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

[emerydc8]

On virtually every one of these arrivals (multiple above/below fixes with multiple speed restrictions), on the 767, we are having to pull the speed brakes out because VNAV leaves us high. So, in their infinite wisdom, the engineers and plate designers, who probably don't even talk to each other, have taken something that was sold to the airlines as a fuel-saving feature and turned it into a fuel waster.

Speed brakes on descent used to only be for pilots' mistakes and emergency situations. Now it's a regular thing on almost every flight -- almost a primary flight control. Yes, you could use FLCH or V/S, but the workload increases dramatically trying to ensure that you thread the needle with all the altitude and speed constraints, then deal with how you're going to slow to 250 by 10,000 (V/S?). I think they've gone way over the top on these arrivals. Maybe the Airbus can deal with them better, but in the nearly 1000 hours I've flown the 767 now, I can count on one hand the number of times it has been able to make every speed constraint on a complex arrival like this.

[Magoo]

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?

With the next Gen FMC, surprisingly often!

[double-alpha]

On virtually every one of these arrivals (multiple above/below fixes with multiple speed restrictions), on the 767, we are having to pull the speed brakes out because VNAV leaves us high. So, in their infinite wisdom, the engineers and plate designers, who probably don't even talk to each other, have taken something that was sold to the airlines as a fuel-saving feature and turned it into a fuel waster.

Speed brakes on descent used to only be for pilots' mistakes and emergency situations. Now it's a regular thing on almost every flight -- almost a primary flight control. Yes, you could use FLCH or V/S, but the workload increases dramatically trying to ensure that you thread the needle with all the altitude and speed constraints, then deal with how you're going to slow to 250 by 10,000 (V/S?). I think they've gone way over the top on these arrivals. Maybe the Airbus can deal with them better, but in the nearly 1000 hours I've flown the 767 now, I can count on one hand the number of times it has been able to make every speed constraint on a complex arrival like this.

Thank you emerydc8 for your feedback.

I agree with you.
IMO, Airbus 340 doen't seem to deal better than the 767...
IRON MAN arrival in VNAV mode is impossible without speedbrake, and I often have to use OP DES mode (similar to FLCH) or VS to adjust the rate of descent when the aircraft goes above the desired profile (FMGS alone is  not able to manage a such complex descent...)

Are you ready to fly without a human Pilot? hehehe

[Hardy Heinlin]

P.S.:

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

By "particular" I mean idle descent paths through altitude constraint tunnels that can only be flown with speedbrakes extended. The FMC's idle path calculation always implies clean configurations. It doesn't even provide dynamic waypoints on the ND to indicate speedbrake extension/retraction points. It only provides speed limit deceleration points for this matter (and flap speed points in the NG FMC).


|-|ardy

[Mariano]

Will,

Speaking from a 767 perspective only, it is rare that we have to intervene with FLCH. The few times I have done so were after very late descents by ATC or ATC requesting we make a tight, non-depicted restriction (i.e. cross eighty miles north of ABCDE at FL###"). In most of these cases, VNAV SPD with speedbrakes catches the path in time. If needed, speed intervenention with speedbrakes helps a lot as well. I only use FLCH to fix these clearances while VNAV computes the new path (sometimes VNAV SPD is quite gentle at reducing thrust and pitching down, wasting precious distance).

When a fix has a hard altitude crossing restriction AND a speed restriction, the deceleration segment might sometimes slightly mess up VNAV (it will stay on path but might not make the speed restriction). I use ten knots per mile as a rule of thumb (767) and get ready to deploy speedbrakes once inside the milage threshold (speed trend vector is very useful when trying to determine if speed restriction will be met and how much speedbrake input is needed, if any).

Regards,

Mariano

[double-alpha]

Will,

Speaking from a 767 perspective only, it is rare that we have to intervene with FLCH. The few times I have done so were after very late descents by ATC or ATC requesting we make a tight, non-depicted restriction (i.e. cross eighty miles north of ABCDE at FL###"). In most of these cases, VNAV SPD with speedbrakes catches the path in time. If needed, speed intervenention with speedbrakes helps a lot as well. I only use FLCH to fix these clearances while VNAV computes the new path (sometimes VNAV SPD is quite gentle at reducing thrust and pitching down, wasting precious distance).

