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Andrew C. Boyd |
This article is Copyright © 1996 by Andrew C Boyd. All rights reserved.
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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.
Your basic no-brainer NDB approach
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
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 inbound—the 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.
What about crosswind correction?
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.
Your slightly-enhanced half-brainer NDB approach
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!)
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