Normally, the construction "<object> of <substance>" in English would be understood as describing an <object> composed of (or, at least, appearing to be composed of) <substance>.
(At least, this would be the default interpretation unless some other, more plausible interpretation overrode it. One such case would be if the object mentioned was a container suitable for storing or transporting the substance: a "cup of coffee" would normally be understood as describing a cup filled with coffee (or the amount of coffee needed to fill one cup), not a cup made of coffee. On the other hand, a "box of steel" would generally be taken to describe the material the box is made out of, while a "box of chocolate" would more likely describe its contents. Basically, given two (or more) possible interpretations of a phrase, we normally choose the one that seems to make the most sense in context; this is not limited to English, or to this particular construction.)
Thus, in a literal sense an "aeroplane of light" would describe an aeroplane made out of pure light. Of course, such a concept, taken literally, would be nonsensical, so one would normally assume that the phrase is intended to be merely descriptive or perhaps metaphorical: that is to say, that it refers to something that looks like, or in some other (possibly obscure) sense resembles, an aeroplane made out of light, even if it might not actually be a real aeroplane, or really made out of light.
Indeed, the full context confirms that this seems to be the most likely interpretation:
"According to the records of Reverend General Vicar of Leiria, one of the eyewitnesses, the lady came in an ‘aeroplane of light, an immense globe, flying westwards, at moderate speed. It radiated a very bright light’."
It appears that the speaker being quoted is trying to describe a strange phenomenon for which he has no precise words, so he's describing it by what it resembled to him. From context, it's clear that he isn't describing a real aeroplane — real planes are not "immense globes" — but something that behaved like an aeroplane, in the sense that it flew like an aeroplane (and, apparently, carried a person inside it). It is also described as radiating "a very bright light", such that, even if it was not in fact made purely out of light, that light was the only part of it that could be seen.
Thus, the strange object being described is sort of like an "aeroplane [made out] of light" — it was big, it flew, it carried a person inside it, and the only thing the speaker could tell about its substance is that it shone a very bright light, as if that was all it was made of.