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

http://www.avgen.co.uk/DME.jpg

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.

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

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.

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

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

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

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

add an l to intelligent.

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?

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.

Thank you Chris Scott for a most informative answer.

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

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.

And I get "intellligent". :ugh:

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

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. ;)