By Juergen Theerkom
Not all runways are smooth, straight, and flat. A shocking reality, but there it is. As we fly around in the valley of the sun, we use large, controlled, paved airports, fewer and fewer new pilots have had experience dealing with some of the more challenging aspects of working out of airports and aerodromes that present a different, more irregular environment. I say that because just the other day I watched a business jet arriving and landing in Sedona (SEZ) using the down-slope runway. Yes, the winds were favoring that runway by 3 knots, and he/she made the landing without a problem, but it kept me thinking.
We discuss the illusions and challenges of dealing with sloped runways during training, but it’s a whole new experience when we are faced with reality. If we combine slope with a few other interesting challenges — obstacles, wind shear, sloping terrain, and perhaps some density altitude (that’s a given for SEZ in summer or even early spring), we can discover we have our hands full. In order not to scare ourselves, let’s tackle the problem a bit at a time. Let’s focus on slope.
Landing and taking off from a sloped runway is neither a good nor a bad thing. It is just a bit different than working with a nice, flat landing surface. Of course, it is the flat surface that is used to give us the baseline performance standards in an aircraft POH. There are three immediate consequences of dealing with a sloped runway: the effects of the slope itself, the factor of wind, and the visual illusions encountered. To keep things simple, I’ll not talk about factors like density altitude, surface, and wind shear. In simple terms—all else being equal—if we have a choice, we would opt to land uphill and take off downhill. The up-slope will shorten our landing roll, and the down-slope will shorten our takeoff role. We will have that wonderful and inexplicable force called gravity working for us for a change. On a downhill takeoff, a portion of our weight vector will be acting as if it were thrust; on an uphill landing, a portion of our weight vector will be acting as if it were drag. Of course, there is a mathematical way to calculate the effect of an increased thrust or drag vector, but I‘d like to talk about some Rules of Thumb. However, it’s important to remember that Rules of Thumb are only that: a simple way to arrive at a ballpark solution. For specific answers to questions, it is necessary to go directly to the Pilot Operating Handbook or Aircraft Flight Manual for your aircraft and work with the tables and charts provided. The lower the margin of error you can live with, the greater the accuracy you must work with. A good Rule of Thumb for estimating the advantage or disadvantage of a sloped runway is that a 1.0% runway gradient (an increase or decrease in altitude of 10’ for every 1000’ of runway length) is equivalent to a 10% increase or decrease in effective runway length.
Where can I find the correct up/ down-slope information for the pre-flight calculation? Well, the first logical place I would look is the airport diagram to find the slope along the runways. Electronic Flight bags may or may not give this information, so the best source is the AF/D. You will need to look at the AF/D section within the corresponding chart supplement for the area of that particular airport and read it among the PCN (Pavement Classification Number) information, and even then it can be rather cryptic. All it says is 1.8% up NE for Sedona, to come back to my example. It is up to the pilot to figure out it is referring to Runway 03 for an upslope and Runway 21 for a downslope.
Just as density altitude can be thought of as performance altitude, the altitude at which the aircraft “thinks” it is operating, so too effective runway length can be thought of as the performance length of the runway, the length of the runway the aircraft “thinks” it has to work with. For the example of SEZ, landing on a 5132’ runway with a 1.8% up-slope will give us an effective runway length, a performance length, of almost 6000’ (5132’ x 1.18 = 6055’). Landing downhill on that same runway will give us an effective runway length of just over 4200’ (5132’ x 0.82 = 4208’).
Sadly, life isn’t always quite so simple and straightforward. One aspect of takeoffs and landings that must always be considered is wind. As a general Rule of Thumb, a 10% increase in groundspeed results in a 20% increase in ground roll. This is a strong argument against being casual about approach speed on landing and about landing or taking off with a tailwind. If our touchdown speed is, for example, 50 knots, and we require 500’ to execute our landing roll in no wind conditions, landing with a 5 knot tailwind will increase our landing distance by approximately 100’ (5/50 = 0.1 or 10%; 1.2 x 500’ = 600’). On takeoff, with the same 5 knot tailwind, our ground roll will be increased approximately the same 20%, perhaps slightly more. Putting the two factors of slope and wind together, we can determine that approximately 1.0% of slope is equivalent to 2-3 knots of wind in its effect on takeoff and landing performance. It will require at least a 3.0 % up-slope to counteract the effects of a 6-10 knot tailwind on landing. For most general aviation aircraft, takeoff with tailwinds greater than 10 knots is not recommended under any circumstances. Ideally, we would choose to take off downhill with a headwind and land uphill with a headwind, but this isn’t always possible.
In the event that we must take off or land with the wind at our tail, particularly on shorter fields, we must make some careful calculations. A basic Rule of Thumb for all flying is, “If you’re not SURE it’s safe, don’t do it.” It may be more prudent to wait things out and live to fly another day, rather than trusting luck and hoping things will work out.
Understanding visual illusions is also important, particularly when setting up for landing on a sloped runway. Illusions in themselves are not a problem, but the problem comes from failing to recognize you are experiencing an illusion and responding to visual information as though everything your poor, old brain is telling you is the whole truth and nothing but the truth. The easiest way to simulate the illusions resulting from a sloping runway is to hold your arm straight out from your shoulder, palm down with your hand flat. This is what a level runway looks like on a normal approach. Now, tilt your hand up. This is the view you see when setting up for landing on an up-slope runway. The illusion tells you, you are too high. The potential danger is that you will respond to the illusion rather than the reality and come in too low. Tilting your hand downward simulates the illusion of the down-slope runway. The illusion is that you are too low and, thus, the potential difficulties arise when you approach the runway at too high an altitude. Landing on a down-sloped runway is particularly difficult because, as you flair, the runway drops away and you risk running out of airspeed while still well above the landing surface. Easy does it. We already saw that landing on a down-sloping runway will increase our landing distance by virtue of the addition to our thrust vector provided by the weight of the aircraft. When we combine, “If you’re not SURE it’s safe, don’t do it,“ and the potential danger that you will respond to the illusion rather than the reality and come in too low, with the natural tendency to approach too high resulting from the visual illusion, plus the difficulty of finding the surface after flair, we must be very alert to the rapidly shrinking options before us.
There are airports that do not fit the typical up or down-slope situation, but may have other factors such as cliffs on the approach end, a hump in the middle obscuring the other end of the runway, a dip, or combinations of them all. Payson and Catalina Island are examples that come quickly to mind. Always be ready to execute a missed approach: add power, level off, and go around for another try.