Bjorn’s Corner: Keeping airliners operational

By Bjorn Fehrm

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.

Figure 1. The first modern maintenance program was formed around the Boeing 747, here at the roll out in 1968. Source: Wikipedia.

Keeping an airliner fit

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?

Aircraft maintenance

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.

The development of on-condition maintenance

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.

 Maintenance Steering Group

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.

11 Comments on “Bjorn’s Corner: Keeping airliners operational

  1. Thanks Bjorn. I have mixed feelings as you delve into the safety inspections required by airlines. Om the one hand its essential. On the other hand its a prime target for overegulation and resultant excessive cost to the airlines which can be abused by those who are for lack of a better word, anti-progress. Are we at the right place now in terms of safety or is more regulation needed? How many multiples safer than a bus do we have to be before its overkill.

    “It’s a small gas turbine and these love to run forever, not start and stop.”

    Are alternatives to gas turbines being investigated in the age of the battery.

    • The whole point of regulation is to set a minimum standard which every operator has to meet. This levels the playing field in the interests of the paying passenger; every operator has the same minimum maintenance costs, and they dare not underspend on maintaining their aircraft. Sure there’s ways in which costs can be reduced – pooling of facilities – but the same amount of maintenance time is required no matter who does it.

      And given the price of a ticket on a LCC flight, maintenance cost is clearly not a dominant burden on the operations of an airline.

      Far from being over regulated, many passengers see the airlines as being under regulated. They’re allowed to over book flights and cram passengers into cramped seat dimensions. There’s growing political pressure to introduce regulation on some of these aspects of airline operation, and the sooner the better I think.

      Would that affect profit? In the short term, yes. But in the long term it would expand the market further; there’s a lot of people who at the moment choose not to fly somewhere because it’s a terrible experience. For example Ryanair are profitable, but there’s lots of people I know who would never dream of setting foot on one of their aircraft ever again. Same increasingly is the case with BA.

      The horrible experience limits the market size. It’s hard to grow a market when what you’re selling is seen as being awful. Growing a market means persuading people to make that discretionary purchase, an impossible task for the LCC cattle herders.

      If seating dimensions became regulated, it sets a minimum ride quality below which no operator can fall. That ought to be a bit bigger than is typical at present. The public wants flying to be nice again.

      • Agree with your comments, very soon passengers will be fed up from airliners who aim to squeeze our pocket, introducing less comfortable seats to get more profit to their pockets. I wonder what is the roll of FAA & CAA groups here. I think they have to look at the comfort of the passengers beside whether the plan is safe or not. What I meant is not to narrow their interest to the reliability of the plane to keep it running in safe manner only.. I am a skinny person, I keep standing most of my flying time because the economy seats are quite small and not comfortable. I wonder every time while I am in the plan. what will happened to those handicap persons in case an emergency landing was there.
        Planes now a day looks like a chicken boxes and soon they will be converted to sardine cans if no authority put a limit to control that.

    • The series about keeping an airliner fit will dwell into this difficult trade. The Maintenance Steering Group (MSG) analyses, works out and mandates the rules for how to maintain airliners. It does that, taking into account all the safety and operational data available. As such it’s the best knowledge available which forms the trades and subsequent rules for the maintenance of airliners. It’s up to the OEMs to make better aircraft, based on these experiences, to bring down maintenance costs. And they do, each generation of air-frames costs less to maintain.

  2. I hope its over regulated, safety comes first! I say that as im about to hop on a plane in a few hours lol.
    Great piece once again bjorn.

    • Over regulation does not necessarily increase safety. The goal should be enough to maximize safety while keeping flight cheap enough for the maximum benefit.

  3. I think this subject matter is difficult to tackle because of it’s enormous scope. To me it starts with weaving reliability and safety into the design. I’ll bet many would be surprised at the fraction of the development budget devoted to reliability and safety engineering. No amount of maintenance can correct an unreliable design.

    This is so much so that Reliability Engineering has evolved into a separate discipline. I also think that the aeronautical industry are leaders in this field and have develop many of the tools and methodologies that other industries have adopted.

    That is why modern airliners are so reliable.

    As a product developer I always thought of the reliability and safety guys as antagonists, but they are part of the recipe.

    I look forward to the next installment.

    • I would like to reply to your comment with respect to the part where the aviation industry is a forerunner. After lots of reading and experience in aviation (Airworthiness Engineer), I have realised that the aviation industry is not even close to be considered a forerunner. Ideas from other sectors (e.g. Automotive, Shipping) are always underestimated and never applied unless someone forces you to do so.See as an example James’ Reason Swiss Cheese model, which is now a major theory in Aviation Human Factors.It was developed after a series of Shell Oil Rig accidents and was quickly adopted by them due to the cost savings risk analysis allowed them to have.In aviation on the hand, it was not until EASA that it became “important”. Nevertheless, Aviation can be summed up by this statement: “How many more people can I ferry around, with the lowest ACCEPTABLE level of safety, at the highest Allowable price at any time?”

  4. What are you lot on about. Flying so high that you need a life support system and so cheap that people commute and even unemployed westerners can afford the occasional flight. It’s unbelievable.

  5. Consider a modern airliner can routinely spend up to 15 hours in the air, flying a third of the way around the world, land for a couple of hours and do it all again. That flight could be mainly over water, with only two engines, and the nearest airport many hours away. that happens many times a day. The safety and reliability of modern air travel is truly amazing. The industry is obviously doing almost everything right. I wouldn’t want to see any less regulation or government oversight, just in case we discover that we only had just enough in the first place.

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