ILS approaches and stepdowns

[Ben Dover]

This topic has been discussed before, but I still have two questions:

Here's the previous threads if you're interested:
http://forums.flightinfo.com/showthread.php?t=74454&highlight=glideslope+intercept+stepdown

http://forums.flightinfo.com/showthread.php?t=19298&highlight=glide+slope+intercept+stepdown

OK, so given the scenario where you are cleared to "maintain xxx thousand until established, cleared for the ILS XYZ approach." My questions are:

1. If you elect not to descend to charted stepdowns and simply intercept the glideslope and follow it down. And, assuming it's the true glideslope and not a false signal, while ON the glideslope you descend below one of these charted stepdowns. Are you in violation of any FARs, or the intent of the approach prodecure? (Granted an unlikely scenario, but the questions stands)

2. If the service volume for the localizer is 18NM as we learned from the threads posted above, wouldn't it be illegal for me to accept, or ATC to issue a clearance that provides an intercept outside 18NM? (20-30 mile intercepts seems fairly common from my experience)

 

[flyboyike]

1. I don't see how it would be, especially since charted stepdowns correspond with the glideslope at their respective distance, so you'd never be below them, if you stay on the G/S.

2. I don't think it would be, though I don't think they would issue such a clearance. They might put you at the minimum intercept altitude way far out, but I don't think they would have you intercept at MIA+2000' 30nm out or whatever.

Keep in mind also that even if you are on the GS, you are not considered past the FAF until you are on the G/S and at or below the MIA. Therefore, if the weather were to go sour while you were on the G/S but way above the MIA, you'd have to go missed.

 

[T206driver]

I don't mean to sound stupid but do you mean that MIA = FAF were the lighting bolt is on the chart?

 

[ChiFlyer]

I don't mean to sound stupid but do you mean that MIA = FAF were the lighting bolt is on the chart?

Yes, I believe so.

Check out the instrument procedures handbook, page 5-38.

The step down fixes that you are referring to are only for the localizer approaches. Usually there's a note on the approach plate telling you this. So if you lose your glideslope you would not descend to the MDA, stop at the step down fix and then after passing it descend to the MDA.

 

[A Squared]

1. I don't see how it would be, especially since charted stepdowns correspond with the glideslope at their respective distance, so you'd never be below them, if you stay on the G/S.

Theoretically, yes. If the ILS approach is designed in accordance with the current TERPS, any stepdown in the intermediate segment would be below the glideslope. That being said, there is anecdotal evidence that some approaches may have been designed with stepdowns that take you above the glide. I haven't ever seen one that clearly does, but be aware that the TERPS requirements can be modified in special circumstances, there just might be a special case with a stepdown above the glideslope. Regardless, be very aware of your position along the localizer and your altitude relativce to any intermediate stepdown fixes. Going below an intermediate altitude just might kill you. They don't add stepdowns to approaches just for entertainment. there's a reason, and often that reason is much tougher than you or your airplane.

The step down fixes that you are referring to are only for the localizer approaches.

This needs some clarification. For the stepdowns shown inside the FAF, your are correct, those apply to the LOC only procedure, not the ILS. The ones outside the FAF though, you do want to heed; those apply equally to the ILS or the LOC only approach.

 

[T206driver]

Thanks for the replies. I learned the hard way at an interview the difference between the maltese cross and the lighting bolt. OOPS. Well, there staring pay scale was in the toilet anyway. I do understand that step down fixes outside the FAF are mandatory thanks to something else that is very hard. The fixes inside the FAF are for when you don't have the glideslope. Personally, on a localizer only or even a VOR, why drop like a rock to the MDA? Just set your VSI at a decent rate to where you arrieve at the MAP over the end of runway. Like 120kias you need about 550 fpm decent. This makes for a smoother ride.

 

[tathepilot]

1. Thanks for the replies. I learned the hard way at an interview the difference between the maltese cross and the lighting bolt.

2.Personally, on a localizer only or even a VOR, why drop like a rock to the MDA? Just set your VSI at a decent rate to where you arrieve at the MAP over the end of runway. Like 120kias you need about 550 fpm decent. This makes for a smoother ride.

1. really?? wow..

