Need some insight about IFR minima

[dALre]

First off, I'm sorry if this question sound dumb.
I need some help in calculating theses numbers since some of the orders are not clarified I believe.

I'm working on the INRAT now and it is about "IFR alternate minimums in setting higher than 31.00 in Hg"
Obviously you add extra values to your minimums depends on the value exceeding 31.00 in Hg.

But the issue I have here is, do you add the extra values(from excessive high pressure) to minimums BEFORE comparing whichever is greater?


ex)ONE precision approach available, DH being 200, advisory vis 1/2SM @your alternate airport. Forecast to be 31.20 in Hg.

#BEFORE case: 600-2/300-1, compare between 600-2 and 700-2[200-1/2(DH) +300-1(minima) +200-1/2(0.2 in Hg),] whichever is greater -> 700-2 is now alternate requirement.
#AFTER case: 600-2/300-1, compare between 600-2 and 500-1 1/2SM[200-1/2(DH) +300-1(minima)] whichever is greater -> 600-2, add 0.2 in Hg (+200-1/2)
so alternate requirement in this case being 800-2 1/2SM


Appreciate for any inputs!

[digits_]

It's been a while, but why would you add those values to your alternate minima?

I'm not aware of any Canadian aviation weather forecast including altimeter settings, so how would you even determine your altimeter setting at time of arrival?

[dALre]

It's been a while, but why would you add those values to your alternate minima?

I'm not aware of any Canadian aviation weather forecast including altimeter settings, so how would you even determine your altimeter setting at time of arrival?

I believe if you don't add those values to alternate minima you might see the ceiling much lower than forecasted one by your altimeter. So instead of not seeing any signs of runway at minimums and going for the missed, we raise the alternate minima. At least that is my understanding. Might be wrong...

and regarding the TAF that's what I wonder as well but since my prep test asks me I don't have much choice

[Conflicting Traffic]

ex)ONE precision approach available, DH being 200, advisory vis 1/2SM @your alternate airport. Forecast to be 31.20 in Hg.

#BEFORE case: 600-2/300-1, compare between 600-2 and 700-2[200-1/2(DH) +300-1(minima) +200-1/2(0.2 in Hg),] whichever is greater -> 700-2 is now alternate requirement.
#AFTER case: 600-2/300-1, compare between 600-2 and 500-1 1/2SM[200-1/2(DH) +300-1(minima)] whichever is greater -> 600-2, add 0.2 in Hg (+200-1/2)
so alternate requirement in this case being 800-2 1/2SM

it looks like you're only correcting one of your options for the altimeter setting. Correct both. Then it won't matter if you correct before or after.

[challenger_nami]

and regarding the TAF that's what I wonder as well but since my prep test asks me I don't have much choice

Altimeter setting is not included in a Terminal Area Forecast (TAF). However, it definitely is part of the forecast on a GFA. It is indicated by those useless looking isobar lines.

For example:
From the attached Pacific GFA for 0600Z ON 13 April 2021 (9 hours from the time of writing this), I can expect the Altimeter setting at YVR to be roughly 1027 millibars which is 30.33 inches of Hg.

.

[digits_]

It's been a while, but why would you add those values to your alternate minima?

I'm not aware of any Canadian aviation weather forecast including altimeter settings, so how would you even determine your altimeter setting at time of arrival?

I believe if you don't add those values to alternate minima you might see the ceiling much lower than forecasted one by your altimeter. So instead of not seeing any signs of runway at minimums and going for the missed, we raise the alternate minima. At least that is my understanding. Might be wrong...

and regarding the TAF that's what I wonder as well but since my prep test asks me I don't have much choice

I think you might be confusing some things. Remember, you *are* using a corrected altimeter setting, that's why you'll have a local altimeter setting when arriving at your arrival. That local altimeter setting ensures your altitude above ground during the approach will be within safety limits. You'll notice some approaches use an altimeter setting at another airport nearby, which means the minima will be a bit higher.

Cold weather corrections are applied in cold weather operations. You are using temperature for that correction, even though the altimeter setting won't be exactly 29.92 either. The cold weather affects the air density during the approach, which could result in you being too low if you don't apply them. The lower your minima (eg ILS 200 ft AGL vs RNAV with 500 AGL), the smaller the error will be.

[Conflicting Traffic]

I think you might be confusing some things. Remember, you *are* using a corrected altimeter setting, that's why you'll have a local altimeter setting when arriving at your arrival. That local altimeter setting ensures your altitude above ground during the approach will be within safety limits. You'll notice some approaches use an altimeter setting at another airport nearby, which means the minima will be a bit higher.

