Hydraulic

Hydraulic maintenance lessons from the Qantas mid-air emergency

You probably recall hearing about the Qantas 747-400 which had an oxygen bottle blow a hole in its fuselage. This resulted in decompression of the cabin, requiring the passengers and crew to don oxygen masks and the pilots to perform an emergency descent and landing. No one was injured.

Nothing grabs the attention of the travelling public like an emergency involving a commercial jetliner. And so this incident made headlines all over the world. Now the maintenance of the aircraft is under scrutiny.

It's important to understand that the commercial aviation industry is a leader in maintenance strategy and procedure. And for good reason too. Their business depends on it. If the public lost confidence in air travel as a safe means of transport, the airlines are out of business.

Look at what happened to Concorde. After 25 or so years operating without incident, one aircraft goes down with the loss of everyone on board and now it's a museum exhibit.

The other side of the airlines' operational safety imperative is the need to intelligently manage maintenance expenditure and downtime. And so it was the civil aviation industry who gave us Reliability-centered Maintenance, or RCM.

The RCM framework evolved over a period of 30 years but really gained momentum when Boeing were developing the first 747. Due to the size and complexity of the 747, it soon became apparent that without a more strategic approach to maintenance than was the practice at the time, the 747 would spend more time in the hangar than it would in the air.

In a nutshell, RCM considers the probability and consequences of failure in the maintenance task selection process. From John Moubray's definitive text on the subject, RCM II:

If a failure has operational consequences, a proactive task is only worth doing if the total cost of doing it over a period of time is less than the cost of the operational consequences and the cost of repair over the same period. In other words, the task must be justified on economic grounds. If it is not justified, the initial default decision is no scheduled maintenance.

For failures with safety or environmental consequences, a proactive task is only worth doing if it reduces the risk of that failure to a very low level indeed - if it does not eliminate it all together. If a task cannot be found that reduces the risk of the failure to a tolerably low level, the item must be redesigned or the process must be changed.

RCM can be an evolving process because it's virtually impossible to anticipate and mitigate every possible failure. So assuming a failure in the oxygen system was the root cause of this incident, it's likely the maintenance of this system will be reviewed within the above framework.

Sitting below the maintenance task selection process are the procedures and check lists which help ensure selected tasks are carried out correctly.

In a previous article, I talked about the disturbing number of accidents in the aerial access industry. It seems to me that the aerial access and aviation industries have something significant in common. And that is, if you use a mechanical device to defy gravity, it's wise to leave as little to chance as possible. If a boom lift or a jetliner comes down in an uncontrolled fashion, people get killed or injured.

In the aviation industry, nothing happens by way of aircraft operation or maintenance without a procedure and a corresponding checklist. And this is fundamental to the airlines' excellent safety record - which as already mentioned, is the premise on which the public's confidence in air travel is based.

The same can't always be said about the aerial access industry specifically - and the hydraulics industry generally. In fact, the use of procedures and checklists is such a valuable and yet much overlooked aspect of hydraulic equipment maintenance, that I spend a whole chapter outlining its many benefits in Insider Secrets to Hydraulics.

There's two parts getting any maintenance task right: knowing what to do; and remembering to do it. When changing out a piston pump for example, you can't pat yourself on the back for filling the pump housing with clean hydraulic oil, when you forgot to open the intake isolation valve before starting the engine.

This sort of mistake is easily prevented by using a procedure and check list. When followed, it eliminates human error. That's why the airlines use them. And why the Qantas incident will most likely turn out to be a maintenance task selection issue, rather than a maintenance procedure issue.

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