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With Airbus' Zephyr drone pushing the limits of aviation to reach an altitude of 74,000ft in the stratosphere during test flights, we would like to ask - where do you think the atmosphere ends and space begins?

Industry Opinion

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Highs and lows

Garnet Ridgeway says: When conducting flight test activities, the effects of altitude are surely beyond compare in terms of challenging conditions. This is particularly true for manned test activities because the human in the loop simultaneously provides the majority of both risk and risk mitigation.

A physiological fact is that humans are poorly adapted to operating at high altitude. Activities as simple as walking up a high mountain are transformed into epic feats of human endurance by the reduced air density; even elite athletes trudge at a snail’s pace, feeling the throbbing of their own racing heartbeats. For an aircrew, these effects are usually mitigated by their aircraft, but their presence is a latent menace that sets the context in which high-altitude operations are conducted.

Hypoxia (the deprivation of an adequate supply of oxygen to the body) features several characteristics that make it the worst nightmare of the team responsible for flight test safety. Its onset can be subtle, starting with a sensation of being lightheaded and reduced general well-being, which makes self-assessment of its occurrence extremely difficult. Unless rectifying action is taken immediately, symptoms quickly develop into hallucinations, irrational behavior and loss of consciousness. Given the emphasis placed on the human operator to provide risk mitigation to the test activity, this really is a worst-case scenario. Not only is the primary source of risk mitigation suddenly removed, but an operator acting irrationally introduces a plethora of hazards that need to be considered during test planning.

Another facet of hypoxia is that is it not fully understood, particularly its subtler effects. For example, recent research has shown that some individuals exhibit mild symptoms such as a degraded ability to distinguish between certain colors at altitudes as low as 8,000ft – far lower than traditionally understood. While the safety implications of this are not as obvious as for the scenario described previously, it could lead to warning lights or other safety-critical information being missed.

While it is true that height can be a pilot’s best friend in many aircraft emergencies, this is not always the case. In the event of a catastrophic failure, for example one initiated by hypoxic crew, it simply means a longer fall to the ground.

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: Proximity to terrain introduces the potential for a hazardous situations to very quickly become an accident, the pilot having much less time and vertical height available to execute a forced landing. Landing site choices will also be limited to what is immediately available and within reach. Ground proximity also makes real-time fault diagnosis very difficult, leaving pilots having to make split-second life-or- death decisions.

Low-level flight also introduces potential terrain-related phenomena like whiteout or brownout, caused by recirculating snow, dust or sand, which causes spatial disorientation and loss of situational awareness, leading to accidents. In maritime environments, spray can cause similar problems. Loss of visibility can also put aircraft into situations where they face risk of dynamic rollover on uneven terrain. Brownout has caused more helicopter accidents in recent military operations than all other threats combined; testing in these conditions can be just as risky. Add to this the complexities of testing some of the novel systems designed to restore visual cues in this type of environment, such as helmet-mounted displays or head-up displays, and what could be perceived as a simple trial can very quickly become incredibly complex.

Alongside this complexity, flight in low-level environments is already a high workload task; introducing any additional elements (such as giving pilot ratings) for evaluation purposes cannot be done without very careful consideration. Low flight also involves far more pilot interaction with the aircraft than at high altitude, meaning the overall pilot-vehicle system is more complex and therefore more difficult to assess.
The low-level environment can be unpredictable and changeable. Terrain masking, a smaller landscape feature obscured by a larger one, can result in collisions. Previously inspected test sites are liable to change through human interference – with masts or wires appearing in a matter of minutes.

The bottom line is that if something happens when you are testing in the low-level environment, it will happen faster, in a much less predictable setting and with potentially much more severe consequences than at altitude, making low-level flight one of the more challenging areas of test.

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.



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