1 hour ago, anavsun said:

12 hours ago, SAS443 said:

A quick glance on the chart's lower left corner reveals what design criterias is being used (TERPS/PANS-OPS etc), if OP wants to know.

 

Thank you. Yes, I'd like to know. Looking at the lower left corner of the chart, how does one know if TERPS or PANS-OPS is used? How are they shown differently in the chart?

@SAS443's comment relates to Navigraph (Jeppesen) charts, which show this information. You have an FAA chart, which doesn't show this information - presumably because all FAA charts are for approaches that are designed to TERPS rules.

Edited February 16, 20251 yr by martinboehme

10 hours ago, Stearmandriver said:

The final approach course is defined on the approach chart.  If it differs from the runway heading, then it's offset.  But you don't do anything to make that happen; in fact, it's forbidden to modify courses or altitudes on an approach procedure inside of the final approach point.  The correct course will be loaded in your FMC when you load the procedure.  (As an aside, while setting the MCP course window to final approach course can be a useful SA tool, it is irrelevant from a technical standpoint on an RNAV approach flown in LNAV / VNAV.  The plane doesn't use the value in the course window at all in this case.)

If the pilot doesn't do anything to make an offset happen, how exactly does an "offset" happen then?  Is it automatically computed by the FMC?

If all the pilot has to do is aim the aircraft at the IAF, and "it's forbidden to modify courses or altitudes on an approach procedure inside of the final approach point" then doesn't the aircraft in effect auto-landing itself?

In landing on an IAN approach (non-ILS) mode, I've seen the FMA change from "LNAV - VNAV Alt" to "LNAV - VNAV Pth", and to "FAC G/P". I've learned that the FMA is what the aircraft is actually doing or intends to do. If an offset is happening, would these messages or alerts change in the FMA?

10 hours ago, Stearmandriver said:

But you don't do anything to make that happen; in fact, it's forbidden to modify courses or altitudes on an approach procedure inside of the final approach point. 

 

24 minutes ago, martinboehme said:

The only thing that's different is that when you get close to the runway, you won't be lined up with the centerline, so you need to maneuver manually to get onto the centerline and land.

Apologies in advance, but the two statements in bold above muddy the water for me (maybe it's just me?).

16 minutes ago, anavsun said:

If the pilot doesn't do anything to make an offset happen, how exactly does an "offset" happen then?  Is it automatically computed by the FMC?

You're overthinking this.

The FMC contains an RNAV approach with a final approach course of 172. It provides guidance to fly the aircraft along this final approach course.

It just so happens that the runway at the end of the approach doesn't have a runway heading of 172, but 177. (Edit: This could be called a Cardinal error. 😉 Sorry, could not resist.)

How did the offset happen? Simply because the course the procedure designer put on the chart (172 degrees) isn't the same as the runway heading (177 degrees).

There's nothing the pilot or FMC does to "make the offset happen". The person who designed the approach made the offset happen. The FMC doesn't do anything special to provide course guidance on the approach. The pilot doesn't do anything special to set up the FMC for the approach or to fly the approach -- except that at the point where the pilot visually acquires the runway, they will need to manually fly themselves onto the centerline.

Edited February 16, 20251 yr by martinboehme

Just now, anavsun said:

10 hours ago, Stearmandriver said:

But you don't do anything to make that happen; in fact, it's forbidden to modify courses or altitudes on an approach procedure inside of the final approach point. 

 

29 minutes ago, martinboehme said:

The only thing that's different is that when you get close to the runway, you won't be lined up with the centerline, so you need to maneuver manually to get onto the centerline and land.

Apologies in advance, but the two statements in bold above muddy the water for me (maybe it's just me?).

It's forbidden to modify the courses or altitudes that are programmed into the FMC (after the final approach point). But once you have visually acquired the runway, you can then maneuver visually and deviate from the final approach course to put yourself on the centerline of the runway and complete the landing.

(The last part of every landing that's not an autoland is flown visually. This is why you have to have the runway environment in sight at the missed approach point / decision altitude -- otherwise, you have to go around.)

Sometimes the safest way to approach an airport is at an angle to the runway. This is called the "offset." It's established when the approach is designed by the people who publish the procedures. If an offset is necessary, you can see it on the chart by comparing the final approach course to the runway heading. Your chart shows the final approach course being 172 degrees. The runway itself has a heading of 177 degrees (which you can get from the airport diagram chart). Thus the offset is 177-172 = 5 degrees. 

