Interesting that there are so many opinions here. Once on the localizer, I would go down to 1700 to intercept. As I recall, you can sometimes get false GS's if you intercept too high. Unless I was very familiar with the approach and the terrain, I would not deviate from the plate. I agree with your interpretation of the plate.
ILS GS intercept
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azure
Final Approach
I mentioned that only because it seemed that Spike had not looked at the plan view for segment altitudes, or he would have likely noticed that one.
Yes, agreed. Approach clearance is sufficient to descend.
I'm more interested in Wally's comment though. Why would the controller likely not issue an altitude restriction with the approach clearance at this location?
The false glideslope occurs not from "intercepting too high" but "intercepting from ABOVE the guideslope". You'd need to descend down onto the glideslope rather than flying level into it as mentioned. Leaving NUMBE at 2800 will still put on course to intercept from below.
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I did notice that, but also observed that the plan view has you at or above 2800' until established inbound on the LOC. The No PT feeder simply mandates that you be at 2800' from the VOR. I do see that the LOC segment between NUMBE and RICKE allows 1700', but since the LOC intercept is well outside the GS intercept, I cannot imagine purposely setting up the extra complication of diving and driving to RICKE, when I could just slide down the rail nice, easy and stable. Just me.
Of course, if I have the GS flagged, LOC rules apply and that's how I'd go there.
azure
Final Approach
If flying it for real, I'd probably fly it the same way -- I'm not crazy about dive and drive and even followed advisory glideslopes on LNAV+V approaches until they were removed from the database (most of them, anyway). All I was saying is that it's NOT true that 1700 is only a FAF cross-check altitude, it's legal to descend to it once you're established on the LOC inside NUMBE.
ARFlyer
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I'd descend to 1700'.
You always want to intercept G/S from underneath and NEVER from above. This protects you from intercepting a false glideslope. Also the G/S is only FLIGHT checked from UNDERNEATH at 10NM out.
You always want to intercept G/S from underneath and NEVER from above. This protects you from intercepting a false glideslope. Also the G/S is only FLIGHT checked from UNDERNEATH at 10NM out.
jesse
Touchdown! Greaser!
That, if I were established inbound on the localizer and I'm within 10 miles, I'm going to descend to 1,700 and intercept the glideslope there...
An ILS is best captured from below, so that's what I do.
I personally consider doing otherwise is "lazy" and has negative consequences (risk of false glideslope) which few pilots seem to know about until they have it happen to them one day.
Hint, if you're rate of descent isn't making sense, as in it's way too damn much..you're on a false glideslope.
Granted you have to be coming in pretty high to catch the false glideslope, but plenty of airliners have had it happen to them....
At 3 degrees all is well, at 6 degrees, the glideslope is effectively reversed, and at 9 degrees it works fine but your rate of descent will be through the roof. The joys of the ILS...
To sum it up -- intercept from above and you risk the chance of there being a false glideslope that'll either be backwards, or work fine and your RoD will be incredibly high. Intercept from below, as designed, and you won't have to deal with it.
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Again, staying at 2800 from NUMBE until the needle centers will NOT put you in the case of intercepting the glideslope from above.
RotorAndWing
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While that may work most of the time, intercepting from above is used daily at airports all over the world.
Even Boeing and Airbus have printed guidance on how to configure for an above intercept.
The key is situational awareness and to maintain the approach profiile at the various gates.
Aztec Driver
Line Up and Wait
I fly this approach almost every weeknight. While it is certainly permissible and safe to descend to 1700 after NUMBE, it is much more relaxing and work load reducing to stay level at 2800 (0r even 3000, last cleared altitude) and have only one configuration change, than to descend, level off, then descend again. Not terribly hard, just more work. It can be done either way, but I prefer to be farther away from the hill and towers than the minimum.
aterpster
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Why would he/she if you are flying published routing?
azure
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Agreed, but a quick calculation shows that the GS is up just under 3400 at NUMBE, so intercepting the GS from 2800 out there is definitely NOT capturing from above.
azure
Final Approach
Many times I've been told to maintain an altitude above the charted altitude for a given segment of an approach, or feeder route, as part of the approach clearance. But I almost never know the specific reasons. I've never flown the approach we're discussing, and you said "this location", so I assumed you meant that something about the airspace there made such a restriction unnecessary.
aterpster
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You apparently must fly in congested airspace a lot. That is a last-resort tool for traffic passing under the approach segment. And, that can place you too high and in an unsafe position to continue the approach. I don't see that happening with the example approach.
azure
Final Approach
I used to fly in DTW approach airspace, and yes it was routine there (even more so around KFNT), but I've since moved and most of my flying in the last 9 months or so has been fairly "local" to VT. But even on the RNAV 35 @ KMPV, with transition LEB, I've been told to maintain 5400 until XIMKY, even though the charted altitude for that segment is 5000. That was a few months ago, at night. It didn't seem to be traffic related, though I can't be 100% sure.
Now this is not an LPV approach, there is no "real" GS (and no longer an advisory one). Does that make a difference?
