NDB

So long as NDB approaches still matter...

93/12/08 08:02

From: E-Pilot To: DIPLOCAT Judith Bradt

It's knife, fork and spoon time.

I never thought of it in terms of how many feet that means one is off before; I was still sitting there thinking: "That's 1/6 of a mile drift per minute, when the airplane is going 1.5 nm in that minute..." I can see how it becomes a pretty small amount between the FAF and the threshold. On the approaches I've been flying, the NDB is on the airport. It's about (3 minutes at 90 knots, so...uh.1.5x3...) 4.5 miles from the point where one leaves the final leg of the procedure turn for the inbound leg to the beacon. For every minute, in your example, one would drift 1 foot for every nine flown. With 20 knots crosswind, would that make the ratio 9:2, though, wouldn't it? That would mean in 4.5 nm, I might drift 4 nmif I only held the published heading, wouldn't it? And if visibility or ceilings are close to minimums, I might have to miss the airport...

Your idea about turning back onto the published heading every so often to check things would work if I could establish heading precisely and keep everything else stable for long enough to check, interpret, and respond as needed to whatever I determine the instruments are telling me. So far, the combination has been difficult.

Techniques are a great deal like a knife fork and spoon: each has an appropriate use.

First of all, holding the inbound course as a heading after passing the NDB is useful ONLY when the NDB is located off-field. It doesn't make any sense whatsoever to use that technique when the NDB is located on the field, as the NDB is the missed-approach point. The technique that I use is employed only when the NDB is the final approach fix (FAF) -- not the MAP.

In your example, you used a 20 knot (I assume, direct) crosswind, stating that you might drift 4 NM in flying 4.5 NM. No no no. In flying 4.5 miles, you'd only drift ONE mile. (Look at the ratios: 9:2 = 4.5:1 ) Secondly, you can't land a Cherokee in a 20 knot direct crosswind, if my memory serves me correctly, the max allowable crosswind is 15 knots, isn't it? So, you'll have to use a circling approach: thus, higher minimums would apply in any case. But in your example, you're approaching an airport where the NDB is located ON THE FIELD -- so you'd just navigate as normal while inbound to the station. Worrying about drift (the ratio I gave you) applies only where the NDB is located OFF the airport.

Get it?

In the case where the NDB is located off-field, and is the FAF, the technique of periodically turning back to the outbound course (FAF to MAP) as a heading is useful only to check your amount of drift during the final approach segment.

The problem is that with station passage, too many pilots will NOT hold a heading within a 1.5-2 mile radius of the NDB and try to chase the needle to hold their course. (A great deal of ADF navigation is having the patience to sit on your hands for a few minutes and do nothing at all.) Thus, when the pilot is chasing the needle and passes the station, they'll usually make some radically drastic and inappropriate heading changes while yet fairly close to the station. The results are disastrous, particularly if the pilot has to make altitude changes at the same time.

So, teaching a student to cross the station and DELAY making a heading change to re-intercept the outbound course (FAF to MAP) allows them time for the needle to stabilize somewhat. Then, 10-15 seconds after station passage, a heading change can be made to re-intercept the outbound course to the airport. Every 30 seconds after that, however, I taught that the pilot should turn to a compass heading equivalent to the outbound course (FAF to MAP) to check that they haven't flown THROUGH their course -- WHICH IS NOT A REAL GOOD IDEA WHEN YOU'RE GETTING CLOSER AND CLOSER AND CLOSER TO THE GROUND.

After making a check of their position relative to the outbound bearing, I had the pilot return to their heading to re-intercept the outbound course, only to check again in 30 seconds. (Don't forget that WINDS CHANGE VELOCITY, that is, speed and direction, WITH CHANGES IN ALTITUDE. Therefore it follows that as you get closer to the ground, the amount of corrections you'll need to intercept or maintain a course is going to change -- OFTEN RAPIDLY. Thus the technique of returning to your outbound course as a heading is particularly useful for it allows the individual to check and re-check drift and position relative to the outbound course bearing while flying FAF to MAP.)

Once the pilot intercepted the outbound course (FAF to MAP), I'd have them hold a wind correction angle equivalent to ONE DEGREE PER EVERY FIVE KNOTS OF DIRECT CROSSWIND as a starting point.

This is the opposite of what the airlines teach. They don't want ANY turns after crossing the station. Hold the outbound course as a heading. The amount of lateral drift is minimal and well within protected airspace tolerances.

My point is this: don't make any radical changes in heading when you're close to the station, inbound OR outbound. Many student instrument pilots (and their instructors) try to make non-precision approaches precise: in contrast, the airlines adopt techniques that are within tolerances and are more realistic.

I had been working out sample solutions to the ADF problems, but at large deltas I tended not to get confused...When I get there, I'm going to want to leave some correction angle in for the wind, so I'll maybe turn 260 and see what happens. Any glaringly dangerous logical processes there?

Well, I commend you for starting to think of ADF navigation with respect (and relative) to the tail of the needle. That's the first indication that you're learning, or at least you're reading what I'm sending you.

In fact, it's the first indication of ADF smarts on your part. That's what the pros do. Keep thinking in those terms. As for the rest of your thought processes...

The formula MB=RB+MH is intended to get you to start thinking about the needle of the fixed-card ADF instrument RELATIVE to your accurately-set compass card, what I assume you're referring to as the HI. Pros simply overlay the ADF needle on the compass card, and the rest is easy, relatively speaking.

Most instructors teach the NDB as a necessary evil, and don't really emphasize NDB navigation and holding enough -- or early enough in instrument flying -- because of the over-reliance on VOR and ILS approaches. As I stated before, YOU CAN'T FLY A PRECISION APPROACH UNTIL YOU'VE MASTERED NDB NAVIGATION, HOLDING AND APPROACHES AS THE PRINCIPLES ARE ALL IDENTICAL.

