My local airport (NZHN) has a training facility based there which is specifically set up for easyJet trainees from the UK. Recently they have been practicing a lot of VOR/DME and NDB/DME approaches, I know this because I have been listening to my new scanner for rediculously long periods of time! 😉
My question for my fellow aviators today is what exactly is the point of a NDB/DME approach when the NDB is not even on the approach path? I get the DME part of the approach.
For reference I have left you a couple of links to the related aeronautical charts:
The basic purpose of any instrument approach is to get the aircraft to a three dimensional point in space, considering terrain clearance and visibility, from which you can see the runway environment. Sometimes the navigational aid for an approach cannot be located in the most ideal location. This could be because the NDB was already there before the approach was established or the airport does not have ownership of the land located at the best point for NDB placement. The local terrain may also be a factor. An offset navigational aid can still give guidance to this three dimensional point as long as the pilot is able to use normal maneuvering technique to align the aircraft for landing. Normally the runway is within 30 degrees of approach course. This allows more airports/runways to have an approach even if not ideally configured.
Why do you say that the NDB is not on the path? NDB approaches are a little harder than the rest of them (although many real pilots disagree and I can see why). It requires a little bit of math, but otherwise it's very doable. Try flying in Russia some time. See how many VORs you get over Siberia 😛 Most airports there (even the big ones) have NDB approaches. That's the point of them - there are many still left in the world, thus pilots need to be proficient in them.
I did not review all the charts but the chart for the Rwy 18 approach shows that the NDB is on the approach path. On option 1, it appears you fly a DME arc to the initial point for the approach, make a procedure turn over the NDB, and intercept the NDB beacon for an inbound course of 184 degrees for Rwy 18. The second option does not have the DME arc but requires you to remain within 10 nm of the DME during the approach.
In both cases, the NDB also establishes the holding point for missed approaches.
I agree with Bindolaf that NDB approaches are more difficult to learn, however, once learned and practised, they are no more difficult to fly than any other non-precision approach. In fact, I think an ILS is the most difficult approach to fly (by hand) consistently because it requires precise control of both heading and altitude rather than just heading in a non-precision approach.
Before everyone jumps on me about precise control of altitude, let me explain. An ILS requires you to be at a specific place in three dimensional space at all times during the approach. A non-precision approach requires specific placement in terms of course but allows the pilot to descend at will within the parameters set by the approach. You can descend at 200 fpm, 500 frm, or 1000 fpm as long as you do not violate descent minimums. So, with the ILS, you have to precisely control course, altitude, and to some extent, airspeed. With a non precision approach, you must precisely control heading but the requirements for speed and altitude control are non-precise.
Thank you very much Don, Bindolaf, and CRJCapt, your explanations helped out a lot. 👍