Extended patterns: when to start down
One challenge faced by pilots early in their career is figuring out how best to respond when you get extended on the downwind. Either tower extends you, the traffic ahead is flying bomber patterns, there’s a bunch of airplanes in the pattern, or there’s a straight-in that messes up your spacing, and your carefully-planned pattern goes out the window. Having a few rules to follow will make the process of descent planning on an extended downwind much easier. Let’s dive into the math and then use it to make up some simple rules.
The key concept I like to teach when doing nonstandard patterns, or straight-ins for that matter, is to maintain the same descent profile and steps, but “unwind” the ground track as you need in order to adapt it. If you typically fly a 1,000-foot pattern with a ¾-mile offset and turn base at 45 degrees, then you’ll cover .75 * 3 = 2.25 nm in your descent. To calculate the average descent glide path angle (GPA), we need to convert nm to feet, then take the arctangent of height/distance:
GPA = atan (1000/(2.25*6076)) = 4.18 degrees.
The key concept I like to teach when doing nonstandard patterns, or straight-ins for that matter, is to maintain the same descent profile and steps, but “unwind” the ground track as you need in order to adapt it. If you typically fly a 1,000-foot pattern with a ¾-mile offset and turn base at 45 degrees, then you’ll cover .75 * 3 = 2.25 nm in your descent. To calculate the average descent glide path angle (GPA), we need to convert nm to feet, then take the arctangent of height/distance:
GPA = atan (1000/(2.25*6076)) = 4.18 degrees.
I’ve added a little spreadsheet here if you want to play around with different spacing options.
Let’s say we want to stick with our 4ish degree GPA. That means that our pattern TOD needs to be 2.25 nm away from the touchdown spot. On a straight-in that’s pretty straightforward: be at pattern speed at TPA 2.25 nm out, then do your normal setup and slowdown process, with any slight adjustments for wind:
If you expect to be triple your normal base-to-final (BTF) distance out from the runway, stay at TPA through your base and then treat it as a straight-in and start your normal setup as you turn final. For a 2 BTF extension: when you get over your normal base turn spot (the pine tree in the sketch), look 45-degrees forward and pick an object that’s the same distance from the aimpoint as your normal BTF point (the house) and a new base spot (the triple tree).
In this case, you see the barn as your 2 BTF spot. Turn base over the triple tree to overfly the barn. Another cue is that your aimpoint will be at your 7-o’clock position about 60 degrees behind you because atan(2/1)=63≈60. Because your base turn will be at your 2.25 nm track distance, start your normal descent then.
So far, these adaptations are all pretty simple: unwind your normal setup based on how far you get sent out. The one slightly tricky scenario is if you stay at TPA but get cut in between the house and the tree. Now you need to lose altitude a bit faster than normal. Let’s say you’re over the single tree and get cleared to turn base. Now you’ll follow the purple track. You can give yourself a little bit of space to get it started, let’s say a third of the normal distance. This will give you .25+.75+.25 = 1 nm to do what normally takes 1.5 nm.
So far, these adaptations are all pretty simple: unwind your normal setup based on how far you get sent out. The one slightly tricky scenario is if you stay at TPA but get cut in between the house and the tree. Now you need to lose altitude a bit faster than normal. Let’s say you’re over the single tree and get cleared to turn base. Now you’ll follow the purple track. You can give yourself a little bit of space to get it started, let’s say a third of the normal distance. This will give you .25+.75+.25 = 1 nm to do what normally takes 1.5 nm.
That means that in order to hit your 400-foot gate, you need to descend faster. If you normally descend about 500 FPM in the pattern, you’ll need to do about 800 FPM until you get to the 400-foot gate over the house, then resume your standard setup. In a 172, this basically means pulling throttle to idle, then as speed permits, go flaps 10 and 20 in quick succession. Mid-base, give the runway a good look to gauge your profile, then either shallow the descent or go flaps 30 a bit early to increase drag and steepen your profile. Because you’ll be slightly further out than normal, expect to be about 500 AGL coming out of the turn, then 400 over the house as normal.
pattern spacing with other aircraft
A good rule for when same-speed airplanes are in the pattern is for you (blue airplane) to turn base when the traffic on final (orange airplane) is abeam you:
Using your pre-selected pattern checkpoints can help you plan out your descents when you see the other traffic. For example, if the orange airplane is over the barn (1), you know that it will be over the house when it’s abeam (2), so start planning your power reduction to turn base over the tree (2). Generalizing this concept: if an aircraft is 30 degrees off your nose, then your abeam point will be 45 degrees off. This should be fed back into your descent planning to avoid any surprises.
conclusion
Having a few set rules and standard procedures will help make pattern extensions easier to learn. As you gain experience, it will become easier to eyeball each lap and make corrections as needed. Send me any other helpful ideas you’ve come up with.