Runway Considerations

According to Darwinian Theory, species that do more than survive and actually thrive tend to be the ones best able to adapt to their circumstances. Transferring that perspective from biology to aviation, we can see a direct parallel: The best pilots adapt to their situation-or conditions-depending on the circumstance. Got a forecast for Level 4 or 5 weather along the route? We adapt by making a decision, maybe go around the turmoil, maybe wait for a better day.

Runways are a good example. If the runway we need is the one weve got, we cant automatically say, “Not going.” Delaying the trip may still be the only smart response. But often, the best response is to adapt. Wet runways, icy runways, snow, slush, slopes, peaks and lengths all complicate the published runway-

performance numbers for a given airplane. And all are generally surmountable, as long as the pilot-in-command knows how and why to adapt to the variables.

Lets take a look at considerations for a sloping runway, regardless of which direction. For example, we know that managing our approach speed is critical to a good outcome. But adapting to a downhill-sloping runway requires more of us than accurate speed control; it also requires us to be as slow as we can get away with, and that we touch down as close to the threshold as possible.

Heres what happens if were too fast when flying a generic airplane, according to a presentation by Sam Harris of V1 Aviation Training LLC: If your approach speed is five percent high, your landing distance can increase by 10 percent. For every degree of downhill slope, count on an increase in landing distance of 200 feet.

Of course, those numbers can quickly add up. Put together a 75-knot approach when 68 knots is called for, throw in a three-degree downslope runway, and your normal 1800 feet to clear a 50-foot obstacle can stretch into 2490 feet of need. The math looks like this: (1800 x 1.05) + 600 = 2490.

Even if you keep your speed on target, that 1800 feet youd normally expect to need will actually be 2400 feet for the same downhill-sloping landing strip. If the strip is 2500 feet long, the margin of error available totals a scant 100 feet. If youre hot, youve got 10 feet to play with.

Of course, if your landing numbers are already higher than this, your problems are proportionally greater and they wont get better, no matter how good your technique. . Maybe this is the time for the new decision to be, “Go somewhere else if at all possible.” And if the runway sports any form of contamination whatsoever, making a new decision seems in order-youre rolling loaded dice against a high probability of running out of runway.

While all of these examples are predicated on no wind, we all know how infrequently “No Wind” is the report. Headwinds can mitigate some of the approach-speed issues but cant overcome excess airspeed completely, a downhill slope or the impact of runway contamination.

So, remember that youre best decision may be to accept a little tailwind to land upslope. The impact of upslope is opposite of the downslope-able to shorten your effective runway needs enough that a little bit of tailwind wont hurt…as long as you manage your speed by the dial and not the view.

If your approach speed is 68 knots and you have no choice but to accept a tailwind, of course you know instinctively youll travel farther over the ground. But how often do you calculate the actual impact for a downwind landing? Do you know the impact? Lets take a look at a couple of examples.

That 68-knot approach speed equals 113 feet per second (fps); throw in five more knots and suddenly your ground speed on approach is now 122 fps. The nine-feet-per-second difference may seem small, but in certain circumstances that well examine in a moment, that difference can combine with other problem issues to make a difference-a bad difference. Push the tailwind to 10 knots and the over-the-runway velocity increases to just over 130 feet per second-27 more fps than at zero wind.

Conversely, a five-knot headwind can knock that 113 feet per second groundspeed back to 104 feet per second; the 10-knot headwind can pull the number down to about 96 feet per second.

These numbers can help when we calculate the impact of runway slope and contaminants-an issue of significance on even a perfectly level runway.

Water, slush, loose snow, compacted snow, ice. Each of these conditions reduces the friction between tires and the surface-and when that friction deteriorates, so does the aircrafts brakes ability to slow and stop. Issues with contaminated surfaces rose to such a high level for commercial and business-turbine operations that several years ago the Flight Safety Foundation formed an Approach-and-landing Accident Reduction Task Force-known in professional flying circles as the FSF ALAR. The ALAR Task Force took to a higher level existing FAA guidance on landings in the Safety Alert for Operators (SAFO) bulletin 06012.

