Aluminium alloy tubes have been the main structural element of many microlights since there inception.
They have been used for wing construction, fuselage construction and cockpit construction.
This makes them the single most important component in any aircraft using them.
Tubes of any type are going to be prone to many forms of stresses and these all need to be checked for.
AC43.13-1b has a large section on tubes of all types but aluminium tubes need special attention with regard to microlights.
Dents:
Dents in the wrong area of tubes can reduce their strength dramatically especially if the tube is under compression. Tubes including struts are under tension most of the time but can be exposed to negative loads in rough air, poor handling or aerobatic manoeuvres.
Dents will normally be caused by mishandling on the ground, i.e. knocking a wing against a hanger door or over tightening a bolt.
To check for dents run your hand along the tubes to feel for irregularities and sight along the tubes.
Bends:
Bent tubes also have their compressive strength reduced dramatically.
Bends can also be caused by poor ground handling, some form of accident or incident involving a hard landing or by excessive G loads.
If you know an aircraft has been involved in the afore mentioned it will require a thorough check, sight along all wing spars and compare side to side.
Any irregularities need to be investigated further.
Enlarged holes
Vibrations and general movement in flight can cause bolt holes to become flogged and elongated. The process of enlargment weakens the tube but will
also stress the area around the hole and may lead to cracks forming. Check for enlarged holes and reject or sleeve as required.
Cracks:
Cracks can cause catastrophic failure and are most likely to be found around bolt holes and rivet holes, these areas are weaker than the surrounding
tube and are areas where stresses are transferred.
Cracks can start small and enlarge quickly, they may be small enough to allow normal flight but break under higher or unexpected loads.
Cracks in tubes can not be tolerated and must be checked for. Tell tale signs are normally thin black lines radiating from the source of stress. Strut, wire and cross tube junctions need particular attention.
Corrosion:
Corrosion is covered in great detail in AC43.13-1b but will still be mentioned here as it can be very destructive to aluminium tubes.
Any aircraft flown on or near the beach will have a much higher chance of suffering corrosion than an aircraft flown exclusively inland. None the less corrosion needs to be checked for. Any tube which has a sail or some other component rubbing on it may have its protective layer of anodising or paint system worn or compromised in some way. These areas need special attention. A trike for instance which has been rigged on the beach may even have sand inside the sail, this is very abrasive and needs to be removed and the wing washed to remove any trace of salt.
Any hole drilled through a tube will be an unprotected area as will the entire inside of a tube (anodising does not form on the inside of tubes due to the way the electric field works during the anodising process).
Pitting can occur which can lead to deep holes, causing damage much larger than the appearance would lead one to believe. If corrosion forms around a bolt or rivet hole, it is not hard to imagine how the tube strength can be compromised.
All the above need to be inspected and checked for when inspecting tube aircraft, AC14.13-1b describes methods or repair for small areas of corrosion but large areas or deep penetrations will render the tube un-serviceable similarly bent, dented or cracked tubes may also be rendered u/s.