Whenever there is an accident, pilots should get into that old "learning from events" mode. The recent 737 runway overrun at Chicago Midway created a lot of food for thought.

In a preliminary NTSB report, it was noted that the dispatchers and the crew had calculated that Runway 31C at Midway was okay for landing even though it was snow covered and there was a downwind component of almost 10 knots before the airplane landed.

After the accident, the NTSB disagreed with their calculations, showing that the airplane would definitely go off the end, given the conditions and the touchdown point, which was with 4,500 feet of runway remaining. The runway is 6,522 feet long with 5,826 available for landing because of a displaced threshold. Further, if the glideslope is followed to the runway there is but 4,925 feet available for stopping. Where the media led the public to believe that the crew gave away 2,000 feet of the runway, that was, as are so many things in the media, incorrect. It was a fairly normal touchdown point.

One other runway length item of note is that air carriers are only allowed to calculate the use of 60 percent of the available runway for landing. So, on this approach to minimums, with a snow-covered runway and a tailwind, they would presumably have had to calculate the ability to fly the glideslope, land, and stop in 2,955 feet.

In the NTSB report it was noted that had Runway 13C been used, in the other direction, with a little headwind instead of a little tailwind, the landing might have been successful. The controllers were using 31C because it has slightly lower runway visual-range landing minimums, and they felt airplanes would be more successful approaching that runway because the runway visual range was higher than on 13C.

Finally, in relation to the runway, the mindless folks who always come up with something like this after an accident said the runway was too short and didn't have adequate overrun. A runway is an inanimate object that has known quantities, like length. It is what it is. The runway at Midway has been deemed adequate for 737 operations for a long time and nothing has changed. To suggest that the runway or the airport had anything to do with an accident is to show complete ignorance on the subject. It is also an insult to the airlines and the pilots who operate there routinely, day after day.

Here we come to the first lesson. It's important for GA pilots because in the 10-day period in which the 737 went off the end, six general aviation airplanes did the same thing. Project that out to a year and we would have about 214 general aviation airplanes a year running out of runway before they stop. In many cases the airplanes are not damaged enough to become an NTSB accident so only the daily FAA reports have meaningful information on this.

Ten knots is the downwind component limit for most airplanes. Land in a 10-knot downwind and you'll see why it is a limit. Nothing feels right and if the component is a quartering tailwind the airplane can be squirrelly.

Another important thing is that a 10-knot tailwind at the surface can be effectively greater than 10 knots unless the pilot understands the effects of changing wind with altitude and has a plan to deal with that.

Let's say the wind at 1,000 feet is 20 knots, not unusual when it is 10 at the surface. In this case, if the airspeed is bang on Vref while descending through 1,000 feet, that means the groundspeed is Vref plus 20 and the pilot has to deal with all 20 knots for the landing to be precise and within acceptable parameters. Ten of the 20 have to come off as the airplane descends and the other 10 is dealt with in the stopping process. Presumably the autothrottles system on a modern jet would handle this.

This makes our GPS a key to managing such a situation. GPS gives up-to-the-minute groundspeed and that is what counts. If descending for an approach in a stratified wind condition, the GPS will give a heads up on the wind change to anticipate. If the groundspeed is 20 higher than the indicated airspeed going through 1,000 feet, that's a clear message that you are going to have to work a little harder to keep from arriving at the threshold with too much airspeed.

In a condition like this, at what altitude is the change in wind velocity likely to occur? Certainly within 1,000 feet of the ground and probably gradually. The wind change should be complete by the time the airplane is 200 to 300 feet high. Wind changes in descents are often accompanied by light jiggles in the atmosphere because while not severe, such a condition is wind shear.

Another lesson is in runway length versus technique. In piston airplanes we seldom fly ILS approaches to runways that, for our airplanes, would be considered "short." However, a tailwind and a slick runway can make what seems to be a long runway a lot shorter.

Let's do the numbers. If we follow airline rules, never a bad idea, the calculated landing length must result in a full stop within 60 percent of the available runway.

Using my P210 as an example, at 20°C the distance shown for coming over the end of the runway at 50 feet, landing, and stopping, is 1,930 feet. That is using short field procedures, which we seldom do on an ILS.