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"On-Approach" Logic on STAR?

Started by emerydc8, Sat, 1 Aug 2015 10:56



I was playing around with the SIERA 6C arrival for the ILS 07L into HKG today and noticed that on the descent, the aircraft is going into on-approach logic at SIERA, which is almost 30 miles from HKG and is the initial waypoint on the STAR. It should not be going into on-approach logic until sequencing the course fix for 07L (CI07L).

I can tell that it's going into on-approach logic on the descent right at SIERA because I kept speed intervening and watching it go from | HOLD | LNAV | VNAV PTH | to | HOLD | LNAV | VNAV SPD | when I speed intervened. As soon as it cycled past SIERA, it stayed in VNAV PTH when I speed intervened and the autothrottle went straight to | SPD | even though it was on the path. This is consistent with on-approach logic.

As far as the descent profile itself goes, it made the altitudes and speed restrictions perfectly, but the FMA's were not accurate here:

Regardless of whether you are in on-approach logic or not, if you are doing a VNAV  PTH descent and you get more than 150' away from the path, it will drop out of VNAV PTH and into VNAV SPD. If the autothrottles are active, VNAV should not let you get more than 150' below the path. It will add power to keep you on the path.

But if you are descending in VNAV, in on-approach logic, and you get above the path by more than 150' (as happened on this arrival), you should see | IDLE/HOLD | VNAV SPD | as soon as the aircraft gets more than 150' high. It will not pitch down and increase speed to get back to the path once in VNAV SPD. If you catch the path at idle or use speed brakes, that's fine, but once the airplane gets kicked into VNAV SPD (more than 150' high), it's not going to increase airspeed to catch the path. If you do manage to get it back down to the path, it will shift from VNAV SPD back to VNAV PTH when within 150' of the path. Note that if you are not in on-approach logic, the speed window must be closed for this to happen (you cannot descend in VNAV PTH with the speed window open unless you are in on-approach logic).

This is a strange arrival and I can see it would be difficult for the FMC to get confused. At one point, between SIERA and BORDA, you are actually flying away from the airport. During this situ, I kept PROGS 2/3 open to get a digital reading of VTK Error. There were a number of times that it got well past 200' high, but never dropped into VNAV SPD.

So, one issue is that it's going into on-approach logic early. The other is that it is not shifting to VNAV SPD when more than 150' above the path.

I made a situ file for anyone interested in dropping it into your situation file folder and flying it for yourself. It starts with the motion frozen so you can assess everything before cutting it loose.

I have done this arrival quite a few times in the real airplane and it never went into on-approach logic on the STAR. I don't think the -400 would go to on-approach logic even if the STAR was coded with a gradient path the whole way down, but this is not the case on the SIERA 6C arrival anyway.

I would be interested in any thoughts on these issues.

BTW, here's a copy of the SIERA 7C arrival which is essentially the same as the SIERA 6C.

Jon D.

Hardy Heinlin


different pilots, different opinions. Unfortunately, I can't make everyone equally happy:




I confess I get completely lost reading this threads, but tonight I'll make my best to discuss it with my hosts, Jeroen and Cristina, inspired, so I hope, by good Portuguese food and drink, but..., allow me a question gentlemen...

Can this mode transitions be a function of the programmed STARS / SIDs, or is there a kernel of the Logic that is only up to the FMS to control ?

I ask based on the OP, and on HH's answer, pointing into a thread where another pilot ( ? ) describes apparently the opposite, and basis his observation on his company SOPs...

So, while I try to understand every bit of it ( good red wine will help latter today... ) I would really like to know if:

- Can this sort of Logic depend on the way the procedures are programmed ?


- it depends only on the FMS, and is invariable for the way STARS are programmed ?


Thanks, Hardy. I don't think S1ncr0 was talking about having it go into on-approach logic while on a STAR at 22,000'. I would be interested in input from others who have flown the airplane because this is definitely not what the airplane would do on this arrival.

The FMC transitions to "on approach" under the following conditions:

. . . .

