woodstock
Final Approach
I hated this in ground school! and now I gotta re-read up on it. bleh.
next lesson - VOR usage
- Thread starter woodstock
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Let'sgoflying!
Touchdown! Greaser!
I hope it soon becomes the great friend that it really can be to you.
gismo
Touchdown! Greaser!
woodstock
Final Approach
I spent the morning reading Rod Machado's explanation. makes MUCH more sense now. he's a god!
poadeleted20
Deleted
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A lot of folks have trouble tracking the VOR/LOC needle, usually because they’re chasing it to an unnecessary degree trying to center it. The fact is that centering the needle, particularly on an approach to a station, is not absolutely necessary. It’s far more important to stabilize the needle, even if it’s a bit off center, than it is to have it in the center but swinging when you hit the missed approach point (MAP). I am indebted to long-time North Carolina Designated Examiner John McLain for this technique.
Let’s look at a typical VOR-on-the-field approach. We’ll say the VOR is halfway down the 6000-foot runway, and the MAP is at the end of the runway. Now, we all know we don’t want to break out right at the MAP, so let’s say that the critical point is about 1/2-mile short of the runway – far enough to let down comfortably from the MDA of, say, 400 AGL. Thus, the visual descent point (VDP) is 6000 feet from the VOR station.
Let’s say you’re chasing the needle left and right, with increasingly large oscillations. You hit the VDP with the needle swinging through the center but your heading 15 or 20 degrees off. If you think enough to look in the correct direction (remember, the nose is pointed 15 or 20 degrees away from the runway), maybe you see the runway. Even if you do, you’re heading away from the runway, and by the time you react and turn back, you’re on very short final and well off centerline. Not good.
On the other hand, let’s say you’ve got a half-scale deflection, but it’s rock steady. Where is your ground track going? RIGHT AT THE STATION – right where you want to go. Oh, so you’re worried about being off centerline? Well, just how far off are you? Half-scale is 5 degrees. One degree is one foot in 60, so 5 degrees is 5 feet in 60, so you’re 500 feet off to one side, 3000 feet from the runway end – about a 10-degree angle off, but headed straight for it. This is a piece of cake in a light plane. With a localizer, at decision height on an ILS, half scale is only 175 feet off, and again, headed straight for the runway.
So, what does this mean? We’re going to strive for a stable needle, even if it’s a bit off center. How do we do this? We use the needle movement, not the needle position, as a roll command. Very simply, if the needle is moving, we bank in the direction it’s moving, with angle of bank proportional to the speed it’s moving – slow movement, small bank; fast movement, larger bank (you’ll pick up the rate/angle correlation pretty quickly). When the needle stops, we level the wings, and keep them level until it moves again.
Once you’ve got the basics, you can start working on finer control by using this technique to make corrections to put the needle closer to center. If the needle is off to the right, roll into a right bank and hold the bank until the needle starts to move. Then roll wings level. The needle will continue to move left until you apply the original technique (needle moving left, bank left until it stops, then level the wings) to stop it. With a little practice, you’ll find this a very simple, natural technique.
You’ll notice the absence of one word you were probably expecting – “heading.” A lot of folks teach people to correct their heading by X degrees for Y amount of needle deflection, then try to compute in their heads a drift correction angle. Who cares? You’re not trying to compute the wind direction and velocity – you’re just trying to get to the end of the runway, and the one thing that will get you there is a stable needle. And winds change significantly between FAF and MAP, so what worked at 1500 AGL may be well off at 600 AGL.
Once you've got this down laterally, you can translate it into tracking the glideslope needle for ILS approaches, using pitch/power changes rather than bank angle changes. You'll find that using this technique produces a smoother, stabler, trip down final, and your passengers will appreciate it.
Let’s look at a typical VOR-on-the-field approach. We’ll say the VOR is halfway down the 6000-foot runway, and the MAP is at the end of the runway. Now, we all know we don’t want to break out right at the MAP, so let’s say that the critical point is about 1/2-mile short of the runway – far enough to let down comfortably from the MDA of, say, 400 AGL. Thus, the visual descent point (VDP) is 6000 feet from the VOR station.
