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How Does A VOR DME Approach Work?  
User currently offlineEKSkycargo370 From United Kingdom, joined Mar 2006, 150 posts, RR: 0
Posted (7 years 6 months 2 weeks 5 days 5 hours ago) and read 32767 times:

Hi,I was just wondering how a VOR DME approach works? If there is no ILS present or VASI how does the pilot judge his glideslope especially in low visibilty?

26 replies: All unread, showing first 25:
 
User currently offlineDLCnxgptjax From United States of America, joined Oct 2005, 353 posts, RR: 0
Reply 1, posted (7 years 6 months 2 weeks 5 days 5 hours ago) and read 32767 times:

To give you a simplified answer, you basically track the specified VOR radial to the runway. After passing the final approach fix you descend down to a Minimum Descent Altitude (MDA) until reaching the missed approach point. The missed approach point and final approach fix, in most cases, is identified by using DME. If you do not see the runway, or the approach lights at the missed approach point, you go missed. There isn't a glideslope for a VOR approach, therefore it is a non-precision approach.

I hope this answers your question.

[Edited 2007-04-05 17:00:49]

User currently offlineHighFlyer9790 From United States of America, joined Jul 2005, 1241 posts, RR: 0
Reply 2, posted (7 years 6 months 2 weeks 5 days 5 hours ago) and read 32767 times:

How would a RNAV/GPS approach work?


121
User currently offlineGeo772 From United Kingdom, joined Jul 2004, 519 posts, RR: 0
Reply 3, posted (7 years 6 months 2 weeks 5 days 5 hours ago) and read 32767 times:

GPS cannot be used for approach, the main reason for this I suspect is that the owners - USA - can program in an offset and or reduce the accuracy of the system with no warning.

This would not produce a failure indication on the flight deck, whereas if there is a problem with a regular radio navigation beacon the flight crew see a failure flag.



Flown on A300B4/600,A319/20/21,A332/3,A343,B727,B732/3/4/5/6/7/8,B741/2/4,B752/3,B762/3,B772/3,DC10,L1011-200,VC10,MD80,
User currently offlineEKSkycargo370 From United Kingdom, joined Mar 2006, 150 posts, RR: 0
Reply 4, posted (7 years 6 months 2 weeks 5 days 5 hours ago) and read 32767 times:

When you mention the MDA,is that descent in feet per minute from the specified VOR/DME?

User currently offlineThePinnacleKid From United States of America, joined Feb 2005, 725 posts, RR: 8
Reply 5, posted (7 years 6 months 2 weeks 5 days 4 hours ago) and read 32767 times:

GPS can be used for an approach in the US... that's why we have GPS approach plates (and why I've flown them on checkrides, flown them in real IMC, and taught students how to fly them...) they are at this time though primarily non-precision (that I'm aware of) and treated like VOR, NDB, ASR etc approaches... they are working on GPS approaches become precision approaches like ILS, and PAR's... I personally do not know where we are in the stage of transitions because now flying for an air carrier we do not have the crew training completed to allow us to fly them... but I promise GPS can and is used for approaches at least here in the States.

Chris



"Sonny, did we land? or were we shot down?"
User currently offlineDLCnxgptjax From United States of America, joined Oct 2005, 353 posts, RR: 0
Reply 6, posted (7 years 6 months 2 weeks 5 days 4 hours ago) and read 32767 times:

Quoting EKSkycargo370 (Reply 4):
When you mention the MDA,is that descent in feet per minute from the specified VOR/DME?

The MDA is the minimum altitude that you may descend down to until the Missed Approach Point (MAP). So say the MDA is 400ft, and the MAP is 0.7 DME. That means that you cannot go below 400ft until you reach 0.7 DME unless you have the runway insight. If you have the approach lights insight you can go to 100ft above touchdown zone elevation until you have the runway environment insight. If none of these things are visible, you must go missed.

[Edited 2007-04-05 17:43:56]

User currently offlineEKSkycargo370 From United Kingdom, joined Mar 2006, 150 posts, RR: 0
Reply 7, posted (7 years 6 months 2 weeks 5 days 4 hours ago) and read 32767 times:

I am assuming this type of approach cannot be carried out in poor visibility?

User currently offlineDLCnxgptjax From United States of America, joined Oct 2005, 353 posts, RR: 0
Reply 8, posted (7 years 6 months 2 weeks 5 days 4 hours ago) and read 32767 times:

As far as I know, you can do any instrument approach in poor visibility, as long as the visibility meets the requirements of that particular approach (published on the approach plate).

