A Material World
So what is so different about aircraft materials and hardware, what sets them apart from the normal ‘commercial’ variety that we can purchase from a hardware shop? The answer is both simple and complex – we know exactly what we are getting when we buy the aircraft grade stuff! It is not that the aircraft grade is necessarily stronger and better, but it is consistent. It always meets at least the minimum specifications that the aircraft designer expected when he designed the aircraft in the first place. When purchased through an authorised source the product will be supplied under cover of an ‘Authorised Release Certificate’ (ARC), also commonly referred to by the now superseded (and incorrect) term, ‘release note’. This is the certification that the product meets the specifications relating to it and that the documentation trail can be traced right back to the original manufacturer or overhauling facility as the case may be. The ARC is a legal document and can only be issued by an organisation approved to do so by CASA or corresponding overseas regulatory authority.
Now, as we know, most aircraft on the Australian register are American or are of American origin, so naturally the majority of aircraft materials and hardware used in aircraft construction are also of American origin and meet American specifications. This is not necessarily a problem except that the good old USA has not transitioned to the metric system of measurement, so goods sourced from there, including aircraft hardware and materials are measured in the old imperial units (inches, feet, pounds etc), not the metric units we are familiar with in Australia. This, of course, introduces the first incompatibility between commonly available aircraft grade materials and hardware and commercially available ones – the sizes differ slightly. For instance a ¼” aircraft bolt is actually 6.35mm diameter, so if we try to replace it with a 6mm bolt the bolt would be sloppy in a ¼” hole and, of course, a 7mm bolt would not fit.
So, leaving the dimensional incompatibility aside, what else sets aircraft materials and hardware apart? Aircraft grade material and hardware are certified as meeting at least the minimum strength specified and assumed by the aircraft designer! Furthermore, unlike commercial products that regularly become obsolete and discontinued, on the whole aircraft materials and hardware do not. For example even today you can still readily buy a bolt, such as an AN series bolt, that may have been specified for an aircraft that was built in say 1940. The AN bolt you buy today will exactly match the strength and dimensioning of the 1940′s bolt and, consequently, it will be a direct replacement. With the long product life cycle of aircraft this is an important consideration.
Let’s look at some of the most commonly available materials and hardware. Firstly by far the most commonly used aircraft grade material used in aircraft construction is 2024-T3 aluminium alloy. This material is used for some 80-90% of the structure of an average metal aircraft as it is strong, light, tough and relatively easy to form. When compared to mild steel, 2024-T3 is stronger than the steel and is only about one third of the weight. Other aluminium alloys are used in aircraft construction of course, some being stronger, some being softer and easier to form, but 2024-T3 remains the most common. So what about commercially available aluminium alloys? Most commercial products constructed from aluminium use softer, more readily formed grades. Often the particular grades chosen are designed specifically for commercial applications such as being easy to extrude (example: window frames), decorative (brushed aluminium lighting fixtures), resistance to corrosion (boats), etc. For aircraft of course, strength, toughness and lightness are the primary considerations. The bottom line is not to use commercial grades in your aircraft: not only are they usually inferior in strength but their quality may be variable between batches, manufacturers and suppliers. There is usually no documentation provided with them to certify that they meet a particular standard or specification and, often, they carry no identifying markings: you really don’t know what you are getting.
Just as for the aluminium alloys there are many types and specifications of aircraft grade hardware, particularly fasteners, used in aircraft. Nevertheless, for light aircraft at least, the most commonly encountered will be AN series bolts and nuts, MS20470AD (universal head) and MS20426AD (countersunk head) rivets. Airframe screws will usually be AN or MS series but there are too many varieties to expand on here. As before, these items are not necessarily stronger than commercially available items, but their quality and properties are consistent and are certified under an ARC to meet the relevant specifications so can be used in your aircraft with confidence.
Obviously the aircraft design engineer has to have an authoritative source of data and strength values of all of the available varieties of aircraft grade materials and fasteners. Two US documents can be considered the ‘bibles’ on the subject. The first is FAA Advisory Circular AC 43.13-1B, ‘Acceptable Methods, Techniques, and Practices Aircraft Inspection, Repair and Alteration’. Every aviation buff should have a copy of this: it is readily available from most aviation bookshops and suppliers. The second is the US Department of Defence 1700 page document titled ‘Metallic Materials and Elements for Aerospace Vehicle Structures’ – also known as ‘MIL-HDBK-5’. This handbook is not so readily available and is not for the faint hearted, but it is the authoritative reference. So for the serious aircraft designer out there go get yourself a copy, but don’t try to read it in one sitting!