AC NO: 20. DATE t wvm 71 ADVISORY CIRCULAR DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION SUBJKT: DUAL LOCKING DEVICES ON FASTENERS 1. PURPOSE. This advisory circular provides guidance and acceptable means, the sole means, by which compliance may be shown with the require- for dual locking devices on removable category airplanes. and transport fasteners installed in not ments rotorcraft: 2. REFERENCES. Federal Aviation Regulations 25.607, 27.607, and 29.607. 3. to secure fasteners device. locking (i.e., bolts, Service experience that due to maintenance, manufacturing, is not adequate category airplanes. for critical integrity loss of is common practice locking device and transport fastener BACKGROUND. It screws, nuts, pins, etc.) with a single has indicated a single rotorcraft of these aircraft, locking the transport craft or the environmental given to installation fastener for fastener; that amended to specify for critical category airplanes, conditions in determining therefore, two separate devices on all applications. removable involving fastener loss of locking could the fasteners jeopardize it was considered necessary or design errors, applications Due to a number of instances installed on two separate in such fasteners to require in any installation the safe operation and that consideration associated with a particular the appropriate locking in which of rotor- must be devices FARs 25.607, 27.607, and 29,607 were devices on removable fasteners 4. REMOVABLE FASTENERS. By a removable attachment is meant the conventional screws, pins, etc., which can be readily replaced by other fasteners. identical fastener devices such as bolts the referenced in removed and reinstalled and nuts, or rule.8 5. LOCKING DEVICE. By a locking device on the the fasteners may be either fastener which will its proper fastener in prevent installation. the is meant the loss of this feature incorporated fastener and retain friction or nonfriction types. The locking devices on the Permanent, solid, Initiated by: FS-120 

AC 2C- 71 nonmetallic locks provided friction they inserts perform are their considered intended acceptable functions, as friction-type 6. 7. DUAL LOCKING DEVICE. two separate locking A dual features locking described device on a in paragraph fastener 5. consists of any BOLTS SUBJECT TO ROTATION. if of bolt and one or more as an axis serves it components A bolt rotation considered is producing to the attached relative bolt. to be subject motion to rotation between the 8. APPLICATION. or The most dual locking system where loss critical airframe power helicopters safe flight individually locking determined, be carefully and with fastener feature stripping between the to s similar features. introduce etc., under a. b. c. reduction applications and transmission obvious critical would devices the failure of application be in the of a single of aircraft’s fastener control such as non-failsafe capability. landing gear system loss landing. to determine where and in many aircraft and primary powerplant of a single fastener Each application critical joints devices are necessary. fastener incorporating Once such considering the the selected functional performance dual structural of the removable fasteners primary joint However, structural as well can there joints joints as control result are with in other on in the the could should where a critical locking continued of controls preclude be evaluated fasteners with dual has been should strength conjunction joint devices fatigue in and the proper the environmental installation. in a bolt-nut conditions Provisions torque nut fastener, and may have and standard In addition, detrimental bolt bolt-nut the the interference reduced when this fastener Installation with all operating conditions. fastener associated with to accommodate example, for dual altered the particular the may have area fastener double fastener structure, locking in bearing the bolt-nut without of the adjacent locking the the thread is compared should not equipment, should loss of etc., Care the nut, including be exercised the fastener under the most vibration and that to assure as a bolt, such severe expected temperature, where each due lock will to backing in off fact of prevent the operating applicable. conditions,. that Assuming fastener should such malfunctioning device fail to one of not two required properly the perform or failure should separate or should not cause locking fail the for other devices any on reason, the locking or become ineffective. Page 2 Par 5 

13/8/W AC 20- 71 d. Fasteners which have been removed for any reason during inspection evaluated incorporated of for continued air- locking devices Fasteners determined to be worn and and proper and maintenance should be carefully worthiness before unairworthy for inadvertent but which nevertheless reinstallation. reinstallation functioning return should be marked conspicuously to service. give the appearance of suitability their to prevent e. Bolts which are subject type l&k. a friction The other lock, or nonfriction-type if dual lock. to rotation must have at least one nonfriction- locks are required, may be either f. Fasteners which are not subject type if dual locks are required: locks to rotation may have the following (1) Two nonfriction-type locks. (2) Two friction-type locks. (3) One friction-type and one nonfriction-type lock. one-of-a-kind that many designs of fasteners; It should be noted custom-designed, is made to porate dual approval of a particular in considering application it has been determined if with FARs 25.607, 27.607, or 29.607. illustrate locking any particular devices. The following joints may require therefore, no attempt types of fasteners which incor- checklist may be useful fastener for a specific that compliance is required 9. CHECKLIST. a. b. C. d. e. f. Are two locking devices or features on the fastener? Would either itself installation? prevent one of the the loss of two locking devices on the the fastener and retain fastener it in by its proper Will of either one locking device effectively lock and prevent the loss the fastener when the other locking device fails or malfunctions? Has the fastener been substantiated for adequate structural strength? Has the appropriate ? fastener been substantiated for fatigue strength, when Has the suitability substantiated temperature of fastener, for all environmental the , susceptibility to corrosion, including conditions devices, locking such as vibration, etc., where applicable? been Par 8 Page 3 

8* h. i. k. 1. the Will condi tiona ? fastener perform its intended &xtione under all operating the Will interference under all installation fartener with adjacent dtructufe, the of operatiug conditions? introducs coaQomnt8, equipment, etc., no detrimental the fastener If two locking (bolt) is eubject devices a nonfriction-type? to rotation, is at least one of the the If been determined, device8, before including installation? airworthinees locking fastener has been ueed previously, ha8 continued proper functioning of incorporated correct detrimental the If duce incorrect .installation? or incomplete effecte, inetallation proper are mean8 of a fastener would provided to prevent intro- an the design especially Are and, functioning, feature8 the locking and proper and physical features, positioning characteristic8 that such the of purpoee, faetener their is easily recognized? Page 4 Par 9