121 instrument approach question

aaronwexler

Well-Known Member
So I was looking over some questions people had posted other places and I came across a couple I can not find an answer to.

1) The airplane before you reports 1/4 miles visibility. Is this in nautical or statute miles and if the approach requires 1/2 miles vis can you shoot the approach.

2) Can you join a DME arc inside of the IAF? In other words do you have to start the arc at the IAF or can you start it in the middle of the arc assuming you can provide your own terrain avoidance, or are getting vectors.

Keep in mind I am asking as it pertains to part 121 not part 91

Thoughts?
 

SpiraMirabilis

Possible Subversive
1. Visibility should be reported in statute miles, and "probably not." If you've started the final approach segment of the approach prior to "noticing" the new AWOS with the 1/4 mile vis then you may continue it and use your judgement as to what the actual "flight visibility" (rather than reported visibility) and if you see the runway you can land. Freight Dawg Tip: When it's close to minimums, you check the AWOS only once and then get on the mic and block ATC if they try to inform you of the new ATIS (half-joking here) ;)

2. No, the DME arcs are in many cases designed to keep you around things. Going into the middle of one may defeat the purpose depending on where you intercept it from. ATC may be able to give you vectors to intercept the arc in the middle, but if you're going to an uncontrolled field you have to start from the IAP. I have asked for vectors for an arc during a practice approach coming back home into ROW one day because it takes like 10 minutes to do the full arc and I got it.
 

z987k

Well-Known Member
So I was looking over some questions people had posted other places and I came across a couple I can not find an answer to.

1) The airplane before you reports 1/4 miles visibility. Is this in nautical or statute miles and if the approach requires 1/2 miles vis can you shoot the approach.

2) Can you join a DME arc inside of the IAF? In other words do you have to start the arc at the IAF or can you start it in the middle of the arc assuming you can provide your own terrain avoidance, or are getting vectors.

Keep in mind I am asking as it pertains to part 121 not part 91

Thoughts?
What the airplane in front of you reports means nothing. RVR (or the vis given by the asos/atis)is controlling. Also if I actually heard someone say on the radio that vis was 1/4, when I landed, I'd have a talk with them about how retarded they are. Oh yeah it's 1/4? You can really tell between 1/4 and 1/2. Uh huh.
 

zmiller4

Well-Known Member
So I was looking over some questions people had posted other places and I came across a couple I can not find an answer to.

1) The airplane before you reports 1/4 miles visibility. Is this in nautical or statute miles and if the approach requires 1/2 miles vis can you shoot the approach.

Should be in sm, but who knows what they mean. I'd say you could proceed with the approach if you haven't received an official weather report below mins, but I'd probably think again if there was a fed in the jumpseat.


2) Can you join a DME arc inside of the IAF? In other words do you have to start the arc at the IAF or can you start it in the middle of the arc assuming you can provide your own terrain avoidance, or are getting vectors.
You can join an arc inside an IAF if you're getting vectored. If you're in a non-radar environment, you have to fly it from an IAF.
 

BobDDuck

Island Bus Driver
You can join an arc inside an IAF if you're getting vectored. If you're in a non-radar environment, you have to fly it from an IAF.
I've been fighting our training department (and Flight Standards) about this for a year now. When we go into Freeport the route we are on dumps at the Freeport VOR with no way to get on any approach. The airway we are filed on crosses an arc, inside the final approach fix, for the ILS (and VOR) approach and the preferred method is to just join the arc inbound. I've pointed out several times that this isn't legal but everybody just kind of shrugs.
 

z987k

Well-Known Member
I've been fighting our training department (and Flight Standards) about this for a year now. When we go into Freeport the route we are on dumps at the Freeport VOR with no way to get on any approach. The airway we are filed on crosses an arc, inside the final approach fix, for the ILS (and VOR) approach and the preferred method is to just join the arc inbound. I've pointed out several times that this isn't legal but everybody just kind of shrugs.
Ya, I used to get "cleared VOR/DME-B approach" while on an airway that did not take me to any IAF. I guess if you're at the MEA for the airway and you turn on to an arc and are above that altitude as well, you're not going to hit anything.
 

cmhumphr

Well-Known Member
What the airplane in front of you reports means nothing. RVR (or the vis given by the asos/atis)is controlling. Also if I actually heard someone say on the radio that vis was 1/4, when I landed, I'd have a talk with them about how retarded they are. Oh yeah it's 1/4? You can really tell between 1/4 and 1/2. Uh huh.
The difference between 1/2 and 1/4 is what you can see in the runway environment. They had a slideshow in our ground school that demonstrated this, however in real life everything is moving so rapidly, you can't really tell.
 

SpiraMirabilis

Possible Subversive
I didn't understand the question, it seems like. I've never heard of a pilot "reporting" a visibility, and my opinion on a pilot "reporting" a visibility that is less than the minimums on the approach is very similar to this:

 

z987k

Well-Known Member
The difference between 1/2 and 1/4 is what you can see in the runway environment. They had a slideshow in our ground school that demonstrated this, however in real life everything is moving so rapidly, you can't really tell.
Ya, part of my point. You can't actually tell 1/4/ from 1/2, and if you miraculously can and report it, F YOU!

I didn't understand the question, it seems like. I've never heard of a pilot "reporting" a visibility, and my opinion on a pilot "reporting" a visibility that is less than the minimums on the approach is very similar to this:

Yep. Even though it's never controling
 
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