make a mental judgment as to how far the plane will travel. >> at 3,000 feet you're probably only going to travel a few miles. at 40,000 feet, you're going to be able to travel 100 miles, roughly. >> michelle summers halloran spent 15 years as a commercial pilot. she's now an associate professor in florida. so now you've got to failure out based on how long this plane will stay in the sky as to what you can reach. >> that is correct. >> you're doing very basic math. >> you're doing a lot of mental math in public, yes. >> captain sullenberger has nearly 20,000 hours of flight time under his belt. so it doesn't take long for him to do the math and realize at their altitude and rate of descent, it's too risky to try to glide all the way to an airport over such a densely packed area. it's now 3:28 p.m. flight 1549 has less than two minutes until impact.