2. In my opinion, its better to be at the mda 'before' you reach the map.. it makes for a better transition if you have to go missed

 

[time builder]

Or.... you could arrive at the MDA at the VDP, if you're into the whole "smooth ride" thing.

 

[Ben Dover]

why drop like a rock to the MDA? Just set your VSI at a decent rate to where you arrieve at the MAP over the end of runway. Like 120kias you need about 550 fpm decent. This makes for a smoother ride.

Because it's more likely you will meet the requirements of Part 91.175 (c) and be able to land, rather than go missed.

OK, back to the original topic. Anyone want to explain why it's legal to intercept and track the localizer beyond 18 nm miles out as issued in many an approach clearance?

 

[flyboyike]

I don't mean to sound stupid but do you mean that MIA = FAF were the lighting bolt is on the chart?

Yes, that would be it.

 

[FL420]

OK, back to the original topic. Anyone want to explain why it's legal to intercept and track the localizer beyond 18 nm miles out as issued in many an approach clearance?

I can't speak strictly to the "legality" of it but be aware that some localizers are flight-checked and certified to have a service volume beyond 18 miles. I don't know offhand how a pilot can readily find that information. Perhaps from the navaid section of airnav.com?

 

[FL420]

Personally, on a localizer only or even a VOR, why drop like a rock to the MDA? Just set your VSI at a decent rate to where you arrieve at the MAP over the end of runway. Like 120kias you need about 550 fpm decent. This makes for a smoother ride.

Not a good idea, especially in higher performance aircraft or in aircraft operated under 121 or 135. If you don't get to the MDA before you get to the MAP on a NP IAP, you will likely not be in a position to land in the designated touchdown zone or the first third of the runway without performing an unstabilized approach with an excessive rate of descent.

 

[A Squared]

Not a good idea, especially in higher performance aircraft or in aircraft operated under 121 or 135. If you don't get to the MDA before you get to the MAP on a NP IAP, you will likely not be in a position to land in the designated touchdown zone or the first third of the runway without performing an unstabilized approach with an excessive rate of descent.

Yep, what he said.

If you did this in part 135 or 121 ops, you would never land legally, or alternately, every landing would be a violation. Remember that under those parts, you must use a normal descent and touch down in the touchdown zone. Now, I'll admit to being a little hazy on the exact definition of "touchdown zone" but I seem to recall that it's the first 1/3 of the runway. If we go with that definition, and add the fact that a "normal descent" is about 3 degrees, it would take 4470 ft to descend from 250 feet above the threshold (Standard location of the MAP on a straight in approach is directy over the threshold) at a "normal descent", so you could never land legally on a runway shorter than 14,310 ft using this technique. Next time you're out doing some discretionary flying, level off at 250 feet above the touchdown, and fly directly over the threshold of the runway. Notice how high you are and how steeply you'd have to descend to get down from this altitude to the runway.

Anyone got a reference for the definition of "touchdown zone" ???

 

[SSDD]

The first 3000' of the runway, or half the length if less than 6000'.

 

[A Squared]

The first 3000' of the runway, or half the length if less than 6000'.

Thanks, got a source for that?

 

[VNugget]

P/CG:

TOUCHDOWN ZONE - The first 3,000 feet of the runway beginning at the threshold. The area is used for determination of Touchdown Zone Elevation in the development of straight-in landing minimums for instrument approaches.

 

[Fedora]

Outside the FAF, you need to heed the stepdown altitudes. Riding the GS can take you below these and result in a violation. (has happened on the Paradise arival into LAX.) The step downs keep you clear of both VFR coridor and crossing trafic to the north side runways. Riding one dot high keeps you legal on the step down altitudes until reaching the FAF (tail of the feather)

 

[nosehair]

I've seen this question a lot, but cannot recall if such an approach exists. Step down minimum altitudes which are higher than the glidslope.
Anybody got a copy of one?

 

[Seadogrun]

I've seen this question a lot, but cannot recall if such an approach exists. Step down minimum altitudes which are higher than the glidslope.
Anybody got a copy of one?

I'm not sure there should be one.

It would take some math to figure out if the stepdowns are actually below the GS as the profile views are not in scale.

It would be interesting if someone could find one.

 

[Say Again Over]

I don't have the plate handy, but I heard this is true at KLAS also, whereby the G/S goes below the floor. Just a rumor.