Fully accredited for the overhaul and repair of landing gears, flap tracks and flap carriage systems, and hydraulic systems on all models of Boeing aircraft (including the McDonnell Douglas MD-11), Hawker Pacific Aerospace is one of the world’s leading providers of aircraft landing gear maintenance services. As Solutions learns, the work it carries out is governed by stringent quality procedures, specifications and processes.

Hawker Pacific Aerospace is one of four similar facilities within Lufthansa Tecknik – the engineering division of the German airline – located in close proximity to major international airports in the UK, Germany, China and the USA.

Its latest acquisition is a Danobat HG 92 heavy-duty CNC cylindrical grinding machine installed in purpose-built premises in Hayes, West London, in August 2009. This machine accommodates grinding wheels up to 1,015mm diameter, and is capable of grinding an 850mm maximum diameter on workpieces weighing up to 5,000kg held within a maximum between centres distance (chuck to motorised tailstock) of 6m.

Part of the Danobat Group, Newall UK is responsible for sales and support of all Danobat products within the UK and Ireland, and has taken a lead role in the successful delivery and commissioning of the new HG 92. In its long history both Danobat and Newall have produced some of the most advanced grinding machines ever built, and their long association with innovative engineering is a matter of record within aerospace companies such as Hawker Pacific and in other high technology industry sectors. 

Complete responsibility

“Although Hawker Pacific is part of the Lufthansa Group, it operates as a separate entity here in the UK,” says Les Flitcroft, Hawker Pacific’s process manager, ”which means we are responsible for every aspect of the business, from securing orders to ensuring a customer’s aircraft is back in service as soon as possible. As part of this remit we have to be able to deal with every eventuality, including AOG (aircraft on the ground) situations typically involving axle changes, brake sleeve changes and shock strut seal changes.

“Our priority is to ensure that we are fully equipped to deal with long range wide bodied airliners like the B787, so our tendering procedure for the new machine was based on the size of component that we expect to be grinding for this type and size of aircraft. We eventually opted for Danobat, in part because of our past experience with Newall, but also because the HG 92 makes use of in-house designed and manufactured modular units rather than sourcing from other OEMs – and because of Danobat’s IDEKO technological R&D centre, which is highly regarded worldwide.
“Danobat’s vision of the future is in step with our thinking,” he continues, “and it has demonstrated its innovative expertise by developing a high precision grinding machine with the swing and between centres distance that we require, together with the option of both internal and external grinding.”

Burnt fingers

Although the internal grinding capability is not needed at this point in time, the option is there, courtesy of the HG 92’s B-axis (continuous with a minimum programmable increment of 0.0001°), and only requires the additional spindle and motor to be fitted. However, the HG 92 is equipped as standard with the DanMDM multi-diameter absolute gauging system for in-process gauging, and is also equipped with probing for length, face and taper measurements.

Measurements can be carried out before and after grinding, with the Windows-based DanOp front end programming system making any necessary corrections as a function of measured values. The DanOp software minimises set-up as it enables the user to complete the machine settings, the wheel dressing management and the grinding programming quickly and easily. The Siemens 840D conversational control is used to call up standard programs and to enter all relevant component data without any requirement for the operator to have experience of ISO programming.

“Although we employ highly skilled grinders rather than machinists that can turn their hand to a bit of grinding, we wanted everything with DanOp to be as simple as possible to minimise the possibility of error,” Mr Flitcroft adds. “Another major requirement is to prevent burning of the workpiece. Typically the material being ground is high tensile 4340 alloy steel that is extremely hard (300ksi) but it is very easy to burn.

“Should this happen the material becomes very brittle and the component is scrap so it is not so much a case of speeding the job up but more a case of avoiding possible damage to high value components. This requires a safety mechanism to prevent the grinding wheel from crashing into the workpiece on its approach, and a sensing mechanism to detect pressure build-up resulting from glazing of the wheel. If a pressure build-up is detected, the wheel is backed off automatically and an additional dressing cycle completed before machining recommences.” 

Play it by the book

All work undertaken by Hawker Pacific is governed by stringent quality procedures and the detailed specification and designated overhaul processes defined in Boeing’s Component Maintenance Manual (CMM). It is a 24 hours a day, seven days a week operation overseen by a computerised production control system that is subject to change on a daily basis. This system, which includes materials requirements planning, barcoded routing systems and electronic data order processing, tracks every job through every repair operation while providing an essential real time overview.

“Because we are an overhaul and repair facility we don’t know what we may be faced with on a day to day basis,” Mr Flitcroft affirms. “Landing gear can come in here at any time from any airline around the world and can be different every single time, although the overhaul process is basically the same. We remove all bushes and bearings, subject each casting or forging to a complete visual inspection, and then carry out bakes, machining, NDT, plating and painting.

“Every aspect has to be inspected before re-assembly and issuing of final certification that the work has been done in accordance with the CMM.”

Newall UK
www.newall-uk.com