ADF frequency indication on ND

[United744]

Due to the way ADF works, I can't see how it can determine if it is "in range" or not of any station. Unless it relies solely on morse decode, there is no way to determine if "a" signal being received is valid or not, so I would expect it to be shown full-time.

[John H Watson]

The in-range assessment could be done by purely looking at FMC nav data and present position.

[Hardy Heinlin]

There are many 1-letter and 2-letter NDBs and the radio range is enormous; I think it's impossible to determine the location by assigning the received identifier to one specific identifier in the FMC's NDB database. The database doesn't even include the frequencies.

My definition of "in-range" is simply a certain dB minimum above which the ADF pointer becomes alive. When the amplitude is too low, steam gauge ADF pointers turn to 9 o'clock, and EFIS ADF pointers just disappear. That's a plain electronic function, not a database function.


|-|ardy

[Markus Vitzethum]

Hi Hardy,

quick side comment:

> The database doesn't even include the frequencies.

Actually, the real world 747-400 NavDatabase does include ADF frequencies (I just checked). The NDB database table consists of 1-3 letter ident, database revision (active or inactive DB), lat + long and frequency.

We sure all agree that this information doesn't seem to be used and that the FMC does neither auto-tune or allow to NDB identifier entry. But it is present.

I think this might be due to database format communality (not 100% but close) with e.g. the A320 or MD-11 Honeywell FMC/FMGC. If I'm not wrong the A320 FMGC can tune the ADF receiver by NDB identifier.

Markus

[Hardy Heinlin]

Hi Markus,

this must be a new feature of the newer FMC versions which have more memory. Older documents say that NDB frequencies are not included -- which makes sense: Why should they bloat that expensive and small memory and then even hide the NDB frequencies on the SELECT DESIRED WPT pages?

Which version did you check? Does that mean that newer FMCs include NDB frequencies on the SELECT DESIRED WPT pages?

Could someone enter "AA" in 4L on the PROGRESS 1/x page in an NG FMC or 767 Pegasus and see if there are any NDB frequencies on the SELECT DESIRED WPT pages? :-)


|-|ardy

[Markus Vitzethum]

Hi Hardy,

I am talking only about the contents of the NavDB (that 1 Megaword file). The databases I was looking at are from 2005, so definitely the classic 747-400 FMC, long before the more advanced FMC versions were available.

A global database of that time stores about 7500 NDB stations, at a cost of 12 bytes (= 6 words) each (6 for lat/long, 4 for identifier (incl. some flags), 2 for frequency, so about 7.5 Kilowords are used for NDB frequencies.

I am not talking about any FMC features using the frequency information. Actually, I'm not aware of any FMC page using the frequency information.

I assume that the presence of the NDB frequencies is a legacy feature from other FMC versions (A320 Honeywell, B757/B767 and A310/MD-11 have a very similar architecture, hardware-wise and database-wise)

Edit:
for comparison: the NDB data takes about 5% of the FMC memory and has global coverage. In comparison, terminal procedures (SID/STAR/IAP) consume about 35% of the FMC memory and cover only a few hundred airports.
(That does not include airport or runway data, these are different database tables.)

Markus

[Hardy Heinlin]

I see, Markus. Thank you.


Hi all,

just learned from a kind mail that the cyan ADF text on the ND can indeed hide both the frequency and the ID. But this seems to happen only when the NAVRAD page displays dashes in the related ADF field, and this may be the case only after powering up a dark ship. Once a frequency is entered, the dashes cannot reappear.


Regards,

|-|ardy