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For instance, at Manchester (EGCC) the approach procedure for RNP 23L has the MSA based off of RNAV waypoints; DOMIG bearing 142 to 322, OSNAP bearing 322 to 052, TINVA bearing 052 to 142.

From NATS UK: EGCC RNP 23L

If you were flying this approach how do you determine which MSA sector you are in? Is there a way in the FMC to see real-time your bearing and distance from a given waypoint (like you would do traditionally with VOR and NDB needles)? Or do you just have to use common sense and the ND to approximate it?

Likewise some airports use the ARP as the reference point for MSA, how would you know which sector you're in?

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    $\begingroup$ Serious question: how would that knowledge affect how you operate the aircraft? ((I've briefed lots of MSA numbers, but never cared about any of them after that. Of course, EGPWS is a great security blanket that way.)) $\endgroup$
    – Ralph J
    Commented Dec 16, 2021 at 4:20
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    $\begingroup$ @DeltaLima Flying the altitudes on the instrument procedure (which absolutely can be below Min sector altitudes) will keep you safe. EGPWS is a great tool for situational awareness of terrain - far far better than MSA quadrants are - and a magnificent safety enhancement, but it's never the primary navigation. Min sector altitudes, IMHO, have extremely limited utility. Those are distinct from each leg's min safe altitude, which I agree are sacrosanct, even if EGPWS looks clear. $\endgroup$
    – Ralph J
    Commented Dec 16, 2021 at 11:19
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    $\begingroup$ @RalphJ I am glad that we are on the same page with respect to adhering to minimum safe altitude! I interpreted MSA as Minimum Safe Altitude (which is used in ATC monitoring & warning systems such a MSAW ) instead of MSA as Minium Sector Altitude, which is indeed a very conservative measure based on 1000 ft clear of the highest obstacle in a 25NM radius. $\endgroup$
    – DeltaLima
    Commented Dec 16, 2021 at 11:25
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    $\begingroup$ @DeltaLima Definitely doesn't help clarity when the same "MSA"acronym is used for two distinctly different, though not entirely unrelated, concepts! My initial comment above was, as I believe the OP was, using MSA in the "sector" version... which I've often briefed (it's a standard part of the brief where I fly) and never used. I agree w your answer below; overall situational awareness matters; knowing the point that we cross into "this" sector, not so much. $\endgroup$
    – Ralph J
    Commented Dec 16, 2021 at 17:09
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    $\begingroup$ I think there is a misunderstanding in the question: while the distances are off of the 3 IAFs, the bearings are all off of the same point, C23LI. And, those are TAA's, not MSA's. The MSA is 3,500', centered around the MCT navaid, and has no sectors. $\endgroup$
    – Ralph J
    Commented Dec 17, 2021 at 9:16

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There is a way to do this in a 737NG using the FMC FIX page:

FMC FIX page

1 FIX Name
Enter the desired fix.
Valid entries are airports, navaids, waypoints or runway identifiers from the navigation database. [...]

3 Radial Entry (example)
Enter a radial from the fix. Radials are displayed on the navigation display map mode as green dashed lines from the fix.
When the radial intersects the active route, the ETA, DTG, and predicted altitude at the intersection are displayed. [...]

6 Radial/Distance From Fix (RAD/DIS FR)
Displays the radial and distance from the fix to the airplane. This information is continually updated as the airplane position changes.

(Boeing 737 NG FCOMv2 11.42.57 - Flight Management, Navigation - FMC Cruise)

For your Manchester example you would define 4 fixes:

  • C23LI: The radials are all based off this waypoint, so you would enter 142/, 232/ and 322/ into LSK 2L to LSK 4L to draw green dashed lines for the three sectors onto the ND, which makes it easy to see in which sector you currently are (or if you are approaching a different sector).
  • DOMIG, OSNAP and TINVA: The distance of 25 NM is based off these waypoints. You can optionally draw green circles around each of them by entering /25 into LSK 2L on each fix page.

Additionally, the radial and distance information is continuously displayed at the top of the fix page. You would just have to cycle to the correct fix using the NEXT PAGE and PREV PAGE buttons.

Since the fix page also allows airports as a FIX Name, this will also work if the MSA is given around the airport reference point.

Having said this, I don't think any operators actually teach their pilots to do this. As Ralph J said in a comment above, there is really no operational need to do so. While on the approach (or transition, or missed approach), the altitude constraints on the waypoints will keep you safe. When under radar vectors, the limiting altitude is MRVA (minimum radar vectoring altitude), which can be lower than MSA anyway.

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  • $\begingroup$ Good info, but why wouldn’t you simply reference ownship position on the MFD map? $\endgroup$ Commented Dec 17, 2021 at 16:04
  • $\begingroup$ @MichaelHall That's what I would do, I'm just saying you can draw these lines, if you really want to. And of course there are airports with more complicated MSA sectors than Manchester (usually in very mountainous terrain). $\endgroup$
    – Bianfable
    Commented Dec 17, 2021 at 16:10
  • $\begingroup$ I would just suggest that you lead off with what you would do then. A screenshot of the superior SA of the MFD map would be bonus points. Because the way I read this was like “how can I undo this bolt, can I use visegrips?” And the answer focusing on how to adjust visegrips to avoid stripping while avoiding the obvious first choice of using a wrench. $\endgroup$ Commented Dec 17, 2021 at 16:38
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I would say you use the ND and general situation awareness to judge your position.

For the Manchester/EGCC case, the three reference RNAV waypoints are all IAFs. They are laid out in standard fashion, with the MSA as described in your post clearly describing the 10 x 5 NM area spanned between them. I don't think you need the FMS to measure the angles, just a brief look on the navigation display should do it. I guess you know your position w.r.t. the IAF during the approach.

Part of the RNAV approach chart fro RWY 23L / EGCC, showing the waypoints DOMIG, TINVA and OSNAP, with the MSA drawn in

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The short version is that, when the MSA is a TAA like this, you will always be given the IAF that is the basis for the sector you are in (or are approaching).

For instance, if you are due west of the airport, you would be “cleared direct TINVA, cleared RNP 23L approach”. You then look at the TAA sector for TINVA and see the MSA is 3500, so as soon as you are within 25nm (if not already), you could descend to 3500 in preparation for the approach. You would also have that approach (including the selected IAF) set up your FMS, which will always show your current distance and bearing to the next fix, so you could verify you are within the appropriate sector until you pass TINVA, and after that it doesn’t matter since you’re established on a segment of the approach.

There is nothing here specific to any model of aircraft.

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If you were flying this approach how do you determine which MSA sector you are in?

While some people are very adept at processing digital information and creating a mental image in their brain, for most people a visual display is superior in providing almost instantaneous situation awareness. Modern multi-function displays, (MFDs) have a map mode that performs this function quite well.

Once an approach is loaded, (unfortunately the example image below on the right side MFD does not show the same approach as depicted in the question) it should be a simple matter of orienting your ownship position relative to the MSA/TAA quadrants shown on the approach plate by taking a quick glance at the map.

If you wanted to be precise and had time you might figure a way to draw additional lines on the display, but it really isn't difficult to imagine a dividing line on the map and determine whether it is forward or behind you.

enter image description here

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