NDB Approaches - Download zip file of this lesson here (400k)
Today well be talking about the least common ground-based non-precision approach, the NDB approach. As the name implies, NDB approaches are approach procedures which use NDBs as the primary navigational aid. Like VORs, NDBs can be located either on the airport or at a remote location. Approach procedures can be based on either. NDBs are very old and and their operation is very simple, yet flying them precisely is difficult. You will notice that approach minimums for NDB approaches are higher than those for a VOR or other. This is because NDB signals are less precise and they are subject to several types of interference. I wont discuss the use of the ADF in this lesson, so if youre not familiar with its use, you may want to skip this lesson for now. The key to remember is that ADF needle always points to the station. Unfortunately, we only have a fixed-card ADF in X-Plane which makes it even harder. Many aircraft are now equipped with RMIs which have movable cards, which are much easier. Maybe we can push Austin to include this on X-Plane RMIs. One other thing to remember is that we dont get DME information from the NDB, so we have to be more aware of our position. So, now that we know a bit about NDB approache, lets brief our chart and then go flying...
Since weve already gone over chart symbology, Ill just touch lightly on most of these things. "A" tells us that were going to be doing the NDB(or GPS) Rwy 11R approach at Vero Beach. Because the approach is categorized with a runway number(11R), we know that the final approach segment of this approach will bring us to within 30 degrees of the extended centerline of 11R. We also see that the primary navaid is the Vero Beach NDB(VEP), its frequency being 392. "B" points to VEP and we see above the ID box that this is the Initial Approach Fix(IAF). There are no other IAFs for this approach. "C" shows us our outbound bearing of 265 degrees. "D" shows us our procedure turn, the course reversal we will fly to establish ourselves on course. "E" indicates our MAP. We see that the MAP is located at the NDB. "F" shows us our mimimums, 760ft MSL/1 mile vis. for both straight-in and circle-to-land. Something youll notice about this approach is the abscence of a Final Approach Fix(FAF). Because we simply make our procedure turn and then descend to our MDA, the beginning of the Final Approach segment will depend on how far out we make our procedure turn. In this case, we have what is called a Final Approach Point(FAP) which is wherever you complete the procedure turn. "G" shows us our time to the Missed Approach Point(MAP), which in our case, isnt applicable. It simply says that the MAP is at the NDB. So, we dont need time to find our MAP, we will know because well pass over the NDB and then ADF needle will flip. OK, now that weve briefed our plate, lets do this...
Todays weather is ceiling 850ft overcast, visibility 3 miles. Well be practicing with no wind because wind makes NDB procedures really tough. Well be doing the full procedure, starting at VEP. So, what were going to do is take off from Vero Beach, climbing to 3000ft, and request the full NDB 11R approach. At that point, Center will say "9246F proceed direct to the beacon, cleared full NDB 11R approach, contact tower 126.3 when procedure turn inbound". So, now weve tuned in VEP and turned so the head of the needle is pointing to 0 so we know were headed directly to the NDB. Were level at 3000ft and proceeding to the NDB. We see that our outbound bearing for the initial approach segment is 265 degrees. So, as we pass over the NDB, we will turn to a heading of approximately 265 degrees, looking for the head of the needle to swing around so the tail is now pointing to 0. Were now actually on the approach, proceeding outbound. Since we dont have DME to give us our distance from the NDB, we will start our timer so that we have an idea of how far out we are. Since were going 90kts, it will take us about 6.5 minutes to go 10NM from the NDB. Another trick that we use in this particular case is to reference the DME readout from the VRB VOR. Since we know from yesterdays procedure that the VOR is 3.5DME from the MAP. Add another 0.5NM since the NDB is in the middle of the field and weve can say that we dont want to go out any further than about 6DME from the VOR and that should keep us within 10NM of the NDB. This is a very general backup and you wouldnt depend on it entirely, but it can be helpful. In the profile view, we see that we can also descend to 2000ft until the completion of the procedure turn. So, well start a descent to 2000ft.
Im going out about 3 minutes from the NDB before I start my turn. You can see at "D" that the first turn is to a heading of 310 degrees. This is a heading, NOT a radial from a VOR. So, when the timer says 3 minutes, Ill make a turn to 310 degrees and start my timer again. We want to fly the procedure turn outbound for 1 minute. When 1 minute is up on our timer, well start a 180 degree turn to a heading of 130 degrees and then fly inbound. MAKE SURE to make standard rate turns(wings set on the turn index line on the turn coordinator) or else the procedure turn wont work out. At this point, were inbound on the procedure turn, so its time to call the tower and report where we are. The tower will then say "9246F roger, runway 11R cleared to land". Now, heres where it gets a bit tricky. Our needle is going to be deflected off to the left a bit. So, how do we know when weve intercepted our course? Well, were now on a heading of 130 degrees and we want to fly inbound on a bearing to the NDBof 085 degrees. Thats a 045 degree difference. So, when the needle swings to the left 045 degrees from 0(in this case it will be pointing to 315 degrees), we know were on course.
Now we want to start a turn to a heading of 085. As we do this, the needle will swing slowly so that the head should line up on 0 when we roll out on a heading of 085 degrees. Once were established inbound(on the inbound bearing), we may begin our descent down to the MDA of 760ft MSL. Were now at the FAP, so lets drop the gear. Our time to descend to the MDA is dependant on how far we went out on the procedure turn, so we must plan our descent accordingly to give us time at the MDA to look outside for the runway. We dont need to start our timer since the MAP is at the NDB
Now were done with communicating and configuring and all of our concentration is on flying. We going to level off NO LOWER than 760ft MSL and proceed along course until reaching the MAP, looking up every few seconds for the runway. Hopefully we would get the runway in sight before reaching the MAP. If not, we would immediately begin the missed approach upon reaching the MAP, calling the tower to let them know were going missed. In this case however, weve got the runway just in sight, so well continue the approach and make a normal landing.
You can see that the approach course or bearing doesnt lead us directly to the runway threshold in every case. Thats another drawback of non-precision approaches. You can also see from the overhead view that even though I flew the course as best as I could within the precision of the instrument, the inbound leg is not exactly lined up with the outbound leg. Such is the inaccuracy of the NDB. Even though we get less information from it, it is still perfectly usuable and excellent practice. Have fun!