User currently offlineEKSkycargo370 From United Kingdom, joined Mar 2006, 150 posts, RR: 0
Reply 9, posted (7 years 6 months 2 weeks 5 days 3 hours ago) and read 32767 times:

If you are attempting this kind of approach in poor visibility,how are you supposed to maintain the centre line?

User currently offlineIAHFLYR From United States of America, joined Jun 2005, 4790 posts, RR: 22
Reply 10, posted (7 years 6 months 2 weeks 5 days 3 hours ago) and read 32767 times:

Quoting ThePinnacleKid (Reply 5):
they are working on GPS approaches become precision approaches like ILS, and PAR's... I personally do not know where we are in the stage of transitions because now flying for an air carrier we do not have the crew training completed to allow us to fly them... but I promise GPS can and is used for approaches at least here in the States.

And he promises correctly as others have mentioned. Using an RNAV (GPS) approach is non-precision, the RNAV (RNP) approaches that are starting to show up will in most cases if the RNP value is tight enough, say RNP .15 or so, you get a decision altitude (DA) that may be only a 100' above CAT I decision height (DH) on an ILS approach, and yes it is considered a precision approach providing vertical guidance as well.



Any views shared are strictly my own and do not a represent those of any former employer.
User currently offlineThrottleHold From South Africa, joined Jul 2006, 657 posts, RR: 1
Reply 11, posted (7 years 6 months 2 weeks 5 days 3 hours ago) and read 32767 times:

Quoting Futurecaptain (Reply 10):
Most people I know, my self included use the "chop and drop" method. As soon as I'm allowed to decend I will get to the MDA as fast as possible and level off there. That way all I'm doing is looking for the airport or the missed point.

Most operators now prefer to fly a Constant Descent Approach. That involves arranging the descent rate so not to have any level flight at MDA, therefore maintaing close to a standard glidepath as possible.
MDA is a minimum altitude which cannot be passed without the required visual contact. Initiating a go-around at MDA could result in the aircraft sinking below this altitude, therefore we add 50ft to all MDA's on NPA's. A DH on an ILS can be broken in the event of a go-around.


User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 12, posted (7 years 6 months 2 weeks 5 days 2 hours ago) and read 32767 times:

Quoting EKSkycargo370 (Reply 11):
If you are attempting this kind of approach in poor visibility,how are you supposed to maintain the centre line?

You arn't necessarily maintaining centerline on a VOR approach. You are simply tracking a radial off the VOR station and looking for the airport. If an approach brings you in far enough away from the runway heading it is called a VOR-A approach rather than having the runway number in the name such as VOR-1L. Some VOR-A approaches even bring you in from a close to 90 degree angle from the runways.

Quoting ThrottleHold (Reply 13):
Most operators now prefer to fly a Constant Descent Approach

True, its probably different for the pilots of the jets rather than us Cessna guys.


User currently offlineXJRamper From United States of America, joined Dec 2003, 2462 posts, RR: 51
Reply 13, posted (7 years 6 months 2 weeks 5 days 2 hours ago) and read 32767 times:

Quoting EKSkycargo370 (Reply 7):
I am assuming this type of approach cannot be carried out in poor visibility?

What do you mean by poor visibility? The reason for an approach is bad weather, whether its rain and clouds, snow, or fog. The non-precision approach just doesn't get you down as far as a precision approach would, which usually isn't more than 300 feet in some cases.

Case in point....

Toledo Express Airport has almost every kind of approach into there, GPS, ILS, VOR, even has an NDB off of runway 7. Coming in on the ILS Runway 7, the DH (Decision Height) is 883. At that point in time, whoever is flying the approach, cannot descend below that altitude until a few things happen. Basically at the MAP (missed approach point) you either see the runway, runway environment, or you go missed. Runway environment basically means the "rabbit" lights, runways lights, etc. At that point only can you descend another 100 feet to see if you have the runway in sight.

The few distinctions between this and a non precision approach:

In the same locality, there is an airport that has a direct VOR/DME approach into Wood County airport. If you notice, the MDA is 1120. This altitude is only 237 feet above the precision approach DH.

Another kind of VOR approach is whats called a VOR-Alpha approach. Because some airports have a VOR approach but do not have a direct "line of sight" to either runway and the offset degree is more than 30 degrees off the runway heading.

An example of a VOR-A is Toledo Suburban airport. Off the same VOR that the above airports use, and in fact exactly 180 degrees reverse of the VOR Wood County, is the VOR-A on the 360 radial. If you notice, the approach intersects the only runway 9-27. So the only option is a circling to land approach.

