Mins before FAF

[juanchie]

Say you are shooting an ILS under part 121. Before the FAF, the weather reported has a adequate visibilty, but a ceiling below the mins. Are you still legal to shoot the approach?

 

[PFD1]

Depends on ops specs for a specific airline. For the most part airlines are vis only. If RVR is reported it is controlling.

 

[PFD1]

so yes to answer your question...you would be legal to shoot the approach

 

[juanchie]

I appreciate it

 

[CA1900]

Note that you have two blocks at the bottom of a Jepp plate: There's the DA or MDA on top, and the various minima below. Note that, for a given approach, the minimum is a visibility only. And that's the only controlling factor of whether the approach is legal to shoot under 121. The report can be Indefinite 100 all the way down, but you can proceed past onto the final approach segment and land as long as the visibility or RVR is at or above the minimum on the plate.


One other important misconception that I saw constantly, both on the line and in the simulator: The Maltese cross is not the final approach fix on a precision approach -- that's the FAF for the localizer-only ("GS Out") approach, and is only advisory on a precision approach to verify the glideslope is accurate. (I've also frequently heard that published altitude over the Maltese cross referred to as the "GSIA" -- again, not sure where that came from, but it's incorrect.)

The intersection of the published glideslope intercept altitude and the glideslope is where the final approach segment begins on a precision approach. If you're already on your way down the glideslope from the published GSIA, but outside the maltese cross, you are on the final approach segment, and thus do not have to abandon the approach if the visibility dips below the approach minimum at that point.

Hope that helps.

 

[juanchie]

Wow, that really clears up my question. One other side question, i you join the loc above the gsia and cleared for the approach, do u descend to the gsia before you intercept the gs (kinda like a step down fix) or just wait for the glideslope?

 

[CA1900]

Wow, that really clears up my question. One other side question, i you join the loc above the gsia and cleared for the approach, do u descend to the gsia before you intercept the gs (kinda like a step down fix) or just wait for the glideslope?

Generally, you can do it either way. If it's within a few hundred feet and ATC doesn't require otherwise, I'll usually just intercept the glideslope from where I am in the interest of a smoother ride.

If you do that, though, make sure you take note of the altitudes on the profile view as you go down, for a couple of reasons. First, you want to ensure you stay above the minimum altitudes along the way -- I've seen approaches where staying on the glideslope way out will put you below some of the stepdowns. Be careful with that. Also, you need to ensure that there aren't any mandatory altitudes (as opposed to minimum altitudes) along the way -- they'll have the word "Mandatory" by the altitude on Jepp charts, or have lines both above and below the altitude on the NACO charts.

 

[PFD1]

Technically if you are established on the approach and within the specified distance on the approach plate you may descend to the published altitude for that segment if you are cleared for the approach. However, in most cases, ATC will say, "Maintain ____Altitude till established on the localizer, cleared ILS rwy whatever." They usually give you the intercept altitude at that point or whatever altitude they would like you to intercept at. If you look at different approach charts, some approaches have different intercept altitudes when assigned by ATC. At that point you would maintain your altitude wait for the glide slope intercept and proceed.

 

[juanchie]

Thanks, you two have been very helpful

 

[A Squared]

-- I've seen approaches where staying on the glideslope way out will put you below some of the stepdowns.

I'm skeptical that this is true. The Maximum descent gradient for an intermediate segment is 300 ft/nm, while a glideslope slopes at 3 degrees which is 318 ft/NM. If both of these conditions are met, it's impossible to a point in the intermediate segment to be above the GS. I suppose that it's possible that a lower than standard GS might be combined with a maximum descent step-down on the intermediate segment, but it wouldn't make sense to combine a steep intermediate with a shallow GS. Do you recall the specific approaches which you think have stepdowns above the GS? I'd be interested to take a look at them.

All that aside, your comment to keep track of where you are vs intermediate altitudes while following the GS is good advice.