COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY
SCHEDULE OF AIRWORTHINESS DIRECTIVES
Robinson R22 Series Helicopters
AD/R22/35
Amdt 1
Rotor Mast Shaft Nuts 12/92
Applicability: All R22 series helicopters in service as at 30 June 1992 with the exception of the following:
- Helicopters S/No 2178 and on; and
b. Helicopters with main rotor gearboxes manufactured or overhauled by the Robinson Helicopter Company after 15 June 1992.
Requirement: 1. Unless already internally inspected for mast shaft nut looseness since 30 March 1992, remove main rotor gearbox from aircraft using procedures specified in the R22 maintenance manual.
Note: Any gearbox previously inspected for nut looseness since 30 March 92 may remain in service subject to a log book certification declaring that compliance with the AD is not applicable and referring to a statement of actual work carried out during such inspection.
2. Remove sump of gearbox noting number and position of shims between case and sump and inspect both shaft nuts, P/N A153-1 and lock washers, P/N A269-1 for looseness or damage.
3. Using special tool MT 124-1 remove both nuts, clean shaft and nut threads using MEK.
4. Inspect gear teeth for signs of damage and also, inspect dowels on gear carrier for damage. Report any damage or looseness of nuts to the CAA immediately.
5. If undamaged and no wear is apparent, fit a new lock washer on shaft ensuring the lock washer dowel pin locating holes are correctly mated with the dowel pins on the gear carrier. Coat threads of the lock nut with Loctite 620 and, while still wet, assemble nut on rotor shaft.
6. Using special tool MT 124-1, or suitable equivalent, torque nut from 135 to 140 ft-lbs. Ensure 2 tabs of the lock washers align with nut. Do not untorque nut to align lock washer tabs with nut.
7. Install second tab washer and align 2 tabs with first nut. Apply Loctite 620 to threads of lock nut and, while still wet, install. Torque nut from 75 to 90 ft-lbs using special tool MT 124-1 or a suitable alternative.
8. Ensure lock washer tabs are correctly bent and not cracked, four tabs to upper lock nut and two to lower locknut. Repeat requirements 5, 6 and 7 if tabs found cracked.
9. Install new o-Ring P/N A215-271 on sump. Lubricate with A257-2 oil and install sump.
COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY
SCHEDULE OF AIRWORTHINESS DIRECTIVES
CAUTION
Be sure all sump bolts have the same shim stack-up as measured in Requirement 2.
10. Torque the sump bolts and drain plug as follows:
Eight locknuts on the AN4 bolts, 70-80 in-lbs plus nut drag. Two MS20074 bolts (cap screws), 60 in-lbs and safety wire. Drain plug A7620, 150 in-lbs and safety wire.
Note: Be sure that the A059-62 ground wire is re-attached under the nut located aft of the fwd RH mount.
11. Using procedures specified in the R 22 maintenance manual re-install gearbox in helicopter and carry out associated maintenance manual requirements.
Compliance: At the next inspection for issue of a maintenance release after 1 August 1992 or by 31 March 1993, whichever occurs first. Requirements 2 to 10 are to be carried out by organisations specifically approved by the CAA to carry out this directive.
Note: Arrangements have been made for certain acceptable organizations to obtain the necessary spares and tooling to carry out this work. A listing of those organizations can be obtained by contacting a CAA District Airworthiness Office.
Should an operator be unable to achieve compliance through this source, consideration will be given to extending the approval to other organizations highly experienced with helicopter gearbox heavy maintenance, upon request, provided they can obtain the necessary spares and tooling support.
Background: Amendment 1 to this AD is raised to exempt specific helicopters and main rotor gearboxes from teardown, inspection and reassembly because the Robinson Helicopter Company has introduced new inspection, torqueing and assembly procedures that make it unnecessary for the nominated helicopters and main rotor gearboxes to comply with this Directive.
The Directive was raised initially because there have been 2 reported cases of failure of the mast support structure due to the rotor shaft nuts becoming loose and allowing the rotor shaft to pull through the retention bearing in the gearbox. Both incidents occurred as collective was being pulled on the ground and were characterised by the shearing of the rivets attaching the mast bearing outer housing to the main rotor shaft as the vertical loads transferred from the main gearbox bearing to the top of the mast.
The failure was accompanied by a loud cracking sound and in one case an increase in manifold pressure as the effective length of the pitch change links altered the rotor pitch setting as the mast shaft moved vertically. Several more instances of loose nuts have been confirmed by inspection; one with fretting of tab washer and nuts, another with gear wear/spalling and others with no other apparent damage.