Planes on intersecting paths will be placed in separate flight levels. The standard is 35,000ft or “FL35”, and usually East bound traffic gets odd FL and West b

Planes on intersecting paths will be placed in separate flight levels. The standard is 35,000ft or “FL35”, and usually East bound traffic gets odd FL and West bound gets even. It’s likely the plane you saw would have collided with yours had it been on the same FL, but it was likely off by 1,000ft. The relative addition of two cruising speeds make it more dramatic too.