It’s a Question of Balance
Wheel and tyre balance is an area that is often overlooked during aircraft maintenance. Most aircraft maintenance manuals provide at best only minimal information regarding the importance of balancing aircraft wheels and tyres and, as a result, there may be no official maintenance requirement for it, even CASAs own ‘Schedule 5’ makes no mention of balancing.
This state of affairs is truly extraordinary as I am sure all of us have driven cars when the tyres are out of balance, let’s face it no tyre fitter worth his salt would install new tyres on your car and not balance them – this is standard practice – and for good reason. With our aircraft, of course, out-of-balance usually manifests itself most dramatically just after take-off when quite violent shaking can occur until the wheels stop rotating. This is clearly not good for either our aircraft or our own heartbeat.
Apart from the aircraft manufacturers maintenance manuals, the aircraft engineers’ maintenance ‘bible’ is the FAA publication AC43.13-1B. In relation to aircraft tyres Chapter 9 of AC43.13 states: ‘Correct balance is important since a heavy spot on an aircraft tyre, tube, or wheel assembly is likely to cause that heavy spot to hit the ground first when landing. This results in excessive wear at one spot and an early failure at that part of the tyre. A severe case of imbalance causes excessive vibration during take-off and landing, especially at high speed.’.
When we fit new certified aircraft tyres to certified aircraft wheels there are specified installation procedures designed to provide ‘adequate’ (although not perfect) balance. Certified aircraft tyres and tubes are marked by the manufacturer with their light spots and heavy spots to aid balancing. Correct assembly using these marks is addressed in Chapter 9 of AC43.13 which states: “The correct assembly of the wheel affects the balance of the tyre. After the wheel halves and bolts/nuts have been inspected and found serviceable, put a little talc on the tube and insert it in the tyre. Align the heavy spot of the tube (usually marked with a yellow line) with the light spot of the tyre (usually marked with a red dot). If the tube does not have a balance mark, align the valve of the tube with the balance mark on the line”. Unfortunately that is the extent of the balancing information provided.
Nevertheless, many maintenance organisations do make an attempt to better balance wheel and tyre assemblies using a simple balancing rig such as a round bar through the wheel bearings supported either side on level horizontal knife edges. Such simple balancing rigs are commercially available but can just as readily be home made. It must be noted that full dynamic balancing, as is done for cars, is not really necessary as long as static balance is achieved. The simple balancing rig described is adequate for this. Dynamic wheel balancing rigs as used for car tyres are not suitable for small aircraft tyres as they will not work properly with the small tyre diametres involved.
Using the simple balancing rig described is easy. Once the wheel and tyre assembly is set up on the balancing rig, the tyre is left to rotate such that the heavy point finishes down. An automotive type mag wheel ‘stick-on’ weight is then added to the high point of the wheel rim. This process is repeated until the wheel and tyre assembly no longer comes to rest at the same spot but stops at random positions. This indicates that the assembly is in static balance.
Many modern sport and experimental aircraft have commercial non-certified tyres, wheels, or both, and have no balancing marks. Some of these sporty aircraft have quite high ground speeds during take-off and landing and with the small diametres involved, the wheel RPM can be quite high, so wheel and tyre assembly balancing becomes even more important. With small diametre wheels, however, there can be a problem with the stick on wheel weights. Firstly, the moment arm is small so a fair amount of weight can be required to achieve balance. Secondly, some wheels do not have a suitable surface to stick the weights onto so they tend to fall off in time. Additionally, it can be difficult to trim the weights accurately as they are usually about 4-6 mm thick. Years ago, when I had a Long-Ez, I came up with a simple solution that is very simple, effective and works well on even the smallest diametre wheels – I don’t use stick on weights but use sheet lead inside the tyre instead!
How you say? Easy! First off, I use the simple balancing rig described but, instead of trying to cut the thick stick on lead weights, I stick a piece of thin sheet lead (common plumbers’ variety) on the tyre tread with cloth type packaging tape. This is easy to trim as required with scissors. When balance has been achieved I index the wheel and tyre then deflate it. I then put the lead on the inside of the tyre in the exact position where it had been on the outside securing it with the same bit of tape. This tape holds it in position until the tube is inflated. The inflated tube then holds the lead very securely in place, the tape of course protects the tube from any sharp edges on the lead.
Now, it could be said, that my method is not an ‘approved’ method – well, maybe that is the case for certified aircraft but, on the other hand, maybe it is. Who knows? Nevertheless, the process works extremely well and with sport and experimental aircraft in particular nobody can really object. In all the years I have been using this method I have never had any problems and the wheel and tyre assembly stays in balance until it is time to change the tyre.