Approach Vis mins

///AMG

///AMG

Well-Known Member
I'm wondering if anyone can explain to me the reasoning for the difference between the visibility minimums for prevailing weather and RVR for an approach. I.E you have approach mins listed as 560/24 with weather mins of (400 - 1/2) or something like that. Is this just to account for the difference between prevailing visibility and actual RVR which could theoretically be increased with certain runway lighting systems?
 
///AMG said:
I'm wondering if anyone can explain to me the reasoning for the difference between the visibility minimums for prevailing weather and RVR for an approach. I.E you have approach mins listed as 560/24 with weather mins of (400 - 1/2) or something like that. Is this just to account for the difference between prevailing visibility and actual RVR which could theoretically be increased with certain runway lighting systems?
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From FAAO 8900.1:
C. Instrumentally Derived Value. RVR is an instrumentally derived value that reflects an artificially created seeing condition on or near the portion of the runway associated with the RVR report. This artificially created seeing condition is achieved by using high intensity runway edge, touchdown zone, and centerline lights. These lights increase the conspicuousness of the landing surface and “reach out” to the pilot thereby creating a seeing condition which is significantly better than the reported ground visibility or tower visibility. For any particular fog density, RVR will be significantly greater than reported visibility because RVR is based on the use of high intensity lights. Since RVR is based on high intensity lights, an RVR report only has meaning when associated with the seeing-conditions on or near the portion of the runway where the report was obtained (TDZ, MID, or Rollout).
​
 
tgrayson said:
From FAAO 8900.1:
C. Instrumentally Derived Value. RVR is an instrumentally derived value that reflects an artificially created seeing condition on or near the portion of the runway associated with the RVR report. This artificially created seeing condition is achieved by using high intensity runway edge, touchdown zone, and centerline lights. These lights increase the conspicuousness of the landing surface and “reach out” to the pilot thereby creating a seeing condition which is significantly better than the reported ground visibility or tower visibility. For any particular fog density, RVR will be significantly greater than reported visibility because RVR is based on the use of high intensity lights. Since RVR is based on high intensity lights, an RVR report only has meaning when associated with the seeing-conditions on or near the portion of the runway where the report was obtained (TDZ, MID, or Rollout).
​
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Which is why I believe RVR is measured with a runway light setting of 3, or MIRL. And tower usually advises when RVR is being measured with HIRL, or 4 or above setting.

As an aside too, AMG, you won't see RVR reported unless PV is below 2 miles or RVR value is less than 6000
 
///AMG said:
I'm wondering if anyone can explain to me the reasoning for the difference between the visibility minimums for prevailing weather and RVR for an approach. I.E you have approach mins listed as 560/24 with weather mins of (400 - 1/2) or something like that. Is this just to account for the difference between prevailing visibility and actual RVR which could theoretically be increased with certain runway lighting systems?
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What's this (400-1/2) thing? Is that really an MDA/DA(H) reduction when using vsibility vs. RVR? Got a plate you can link? I'd like to take a look at it.

-mini
 
Sorry, yeah what MikeD said...I should have specified that previously. Put simply (save a couple loopholes) we can't legally commence an approach unless the reported wx is at or above those specified mins in the parenthesis, though we can continue down to the hard numbers listed on the left side if we are already on the approach. This is kind of why I was originally wondering about this difference. Thanks for the help gents!
 
///AMG said:
Sorry, yeah what MikeD said...I should have specified that previously. Put simply (save a couple loopholes) we can't legally commence an approach unless the reported wx is at or above those specified mins in the parenthesis, though we can continue down to the hard numbers listed on the left side if we are already on the approach. This is kind of why I was originally wondering about this difference. Thanks for the help gents!
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You can legally commence an approach regardless of what's reported. You might have to explain how you determined that the flight visibility was above the required published minimums.

Take your alternate for instance, say you used the 600-2 for a PA during your planning, and you couldn't get into your primary for whatever reason...doesn't matter what's being reported at the alternate, you were legal to file there, and you just may get in. Whether that's a good idea or not is another story.
 
