Alternate Weather minimums very close to approach minimums

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pstan

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Stan
I always thought that alternate weather minimums were meant to provide a reasonable buffer between the forecast ceiling and visibility, and the minimums for the approach that may be flown. However, I see for KPQI Presquile, Maine, that this is far from the case

Eg, Cat c aircraft with vor (no dme) and approach certified gps with lnav minimums capability only

For the runway 19 VOR approach, I see minimums of 1280 feet and 2 1/4 miles (no dme), giving a height above TDZE of 746 feet. The alternate weather minimums are 800 and 2 1/4. So I am OK to file this as my alternate as long as the forecast is 800 and 2 1/4 or better. Not much buffer there at all

Similarly, for the runway 19 RNAV, using LNAV minimums only, I see 1260 MDA with 2 miles, that's 726 feet above TDZE. The alternate weather minimums are 800 and 2. Again, not much buffer.

Have alternate weather minimums always been that close to approach minimums?

Stan
 
dtuuri said:
Your question piqued my curiosity, so I looked it up for you. Check page 3-24.

dtuuri
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You are a better man than I. I found it also but it was too much work to try to understand it (story of my life when reading TERPS). Care to take a shot at an English translation and what it means in terms of Stan's example?
 
midlifeflyer said:
You are a better man than I. I found it also but it was too much work to try to understand it (story of my life when reading TERPS). Care to take a shot at an English translation and what it means in terms of Stan's example?
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Objection! Calls for speculation.

Edit:
3.4.2 a. (2) When either the ceiling or visibility from the applicable no-light minimums is greater than the standard, establish alternate minimums as the higher of the standard or the no-light value. Note: Ceiling values are based on the DA/MDA minus airport elevation, rounded to the next higher 100-ft increment (e.g., 601 through 699 round to 700).

3.4.2 b. When required, alternate minimums are based on the NPA line with the highest ceiling or visibility on the same chart. For procedures without an NPA line, alternate minimums are based on the PA/APV line with the highest ceiling or visibility on the same chart.
​
So... the "safety buffer" could be merely the advantage of having approach lighting AND having a radius of action extra fuel supply, which is what I think the rule was meant to achieve all along.

dtuuri
 
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So, if I understand correctly (and I'm still not sure)...

If the no-light, non-precision minimums for an IAP are higher than standard alternate minimums, the higher no-light minimums are used as the alternate minimums.

?
 
midlifeflyer said:
So, if I understand correctly (and I'm still not sure)...

If the no-light, non-precision minimums for an IAP are higher than standard alternate minimums, the higher no-light minimums are used as the alternate minimums.

?
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I think you are correct midlife. So...not much buffer at all, by design I guess. On a 200 and half approach for an ils you would have 400 feet and 1 and half miles buffer normally. On a 250 foot (theoretically possible) non precision approach and 1 mile, you would have 550 feet and 1 mile buffer. With better weather, eg 800 foot ceiling and 2 miles, and a different location, you could have a non prec app with 800 and 2 as the approach limits, and now almost no buffer.

And Dtuuri, thanks for your work. Wouldn't there also be a buffer equal to the extra distance between the mda's height above TDZE, and the forecast ceiling, which works out to be a number between 0 and 99 feet?

Re no light minimums, seems I recall they used to be on a lot of charts. I'm not sure I'm seeing them on as many these days....

Stan
 
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midlifeflyer said:
So, if I understand correctly (and I'm still not sure)...

If the no-light, non-precision minimums for an IAP are higher than standard alternate minimums, the higher no-light minimums are used as the alternate minimums.

?
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Even if, say, the ceiling is the same and just the vis is higher--use them both as the alternate minimums (see Table 3-11 on the following page).
 
