The wind blows, the sun shines, a storm brews, and rain falls. Weather is the ultimate everyday talking point. Stand at a bus stop, start a conversation and it’ll likely be about the weather. Snow, sleet, ice or hail the atmosphere can be hostile to our best laid plans. It’s important to us because it affects us all. It has a common effect.
We started a discussion of common-mode failures in earlier paragraphs. We’ll follow it up here. Aircraft systems employ an array of strategies to address combinations and permutations of failure conditions. That said, we should not forget that these can be swamped by common-mode effects.
Environmental effects are at the top of the list of effects to consider. It’s a basic part of flying that the atmosphere changes with altitude. So, aircraft systems and equipment that work well on the ground may have vulnerabilities when exposed to large variations in temperatures, atmospheric pressure, and humidity.
Then there’s a series of effects that are inherent with rotating machinery and moving components. Vibration, shock impacts and heat all need to be addressed in design and testing.
It is possible to apply statistical methods to calculate levels of typical exposure to environmental effects, but it is more often the case that conservative limits are set as design targets.
Then there are particular risks. These are threats that, maybe don’t happen everyday but have the potential to be destructive and overcome design safety strategies. Electromagnetic interference and atmospheric disturbances, like lightning and electrostatic discharge can be dramatic. The defences against these phenomena can be to protect systems and limit impacts. Additionally, the separation or segregation of parts of systems can take advantage of any built-in redundancies.
Some common-mode effects can occur due to operational failures. The classic case is that of running out of fuel or electrical power. This is where there’s a role for dedicated back-up systems. It could be a hydraulic accumulator, a back-up battery, or a drop-out ram air turbine, for example.
Some common-mode effects are reversable and tolerable in that they don’t destroy systems and equipment but do produce forms of performance degradation. We get into the habit of talking about failure as if they are absolute, almost digital, but it’s an analogue world. There’s a range of cases where adjustments to operations can mitigate effects on aircraft performance. In fact, an aircraft’s operational envelope can be adjusted to ensure that it remains in a zone where safe flight and landing are possible, however much systems are degraded.
Probabilities can play a role in such considerations. Getting reliable data on which to base sound conclusions is often the biggest challenge. Focusing on maintaining a controllable aircraft with a minimum of propulsion, in the face of multiple hazards takes a lot of clear thought.
John Vincent 