WIND AND LANDING
For students, flying the proper traffic pattern is one of the most difficult concepts to master. They must learn to make judgements and decisions rapidly and correctly. As mentioned in an earlier article the traffic pattern is a dynamically changing concept. Heights and angles need to change based on various conditions. The most important condition that needs to be addressed is wind. For a student (and many pilots) wind is an enemy. Wind requires that the student change everything about his traffic pattern. What he has learned on a calm day is now wrong. Be it altitude or angles they now must be adjusted if the student is to have a safe traffic pattern. How many times have we, as instructors, told new solo students they cannot fly that day because the wind is too much? This article will deal with headwinds. Another article will discuss crosswinds.
Flying the traffic pattern in windy conditions is really a fairly simple concept. Mastering the concept and applying it to real life is another issue. Every training manual discusses the concept. To review, most books say to fly faster in windy conditions. The general rule of thumb is to add 50% of the maximum estimated gusts to your normal airspeed. The Schweizer manual even say to add, not half, but the entire wind gust to the normal airspeed. However, many books do not go further. There are actually three things that must be done to fly a safe traffic pattern in windy conditions. The first is indeed to increase the airspeed. However, there are two other thing that must be done to be safe. You must stay higher throughout the traffic pattern and you must turn your base leg in closer to the end of the runway. Flying faster does indeed offset some of the effects of wind on the traffic pattern. That is, you will penetrate better into the wind if you fly faster. However, flying faster (above your best L/D speed) also makes you descend faster. So flying faster has a drawback. Your decent rate increases. Your distance covered for each foot of altitude lost is still reduced versus a calm day. Therefore, you have still not made up completely for the headwind if all you do is increase your airspeed. You are still sinking faster than you would in a no wind condition. Thus, you must do more to offset the adverse effects of a headwind. The most obvious thing you can do is to teach your students to maintain extra altitude to make up for the increased rate of decent. More altitude is the key. How much? As a general rule of thumb I have students add 100 feet of altitude (Using angles is a bit more complicated as the change in angle depends on how high you are. At the beginning of the traffic pattern, approximately 1000 feet agl the angle increases by 10%. Yet as you turn final the angle for adding 100 feet is an increase of near 30 to 40%) for every 10 mph of wind. The worst thing the student can do is not adding enough. Hence, add even another factor for the pattern. Turn the base leg in earlier. This will give the sailplane less distance to travel to get to the end of the runway and thus, require less extra altitude.
Let’s look at some numbers. The L/D of a 2-33 flying at 55 mph (the normal pattern speed) is about 20/1. For a 2-33 flying into a 20 mph headwind, flying at 65mph (55 mph normal pattern speed plus half the estimated wind) the L/D with spoilers closed is approximately 11/1. With full spoilers the L/D of a 2-33 on a calm day is about 7/1. In our example of a 20mph headwind it becomes about 4/1. Our L/Ds with spoilers closed and with spoilers full open are both reduced by approximately half. Let’s assume that we try to keep our glide slope in the middle of our no spoiler versus full spoiler glide slopes on all days for all conditions. In other words keep the same safety margin. Then for our windy day example we need to be twice as high (if we turn final at our usual position) to still keep our new glide slope in the same relative position between our new (windy) no spoiler and full spoiler glide slopes.
What about high performance sailplanes. All new sailplanes that meet the JAR 22 standards must have spoilers that reduce the L/D to 7/1 or less. That is basically the same L/D with full spoilers as a 2-33 with full spoilers. Let’s say our high performance sailplane has a 40/1 L/D at 55 knots and to keep it simple let’s say 55 knots is the normal pattern speed. In our example of a 20-mph headwind (17 knots) we should increase our pattern speed to 63 knots. If we assume a 10% loss of L/D by flying 63 knots versus 55 knots, then flying into a 20 mph wind will reduce the L/D to 26/1 (46 knot ground speed divided by 63 knots indicated airspeed times 36). The high performance sailplane in a 20-mph wind will need to be at least 50% higher on the turn to final to be in the middle of the new (windy) no spoiler versus full spoiler glider paths.
But wait! What if the student has too much altitude and turns the base leg in early. Won’t he fly through the runway/field and crash on the far end of the runway/field. Not likely if you have taught him/her to slip and use full spoilers once he/she is sure that the field will be made. In our example of a 20mph wind let’s have the student cross the end of the runway extremely high, say 500 feet, above the ground. The sailplane will touch down 2000 feet down the runway with full spoilers and no slipping. (Slipping will certainly shorten that distance.) No where near the end of most runways used when training students. With a runway of 3000 feet the student would have to cross the end of the runway at an altitude of over 700 feet before he/she might run out of runway. Amazing, huh? And this is without using a slip to increase the rate of decent.
Conclusion: We as instructors must teach our students to adjust for windy conditions. They must be taught that it is much better to err on the high and fast side versus the low and slow side. I show them this concept by having them turn final during a training flight at what they think is very, very high and let them see that the sailplane can still be landed way before running out of runway. Certainly there is a height that can be too high to land safely. However, our current accident experience shows that our problem is not flying through the runway but rather getting to it in the first place. To summarize in simple terms my rule for students when flying in wind is as follows:
Fly Safely and Have FUN!!
Frank Reid
Bermuda High Soaring