Successfully mastering the NDB facilitates everything that you do in a precision approach. You think of the glideslope and localizer in the same way that you do the final approach segment of an NDB approach (opposite angles are equal).

All the mental gymnastics of MB=RB+MH are dangerously distracting. Because of this, I STRONGLY recommended that you get into an airplane equipped with an RMI -- thus eliminating the need to distract yourself mentally trying to figure out what bearing you're on, etc, etc while flying. What matters is WHERE you are, WHERE you're going, and HOW you get there. A controller will never ask you what bearing you're on; the only time that being on the exact bearing matters at all is when you're holding on an NDB or flying an NDB approach. Even then, if you're like 99% of the pros, you'll only be able to bracket the bearing while holding or on approach.

True, the brackets are small, and the distance from the bearing is measurable in terms of hundreds of yards, not miles.

Since you've asked me what I think about I think about your thought processes, let me state this:

"The trick of successful ADF navigation and approaches is in realizing that in geometry, opposite angles are equal."

Too much emphasis is placed on the RB+MH=MB -- people get too wrapped up in the mathematics, particularly in flight. If you can just mentally overlay the needle on your compass card, more than 75% of the battle is won.

Understanding this, you make the shortest turn to put the head or the tail of the ADF needle in the upper half of the ADF instrument. Once you're close to intercepting a bearing and have turned to intercept and fly inbound/outbound, it's a matter of balancing needle drift/displacement with an opposite correction angle.

This all takes me a long time to work out, however -- and it's very distracting to try and do it in flight, let alone on the ground. I believe this is the reason why studying and practice and studying and studying are recommended.

Yes -- exactly what I've been talking about.

Believe me, I know and remember well the frustration of trying to learn the ADF and in doing NDB approaches. I share your frustration with the mental gymnastics. I knew what was coming and, because I like you, wanted to spare you this frustration by recommending flying an HSI and RMI equipped airplane. Many, if not most, of all people who quit an instrument rating course do so when the lesson plans come to ADF navigation and NDB approaches. Most people who fail their instrument check ride do so because they really haven't learned the ADF, and can't do NDB approaches or NDB holding. It plagues the airlines, too, because many of the people I've flown with simply have never mastered the NDB -- and many of them have worked as Instrument Instructors.

Amazing, isn't it?

The telling truth of this matter is that I probably couldn't fly an NDB approach using a fixed-card ADF. Well, I could, but not without some difficulty. But it's that very distraction of the mental gymnastics which make the flying difficult.

Stay with it. Don't just try to get through ADF navigation -- break through it. It's a good fight.

And keep thinking in terms of the tail of the needle.

I remember the frustration well that you are now feeling. Don't worry: you'll get over it. I don't mean that you'll not feel frustrated -- I just have the feeling that you'll stick with this until you master it. You're that type of pilot, I believe. If you're not, it will come as a shock. For the moment, I'll not offer any advice about flying the ADF. When you're ready, let's exchange a few letters about what you DO understand about it, and I'll try to fill in the missing blanks, if any.

The truth is, Judy, that if you can't fly an NDB approach, you probably can't fly a precision approach. The elements are all the same. That's the bad news.

The good news is that you probably know already how to navigate by and use the ADF but just don't realize it. Non-precision approaches are tough. Of those, the NDB is the toughest nut to crack. You can do it. And you're so close to finishing, too. I listened to the new president of Honda Motor say that, at least at his company, sucess is 96% failure. So why should you be any different?

NDB involves things like addition, subtraction, left, right,inverses, and simple multiplication. To be as positive as I can muster at the moment, these things are elements of flying that I find occasionally pose a challenge...

Excuse me while I tear my hair out and step outside for the gulp of air that will precede a primal scream.

I DO know the feeling. I spent an hour breaking light bulbs and bottles, crushing cans and chasing chickens to vent my frustration with a particularly mean FRASCA simulator.

The assigned exercise in the meantime is to make up a dummy ADF and HI, and work through some examples of "where am I, what do the instruments read, and where would I have to turn" to get better at this.

This is a valuable exercise. Don't neglect it.

Why is it that I continue to take up activities at which one must continually work to improve, perfect, maintain and practice skills, rather than ones for which one can be permanently certified once and never have to re-prove or -qualify? I suppose I would be having less fun if I had an avocation that did not give me cause to shout with frustration inside my car as I hurtle down Sudley Road considerably above the speed limit.

If this was easy, then everybody would be doing it, and doing it very well, right? Then, the problems you're having right now would mean, what, that you're a moron? So, what is your problem? ARE you a moron? I didn't think you'd feel this way about yourself. So you must be normal, right?

Judy, I haven't met a good pilot yet who didn't have some threshold they had to crash over or through at one point or another in their training and education. Any idiot can eventually drive a golf ball 300-400 yards. It's the skill-development and practice which makes the difference between a 80's handicap and a hacker's 200. I've seen grown, mature men wrap golf clubs around trees in frustration -- and they're NOT learning skills which will someday preserve or save their lives.

The truth of the matter is that you already know how to navigate using an ADF -- you just DON'T realize it. If you know the basic fundamentals of boat handling, you already know how to navigate using an ADF, just as an example. Even more basic, what you've already learned about flying approaches (VOR, LOC and ILS) applies to NDB approaches: the ADF is a slightly different way of looking at the same information that a VOR provides. Change your perspective, and you're Columbus of the New (Instrument) World.

Could you manage not to tell me how wonderful HSI's are, and how frustrated people should not get in their cars and drive anyplace? Thank you.

You're welcome. Actually, I'll pretend not to be insulted about caring for you, and being told not to. And in this case, it's RMI, not HSI.

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