ILS intercept preferences

[spiderweb]

You've been vectored towards the FAC of an ILS with the instructions to intercept; cleared for the approach. Now, once you've intercepted the localizer, the plate says you may descend to a certain altitude and then intercept the GS.
You under the GS now, so the question is: do you descend and intercept or do you stay at the current altitude and wait for the ball?

 

[kgruber]

Depends. Am I in an icing layer right now? Do the clouds below look turbulent?

Probably wait. Being inherently lazy and wanting less configuration changes.

 

[Palmpilot]

There have been recent discussions touching on this. I believe the conclusion was that you can do it either way, but if there are intermediate stepdowns between where you intercept and the depicted interception point, then you must make sure that you don't descend below those stepdown altitudes. This can be a factor because your true altitude on a stepdown is affected by barometric pressure, whereas on the glideslope it is not.

 

[spiderweb]

Depends. Am I in an icing layer right now? Do the clouds below look turbulent?

Probably wait. Being inherently lazy and wanting less configuration changes.

Good points.

 

[Kiloalphadriver]

There have been recent discussions touching on this. I believe the conclusion was that you can do it either way, but if there are intermediate stepdowns between where you intercept and the depicted interception point, then you must make sure that you don't descend below those stepdown altitudes. This can be a factor because your true altitude on a stepdown is affected by barometric pressure, whereas on the glideslope it is not.

Keep in mind the glide slope has a 10 mile usable distance limitation from the transmitter unless there's a lower flight check limitation.

 

[denverpilot]

Aren't there also antenna lobe issues? You want to intercept glideslopes from below, not above, my radio-head is saying.

 

[bbchien]

Above the ice there are two choices - stay up, dive to get to the intercept altitude at least 2 miles from the fix; and intercept, or drive down the extended GS if you have a good signal.

I tend to do the dive if the IAF is going to be in the clear; I tend to try the extended GS if there's clag at the IAF, e.g, extended stabilized course, but I'll fly it pretty fast- say 120 knots.

 

[Palmpilot]

I flew over LVK above the class D ceiling with the ILS tuned in not too long ago, and I got a chance to actually see false glide slopes for the first time. As I understand it, they are integral multiples of the main glide slope height, which makes them high enough so that it doesn't seem all that likely to mistake them.

 

[denverpilot]

I flew over LVK above the class D ceiling with the ILS tuned in not too long ago, and I got a chance to actually see false glide slopes for the first time. As I understand it, they are integral multiples of the main glide slope height, which makes them high enough so that it doesn't seem all that likely to mistake them.

Yeah, they're quite high the further out you are from the antenna. Degrees above the main lobe. Thus, quite steep if you were to intercept one. I just pointed it out because most procedures have you intercepting from below for a reason...

 

[poadeleted20]

As noted by Ray, you don't want to try intercept the GS outside 10 miles unless there's a GS intercept lightning bolt out there. Absent one of those lightning bolts that far out, follow the published altitudes until you're within 10 miles. In addition, some approaches in crowded airspace have "not above" altitude restrictions (ones with a line over the altitude, not below) on them to keep you below traffic on procedures crossing over the top of the one you're on. Obviously, you have to stay below them. Beyond that, it's up to you. Just make sure you don't go through the GS and then try to reintercept from above, because a) that's where the false signals are, and b) if you're on autopilot, most of them will capture only from below.

 

[flyingron]

And don't forget if you intercept the GS prior to the lightning bolt it's possible in some cases to bust a minimum altitude so watch those...

 

[John Collins]

Ron,

From my experience, most autopilots can capture GS from above or below, although from below is the normal method. Capturing from above may not be automatic and even if it is, it can be a steep ride.

Autopilots that I know that require capture from below are the Century III and IV.

Autopilots that can capture from above or below: Century 31,41, 2000; Stec 60-2, 65, 55, 55X; Bendix King KFC200, KFC150, KFC225; Garmin GFC700.

I am not sure about the KAP140.

 

[Kiloalphadriver]

As noted by Ray, you don't want to try intercept the GS outside 10 miles unless there's a GS intercept lightning bolt out there. Absent one of those lightning bolts that far out, follow the published altitudes until you're within 10 miles. In addition, some approaches in crowded airspace have "not above" altitude restrictions (ones with a line over the altitude, not below) on them to keep you below traffic on procedures crossing over the top of the one you're on. Obviously, you have to stay below them. Beyond that, it's up to you. Just make sure you don't go through the GS and then try to reintercept from above, because a) that's where the false signals are, and b) if you're on autopilot, most of them will capture only from below.

Ron, I'm surprised you didn't say that the reason for the 10 mile GS limit is because the FAA thinks all pilots are idiots and they're too stupid to intercept beyond that. You know, like with your fuel analogy.
:wink2:

 

[gismo]

As noted by Ray, you don't want to try intercept the GS outside 10 miles unless there's a GS intercept lightning bolt out there. Absent one of those lightning bolts that far out, follow the published altitudes until you're within 10 miles. In addition, some approaches in crowded airspace have "not above" altitude restrictions (ones with a line over the altitude, not below) on them to keep you below traffic on procedures crossing over the top of the one you're on. Obviously, you have to stay below them. Beyond that, it's up to you. Just make sure you don't go through the GS and then try to reintercept from above, because a) that's where the false signals are, and b) if you're on autopilot, most of them will capture only from below.

While I agree that capturing from above is generally undesirable, it's still possible while remaining within the 10 nm standard reception range as the FAFs are often about half that distance from the GS transmitter. That said, if my reason for dropping onto a GS was ice avoidance I'd normally just make a steep approach to reach the intercept altitude a couple miles before the FAF then level out at or just above the pre-FAF published altitude briefly and intercept from below. I've found that to be a lot easier than trying to capture from above and I think it's a very bad idea to attempt a topside capture beyond the FAF where the obstruction clearances are diminishing rapidly and the GS pointer is getting rather sensitive.

 

[Palmpilot]

As long as you make sure you're above the published minimum altitude for your current location, what difference does it make if you intercept the glide slope outside the flight checked distance? If the signal is too flaky to provide a stable descent, you can just revert to nonprecision descent techniques until you're within 10 NM, no?

 

[spiderweb]

Aren't there also antenna lobe issues? You want to intercept glideslopes from below, not above, my radio-head is saying.

Right. This was assuming that once you've intercepted the FAC you are below the GS. Otherwise I would definitely descend to the intermediate MDA and wait for the ball.