Rivets and Airplane Maintenance

Rivets and Airplane Maintenance

Junkers F 13

Junkers F-13 CH-59 displayed at the Aviation and Space Permanent Exhibition, Budapest. Photo: Attila Szabo, Curator

Universally taught at all aircraft mechanic schoolsairplane rivets are a time-honored method of attaching two metal surfaces together, for airplane maintenance.  Rivets are generally superior to welds when it comes to airplane maintenance.  Solid shank aluminum rivets first made their appearance in 1919, with the German Junkers F-13, which was the first civilian all-metal airplane. Also, the Junkers F-13 was the first aircraft to appear without spars. This innovative aircraft manufacturing technique has proven to be of lasting appeal through the ensuing decades, due to the inherent advantages of rivets for simplicity, cost, reliability and weight. An aircraft designer specifies rivet fasteners in their blueprints for the airframe, and will identify a certain rivet type by using rivet part numbers, a drawing symbol or a head marking. For airplane maintenance, it is essential an aircraft mechanic understand how to read the manufacturer blueprints and understand what type rivets are needed in a particular airplane.

Junkers F13

Credit: Wikipedia - Avion Junkers F-13 en el Deutschen Museum

Rivet heads come in various sizes and head styles, including

  • Universal
  • Countersunk
  • Flat
  • Round

By convention, rivet sizes increment in steps of 1/32″. Rivet diameters range from 3/32″ to 1/4″.   Stock rivets are usually cold formed from aluminum alloy that has been annealed. These rivets are usually called “Manufacturer’s Heads”. Common types of manufacturer’s heads are Universal Head rivets and Flush Head Rivets (Countersunk Rivets). A common Universal Head rivet is designated as rivet type MS20470, which is the most common, and strongest solid aluminum rivet. It is used in internal airplane structures, where a protruding head is not significant. On external aircraft surfaces, a countersunk rivet becomes essential. The following video gives some fundamentals of flush head riveting, and is very instructive! These are key points to keep in mind for airframes and airplane maintenance.

Some Aircraft Rivet Conventions in Airplane Maintenance

MS20470 Universal head rivets, used in airplane maintenance

MS20470 Universal head rivets

Rivet lengths go up in steps of 1/16″. In common usage, popular drill bit sizes for solid rivets are

  • #40 drill bit    =>  installs 3/32” rivets
  • #30 drill bit    =>  installs 1/8” rivets
  • #21 drill bit    =>  installs 5/32” rivets
  • #11 drill bit    =>  installs 3/16” rivets

A properly installed rivet will have a diameter about 3 times the thickness of the thickest airframe sheet used.  A good rule of thumb is to assure the rivet length protruding from the far side of the hole is approximately 1.5 times the diameter of the hole in the metal. In the aviation industry, several rivet types are in common usage. They are categorized as one of:Rivet head types

  • plain (“A”)
  • dimpled (“AD”)
  • double dimpled (“M”)
  • raised dot (“D”)
  • raised double dash (“DD”)
  • raised cross (“B”)
  • raised circle (“E”)
  • three raised dashes

With aluminum rivets, the shear strength can vary from 10,000 psi to 41,000 psi. The AN or MS number of a rivet identifies the head shape, while the letters identify the alloy in the rivet. Rivet standards are supplied by three organizations, Aluminum Association (AA), National Aerospace Standard (NAS), and Military Standards (MS).  A specific rivet part number follows the coding convention of:

  • Style of Head
  • Rivet Material
  • Diameter of the rivet Shank
  • Rivet Length

For instance, a MS20470AD5-6 rivet would be broken down into:

  1. MS20 :  Military Standard category 20
  2. 470:    Head Style Code
  3. AD:    Rivet Material Code
  4. 5:  Shank Diameter in units of 1/32”
  5. 6:  Rivet length in units of 1/16”

 

Rivet quality is addressed in the U.S. Fastener Quality Act of 1999. Materials used in a rivet must adhere to certain standards specified in this act.

The importance of rivets in airplane maintenance

To show you just how important it is to use correct rivet technique in airplane maintenance and manufacture, a terrifying incident on a Southwest Airlines 737 airplane occurred in 2011, in which a 5 ft. long section of aircraft skin ripped off in flight at 30,000 ft. This failure exposed passengers to the sky, it might have been fatal, and was ultimately found to be created by a manufacturing defect in which rivet holes were not sized correctly during manufacture 15 years before. At a seam in the fuselage, where one piece of metal overlapped another, some of the rivet holes were not sized correctly during manufacture. As a result, two aluminum skin pieces were not fastened together tightly enough at the seam, during manufacture. As time passed, apparently the area was stressed, resulting in the cracking.  Afterwards, investigators found widespread cracking in the surface metal.  Luckily, no one was injured from this incident.

Lastly, returning to early rivet history, here is a short video, showing an operational Junkers F-13, the aircraft in which rivets were first used.  At around 0:10 in this video, you can see some of the rivets on the Junkers F13 aircraft skin.

Aircraft rivets are derived from decades of development in airplane maintenance, and are necessary training at any aircraft mechanic school.

by Steve Adams

Junkers F13

Junkers F-13 (Credit: Andi Szekeres, www.idflieg.com)

 

 

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3 Responses to “Rivets and Airplane Maintenance”

  1. Rivet history | Zurichgnome Says:

    [...] Essential Rivet Knowledge for Aircraft Mechanics … [...]

  2. Airplane blueprints | Hatcarl Says:

    [...] Essential Rivet Knowledge for Aircraft Mechanics … [...]

  3. A and P License Says:

    One thing my teacher at my aviation maintenance school promise him was that I would never practice sheetmetal repairs! Lets just say it wasn’t my strong area. I give major props to the sheetmetal artists out there who keep our old plans flying safely!