VHOJT

RNAV LNAV/VNAV approach problems

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Hi chaps,

I am having a few issues getting the 744 to behave on an RNAV LNAV/VNAV approach.  Well - it may be the way I'm doing it, the way the real thing does it, or the way PMDG modelled it, I am just not sure...

The approach I am doing is the RNAV-Z onto RWY 34 at YMML (http://www.airservicesaustralia.com/aip/pending/dap/MMLGN01-152.pdf)
It is a basic RNP-1.0 approach that the 744 is capable of doing, with no fancy curvy turns or anything.

I am joining the approach from the WENDY5A RNAV arrival (http://www.airservicesaustralia.com/aip/pending/dap/MMLSR35-152.pdf)

(Now, I know they are not in effect until August - for some reason the current Navigraph database has them in there and has removed the previous versions).

My issues are:

1) Turning from AKDEL onto the final approach course is a 90 degree left turn from 070 to 340.  The 744 seems to cut the corner considerably and ends up left of centreline, before then correcting to the right, and back to the left again.  I did read in another thread (https://www.avsim.com/forums/topic/506253-erratic-lnav-turn-behavior-during-approach/#comment-3589467) that this may be a known issue with the 744?  If so, no problem, I can heading select the turn.

2) Configured at Flap 10 and 170 knots, the autopilot starts the turn.  PFD is showing IDLE | LNAV | VNAV PTH.  During the turn (have tried it twice now) - VNAV allows the speed to decay to ~ 155-160 knots before aggressively applying thrust (SPD | LNAV| VNAV PTH) to bring the aircraft back to 170.  In the process, it almost levels off, loses its VNAV path and, once recovered at 170 knots, reverts to IDLE | LNAV | VNAV SPD.  We are now 400-500ft above our ideal vertical profile, so no chance of re-engaging VNAV PTH until I V/S down at quite a rate to make up my lost 500ft.  Should it allow the speed to decay so far at this stage of the approach in VNAV PTH before switching the autothrottle from IDLE to SPD?  Am I better off using V/S for the initial phases of an RNAV approach?

3) It looks like (from a 744 FCTM) that it is certainly possible to remain in VNAV PTH prior to final approach, and that VNAV PTH will stay locked "on" (i.e. it will not revert to VNAV SPD if you intervene, and providing you are not high) when approaching the FAF).  It doesn't seem possible to do as the manual suggests in this case from what I experienced earlier.

4) I have noticed in other planes as well (the NGX, FSL A320) - that when passing a waypoint on an approach, the vertical deviation caret can jump around a bit up or down, making the approach not very smooth - VNAV PTH has to pitch up or down to regain its "new" profile.  Has anyone else noticed this - and is it a real-world behaviour, or something I can do something about, or neither?

5) Any real-world experience with doing LNAV/VNAV approaches in the 744?  Are there a few traps for the unwary?

Cheers for any advice or insight,

Rudy

 

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I am pretty sure the B744 is not equipped for RNP approaches.  Most obvious are the missing navigation performance indicators.  This beast originated with IRS was king and is still the primary mode of navigation with GNSS/GPS playing a supporting role.

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5 hours ago, downscc said:

I am pretty sure the B744 is not equipped for RNP approaches.

Correct, but in this case what Rudy is referring to is not RNP AR with RF legs (which the B744 cannot do) but a 'standard' RNAV non-precision approach, which the B747 (provided the particular aircraft is equipped with GPS) is quite capable of flying.

1) is largely to be expected: as previously mentioned, the thing about the B747 is that it cannot (with the legacy FMC at least) do RF legs and therefore there is no guarantee of how it will track in a turn like that. You can help it by not going too fast, but you may want to try and reduce the angle of intercept if you can.

Now let's discuss 2) and 3)...

With regard to the height and speed keeping -- I'm not sure but in honesty that doesn't sound terribly accurate to me; I've not heard lots of complaints about the B747 VNAV in this respect. It may be a 'sim-ism' and, again you may need to help it out by stabilising the path a little earlier.

You are correct with regard to the 'paradox' of having VNAV PTH annunciated with the speed window open. For those who may not be familiar: in the descent normally the aeroplane will be in VNAV PTH with the speed window closed. The FMC will then vary the speed in a range around the target with, ideally, idle thrust in order to maintain the path.