Cold weather corrections are applied in cold weather operations. You are using temperature for that correction, even though the altimeter setting won't be exactly 29.92 either. The cold weather affects the air density during the approach, which could result in you being too low if you don't apply them. The lower your minima (eg ILS 200 ft AGL vs RNAV with 500 AGL), the smaller the error will be.

Some altimeters can't be set to above 31.00"Hg, so you have to add to your minima accordingly. If you have an altimeter that can be set to above 31.00"Hg, you set the correct setting during the approach and this whole discussion is moot.

[dALre]

ex)ONE precision approach available, DH being 200, advisory vis 1/2SM @your alternate airport. Forecast to be 31.20 in Hg.

#BEFORE case: 600-2/300-1, compare between 600-2 and 700-2[200-1/2(DH) +300-1(minima) +200-1/2(0.2 in Hg),] whichever is greater -> 700-2 is now alternate requirement.
#AFTER case: 600-2/300-1, compare between 600-2 and 500-1 1/2SM[200-1/2(DH) +300-1(minima)] whichever is greater -> 600-2, add 0.2 in Hg (+200-1/2)
so alternate requirement in this case being 800-2 1/2SM

it looks like you're only correcting one of your options for the altimeter setting. Correct both. Then it won't matter if you correct before or after.

Well I didn't know that it need to be applied to both. Apparently, my online ground school is teaching me wrong and solution for the quiz seems not enough to help
Big thanks btw

[challenger_nami]

Don’t cause confusion for yourself by consulting AvCanada.
To pass your test, Just do whatever your Online Ground School or your instructor tells you.

When you start flying commercially, you will see that things are generally done differently... for better or for worse.

At that point you will need to adjust to it accordingly.


.

[dALre]

Don’t cause confusion for yourself by consulting AvCanada.
To pass your test, Just do whatever your Online Ground School or your instructor tells you.

When you start flying commercially, you will see that things are generally done differently... for better or for worse.

At that point you will need to adjust to it accordingly.


.

Understood. But issue is that the online ground school is telling me a different way to calculate from their own solution for some following up quizzes.
It just drives me crazy sometimes.

[ZaSz]

Trying to study this mysef too. See below references about how it works:

1.5.9 Abnormally High Altimeter Settings
Cold dry air masses can produce barometric pressures in excess
of 31.00 in. of mercury. Because barometric readings of 31.00 in.
of mercury or higher rarely occur, most standard altimeters do
not permit setting of barometric pressures above that level and
are not calibrated to indicate accurate aircraft altitude above
31.00 in. of mercury. As a result, most aircraft altimeters cannot
be set to provide accurate altitude readouts to the pilot in these
situations.
When aircraft operate in areas where the altimeter setting is in
excess of 31.00 in. of mercury and the aircraft altimeter cannot
be set above 31.00 in. of mercury, the true altitude of the aircraft
will be HIGHER than the indicated altitude.
Procedures for conducting flight operations in areas of abnormally
high altimeter settings are detailed in AIP Canada (ICAO)
ENR 1.7.

When the barometric pressure exceeds 31.00 inches of mercury, the following procedures take effect: Altimeters of all IFR, controlled VFR flight (CVFR) and VFR aircraft are to be set to 31.00 inches of mercury for enroute operations below 18 000 feet ASL. All pilots are to maintain this setting until beyond the area affected by the extreme high pressure or until reaching the final approach segment of an instrument approach for IFR aircraft or the final approach for VFR aircraft. At the beginning of the final approach segment, the current altimeter setting will be set by those aircraft capable of such a setting. Aircraft that are unable to set altimeter settings above 31.00 inches of mercury will retain a 31.00 inches of mercury setting throughout the entire approach. Aircraft on departure or missed approach will set 31.00 inches of mercury prior to reaching any mandatory or fix crossing altitude, or 1 500 feet above ground level (AGL), whichever is lower. For aircraft operating IFR that are unable to set the current altimeter setting, the following restrictions apply: To determine the suitability of departure alternate aerodromes, destination aerodromes and destination alternate aerodromes, increase the ceiling requirements by 100 feet and visibility requirements by 1/4 statute mile (SM) for each 1/10 inch of mercury, or any portion thereof, over 31.00 inches of mercury. These adjusted values are then applied in accordance with the requirements of the applicable operating regulations and operations specifications.Example: Destination altimeter setting is 31.28 inches, instrument landing system (ILS) decision height (DH) is 250 feet (200-1/2). When flight planning, add 300-3/4 to the weather requirements, which would now become 500-1 1/4. During the instrument approach, 31.00 inches of mercury will remain set. DH or Minimum Descent Alti tude (MDA) will be deemed to have been reached when the published altitude is displayed on the altimeter.

