Loss of lubrication to the main gearbox (MGB) has been responsible for several crashes, ditchings and precautionary landings in large transport category helicopters in recent years. These accidents set off broad debate as to exactly what is required of main gearboxes under Part 29 certification of the U.S. Federal Aviation Regulations, even as OEMs devised fixes for actual or perceived deficiencies of these components and their related lubrication systems. Chief among the certification requirements, that has caused some confusion, is the assumption that gearboxes must be shown capable of running without lubrication for up to 30 minutes.
AIN spoke to Giuseppe Gasparini, head of transmission systems design and development at AgustaWestland, to find out his views on the regulation and the company’s design strategies to meet requirements. Gasparini has more than 30 years of gearbox design and development experience. Earlier this year AgustaWestland announced that its new AW189 medium twin, the civil variant of the AW149, could continue to run for 50 minutes after loss of gearbox lubrication.
Q: What does the 30-minute run-dry requirement really mean?
A: I prefer to say “Loss of Lubrication,” or LoL, and not “run dry” or “run without oil”; we are demonstrating the capability of the gearbox to operate and transmit torque after the loss of most of its original lubricant when the oil is suddenly lost, but some residual lubricant is still inside the gearbox. This means the MGB can continue to run for 30 minutes without loss of drive or any other catastrophic failure after the pilot notes a major lubrication system failure, normally identified as a low pressure warning, and immediately reduces power to the minimum required to maintain forward flight and lands as soon as possible.
Q: What types of gearbox designs are more susceptible to lubrication failure?
A: The loss of oil is more likely with gearboxes that are pressure-lubricated. Decades of development and service experience has demonstrated that there are some weak points on the MGB lubrication system that can cause rapid and complete loss of oil: mainly the oil pipes, fittings, connecting coolers, filters and other components of the lubrication system itself.
Q: So what is the fix for the problem?
A: For many years now, our MGBs have been designed to exclude the use of any external pipes and fittings. This is not easy because the filter, cooler, fan etc. have to be fully integrated in the MGB castings and all the pipes are replaced by cored passages in the castings, but it represents a dramatic improvement over conventional designs and in reducing the probability of actual gross oil leakage. Also, any cover subjected to oil pressure such as the filter head of the MGB is secured with multiple fasteners and tested for loss of at least one fastener. Lubrication to our MGBs is supplied by dual pumps working in parallel. If one fails or jams for any reason, it is automatically excluded and the remaining one provides the additional flow of oil. This failure condition still generates the MGB oil pressure warning, but the pilot is able to immediately distinguish it from a total loss of oil and should be able to complete the flight.
Q: What other design factors can mitigate MGB failure?
A: There are five main factors determining the capability of an MGB to operate for a prolonged period of time after “loss of oil.” The design of the MGB must balance all of them to achieve and maintain for the longest possible period of time a thermal equilibrium condition whereas the critical temperatures, although high, are constant or increasing at a low and controlled rate.
These factors are:
1. Low friction at gear and bearing contacts. This is achieved by keeping a low friction coefficient by means of the residual oil lubrication and by low gear surface roughness by super-finishing the gears and by using special low-friction coatings.
2. Low sliding at gear and bearing contacts. This comes down to design choices. Coarse pitch gears are stronger but have more sliding velocity. Journal bearings have to be excluded because they will seize in seconds after lubrication supply reduction. Rolling bearings, despite of their name, experience rolling and sliding motion. In order of decreasing sliding we can list: cylindrical rollers, ball, spherical rollers and tapered rollers. In particular tapered rollers involve kinematically a significant sliding. For this reason at AW they are not used at high or moderate speeds but only at very low speeds.
3. High “hot-hardness” of gears and bearings such that they maintain their original size, shape and roughness up to the highest temperature
4. Clearances and plays must be maintained at all gear and bearing contacts throughout the highest expected temperatures. Loss of gear teeth and bearing backlash will generate unwanted tightness causing an uncontrolled and exponential increase of the contact forces.
5. Heat removal from the hot spots by conduction and convection to dissipate at the highest possible rate the heat produced by friction at the sliding contacts. It is even more important to avoid thermal gradients and allow uniform and progressive thermal expansion of all the parts.