1937 Airplane Airworthiness Regulations - Wires

[Mechta]

Several times in the past I've come across statements that particular aircraft designs stopped production, or only continued under grandfather rights, due to airworthiness regulations introduced in 1937. The Douglas DC-3 and Aeronca C-3 being two aircraft in particular.

In the case of the Aeronca C-3, it has been written that its multi-strand wire bracing was outlawed, hence the introduction of the strut equipped Aeronca K. Given that a lot of modern ultralights/microlights use multi-strand wire bracing, why was it banned, and what makes it acceptable today?

In the case of the DC-3, I believe it was the control cable sizes which were the issue. Why was it deemed necessary to go to larger sizes, as clearly the ones used must have been sufficient?

I presume these are the regulations in question:

DOT Online Database (http://specialcollection.dotlibrary.dot.gov/Document?db=DOT-CARS&query=(select+6+(byhits+(field+DOCUMENT+(phrase+CIVIL+AIR+RE GULATIONS)))))

[onetrack]

Mechta - I can't give you a specific answer to your question, but multi-strand wire rope has specific peculiarities that must be addressed in use and in application.

In positions where regular loading is applied, steel wire rope is prone to stretching, unlike solid struts.
Steel wire rope is prone to internal corrosion and fretting (wear of the strands) via constant movement.
Nearly all SWR has its wire strands lubricated or coated during manufacture with a product that ensures either lubrication of the wire strands or corrosion protection of the wire strands - or both.

In use, the lubricant or coating can be washed out under pressure, or it can be affected by salt or other aggressive contaminants that remove the lubricant or coating, thus leading to internal corrosion and therefore weakening of the tensile strength of the SWR.

To try and combat corrosion, wires are constructed from stainless steel or galvanised (or tin-coated) steel. However, S/S wire is of lower tensile strength than the higher grades of carbon steel (size-for-size), so the wire must be larger diameter if made from S/S, to have the same tensile strength as a smaller diameter carbon steel wire.

Galvanised or tinned coatings can be removed by abrasion and chemicals, thus initiating corrosion, so galvanising or tinning is not the total solution.
Many designs of SWR contain a hemp or cotton core which holds a reservoir of lubricant, which extends the life of SWR.
However, SWR with this style of core is banned from aviation use, as it is much weaker as regards tensile strength - and as you could imagine, replacing a specified steel-core rope with a hemp-core rope, would lead to disaster.

Steel core rope is used in all applications where high strength is required - in cranes, in elevators, in aviation applications. The common material in SWR used outside aviation use, is the high-strength carbon steel known as Plow Steel.

The other potential problem area with SWR is the various methods of securing the ends of the SWR. This can be done by a number of methods, but a common method is the SWR wrapped around an eye or thimble and secured by a ferrule.
Wire rope grips can be used to secure the ends of the SWR as well, instead of ferrules, but wire rope grips are banned from use in aviation.

There have been a wide range of methods of securing the ends of SWR rope used over the decades. Many are totally unsuitable for aviation use.

As regards the control cables, I would guess the problem was purely one of engineering miscalculation, in not allowing for practical problems, such as miscalculated wear rates, and inadequate provision for allowance of the real-world problems of dust, grit and accidental kinking - all of which pose serious shortening of planned life-expectancy of SWR and control cables, which utilise stranded wire rope in their construction.

The below article has a very good outline of designs, definitions, descriptions, uses, potential problems and solutions, associated with SWR and cables, as utilised in microlight aircraft. The information starts from section 5.5.

http://www.bmaa.org/files/044_2_sigma_2.0_combined_binder_accepted_final_01-2010.pdf

[Fareastdriver]

The problem with steel wires can be seen with the Forth Road Bridge. This is a multi-wire rope suspension bridge built about twenty five years ago. The cables are losing their strength through corrosion and a replacement bridge is being built alongside.

When the new bridge comes into operation all the heavy traffic will go that way leaving the old bridge for pedestrians and bicycles; until it gets too expensive to maintain, say ten years.

[Mechta]

Onetrack, thank you for the detailed answer, I can see the problems the 1937 regulations were aimed at overcoming. The microlight inspector's guide should be compulsory reading for anyone who flies. It certainly makes the reason for introducing Section S apparent. I only hope every would-be SSDR designer reads it.

Fareastdriver, A good example which was already entering my thoughts halfway through onetrack's response!

[Herod]

The original Austin Maxi had a cable-operated gear-change. A bit of stretching introduced all sorts of fun, I gather. I had one of the later ones, where common-sense prevailed and the cables were replaced by rods.

[Democritus]

The problem with steel wires can be seen with the Forth Road Bridge. This is a multi-wire rope suspension bridge built about twenty five years ago.

Nit-picking I know, but the Forth Road Bridge was opened by the Queen 52 years ago tomorrow on 4 September 1964. Mrs D was at the opening as a 9 year old and remembers it vividly. It's been fascinating watching the replacement bridge (Queensferry Crossing) growing since they started work in 2011. It's planned to open next May.

[Wander00]

Democritus - I too was there, as a Cranwell cadet on the "Navy visit" -in fact I was one of two cadets in HMS Rhyl. The Queen could not see the flotilla because of the fog, and the following day on departure for a week's courtesy visit to Stockholm, one of the frigates sailed through HMS Lion's anchorage, but unfortunately the destroyer was still there, and there was a loud clang and a big hole in Lion. The run ashore in Stockholm was pretty good though. Indeed the lovely Iain A-Rees, sadly no longer with us, who was the other cadet in HMS Rhyl, and I for years after greeted each other "They don't dance like that in Pinner" (where we had both grown up, but had never met before Cranwell, a reference to the very close dancing style of the statuesque Nordic blondes at the various parties.