Simviation Forum - ILS and Distance

Oct 23^rd, 2007 at 12:13pm

Schnyd Offline
Colonel

Posts: 46

I was curios as to why some ILS don't give you a distance readout. For example I frequently fly out of KBLI and the ILS there does not tell me how far away I am. Where as a larger airport such as KSEA does.

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Reply #1 - Oct 23^rd, 2007 at 1:31pm

Brett_Henderson Offline
Colonel
EVERY OUTER MARKER SHOULD
BE AN NDB

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Posts: 3593

Some ILS have DME, some don't.

It's semi-redundant information; because once you're established on an ILS, you already know how far out you are. It's not like a VOR/DME where distance is important data.

I can be comforting to know how far out you are, before becoming established, but 99 times out of 100 ATC has vectored you in and will tell you. Plus, you'll have the approach plate in front of you... AND between outer, middle and inner markers.. AND knowing where the IAF (initial approach fix) is, on top of knowing where the FAF (final approach fix) is... AND your missed approach point ... there's never any doubt how far out you are....

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Reply #2 - Oct 23^rd, 2007 at 3:52pm

Mobius Offline
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Highest Point in the Lightning
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Wisconsin

Posts: 4369

You forgot timing Brett. Cheesy

My DE got on me a bit on my checkride because I didn't start my timer at the FAF in case the glide slope went out of commission. That way you would be able to continue the approach down to the localizer minimums and find the MAP with timing (no DME). Still strange though, there wasn't even timing on the IAP for the localizer approach that I was flying, which I asked the DE about, but he didn't really have an answer. Oh well, I passed... Grin

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Reply #3 - Oct 23^rd, 2007 at 4:25pm

Brett_Henderson Offline
Colonel
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:
Posts: 3593

Oh gosh.. there's so much ambiguity there, that you'll likely get as many answers to those questions, as you will people you ask Roll Eyes

Good point though.. that is indeed another distance reference..

Off the top of my head, most of the ILS with no timing grid ARE for runways where the ILS has a DME distance for the MAP..or some other reference, like the middle marker (or require you to go missed if you lose the glide-slope).

Which approach was it ?

psssst... (just between us... I've always found timed descents near useless. If you show me a pilot who says he has (or would) switch his MAP reference to a timer in the middle of an approach, I'll show you a liar lol... When was the last time you held a perfect 90 or 120 knots for an entire approach ? Throw that whole thing off by a few seconds(knots) and you're guessing anyway (not to mention that you don't know your ground speed to begin with). If he was established on a working glide-slope, you can bet he's going to keep going to descision height, and then go missed, no matter WHAT that timer says.. I've always thought of timed descents along the lines of student-pilot flight planning. You spend all that time adjusting for wind, compass variation , compass deviation to calculate an exact heading THAT NOBODY ALIVE could hold, +/- 5 degrees for an entire leg Roll Eyes

It's all just to drill a frame of mind home.. so that all those factors are in you head and you consider them.

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Reply #4 - Oct 23^rd, 2007 at 4:58pm

Mobius Offline
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Highest Point in the Lightning
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Wisconsin

Posts: 4369

I totally agree Brett. Wink

The approach was the ILS 36 into Madison (http://204.108.4.16/d-tpp/0710/00245ILD36.PDF). ; I agree, I've never flown an approach and held 90 kts exactly, maybe once or twice when there's no wind or turbulence or anything. Whenever I would time an approach I would check the time at the physical MAP and I would always be off, sometimes by almost a minute. It can be dangerous; imagine flying an approach and you depend on timing for the MAP, but you have a hefty tailwind that you try to account for, but don't get exactly right (because you can't, no matter how hard you try), so you're at the MDA still in the clouds, waiting for your time to come up, but you could already be past the MAP, even by a mile or two, which could put you into the side of a mountain. No thanks, I'll rely on my DME and GPS and VOR intersections and whatever else I can before I trust my life with estimating wind and my ability to hold an exact airspeed. Not to mention the situation when the controller says the magic words , "maintain best forward speed", now what? Do you have to go missed because you're timing is off? That's probably not a good idea. It's just one of those things they teach you and you have to know, but never really use. Tongue

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Reply #5 - Oct 23^rd, 2007 at 5:57pm

Nexus Offline
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The greater of two evils...

