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The title to Part 5 lists, "flying to an NDB".

Just like discussing VORs can not be only, "tracking a radial"... NDBs can not just be flying to one. Although, that simplest of NDB tasks also gets into one of the more difficult aspects of NDB navigation... wind aloft corrections.

Basically, and NDB (non directional beacon) is nothing more than a dumb transmitter. It just sits there radiating radio waves in every direction (non directional). All the intelligence is on the receiving end... and it pretty much amounts to a directional antenna. Remember holding your TV's rabbit ears, trying to get a better picture.. and part of that process was "aiming" them ? As you'd rotate them, the picture would get better, and then worse ?  If, when the picture quality peaked, and you knew exactly which direction to that station's transmitter, you could glue a pointer to the rabbit ears, you would have a crude ADF (automatic direction finder, the radio and gauge used to track NDBs).

Your airplane is the rabbit ears, and the ADF gauge tells you which direction to point them, to give you the best reception..

The ADF gauge itself gives no heading information. With the VOR you can spin the OBS to get the exact heading TO that station, but all the ADF gauge tells you, is a relative bearing. If the ADF needle points to 12 o'clock, the station is straight off your nose; 6 o'clock, off your tail; 3 o'clock, directly to your right... etc..  In other words.. if you took your chart in hand (oriented so that "up" is your heading), and could slide it behind the ADF needle so that the needle is your current position.. the needle would point to the NDB on that chart.

The standard ADF gauge has a compass card (kinda like an OBS ring, but utterly functionless, other than a reference). The theory is that if you spin that card to match your directional gyro, the ADF needle would now point to the heading (on that card) that is indeed directly to the station. The downside to this, is that every time you change headings, you'd have to manually change the compass card. The first skill you need to practice for efficient NDB navigation, is to mentally transpose the ADF needle onto you directional gyro. It's not completely accurate, but neither are NDB/ADFs anyway.. and saves you a step and some time that will be precious time when you're flying an NDB approach in 1 mile visibility.

The MSFS C172 is equipped with a classic ADF gauge. There are a few "modern" ADF gauges out there, that have slaved compass cards (they automatically adjust for your heading), and those are really more like an RMI (radio magnetic indicator). The default Baron and KingAir have these RMIs that also include a VOR needle (associated with the Nav-2 radio ). RMI navigation is a whole different beast that we'll get into later. I'll just add that the ironic twist to an RMI, is that it reduces the VOR to and NDB (no CDI or exact radial selection.. it just "points" to the station).

ANYhow... as you're seeing; NDB navigation is relatively simple, until you add some wind.

Imagine that you're flying to a small airport and the visibility suddenly deteriorates. Luckily, there's an NDB right at the airport (and you knew this because of your professionally detailed flight planning, and already have it tuned in). With no wind to deal with;  you'd just turn the plane until the ADF needle was at 12 o'clock and it would take you straight to the airport. As I mentioned..the compass card is to be ignored..All we care about is the needle and its relative bearing.



However.. wind has its say. With a  VOR, you'd have the CDI needle to keep you on that initial, straight line. If you just "point" your plane in the direction indicated by the ADF needle.. the wind will start blowing you off that line... and as you keep adjusting your heading to keep the ADF needle at 12 o'clock... you'll end up flying a big "arc" to the airport. This will get you there, if time and fuel are of no concern, but it's hardly efficient nor good piloting... AND could take you into restricted or controlled airspace.. OR.. right into some obstacles or terrain you weren't prepared for.







The trick is to note the heading when you first turn directly to the NDB.. and then make your heading adjustments to keep the ADF needle on THAT heading (by super-imposing it on the directional gyro (or if you insist, by the compass card  IF you're constantly adjusting it to match the directional gyro). Again, I've intentionally left the compass card at a random setting in the diagrams.. and it is to be ignored. I've also tried to show the mentally transposed ADF needle on the directional gyro (translucent magenta). Now you're flying a wind-corrected heading of ~060 to keep the ADF needle on ~030.



Don't worry if this doesn't make sense right off the bat. Just like VOR navigation.. a little discussion, with questions and answers, will have it crystal clear in no time...

It's important to mention why it can be critical to fly directly to an NDB on a SPECIFIC course. Aside from wasting time and fuel; if you're flying an NDB instrument approach... that course will be most likely be runway heading (you don't want to pop out of the clouds at 300agl perpendicular to the runway).. or sometimes the NDB approach cousre is to keep you clear of obstacles (it would be even worse to pop out of the clouds with a radio tower filling your wind-screen)...

Excellent !  

Wait until the "real" thing happens... When you sit down to fly to your favorite, $100 hamburger place. You know the route well so pre-flight begins with a glance at the weather http://adds.aviationweather.noaa.gov/ to decide whether or not to file IFR....It looks "OK", so you decide to go VFR...  Fire up the sim  run real weather.. go to spot view for a walk-around... sit yourself in the VC... breeze through the checklist (tune in the NDB at the field, just in case)...  take off... dodge a few clouds.... get up to 4,500msl and start enjoying the view...

