6 minutes ago, anavsun said:

Ok, I think I get it. The final approach has two alignment requirements. Initially its 172 degrees at the IAF as programmed in the FMC and entered into the MCP but as soon as the aircraft hits the SAKYY waypoint the requirement changes and the pilot has to manually eyeball the runway, make adjustments, and steer the aircraft to 177 degrees. Do I have that right? (I hope).

Yes.  👍

9 minutes ago, anavsun said:

Ok, I think I get it. The final approach has two alignment requirements. Initially its 172 degrees at the IAF as programmed in the FMC and entered into the MCP but as soon as the aircraft hits the SAKYY waypoint the requirement changes and the pilot has to manually eyeball the runway, make adjustments, and steer the aircraft to 177 degrees. Do I have that right? (I hope).

Mostly. Quoting various bits again:

9 minutes ago, anavsun said:

Initially its 172 degrees at the IAF

The IAF is the initial approach fix -- in this case HETRY or HUYJO. The segment that has you flying at a course of 172 degrees starts at the final approach fix JEZYY.

9 minutes ago, anavsun said:

and entered into the MCP

As @Stearmandriver has pointed out, on an RNAV approach you don't need to dial the final approach course into the course window of the MCP (though you can do so as a reminder).

9 minutes ago, anavsun said:

as soon as the aircraft hits the SAKYY waypoint the requirement changes

Yes, because SAKYY is the missed approach point, and you're required to have the runway environment in sight at this point, or you have to execute the missed approach.

If you have the runway in sight before SAKYY (and are sure you won't lose it), you could also visually deviate from the final approach course earlier than SAKYY -- though in this case, that's probably not necessary as SAKYY is still 2.6 miles from the threshold.

Edited February 16, 20251 yr by martinboehme

6 minutes ago, prolixindec said:

 

Correct. You stay on the published approach until the missed approach point (or "MAP") which in this case is SAKYY, and then, assuming you have the airport in sight, you maneuver to land--which means lining up with the runway. If you don't see the airport at SAKYY, you fly the missed approach procedure.

Can a pilot simply delete the two waypoints before SAKYY ie. delete JEZYY and SERVY?

Just now, anavsun said:

Can a pilot simply delete the two waypoints before SAKYY ie. delete JEZYY and SERVY?

No. As @Stearmandriver has pointed out, you're not allowed to modify the approach in the FMC after the final approach fix.

Anyway, why would you want to do this?

1 minute ago, martinboehme said:

No. As @Stearmandriver has pointed out, you're not allowed to modify the approach in the FMC after the final approach fix.

Anyway, why would you want to do this?

Keeping it simple?

Just now, anavsun said:

Keeping it simple?

You can always keep it simple and just look at the runway and land, if the weather is good enough to accept a visual approach.  The instrument approach procedures are for poor weather or at night, when you can't clearly see the terrain.

If you've got the visibility to see the runway and all terrain, just look out the window and fly a visual approach.  It's much simpler than trying to figure out how to modify a coded procedure.  This is what I meant before about degrading the level of automation used as appropriate - sometimes it's simpler to just look out the window and fly the airplane yourself, than sit there trying to figure out what will happen as you modify the automation.

9 minutes ago, anavsun said:

Keeping it simple?

It might be simpler, but you'd be doing something else than ATC cleared you to do, and you'd be flying your own made-up procedure at that point, with no guarantees that it's safe. (You can do this in the sim, of course, but never in real life.)

Where you're probably coming from is that going from OBAVE direct to SAKYY would put you even further over water -- so surely that couldn't be a problem then? If you as the pilot aren't allowed to "simplify" the procedure in this way, why didn't the procedure designer do this to keep things simple?

For one thing, this would have increased the offset between the final approach course and the runway heading -- and they probably wanted to keep this offset as small as possible.

Also, this would have required a slightly larger turn at the missed approach point, which might have required moving the missed approach point even further from the runway to keep it at the required distance from terrain.

Only the procedure designer could say for sure. I assume there were many tradeoffs that went into designing this procedure -- but at the end of the day, the only way you can be safe is to fly the procedure as charted.

Edited February 16, 20251 yr by martinboehme

11 minutes ago, anavsun said:

Can a pilot simply delete the two waypoints before SAKYY ie. delete JEZYY and SERVY?

No, as Martin said, you can't do that. For two reasons.

First, that protected volume I told you about is based on the approach track as published. If you deviate from it, your airspace is no longer protected, and you aren't guaranteed obstacle clearance. Sure, in this example, the course is over a lake, but there could be some hypothetical 1000-foot windmill sticking up out of the lake that requires you to fly the course as published.

Second, you have to fly in your airspace, and other aircraft are going to fly in their airspace. If you modify the course and fly it differently, you might get in the way of other aircraft. Your aircraft also has a protected volume around it, typically a 3-mile "bubble" that goes out laterally for three miles and up and down for 1000'. You can't have the edge of your bubble touching the bubble of an aircraft nearby, and the courses on approaches and your ATC clearances take that into account.