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I have yet to come across an ILS apporach here which allows us to actually use timing to define the MAPt whenever the GP is inop.
We're simply not authorized to do it.
And if we dont have ILS DME the MM usually acts as MAPt.

I learned a nifty trick when I flew (this only works if you have groundspeed readout)
: Instead of looking the chart for appropiate descent rate whenever your GS changes, simply multiply it by 5 in order to get a fairly precise descent rate.
Only works on standard glidepaths (3.0 degrees, 5,2%gradient)

But in general, we (in my country) cant use timing in order to define the MAPt on an ILS approach incase loss of glidepath signal. We suck.

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Reply #6 - Oct 23^rd, 2007 at 6:12pm

Mobius Offline
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Highest Point in the Lightning
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Wisconsin

Posts: 4369

Nexus wrote on Oct 23^rd, 2007 at 5:57pm:

I have yet to come across an ILS apporach here which allows us to actually use timing to define the MAPt whenever the GP is inop.

That's because an ILS approach becomes a localizer approach when there is no glideslope. No ILS uses timing to define the MAP. DH is the only thing that defines the MAP on an ILS approach. Wink

Sorry, I had to do it because it's been done to me so many times....

One of those stupid terminology things that seem to pop up so often in aviation... Wink

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Reply #7 - Oct 23^rd, 2007 at 6:38pm

Brett_Henderson Offline
Colonel
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:
Posts: 3593

I just looked at that plate... It's a "DME OR RADAR REQUIRED" approach. Meaning, if for some reason ATC just told you, "cleared for the approach".. instead of vectoring you to the ILS intercept, you'd be expected to fly the whole thing, including procedure turn (unless you navigated in via the Janesville VOR radial 328). Your only reference for the IAF/FAF (OZMIX) is 6.0 DME. There is no NDB at the outer marker (see my avatar text ..lol)... and your MAP is DH while on the glideslope, with a 1.1 DME as reference...

SO..... if you can't get radar-vectors to intercept the ILS, and you have no DME, you can't fly this approach. Even if you WANTED to trust a timer; you wouldn't know when to start it... Your DE was wrong to chastise you for not starting the timer Wink

Even if his excuse was that you had a "DME" by having KMSM programmed into the GPS (or even OZMIX), because you were shooting an ILS/LOC-DME... not a GPS approach. If you're using the GPS as a backup (or DME) (who wouldn't), you can SEE when your at the MAP... and that's why there's no timing grid..

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Reply #8 - Oct 24^th, 2007 at 5:11am

Nexus Offline
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The greater of two evils...

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Mobius wrote on Oct 23^rd, 2007 at 6:12pm:

Nexus wrote on Oct 23^rd, 2007 at 5:57pm:

I have yet to come across an ILS apporach here which allows us to actually use timing to define the MAPt whenever the GP is inop.

That's because an ILS approach becomes a localizer approach when there is no glideslope. No ILS uses timing to define the MAP. DH is the only thing that defines the MAP on an ILS approach. Wink

Sorry, I had to do it because it's been done to me so many times....

One of those stupid terminology things that seem to pop up so often in aviation... Wink

Maybe it's a misunderstanding of terms, but during those 3 years that I've been flying, I've never heard it like you described it. I feel a little embarassed Embarrassed

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Reply #9 - Oct 24^th, 2007 at 10:25am

Mobius Offline
Colonel
Highest Point in the Lightning
Storm
Wisconsin

Posts: 4369

Nexus wrote on Oct 24^th, 2007 at 5:11am:

Mobius wrote on Oct 23^rd, 2007 at 6:12pm:

Nexus wrote on Oct 23^rd, 2007 at 5:57pm:

I have yet to come across an ILS apporach here which allows us to actually use timing to define the MAPt whenever the GP is inop.

That's because an ILS approach becomes a localizer approach when there is no glideslope. No ILS uses timing to define the MAP. DH is the only thing that defines the MAP on an ILS approach. Wink

Sorry, I had to do it because it's been done to me so many times....

One of those stupid terminology things that seem to pop up so often in aviation... Wink

Maybe it's a misunderstanding of terms, but during those 3 years that I've been flying, I've never heard it like you described it. I feel a little embarassed Embarrassed

No, no, it's completely a misunderstanding of terms. You were correct in what you said, it's just one of those things where you can call it 10 different things and everyone will know what exactly what you're talking about. Wink

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