Post by brian whatcottI am going to suppose your old data for these UK fields is correct.
And I am going to suggest that a current DAFIF database will show the
great majority of (US) ILS glideslope transmitter placements is about
1200 feet from the runway threshold.
Post by bin mePost by brian whatcottQuite. Just about never "a third the way down the runway" in fact.
I was giving a few examples of where "just about never" did not apply.
The examples were around or greater than 1/3 of the way along the
runway. I was not suggesting this was the normal situation, its just
one of many variables used to ensure an ILS meets specification.
As you quote USA documentation, I hope you are familiar with the FAA
publication AIM. The current online version is here:
www.faa.gov/air_traffic/publications/atpubs/aim/Chap1/aim0101.html#1-1-9
I quote:
2. The glide slope transmitter is located between 750 feet and 1,250
feet from the approach end of the runway (down the runway) and offset
250 to 650 feet from the runway centerline. It transmits a glide path
beam 1.4 degrees wide (vertically).
The FAA use this as the basic guidance as the CAA do. However, it does
not mean in either Country it can't be varied to suit conditions. In
the USA generally, more land (real estate) is available for airport use,
and the approach can usually be accommodated with a GS position 750 -
1200 feet into the surface. In the UK with less land available, the
approach on occasions has to be modified to accommodate obstacle/terrain
clearance (lots of other reasons) which change the GS position - as
shown in the list in the last post.
Similarly, the FAA use (or in the past did, not sure about now with GPS)
marker beacons, while in the UK we tend to favour DME. DME takes up
less land, also maker beacons (all same frequency, different modulation
for type, OM, MM etc) are less precise in areas with close by airports
with ILS approaches. An accident occurred in the UK over pilot
confusion. The pilot was approaching airport A but was a little off
course and picked up airport B OM and started the descent - straight
into a hill. Since then, the UK favoured DME. Our airports can be
closer together, hence the reliance on DME when land is more difficult
to obtain. Another example of how the "general arrangement" is modified
to suit local conditions.
Post by brian whatcottIt may be helpful to review ILS instrument approach plates, which show
height above threshold, when on the glideslope.
This is normally around 55 foot elevation at the threshold.
If you recall that the usual glideslope angle is 3 degrees, I think you
will be able to work out the distance from threshold to the glideslope
that this elevation represents, with elementary trigonometry.
That's 50 / (tan (3 deg) feet.
Again from AIM, Fig 1-1-7 ILS: gives the 55ft (+/- 5ft) runway
threshold crossing height.
Again, this is not a limiting factor! It is a "general arrangement" -
it will be modified somewhere to take account of local conditions.
As this group is UK_rec_aviation, I hope nobody takes offence to the
frequent USA references.
Regards
Alan
Post by brian whatcottPost by bin mePost by brian whatcottPost by bin mePost by brian whatcott/snip/
the localiser array is at the end of the runway and the
glideslope array is around (lots of variables) one third of the
way down the runway, but always adjacent to the TDZ marked on the
Alan
A better guide for glideslope antenna positioning is around 1000
from the runway end, near the PAPI and VASI sites, for obvious reasons.
Brian W
TDZ is the place for the G/S aerial by International agreement,
and practical considerations./snip/
Quite. Just about never "a third the way down the runway" in fact.
Brian W
Brian,
As I said earlier in the thread, "the glideslope array is around
(lots of variables) one third of the way down the runway". I did not
say it was "a third of the way down the runway".
It depends on many factors, mainly designed for commercial transport
standards. To make it absolutely clear, it can be well into the
runway with a third being around the typical maximum with lots of
variables, (but somewhere in the world, including Farnborough in the
UK, it is/could be more than a third). It will always give an
undershoot area, the length of that area is again variable. As I also
stated in the same post, I have been out that loop for more than 10
years and things do change.
Take some examples, all using very old date, but it does show the point.
London City 28 (5.5 degree), overall length is around 4948ft.
3962ft available from TDZ. That is about 20% in.
Coventry 23 (3.0 degrees) is 5988ft overall, 4396ft available from
TDZ. That is 27% in.
Sumburgh 27 (3.0 degrees) is 3871ft overall, 2796ft available from
TDZ. That is 28% in.
Edinburgh 24 (3.0 degrees) is 8399ft overall, 5997ft available from
TDZ. That is 28.5% in.
Waterford, Ireland 21 (3.33 degrees) is 4701ft overall, 3201ft
available from TDZ. That is 32% in.
Isle of Man 08 (3.0degrees) is 5755ft overall, 3861ft available from
TDZ. That is 33% in.
Bournemouth 08 (3.0 degrees) is 7451ft overall, 4996ft available
from TDZ. That is 33% in.
Farnborough 24 (3.0 degrees) is 8005ft overall, 5011ft available
from TDZ. That is 37% in.
The conclusion is many factors are taken into account when designing
an ILS. The glideslope aerial/TDZ is one variable which is/can be,
used to ensure an A/C following the ILS will arrive at the TDZ if all
procedures are followed. You will often find the aerial position
gives a 55ft runway threshold crossing height - but not always.
Every ILS is "tailor made" to suit the circumstances for the field
it is installed on. Hence, every approach has a different approach
plate which lists some of the variables, mainly the important
variables for pilots.
Hope that clarifies matters.
--
Alan