Saturday, May 10, 2008

Muddy Boots

Rubber Boots + Muddy Holes = Muddy Boots

So goes the memory device used to remember the formula for calculating the magnetic bearing to an NDB station. The formula is: Relative Bearing + Magnetic Heading = Magnetic Bearing to the station. Let's forget for a moment that no one actually uses this formula in the air while performing Automatic Direction Finder (ADF) navigation. We're talking about a sacred aviation tradition here!

My recent comments about non-directional beacons (NDBs) and the FAA's test questions about them drew some emails from a several readers. Yes, there are still a few pilots out there who actually like using NDBs. For all we know there may even be a few pilots still alive who have fond memories of the A-N range system, though it's hard to carry on a conversation with them since they all went deaf listening to the static and the A-N tone while trying to stay "on the beam."

Let's set sentimentality aside: NDBs and the ADF receivers that use their signals are not terribly accurate and we have significantly easier-to-use navigation systems, namely GPS. With all the emphasis on GPS/WAAS, many pilots seem to have forgotten (or they never learned) that GPS also relies on ground-based facilities. WAAS requires a series of ground stations to calculate the correction messages that provide the 3 meter accuracy offered by differential GPS. In fact, the GPS satellites themselves depend on ground-based support in order to function properly.

The possibility of a GPS failure at a systems level, however remote that might be, is something that many pilots simply don't want to consider. And many pilots don't understand that GPS receivers can and do fail. So while I'm not sentimental about NDBs and LORAN, on more than one occasion it was a lowly ground-based VOR, NDB or marker beacon that saved my bacon when my GPS receiver failed. So the big deal with ground-based navigational aids like NDBs, marker beacons, and LORAN is that they provide something that should be of interest to all pilots - redundancy.

Every few years, the FAA insists that VOR stations will eventually be phased out. Many NDBs have been or are slated to be decommissioned. I don't have access to an aircraft with a functioning LORAN receiver, but I know at least one pilot who does and uses it regularly as a backup. Even marker beacons are being eliminated, presumably to save money, though strangely the old CASES outer marker (which used to be part of the Oakland ILS RWY 27R) still continues to function even though it is no longer associated with any instrument approach or departure procedure. Think about that for a minute - the real estate is still being used, the antenna is still there, and the marker beacon transmitter is still functioning, it's getting electrical power, the electric bill is being paid by someone (probably taxpayers): Talk about the lights being on and no one being home!

Personally, I like the situational awareness that an NDB provides when it is associated with an ILS approach because the ADF needle points in the general direction of the final approach fix. Unlike other radio navigation systems, the ADF provides instant situational awareness once you accept that the needle usually-kinda-sorta points to the station. Virtually all of the planes I fly do not have an ADF or the ADF is broken, so what's a G1000 pilot to do if they want to maintain proficiency in using ADF-style navigation?

If you're getting vectors to intercept an ILS and you've selected the Activate Vectors to Final option, you can get the G1000 to emulate the behavior of an ADF by pressing the PFD softkey, then selecting one of the bearing pointers to use the GPS.

With vectors-to-final selected, the current GPS waypoint is the final approach fix and the bearing pointer will act just like an ADF needle, albeit much more accurately. In the example below, it's pretty easy to see that you are on a left base vector to the localizer.

The ADF-style pointer is also a great way to see when the controller has forgotten about you and you are about to go through the localizer.

A big advantage with the G1000's bearing pointer is that it is superimposed over a slaved heading indicator. Most of the older ADFs in GA aircraft have a fixed card or moveable card that is not slaved. It's even possible to practice NDB-style navigation to any waypoint you choose, so enterprising instructors can still expose instrument students and pilots to a bit of aviation tradition. You can practice wind correction angles, tracking bearings to and from the waypoint, the whole enchilada. You won't even have to get your boots muddy.

1 comment:

IFR Pilot said...

I learned this particular mnemonic as My Body Must Have Red Blood.

I'm sure there are plenty of other variants as well.