VOR/DME approach

Oscar Bravo

Could anyone explain to me what a a VOR/DME approach means,example to Knock,and the term "Oscar Kilo"? I guess its to do with having no ILS and so you would have to use Distance Measuring Equipment,so you(a pilot) would report "established outbound",followed by "turning inbound".is it basically a visual approach?

Delta Kilo

Now Im not a pilot but my understanding of it is as follows:

VOR is short for VHF omnidirectional radio range and DME is distance measuring equipment.

So basically the VOR is located at the airport somewhere and you know the runway bearing is say for example East, or 90 degrees. The idea is to intercept this bearing a few miles away from the airport. So you would fly North or South depending on the direction you are approaching the airport from, with the frequency of the VOR tuned into the VOR Navigational equipment in your aircraft. You intend to intercept the 90 degree bearing so you would turn the wheel to 90 degrees also. As you fly North or South, a needle moves across the device and when it is in the middle, you are at the point of intersection ie you have crossed the bearing.

The idea is to follow this bearing (by keeping the needle in the middle) and it will bring you to the airport along the bearing of the runway.

Where the DME comes in is so you know how far away you are, thus being able to find out what your altitude should be. There are also "markers" on the approach such as the Outer Marker, which will beep when you pass them and on your chart it will say what altitude you should be at this point.

So by following the bearing and working out what altitude you should be, you will find the end of the runway.

While it is not a visual approach, it is not near as accurate as ILS, due to the VOR not being at the end of the runway and inaccuracies in the instrumentation, so good visibility is required.

Thats my understanding of it anyway, If Im wrong feel free to correct me.

highlydebased

THis chart http://www.iaa.ie/safe_reg/iaip/Published%20Files/AIP%20Files/AD/Chart%20Files/EIKN/EI_AD_2_EIKN_24-6_en.pdf

Basically a vor/dme is an ILS without the glideslope, for want of a better explanation. you fly a certain radial onto a VOR and descend as per the altitudes on the chart, it tells you what altitude you need to be at at a certain DME. When you are visual with the field you take it from there.

basill

Just to clarify a few things. Not all VOR’s or for that matter NDB’s are located on airfields. Depending on the location of the beacon there can be a considerable offset. Alicante is a good example. A large offset will require you to make a considerable turn to line yourself up with the runway. The published approach will take into account these factors as well as surrounding terrain when determining the minimum altitude that you can descent to in order to become visual with the runway. If you do not get visual then you will commence a missed approach. Other approaches have a VOR/DME that is located on the final approach track but some miles away from the airfield. These can be confusing when you are first training as the DME will count down as you approach the beacon and then count up once you pass overhead the beacon. If landing on the reciprocal runway then you will never pass over the beacon on your approach.

VOR approaches are termed non-precision approaches. They are not designed to have you lined up perfectly with the runway. I personally like to think of them as cloud break procedures and nothing more. If flown accurately then they will allow you to safely descend through cloud (Instrument Meteorological Conditions) until you become visual without hitting any granite.

In the example at EIKN which was linked to from the Irish AIP it is a relatively straight forward procedure. The full procedure requires you to be at 4,000ft overhead the Connaught (CON) VOR. Depending on your aircraft type you then commence an outbound leg descending to 3,000ft. The radial you track outbound on depends on your aircraft category. An 737/A320/330 is categorised as category C whereas a 747 is D. An ATR would be in the A/B category. An A320 would track out on 066 degrees to 12DME before turning to the right to pick up the 267 inbound radial to the CON VOR. Incidentally at 12DME there is a hold as well with right hand turns and 1 minute legs to be flown at 220kts. At 7.8DME a descent is commenced from 3,000ft. The check altitudes are published bottom right on the chart. For example at 4 DME from CON you should be at 1,790ft. Once visual the airfield and runway will be in front of you and you will then need to align with the runway to land allowing for any wind.

Now I used the term full procedure above. As a general rule we don’t fly full procedures. They are a waste of time and fuel. The only times we would fly them are if there was no radar control or if there were special rules requiring us to do so taking into account things like high terrain etc. An example would be dropping into a valley with high surrounding terrain where we couldn’t do a straight in arrival. Think somewhere in the Alps. Generally though ATC would provide us with a radar vector to intercept the 265 inbound radial. Once established on the inbound radial we would advise ATC and be cleared to descend with the procedure. Again depending on surrounding terrain ATC may descend us down to say 3,000ft in the example at EIKN but often they may leave us a little higher and establish us further out. Say at 15DME from CON. In that situation we will have previously worked out that for each nm on the procedure that we need to lose 320ft (2,750-2,430) so it’s a pretty simple calculation to work backwards.

Hope that was of some use. Enjoy.

Oscar Bravo

Thanks for all the reply's gang.and thanks Basill for the EIKN example.the reason i asked is that with the E,NE wind lately there have been quite a few VOR/DME approaches to Knock. It was straight in ILS today even with a tail wind,nothing more than around 10kts though.However ive often seen the Aer Arann land on 27 with a considerable tail wind,i suppose he has plenty of runway to play with.