When a fix has a hard altitude crossing restriction AND a speed restriction, the deceleration segment might sometimes slightly mess up VNAV (it will stay on path but might not make the speed restriction). I use ten knots per mile as a rule of thumb (767) and get ready to deploy speedbrakes once inside the milage threshold (speed trend vector is very useful when trying to determine if speed restriction will be met and how much speedbrake input is needed, if any).

Regards,

Mariano

Hello Mariano,

If you have time, could you try the iron man arrival and let me know if you succeed without using FLCH mode? And if you think PSX behavior  would be similar to your 767 (even if 747 is not a 767)?

Regards

[torrence]

This has been an interesting discussion for me (not real world pilot).  I kept wondering why I had to keep using the speed brakes on various STARS and assumed it was my fault (or the PSX ATC generated commands).  Makes me feel a little bit better that this is tough in real world also 

Cheers,
Torrence

[Mariano]

Double-alpha,

I wouldn't mind at all. Do you perhaps have the .situ just before starting the arrival you could share with me?

Torrence,

We use speedbrakes in EVERY sector. I can't recall not using them. Sometimes with VNAV engaged and sometimes while maneuvering in the terminal area positioning for the approach.

Best regards,

Mariano

[double-alpha]

@ Mariano,

Thank you if you can try.

Here is my .SITU :

https://www.dropbox.com/s/7nkkpjdrzcf1x42/IRMNM1.situ?dl=0

FYI :
FMC ROUTE = IRNMN1 STAR followed by an ILS 24R
you are about 6 NM from top of descent
GW = 256 T
Wind calm at KLAX
qnh 30.18

MCP ALT knob is already set to 6000 ft (final altitude at DAHJR)

I use VNAV mode only and I extend full speedbrake when vertical path deviation indicator is below the descent profile (I only rely on this indicator : I do not take into account my own rate of descent calculations, so I do not use FLCH or V/S mode)
I do not use speed intervenention

I missed MDOTS (1000 ft high) with full speedbrake
BIKNG (1000 ft high) although vertical path deviation indicator shows me too low on the decent profile then too high a few miles before BIKNG
RUNNN (1000 ft high) with full SPD BRK after BIKING
IRNMN : OK
SYMON : 500 FT HIGH
The rest of the STAR is OK until DAHJR at 6000 FT

Best Regards

[Mariano]

Thanks, Double-alpha. Perfect setup.

Will try it tonight.

Best regards,

Mariano

[double-alpha]

No worries , take your time...

[Dirk Schepmann]

Interesting, I always thought that the mountain arrivals into KLAX from the east (e.g. BASET) are already tough. Seems I have to try something new. 

[Jeroen Hoppenbrouwers]

Flew (in the back) on a A321 MIA-LAX and a B777 LAX-MIA last week.

Both times, the approach was basically straight-in. Not more than 5 degrees correction required at any one time after leaving cruise alt.

Both times, ATC saw us coming from 1000 nm out.

There was some limited speed correction done, but once the engines came back, both aircraft descended around 1250 ft/min for a bit and then, out came the boards. And in. And out. And in. And out. And in, until 2000 ft and then it was final approach speed.

AA has a reasonable IFE on both types so I am pretty sure about the figures.


Hoppie

[emerydc8]

On the 767, I've just accepted the fact that speedbrake usage on these arrivals is the easiest and safest way to deal with making the speeds. VNAV will usually stay in path but it just can't cope with all the speed restrictions without help.

Going into SFO on the SERFR 3 Arrival to 28R, if you cross EDDYY at 6000 and 240K (as published), they'll clear you for the approach after EDDYY and it's a slam-dunk to make AXMUL (FF) at 1800 and 180 knots. You have to lose 4200' in 14.6 miles and you are starting at 240 knots at EDDYY. Even sitting at Flaps 1 at EDDYY (our flaps 5 max placard speed is 230), I have found that the gear is the best way to get down, because if you wait to slow to your flaps 5 speed after EDDYY, you will be really high at that point. And if you pull the speedbrakes out at max landing weight as you are making the turn to intercept the 28R localizer, you're going to be well into the amber band in a 30-degree bank. It makes me wonder about whether the creator of this jack-up arrival has any clue as to what the 767 can and cannot do. At least on the 744 you could be at flaps 5 crossing EDDYY (flaps 5 max placard is 260). I haven't tried this arrival in PSX but it would be interesting to see how it does.