Although it's interesting to see how somebody else processes the alternate requirement, for me it's as uncomfortable as wearing somebody else's shoes. I never thought about "buffers", probably because I never felt like I was actually going to wind up at the alternate airport, at least in most cases. I just wanted to make sure I met the regulation and had enough range to get that far away from the destination toward better weather, including trying another airport en route to the better weather. Never thought the NWS' forecaster was accurate enough of a shot to hit a moving target like the weather.

pstan said:
I think you are correct midlife. So...not much buffer at all, by design I guess. On a 200 and half approach for an ils you would have 400 feet and 1 and half miles buffer normally. On a 250 foot (theoretically possible) non precision approach and 1 mile, you would have 550 feet and 1 mile buffer.
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I think I understand you this far.

pstan said:
With better weather, eg 800 foot ceiling and 2 miles, and a different location, you could have a non prec app with 800 and 2 as the approach limits, and now almost no buffer.
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Lost me here. "Better weather"?

pstan said:
And Dtuuri, thanks for your work. Wouldn't there also be a buffer equal to the extra distance between the mda's height above TDZE, and the forecast ceiling, which works out to be a number between 0 and 99 feet?
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I believe that's the meaning of par 3.4.2 a. (2).

pstan said:
Re no light minimums, seems I recall they used to be on a lot of charts. I'm not sure I'm seeing them on as many these days....
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I believe you need to refer to the Inoperative Components Table if you use government charts. Jepps are probably still on the plate, but I don't know for sure.

dtuuri
 
With better weather, eg 800 foot ceiling and 2 miles, and a different location, you could have a non prec app with 800 and 2 as the approach limits, and now almost no buffer.​
Lost me here. "Better weather"?

Dtuuri, yes poor choice of words on my part. I was trying to say the weather can be 800 and 2 forecast at the alternate, just barely acceptable as a legal alternate for a non prec approach, while the approach can have limits of 250 feet and 1 mile (should be easy to land), or the approach can have limits of 800 and 2 ( a true minimums approach). Stan
 
dtuuri said:
Although it's interesting to see how somebody else processes the alternate requirement, for me it's as uncomfortable as wearing somebody else's shoes. I never thought about "buffers", probably because I never felt like I was actually going to wind up at the alternate airport, at least in most cases. I just wanted to make sure I met the regulation and had enough range to get that far away from the destination toward better weather, including trying another airport en route to the better weather. Never thought the NWS' forecaster was accurate enough of a shot to hit a moving target like the weather.
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I think on this one, both your and my curiosity was raised so we went looking for an answer (a good thing). But, again perhaps like you, when it come to approach design, I am less concerned with figuring out why TERPS requirements are what they are - I leave that for Wally - and I don't terms of buffers either. I'm interested understanding how the rules apply to me and my flights.

@pstan, here's why I don't think of it in terms of a buffer. It's the old "legal does not mean safe." I see three "legalities"
  • The rule tells me when I need to file an alternate.
  • The alternate minimums, standard or otherwise, just tell me what airports I am allowed to choose for the purpose.
  • And most important, selecting an "legal" alternate does not mean I have to go there. The regulatory requirement for an alternate is a flight planning requirement that can be completely disregarded once the wheels leave the ground.
So once I get past the rule and actually select an alternate or alternates, it's a different show with different goals. I don't care about matching some legal standard. I'm interested in finding a place where I can comfortably and safely go if things crap out, rent a car, find a place to eat and, if necessary, find a place to stay overnight. So I may have in mind more than one alternate. One of the rule and one for me. For example, if I'm heading to a coastal airport, I may well select the nearest big airport, even if it is closer to the coast. That could well mean that, even though I have complied with the reg, the weather could easily be far worse at my alternate than at my destination. So, I may also be thinking in terms of an alternate alternate - one further inland where I can expect the weather to be better. For that one, i don't care if it's a legal alternate or not.

IOW, I don't expect the rules to give me a buffer. I expect me to give me a buffer.
 
Sorry to resurrect an old thread, but the alternate minimums for Boston KBOS have me baffled. I'm hoping someone can shed some lights on this. All the ILS's are 200 and half, for a categories of aircraft, so based on Dturri's excellent posts from the TERPS, I would have expected to see 600 and 2 as the alternate minimums.

However, they are 700 and 2 for cat C and D. Ok, that's pretty close to the expected 600 and 2.

For cat A and B they are listed as 1000 and 2.

