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Gramsca

How do you determine what altitude you should be at for minimums?

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The title may not make sense but when you go to do the decent brief it says minimums. I know it is the "minimums" and what it is and all that but I don't know what to set it to. I just set it to 300 ft all the time. I have PFPX and TOPCAT installed.

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Look at the approach plates of the airport,  the mins are on the bottom of the table

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Hi Brennan,

I can give you a quick rundown on what to put there (this is a simple overview of the system - in reality there is a lot more to consider then what I've shown here!).

 

Firstly, a quick overview. There's currently three types of ILS approaches available (not at every airport as it depends on infrastructure amongst other things) - Category (CAT) I, II and III (which is further broken down into IIIa, IIIb and IIIc). Each approach type has a general decision altitude/height:

 

CAT I     - 200ft, Minimum Visibility of 1,800ft

CAT II    - 100ft, Minimum Visibility of 1,200ft

CAT IIIa - No Decision Height, Minimum Visibility of 700ft

CAT IIIb - No Decision Height, Minimum Visibility of 150ft

CAT IIIc - No Decision Height, No minimum Visibility

 

(CAT IIIc have a substantial number of ground requirements such as taxi guidance for zero visibility as well as the actual approach infrastructure).

 

The design criteria of the approach (as well as obstacles in the missed approach path) determines the height at which the decision height or altitude occurs. CAT I approaches and any non precision approach (such as VOR or RNAV arrivals) use an altitude (DA) that is reference to a barometric altimeter (BARO mode on your EFIS Display Control Panel - near where you select the current QNH). CAT II and all type III approaches use a height (DH) that is referenced to a radio altimeter height or height above ground (RADIO on EFIS display control panel or DCP). You'll notice if you change between RADIO and BARO on the EFIS DCP, so too will the title of the minimums section of the FS2Crew briefing panel. I'd also add that the point of a missed approach point (or DA/DH) is a safe point to conduct a missed approach from (without hitting anything) but also safe to conduct a landing if you can see the runway with the required visibility (and that's a whole other topic!!).

 

So, where do you get this information from? As Peter suggested, approach charts give you all this information including whether the runway you intend to use is capable of accepting CAT II or III arrivals. There are multiple websites out there to use or payware files. However I believe skyvector.com has most of the US airports.

 

As an example if you look at the CAT II approach at Los Angeles (KLAX) ILS 24R you'll see it is capable of CAT II and IIIa & B arrivals. For CAT II it says RA 115 so in this case you would set a your MINS selector to RADIO (which also changes the FS2Crew briefing panel to RADIO) and a height of 115ft. If you were conducting just the CAT I approach the altitude listed on the separate chart is 320ft (or 200ft above the ground) so you would set the MINS selector to BARO and set 320ft. As you may know the altimeter displays a height above mean sea level (AMSL) corrected for the current atmospheric pressure so if you add the threshold elevation of 120ft to the 200ft for a CAT I approach, you get.... 320ft!

 

As far as a VOR or RNAV approach approach goes you would use BARO.

 

For a visual approach, that would most likely vary from airline to airline and your virtual airlines may be able to provide some guidance as to what they recommend to set.

 

As a general rule for a visual approach, Boeing recommends that you have the aircraft in a stable approach configuration (fully configured with gear and landing flaps extended, and speed not more than 20kts above your landing speed - amongst a few other things) by 500ft. So in this case you could set a BARO altitude of 500ft plus whatever the height of the aerodrome is, and this can be found on any approach chart or on the main aerodrome chart. So again using LAX - the overall aerodrome has an elevation of 128ft so 500 + 128 is 628... pretty close (and always safer to round up) to 630ft for the BARO altitude. In same cases this could be quite high depending on where you're landing (such as Las Vegas which has an elevation of 2181 so that would be 2681 or 2700ft AMSL, yet still only 500ft and a bit above the ground!!). 

 

Again this is a very simple overview but I hope it answers your question and helps you understand what to set and why - aviation is all about learning!

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Wow, Haydn.That is such a well-written well-thought out explanation, I reckon it should be pinned. Nice one,

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You're most welcome Rick! I was concerned even that was too detailed and people wouldn't want to read through the whole thing - it certainly ended up a little more than a "quick rundown!" Good to know it was of use.

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The only other aspect you may have to take into account is - operations under really cold weather - due to the higher density of the airmass you'll be in.

 

Tables exist for this kind of situations, based of deviation from ISA.

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That's the main reason I prefaced it with being a "simple overview" with a lot more to consider in real operations, but you are indeed correct.

 

There's also Pressure Error corrections for some altimeters, glide path failure consideration, ILS critical area protection and autoland along with missed approach performance on some approaches where there's a different DA based on the performance you can maintain in a missed approach.

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