One Mans Opinion by Tony Markl Traffic Pattern Power Off Approach - When to turn Base

The 45* Base Leg Myth

Many books on flying teach the reader to turn to base leg when his aircraft is 45* past abeam the touchdown point of the runway as gospel and as FAA-approved. When I ask my tailwheel students (all of whom are pilots and have licenses) when or where they will turn base leg this is what they tell me. This is OK as an approximation of where to turn when teaching a novice, but should be abandoned after the pilot gets experience.. A moments thought will show that this procedure cannot work except in one wind condition. .

If you had an instrument to tell you when you were exactly 45* past the touchdown point and if you had another pilot to watch out for traffic in the pattern while you spend all your time watching the runway and if you then turned to a drift-corrected base leg heading and then turned to final (this sounds better than "made your final turn") and the airplane touched down at the touchdown point in any given wind condition then it is obvious that you would miss the touchdown point with any other (more, or less) wind condition.

Rather than spending all my time with my head twisted to the left watching a runway which does not move and cannot hit me when I should be looking in all directions for other traffic which does move, and can hit me , I count seconds. I look at the runway when abeam the touchdown point, close the throttle (for a power off approach), and then do not look again until halfway through the base leg. I count somewhere between 10 and 30 seconds depending on runway headwind component. You may say that this is a wide range of times but if we say that 30 seconds is about right for a no headwind final, and 20 seconds will work for 8-10 kts, and 10 seconds for 15-20kts headwind then it is more accurate. This technique works best for aircraft that fly any fixed speed, fixed power setting, and fixed flap setting during the approach

To learn this technique dont worry about being perfect in counting seconds. The point is that you count at your own pace each time and you make the same medium banked turn each time. If you turn base after counting 18 on your first pattern and you end up long, then add 2 seconds to the next count. A second is 100 at 60 kts so you can move yourself closer to the touchdown point by 100 increments until you are right on. Your third pattern should be right on for that headwind. Do this for No wind, 5-10 kt headwind, and 10-15 kt headwind, and you have broken the code!! This will work at any airport for the rest of your flying life.

Lets use the example of an aircraft which flies at 60kt for the approach This aircraft is moving 100/second. If you want proof --- 60kt is 1 nautical mile per minutethis is obvious. What is less obvious is that 60kt is also 6000 in 60 seconds. Even less obvious is that 60kt is 100 per second. An aircraft flying 90kt is doing 150 per second. Feet per second for speeds that are plus or minus 10% from these two speeds can be figured out mentally and cover all the speeds small aircraft commonly use.

Lets suppose that this aircraft descends at 600 per minute with power at idle (pretty close for low wing loading aircraft). At 600 per minute the aircraft descends 10 per second.

Lets start our traffic pattern at 1000 AGL. At 600 per minute rate of descent (or 10per second) we will be on the ground in 100 seconds ignoring for the moment the fact that after closing the throttle abeam the touchdown point there will be about 5 seconds used up to slow to approach speed.. In a no wind pattern we can go 30 seconds and turn base. If base leg is 5-10 seconds long and final is 30 seconds long, then there are about 30 seconds left to make the two turns with

If we roll out on Final at 200AGL as is recommended, it would take 20 seconds to reach touchdown. If the turn to Final is a shallow bank as is recommended, it might take 30 seconds for the turn to Final This means we would be at 500AGL when starting the turn to final. If we would like 5-10 seconds of straight flight on our Base to check Final for traffic (called "Enemies " by me), and to judge wind drift, then we would be at 550-600 when rolled out from the base turn.. If a Medium bank is recommended for turn to Base this would certainly take 10 seconds which means we need to start this turn at 650-700 AGL. If our no-wind Final was 20 seconds long (it was) then our distance (time) from Downwind abeam touchdown point to start of Base turn should be almost 20 seconds long also. Remember that we are slowing from traffic pattern speed to approach speed (abeam the touchdown point) and our average speed here is a little higher than approach speed. However, 20 seconds would mean that we would be 200 higher than start of Base turn or 850 AGL. This would fit an 800 AGL (which was the normal altitude years ago) traffic pattern very well. Turning Base 5 seconds later would add 10 seconds of flying time and 100 of altitude loss and fit the 1000 AGL pattern very well. All of this is not meant to say one should fly a traffic pattern like a robot, but rather to know that you are close to correct, and can repeat the pattern, or adjust it if necessary.

How do you succeed in flying good traffic patterns? By controlling the variables so that you can predict the outcome. What are the variables we need to control? Aircraft altitude, direction, rate of descent,and speed; pilot idea of distances to keep away from the runway, pilot idea of what banks to use in turns, and strength and direction of the wind. Here are seven variables and we have control of the five in the middle. Government rules generally require us to fly 800 or 1000 feet above the ground and the wind is not subject to our control or the governments. This means we need to have good control over aircraft speed, direction, rate of descent, and distance away from runway and angle of bank in turns. If we let any or all of these vary from one pattern to the next then we, ourselves, are making it hard to fly a good (repeatable) pattern. When I get a new student I let them fly the pattern however they are used to as long as it produces consistently good results. Commonly they let the distances, speeds, power settings, and banks vary from one pattern to the next. After three patterns there is no visible improvement. A good landing comes from a good pattern. They are so busy in the pattern fixing problems that the short final is different each time and they are not ahead of the airplane and the landings are humorous or poor. A series of poor landings will make them lose their confidence so I put order and priorities in their pattern.

What is a good traffic pattern? One that produces the desired outcome repeatedly. What is the desired outcome? To fly safely in a pattern that is recommended or approved for all pilots such as that in the AIM.

The AIM only talks about direction of turns and nomenclature of the traffic pattern parts. No advice or recommendation is given on distance, size, or speed.

If you fly an airplane with flaps and intend to use them, then fly a closer base leg (perhaps 5 seconds less) and put some flaps down on base leg. Full flaps on final when you see you have the touchdown point made.

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If you fly a power approach at a fixed power the same concepts can be used but a longer time before turning base is required as your rate of descent will be less. After three tries you should know how many seconds to add from your power off to your power on pattern.

Numbers used here were chosen to simplify math. My L-16 actually descends 640fpm at 60 mph for a glide slope of 8.1/1. A Stearman will be more like 6/1 and a C172 about 10/1 but the logic will be the same.