• a published instrument approach has been selected and
incorporated in the active flight plan and the airplane has
sequenced the first waypoint on the published approach

A STAR is not a "published approach."

Jon D.

Hardy Heinlin

Then perhaps we're talking about two different things.

You call the usage of VNAV PTH at +150 feet above the path "on-approach logic"? I don't. It may be a function of the idle path descent mode. When the aircraft is high and the trend is such that it will recapture the path, PSX will keep VNAV PTH. Only when it tends to leave the path and cannot keep the target speed+15, PSX will change to VNAV SPD.

I never heard of a fixed 150 feet value for the mode change-over trigger. Is this a value you observed on the big sims?

PSX will switch to on-approach logic only when the active leg is coded as an approach leg. The code is different to that of a STAR leg.



No, being more than 150' above or below the path and being in on-approach logic are two different things.

The Boeing FMC Users Guide describes being above the path as more than 150'.
Quote(When the aircraft is greater than 150 ft higher than the path, it is considered to be above the path.)

In this case, if it was descending in VNAV PTH and got above the path, it would shift to | IDLE/HOLD | VNAV SPD|.

Just like when you don't reset MCP altitude at the top of descent, it will not stay in VNAV PTH, it will go to VNAV ALT because it is more than 150' above the path.

QuotePSX will switch to on-approach logic only when the active leg is coded as an approach leg. The code is different to that of a STAR leg.

Is SIERA coded as an "approach" leg? It is the first fix on a STAR -- not the ILS approach.

This is also published in the Atlas check airman publication:

 The VNAV descent path is based on altitude constraints, speed, and idle thrust. The path approximates a 3:1 ratio.
 VNAV PTH will convert to VNAV SPD if 150' above or below the programmed path.
 When the On-approach function is not active:
 The IAS/MACH window must be closed to remain in VNAV PTH. With the IAS/MACH window closed, the speed target is the FMC speed. The FMC speed is displayed on the VNAV DES page (ECON SPD or SEL SPD). Speed/altitude constraints are automatically complied with.
 The autothrottle is in the idle mode (IDLE).
 ATC speed changes are set on the VNAV DES page (SEL SPD) in order to remain in the VNAV PTH mode.

Hardy Heinlin

I'm now flying your situ, and just overflew SIERA. Now I see what you mean. This is not "on-approach" logic. "On-approach" would allow you to set the MCP altitude above the aircraft (for the missed approch preparation). You can't do this after SIERA. If you do, you will get CHECK ALT TGT and the aircraft will level off.

The effect after SIERA is intentional. PSX is in idle descent mode until the first altitude constraint after T/D (in this situ at SIERA). Thereafter the descent profile is typically shallower to connect the following altitude constraints together in straight path lines. Therefore PSX goes to SPD | | VNAV PTH, unless it's very high above the path. But in your scenario it's not high above the path, hence it remains in VNAV PTH and descents directly to the next constraint. This is not "on-approach" logic. This is STAR logic.

I don't understand why you expect it to change to VNAV SPD if it's on the path.




On the descent in VNAV (with the speed window closed), you should see IDLE/HOLD | | VNAV PTH if you are on path.

If you start getting low, you should see THR as the throttles come up, but it should still remain in VNAV PTH. Once it recovers its speed (it pitched up and slowed down to try to stay on path), it should go back to ILDE/HOLD. It should not go into SPD || VNAV PTH if power is needed to maintain the path on a descent.  

If you get high (more than 150' above the path), not only should the autothrottle FMA indicate IDLE/HOLD, but the pitch FMA should go from VNAV PTH to VNAV SPD.

In any case, you should never see SPD || VNAV PTH while descending on a STAR. I have never heard of STAR logic. Is this published somewhere?

Hardy Heinlin

Quote from: emerydc8In any case, you should never see SPD || VNAV PTH while descending on a STAR.
This is new to me. But I believe you until someone proves it may happen on a STAR as well :-)

Quote from: emerydc8I have never heard of STAR logic.
There's no such term. I just used this wording above to emphasize that this is not "on-approach" logic, and that PSX is not "on-approach" logic when on a STAR.