Why would cat A and B require higher alternate minimums than C or D when all the ILS's are 200 and half for all categories?

anyone care to take a shot at that?

Stan
 
Don't confuse alternate mins with actual -- there's no correlation. Alt mins are there as a flight planning parameter to ensure you have adequate fuel and options based upon the forecasted weather and the types of approaches (precision vs non-precision) available. The idea of having alt mins higher than the actual mins is to increase the odds of being able to safely complete the flight.
 
Sometimes, you need to reject a legal alternate. For example, if I'm flying into my grass strip (no IAP), I have to have an alternate no matter what the weather is. Now 6NM north of me there's a field with a good ILS, so if it's 600-2 there All I need is 3 minutes worth of fuel to get there.

Now if I wanted to file that field as a destination, the nearest legal alternate is about 25 miles away. Frankly, I'm not going to count on a 6 mile alternate on my grass strip. 35 miles away there's a primary class B airport with big runways and real bright approach/runway lights.
 
tsts4 said:
Don't confuse alternate mins with actual -- there's no correlation. Alt mins are there as a flight planning parameter to ensure you have adequate fuel and options based upon the forecasted weather and the types of approaches (precision vs non-precision) available. The idea of having alt mins higher than the actual mins is to increase the odds of being able to safely complete the flight.
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Well, if we are to believe what dturri repeated from the terps, there is a correlation. For example, if the no light minimums are less than 600 and 2 for a precision, then the alternate minimums are 600 and 2. If the no light minimums are greater than 600 and 2, then the alternate minimums are equal to the no light minimums.

However, that does still not explain why Boston shows cat A and B alternate minimums as 1000 and 2. And that's what I was asking
 
flyingron said:
Sometimes, you need to reject a legal alternate. For example, if I'm flying into my grass strip (no IAP), I have to have an alternate no matter what the weather is. Now 6NM north of me there's a field with a good ILS, so if it's 600-2 there All I need is 3 minutes worth of fuel to get there.

Now if I wanted to file that field as a destination, the nearest legal alternate is about 25 miles away. Frankly, I'm not going to count on a 6 mile alternate on my grass strip. 35 miles away there's a primary class B airport with big runways and real bright approach/runway lights.
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Ron, I understand what you say, however when the FAA decides what alternate minimums to put on the approach plate, I don't see how an aircraft's departure point could be taken into consideration. If the FAA lists 600 and 2 as alternate minimums for a normal ils, why would a facility such as Boston with long runways and multiple ILS's require forecast weather of 1000 and 2 for a category B aircraft?
 
Who said anything about departure point?
pstan said:
Ron, I understand what you say, however when the FAA decides what alternate minimums to put on the approach plate, I don't see how an aircraft's departure point could be taken into consideration.
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Who said anything about departure point?
 
pstan said:
Ron, I understand what you say, however when the FAA decides what alternate minimums to put on the approach plate, I don't see how an aircraft's departure point could be taken into consideration. If the FAA lists 600 and 2 as alternate minimums for a normal ils, why would a facility such as Boston with long runways and multiple ILS's require forecast weather of 1000 and 2 for a category B aircraft?
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That's a good question. In fact, in the same Alt Min document for New England, the BOS alt mins are the only ones I can find where there is a bonus, instead of a (possible) penalty for cats C,D vs. A,B.
Terpsters?
 
flyingron said:
Who said anything about departure point?

Who said anything about departure point?
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You're right Ron, I meant an alternate close to destination. Still, I don't see why 1000 and 2 is the alternate mins for cat B.
 
