Never assume that an aircraft will be audible. They often aren't.
Even when you'd think they should be. Aircraft can make brilliant,
often dazzling reflections in wonderful shades of gold to deep orange
(in the evening) regardless of their angle to you or the sun or their
distance. The brilliance often inflates the apparent size of the
object seen. This can occur even when you think the sun has set and
the aircraft is very low in the east. This happened to me only a month
or so ago. I was (almost) convinced my sighting was a real UFO. Then I
repeated exactly the same observation the following evening and the
next at the same local time (10pm). I am often surprised how bright
aircraft can appear to the naked eye. Normally airliners can be
recognised for what they are even at very high altitiudes.Though they
don't always drag a vapour trail they are usually visible as a tiny
cross thanks to their considerable real size. Presumably most military
aircraft have low albedo coatings to avoid such brilliant reflections.