Okay, I hope to get some input on this from others. While it may not be in full-blown on-approach logic during that arrival, it is about half way there because you can speed intervene and it will stay in VNAV PTH. That's not supposed to happen unless you are in on-approach logic.

Hardy Heinlin

What if you have a level-off segment on the STAR, e.g. two consecutive "AT" constraints at FL150? Will you maintain FL150 with THR || VNAV PTH?


If you are level and flying the level segment, then you would see SPD || VNAV PTH. But you would not see that while "descending" on a STAR.

Let me do some more research on this today.


Okay, I think I was wrong about never seeing SPD || VNAV PTH on a descent. In the case of a flight plan modification or unknown winds, if you get 15 knots below the target speed while attempting to maintain the path the autothrottles will go to SPD in order to accelerate back to the FMC target speed, then back to IDLE/HOLD once it reaches that speed. It should not sit there in SPD || VNAV PTH once it regains the target speed. If the autothrottle is not active you will get a "THRUST REQUIRED" on the scratchpad.

In the case of this arrival, there were no unforecast winds (I uplinked the descent winds) and no flight plan modifications; and you are never close to being 15 knots below the FMC target speed, so you should not be seeing SPD in the autothrottle FMA. It should be descending at IDLE/HOLD. The reason I felt something was wrong here is that you should not be staring at SPD || VNAV PTH during the entire descent. That should just be transitory until you regain your speed, then back to IDLE/HOLD.

The only at-or-below restriction is at MURRY (FL130-FL110). All the rest of the altitdue restrictions are at-or-above FL130. Because the database did not have the SLP speeds in it, I had to enter at-or-above altitudes in order to assign the speed limits at SIERA (280) and BORDA (250). So, it should have been able to descend at idle the whole way. There were no level or shallow segments.

It would be appreciated if any -400 guys could try flying this situ and giving some input.

Hardy Heinlin

If it can't maintain the idle descent path at -10 knots with idle thrust, I think the mode should change to THR, not SPD.


That's what I initially thought too -- and maybe you are right. But from doing some research on it, I'm now leaning towards it going to SPD to accelerate back to the FMC target speed, then back to IDLE/HOLD.

I'm not sure why it can't do an idle descent all the way to MURRY on this. All the other altitudes are at-or-above. Crossing BORDA, ROCCA and CANTO at-or-above FL130 should not be restrictive.

Hardy Heinlin

Set all constraints before MURRY to an altitude lower than that at MURRY. MURRY should have the highest constraint AND it shouldn't be an altitude AB window but an exact AT constraint. Maybe this helps. I haven't tried it.


Hardy Heinlin

Just tested. It won't help.


Me too. It initially starts out approaching SIERA and says it's 7000 high. When I extend the speedbrakes and try to get down, I eventually do, but it's not until the segment between BORDA and ROCCA.

During this descent, the initial descent FMA is correctly in | HOLD | | VNAV SPD | because it's indicating more than 150' above the path.  Then, when I finally do get down to the path, it goes back to | SPD | | VNAV PTH | the rest of the way and the power is sitting at about 50% N1. So, it probably wasn't really 7000' high since it's taking a good amount of thrust once I do get on the FMC path to fly the rest of the arrival.

Ideally, it should go to | IDLE/HOLD || VNAV PTH | once it finally does capture the path and the airspeed should decrease below the FMC target speed to try and maintain the path before power is ever added.

When the manual refers to passing the first "altitude constraint waypoint," would that be an at-or-above waypoint like SIERA (280/FL130A), or is it looking for a hard or at-or-below altitude like MURRY (FL130BFL110A)? Since the database does not have the speed limit points coded in, I had to assign an at-or-above altitude in order to assign a speed at those waypoints. It's either that or speed intervene.

Hardy Heinlin

It checks out which constraint projects a straight 3° corridor back up to CRZ ALT without touching any other descent constraint (checking that no knee is in the way), taking predicted groundspeeds and all that forecast stuff into account. This is a very complicated interactive task and that's why the VNAV descent program is so terribly sensitive regarding modifications.