For each type of contamination, the SAFO provides an expectation of typical braking-action level and a multiplication factor for adjusting your calculated runway need for landing. As one example, presume a runway covered with uncompacted snow and “good” as the reported braking action. In this example, the factor applied to a calculated dry runway landing distance in the SAFO is 0.9. So, for our 1800-foot example from earlier, the calculated land-and-stop distance is 1620 feet. Again, the math looks like this: (1800 x 0.9) = 1620.

Right about now, youre saying to yourself, “What?” As you noticed, SAFOs numbers are actually 10 percent better than the dry-runway number. We have reason to be hesitant to use it since wet or dry snow seldom gives better traction than clean, dry pavement. And things get more interesting from there.

For packed or compacted snow, for example, the “fair”/”medium” braking action rating calls for a fudge factor of 1.2, making our 1800-foot need under normal conditions grow to 2160 feet.

For wet snow, slush, standing water or dry, hard ice, the adjustment grows to a factor of 1.6; now our 1800 feet properly adjusted goes to a whopping 2880 feet.

Are you getting convinced? Well, weve got one more example from the FAAs SAFO: wet ice. With braking action considered nil, the FAAs SAFO says simply, “Landing is prohibited.”

Think for a moment about how you might try to tackle this. No tire traction, the likelihood of a crowned runway-used to promote water runoff-and maybe winds a little off the runway centerline. Who knows where the plane might wind up? In a ditch, plowing through edge lights, maybe into trees or a fence? Wherever it may go, youre no longer PIC-youre now a passenger in peril.

The guidance provided by the FSF ALAR Task Force is more detailed in nature and, to me, more realistic and useful, adding to the considerations elements such as field elevation, higher-than-usual approach speeds, arrival over the threshold 100 feet high-even a delayed flare. Each of these added elements is good for an added 0.2 to the equation.

So lets put several together with even a dry runway for our 1800-foot example.

A hundred feet high at the threshold? Add 360 feet.

Delay the flare or touchdown? Add 0.3 if your delay is only five seconds; thats another 500 feet.

Arrive five knots too fast? Add another 0.1 or 180 feet.

These three small failures to adjust to the runway total an added 1040 feet on their own! That converts our 1800 feet of runway needed under normal conditions a whopping 2840. And thats for a perfect, dry, level runway. If its only 2500 feet long, youre in trouble.

Now, give it even a tiny one-degree downslope and add another 200 feet-your runway needs to be at least 3040 feet. Throw in even a minimal amount of contamination and add yet another 616 feet-for a total of 3656. If youre paying attention, thats more than twice the runway required if it was dry and level, and your technique was “by the book.” Get the picture?

Speed control, something we discussed last month, can make a huge difference. For every five knots of excess speed, you add a couple hundred feet to the runway you need. So mind your speed and exercise control.

Also focus on a touchdown spot and arrive there affirmatively-even if it means dragging in with power at a steeper-than-usual angle. The goal is to eliminate any float from your arrival and using an approach angle that firmly plants you on the runway rather than one causing you to cruise in ground effect until the speed bleeds off. The ALAR notes this latter event as delayed flare. And it adds up fast.

As discussed, at our mythical 68-knot approach speed, were covering 113 fps. Delay the flare for a second and youre 113 feet farther down the runway; delay touchdown with a five-second flare and youre 565 feet from your arrival point-and youve not yet touched down. Put another way, a three-second delay in flaring more than covers the length of a football field, including the end zones. Can you afford to give up that much yardage in your landings?

Finally, know the runway and its peculiarities, not merely its length. Information on slope-how much it slopes and in which direction-is available from data in the Airport/Facilities Directory, on instrument-approach plates and other directory sources, both online and in print. But, by itself, knowing the runway isnt enough.

Take the time to calculate the actual ideal numbers for your airplane, taking into account the density altitude of the day-and then calculate the various factors at play, whether water or wind, ice or slush and slope.