Constant speed propellers are wonderful inventions – they automatically adjust to suit your engine’s peak power and torque and thus best efficiency. This is really useful for takeoff performance and comfort and ease in the cruise. It’s like a car’s automatic transmission but better because it does not sap energy through an inefficient torque converter.
The only drawback with a constant speed (CS) prop is that it weighs a bit more than a fixed pitch prop – and it needs regular maintenance. The propeller manufacturer provides maintenance intervals for overhaul on all their propellers by flight hours or calendar months, whichever occurs first. OFH has a 1997 Hartzell prop which has an overhaul interval of six years or 2,400 flight hours. In South Africa, if a prop has used less than half of its time between overhaul (TBO) hours limit it can have a detailed mid-life inspection at five years and then a full overhaul every ten years.
Our current steed, ZS-OFH, is just 19 years old, but the prop was due for its second full overhaul before the next MPI was due.
Seven years ago, our Cessna 182, ZS-FPI, needed her prop overhauled. I was recommended to Joc-Air and was very impressed with their service, and especially their aftersales care when one of their engineers came out on a freezing winter Saturday morning to try to dynamically balance the prop before we left on our Botswana safari. (The vibration turned out to be an ignition timing issue and not the prop’s fault.) So, it made perfect sense to courier the prop up to Lanseria while OFH was getting her MPI.
The prop was removed and then beautifully crated by Side-slip Aviation at Morningstar Airfield. A few days later it was delivered by truck to Lanseria. When it was unpacked by Joc-Air, the prop was first visually inspected to determine if it qualifies for an overhaul – or has to be scrapped.
The reasons that a prop would not qualify for an overhaul are usually damage from a ground strike or severe corrosion. If the prop passes the visual inspection, it is placed on a stand and, using special tools and much muscle power, is disassembled and the parts stripped and cleaned. This is reminiscent of open heart surgery. The workshop seems as clean as an operating theatre and the hub halves are prised open like a chest cavity, revealing the complex heart of the propeller hub with its thick green grease instead of blood.
Once the blades have been removed and cleaned, the blades’ width, thickness and length are measured at specific stations as prescribed by the manufacturer. Tolerances are measured on the many components that make up the hub of a constant-speed prop and part numbers are checked for provenance. The blade measurements and other measurement inspections become part of the work order folder. Each part must be inspected, measured, and/or replaced as necessary to meet the manufacturer’s guidelines.
The paint is stripped off the aluminium and steel components – generally through bead blasting. The aluminium parts are then etched with a caustic solution. All parts will then be inspected for cracks and defects using Non-Destructive Testing (NDT) consisting of magnetic particle, eddy current inspection and fluorescent dye penetrant using an ultraviolet light.
The prop is then reassembled and set to the specified torque values and blade angle settings. The overhaul manual for each manufacturer has an established list of parts that must be replaced at each overhaul, including seals and O-rings, mounting studs and nuts, blade ball and needle bearings and all hardware. Airworthiness Directives (ADs), Service Bulletins and Service Letters are checked and done as required. The details regarding the work done and the replacement of parts become a permanent part of the work order folder and thus the history of the prop.
Finally, the hub is lubricated, and the prop blades reinstalled and statically balanced. OFH’s prop was carefully painted in the original matt grey with white tips and even new stickers were applied. The prop was then carefully repacked in its shipping crate, which looked like a giant pizza box, and couriered back to Morningstar.
Once the prop has been refitted to the aircraft, a dynamic balance should be performed, but I am very pleased to be able to report that even without a dynamic balance it is running far more smoothly than it was before the overhaul.
Budget around R23,000 for a three-bladed prop overhaul – but it is money well spent on your prop’s longevity and your peace of mind. More than one pilot has had to do a forced landing because of a prop runaway when it runs to the full fine limits and effectively stops delivering thrust. It happened to a fixed gear Saratoga a few years ago and the pilot managed to save the day with a forced landing on the N1 highway north of Bloemfontein.
Joc-Air has been in business since 1992. Their services include overhaul, midlife inspection, new parts, forward exchange propellers and blades, dynamic balancing, assembly, disassembly, lightning strike repairs and repaints.
They also provide away-from-base services, including dynamic balancing, assembly and repairs worldwide. Notably too, they have 17 years of experience in composite blade overhaul and repairs.
For OFH’s Hartzell propeller Joc-Air was able to provide cold compression rolling and pitch change knob shot peening. Over and above the knob shot peening and cold compression rolling, Joc-Air also have Hartzell approval to perform blade airfoils shot peening and composite blade overhaul, including leading edge erosion shields replacement.
Joc-Air is located on the south side of Lanseria at Hangar 36, through Gate 13.
You can contact Andre at +27 11 701 3114 or Leon at +27 82 743 0086 or e-mail them at email@example.com