Low Px altitude correction on Appraoch

[FOD_Vacuum]

Just want some input on a dead end we have about this topic. It is not every day that you get unusually low pressure (below 29.00') but as some of you in Manitoba may have experienced yesterday, our pressure was fairly "low being in the high 28.xx" being the lowest we had all night.

When I asked my copilot if he knew anything about the altitude correction formula in extreme low px setting, he gave me a blank look and said he had never heard of it. I do remember seeing this somewhere that if the px is below 29.00 or 28.00, I can't remember which one, it is advisable to add 10 feet to your DH or MDA for every .01' below that value. I think I saw it in a book but can't remembr which one as it was many years ago.

Does anyone know if this is an actual thing? Thanks!

(Edited 28.10 to high 28.xx**)

[Sidebar]

I think I saw in a book that if you can't set the altimeter, you shouldn't be flying an approach.

[photofly]

Which airport in Manitoba, yesterday?

As long as you can set the correct altimeter setting you shouldn't have to adjust any heights because of pressure. (That's what the altimeter setting does). Altimeters are usually adjustable down to 28.00". CAR571 Appendix B suggests altimeters have to be tested on a setting of 28.10".

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AIP CANADA (ICAO) ENR has a section on operations where the altimeter setting exceeds 31.00" but it's silent about settings below 28.00".
https://www.navcanada.ca/EN/products-an ... nr_eng.pdf

[Ki-ll]

There is guidance in CAP Gen for operations when altimeter setting is above 31.00, but not when below 28.00.

[confusedalot]

Count me in as one who has never thought about correcting DA or MDA for a low altimeter setting. (after 39 years)

However, I suppose that it could be easily done on a circular CR-3 or CR-5 slide rule by doing the true altitude determination. That is, if anybody on this board actually remembers the jepp circular slide rule

Suspect that the true versus indicated altitude difference would be negligible.

[oldtimer]

Having been in both situations where the altimeter setting is below 29.00 and again when it was above 31.xx, there is no adjustment to the MDA/DH required. If the setting is at 28.99 or lower but within the range of their instruments, ATC/FSS will say the altimeter setting twice. If you can set it, you can use it. If the altimeter setting is far enough above 31.xx and the setting is greater than what FSS's Kollsman altimeter can indicate and therefore FSS cannot accurately read the altimeter setting, the procedures to be used are in the CAP Gen as someone noted. Basically standard altimeter setting is used for cruise and an ILS approach may be required for approach and landing. When it happened to me many years ago, ATC could not get an accurate altimeter setting because the Kollsman setting was off the scale. FSS had a much broader range in their instruments but precautions were in place. Environment Canada's mercury barometers had a larger scale and was considered more accurate but I do believe caution dictated the procedures in CAP Gen were required. Basically once visually above the MOCA/MEA, flight levels were used. When it happened to me, I was in Fort Chip and altimeter was above FSS's scale but the Edmonton altimeter setting was within range of their instruments so we could get an accurate altimeter setting for approach and landing but at Fort Chip, we had set our altimeters to field elevation and climb visually to the flight levels. Because I was in a Cheyenne, it was no problem but if I had to land in YMM, I would have had to do an ILS. Since this was the very first time this problem had ever surfaced, this was the quick fix. At FL190, there was a 2000 foot difference between the altimeter setting and FL190. From there, we had to apply the temperature correction for landing because it was cold enough to freeze the balls off a brass monkey.

[Beefitarian]

I know it's not tough to extrapolate you must mean pressure but.

Doesn't Px stand for passenger?