NEXT THING YOU KNOW, you're VFR on top and can't see the ground well enough for pilotage.

You're only 30 miles out, and a check of the destination AWOS confirms VFR, with a broken layer you'll have to descend through... so you decide not to bother ATC with a "file on the fly" (but are prepared to, if things get worse)..

The runway there is 09/27 with the wind favoring 27... You go ahead and start turning to fly a course that will put you on an extended final to runway 27... When the mentally, super-imposed ADF needle points to 270 on the directional-gyro, you turn 270. Since you checked winds aloft during preflight, you know the appproximate correction, and sure enough.. as the needle starts drifting; you correct your heading to about 275 to hold the needle on the gyro-imposed 270, adjusting as needed.

Now.. this brings us to your  GOOD  question.. How far out are we ? When will it be safe to descend ? If it were at all in question, we'd have to break off... climd to a safe altitude, and file IFR, and fly the published approach. That gets into instrument training, so I'll just touch on it now.. but we'll have a blast with it later  

On this flight.. you still have enough visibility to descend at your discretion, and you're just using the NDB to keep you from getting completely turned around. If you know you're only 10 miles out and have to punch through a broken layer; it's comforting to know where the airport is, and that you're on runway heading.

If it were a genuine NDB approach; a typical, published approach would have you fly FIRST to the NDB (at the field in this example)... then you'd fly outbound on the reciprocal of runway heading for a set time.. then make a procedure turn to reverse your course, and fly a wind adjusted time back to the airport. Since you have an initial approach fix (IAF) and timed your way outbound... you'll have a good idea of how far out you are, after the procedure turn... so you descent accordingly on the inbound leg.

Here is a simple example of an NDB approach (we'll get more in-depth when I start the instrument lessons).. You fly directly to the NDB staying above 3100msl.. then at the NDB you turn 095, fly outbound for 2 minutes, while descending to 2600msl... then the procedure turn.. fly 050 for 1 minute, then turn right (always turn away from the airport to keep from drifting too close) to 230 and wait for the super-imposed needle to reach 275... then turn 275 and adjust your heading to keep the super-imposed needle on 275 while descending. You can descend all the way down to 1620 (NO lower) and stay there looking for the runway.. if the NDB needle swings (meaning you just passed over it and missed the runway).. you fly the published missed We'll get into that later).....

Luckily I learned to read charts using the built-in lessons in the simulator, so that was easy for me to understand. I think later tonight I'll fly a flight to an airport with an NDB approach and let you know if I have any questions.

-Andy

Brett_Henderson wrote on Jan 13^th, 2008 at 10:21am:


I have to admit I think the way the US plates show the missed approach procedure is really useful. Very clear and very succinct - even a half trained pilot monkey could follow those instructions with comparitively little bother...

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Agreed.. but when you're already flustered from missing the runway.. can't see your wing tips for the IMC, flying to the published hold is just the beginning... Entering and flying that hold.. blind and with a wind can be a task and a half

The compass ring on the ADF is nearly useless to me... It's just easier look at the ADF needle and then place it mentally on top of your directional gyro.

Again, it sounds like you're getting it, but you're just a little confused.

When you made your original, corrected turn to 335, the ADF needle should not have been "straight" (if by straight you mean pointing to 12 o'clock).. it should have been pointing ~11 degrees to your right (about 1 o'clock) and your goal is to keep it to the right so that where it would be pointing, if it were super-imposed on the directional gyro, would be 335.

In other words... You're  heading will be into the wind by 11 degrees so that your course takes you straight to the NDB (your nose is not pointed at the NDB, so the ADF needle can't be at 12 o'clock)...

Keeping the ADF needle at 12 o'clock is what makes you fly that big arc.. that keeps the NDB straight off you nose, but the wind will keep blowing you off course.. and as you "chase" the needle to keep it at 12 o'clock, you kind of wind-vane around the NDB..

Take another quick look at the diagrams at the beginning of this thread..

Ok that clears it up much better. I'll fly again tomorrow as I'll have a little bit of time before having to study for mid-terms.

I think my problem is that I'm almost afraid not to have it straight, because when I have it straight, I know that I'm right on track and will fly right over the NDB. On the other hand, even though I'm on track for the NDB with the corrected heading and the needle not on the twelve, I'm afraid I'll miss the NDB when I fly near/over it. That's something I'll have to get used to, but I'm really starting to understand it more. Thanks for clearing that up.

Does the sim use an ADF with a fixed compass card or is it a rotatable compass card?  Or is it just an RMI?  I haven't used it in a long time, and I'm not anywhere near FS right now so I can't check.

Also, with the RMI, you can tune it to either NDBs or VORs, can't you?  Or is it just NDBs?  I've never actually used one, but it seems like if you could tune it to a VOR and have a CDI for the same VOR, it would be too easy almost.  But now I'm just talking about a HSI, aren't I?  I think it's too late and I'm just confusing myself now...

I think I just re-invented the wheel and sliced bread.