IRON MAN ARRIVAL at LAX : unable to follow the VNAV profile

[double-alpha]

And, btw, what are the consequences IRL, when a given waypoint is crossed above a max / bellow a min restriction?

If it's more than 200' it's a bust. Depending on the situation, you may be given a phone number to call when you get on the ground or you may get a letter of investigation in the mail from the FAA. Best to file a NASA report to CYA.

Jon

Fortunately my messy approach happened with PSX, otherwise I would have been sent directly to prison after landing

[double-alpha]

Well,

I'll keep trying, but I think it is impossible to follow with the proposed SITU ( ? ) at least in our PSX 744 ...

Even if you follow Hardy or Mariano 's advice??

[emerydc8]

Fortunately my messy approach happened with PSX, otherwise I would have been sent directly to prison after landing

It couldn't have been any worse than the training I just got in the sim. Imagine another check airman being told to do his best to crash the sim just to see how far you'll let him go down that road. That's when you hear all the horror stories about what prior students have tried to do on OE. I'm sure Peter has a few stories.

[cagarini]

Well,

I'll keep trying, but I think it is impossible to follow with the proposed SITU ( ? ) at least in our PSX 744 ...

Even if you follow Hardy or Mariano 's advice??

No, just using default VNAV + Speedbrakes + manual thrust, with the initial original fix constraints. Still have to give their suggestions a try :-)

[cavaricooper]

Towards self-education... is the 280 at MUPTT a mandatory speed or a maximum speed limit?  Using the SITU and immediately entering a 240 VNAV DES made things much more manageable, but I think I know the answer. 

I suppose the issue is traffic stacking up behind with closure rates that tend to upset the apple cart when one dwaddles on the arrival.  Minimum separation would go all amok...

Interesting scenario, and one that highlights the supremely educational component of PSX- ta!

C

[double-alpha]

You have to be AT 280 kt at MUPTT, not less (otherwise you would read 280 MAX).

Despite these speed constraints in the STAR , ATC sometimes request us to reduce speed (to manage the flow) and/or give us a heading to go off the track for a while

[double-alpha]

Fortunately my messy approach happened with PSX, otherwise I would have been sent directly to prison after landing

It couldn't have been any worse than the training I just got in the sim to be a line check. Imagine another check airman being told to do his best to crash the sim just to see how far you'll let him go down that road. That's when you hear all the horror stories about what prior students have tried to do on OE. I'm sure Peter has a few stories.

I guess ... a real nightmare 

[Mariano]

Adrien,

At least in this case, with PSX, and with the current atmospheric and anti-ice settings, if you edit CROWLY to be crossed AT FL260, the rest of the arrival should go well (since all the other "between" constraints will be met toward their lower limits). Just edit CROWLY and not the others.

Let me know how it turned out for you (switch to QNH exactly at FL180 and not a second  later! ;-)

You might want to type a fifty-knot tailwind in DES FORECAST and add anti-ice on altitude and see if you can do the arrival without editing CROWLY (you must do this early enough to allow the FMC to move the T/D backwards).

Regards,

Mariano

[Panos Bilios]

Mariano,

I would like to report that your recomendation worked fine for me(  CROWY AT FL260)
it flew the rest of the star without any problems


Regards
Panos

[Hardy Heinlin]

Or, shortly after T/D, engage VNAV SPD by opening the MCP SPD window (for 280 kt). Set idle thrust, extend the speed brakes by 50%. Ignore the path deviation indicator. Altitude constraint protection remains active. After RUNNN close the MCP SPD window to continue with speed constraint protection.

By the way, all FMC speed constraints and restrictions are always coded as at-or-below speed limits. E.g. when approaching a speed constraint or restriction of 280 while the current SEL SPD is 270, the FMC will not increase the SEL SPD to 280. The FMC will only decrease the SEL SPD -- and that only when the current SEL SPD exceeds the limit.


Regards,

|-|ardy

[double-alpha]

Or, shortly after T/D, engage VNAV SPD by opening the MCP SPD window (for 280 kt). Set idle thrust, extend the speed brakes by 50%. Ignore the path deviation indicator. Altitude constraint protection remains active. After RUNNN close the MCP SPD window to continue with speed constraint protection.