Edited February 16, 20251 yr by prolixindec

This is actually an interesting thread.

I don't fly the pmdg much but I too was using APR when flying an rnav with vertical minimums.  I'd stay in lnav/vnav if the mins were non LPV.

I've cleared tons of airliners for rnav approaches but yeah, most of them would prefer an ils due to minimums.  Seems like the newer jets have waas - I wonder if they engage APR when flying LPV mins?

In the prior thread here in Avsim (in the "is landing in KSJC possible" thread) and I think it also happened here while doing the final approach to KTVL I noticed that the FMA quickly changes from "LNAV VNAV Alt" to "LNAV VNAV Pth" and finally to "FAC G/P" in rapid succession. What causes the changes?  Is that in reaction to changes that is detected by the aircraft sensors (eg.  by the pitot, radio altimeter, throttle because of changes to the wind, altitude, speed, etc.) or something else?  Are these 'alerts' or 'messages' in the FMA strictly informational or are they supposed to be reacted upon by the pilot?

55 minutes ago, Stearmandriver said:

The tradeoff is higher minimums.  A better approach like an RNP (AR) could make the close-in alignment with the runway while still in the clouds to an acceptable degree of accuracy and so would have lower minimums but not everyone can fly those.

Yeah

If I paid attention during my instrument classes (cherry coke and coffee saved me), standard RNAV procedure must still adhere to the conventional primary/secondary protected areas for MOC, whereas an RNP AR is exempt from any secondary area and primary area on either side of track is 2 * RNP. 

This is one of the reasons why many are advocating for RNP AR in mountainous regions, to reduce the obstacle clearance tolerance (both on approaches and departures)

@anavsun this would basically allow for wpt JEZYY on the RNAV GPS into Lake Tahoe to be re-located more easterly than it currently is, thus reducing the offset angle (not sure how much, Maths was never my forte)

 

Edited February 16, 20251 yr by SAS443

22 minutes ago, ryanbatc said:

This is actually an interesting thread.

I don't fly the pmdg much but I too was using APR when flying an rnav with vertical minimums.  I'd stay in lnav/vnav if the mins were non LPV.

I've cleared tons of airliners for rnav approaches but yeah, most of them would prefer an ils due to minimums.  Seems like the newer jets have waas - I wonder if they engage APR when flying LPV mins?

I'm not sure I understand why an LPV is not considered a precision approach. Is it only because it is satellite based and not ground based?

I'm curious if satellite-based or ILS approaches use the same satellites or systems in Europe and Asia.

7 minutes ago, SAS443 said:

this would basically allow for wpt JEZYY on the RNAV GPS into Lake Tahoe to be re-located more easterly than it currently is, thus reducing the offset angle (not sure how much, Maths was never my forte)

Wouldn't it be better to move towards the middle of the lake or in the opposite direction to avoid unpredictable thermals coming from the mountains in the east?  This is the reason why I thought it would be simpler had the procedure designers simply moved JEZZY and SERVY waypoints more to the west towards 177 degrees to begin with.

24 minutes ago, anavsun said:

Wouldn't it be better to move towards the middle of the lake or in the opposite direction to avoid unpredictable thermals coming from the mountains in the east?  This is the reason why I thought it would be simpler had the procedure designers simply moved JEZZY and SERVY waypoints more to the west towards 177 degrees to begin with.

The big picture. There's more going on than just your approach! If you read the airport info you will notice noise abatement, heavy bird presence and the mountain thermals mentioned. 

More important than that is that r18 is preferred landing and r36 takeoff. To avoid planes flying right at each other the only SID for ktvl takes your plane on a left turn after takeoff and underneath a jetway that passes high over the lake. The left turn is to avoid your approach that is routed close to the shoreline to keep you well clear of departures. 

37 minutes ago, anavsun said:

I'm not sure I understand why an LPV is not considered a precision approach. Is it only because it is satellite based and not ground based?

I'm curious if satellite-based or ILS approaches use the same satellites or systems in Europe and Asia.

An LPV is a precision approach.  But 737s are not certified for LPV (minus the Max which can do it with its MMR nav receiver, but which almost no operator uses.). The airline world by and large went the RNP route instead, before LPV was really a thing.  An RNP .10 approach yields basically the same minimums as an ILS or LPV.

But no need to get too down in the weeds.  Just know that you cannot use LPV minimums in your 737 NG.  Even if you fit the MMR, your operator almost certainly has not paid to get it added to your ops specs 😉.  So, you need to use LNAV / VNAV minimums. 

In reality, if an airline was serving this airport in 73s, the operator would likely build a non-public,  tailored approach that only they can use.  We have dozens of these, even excepting SE Alaska where we've built an entire network of them.  SAN, DCA, SBA, etc etc.