If you press the SPD knob to open the speed window, the aeroplane will pitch to maintain the selected speed and the mode will change from VNAV PTH to VNAV SPD; this is essentially the aeroplane telling you "well, I can do one or the other: either let me vary the speed to maintain the path or we'll ignore the path and pitch for speed."

This is obviously an issue on an approach using VNAV, where we want the aeroplane to maintain a specified glide path, but we also want it to fly the speed we want (i.e. VRef + an additive). To get around this, Boeing came up with something called "On Approach Logic".

The FMC transitions to "On Approach mode" based on a various pieces of witchcraft but primarily what it must have is a glide path angle coded in the Legs page. If this is the case then provided the flaps are out of up and you are approaching the FAF (or thereabouts -- depending on how the procedure is coded in the database).

"On Approach Logic" allows us to do various things that we would not normally be able to -- for example, not only will it allow VNAV PTH with the speed window open, it will also enable us to set the missed approach altitude once we are at least 300ft below said altitude, without commanding a climb.

So what we must do is:

1) Make sure there is a glidepath angle coded in the database on the LEGS page (and even if it were possible, we wouldn't be able to "cook our own").

2) Make sure that we are approaching the FAF in a reasonable configuration and speed at or approaching the platform altitude for the procedure.

Once we have done this all we have to do is make sure that we are in VNAV PTH with the speed window open and wind the alt sel down to the MDA -100ft. All being well, the aeroplane should start following the path and we can then configure for landing as normal. Once we are at least 300ft below our missed approach altitude, we can then wind the alt sel back up to the missed approach altitude.

Things to watch out for:

- VNAV ALT. We could be in VNAV ALT in one of two situations:

1) We have levelled off because the MCP alt sel is set above the FMC commanded altitude

2) We have levelled off below the FMC coded altitude for the FAF.

Case 2) is easy: all we have to do is wind the alt sel down to MDA -100 before we intercept the path and, all being well, the aeroplane should pick up the path when it intercepts it and start descending in VNAV PTH.

If it is case 1) however, we will need to press the alt sel button to start down (again, after setting MDA -100 in the window).

- Radar Vectors. If we have been vectored to the FAF, we may well be in, for instance, SPD | ALT | LNAV as we approach the FAF. Obviously we need to get in to VNAV PTH to start descending on the glidepath. So how to do this?

Well, we could just press the VNAV button on the MCP. The problem with this is that if we do so, the speed bug will jump to the active VNAV speed. Usually this will be the speed on the VNAV DES page, but very occasionally if you are in an odd mode you might be in VNAV Cruise mode or even VNAV Climb. The way to find out what VNAV mode is active is to press the VNAV key on the CDU once. Whatever speed is in there is what the aeroplane will attempt to fly, and this might be very undesirable indeed (for instance, the default VNAV descent speed is 240 kt). Therefore, if you are approaching the FAF and NOT in VNAV, it is a very good idea to press the VNAV key once to establish the active mode and then overtype the speed with something sensible (e.g. your current airspeed). Then when you press the VNAV button nothing will happen with the speed.

- Altimeter setting. Remember that VNAV knows nothing about where the ground is. Is is completely dependent on you having the correct QNH set to fly an accurate glidepath. If you have the wrong QNH set, the aeroplane will indicate that it is perfectly on path but the true altitude and hence actual glidepath flown will be wrong, perhaps catastrophically so. In real life, temperature will also affect Baro-VNAV paths -- this is why all Baro-VNAV procedures will have a box on the chart stating "PROC NA BELOW (some temperature)". This is because if it is hotter than standard, the aeroplane will fly a slightly steeper path, but if it is colder than standard, the aeroplane will fly a much flatter glidepath and below a certain temperature this glidepath will be so flat that obstacle clearance can no longer be assured. However, temperature error is not simulated in FS so we don't need to worry (though if you are striving for realism you should fly an alternative approach if the temperature is out of the limits stated on the chart!).

Things you should also do:

- In real life, the PNF would monitor lateral and vertical track deviation during the approach on PROG page 2 -- if the cross track deviation exceeds half the RNP for the segment of the apporach (i.e. +/- 0.15NM in the final segment and +/- 0.5NM in all other segments, and no more than 50 ft below the vertical path in the final segement). If these parameters are exceeded (other than very momentarily during/immediately after turns in segments other than the final segment) then a go around should be flown.

- You must fly the approach in LNAV and VNAV, and remain in VNAV PTH (with the speed window open) after the FAF

I think that's all I can think of - enjoy!

