Aha! Here we go. Here's a telephone pole: https://earth.google.com/web/search/39.0303298,-93.58495/@39...

We know that telephone poles stick about 32 feet from the ground. Measuring the shadow in Google Earth above, it's about 20 feet. Now, it's a simple ratio.

Using Earth, I measured the distance from the plane to its shadow at about 2430 feet, that would put it at 3,900ft, which is surprisingly low, but I don't know the height of that pole for sure. It's the right ballpark, though.