C Kingsbury wrote:

Debbie,

A large transport jet (such as a DC-10 operated by FedEx) can climb, lightly
loaded, at speeds around 150mph. This would be unusual (170-200mph is more
normal) but well short of impossible. It might have been a maintenance
flight, for instance.

Now, with that in mind let's apply a few more factors. First, windspeed.
Let's say the wind is blowing at 40mph. This would not be unusual for the
Northern half of the country this time of year. An airplane flying into that
wind would only need to move at 110mph relative to the ground. Windspeed can
vary strongly from the ground to the first few thousand feet as well, so you
might not have noticed the wondspeed on the ground. Now, you also mentioned
you were in the car going at least 40mph, so relative to you, the plane
would only appear to be moving at 70mph.

Last, you need to do a little trigonometry, I'm afraid. Imagine putting a
ladder up against the side of your house. The ladder goes up ten feet, and
at its base is set back perhaps three or four feet from the house. Now, when
you climb that ladder, you are traveling ten feet, but you only move forward
three or four. A similar thing is going on when you compare your speed in
the car to the plane's speed in the air. So we can reduce the difference to
perhaps 55 or 60mph.

What you perceive as speed is going to be based on changes in angle, in
other words, you expect to see the plane "pulling ahead of you" at a certain
rate. It turns out that the human eye is quite poor at judging small angles.
A speed difference of 60mph at a distance of half a mile is going to produce
very small angular changes, which your eye will not perceive well. So, even
though the plane is moving at sufficient speed to fly quite safely, to your
eye it will appear to be standing still.

I can assure you that even us pilots, who know all these things, still often
see the very same illusion you describe, especially with very large
airplanes. It's simply a combination of factors that produce an overwhelming
optical illusion.

-cwk.

yes, I've imagined the whole trig thing. That's why I was looking at the
buildings and other higher geographic markers to get a fix on how it
appeared to be moving relative to those things. Plus, for a moment I was
stopped and pretty much next to it, although of course I was on the
ground and it was probably a couple of thousand feet up or less. And, if
it makes a difference in possible explanations, the plane was headed
east and I was moving west. I'm 99.999% sure it wasn't moving or was
moving very slowly.

Note, I've driven in this particular area for more than a decade and
I've seen hundreds of planes moving in the sky from many vantage points
under many different weather and cloud conditions. I'm pretty sure of
what I saw. Your explanation makes great sense but I'm still not
convinced that it was an optical illusion. Really hope so though because
it was quite disturbing.

Debbie