There's nothing wrong from intercepting high with modern avionics ... Most systems have VNAV "ghosted" onto the GS display so you can see the agreement between the VNAV and GS... I would imagine the false glide slope is more probable with conventional/older "steam" avionics systems . I have seen many false localizers and so far not one false GS ( not suggesting they aren't out there) but it has to be pretty rare.
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Palmpilot
Touchdown! Greaser!
That, if I were established inbound on the localizer and I'm within 10 miles, I'm going to descend to 1,700 and intercept the glideslope there...
An ILS is best captured from below, so that's what I do.
I personally consider doing otherwise is "lazy" and has negative consequences (risk of false glideslope) which few pilots seem to know about until they have it happen to them one day.
Hint, if you're rate of descent isn't making sense, as in it's way too damn much..you're on a false glideslope.
Granted you have to be coming in pretty high to catch the false glideslope, but plenty of airliners have had it happen to them....
At 3 degrees all is well, at 6 degrees, the glideslope is effectively reversed, and at 9 degrees it works fine but your rate of descent will be through the roof. The joys of the ILS...
To sum it up -- intercept from above and you risk the chance of there being a false glideslope that'll either be backwards, or work fine and your RoD will be incredibly high. Intercept from below, as designed, and you won't have to deal with it.
Wouldn't the aircraft have to be at twice the normal height to be on the first (reversed-sensing) false glideslope? And wouldn't the required rate of descent be doubled as well?
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Gucci Pilot
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Tell that to some of the ICAO controllers in other countries...
aterpster
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All they are doing is costing themselves a cardinal altitude. Or, since it is Boston Center, perhaps they lose you on radar below 5,400 and want to see you until XIMKY.
Shouldn't.
MAKG1
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I've always been concerned that the GS signal might reflect off something. 9 deg is a truly ridiculous GS angle; even the poster child for outrageous approaches (KASE) has less than 7. To meet that, you would need 1500 FPM descent at 90 knots. That's rather difficult to do in a small spam can without slipping. You could do it at 1000 FPM at 60 at idle and with the flaps out, but the need to do that would be pretty obvious, wouldn't it?
But, suppose there were an ILS on one of those mesa airports we find on occasion. Could it not reflect off the rocks below, giving a false glideslope into terrain? Maybe that's why I can't find any examples (even KTEX has a LOC/DME, not an ILS).
Closer to home, it is not that unusual to find an ILS around mountainous terrain, e.g. KSNS or KNUQ. What prevents the GS from reflecting laterally off the (very) nearby mountains, yielding an arbitrary descent rate? Is the receiver polarized and tuned to reject incorrectly polarized signals? Is it just a really narrow signal? The KNUQ IAF is distressingly close to the summit of Mt. Umunhum. Maybe that's why the GS intercept is 60% down the localizer with step-downs ahead of it. That approach is meant for jets, not spam cans, so the 2000 AGL GS intercept seems pretty low. Even so, the obstacle clearance on that approach seems insufficient. It's supposed to be 2000 feet, right? It's only 1300 at one point, over the "cube" at former Almaden AFS (minimum altitude is 4800, obstacle height just under 3500).
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aterpster
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ILS final is not supposed to be more than 10 miles in length. Flight inspection is very thorough in their annual flight check of an ILS.
Palmpilot
Touchdown! Greaser!
That's one of the reasons why instrument approaches are flight checked periodically, as Aterpster mentioned.
That's my thinking as well. I was taught to use the VSI to maintain a target rate of descent, as a tool to make it easier to stay on the glideslope, so if I saw that it took double the expected descent rate or more, it's "uh oh" time!
azure
Final Approach
That could be it, though at other times they've given me 5000 to XIMKY, and at others they just give me an altitude until LEB.
It's beginning to sound like maybe individual controller preference is a factor here...
Kritchlow
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I am a fan of descending in the GS from my last assigned altitude (observing all published restrictions).
I favor this for a couple reasons, the main reason being wake turbulence avoidance. That may have been mentioned already and I may have missed it.
I favor this for a couple reasons, the main reason being wake turbulence avoidance. That may have been mentioned already and I may have missed it.
RotorDude
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Me too. I never descend unless I have to, and the GS provides a safe, stabilized and comfortable way to descend, all the way to the threshold. Another reason for altitude is potential engine loss in a single engine. You might not make it to the runway from a 3 degree glidepath, but you'll have more time and more options.
MAKG1
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No way you'll make it with a 3 deg glide path.
500 FPM works at 90 knots. In a 172, you'll be doing 65, and still 500 FPM. You'll only make it 70% of the way.
The difference between the GS and step-downs appears to be no more than 1 minute of glide time. Which makes absolutely no difference if you're in IMC. I suppose you might find a slightly better field in VMC, but this doesn't seem like a very big issue. You can guarantee you make the field by increasing your altitude by 30% and descending power off at best glide. Not that I'd recommend that.
