ILS Approach

[RJET]

There are different ways different companies and airlines excute ILS Approaches.
SOme Excute the stepdows and intetcept the Glide Slope at GSIA.
Other stay at higher altitude and intercept it from there.
I looked up the definition of Glide slope intercept altitude and it states that is the minimum altitude at which you should intercept the glide slope. It does not state that it is the altitude to intercept the glide slope.
NOw I know that it some cases there is a mandatory altitude. In those case, it is self-explainatory.

I was just curious what guys think about it.

 

[A Squared]

Strictly speaking, you shouldn't be using the Glide Slope for primary vertical navigation above the Glide Slope Intercept Altitude. The Glide Slope is only fully flight checked to GSIA, it might be checked further in some cases, but you can only be sure that it has been checked to GSIA Out further, it is flight checked below the glideslope for proper fly-up indication, or in other words they check that you don't get an above GS indication when you are below the GS.

So, you're out there on the localizer and you have a stepdown coming up, and your gs needle is coming alive, soooooo.......do you dive for the next altitude then level off again? or do you follow the glideslope? Again, you're not supposed to be using the GS for *primary* vertical guidance above GSIA, but what's wrong with using it as a "guide" for your descent that you're going to make anyway? Personally I don't see any compelling practical or regulatory reason not to use it as a guide for your descent.

There are a couple of objections which come up. One it the fear that you might get on the wrong lobe of the glideslope. Well, let's think about it. An ILS would not have a GS lobe below the correct GS, It would be a disaster, planes would be hitting the ground at the inner marker one after the other. Additionally, that's why they flight check for fly-up indication, to make sure that there's no false GS below the correct GS. There are GS lobes *above* the correct GS, the first one is has a 6 degree slope and has reverse GS needle sensing, you should be able to tell if you're tracking that one. The next one is at 9 degrees and has corect sensing. If your normal final approach speed requires a 700 ft/minute descent to follow the 3 degree GS, the 9 degree GS will require better tan a 2000 ft/min descent to follow it. That should be your tip off.

The false GS shouldn't be an issue unless you're really, really unaware of your descent rate.

The other objection is that the GS might take you below an intermediate stepdown altitude. This is a little more credible concern. I understand that there are ILS approaches that are like this in the US. I don't know where, but I was told by a flightcheck guy. I assume he knows what he's talking about.

Well, here's the deal, like I said, don't use the GS as your *primary* means of vertical navigation. Presumably, you have navigation equipment sufficient to identify the stepdown fixes and determine that you are above minimum altitudes at all times, right? If not, you shouldn't be flying that approach. So, you use that equipment (DME, Altimeter, etc) as your primary guidance to ensure that you're above minimum altitudes, and you use the GS as a guide to help you achieve a nice smooth descent.

Disclaimers:

1) There are ILSs with *mandatory* stepdown altitudes. IAH is one. Like RJET said, those are self explanatory.

2) If you're doing an oral or a checkride, the best answer is that you intercept the GS at GSIA. If the examiner is enlightened, he can then explain to you the same thing I just explained. If he is unenlightened, that is the only correct answer.

3) You are not inside the final approach fix untill you have passed the GSIA on the GS, intercepting the GS 10 miles out does not constitute the FAF

4) Your company policy may vary.



regards

 

[Checks]

You can intercept the gs above the gsia but you must double check the altitude when crossing the om/FAF.
Thats why the GlideSlope Altitude at Outer Marker/FAF is published on the approach plate.
For example, you intercept the glideslope at 10,000ft and at the OM/FAF the altitude is reading 2900ft while the approach plate says you should be at 2083ft. This means you are on a false gs. If on the correct gs the altitude should read pretty close to 2083ft at the OM/FAF.

 

[The FNG]

Could mean the altimeter setting is off...

 

[A Squared]

Could mean the altimeter setting is off...

Much more likely. Even if you had intercepted the 6 degree false glideslope and were following it down not noticing that the GS needle was acting backward, and you were descending at double your usual rate, you'd cross the outer marker at about 3200 ft above the airport instead of the usual 1500-1600 ft HAA. If you were on the lowest false GS that gives correct sensing, you'd be crossing the OM at about 4800 ft HAA. If you're crossing the OM 800 ft high with correct GS needle sensing, a mis-set altimeter is a lot more likely

BTW Checks, I've alweays wondered what your little quote about the bottles means.


regards

 

[RJET]

Thanks for the info.

 

[Checks]

Asquared, you must not be a Cleveland Browns fan. That is what our former owner said after fans belted the officials with plastic beer bottles.

I probably should change it. It is kinda like my pic of Michael Jackson and his monkey, it really isnt funny

 

[embdrvr]

The 3:1 rule is a useful one to remember when it comes to making sure you are not following a false GS signal. 3 degress is 1000'/3nm. So if your ILS DME reads 9 miles you should be 3000 above TDZE. IF the ILS lacks DME look to see if a VOR is colocated on the field and sueuseyou could use your second nav for that also. Even a VOR that is not colocated can be helpful if you can keep the offset straight in your mind. Keep terrain in mind as well. Some glideslopes are greater than 3 degrees. As a general rule I always look at the MSA for the sector I'm operating in if it's an unfamilar airport.
Some guys I've flown with always seem to be in a hurry to descend. Going into busy airports that kind of flying can put you in risk of wake turb. Don't get in to that kind of trap. If you want to stay high though don't let yourself get fast. The most efficient way to get downhill is gear down and props set to max rpm. Many of us prefer to fly a dot high at all times. If you're doing a visual into a large hub airport you may also want to stay a dot offset laterally as well on the upwind side of the LOC course assuming you are not conflicting with traffic for a parallel runway.
I know, I can be a pain in the butt to fly with but these techniques can keep you upright.