Why is ADF required on ILS approach?

An instrument approach requires an IAF. In the case of an ILS
approach, this must be separate to the ILS localiser and glideslope
because of the directive nature of the aid. The most common solution
is to use an NDB to provide the IAF and base the procedure on that.
NDBs installed in conjunction with instrument approach aids are
generally lower power than en-route NDBs. You could, in theory, use a
VOR as the IAF as well, however. Also marker beacons are notorious
for going on the blink, so a co-located NDB with the outer marker
(called "LOC") provides an additional check for ranging on the final
approach. (in some cases, the ILS system might not even have marker
beacons, in which case an NDB will be required at the OM location).
Remember that not all ILS systems have DME associated, so some form of
range checking is required.

The vast majority of ILS approaches in Europe use an NDB as the IAF
and many of them use an NDB LOCator (colocated with outer marker).

My local airport has several instrument procedures: ILS/NDB, NDB
Only, VOR, and VDF. There is no DME on either the ILS or the VOR and
since the VOR is on the A/D and not 7.5 nautical miles out like the
NDB, it can't reasonably be used as the IAF for the ILS approach
without totally throwing totally non-standard timings into the
equation. (as it is already the VOR approach [non-precision] already
has a non-standard outbound timing to begin base turn).

You will find that even on some of the more advanced European
airports, there are NDBs associated with the ILS at the outer marker
location. (Heathrow, Gatwick, Stanstead, Luton, East Midlands,
Birmingham, Paris Orly, Paris Charles de Gaulle, all have NDB LOCs at
around 8 nautical miles out.).

Also most holding procedures in the Europe use NDBs as the holding
fix. (there are currently NO operationally published GPS approaches
in the UK, and the CAA and even JAA are tending to be somewhat wary of
implementing GPS procedures for instrument approaches.)

Regards,

Leland



On Wed, 9 Jul 2003 08:40:13 -0500, "Gig Giacona"
wrote:

Let's see if I have this straight and please forgive any ignorance on my
part.

When I got my PP-SEL in ELD '79-'80 there was no ILS or NDB approach at ELD
just VOR. All the pilots bitched including the Lear and Citation pilots and
their bosses and ELD gets an ILS. But it gets put in requireing ADF that
even then was on the way out.

This is the silliest thing I have ever heard of.


"Thomas Borchert" wrote in message
...
Gig,

the initial approach fix (IAF) is defined by the NDB. how would you
find this point without an ADF? You'd get around that with radar
vectors, but you couldn't fly the full procedure without the NDB. And
the missed approach is a hold on the NDB. Same problem.

BUT: An approach certified GPS with the NDB in it's database can
replace for the ADF.

--
Thomas Borchert (EDDH)