The workload issue with an extra level-off is relevant, but I'm just not seeing the emergency issue.
poadeleted20
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If you're arriving at EMI on one of the several airways leading to it, the controller need not issue a "maintain XXXX until established" restriction -- that is required only when joining the approach from off-airways or when the controller has other reasons (e.g., other traffic, TFR, etc) for limiting your descent prior to the IAF.
poadeleted20
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As noted by others before your post, if you fly 2800 to NUMBE, turn onto the localizer, and then fly 2800 to the GS, you will be intercepting from below. You'd have to cross NUMBE at least 3500 or so to be above the GS at that point.
aterpster
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John Collins
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The quick calculation is low by 333 feet, the more precise calculation is 3733, but I still agree with your conclusion, only more so.
aterpster
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General comment: The annual flight inspection of an ILS is considerably more expensive than that for other IAPs.
This is one reasons why ILS approaches are on the FAA "hit list" except at airports used extensively by the airlines.
Another reason is the equipment is very expensive to maintain, and in some cases replace.
The FAA would love to wave a magic wand to get the airlines switched over to WAAS and GLS.
This is one reasons why ILS approaches are on the FAA "hit list" except at airports used extensively by the airlines.
Another reason is the equipment is very expensive to maintain, and in some cases replace.
The FAA would love to wave a magic wand to get the airlines switched over to WAAS and GLS.
aterpster
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The quick calculation is low by 333 feet, the more precise calculation is 3733, but I still agree with your conclusion, only more so.
My precise calculation is 3642. :wink2:
Then again, I haven't calculated the curvature of the earth.
John Collins
En-Route
My precise calculation is 3642. :wink2:
Then again, I haven't calculated the curvature of the earth.
I included the curvature. I used the TERPS calculation which I have coded in a spreadsheet. A short hand estimate for the delta due to the curvature of the earth for an ILS or LPV measured in feet is to multiply the distance in NM times the distance in NM minus 1. That is 10 x 9 = 90 feet as a rough estimate. It works out pretty close 3642+90 = 3732 and the real answer is 3733.
azure
Final Approach
Yes, my calculation was definitely in error. When I rechecked it I realized that I had used 5280 feet/mile instead of 6076.The quick calculation is low by 333 feet, the more precise calculation is 3733, but I still agree with your conclusion, only more so.
But even redoing it with the correct conversion factor, I don't get 3733, but more like 3640. That's using a 3.00 degree GS (as stated on the chart) that crosses RICKE at 1700, and 6.1 nm horizontal distance from NUMBE to RICKE.
azure
Final Approach
Just saw your followup post -- okay, I didn't include the curvature either. That explains it...
aterpster
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A bit of generic follow-up using the KFDK ILS 23 as a case in point. I have a trainer for a state-of-art FMS that is in a Part 25 business jet. This is an airplane with both WAAS and IFR certified Baro VNAV. In other words typical of a Part 25 airplane FMS, whether a Gulfstream or a 737-800 (well, the 737 doesn't have WAAS). My point is that Jeppesen and the other database vendors code an ILS like FDK the same for any "full up" FMS. I ran the trainer down V-166 from BELAY onto the EMI initial approach segment.
I started at 12,000, then let VNAV do its thing (a 3 degree descent profile). The FMS leveled at 2,800 passing NUMBE where it auto-sequenced from LNAV to LOC. The ILS GS was well above at that point. So, the FMS remained at 2,800 until GS intercept at about 7.6 miles from the runway. No dive-and-drive to 1,700 to intercept the GS at RICKE.
FWIW....
I started at 12,000, then let VNAV do its thing (a 3 degree descent profile). The FMS leveled at 2,800 passing NUMBE where it auto-sequenced from LNAV to LOC. The ILS GS was well above at that point. So, the FMS remained at 2,800 until GS intercept at about 7.6 miles from the runway. No dive-and-drive to 1,700 to intercept the GS at RICKE.
FWIW....
Checkout_my_Six
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Oh, how nice....
John Collins
En-Route
A bit of generic follow-up using the KFDK ILS 23 as a case in point. I have a trainer for a state-of-art FMS that is in a Part 25 business jet. This is an airplane with both WAAS and IFR certified Baro VNAV. In other words typical of a Part 25 airplane FMS, whether a Gulfstream or a 737-800 (well, the 737 doesn't have WAAS). My point is that Jeppesen and the other database vendors code an ILS like FDK the same for any "full up" FMS. I ran the trainer down V-166 from BELAY onto the EMI initial approach segment.
I started at 12,000, then let VNAV do its thing (a 3 degree descent profile). The FMS leveled at 2,800 passing NUMBE where it auto-sequenced from LNAV to LOC. The ILS GS was well above at that point. So, the FMS remained at 2,800 until GS intercept at about 7.6 miles from the runway. No dive-and-drive to 1,700 to intercept the GS at RICKE.
FWIW....
The G1000/GFC700 on other air frames with VNV will also do the same, Baro VNAV to the IF, then LPV or ILS intercept at the IF altitude.