Let’s say you’re chasing the needle left and right, with increasingly large oscillations. You hit the VDP with the needle swinging through the center but your heading 15 or 20 degrees off. If you think enough to look in the correct direction (remember, the nose is pointed 15 or 20 degrees away from the runway), maybe you see the runway. Even if you do, you’re heading away from the runway, and by the time you react and turn back, you’re on very short final and well off centerline. Not good.
On the other hand, let’s say you’ve got a half-scale deflection, but it’s rock steady. Where is your ground track going? RIGHT AT THE STATION – right where you want to go. Oh, so you’re worried about being off centerline? Well, just how far off are you? Half-scale is 5 degrees. One degree is one foot in 60, so 5 degrees is 5 feet in 60, so you’re 500 feet off to one side, 3000 feet from the runway end – about a 10-degree angle off, but headed straight for it. This is a piece of cake in a light plane. With a localizer, at decision height on an ILS, half scale is only 175 feet off, and again, headed straight for the runway.
So, what does this mean? We’re going to strive for a stable needle, even if it’s a bit off center. How do we do this? We use the needle movement, not the needle position, as a roll command. Very simply, if the needle is moving, we bank in the direction it’s moving, with angle of bank proportional to the speed it’s moving – slow movement, small bank; fast movement, larger bank (you’ll pick up the rate/angle correlation pretty quickly). When the needle stops, we level the wings, and keep them level until it moves again.
Once you’ve got the basics, you can start working on finer control by using this technique to make corrections to put the needle closer to center. If the needle is off to the right, roll into a right bank and hold the bank until the needle starts to move. Then roll wings level. The needle will continue to move left until you apply the original technique (needle moving left, bank left until it stops, then level the wings) to stop it. With a little practice, you’ll find this a very simple, natural technique.
You’ll notice the absence of one word you were probably expecting – “heading.” A lot of folks teach people to correct their heading by X degrees for Y amount of needle deflection, then try to compute in their heads a drift correction angle. Who cares? You’re not trying to compute the wind direction and velocity – you’re just trying to get to the end of the runway, and the one thing that will get you there is a stable needle. And winds change significantly between FAF and MAP, so what worked at 1500 AGL may be well off at 600 AGL.
Once you've got this down laterally, you can translate it into tracking the glideslope needle for ILS approaches, using pitch/power changes rather than bank angle changes. You'll find that using this technique produces a smoother, stabler, trip down final, and your passengers will appreciate it.
Dave Krall CFII
Final Approach
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Dave Krall CFII SEL SES, Cmcl HELI
woodstock said:
I guess it's a little late for you, but it's much easier to learn to use VORs in the aircraft first, then go learn how to pass the test questions.
Let'sgoflying!
Touchdown! Greaser!
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Bill
Touchdown! Greaser!
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woodstock said:
And then someday you'll start IR training, and shoot NDB approaches. Then you'll realize what a colossal advance VOR was compared to ADF.
Eamon
Line Up and Wait
Here is a completely off the wall way of using you VOR that might just help you make sense of it all.
Another good way to get a quick fix of your position to the vor before you center the needle with the OBS knob..............
As soon as you tune in the correct VOR Freq & identify it and before you touch the OBS knob ..........
Brake the VOR head into 4 quarters. using a line from left to right and another from top to bottom.
The "to - from" arrow will tell you if the VOR is to the top or bottom 1/2 of the the VOR head and the needle will tell you, depending on deflection, if the VOR is to the left or right 1/2.
If the top of the VOR head was tuned into 060 and the head had a from flag & a left needle then..... to fly to the vor you would turn to a approximate heading of 285
Another good way to get a quick fix of your position to the vor before you center the needle with the OBS knob..............
As soon as you tune in the correct VOR Freq & identify it and before you touch the OBS knob ..........
Brake the VOR head into 4 quarters. using a line from left to right and another from top to bottom.
The "to - from" arrow will tell you if the VOR is to the top or bottom 1/2 of the the VOR head and the needle will tell you, depending on deflection, if the VOR is to the left or right 1/2.
If the top of the VOR head was tuned into 060 and the head had a from flag & a left needle then..... to fly to the vor you would turn to a approximate heading of 285
woodstock
Final Approach
thanks everyone - I did pretty well tonight! still need to get it straight "in action" but I at least understand the concept.