This article is Copyright © 1996 by Andrew C Boyd. All rights reserved.
The NDB approach is generally considered one of the tougher items on the flight test for the instrument rating. If you asked a bunch of instrument-rated pilots to list their favorite approach equipment, you'd probably get a list something like this:
# 1 ILS
# 2 LOC only (no glideslope)
# 3 VOR
# 4 NDB
The ILS (Instrument Landing System) is every IFR pilot's favorite. It consists essentially of two indicators, one for "go right/go left" and another for "go up/go down". The LOC (localizer) only and VOR approaches look pretty much the same as an ILS except that they don't have the "go up/go down" glideslope needle, just a "go right/go left" needle.
Everybody's last choice is the old NDB (Non-Directional Beacon) approach. The important difference between the NDB and the others is that there isn't a single dial in your cockpit that you can refer to, to see your tracking error (go right/go left). You must use both the Heading Indicator and the ADF (Automatic Direction Finder) indicator and combine the information presented by the two, if you wish to accurately track to or from an NDB.
Looking at two dials probably sounds pretty simple, reading this article on your couch eating crackers, but gosh, when you're bumping along in the soup, seemingly with a million other things to do, it somehow gets lots harder!
Being the lazy but crafty person that I am, I've wondered if it was possible to consistently fly an acceptable NDB approach without ever having to track using an NDB.
Don't get me wrong, I have nothing against learning to track using the NDB. Cross-country, there's not much else to do, so why not fiddle with your crab angle? Good visualization practice.
But during the approach, you're busy with other things and you're awfully close to sharp, hard things that stick up into the clag. I worry that people under load may try to get fancy, misinterpret the dials and go the wrong way. That's probably never happened to you, but it sure has to me.
So, let's look at flying the NDB approach to our favorite airport, Sumspot, which has an NDB located 4 miles west of the threshold of runway 09.
You're cleared for the Sumspot NDB 09 approach, so you could simply "home" to the NDB, which is your IAF (Initial Approach Fix).
"Homing" with an ADF simply consists of keeping the needle on the _nose_ of the aircraft, or on the zero at the top of the ADF indicator.
With a crosswind, our path to the NDB may not be pretty, but we'll get there. Spend any excess brainpower (and in a single pilot IFR cockpit without an autopilot, there may not be much!) studying the approach plate. Lots of stuff on the plate, but make sure that you get the important items. I use the acronym "AMORTS":
Approach - have you got the right plate out? (don't laugh!)
Minimums - MSA, PT, FAF, MDA
Overshoot - quickly review the missed approach so you're prepared
Radios - ADF tuned and identified, comm and nav as reqired
Times - get a time to the Missed Approach Point (MAP)
Speeds - based on the groundspeed you expect with the wind
Passing the NDB outbound, fly some kind of procedure turn.
Visualize what the wind is doing to you on each straight leg of the procedure turn, and make a correction for it. A crosswind, crab into it. A headwind, increase your time significantly. A tailwind, decrease your time slightly.
Do your cockpit checks now. Check your Heading Indicator against the compass. You cannot fly an accurate NDB approach without an accurate heading reference.
OK, the procedure turn is complete, we're now intercepting the Final Approach Track.
Turn initially to a heading of 090 and get a descent started if your altitude over the NDB inboundthe FAF (Final Approach Fix)is lower than your procedure turn altitude. Be sure to keep a solid 500 fpm descent rate going, use 600 or 700 fpm if you have a tailwind.
Now, if there's no wind, a heading of 090 will take us right back to the NDB. You should be so lucky! Rather than using some complicated tracking and correction technique after you're established on the final approach track, an idea is to simply home to the NDB. Needle on the nose again, back to the NDB.
If we home to the NDB, we are assured of a good station passage.
Passing the NDB (the FAF) inbound, turn back to 090 and get a descent going again if necessary, down to your MDA (Minimum Descent Altitude).
If you want to keep it really simple, after station passage just fly a heading of 090 until you see the runway, or your timer runs out at the MAP.
Without much wind, this generally works out pretty well. At a a groundspeed of 90 knots, it will take 2 minutes to travel 3 nautical miles to a 1 mile final for runway 09, at which time you should be visual with the runway, and start your descent from the MDA.
But let's say we've got a 30 knot direct crosswind out of the south (which is a lot at 500 feet AGL). If we get a good station passage, and just ignore the wind by flying a heading of 090, after 2 minutes the crosswind will have pushed us 1 nm north of course. If you figure out the trigonometry (pythagorean is easier), instead of being 1 mile from the runway threshold, we will be 1.4 miles, which is a pretty good return for such a simple procedure.
If you think about it carefully, it's actually going to be better than that. With a strong wind from the south, while homing to the NDB after the procedure turn for the FAF, let's say we eventually weathervaned to a heading of 110 at station passage.
After we pass the station inbound, during our turn from 110 back to 090 we will travel somewhat south (upwind) of the desired track, so we probably won't drift a whole 1 nm downwind as we calculated.
Also, if you have a higher groundspeed than 90K, you will be affected less by the crosswind. There's less time for the crosswind blow you off course after passing the FAF.
The neat thing about the above no-brainer NDB approach is that at no time did you ever have to track using the ADF.
Okay, so you're a perfectionist. The "no-brainer" approach just isn't good enough for you. You want to be right over the runway threshold, so that when you pop the hood, you can't even see the runway underneath the nose of the aircraft, right?
Well, we can do that, too, if you insist.
Slightly modify the above procedure. With a groundspeed of 90K, we know that two minutes after passing the FAF we should be visual with the runway, and in position to start descending on final.
So, passing the NDB inbound, turn to a heading of 090 and hold it as accurately as you can for one minute. After one minute, observe the ADF indication. Let's say that it's now 170 relative. This means that if all of our toys are working as advertised (heh heh) we have drifted south (to the right) 10 degrees during the one minute after we passed the FAF.
To correct for it, we turn right 20 degrees to a new heading of 110 for the remaining one minute. This 20 degree correction for a 10 degree drift is exactly the same track correction method you used as a student pilot on your private pilot flight test.
How to fly an NDB approach with the NDB on the field is left as an exercise for the reader. (I've always wanted to say that!)