What's a VOR/DME approach ???

[pkonowrocki]

A question to all u pilots and specialists. I heard that often but I never knew what that is. What's a VOR/DME approach. Eg. when ATC tells u to expect vectors VOR/DME ??? And how does it work ???

[screaming_emu]

Here is a chart for a VOR DME approach.

http://204.108.4.16/d-tpp/0602/05799VD13.PDF

Obviously to shoot the approach, you have to have a VOR as well as DME equipment. DME is usually tied into a VOR or ILS and simply tells you how far you are from the station.

Here's the quick and dirty on how it works. One of the big things about this kind of approach is that there is usually some sort of reason that you can't descend all the way to your minimum descent altitude for one reason or another. Might be an obstructoin, or it might just be that it makes sense not to be that close to the ground for the duration of the approach.

Looking at the chart, you see that the OBS course you set in on the VOR is 126. A VOR works such that if you dial in 126 with a "to" indication (arrow pointing up), you will approach the station from the north west, on a heading of 126.

You can see both on the planview (looking down on the approach) and the profile view (looking sideways) there are big boxes that look like the letter D. What that tells you is your distance from the station at that certain point. Again, looking at the planview, you see that when you first start the approach (say you are getting vectors on to the final approach course and starting at the beginning), you will start the approach at 2,800ft. But once you reach 6 DME, you can descend to 1,800ft. You can't go below that until reaching 3 DME. After that you can go down to your Minimum Descent Altitude (MDA) which is 1,420 (you get that from the chart at the bottom) assuming you are going straight in to that runway. Last, you see the missed approach point is 0.5 DME. If by that point you do not have visual contact with the runway environment, you have to a missed approach.

Hope this helps, dont hesitate to ask any questions if something isn't clear.

Oh yeah, one more thing. Right next to the box displaying your DME, it says which station it is from. Approaches aren't always from a VOR that is on the field, so its always good to check to make sure you have the right one tuned in.

[pkonowrocki]

Thanx Emu for help. Is this kind of approach used often ??? And do pilots like it ? For me it seems kinda complicated

[screaming_emu]

Thanx Emu for help. Is this kind of approach used often ??? And do pilots like it ? For me it seems kinda complicated

yeah, they're used fairly often I guess. Not by the airliners though, they usually shoot an ILS when given a chance because the minimums to land are considerably lower with those. It's really not all that complicated, just center the VOR needle and watch the DME. Once you see a certain number, you can descend to your next altitude. Certainly easier than an NDB approach, those just flat out suck.

[pkonowrocki]

yeah, they're used fairly often I guess. Not by the airliners though, they usually shoot an ILS when given a chance because the minimums to land are considerably lower with those. It's really not all that complicated, just center the VOR needle and watch the DME. Once you see a certain number, you can descend to your next altitude. Certainly easier than an NDB approach, those just flat out suck.

NDB is probably the worst type of approach. I would rather "land by hand" than NDB

[chrisburns]

NDB is probably the worst type of approach. I would rather "land by hand" than NDB

When conditions allow, most land by hand.

[screaming_emu]

When conditions allow, most land by hand.

I always land by hand...maybe when my telekenisis gets a little better I'll be able to do otherwise

[pkonowrocki]

When conditions allow, most land by hand.

I didn't know that. I thought most do ILS when possible.

[G-DALE]

I didn't know that. I thought most do ILS when possible.

Not at all. Most, if not all of the pilots I know or have spoken to definitely prefer to land the aircraft by hand. Pilots like to have a practice whenever possible, plus it would be boring if it landed itself everytime

The only time they are not able to perform the landing by themselves is (mainly) when visibility is too low, then it's up to the autoland to do the job.

Dale

[screaming_emu]

I didn't know that. I thought most do ILS when possible.

I understand why you could make the mixup, but even though Instrument Landing System does make it sound like the instruments are flying the airplane, but in reality you are using the instruments to make the approach. The plane I learned to shoot ILS's in didn't even have an autopilot, let alone one with an auto-lalnd system.

An ILS is made up of two parts. One is the localizer, which tells you whether you are to the left or right of the runway. The second is a glideslope which functions exactly like a localizer, but its on its side. It tells you whether you are above or below the glidepath to the runway. On most approaches, the glideslope is set at 3 degrees.

The way these work is there are two interlapping signals sent out by the antenna (I'll include pictures of both the antennas and what the lobes look like when I get a chance, right now I have to do some last minute cramming for an exam). The airplanes navigation radio senses how much of each signal it is getting and uses that to tell where on the course the aircraft is. Once I throw a picture up there, it should make sense, I should be able to do that in about 2 hours.

This information is put on either the CDI (like what the Cessna has in flight sim) or an HSI (mooney, or any of the commercial aircraft). Most autopilots nowadays can shoot an approach for you, but at least in General Aviation, few (if any) have autoland capability. Pilots love to fly, so usually we shoot the approach, and do the landing by hand.

I would put something in here about autoland sytems but a) I have to get to that studying and b) I'm sure AJ knows a whooooooole lot more about it than I do, so I dont want to risk saying something wrong.

[G-DALE]

Looks like you hit the nail on the head there Joe. Below is a quick and simple example of what Joe is talking about. I have pin-pointed and labled the key features of what makes an ILS approach possible. Hope this helps.



Correct me if I'm wrong,

Dale

[screaming_emu]

yup, that's correct Dale, though the way you should be doing it is instead of having the GPS up on the screen, be looking at the HSI. When you do this, make sure its on VOR mode instead of GPS. Also, go to map mode (where you go to get the freq anyway) and get the course for the approach. Dial this into the course window. And also, its way more fun to hand fly it instead of the autopilot. Anyway, here's the diagram of the instrument landing system. I know I cut some stuff off, but it was hard to take the screenshot of the PDF file and get the whole thing. Any questions feel free to ask and I'll do my best to come up with the answer. Even if I dont know it, I'm sure I have a textbook from some class that will have the answer in it.



This is striaght from the instrument flying handbook, a government published document, so no need to worry bout copyright stuff. Anyway, as you can see, on the left side of the localizer, the antenna is transmitting at 90hz, and on the right side its 150hz. I'm not exactly sure whether the frequency varies as it goes from left to right, but I'll try to find that out. The radio figures out where in that "fan" the aircraft is and places the bar on the HSI or CDI appropriately. If the bar is to the left, then you are to the right of the course.

Same thing happens for the glideslope, only it is turned vertical and transmits in an ultra high freq that corresponds to the High freq used for the localizer.

The marker beacons are positioned along the course so that if you dont have DME (or the localizer doesn't) you can still know where you are along the approach.

[Crism]

Emu quick one here. The localizer is used exactly like the VOR except its more accurate since it basically has the runway instead of a VOR radial? If it was localizer only you'd need DME to track how far you are from the station still am I correct?

[screaming_emu]

Emu quick one here. The localizer is used exactly like the VOR except its more accurate since it basically has the runway instead of a VOR radial? If it was localizer only you'd need DME to track how far you are from the station still am I correct?

yeah, it does work almost exactly like a localizer in terms of how you use it. Main difference is that on a localizer, it doesn't matter what number you have dialed into the OBS (unless you're using an HSI). Either way its a good idea to set your final approach course into the CDI or the HSI. With the CDI (like in the cessna), you're really just tuning it in for reference. But with the HSI, if you have the course turned the wrong way ( say about 180) you'll actually get what is called reverse sensing, meaning that it will react the opposite way you are expecting.

As for the only localizer, you do need something to give you information about where you are on the approach. DME is the easiest, but you can also use marker beacons, or even intersections with other navaids to figure it out.