ILS Sensitivity

[DaveReidUK]

Hmmm.. I always thought the DME was zero ranged to touchdown.

That may well have been the case once, but it certainly isn't nowadays.

[Chris Scott]

Hi DaveReidUK,

I see you are right: the LHR 27L ILS DME currently (well, 2010) reads zero at the threshold, not the touchdown point. I wonder why that should be so, and whether it was ever as HEATHROW DIRECTOR and I think we recall.

[eglnyt]

With a display resolution of 0.1 Nm and an accuracy of 0.1 Nm you'd be hard pushed to tell the difference.

[bubbers44]

Things have changed in the last 10 years for the better. We always had to subract about 2 miles from Localizer DME to calculate touch down DME. This is a great improvement I wasn't aware of.

[flyboyike]

Why 2 miles? What if the runway is less than 2 miles?

[bubbers44]

Then you subtract runway length, what ever that may be.

[flyboyike]

Good thing too, because if you subtract 2 miles from, say, Rwy1 at DCA, you'll be swimming.

[bubbers44]

true, but most pilots are inteligent enough to figure it out.

[bubbers44]

add an l to intelligent.

[flyboyike]

Hey, if we were good at math, would we do this for this kind of money? It's just that until today I never heard the 2-mile figure before. Besides, how would you account for displaced thresholds and such?

[bubbers44]

LAX, MIA and DFW usually have two miles of runway and when the LOC DME was at the far end you subtracted distance to figure touchdown point.

[dcoded]

Thank you Chris Scott for a most informative answer.

My question has been answered and MODs can now lock it if they so desire!

[bubbers44]

Using LOC DME must be different now but even when you had to do the math it gave you a heads up if the glide slope didn't match the 300 ft per mile profile. Nobody uses this as their primary descent profile, it is a backup to verify the ILS or what ever descent profile they are using.

[Ozlander1]

Originally Posted by bubbers44

add an l to intelligent.

And I get "intellligent".

[bubbers44]

Age does screw up your vision doesn't it. I could have sworn I wrote intelligent. I'm only 69.

[bubbers44]

Looking back I did leave out the L so I fixed it by adding another. I hate it when my dead english teacher keeps waking me up in my dreams.

[Ozlander1]

Originally Posted by bubbers44

Looking back I did leave out the L so I fixed it by adding another. I hate it when my dead english teacher keeps waking me up in my dreams.

I still remember mine. And that was a loooong time ago.