14 CFR Part 25.607 – Fasteners

In this post, we will explore the next regulation, 14 CFR Subpart C Section 25.607 – Fasteners.

Before you read on, I suggest you read through this advisory circular: AC20-71

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§ 25.607 Fasteners

(a) Each removable bolt, screw, nut, pin, or other removable fastener must incorporate two separate locking devices if—
(1) Its loss could preclude continued flight and landing within the design limitations of the airplane using normal pilot skill and strength; or
(2) Its loss could result in reduction in pitch, yaw, or roll control capability or response below that required by Subpart B of this chapter.
(b) The fasteners specified in paragraph (a) of this section and their locking devices may not be adversely affected by the environmental conditions associated with the particular installation.
(c) No self-locking nut may be used on any bolt subject to rotation in operation unless a non-friction locking device is used in addition to the self-locking device.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]

In the previous posts, we looked at:

In the last post we studied the importance of fabrication methods and process specifications. In this post we will dig into a very important design feature that must be used for critical fasteners to comply with this regulation.

(a) Fasteners: Dual Locking Devices

As is clear from part (a) of this regulation (see the regulation text above), removable fasteners that are primarily intended to transfer shear load (single or double) between critical joint members must incorporate two separate locking devices or features. This is applicable to bolts that also serve as axis of rotation for the joint members.

It is common practice to include one locking device, such as self-locking nut with a detent in the threads. But for critical applications such as control surfaces rotating using clevis and bolt fulcrum parts, dual locking devices are mandatory as the loss of this bolt (slipping or sliding out due to failure of the single locking device) would impact safe flight and landing.

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(b) Fasteners: Environmental Impacts

This could be high temperatures, corrosion, cracks etc. Therefore, only approved materials with proper heat treatments must be used for critical applications. Common fastener specifications include NAS, MS or Hi-Lok type fasteners that are commonly used for most applications.

(c) Fasteners: Rotating Bolts

If the bolt or fastener is designed to rotate in the applications, or is allowed to rotate, then a self-locking nut or any self-locking device is not allowed. In such cases, other non friction type devices must be used such as cotter pins that pass through the shank and the head of the bolt.

Read this article for more details: Non Self-Locking Nuts

Alright, I think that pretty much sums it up. Cheers! Oh and don't forget to comment and share this post, use the sharing methods below.

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Surya Batchu
Surya Batchu

Surya Batchu is the founder of Stress Ebook LLC. A senior stress engineer specializing in aerospace stress analysis and finite element analysis, Surya has close to a decade and a half of real world industry experience. He shares his expertise with you on this blog and the website via paid courses, so you can benefit from it and get ahead in your own career.