-------------------

Now your original question "how does a VOR/DME work?"

The VOR is basically a horizontal guidance. It lets you know if you have deviated away from the particular radial that you tuned your OBS knob to. When entering past the Final approach fix inbound to the airport, you do a couple of things. One is making sure your Radial is tuned to the correct directional radial. Secondly, making sure that you start time. And lastly speed. If you do not have the correct radial tuned in, you will be off course from the get go, and that right there you have to go missed and try it again. Time is the most important factor in most approaches (GPS approaches being the only that do not require time). Setting the time will be able to accurately define where your MAP will be. In fact, time will be the only way to tell when you are at the MAP when you are flying a stand alone VOR with no DME. Speed is the other important factor. When descending through to the MDA, it is important to maintain your aircraft type's approach speed according to your approach plate. If you maintain (usually) a 500 fpm decent, then you should be able to maintain the correct speed on the approach, which in turn will put you at the correct time when crossing the MAP threshold.

VOR's to require a bit of study, but once you understand them they are a very useful system.

Hope this helps.

XJR



Look ma' no hands!
User currently offlineGoaliemn From United States of America, joined Sep 2005, 463 posts, RR: 3
Reply 14, posted (7 years 6 months 2 weeks 5 days 2 hours ago) and read 32767 times:

Quoting EKSkycargo370 (Reply 11):
If you are attempting this kind of approach in poor visibility,how are you supposed to maintain the centre line?

Larger airports have the center line lit up, plus you can still see the lights on the edges of the runways. At the MDA, you have to be able to see the runway lights, then you complete the rest of the landing You don't do this landing without being able to see anything. The MDA is usually acouple hundred feet off the ground, depending on the airport and aircraft type.

A great site for some approach charts.. http://www.myairplane.com/

[Edited 2007-04-05 20:08:42]

[Edited 2007-04-05 20:10:10]

User currently offlineMustang304 From United States of America, joined Aug 2005, 70 posts, RR: 0
Reply 15, posted (7 years 6 months 2 weeks 5 days 2 hours ago) and read 32767 times:

Quoting ThePinnacleKid (Reply 5):
GPS can be used for an approach in the US... that's why we have GPS approach plates (and why I've flown them on checkrides, flown them in real IMC, and taught students how to fly them...) they are at this time though primarily non-precision (that I'm aware of) and treated like VOR, NDB, ASR etc approaches... they are working on GPS approaches become precision approaches like ILS, and PAR's... I personally do not know where we are in the stage of transitions because now flying for an air carrier we do not have the crew training completed to allow us to fly them... but I promise GPS can and is used for approaches at least here in the States.

WAAS (Wide Area Augmentation System) is coming into effect at several airports in the US. It is currently replacing NDB approaches as that equipment is being retired. GPS approaches with WAAS, will allow for precision approaches (Glideslope, ~200' AGL Decision Altitude). It will also allow for more complex approaches into certain airports. GPS approaches are pretty normal fair in the US. As WAAS is implemented more, I'd expect they would be pretty common place. The WAAS system requires a module for the GPS units to receive the signal from the ground station. In most General Aviation Aircraft it is $1500 to $2000--> a lot of functionality for a little money.

I have a C17 Pilot friend who says the Military does not do GPS approaches, doesn't even practice them. However, they are used pretty commonly by GA and by commercial traffic in the US.


User currently offlineIahflyr From United States of America, joined Jun 2005, 4790 posts, RR: 22
Reply 16, posted (7 years 6 months 2 weeks 5 days 1 hour ago) and read 32767 times:

Quoting Futurecaptain (Reply 14):
If an approach brings you in far enough away from the runway heading it is called a VOR-A approach rather than having the runway number in the name such as VOR-1L. Some VOR-A approaches even bring you in from a close to 90 degree angle from the runways.

If the VOR approach radial is greater than 30 degrees from the runway centerline course then it is gets tagged with circling minima only, no straight-in minima and thus is a circling approach......it may be named VOR-A it could be named VOR-D or any letter associated with the basic NAVAID used for the approach.

As the AVN OKC procedure folks crack open an approach for review or an approach is changed, this new naming convention is to be applied and that letter given to the procedure isn't to be used again in the same approach control area as per the latest/greatest current naming convention for a circling approaches. You might find an NDB-A to one airport and a VOR-A to another airport but over time that will change to match this new naming stuff, unless that changes another 50 times!  Smile



Any views shared are strictly my own and do not a represent those of any former employer.
User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 17, posted (7 years 6 months 2 weeks 5 days 1 hour ago) and read 32767 times:

Quoting XJRamper (Reply 15):
In fact, time will be the only way to tell when you are at the MAP when you are flying a stand alone VOR with no DME.