Some very very strange anomolies I'm reading from aterpster's new link. To calculate alternate minimums, the example's shown are significantly different in 8260.3C provided by aterpster and the old 8260 provided by dturri in post #2 and he explained in post #4. The new 8260 shows that for a non precision approach that also has circling minimum published, the alternate minimums are the lowest no light minimum on chart, including the circling minimums line. This explains why KBOS has such high non precision alternate minimums. The old 8260 makes no mention of using these circling minimums when calculating alternate minimums for a non prec app. Secondly, that infers to me that if the approach had no circling minimums (for example towers in the circling area) then the alternate minimums would, in just about all cases, be even lower since there would be no circling line on the chart. thus...more towers equals lower alternate minimums....
Further...., the new 8260 seems to clearly indicate for a precision approach (eg ILS) that circling minimums, even if on the chart, are not to be used to determine alternate minimums. However the KBOS charts do exactly that, requiring circling limits (1000 and 2 for cat A and B) for this to be a legal alternate.
A fourth peculiarity; explain why cat A circling minimum height above aerodrome can ever be higher than cat D (eg ILS 22L, cat A 940, cat D 620)? Have I missed something new in the TERPS?

Anyone with insight?

caveat; all data taken from jeppessen plates, not from other providers.

Stan
 
pstan said:
A fourth peculiarity; explain why cat A circling minimum height above aerodrome can ever be higher than cat D (eg ILS 22L, cat A 940, cat D 620)? Have I missed something new in the TERPS?
Anyone with insight?
Stan
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As I noted above, I too can't fathom why cat D would ever be lower than cat A. Still waiting for the expert terpsters to chime in on that.
 
RotorDude said:
As I noted above, I too can't fathom why cat D would ever be lower than cat A. Still waiting for the expert terpsters to chime in on that.
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They figure if you hit things faster it won't hurt as much? ;)

(Ducking...)
 
pstan said:
Well, if we are to believe what dturri repeated from the terps, there is a correlation. For example, if the no light minimums are less than 600 and 2 for a precision, then the alternate minimums are 600 and 2. If the no light minimums are greater than 600 and 2, then the alternate minimums are equal to the no light minimums.

However, that does still not explain why Boston shows cat A and B alternate minimums as 1000 and 2. And that's what I was asking
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Go to the following link:

https://www.faa.gov/air_traffic/fli...procedure.results&nasrId=BOS#searchResultsTop

Click on the email link on the same line as Alternate Minimums and submit the question.
 
flyingron said:
Wow Wally...it's about time. They finally got someone to retype those parts that were just scans out of the historic TERPS document. They even redrew the figures. The thing is a lot more legible and useful now!
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Way overdue. They should have made the graphics vector graphics rather than bitmaps, though.
 
Speaking of minimums , was there a final report on the bloomington I'll. Accident that killed several people in a cessna 400 series? As I remember the weather was very bad, at night. High time pilot, Anyone know?
 
I received a short email reply and had a phone conversation with a gentleman after submitting my question about alternate minimums and circling mda. He was patient and courteous.

So....why are alternate minimums so high at KBOS?

He understood my point that alternate minimums were only to be published if the ils approach minimums were above 600 and 2. However his interpretation was that since there was more than one line of minimum published on the ils chart, the higher line minimums now applied. I tried to explain these other minimums were not PA minimums, but loc only non prec minimums, and circling minimums, and the terps pretty clearly says that, however he says the line with the higher minimums is the one considered, that's how the alternate minimums were being selected. KBGR was an example similar to KBOS.

Just to clarify with him, I asked the hypothetical question that if the FAA withdrew all the approaches to KBOS except the ILS with glideslope approaches, so that there was only an ILS line on the chart, what would be the alternate minimums? HIs answer was that they would now all be 600 and 2. I asked if it made sense that when the FAA gives you a whole lot of other choices for an approach other than just an ILS, does it make sense that the alternate minimums should then be higher? He laughed, somewhat agreed, and said something like one never should try to make sense of this sort of thing. I still read the terps differently.

It was my impression that the minimums selected were done by some sort of software, and not by a terps specialist who reads the alternate minimums requirements himself (for refresher if necessary) in order to chose what he thinks they should be listed as. And so, there was no one able to over write what the form says, no one to make a change if a change was needed.

aTerpster, could that be correct?

Re Cat A and B mda minimums for circling being greater than C and D, the explanation was that there is a note on the chart (I missed it) that says "Circling NA for Cats C and D west of Rwys 4L and 15R" and thus a lower cat C and D is possible
 
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