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

[double-alpha]

Hi,

I've tried several times to perform the IRMN1 STAR followed by an ILS 24R approach at the maximum landing weight (about 260 T) at KLAX airport.

It is a tricky descent in real life for the A340 FMGS when the aircraft is heavy (there are speed constraints, and many window altitudes) , but it is doable (with a careful monitoring of the FMGS and use of speedbrakes)

However, with PSX I am unable to perform an "acceptable" descent ... Even with full speedbrakes, I miss a lot of waypoints (1000 ft high ...) .
The vertical path deviation indicator (deviation from the planned FMC descent profile) seems to be unreliable.
Maybe I do something wrong with this approach ... Have you ever tried it ?

Thank you and good flights

[Hardy Heinlin]

Which transitions?

[double-alpha]

REBRG (I disregard CRCUS and SEAVU transition)

By the way, it is a lovely approach over LAX downtown

[Hardy Heinlin]

CRZ ALT?

GW?

Wind?

Anti-ice?

Baro pressure?

Between BIKNG and RUNNN there are only 7 nm and the STD/QNH transition. If the QNH is high the altitude indication will jump up.

[double-alpha]

REBRG (I disregard CRCUS and SEAVU transition)

CRZ ALT = FL 370

GW = 260 000 kg at the Top of Descent

Winds at KLAX = 290/3 kt (today METAR 1943Z)
Winds during descent variable but less than 25 kt

Anti-Ice = OFF

QNH at LAX = 30.14 IN

[Hardy Heinlin]

OK, I'm having the T/D ca. 3 nm before DOUIT. Does this agree with your test?

[double-alpha]

YES My TOD was approximately a few nm before DOUIT

[double-alpha]

Between BIKNG and RUNNN there are only 7 nm and the STD/QNH transition. If the QNH is high the altitude indication will jump up.

I remember to have crossed GRIPR at FL240 (with full speedbrakes / 1000 ft high) and yes I also missed BIKNG and RUNNN

[Hardy Heinlin]

What's your DES speed?

[double-alpha]

Don't remember after the TOD (cost index 100), but I'm sure my speed was 280 kt (SPD constraint) AT and after MUPPT (until the next SPD constraint at SYMON 250 kt)

[Hardy Heinlin]

To hit RUNNN at 17000 I need to set max speedbrakes 3 nm before BIKNG.

With FLCH at idle it flies nicely through the constraint window tunnel. Unfortunately, the VNAV path prediction needs to get recalculated every few seconds because the predicted path constantly touches any A or B constraint, and then has to chose a different target fix and a different path angle.

[double-alpha]

Thank you Hardy, I m going to  try a new attemp.

I also add a Vapp speed constraint at JETSA (Vapp 145 kt  if I am not mistaken)  2200 ft.

Just before the TOD, I set the MCP ALT knob to the final STAR altitude (6000 ft at DAHJR)

I let you know if I succeed

[double-alpha]

To hit RUNNN at 17000 I need to set max speedbrakes 3 nm before BIKNG.

With FLCH at idle it flies nicely through the constraint window tunnel. Unfortunately, the VNAV path prediction needs to get recalculated every few seconds because the predicted path constantly touches any A or B constraint, and then has to chose a different target fix and a different path angle.

Just to be clear : you use FLCH for your descent and then VNAV ? or FLCH all the way to GADDO ?
Because I use mainly the VNAV mode, and set speedbrakes according to the vertical path deviation indicator in case it sees the aircraft too high above the descent path.

I haven't tried yet the FLCH mode for this approach, but I guess I will be able to manage the descent. Airbus has a "FLCH like" mode (OPEN DES) but I use mainly VNAV mode during the descent via a STAR (because OPEN DES deletes the ALT constraints)

[Hardy Heinlin]

I was using FLCH and ignored the VNAV path predictions. I always set the green altitude range arc on the ND onto the next waypoint and checked the MCP ALT indication if it lies within the constraint window at that waypoint. So I could see in advance whether I will get through the RUNNN window.

Somewhere after RUNNN I engaged VNAV.

[double-alpha]

Unfortunately, the VNAV path prediction needs to get recalculated every few seconds because the predicted path constantly touches any A or B constraint, and then has to chose a different target fix and a different path angle.

I still have a few questions (sorry...) :

Is this the reason why you use VNAV only after RUNNN ?

Is this a VNAV PSX behaviour or is it similar to the real 747 ?

I succeed with the use of FLCH like you do, but I still cannot perform the whole STAR with the use of VNAV mode only (like I would do with an Airbus)

[Hardy Heinlin]

1) Yes.

2) In PSX. I don't know how the real 744 FMC behaves on this particular STAR.

[double-alpha]

Thank you Hardy for all

Best regards

[Mariano]

Double Alpha,

If on a complex Descend Via STAR into a busy terminal area, I always set the next two fixes most restrictive crossing altitudes in the FIX pages PRED ALT lines. This will enable to see where the the FMC predicts reaching that altitude in regards to the respective fix. It is not as accurate as the green arc (sometimes slow to move), but if your green arc is way down at the STAR bottom altitude, it helps to stay ahead of any path non-ccompliance issues.

Also, if QNH is high, as soon as you pass transition level, start deploying spreedbrakes gradually to stay ahead of the imminent path deviation.

Best regards,

Mariano

[double-alpha]

Hi Mariano, thank you for your reply

[Will]

So then, real world pilots, how often do you find yourself needing to use FLCH to descend, for the reason that VNAV doesn't get you down fast enough?