Surely LNAV+V must be safer?
Thread Starter
Surely LNAV+V must be safer?
Many aviators know that the Garmin 650/750 equipment can be upgraded with a low or no cost software change to offer LNAV+V.
This would seem like a substantial safety improvement to me, however the people at CASA appear to make out that LNAV+V does not improve safety over the existing RNAV, and it is really required to be Baro-VNAV.
However the ATSB report on Lockhart River states:
If you have LNAV+V installed, the actual descent doesn’t start until the GPS position in the database complies with the start of the (normally) 3 degree approach path. That means, when following the glide slope indicator, either manually or coupled, it would not be possible to start the descent early.
In that case, isn’t it clear that LNAV+V provides a safety advantage over basic RNAV?
Yes, I accept that Baro-VNAV is even better and allows a lower minima, but there are only a limited number of approaches in Australia with Baro-VNAV and in some aircraft the cost would be quite expensive to upgrade. In the case of the Garmin 650/750 equipment the cost is either zero or very low to upgrade the software of existing units.
This would seem like a substantial safety improvement to me, however the people at CASA appear to make out that LNAV+V does not improve safety over the existing RNAV, and it is really required to be Baro-VNAV.
However the ATSB report on Lockhart River states:
“There were 49 respondents who reported that they had been involved in an incident involving RNAV (GNSS) approaches. The most common … was commencing the descent too early due to a misinterpretation of their position.”
In that case, isn’t it clear that LNAV+V provides a safety advantage over basic RNAV?
Yes, I accept that Baro-VNAV is even better and allows a lower minima, but there are only a limited number of approaches in Australia with Baro-VNAV and in some aircraft the cost would be quite expensive to upgrade. In the case of the Garmin 650/750 equipment the cost is either zero or very low to upgrade the software of existing units.
however the people at CASA appear to make out that LNAV+V does not improve safety over the existing RNAV, and it is really required to be Baro-VNAV
In that case, isn’t it clear that LNAV+V provides a safety advantage over basic RNAV?
Is anyone being prevented from using LNAV+V? Dash 8 operator I used to work for used advisory VNAV on normal RNAV approaches with standard RNAV minima. We still had to monitor limiting steps and so on but it worked very well and made for a very low workload approach.
I agree that early descent is a real issue with the current standard of RNAV approach, particularly when tracking via the straight in fix. Using the dog leg rather than the straight in provides additional cues about your position in the approach. I know of many people who have descended, or been about to descend, early on a straight in RNAV. Agreed that any vertical guidance is better than none.
I agree that early descent is a real issue with the current standard of RNAV approach, particularly when tracking via the straight in fix. Using the dog leg rather than the straight in provides additional cues about your position in the approach. I know of many people who have descended, or been about to descend, early on a straight in RNAV. Agreed that any vertical guidance is better than none.
From CAAP 178-1(2)
"however the people at CASA appear to make out that LNAV+V does not improve safety over the existing RNAV, and it is really required to be Baro-VNAV". Please provide evidence of this... The way the CAAP reads is that you can use VNAV but you just need to monitor your descent profile to ensure you don't descend below a limiting altitude.
Extract from the CAAP paragraph 4.4.5 sates: "Some non-APV (NPA) avionics have a VNAV function that displays the vertical path in an ILS-like fashion. In these instances the vertical information is simply a mechanised representation of the designed approach path angle and is not linked to any external vertical navigation source and does not indicate the aircraft’s true relationship with the ground. If the approach chart line of minima indicates S-I or LNAV, then any VNAV indication provided by the avionics is advisory only. If this type of vertical advisory information is used, the pilot is responsible to ensure that the minimum segment altitudes published on the approach chart are adhered to.".
Extract from the CAAP paragraph 4.4.5 sates: "Some non-APV (NPA) avionics have a VNAV function that displays the vertical path in an ILS-like fashion. In these instances the vertical information is simply a mechanised representation of the designed approach path angle and is not linked to any external vertical navigation source and does not indicate the aircraft’s true relationship with the ground. If the approach chart line of minima indicates S-I or LNAV, then any VNAV indication provided by the avionics is advisory only. If this type of vertical advisory information is used, the pilot is responsible to ensure that the minimum segment altitudes published on the approach chart are adhered to.".
