shape of an offset bend. The acceptable limits shown in Figure 3 are as follows:

(i) The offset on one set of plates must not exceed 0.015 inches.

(ii) The maximum difference in offset from side to side (one pair of bolts to the other pair) must be less than 0.007 inches.

(iii) The maximum allowable shim thickness is 0.072 inches (not including bevel washers).

NOTE: The replacement of the five plate coupling with the seven-plate coupling requires a thinner set of beveled washers which are included in Enstrom Kit Number 28-01041-1.

(4) Replace any seven-plate coupling, that either--

(i) Exceeds the airworthiness limits specified by paragraph (c); or,

(ii) Has accumulated 1,200 hours' time in service.

AD 90-17-02 FIGURE 1

Tension Sides Versus Compression Sides Tail Rotor Drive Shaft Coupling Installation. Rear coupling shown, forward coupling similar. NOTE: Tie-wraps eliminated for clarity.

(d) After removal of either seven-plate flex-pack coupling, inspect and replace, as necessary, the coupling hub flanges in accordance with paragraph (b)(1) of this AD.

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

(f) An alternate method of compliance or adjustment of the compliance time, which provides an equivalent levelof safety, may be used if approved by the Manager, Chicago Aircraft Certification Office, ACE-115C, FAA, 2300 East Devon Avenue, Des Plaines, Illinois 60018.

This amendment (39-6688, AD 90-17-02) becomes effective on September 7, 1990.

Figure 2 - Acceptable Limits of Bowed Flex Packs. NOTE: Tie-wraps eliminated for clarity. AD 90-17-02

Figure 3 - Acceptable Limits of Flex Packs with Offset Bends. NOTE: Tie-wraps eliminated for clarity. AD-90-17-02