767 miles to run
 

Hey Wastegate, sitting in YPPH doing the daily gring. I too have heard a heap of different formulas in calculating the track miles req'd. The one I use is a simplified one of yours.

300 kts : 3x Ht +10
250 kts : 3x Ht +7
Ref +80 : 3x Ht +5

Saves on that first step....if you're like me and need every second and cubic millimeter of brain capacity! Catch you on the airwaves. (spoke to devilboy today)

BTW...never thought about the whole 'curviture of the earth' thing on an ILS. Interesting.

4 dogs ... agreed. But at the stage where I'm planning my descent, my GS is a bit different to what it'll be during descent. As a result, I've never bothered to include GS in my scan until after passing the IAF/IAP.

Operated with a particular F/O a few years ago who started to "plan" his descent about TOC. Twisted his Breitling bezel, spun the wiz wheel, punched the calculator buttons...and still got it wrong.
Hopeless. :rolleyes:

4Dogs

I suppose it's what you are used to, but I don't see how with a G/S of 160kts multiplying 160 x 5 can be easier than 16 (just ignore the last digit of the GS) divided by 2 to give you your answer of 8(hundred)fpm ROD?

OZ

Good point, I too am not really interested in groundspeed until I am about to capture the glideslope, until then it's airspeed that I need to be concerned with as that's what I have to lose to take flap etc at the right point.

PP

Young Paul wrote:

Full marks for thinking about the accuracy but the true figure is almost slap bang between the two:

For a 3 degree glidepath, the height is 6080ft * sin (3 degrees) per nautical mile from the touchdown point, which is 318 ft/nm.

As you say, I don't think the difference between any of these is likely to be of operational significance.

GDay SDB.

I must have missed you in YPPH by only a day. Left home on Thurs. night for Auckland, and ended up in Perth. Go figure . . . :confused: :confused:

WG.

Pete ... humble apologies for the delayed response... was travelling to the UK - and then there was booze... and wimmen. Ya know how THAT goes! ;) Anyway, yes, even tho my machine ain't as slippery as yours, the same considerations apply coz it takes a while to slow a B200 from 220 KIAS to 200 for approach flap, then back to 180 for the gear. I like to have meself sorted at GP intercept.

bookworm ... I appreciate your comments but the fact is that, things are changing throughout the approach. In fact, they're changing right from the moment you leave crz alt. The airspeed is changing a bit, the TAS is changing a lot. It is for this reason that a "close approximation" works so well. But I'm sure you already knew that.

Oz,

I can't find my old Blue books since she who must be obeyed tried to clean up my study, so I need a procedure designer's help:

Noting that there are some differences in ILS installations, what sort of vertical distance from the glide path would one expect to see at half scale deflection up and down at say 4 miles from touch down (about where OMs are sited, depending on terrain etc) and 10 miles (about the start of the procedure)?

4dogs ... I'm several thousand miles away from all my references sources at present. Rather than "wing" an answer that could well be wrong, I'll have to bat this one to someone elsewho may be tuned in here.

I'm not due back at work until 28 August, but sure am having fun in the meantime! :) Sorry I can't be more help right now.

4dogs
Whatever formula you get, remember the real world is not flat [increased GP altitude by 90 ft at 10nm], and there are true altitude considerations. Otherwise unexplained crossing altitude errors may be more clearly understood if you do more complete maths.

Here is my rule of thumb:

Feed the FMC with wind data and any altitude/speed restrictions, get the newspaper back out and go down when it tells you to.

altitude x 3 for the TOD
DME x 3 is the formula i use to check if i am on glide when all else fails