IslandFlyer said:
Take your alternate for instance, say you used the 600-2 for a PA during your planning, and you couldn't get into your primary for whatever reason...doesn't matter what's being reported at the alternate, you were legal to file there, and you just may get in. Whether that's a good idea or not is another story.
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Overall true, only difference being he and I have different regs for what requires an alternate and what the alternate WX would have to be to qualify. But agree with the balance of your post.
 
IslandFlyer said:
You can legally commence an approach regardless of what's reported. You might have to explain how you determined that the flight visibility was above the required published minimums.

Take your alternate for instance, say you used the 600-2 for a PA during your planning, and you couldn't get into your primary for whatever reason...doesn't matter what's being reported at the alternate, you were legal to file there, and you just may get in. Whether that's a good idea or not is another story.
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Your regs are probably different, but Navy regs (OPNAV 3710) prohibit commencing an approach to a field that is below mins unless it is a "practice approach". In the training command this means (for us single engine jet types) that we immediately proceed to alternate, don't hold, don't collect $200 and don't pass go. I believe the multiengine dudes get to hold for some amount of time given fuel reserves but we basically have nothing to spare in most cases. Also, for us to legally fly a practice approach, the approach must not be to a filed destination or alternate, and we may not have the intent to land.

Our alternate planning may also differ a bit from yours. If destination +/- 1 hr of ETA is forecast below mins, we have to have 3000/3 at our alternate for that period. If destination is between mins and 1000/3, then we have to take alternate mins and add 200-1/2 to their precision mins, or + 300-1 to their non-precision mins.

There are some other assorted gotchas but that covers the general idea. From your comments I presume your regs are less restrictive?
 
Part 91 yes. For alternate, Precision approach 600-2, Non-precision 800-2 and that is the 1-2-3 rule. +/- 1 hr eta, 2000 foot ceilings & 3 sm viz then we need alt. For part 91 you can commence the report at any time even if the viz or ceiling is less than prescribed.

Part 135 yes also. For alternate, one nav/two nav rules. EDIT -->The biggest thing to make sure of is that the airport you are looking to use has an authorized approach then use the one-nav/two-nav rule. For instance, my company cannot perform GPS or PRM approaches. Not in OpSpecs. The one-nav rule states that as long as you have at least one suitable IAP you add 400 feet and 1SM to the approach minimums and you can legally file that airport as an IFR alternate. For the 2 nav rule if there are at least two suitable straight-in IAP approaches that involve 2 different runways and separate nav aids you may pick and choose your ceiling and visibility. You should add 200 feet to the higher HAT and add a 1/2 SM to the higher visibility. Yeah, it gets complicated. You hope for 2 ILS approaches.[FONT=&quot][/FONT]
 
braunpilot said:
Part 91 yes. For alternate, Precision approach 600-2, Non-precision 800-2 and that is the 1-2-3 rule. +/- 1 hr eta, 2000 foot ceilings & 3 sm viz then we need alt. For part 91 you can commence the report at any time even if the viz or ceiling is less than prescribed.

Part 135 yes also. For alternate, one nav/two nav rules. EDIT -->The biggest thing to make sure of is that the airport you are looking to use has an authorized approach then use the one-nav/two-nav rule. For instance, my company cannot perform GPS or PRM approaches. Not in OpSpecs. The one-nav rule states that as long as you have at least one suitable IAP you add 400 feet and 1SM to the approach minimums and you can legally file that airport as an IFR alternate. For the 2 nav rule if there are at least two suitable straight-in IAP approaches that involve 2 different runways and separate nav aids you may pick and choose your ceiling and visibility. You should add 200 feet to the higher HAT and add a 1/2 SM to the higher visibility. Yeah, it gets complicated. You hope for 2 ILS approaches.
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One "nav aid" or approach.... add 400 and 1 to the minimum and there is your alternate minimum.... for two, add 200 and 1/2 to the highest minimum (which ever approach is higher)
 
cencal83406 said:
One "nav aid" or approach.... add 400 and 1 to the minimum and there is your alternate minimum.... for two, add 200 and 1/2 to the highest minimum (which ever approach is higher)
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different regs, different company. you are correct that is for an authorized approach. the two-nav rule applies to the higher of the two approaches. viz & ceiling.
 