By the way, all FMC speed constraints and restrictions are always coded as at-or-below speed limits. E.g. when approaching a speed constraint or restriction of 280 while the current SEL SPD is 270, the FMC will not increase the SEL SPD to 280. The FMC will only decrease the SEL SPD -- and that only when the current SEL SPD exceeds the limit.


Regards,

|-|ardy

In that case, you set SEL SPD to 270 instead of the restriction of 280 because of ATC request, right ?

[double-alpha]

Adrien,

At least in this case, with PSX, and with the current atmospheric and anti-ice settings, if you edit CROWLY to be crossed AT FL260, the rest of the arrival should go well (since all the other "between" constraints will be met toward their lower limits). Just edit CROWLY and not the others.

Let me know how it turned out for you (switch to QNH exactly at FL180 and not a second  later! ;-)

You might want to type a fifty-knot tailwind in DES FORECAST and add anti-ice on altitude and see if you can do the arrival without editing CROWLY (you must do this early enough to allow the FMC to move the T/D backwards).

Regards,

Mariano

Hi Mariano, it is OK for me , thank you

Regards

[cagarini]

Ok, all OK here too :-)

Used HH's technique... Speed intervention ( 280K ) from DOUIT coupled with FLCH, then back to VNAV at RUNN, with today's weather.

Still had to revert into VNAV here & there to avoid overshooting some alt constraints.

[Jeroen Hoppenbrouwers]

https://www.youtube.com/watch?v=hQ189a3Wnk4

:-)

[cagarini]

https://www.youtube.com/watch?v=hQ189a3Wnk4

:-)

And... HH could start working on he's new Project SSX !!!

[United744]

I can't believe VNAV would be any less capable than FL CH IDLE descent.

As long as the aircraft is below clean idle descent path VNAV should surely be able to make it?

I've always found VDEV to be completely unreliable, and I can't believe the real thing behaves that way.

I wish I could make videos because I've seen and repeated situations in PSX where VDEV has actually increased its deviation despite improving adherence to the required path. It seems the point of the VDEV prediction moves through space, or that the VDEV computed angle changes, which just isn't possible.

[Hardy Heinlin]

As long as the aircraft is below clean idle descent path VNAV should surely be able to make it?

Yes, but this is the opposite of a shallow STAR with clean config below path, where you could add thrust to stay on the path. This STAR requires drag without which the aircraft will be above the path.

The real FMCs (as reported here the Airbus and 767 FMCs included) compute clean paths, not drag paths. They don't compute a specific speedbrake extension nor any specific phantom waypoints for speedbrake extension/retraction. It just expects clean idle, and when it deviates too much, it just triggers a DRAG REQUIRED message.

Regarding altitude constraint protection; let me requote my suggestion from yesterday:

"Or, shortly after T/D, engage VNAV SPD by opening the MCP SPD window (for 280 kt). Set idle thrust, extend the speed brakes by 50%. Ignore the path deviation indicator. Altitude constraint protection remains active. After RUNNN close the MCP SPD window to continue with speed constraint protection."


|-|ardy


United744, when you are suddenly below the path during approach, that is the segment when the FMC switches to the final approach path which is typically 3°. You may have been on level flight or on a shallower path before. When it switches to 3° or whatever, the deviation indicator behaves like a glideslope indicator: The needle is coming down and you can anticipate the capture point.

[Hardy Heinlin]

Hope this sketch illustrates the problem better. The FMC in PSX needs to switch the path reference several times when drag is required to get through a constraint tunnel. I don't know in detail how the real FMCs switch their targets in such scenarios. Fact is: The target is always either the clean path or a constraint collision, whichever occurs first (from the aircraft's perspective); and the clean path is always computed backwards. The aircraft may experience several reference changes when going above a B-constraint, then going below it, then being above the clean path:

[Hardy Heinlin]

I just increased the tolerance of my AB-window intercept checker by 1° on either side, and guess what? I can fly all the way from T/D down to RUNNN to 17000 ft without any path deviation and without speedbrakes. This is because DAHJR (6000 ft) remains the target from the T/D on. No target switching.

That's another example of how a tiny change can cause a giant leap.

I don't know yet what side effects this "tiny change" will have on other STAR scenarios though.


|-|ardy

[Jeroen Hoppenbrouwers]

There must be rules out there for the approach designers. They cannot just throw airplanes around at will, there definitely will be restrictions. However I don't know where to start looking for them. Much seems to be too close to the ground, so to say.

https://www.faa.gov/airports/engineering/

Hoppie