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Environmental testing: hot or cold?
Garnet Ridgway says:Testing aerospace systems in extremes of cold is without doubt one of the most challenging tasks faced by the engineering community.
As anyone involved in testing will appreciate, trials often consist of long periods of inactivity punctuated by intense, time-critical data-gathering phases. In temperate conditions, the inactive periods provide respite and time for reflection for all parties involved, ensuring that everything is in place for the next test point. In cold conditions, these periods still occur, but in an environment in which inactivity can pose significant risks to the safety of test personnel.Aerospace systems, by their very nature, generally produce considerable amounts of heat. Therefore, the stress applied to components as a result of changes in temperature is greatly exacerbated when starting from a cold condition. Perishable components such as rubber seals and hoses are particularly susceptible, and their failure modes tend to be both dynamic and safety-critical.
Conversely, the potential exists for systems to quickly drop below acceptable operating limits in the event of delays in testing; this can prolong the exposure of the test team to hazardous situations.
In terms of the potential for harm to test personnel, injuries associated with extremes of cold tend to be more serious in the long term than is the case for testing in the heat. Conditions such as frostbite, the onset of which can be particularly difficult to detect, can result in irreversible, life-changing injuries.
Although appropriate personal protective equipment can provide a degree of mitigation, such items tend to be bulky and restrictive; this has the potential to introduce further hazards such as flying control restriction and delayed emergency egress. The formulation of a test plan that balances these risks will necessarily be complex, and therefore lengthy and expensive.
As if the challenges purely associated with cold conditions weren’t sufficiently onerous, such conditions are often also combined with extremes of darkness, snow and ice. By comparison, the sun-baked desert under a clear blue sky seems rather appealing!
Garnet Ridgway has a PhD from the UK's University of Liverpool. He has designed cockpit instruments for Airbus and currently works for a leading UK-based aircraft test and evaluation organization
Sophie Robinson says: Although I can wholeheartedly agree with my colleague on the other half of this page that testing in cold climates is one of the most challenging tasks we face as engineers, I’m not sure that a sun-baked desert does seem any more appealing than testing in the frozen North. If anything, testing in hot climates presents its own unique set of challenges, in much the same way that testing in the cold does – some of which can be even more difficult to deal with.
One of the primary issues surrounding testing in the heat is that aircraft and aircraft systems already generate a lot of heat themselves. Dissipating this into the environment in hot conditions can be a struggle.
This struggle is further amplified in hot conditions. This can result in unanticipated system failures, particularly of electronics, that have to be addressed.
Unpredicted performance losses can also cause problems. Reductions in engine performance or aircraft control authority can often be worse than predicted, resulting in potentially dangerous situations. Similar things can happen when testing novel materials; unexpected failures can be catastrophic.
Actually taking measurements with sensors can also be a problem in the heat – meaning it can be difficult to maintain protection against other external hazards such as fires in hot conditions.Thermal stress is another factor to consider. It only gets worse with activity in heat; this applies to both the aircraft being tested and the engineers carrying out the tests. Subjecting your engineers to heat can result in dehydration, sunburn and even sunstroke. Although protecting your engineers from this is of the greatest importance, it can be costly in terms of time and money. Writing from a British perspective, we’re also not naturally used to dealing with the heat – in a country where we experience rain for 30% of our year, encountering a truly hot day is a rarity – and that can create issues all of its own.
Of course, high temperatures are often accompanied by other environmental factors that have to be considered in testing – like sand. To quote Anakin Skywalker: “I don't like sand. It’s coarse and rough and irritating and it gets everywhere.”
Sophie Robinson works at the front line of aerospace testing as a rotary-wing performance and flying qualities engineer for a leading UK-based aircraft test organization. She also holds a PhD in aerospace engineering from the University of Liverpool.
This article was also published in the December 2016 issue of Aerospace Testing International magazine.
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