You must stay on the published course (the 172 degree line on the approach plate) until you cross SAKYY and have the airport in sight. At that point, you turn to align your aircraft to the runway (177 degrees).

The word "offset" here just describes the fact that the approach isn't pointed directly at the runway, it's 5 degrees off. And the reason for the offset, is that terrain requires it. There is no pilot choice here; the approach is what it is, you always fly the offset course so that you don't get too close to the mountains to the east.

1 minute ago, martinboehme said:

How did the offset happen? Simply because the course the procedure designer put on the chart (172 degrees) isn't the same as the runway heading (177 degrees).

I'm certain the procedure designer and the FAA are rational people. One thing I've learned from you and @Stearmandriver (which I highly appreciate) is that there is almost always nothing simple about flying a complex airliner. Why then would the procedure designer and the FAA use wrong info and use 172 instead of 177? There has to be an explanation and that's what I'm trying to figure out. 

4 minutes ago, anavsun said:

Why then would the procedure designer and the FAA use wrong info and use 172 instead of 177?

It's not "wrong"; the offset is needed so that the final approach course doesn't get too close to the mountains to the east. 

Just now, anavsun said:

I'm certain the procedure designer and the FAA are rational people. One thing I've learned from you and @Stearmandriver (which I highly appreciate) is that there is almost always nothing simple about flying a complex airliner. Why then would the procedure designer and the FAA use wrong info and use 172 instead of 177? 

Because if they had used a final approach course of 177, the final approach would lead through a mountain, and the airport authority was too cheap to have a tunnel dug through the mountain. (I'm obviously joking on the last part. 😉)

Obviously, the preference is for the final approach to be aligned exactly with the runway, but sometimes terrain simply makes that impossible. In those cases, the final approach has to be at a slight angle to the runway.

A famous example of this is the Kai Tak airport in Hong Kong (now closed), where the final approach to runway 13 was offset from the runway by 40 degrees IIRC. Aircraft had to make a low-level turn close to the ground to align themselves with the runway. Here's an example:

Hopefully it's obvious why it wouldn't have worked to align the final approach with the runway.

 

7 minutes ago, prolixindec said:

You must stay on the published course (the 172 degree line on the approach plate) until you cross SAKYY and have the airport in sight. At that point, you turn to align your aircraft to the runway (177 degrees).

The word "offset" here just describes the fact that the approach isn't pointed directly at the runway, it's 5 degrees off. And the reason for the offset, is that terrain requires it.

But the terrain is over a lake and flat with no obstacles in sight.

3 minutes ago, anavsun said:

But the terrain is over a lake and flat with no obstacles in sight.

Take a look at the terrain to the east of the lake. The mountains rise to over 9000 feet. If the final approach course was exactly aligned with the runway (177 degrees), that would put the final approach dangerously close to the mountains, if not through a mountain.

Also, the protected airspace is not just a line, it's a volume. That's because there are inaccuracies in the real world navigation equipment: in the aircraft, in the navigation aids on the ground, and in space. (Even GPS has some level of inaccuracy.) When you add up the maximum error from all of these sources, your aircraft still has to miss the mountains, even in the worst case scenario. Therefore the protected airspace is a volume, with the course to be flown going right down the middle of it. To maintain that protected volume, the person who designed this approach set the final approach course to be 172 degrees. That keeps the eastern edge of the protected volume from clashing with the high terrain.

Edited February 16, 20251 yr by prolixindec

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).

29 minutes ago, anavsun said:

 

Apologies in advance, but the two statements in bold above muddy the water for me (maybe it's just me?).

Remember in the last thread, we said that almost no landings in reality are autolands.  The pilot is manually flying almost every landing in the airplane.  The purpose of almost any instrument approach is to put the airplane in a position from which this can be done.  It is understood that the pilot will be disengaging the automation and being a pilot at minimums if not sooner. 

On this approach, the final approach course is "offset" (which is not the same thing as a lateral offset in the FMC, it just means the approach course is not aligned with the runway) to meet the required terrain clearances specified by TERPS.  A straight in final would put the aircraft closer to terrain then allowed. 

Difference types of approaches would provide different tools to accomplish that terrain clearance, but on a simple RNAV this is the easiest way.  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.

Autopilots - there's no case other than an autoland where you'll engage both autopilots.  In fact, it wouldn't work if you tried, it'll just swap the active autopilot from one to the other.

Quote

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?

 

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.

Edited February 16, 20251 yr by prolixindec