[Mariano]

Double-alpha,

I tried your .situ, initially without editing anything in the CDU. I experienced the same thing you (and Hardy) did (descent forecast was accurate).

What I noticed is that VNAV crossed CROWY (first fix) at FL290 even though CDU had FL290B (chart says FL290BF260A). So it made the restriction, but on the upper end of the envelope. After crossing CROWY at FL290, the rest of the STAR became a bit discombobulated VNAV-wise. I did then edit FMC for CROWY at FL290BFL260A but it again crossed it on the upper end.

Next, I edited CROWY to be crossed at FL260 (hard), which is the lower end of the envelope. Because the .situ begins at 6 nm. from TOD, by the time VNAV recalculated the path, I needed a bit of speedbrakes (initially) to make CROWY at FL260. If you run this flight again and program CROWY at FL260 (hard) a bit sooner, there should be no need for speedbrakes at all.

Once I edited CROWY for FL260, I managed to fly the whole arrival without speedbrakes, even with the high QNH. I could have used them a bit after setting QNH, but left it alone to test. Speed deviated just shy of ten knots and all went back to normal soon after. I left flaps up until crossing JUSSE, in order to make CLIFY at 210 kt. (with flaps 1). All continued well thenceforth.

A nice technique when the first arrival fix has a range of crossing altitudes is to set both in the fix pages and see where the FMC will actually cross the fix. If you see that it will be on the high end, you might want to force it into the lower (lowest) end in order to start the descent ahead of the curve (in this case, 3000 ft. ahead, which is considerable).

Best regards,

Mariano

[cagarini]

I'm smarter :-)

Disengaged A/T, and ... well.. almost always between full and half speedbrake, and a bit bellow the scheduled speeds... ( is that ok IRL, i.e.: where I should be at 280 KIAS I was sometimes at 265... )?

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

I still was "above" around 900' at one of the waypoints ( can't recall which one exactly ).

[emerydc8]

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

[Jeroen Hoppenbrouwers]

is that ok IRL, i.e.: where I should be at 280 KIAS I was sometimes at 265...

Usually there's an aircraft right behind you doing 280 so usually it leads to yelling. Airports like KSFO are truly maxed out on the runway capacity. With departures wedged in between arrivals, even on parallel runways, they try to be very precise in separation of the arrivals. Any sloppyness will mean less room for inevitable inaccuracies further down the line.


Hoppie

[cagarini]

Well,

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

[double-alpha]

Double-alpha,

I tried your .situ, initially without editing anything in the CDU. I experienced the same thing you (and Hardy) did (descent forecast was accurate).

What I noticed is that VNAV crossed CROWY (first fix) at FL290 even though CDU had FL290B (chart says FL290BF260A). So it made the restriction, but on the upper end of the envelope. After crossing CROWY at FL290, the rest of the STAR became a bit discombobulated VNAV-wise. I did then edit FMC for CROWY at FL290BFL260A but it again crossed it on the upper end.

Next, I edited CROWY to be crossed at FL260 (hard), which is the lower end of the envelope. Because the .situ begins at 6 nm. from TOD, by the time VNAV recalculated the path, I needed a bit of speedbrakes (initially) to make CROWY at FL260. If you run this flight again and program CROWY at FL260 (hard) a bit sooner, there should be no need for speedbrakes at all.

Once I edited CROWY for FL260, I managed to fly the whole arrival without speedbrakes, even with the high QNH. I could have used them a bit after setting QNH, but left it alone to test. Speed deviated just shy of ten knots and all went back to normal soon after. I left flaps up until crossing JUSSE, in order to make CLIFY at 210 kt. (with flaps 1). All continued well thenceforth.

A nice technique when the first arrival fix has a range of crossing altitudes is to set both in the fix pages and see where the FMC will actually cross the fix. If you see that it will be on the high end, you might want to force it into the lower (lowest) end in order to start the descent ahead of the curve (in this case, 3000 ft. ahead, which is considerable).

Best regards,

Mariano

Thank you Mariano for your test.

So, to comply with this STAR in VNAV mode you need to delete the upper constraint.
Boeing is not able to manage his own descent without intervention on the FMC ; Airbus seems to be a bit better at this game...

Thank you for the tips.

Adrien