The Air Force's version is similar to the Navy's (from what AMG posted), but it's even a little different than that.

Do I need an alternate?
WWW.RUG

If my destination has:

W. Weather- less than 3000 and 3 mi vis at the ETA +/- 1 hour, (or 3000 and 2 miles above the lowest suitable approaches minimum requirement, whichever is higher)
W. Winds- out of limits ETA +/- 1 hour (training command only)
W. Weather - no weather reporting capability
R. Radar is required to fly the approach (either it's a GCA approach like a PAR/ASR, or RADAR is required to identify a point on the approach, etc.,)
U. Unmonitored NAVAID.
G. GPS is the only available approach.




If any of the above is true of my destination, then I must file an alternate. At my alternate, I also have a WWW.RUG set of rules to follow:

W. Weather- 1000 and 2 miles vis, or approach mins. +500 feet and approach mins +1 mi. vis, whichever is higher. (ETA +/- 1 Hr)
W. Winds within limits (ETA +/- 1hr, training command only)
W. Weather reporting capability- must have it... unless I can get from my destination to a point where I can continue VFR to the alternate.
R. Radar required - can't be... unless I can get to a point where I can continue VFR to the alternate.
U. Unmonitored Navaid- can't be... unless I can get to a point where I can continue VFR to the alternate.
G. GPS only suitable approach- can't be... unless I can get to a point where I can continue VFR to the alternate.

Heavies are also allowed to substitute an amount of fuel for holding instead of an alternate if there is no suitable alternate within 2 hours of flying time.

The way guys typically remember the weather #'s is by making them a phone number: 332-1521. The 332 applies to whether or not I need the alternate, and the 1521 applies to the weather at the alternate.
 
The Army's regs on alternate planning are a bit different as well. People sometimes post "military" regs/requirements and I suspect a lot of folks think all services operate under the same rules when in fact each has their own flight regulations.

From AR 95-1

e. Alternate airfield planning. An alternate airfield is required when filing IFR to a destination under any of the
following conditions:
(1) Radar is required to execute the approach procedure to be flown.
(2) The instrument approach navigational aids to be used are unmonitored.
(3) The predominant weather at the destination is forecast at ETA through 1 hour after ETA to be less than—
(a) Ceiling 400 feet above the weather planning minimum required for the approach to be flown.
(b) Visibility 1 mile (or metric equivalent) greater than the planning minimum required for the approach to be
flown.
(4) An alternate is not required if descent from en route minimum altitude for IFR operation, approach, and landing
can be made in VFR conditions.

f. Alternate airfield selection.
(1) An airfield may be selected as an alternate when the worst weather condition for that airfield is forecast for ETA
through 1 hour after ETA to be equal to or greater than—
(a) Ceiling 400 feet above the weather planning minimum required for the approach to be flown and visibility 1
mile (or metric equivalent) greater than the weather planning minimum required for the approach to be flown; or
(b) VFR minimums and descent from en route minimum altitude for IFR operation, approach, and landing can be
made in VFR conditions.
(2) An airfield will not be selected as an alternate except per f(1)(b) above—
(a) If the approach procedure to be used at the alternate is shown not authorized (NA) in FLIP.
(b) If radar is required for the approach procedure to be used at the alternate.
(c) If the instrument approach navigational aids to be used is unmonitored.
(d) If a Class B, C, D, or E surface area airspace does not exist or is not in effect at the airport to be used.
(e) If the global positioning system (GPS) is required for the approach.
 

Attachments

ghogue said:
The Army's regs on alternate planning are a bit different as well. People sometimes post "military" regs/requirements and I suspect a lot of folks think all services operate under the same rules when in fact each has their own flight regulations.
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Not to mention that when the weather is so bad you can't go anywhere IFR, two Army warrant officers in a helicopter are going to get there VFR.
 
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