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6 hours ago, downscc said:

I am pretty sure the B744 is not equipped for RNP approaches.  

Where are you getting that wrong information from?

We do fly t.o., en route, terminal and approaches based on RNP....

Required Navigation Performance

Required Navigation Performance (RNP) is the navigation accuracy required for

operation within a defined airspace. It is expressed in nautical miles. RNP values

have been published for areas of operation around the world. Operations in these

areas require on-board navigation systems to alert the flight crew if ANP exceeds

RNP. The FMC supplies a default RNP value for takeoff, en route,

oceanic/remote, terminal, and approach phases of flight. The flight crew may enter

an RNP value, when required. RNP is on POS REF page 2.

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9 hours ago, VHOJT said:

 

1) Turning from AKDEL onto the final approach course is a 90 degree left turn from 070 to 340.  The 744 seems to cut the corner considerably and ends up left of centreline, before then correcting to the right, and back to the left again.  I did read in another thread (https://www.avsim.com/forums/topic/506253-erratic-lnav-turn-behavior-during-approach/#comment-3589467) that this may be a known issue with the 744?  If so, no problem, I can heading select the turn.

 

i see this corner cutting all the time especially with STARs that have a 180 sort of pattern in them.

like for example in EPWA SORIX4U:

iZC7cyn.jpg

it cuts early, then realizes it cut early and corrects almost 90 degrees back, then corrects again. it's kind of strange i feel like i don't really understand why it starts to turn the corner so early when the magenta line draws a nice smooth path that it seems like it ignores.

one thing i have noticed that really affects it is if you change speed or configuration just before or during the turn ...so i'm guessing maybe it's related to that, like it calculates that turn at one speed but if you drop your speed and your flaps while it is turning it doesn't really update it immediately. generally in the case like this STAR i will try to get to flaps 5 and stable at least 1-2nm before the turn and it seems to deal more elegantly with it. but i find in the 737 for example i never really have to worry about this. maybe just because it's a lot less inertia to deal with? .

cheers,-andy crosby

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In RW, intercepts with localizer or RNAV headings are usually at around 30 degrees. Expecting a large heavy aircraft to turn 90 degrees to intercept a runway heading is a bridge too far. Not to mention spilling all those brandies in first. :(

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10 hours ago, VHOJT said:

1)  I did read in another thread (https://www.avsim.com/forums/topic/506253-erratic-lnav-turn-behavior-during-approach/#comment-3589467) that this may be a known issue with the 744? 

2) Configured at Flap 10 and 170 knots, the autopilot starts the turn.  PFD is showing IDLE | LNAV | VNAV PTH.  During the turn (have tried it twice now) - VNAV allows the speed to decay to ~ 155-160 knots before aggressively applying thrust (SPD | LNAV| VNAV PTH) to bring the aircraft back to 170.  In the process, it almost levels off, loses its VNAV path and, once recovered at 170 knots, reverts to IDLE | LNAV | VNAV SPD.  We are now 400-500ft above our ideal vertical profile, so no chance of re-engaging VNAV PTH until I V/S down at quite a rate to make up my lost 500ft.  Should it allow the speed to decay so far at this stage of the approach in VNAV PTH before switching the autothrottle from IDLE to SPD?  Am I better off using V/S for the initial phases of an RNAV approach?

3) It looks like (from a 744 FCTM) that it is certainly possible to remain in VNAV PTH prior to final approach, and that VNAV PTH will stay locked "on" (i.e. it will not revert to VNAV SPD if you intervene, and providing you are not high) when approaching the FAF).  It doesn't seem possible to do as the manual suggests in this case from what I experienced earlier.

4) I have noticed in other planes as well (the NGX, FSL A320) - that when passing a waypoint on an approach, the vertical deviation caret can jump around a bit up or down, making the approach not very smooth - VNAV PTH has to pitch up or down to regain its "new" profile.  Has anyone else noticed this - and is it a real-world behaviour, or something I can do something about, or neither?

5) Any real-world experience with doing LNAV/VNAV approaches in the 744?  Are there a few traps for the unwary?

Cheers for any advice or insight,

Rudy

 

1) CORRECT - LNAV tracking in particualr is a well known issue with several Addon aircraft and I wouldn't be at all surprised to hear that PMDG and others are working on a fix.

