AN6 Bolts, Castle Nuts, and Cotter Pins on Boeing 737 Max 9
Aviation thrives on meticulous attention to detail, and nowhere is this more evident than in the selection of fasteners. The AN6 bolt, castle nut, and cotter pin trio are used on the Boeing 737 Max 9 aircrafts.
According to an investigation from the National Transportation Safety Board preliminary report released on February 6th, four AN6 bolts that hold the door plug in place on the Boeing 737 Max 9 were missing at the time of the blowout on Alaska Airlines Flight 1282 incident on January 5, 2024.
Source: CNN Key bolts were missing from a Boeing door plug that blew out in mid-air, report says
Demonstration from Pete Muntean, CNN Aviation Correspondent
AN6-32 bolt
Let’s look at the AN6 Bolt, the castle nut, and the cotter pin in more detail.
AN6 Bolt
The AN6 bolt, designated by Aerospace Standard (AS) NAS1438, forms the backbone of this trio that guarantees critical connections remain secure. The AN6 bolts come in various lengths and thread configurations. Below is a standard AN6-32 bolt commonly used in Boeing applications.
Specifications:
Material: High-strength steel (typically 4037 or 8740 alloy)
Diameter: 0.19 inch (4.83 mm)
Thread Length: 1 inch (25.4 mm)
Thread Type: Unified National Coarse (UNC)
Head Height: 0.20 inch (5.08 mm)
Head Type: Hexagonal
Thread Size: 6-32 UNC
Finish: Cadmium plated (per QQ-P-416)
Tensile Strength: 125,000 psi (862 MPa)
Grade: 8
Overall Length: Varies depending on application (typically 1.25 to 2.5 inches)
Applications:
The AN6 bolt finds diverse applications across Boeing aircraft, including:
Control surface attachments: Securing ailerons, elevators, and rudders to ensure precise maneuverability.
Engine mount components: Withstanding the demanding vibrations and loads experienced by engines.
Landing gear assemblies: Maintaining structural integrity for safe landings and takeoffs.
Interior panels and equipment: Providing secure installation for various systems and amenities.
The specific application determines the bolt's length and thread configuration. For example, longer bolts might be used for control surfaces, while shorter ones might suffice for interior panels.
AS315 Castle Nut
Partnering with the AN6 bolt is the AN315 castle nut, identified by AS NAS331. This hexagonal nut features slots cut into its circumference, allowing for the insertion of a cotter pin.
Specifications:
Material: Same as AN6 bolt (high-strength steel)
Diameter: Matches AN6 bolt diameter (0.19 inch)
Thread Type: Matches AN6 bolt thread (6-32 UNC)
Finish: Matches AN6 bolt finish (cadmium plated)
Slots: Two slots spaced 180 degrees apart
Function:
The castle nut's slots allow the insertion of a cotter pin, preventing the nut from loosening due to vibration or other stresses. This redundancy adds an extra layer of security compared to solely relying on thread friction.
AN181 Cotter Pin
The AN181 cotter pin, specified by AS NAS123, acts as the final line of defense. This U-shaped wire component secures the castle nut by passing through its slots and the hole drilled in the AN6 bolt's shank.
Specifications:
Material: Steel or stainless steel
Diameter: Varies based on bolt size (typically 0.048 inch for AN6)
Length: Long enough to bend over both sides of the castle nut slots
Finish: Cadmium plated or unplated
Function:
The cotter pin prevents the castle nut from rotating, ensuring the joint remains secure. Bending the pin's legs over the nut's slots creates a visual indication of proper installation and prevents accidental dislodgement.
The Trio
The AN6 bolt, castle nut, and cotter pin are used for aviation safety. Individually strong, their combined presence multiplies their effectiveness, safeguarding countless lives every time a Boeing aircraft takes flight. While often unseen, these unsung heroes play a crucial role in ensuring the smooth and safe operation of Boeing aircraft like the 737, 747, and 787 Dreamliner.
Further Reading:
Aerospace Standards Information: For information on NAS standards including NAS1438, NAS331, and NAS123:
National Aerospace Standards Committee (NASC)
Society of Automotive Engineers (SAE International): [https://www.sae.org/]
Federal Aviation Administration (FAA):
Aircraft Certification Service
Advisory Circular AC 20-150A - Design and Installation of Mechanical Fasteners
Boeing Fastener Information:
Boeing Commercial Airplanes - Supplier Information Management System (SIMS)
Boeing Fastener Standards Manual
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