AWB 75-1 Issue 1, Cooling fans in piston engine helicopters
AWB 75-1 Issue 1, 15 March 2002
Cooling fans in piston engine helicopters
All piston engine helicopters that employ an engine-cooling fan driven by drive belts or via the main transmission.
To advise operators and aircraft maintainers regarding the proper maintenance techniques of cooling fans.
Reports of piston engine-cooling fan destructive failures have been initiated by fatigue cracking in the fan disc and at the fan blade roots. In many cases, this has been caused by improper maintenance techniques. In one case, failure was attributed to a change in the metal surface finish caused by abrasive scratches and shot peening the surface. This was done as part of the preparation when painting the fan. The manufacturer did not approve any of these changes to the surface finish. Changes to the designed surface finish can reduce the fans resistance to fatigue.
Maintain the fan strictly in accordance with the manufacturers recommendations. For example, do not change the surface finish from the original design by the manufacturer. This includes surface preparation for non-destructive inspection. Extreme care must be taken to ensure excessive abrasion (scratching and bead blasting) does not materially weaken the fan.
Where the manufacturer does not recommend painting, the painting of the fan is not to be carried out as this practice may lead to early fan failure. One of the problems associated with painting the fan, is that torque values may be reduced on the retaining nuts during the operation of the engine as the paint further compresses.
All maintenance must be strictly carried out in accordance with the manufacturers data.
Fan assembly technique
If the bolt torque is incorrect or the mating surface is poorly prepared, the bolt torque may be lost after the fan commences operation. Loss of hub assembly bolt torque means the hub cannot support the disc and blades properly, so resonant vibration motion results.
Rigging of the fixed tabs
When the fan is installed, the tip clearance must be carefully measured and adjusted, otherwise the unbalanced pressures will induce a resonant vibration motion. The small tabs cause the fan to have the same backpressure at each quadrant of the fan area. If the tabs are not in the correct place, have wrong clearance, one is missing or the tip clearances are incorrect, this may lead to fan failure.
Correct drive belt and shaft rigging
The fan is usually driven by a gearbox, which receives power via belts or a drive shaft. The drive belts and rubber couplings in the drive shaft help isolate the vibrations induced by gear meshing. It is essential they are rigged correctly to avoid undue vibration.
Cooling duct inspection plates
Never attempt to run the engine without all the cooling duct inspection plates fitted. Removal of a single duct cover plate upsets the pressure pattern that the fan operates in and causes a violent and destructive resonant type motion. Correct fitment of the plates is vital.