On some approaches the VOR is at the airport and crossing the VOR again inbound is the MAP. No time required.


User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 18, posted (7 years 6 months 2 weeks 4 days 23 hours ago) and read 32767 times:

Quoting Geo772 (Reply 3):
GPS cannot be used for approach

So far from the truth. I've done GPS approaches many times, and in the unlikely event that you do get a bad signal, you WILL get an annunciation in the GPS unit (RAIM or INTEGRITY annunciator).

Like most have said, most are non-precision, but a few CAT I GPS apps are starting to be developed.


User currently offlineConjureMe From United States of America, joined Oct 2006, 36 posts, RR: 0
Reply 19, posted (7 years 6 months 2 weeks 4 days 23 hours ago) and read 32767 times:

Quoting FLY2HMO (Reply 20):
Like most have said, most are non-precision, but a few CAT I GPS apps are starting to be developed.

The technology is definately there. The garmin gns480, which came out in 2004, was the first unit approved for precision gps approaches. I don't believe there are any airports that have commisioned a precision gps yet.



Never let the plane take you somewhere your brain didn't get to five minutes ago.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6407 posts, RR: 3
Reply 20, posted (7 years 6 months 2 weeks 4 days 23 hours ago) and read 32767 times:

Quoting ConjureMe (Reply 21):
The technology is definately there. The garmin gns480, which came out in 2004, was the first unit approved for precision gps approaches. I don't believe there are any airports that have commisioned a precision gps yet.

Well, considering that Apollo Avionics, developer of the CNX 80 (now the Garmin 480) was headquartered at SLE...

http://204.108.4.16/d-tpp/0703/00361RZ31.PDF

LPV Decision Altitude: 540

http://204.108.4.16/d-tpp/0703/00361I31.PDF

ILS Cat III to same runway DA: 410



So, it gets you almost as low...  Wink



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineGoldenshield From United States of America, joined Jan 2001, 6070 posts, RR: 14
Reply 21, posted (7 years 6 months 2 weeks 4 days 23 hours ago) and read 32767 times:

Quoting KELPkid (Reply 22):
ILS Cat III to same runway DA: 410

That's a CAT I approach plate.



Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6407 posts, RR: 3
Reply 22, posted (7 years 6 months 2 weeks 4 days 23 hours ago) and read 32767 times:

Quoting Goldenshield (Reply 23):
That's a CAT I approach plate.

D'oh! You're correct. SLE doesn't have Cat III...

[Edited 2007-04-05 23:32:39]


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinePope From , joined Dec 1969, posts, RR:
Reply 23, posted (7 years 6 months 2 weeks 4 days 17 hours ago) and read 32767 times:

Quoting Mustang304 (Reply 15):
I have a C17 Pilot friend who says the Military does not do GPS approaches, doesn't even practice them. However, they are used pretty commonly by GA and by commercial traffic in the US.

Does anyone know why this is?


User currently offlineMir From United States of America, joined Jan 2004, 21675 posts, RR: 55
Reply 24, posted (7 years 6 months 2 weeks 4 days 16 hours ago) and read 32767 times:

Quoting Geo772 (Reply 3):
GPS cannot be used for approach

Not at all - I've done GPS approaches many times.

Quoting Geo772 (Reply 3):
This would not produce a failure indication on the flight deck

Again incorrect - there would be an indication of failure, at which point a missed approach must be carried out.

Quoting DLCnxgptjax (Reply 6):
The MDA is the minimum altitude that you may descend down to until the Missed Approach Point (MAP). So say the MDA is 400ft, and the MAP is 0.7 DME. That means that you cannot go below 400ft until you reach 0.7 DME unless you have the runway insight.

Not quite right. It means you can't go below 400ft until you have the runway or required lights in sight, and if you don't have them in sight by 0.7DME, you have to go missed.

-Mir



7 billion, one nation, imagination...it's a beautiful day
25 KELPkid : Just hazarding a guess, but I'd imagine that 1) the Air Force, the owner of the system, designed it for en-route navigation, and reserves the right t
26 Mir : Also hazarding a guess, as I've heard of that too (and enroute charts reflect this fact), and it does confuse me a bit: GPS approaches are generally
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