Caveat emptor (Let the buyer beware)

“Caveat emptor” translated to English: “Let the buyer beware,” or translated to Afrikaans this could be: “Goedkoop is duurkoop.”

It would seem that on a weekly basis containers are arriving in this country with homebuilt aircraft that have been purchased in the USA. As we all know, the world economy is in all sorts of problems at the moment and for years the crisis in the USA has forced many to sell off all their toys. As a result we see the arrival of homebuilt aircraft purchased for a 'song.' To the seller, the ever-present legal liability threat is greatly reduced by exporting the aircraft and unfortunately there are some good buys to be had and some really bad ones.

This column will not advise on importation of aircraft, but rather concentrate on the pitfalls and suggest ways of avoiding buying an aircraft that will cost a lot to rectify. Buying a metal or rag and tube aircraft does make a pre-purchase inspection easier, but in the case of a composite aircraft, you really have little chance to see what lies beneath the glossy paint job, as this can hide many mistakes or omissions. The problem is exacerbated by builders who are either too 'clever,' or too lazy to read the plans and just do things as they see fit.

From personal experience with 'imported' homebuilt aircraft, I inspected one almost 3 years ago and the owner is now nearing completion, having had to strip the aircraft completely and rebuild it according to the plans as there were so many error, faults and deviations that the aircraft was considered to be not airworthy. What amazed me was that this aircraft had been signed out by somebody when the annual inspection was done for the change of ownership the previous year. Fortunately the new owner agreed with me and set about correcting the many snags. So, a pre-purchase inspection by a knowledgeable person with that type of aircraft would be money well invested.

Recently another 'imported' aircraft was taken to a friend to do a few repairs due to fuel vaporization problems experienced during flight, all sots of 'fixes' had been incorporated during the 180 hours the aircraft had accumulated to cure the problem. All were unsuccessful, as the high-pressure electric fuel pump had been mounted on the engine side of the firewall where all the pipes, pump, bypass etc. were exposed to heat from the engine. This fuel pump was then moved to the cockpit floor, as called for in the plans, all the 'fixes' were removed and the fuel pipe plumbing returned to design specification. Naturally this cured all the problems with regard to fuel vapourization. Why the FAA inspector and the local AP allowed this aircraft to fly is a mystery to me, as it was patently unsafe considering all the 'fixes.'

This same aircraft had many other snags; the oil cooler had been badly mounted, resulting in high oil temperature. Returning the oil cooler to the position called for in the plans, cured the problem. Strange isn't it… Further inspection found that the anchor nuts (called for in the plans) had never been fitted to the lower inboard ribs of the wings and as a result, the lower fuselage skin had never been attached to the wings' lower skins; reducing the structural integrity of the wings.

This one snag, was enough to advise grounding of the aircraft, removal of the wings and fitment of the anchor nuts as called for in the plans, but here the plot thickens! As the builder had (apparently) not read the plans (which calls for trimming of the rear spar attach structure, so as to ensure the wing leading edges are at 90° to the centerline and in a straight line) the wings had been fitted with a slight forward sweep; as a result the holes in the lower fuselage skin for the attachment screws, ran off the edge of the skin. A second extension to the skin had to be added to allow attachment of the lower fuselage skin to the wings.

Again this could have been picked up during a pre-purchase inspection, and should have been picked up by the local assembler and the local inspector.

In this type of aircraft the main spar carry-through structure and the wings' spars, have their attachment holes reamed to size, for NAS bolts. These bolts are identified by a depression (dent) in the top of the head of the bolt.

These bolts have shanks with a smooth (ground) finish and are larger in diameter than standard AN bolts and are therefore a tight fit. As a result the plans advise the builder to use common hardware store bolts with the threads removed and the shank ground and polished into a bullet shape, when attaching the wings to the fuselage initially, to set incidence angle etc. as the wings could be fitted and removed a few times. A bit of grease helps when fitting these bolts and prevents damage to the reamed holes. For final assembly the NAS bolts need to be cooled to shrink them to assist fitment. A good three star (***) deep freeze will cool to about -16°C, this will help if left overnight. The best way is to buy methanol (methyl alcohol) or as a substitute try MEK (Methyl alcohol, Ether and Keytone) and add some dry ice to this, this will cool the bolts to about -40°C and do that in a minute or two. With these bolts having been shrunk and the spar structure at a nice warm room temperature, the bolts will be fitted relatively easily. Do not take a big hammer and try to smash the bolts in, nails are driven into wood; and NAS bolts are not nails.

A NAS bolt destroyed by a bad builder

Never drill out these holes, so that the bolts are an easy fit! And, yes, it has been done before. Metallurgically, a hole through the structure disrupts the flow of stresses through the metal. In Layman's terms, by reaming the holes and using an 'oversize' bolt, this puts the metal around the hole in compression and the stresses flowing through the metal then do not 'see' the hole and are routed around the hole. Again these bolts should be fitted with the aid of a lubricant such as grease, but better still a corrosion preventative lubricant such as Mastinox.

Closer to home, I recently purchased an aircraft from the family of a late friend. I was familiar with the aircraft, having done the first test flights personally, shortly before the owner/builder's untimely departure. On arrival at my home base, a few niggly little snags were attended to with the most annoying being the movement of all the Plexiglas and the cracking of the paint in the vicinity of the side windows and windscreen. So I decided to see what was going on under the paint. Here I found that peel-ply had been used as 'fiberglass' to secure the windscreen to the forward fuselage. This could not be seen under the filler and paint and was only apparent after destroying the body to windscreen fairing. It was then also discovered that a flexible adhesive had been used to attach the Plexiglas to the structure and not an epoxy glue as called for in the plans.

Unfortunately the glue called for is deemed to be 'Dangerous Goods' and is difficult to import, as a result I now have a flexible filler/adhesive and no paint between the Plexiglas and the structure to mask any movement.

Now considering that I have a big job ahead of me including painting the fuselage, I decided to replace the avionics too so it will be a while before this aircraft flies again. Unfortunately, removing the avionics revealed a few other problems in the wiring and as the autopilot was misbehaving, this was returned to the manufacturer (on the advice of the manufacturer) due to potential damage done by the bad wiring.

When this aircraft does fly again, it will be a safe and reliable form of transport for years to come.

Keep it safe out there.

Chalkie Stobbart - Technical Talk
Aircraft Technical

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