- Thu Sep 27, 2018 8:05 pm
As Don mentions, there are two factors at play here: 1) radar vectoring/sequencing to final approach vs flying the full procedures and 2) flying a non-precision approach rather than an ILS.
The former (procedural approaches) are common at smaller airfields where there is no Radar, because perhaps terrain/traffic levels/investment (to give just a few examples) precludes installing radar and recruiting/training/maintaining radar controllers. Many of the Greek islands, for example, have no Radar.
With no radar, aircraft have to be procedurally separated which is not very efficient as it relies effectively on position reports from the pilots and time and altitude-based separations -- however, the approach procedure provides a convenient means for that to be accomplished whilst keeping the aircraft terrain safe and within a reasonably compact piece of airspace. Thus, an aircraft can be cleared for the procedure whilst another sits in the hold over the beacon at a higher level - once the first approach is completed the aircraft in the hold can be cleared for the approach and so on. Clearly, using radar to sequence aircraft on to a final approach track is much more efficient for busier aerodromes! A procedural approach, however, could be a non-precision approach like a VOR/NDB etc, or it could be to an ILS.
On the other hand, there are some aerodromes where it is impractical to install an ILS. This is often because of terrain concerns -- there are minimum terrain/obstacle clearance surfaces for the glideslope and the missed approach which must be met and terrain can interfere with the ILS signal to the extent that it is not usable. At these aerodromes there may be a VOR or NDB, for instance, that can be used as an approach aid to enable an aircraft to let down through cloud to a position where the pilot can see the runway sufficiently to manoeuvre the aircraft visually to land. Because such aids are less accurate than an ILS and there is no glidepath guidance the minima for such approaches are higher and the final approach track is unlikely to be directly in line with the landing runway.
However, it is quite possible to be radar vectored to the final approach track of a VOR or NDB approach just like an ILS if a radar facility exists, but obviously once established on the approach it is then over to the pilot (just like an ILS).
When would a pilot choose to fly one? In most modern airlines, generally when there is no other approach available as most specify that the most accurate available approach should be flown, which will normally be an ILS if one is installed and serviceable. Otherwise the order of preference would then, nowadays, often be RNAV if one is available and the aircraft is equipped, then VOR and finally NDB.