2 - 4) Simon makes a very important safety point in respect of VNAV and altimeter settings.   Before the days of GPS the recommended technique to join any ILS on the B744 was to use all available aids, including the ILS raw data with IAS and FLCH engaged (or V/S when appropriate) during the intermediate approach.  Even with DME/DME updating the calculated FMC position might still be miles out from where you think you are (e.g.regions with relatively sparse Navaid cover, such as Central Africa).  Incidentally, you should use V/S instead of FLCH if you want to fly a continuous descent/minimum noise type of approach using the 300ft/nm rule to calculate your height versus distance from touchdown.  So, bearing in mind that we are talking about a PC simulator here and its LNAV and VNAV PATHS will probably never be 100% accurate or as smooth as those on the real B744, I suspect the QOTS III was doing its level best to cope with what was being asked of it - and probably performing a bit like the real aircraft too!!

5)  One of the biggest traps for the unwary is the sheer inertia in any large jet aircraft of this size.  The B744 doesn't seem to want to slow down if you are fast and clean and, even worse, it doesn't want to speed up if you are on the wrong side of the drag curve [although not as much as Concorde :-) ].  Accurate speed control is therefore very important; and remember also that the flaps are very large and take some time to travel between UP and Flap5 (especially when using the Alternate system).  Last but not least, even today you should never rely solely on LNAV and VNAV for any departure or approach, especially when operating at or below Safety Altitude.  As the pilot your situational awareness just might be be the last link in the chain which prevents a hardware or software fault, or a NAV database error from causing an accident.

Bertie Goddard   

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P.S.  I forgot to say you should also use HDG instead of LNAV to capture the ILS LOC.

Bertie

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LNAV tracking/behaviour is reported.

 

RNAV

RNAV can be a very complex subject and I drew this picture to better understand the subject, but basically, you will need certification and approval  for each RNAV spec. With each spec you get certified for, the better the yield for the operator with regard to airspace utilisation.

 

I don't know if Boeing offer the RNP-AR spec to 747-400's. If they did, hypothetically, it would probably mean system enhancements to greatly improve the 747-400's position "monitoring" and "alerting". This would probably mean upgrading the FMCS and the IDS. In addition to this, personnel training would be undertaken and a flight performance and maintenance trend monitoring program would need to be set up, at a guess. All this costs $. For an aging aircraft, it's probably not worth buying.

 

The Queen has a very basic position "monitoring" and "alerting" system. I don't know how extensive it's "alerting" has been modelled. In the Queen, you can probably get "virtual certification" for all the specs shown except: A-RNP, RNP-AR and probably RNP 0.3.

This document spells it out.

http://www.icao.int/APAC/Documents/edocs/COSCAP_PBNOPS_HANDBOOK Version 2_4.pdf

 

2ajcldh.jpg

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Nice chart Brian.  Very nice.  This is why I first mentioned that the Queen is not certified for RNP, although several declared me wrong, I think the chart supports my opinion.  I based that simply on the fact that there is no performance monitoring and that is why you do not see RNP/ANP readouts on the ND.  The context I use of course is RNP AR APCH and RNP 0.3... here in the states either an approach is RNAV or RNP and RNP requires the equipment and crew certification.  Broadly speaking, RNP can define a broader range but that isn't how it's used where I fly.

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40 minutes ago, downscc said:

I based that simply on the fact that there is no performance monitoring and that is why you do not see RNP/ANP readouts on the ND. 

The 744 does have RNP performance monitoring: just not on the PFD. You can enter an RNP value in to the FMC and it will alert you if the ANP exceeds this value: further, the cross-track error values (both lateral and vertical) are displayed on PROG page 2 and this is why it is a requirement for PM to monitor them during an RNAV approach. 

As Brian's chart shows, the naming of some RNAV procedures is inconsistent and confusing. The B744 can do 'standard' RNAV approaches (RNAV GNSS, sometimes titled RNP APCH) as these do not include RF legs and do not require RNP values below 0.3.

What it cannot do is RNP (AR)/SAAAR etc: these procedures require RF legs (which the aeroplane cannot fly) and RNP values below 0.3 (which the FMC is not certified for).

Clear as mud?

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Some of the 744F with NGFMC are able to do RF legs, therefore they are able to conduct RNAV SID with RF legs i.e. those noise abatement SID out of Hong Kong.  

However the cost of certifying the type to do RNP (AR) appr is probably too high for the soon-to-be retired airplane, as the 747-8F is becoming a majority of the freighter fleet. 

