I guess it was to be expected that, following Rhys Phillips' recent presentation "Thunder and Lightning Isn’t Very Frightening", in which he discussed the ways in which aircraft are protected from lightning strikes, a story pops up in the newspapers this weekend of a pilot who saves his thunder bolt-struck plane "with seconds to spare".
As usual this tale is not quite what it seems. The event actually happened on 14 December 2014 and the story had been triggered by the release of an interim report from the Air Accident Investigation Branch. Had Rhys Phillips been proved wrong? Was lightning indeed a threat to air safety? Well no. It turns out that the pilots thought their plane had been disabled by the strike but in reality it hadn't. They had been intending to land at Sumburgh airport and had set up the autopilot to control the descent, however bad weather at the airport caused them to change their minds and they altered course. When the aircraft then began its programmed descent the pilots struggled to overcome the actions of the autopilot. With 'seconds to go' our 'brave' pilots finally realised that the autopilot was still active and disengaged it and were then able to recover flight altitude.
That set me thinking about a throwaway remark that Rhys Phillips had made about being safe whilst in a car struck by lightning - the 'Faraday Cage' effect. No doubt many of us in the audience expected him to say that, it is, after all, what is 'in the textbook'.
But is that true? The Faraday Cage is from the age of the classics, the static and the sine wave. Lightning is about the 'impulse', a single, very dynamic 'event'.
The Faraday Cage is supposed to place the charge on the outside of the conductive surface, leaving no charge within and having an equipotential surface. How exactly does an electron 'know' what is external and what is 'internal'? Surely they must flow where they can and act upon each other until charge equilibrium is achieved? The static state beloved of the classic textbook! A body inside an equipotential shell must be safe because there is no potential difference to drive current through that body.
Consider now a car, struck by lightning on its roof. At the instant of the strike the roof is at a higher potential than the ground so a current will flow through the car body. The roof is connected to the rest of the car body by the 'posts' framing the doors and widows. These parts of the structure resemble cables and cables have the property of inductance, let's say 1 mH per post? The initial lightning strike might have a current of the order of 25 kA and a risetime of about 8 μs, a rate of change of about 3 x 109 A/s. This would produce a voltage of 500 kV between roof and body, assuming the current divided equally between six posts. Not exactly the equipotential of the classicists!
How many of us question what we were taught? How many of us pass that on unquestioned to our students and colleagues?
I think my favourite ‘textbook falsehood’ are the smart particles of air that flow over an aerofoil and that ‘know’ they must speed up to catch up with their twin that took the ‘low road’. Does anyone else have a favourite ‘truth’ that they now question?