April 20, 2017, ©. Leeham Co: We expect our flights to depart on time and with 100% safety. At the same time, the aircraft is used up to 14 hours and flies up to 5-6 missions a day.
This means 1,800 flights a year. As airliners will last 25 years, we talk about 45,000 flights with 99.7% dispatch reliability and 100% safety.
It’s clear the aircraft must be cared for in a special way. We’ll discover how.
Aircraft, whether used or standing still, deteriorate. The atmosphere and the air in the cabin cause corrosion of the metals. Seals of rubber or polymers age. The metal in the pressurized cabin is stressed 5-6 times a day as the air-condition system maintains a breathable atmosphere. The metal gets tired from the cycling and cracks.
The brake/tires and landing gear takes a beating six time as day. The Auxiliary Power Unit is started and stopped, running only short cycles when the aircraft is on the ground and during take-off and landing. It’s a small gas turbine and these love to run forever, not start and stop.
We could go on with the 40,000 wires which lead to electrical systems and avionics. These can develop bad contact at the joints. We could talk about the hydraulic or pneumatic systems and the cabin’s myriad of systems. The described parts and systems all need care and attention. How to do it in a rational manner?
And how do you carry out the changes and modifications that become mandated over time by the FAA or other authorities?
The first airliners were kept fit by the pilot and the mechanic who was part of the crew. Gradually, they learned what to check before flight and what needed inspection and be taken apart after a day’s hard work.
Over time, air certification authorities got involved in the what was required to allow an airliner to transport passengers. The de Havilland Comet crashes in 1954 (because of metal fatigue) shocked a world dreaming of fast jet travel. The knowledge gained in finding the cause laid the ground for rules and requirements around modern aircraft structures.
The jet aircraft following the Comet were only accepted for transport of passengers if the manufacturers had devised a maintenance program for their inspection and care. Flight safety and reliability was in focus.
The first maintenance programs put limits for how many flight hours/cycles or calendar months an aircraft could be operated before it was subject to disassembly and inspection of parts. Worn or faulty parts were repaired or replaced and the aircraft was re-assembled, test flown and went back to service.
The maintenance programs were preventive in nature and hard-timed. You flew to the limits, stopped, maintained and flew again. Gradually, system parts were defined as Line Replaceable Units (LRUs) and a chain of such parts formed a loop around a repair shop (hydraulics pumps, brakes, landing gear, electronic boxes…). Units which had reached their limit were dismantled, and a replacement unit, which was zero-timed by the maintenance shop, was mounted and flew again.
In 1960, the FAA and a number of airlines formed a task force to investigate how effective the preventive maintenance was. It wasn’t very effective. The prevention of malfunction of complex systems by dismantling them at regular intervals and replacing functioning units with overhauled units wasted effort and money without significantly increasing reliability. Many times, touching of a running system created new faults.
The task force also found that many systems in the aircraft were not suitable for hard time maintenance. What the systems needed was a monitoring process that allowed disassembly and replacement/repair when certain conditions were met. Complementary on-condition maintenance should be introduced besides the hard-time maintenance.
An on-condition maintenance concept requires that one can continuously monitor what is going in the system. At a prescribed limit of one or several surveillance parameters, an action will be triggered. A simple example is the monitoring of tire and brake pad wear. When the tire tread or pad depth reach a certain limit, the tire/brake pads must be replaced.
Monitoring other systems required new techniques. Oil sampling with detection of metal residues in the oil could trigger actions for hydraulics systems, engines or APUs. This was also true for flight control gearboxes and air conditioning turbines. The first ideas for built-in monitoring and test for system units were developed.
As the 1960s drew to an end, it was clear that the maintenance of aircraft and the decision on which type of maintenance the multitude of systems should have, needed a more structured decision logic. A Maintenance Steering Group was formed in 1968 with participants from FAA, aircraft manufacturers, suppliers and airlines.
This group developed the first modern maintenance program around Boeing’s new 747 airliner. We will look into the structure and principles for such a program in the next Corner.