It doesn't really say if you put it on the standard minimums too or only on the actual approach minimums.
The example uses the ILS minimum, so I guess that is where you add it.
Was it what your ground school was asking you to do?

[dALre]

Trying to study this mysef too. See below references about how it works:

1.5.9 Abnormally High Altimeter Settings
Cold dry air masses can produce barometric pressures in excess
of 31.00 in. of mercury. Because barometric readings of 31.00 in.
of mercury or higher rarely occur, most standard altimeters do
not permit setting of barometric pressures above that level and
are not calibrated to indicate accurate aircraft altitude above
31.00 in. of mercury. As a result, most aircraft altimeters cannot
be set to provide accurate altitude readouts to the pilot in these
situations.
When aircraft operate in areas where the altimeter setting is in
excess of 31.00 in. of mercury and the aircraft altimeter cannot
be set above 31.00 in. of mercury, the true altitude of the aircraft
will be HIGHER than the indicated altitude.
Procedures for conducting flight operations in areas of abnormally
high altimeter settings are detailed in AIP Canada (ICAO)
ENR 1.7.

When the barometric pressure exceeds 31.00 inches of mercury, the following procedures take effect: Altimeters of all IFR, controlled VFR flight (CVFR) and VFR aircraft are to be set to 31.00 inches of mercury for enroute operations below 18 000 feet ASL. All pilots are to maintain this setting until beyond the area affected by the extreme high pressure or until reaching the final approach segment of an instrument approach for IFR aircraft or the final approach for VFR aircraft. At the beginning of the final approach segment, the current altimeter setting will be set by those aircraft capable of such a setting. Aircraft that are unable to set altimeter settings above 31.00 inches of mercury will retain a 31.00 inches of mercury setting throughout the entire approach. Aircraft on departure or missed approach will set 31.00 inches of mercury prior to reaching any mandatory or fix crossing altitude, or 1 500 feet above ground level (AGL), whichever is lower. For aircraft operating IFR that are unable to set the current altimeter setting, the following restrictions apply: To determine the suitability of departure alternate aerodromes, destination aerodromes and destination alternate aerodromes, increase the ceiling requirements by 100 feet and visibility requirements by 1/4 statute mile (SM) for each 1/10 inch of mercury, or any portion thereof, over 31.00 inches of mercury. These adjusted values are then applied in accordance with the requirements of the applicable operating regulations and operations specifications.Example: Destination altimeter setting is 31.28 inches, instrument landing system (ILS) decision height (DH) is 250 feet (200-1/2). When flight planning, add 300-3/4 to the weather requirements, which would now become 500-1 1/4. During the instrument approach, 31.00 inches of mercury will remain set. DH or Minimum Descent Alti tude (MDA) will be deemed to have been reached when the published altitude is displayed on the altimeter.

It doesn't really say if you put it on the standard minimums too or only on the actual approach minimums.
The example uses the ILS minimum, so I guess that is where you add it.
Was it what your ground school was asking you to do?

Sadly, all those examples on AIP are dealing with a destination scenario only, whereas I'm looking for application to an alternate minima.
I left some feedback to my online ground school system so hopefully I look forward to getting an answer.

[Big Pistons Forever]

The few times I have seen an altimeter setting above 31.00 inches the sky was clear and the visibility was 100 miles. While I guess it certainly scores high on the exam trivia spectrum, I would be interested in whether anybody had to actually use a high baro correction in real world ops.

[co-joe]

I don't see why you would need to use the altimeter setting at all when calculating your alternate minima. Alternate minima are based on a formula and the minimums of the approaches in use. Not only that they are EITHER based on the TAF which doesn't forecast altimeter settings, OR based on the GFA, in which case alternate minima are GFA alternate minima of 1000' above the lowest usable IFR altitude, vis >3 sm. Weird question, verging on an SQ, or even a bit of an FSQ unless I misinterpret.

[ZaSz]

dALre if you get an answer I'm interested to seeing it too.
It seems like co-joe is right and it wouldn't really apply to alternate minimums since TAF doesn't forecast altimeters and if we base ourselves on FGA then another set of rules applies anyway.