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16 hours ago, VHOJT said:

Configured at Flap 10 and 170 knots, the autopilot starts the turn.  PFD is showing IDLE | LNAV | VNAV PTH.  During the turn (have tried it twice now) - VNAV allows the speed to decay to ~ 155-160 knots before aggressively applying thrust (SPD | LNAV| VNAV PTH) to bring the aircraft back to 170.  In the process, it almost levels off, loses its VNAV path and, once recovered at 170 knots, reverts to IDLE | LNAV | VNAV SPD.  We are now 400-500ft above our ideal vertical profile, so no chance of re-engaging VNAV PTH until I V/S down at quite a rate to make up my lost 500ft.  Should it allow the speed to decay so far at this stage of the approach in VNAV PTH before switching the autothrottle from IDLE to SPD?  Am I better off using V/S for the initial phases of an RNAV approach?

 

And to effectively mitigate this, what the Boeing people normally do is to harden up the Altitude constrain over AKDEL from 3000A to just 3000ft and put a speed of something like 180kts along with the altitude constraint as well (actually same on the airbus as well). And try to prevent the airplane from going above VNAV path within the last 20 track miles to go from the runway, so that you get to  SPD LNAV VNAV PATH quite early after flap 1. (activate the approach logic)

I must agree that the PMDG VNAV PATH does not react as fast as the real airplane, and it doesnt quite follow the PATH as actively as it should be. Therefore in order to do a smoother approach, I recommend to do a short level segment between OBGAL and AKDEL at 3000ft to let the airplane settle down. 

The MEL runway 34 Rnav approach has always been quite "problematic" in real life even with the airbus FMC which I used to deal with a lot of times, by all means have a try my advise and see how it works out to you. :smile: 

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10 hours ago, skelsey said:

The 744 does have RNP performance monitoring: just not on the PFD.

It is not only the RF legs, but in general the ability to maintain the required navigation performance.  Also, "not on the PFD" is just what is required as part of the certification process I believe. I'm pretty sure we are in agreement, just have emphasis in different areas.

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1 hour ago, downscc said:

It is not only the RF legs, but in general the ability to maintain the required navigation performance.  Also, "not on the PFD" is just what is required as part of the certification process I believe. I'm pretty sure we are in agreement, just have emphasis in different areas.

Yes indeed -- I think the issue is the naming convention for RNAV approaches!

Originally, there were RNAV (RNP) approaches, which require special certification, authorisation etc (e.g. Queenstown NZ etc). These procedures may require less than the standard RNP values in any of the segments (i.e. less than RNP 1.0 in the initial/intermediate/missed etc, less than 0.3 in the final segment) and/or RF legs after the FAF. I think the FAA tended to title these RNP (AR) or SAAAR (Special Aircraft and Aircrew Authorisaton Required). It is this standard that requires lateral deviation information on the PFD/ND etc.

The "standard" RNAV approach which can be flown with any RNP alerting capable FMC (such as that installed in B744) used to be titled "RNAV (GPS)" or "RNAV (GNSS)". These approaches will never require RNP values below 0.3 in the final segment or below 1.0 in any other segment, and currently only use straight legs. As I say, the B744 is most definitely capable of and certified to fly these procedures, provided the GPS is operative.

However, where it gets confusing is that the naming convention is changing: all procedures that require the use of GPS are, by definition, RNP procedures. Thus, the "RNAV" designation is being phased out to be replaced by "RNP". However, "RNP" in this new naming convention is not the same as "RNP (AR)"!

Therefore:

  • Anything which is or was titled "RNAV (GPS)" or "RNAV (GNSS)" is being changed to RNP APCH
  • Anything which is or was titled "RNAV (RNP)" is being changed to "RNP AR".

Thus, the B744 can indeed fly anything titled "RNAV (GNSS)" "RNAV (GPS)" or "RNP APCH"  to LNAV/VNAV minima, provided the GPS is operative.

It cannot fly "RNAV (RNP)" or "RNP (AR)" procedures.

Edit: my understanding is that the FAA are not planning to bring their IAP naming conventions in line with ICAO. Therefore there are now three naming conventions for RNAV approaches out there:

1) FAA: RNAV (GPS) and RNAV (RNP)
2) Old ICAO: RNAV (GNSS) and RNP (AR)
3) New ICAO: RNP APCH and RNP (AR)

Clearly, this may lead to some confusion!

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