Why MSA circle is not aligned into Magnetic North?

8 Answers

1. 
   

   
   

   Brian on Apr 05, 2020

   Not a great answer, but I can give you a little insight.

   The directions on the MSA are the magnetic courses TO the station, in this case the PUC VOR. The lines defining the segments of the MSA are aligned with True (not magnetic), which is why they appear off from where they should be. If you’re coming in from the west and tracking towards the PUC VOR you’ll be using the 090 course into the station, which is easy to identify on the MSA, and therefore you should stay high.

   They don’t use 360, 90, 180, 270 degrees to determine the safe altitudes because they clump large obstacles together to give you a better idea. If there were massive mountains to the west from 180 clockwise to 010 degrees then the large obstacles would take up 3/4 of that MSA circle – by listing different radials/courses into the VOR they can cluster the big stuff together while maximizing the lower level areas displayed.

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2. Best Answer
   
   

   
   

   Mark Kolber on Apr 06, 2020

   It is simply because the MSA for the approach is based on the VOR. The underlying assumption is VOR is your only nav source.

   As you know, due to VOR declination, the radials are often not aligned with either magnetic or true north.

   So the directions in the MSA circle aren’t random. They are aligned exactly the same way as the plan view. True N at the top, but the courses based on the radials. Combine VOR declination and magnetic variation, and you get a large discrepancy. To really see that, take a look at FOSOV. On the 164 degree radial but pointing almost due N-S.

   But the discrepancy is not where it matters. Consider. If it wasn’t this way. If those numberes were true or compass magnetic, you would need to pull up the enroute chart and the Chart Supplement, read the variation and declination, and calculate the radial/course while dealing with the emergency that led you to use the MSA.

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3. 
   

   
   

   John D Collins on Apr 06, 2020

   You wrote: \”Why the circle has different direction in terms of cardinal direction?\” and \”I have know Idea why the MSA circle is not aligned to the magnetic north, it shows just random direction, and it confuses the users I think.\”

   It appears you are confused about what the purpose of the sectors are for in the MSA. When the altitude difference between all sectors does not exceed 300 feet, only a single MSA altitude is used. Sectors are introduced when there are significant differences in the safe altitude for various directions from the reference navigation facility. God did not align terrain and man did not align obstacles with true north around a navigation facility and VOR\’s are not likely to be aligned with current magnetic north. But the VOR bearings don\’t move and are easy to use to inform the pilot of a safe altitude to use in an emergency when inside the MSA circle. In an emergency, this can be a really good thing to know.

   MSA in the US is not an operational altitude as it is only intended to be available to be used in the case of an emergency. So you can\’t use it as a \”descend to altitude\” when on an approach, unless you are cleared to maintain that altitude by ATC or are on a charted segment of the procedure and cleared for the approach.

   So back to your quoted statements, I have no idea what relevance magnetic north or cardinal directions has to do with emergency guidance of avoiding obstacles.

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4. 
   

   
   

   Russ Roslewski on Apr 06, 2020

   The above answers are all correct, but allow me to offer a bit more about how these are developed.

   The MSA sectors are determined by the specialist developing the procedure. Each sector can be not less than 90 degrees wide. But the exact placement of the sectors is left up to the specialist. They will typically try various configurations until arriving at one that has the lowest possible minimum altitude over the largest possible area. In the case at PUC, the lowest terrain is to the south. So by skewing that sector to extend from the SE to the SW, the lowest altitude was able to be obtained.

   Be assured that these directions are not “random”, but purposefully chosen for maximum advantage.

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5. 
   

   
   

   dgssss on Apr 06, 2020

   Thank you for your answers.

   That helped me a lot in those things –

   – VOR station’s radial is aligned to Magnetic Course
   – MSA is for emergency usage
   – MSA is divided if there is large discrepancies in safe altitude

   – VOR is not “exactly” aligned to Magnetic Course
   – Many components (at least MSA) in the approach chart are not “randomly” created, but by the matriculate work of the designer.
   – MSA’s vor station radial is aligned same as plan view.

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6. 
   

   
   

   dgssss on Apr 06, 2020

   **Cannot edit the answer so I rewrote.

   Thank you for your answers.

   That helped me a lot in those things –

   *What I already knew
   – VOR station’s radial is aligned to Magnetic Course
   – MSA is for emergency usage
   – MSA is divided if there is large discrepancies in safe altitude

   *What I knew from this answers
   – VOR is not “exactly” aligned to Magnetic Course
   – Many components (at least MSA) in the approach chart are not “randomly” created, but by the matriculate work of the designer.
   – MSA’s vor station radial is aligned same as plan view.

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7. 
   

   
   

   Mark Kolber on Apr 07, 2020

   Pretty much correct.

   Expanding in the VOR piece a bit, when a VOR station is installed, it is aligned with magnetic north. Magnetic variation changes with time but the VOR station does not realign itself automatically. It gets realigned when site checked and, apparently (someone correct me) this is not done on a standard schedule.

   The result is, in flight, the VOR radial you are on and your magnetic compass/DG/HSI is likely to show different numbers even with no wind. It also explains the different numbers you see if you center the VOR and GPS CDIs when heading to a station (even if your VOR receiver’s error is zero).

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8. 
   

   
   

   Russ Roslewski on Apr 07, 2020

   Mark, the VOR checks themselves are done on a regular schedule. However, the VOR realignment itself is not. Not until the difference from actual is greater than 5 degrees is the VOR even considered for realignment. (Reference FAAO 8260.19H, para 2-5-3)

   This is purely based on benefit vs workload.

   If a VOR is off by 5 degrees, then your compass and VOR radial may differ by that amount (likely not. due to compass deviation anyway), but you’ll still be flying the same ground track you were before. So it makes no difference to navigation.

   However, realigning the VOR has a ripple effect through many other procedures. Even a relatively remote VOR like PUC has several procedures tied to it.

   For PUC, realigning it would require also amending the following things:
   PUC ILS OR LOC RWY 1 and any fixes on it
   PUC VOR RWY 1 and any fixes on it
   69V VOR-B and any fixes on it
   V134
   V208
   T298 (since the VOR’s Mag Var is used for the “radials”)
   Navigational fix data for ARBIH, YMONT, COPUK, FOSOV, ADIBE
   – and perhaps some more I didn’t catch

   And that’s for PUC! Now consider the case at a VOR in the New York City, Los Angeles, or Dallas areas. Some of those have dozens of procedures using the VOR – SIDs, STARS, approaches, ODPs, airways, etc.

   So VOR realignment is a pretty substantial undertaking that is only taken when it’s viewed as absolutely necessary. You will even see numerous VORs that are “off” by more than 5 degrees for just this reason.

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