A simple hose tells a sobering tale about the inherent complexity of aircraft systems.
Anyone who operates or maintains aircraft needs to know the difference between complicated and complex. It’s not just a matter of how many components a system has but how they interact. The difference is safety-critical. The Harvard Business Review says:
Complicated systems have many moving parts, but they operate in patterned ways. The electrical grid that powers the light is complicated: There are many possible interactions within it, but they usually follow a pattern. It’s possible to make accurate predictions about how a complicated system will behave. For instance, flying a commercial airplane involves complicated but predictable steps, and as a result it’s astonishingly safe.
Complex systems, in contrast, are imbued with features that may operate in patterned ways but the interactions of which are continually changing. Three properties determine the complexity of an environment.
- The first, multiplicity, refers to the number of potentially interacting elements.
- The second, interdependence, relates to how connected those elements are.
- The third, diversity, has to do with the degree of their heterogeneity (or variety). The greater the multiplicity, interdependence and diversity, the greater the complexity.
Under this scheme a jigsaw puzzle is complicated but not complex. There is only one way it can all fit together. In contrast, an aircraft is inherently complex because it satisfies the three criteria: lots of different parts that strongly influence each other’s function.
Complicated systems are predictable: complex ones are not or, at least, not always. The tale of a simple hose on the Eurocopter AS350 Squirrel helicopter makes the point well:
Eurocopter made a running change to the hydraulic hoses used in the flight control system on the AS350 series helicopters from a polypropylene hose to a stainless steel-braided Teflon line hose. The new hose had the theoretical advantage of being more durable, and was not subject to the 5000-hour/six-year limited life of the polypropylene hoses. The new lines were sheathed in an orange-coloured fibreglass and a silicon fire-proof shield or cover was clamped to the pressure hose at each end.
Immediately upon their introduction, however, these new hoses developed leaks. An investigation by Eurocopter found that the Teflon lining in the hoses could charge with static electricity through a triboelectrical phenomenon (friction/rubbing caused by the oil passing through the hose) and discharge into the stainless steel braid when its electrical potential reached the Teflon tube’s insulating limit. This electrical discharge (sparking/arcing) could create a micro-perforation of the Teflon lining in the hose and cause the hydraulic oil leaks, as per Eurocopter Information Notice 2506-I-29.
The ‘complexity issue’ of the simple hose problem came to light when a conscientious operator had, ahead of the requirement to retire polypropylene hoses, installed a set of six of the new stainless steel-braided Teflon hoses, unaware that Eurocopter had already identified that these newly manufactured hoses could develop leaks – but was continuing to supply them.
The operator then had replaced (under warranty) two of these new design hoses, one after the other, on the same helicopter due to leaks. The helicopter was then dispatched to fight bushfires, but during the ferry flight a third new hose failed, developing an undetected leak which was severe enough to deplete the hydraulic reservoir from MAX to MIN in the single ferry flight, lasting about three flight hours. On landing, the contents of the hydraulic reservoir were found coating the interior of the transmission area and dripping on to the ground.
Out of a set of six new hoses, three had failed. But what if they had not failed in succession, but failed together during the same flight and the failure went undetected? What if more than three of the six hoses had failed at the same time? The most obvious outcome would be the loss of hydraulic power for the flight controls. The AS350 has a small accumulator at each actuator to provide a short reserve of hydraulic power, not enough perhaps to approach and land with, but enough for a quick-thinking pilot to reconfigure the helicopter to reduce speed control loads while reverting to manual control—and land immediately.
A more dramatic result for the same type of defect was considered when an in-flight fire occurred in an AS350 when an electrical fault had damaged a single leaking un-sleeved flight control hose. EASA AD2011-0033 describes complex system failure well. An undetected in-flight fire can cause loss of hydraulics, possible shut-down of the engine, loss of control of the main rotor system, fire into the cabin and subsequent loss of control of the helicopter.
Eurocopter offers a replacement for the problem hoses, known as MOD 074239, with another hose series known as MOD 074686, while continuing to provide the Teflon hoses, even though the reliability of the MOD 074239 hoses is not assured.
In Airworthiness Bulletin 29-004, CASA makes three recommendations to Eurocopter operators:
- Do not install the Eurocopter Teflon/stainless steel hoses (MOD 074239)
- Ensure all pilots and maintenance personnel closely monitor installed Teflon/stainless steel hoses for leaks
- Remove the Teflon/stainless steel hoses from service, and install MOD 074686 hoses at their earliest convenience.
The concept of complexity, and the unpredictable dangers that can follow, affect more than Eurocopter helicopters. Explanations of chaos theory often invoke the butterfly effect as an illustration. The theory says: in a complex system the beating of something as tiny as a butterfly’s wings is enough to cause an unpredictable result. That’s why weather forecasting is an inexact science. A similar mythical beast inhabits the fuel system of the AS350 and, no doubt, many other aircraft. The only difference is that in these types it has not yet fluttered its wings.
The way tiny changes in the chemistry of a hose interact with a certain grade of hydraulic fluid to cause an unanticipated effect is food for thought for any aircraft designer, constructor, engineer, or owner considering a modification. Experimentation, innovation and improvisation have a hallowed place in engineering culture, but in aircraft design, construction and repair they need to be accompanied by analysis, conservatism and caution.
Sargut, G., & McGrath, R. (2011). Learning to Live with Complexity. Harvard Business Review, 89(9), 68-76