DN ILS - TCH 75' - Why?
 

Little thing here that bugs me- Why does the Darwin 29 ILS show a Threshold Crossing Height of 75' when every other ILS in Oz has a TCH of 50'?

I've heard the guesses (obstacle clearance, rwy slope (?), aliens, etc) Anybody shed some light on the real story?

I promise to get a life after this:)

Theres only two possible reasons i can think of :

1. the ILS glide slope is greater than 3 degrees ( 4.5 ), or

2. the glide slope intercepts the ground at 1500 ft instead of 1000 ft ( perhaps the ILS was installed then later the runway length increased )

Lefty,

Answer 2 seems correct.

Checking the usable runway length on the Jepp T/O Minimums, the diff at DRW is 429m Vs an average of 300-330 m for most others. which implies the gp intercept of the rw is further in.

On another thread Renurpp mentioned the 29 T-VASIS doesn't align with the 1000' touch down markers so this seems to validate this assumption (Yes I know...never assume anything :O )

Maybe something to do with the Arrester cable position ???

429 metres equates to around 1420 ft, so No.2 it is.

As we are all aware the 1000ft markers are where you will find a majority of skid marks and most bits of an aeroplane that fall off.

It's for A/C with long wheel base (such as 747s and B52s) which use a 1500ft aim point to avoid dragging the wheels short of the threshold. . .

WG

From the ERSA :
- ILS at Darwin is the usual 3 degree's.
- ILS/ Vasis Glide path both intercept the runway at 1448'.
- RWY 29 hookcable is 1525' (465m) from threshold RWY 29.

Many years ago whilst in a jump seat of a Bae146, I noticed the crew brief the approach and landing and to "intentionally" fly 2 dots low because of the 75' threshold crossing height.

'Tis a mystery indeed.

Originally I thought that it was for the arrestor cable, but if that is positioned around 1500', then that don't make no sense.

WG,

If that were the case, why don't all runways that accept aircraft with long wheel bases have same?? (See other thread re pilot eye height)

ITCZ
 

Next time you land 29 take note of where th vasis, (same touch down point as an ILS) are in relation to the touchdown markers and you will find your answer.

I believe to do with the area on the end of the runway that is used for fighters to hold ready for take off, not sure of the correct term, (ORP), it moves the touchdown point for the ILS/VASIS further down the runway improving the obstacle clearance with the above aircraft I believe.

ARP
 

REN

I had the same thought, similar to the Runway end lights. But I can't from memory recall if it is the same for other ILS equiped RAAF/Joint aerodromes. (TVL, WLM)

Maybe one of our RAAF ATC guys can give this one a go, that is if any are still here in Aust and not in Bagdad.
ARP = Aircraft Rediness Platform.


I don't know the answer, but don't be too quick to discount the terrain idea. A quick look at the threshold shows that the Runway slopes up to the west (29) and the terrain to the east slopes down to the threshold. Plus the light poles on AmyJohnsotone Ave may not provide the required obst free gradiant for ILS. Just a thought.

KISS:

I'd say that's because v.few aircraft require a 1500ft aimpoint, and to have a 1500ft vasis would work against the vast majority of aircraft normally using a 1000ft aimpoint.

Furthermore, most of the airports which Qantas flies the 400 into have a 1000ft vasis. Obviously the 1000ft vasis is acceptable to 737 sized aircraft which are still the greatest users of these same airports. So a normal 1500ft aim point requires 2 dots fly down on the vasis. :{ :{

WG.

2 dots low doesnt quite suffice as the touch down point for two dots low in this case is still 1500ft.

ORP = Operational Readiness Platform
ARP = Aerodrome Reference Point :E

No doubt the reason is lost in the mists of time, locked away in the mind of some long retired engineer.
I've been flying 2 dots low ever since I first went there yonks ago.
Doesn't do much for safety when breaking visual in rain at 200ft, trying not to "duck under" and land short, but still trying to hit the 1000ft markers on a wet runway.

The preliminary report on the recent B737 excursion at DN in interesting reading on the Departments website.

Investigators are currently asking 737 pilots using Darwin recently to provide information on their experiences on a no blame basis. Must be something there they are trying to come to terms with, and looking for some honest opinions.

OOPS
 

Thanks Jarse

Lisdexia of the A and O's

Personally cannot see any reason to dive for the 1000' markers. Sounds like bad practice to me, especially in poor weather.

I reckon the markers should simply be repainted in alignment with the VASIS/ILS touchdown point.

Doesn't say anywhere that I can find, BUT the LDA must surely take into account this situation.

I would be supprised if Darwin was at all landing distance limited for any of the regular civil aircraft that use it. B767 being the heaviest!

I agree RENURRP. It is not unknown to see the piano keys and touchdown markings displaced "into" a runway where an obstacle exists for a landing approach to that runway.

I don't buy the fighters on the ORP part though. Runway obstacle gradients are in the range of 1.2% to 2.0%, which meant that moving the aim point in by 150 odd metres only creates room for an aeroplane with a tail not greater than 3m tall. An F-18 stands 4.6m at the tail.

Something else is at work here.

Maybe I am being a bit dim, but lets put it this way. Say we are making a no flap approach into 29 and we are confirming from the Aircraft Flight Manual that we have adequate LDA for a full factors flap zero landing. Do we (1) fly the approach one dot low to confirm a TCH of 50' and put the wheels on the 1000' markers, or (2) is the LDA calculated from the wheels going on at the aim point defined by the intersection of the VASIS/ILS path and the runway?

My thought is that we fly through the 50' gate to ensure that we are using the same datum that the performance guys used to write the requirements.

Fully aware thought that 11/29 is one of the longest runways in the country, so perhaps it is academic. I'd still like to know the answer though :ok:

RENURRP - didn't get to fly the 29 the other night, circled for 11 for practice:8

ITCZ
 

ORP affecting slope guidance?? you may well be correct. I do recall it being discussed at on of the RAPAC meetings in my early days up here. RAAF have different ways of doing things you know.

Re where to touch down flapess, our SOP's say the 1st 10% of no factors and full length with all factors, whatever that means, so I guess 50' becomes insignificant in any case.