Thread Starter
Ftrplt and alphacentauri, I agree that CASA has not said you can’t use LNAV+V, however every letter I get back is relatively negative. Here are a couple of quotes from letters I have received from Mr Carmody.
Firstly, on 3 April 2018:
In a letter from Mr Carmody dated 24 January 2018:
For some reason Mr Carmody is clearly getting advice from others (as Mr Carmody does not have an aviation background) which show no enthusiasm for LNAV+V. Never in any of the letters that have come back to me do they mention that at a minimum, the LNAV+V has a starting point for the descent incorporated in the database. This makes it a lot more difficult to start descending early if the vertical guidance is going to be used.
In all of the replies, CASA just keeps going on about Baro-VNAV, when the cost is far higher for fitment in most cases, and there are not that many Baro-VNAV approved approaches so far.
Firstly, on 3 April 2018:
“I am advised that the type of LNAV+V equipment that you highlight presents an average descent profile based on the length of the approach and the altitude to be lost. It does not consider any obstacles or terrain at the location, nor does it consider whether the average profile displayed will cause a descent below the approach steps. For this reason, CASA guidance … specifically warns pilots not to rely solely on LNAV+V information for their descent … ”
“…some aircraft systems provide advisory vertical guidance (e.g. LNAV+V) which is not certified and based on a GPS derived vertical position … the slope from which the advisory vertical guidance is derived is not subject to certification of the vertical path and may not be within prescribed vertical limits.”
In all of the replies, CASA just keeps going on about Baro-VNAV, when the cost is far higher for fitment in most cases, and there are not that many Baro-VNAV approved approaches so far.
Thread Starter
VH. Note how the CASA CAAP makes no mention of the fact that in such a VNAV. approach that there is an actual GPS position in the data base that starts the continuous descent.
This surely must reduce the chance of starting the descent too early.
Are they suggesting that a company like Garmin would provide a descent profile that runs into a mountain? Gad. They must be more amateur than I thought.
This surely must reduce the chance of starting the descent too early.
Are they suggesting that a company like Garmin would provide a descent profile that runs into a mountain? Gad. They must be more amateur than I thought.
Thread Starter
I have just been informed that the CASA problem with LNAV +V is that it is not ICAO accepted!
The astute Americans have gone and introduced this as it clearly improves safety in a low cost way.
The astute Americans have gone and introduced this as it clearly improves safety in a low cost way.
None of those letters say you can’t use it Dick, they’re just saying you must use alternative means to confirm you’re profile, ie GPS distance plu baro altitude. Advisory VNAV is fine just dont use it as your sole profile reference.
Dick,
You wonder why people get short with you! You're either trying to stir up trouble or perhaps have no idea, but in this instance I'll go with the stir up trouble because of the stupidity of some of your statements e.g. "Are they suggesting that a company like Garmin would provide a descent profile that runs into a mountain? Gad. They must be more amateur than I thought.".
Aerocat, well said
You wonder why people get short with you! You're either trying to stir up trouble or perhaps have no idea, but in this instance I'll go with the stir up trouble because of the stupidity of some of your statements e.g. "Are they suggesting that a company like Garmin would provide a descent profile that runs into a mountain? Gad. They must be more amateur than I thought.".
Aerocat, well said
Last edited by VH-MLE; 6th Apr 2018 at 03:51. Reason: Comment to AerocatS2A
Originally Posted by Dick
So you imply that you can use Baro VNAV as a sole reference.. you could die!
Do I sniff a whiff of a republic?
Baro V-Nav approaches are designed and coded differently to 'basic' RNAV approaches. When a Baro V-Nav approach is loaded into a certified avionics system - yes, you can fly solely off the vertical glidepath with no requirement to monitor intermediate steps (in fact there aren't any intermediate steps beyond glidepath capture), you wont fly into a mountain (as long as the temp is within the certified bounds and accurate QNH is available).
'Basic' R-Nav are not coded with a vertical glidepath to 50ft (usually) over the threshold, they are designed with intermediate altitude steps vs distance. Therefore, no avionics manufacturer can build a vertical glidepath capability that can be solely relied upon as the data is not designed into the approach, and therefore not coded into any database, hence the advisory nomenclature.
The FAA say no different to CASA in relation to use of advisory VNAV.
'Basic' R-Nav are not coded with a vertical glidepath to 50ft (usually) over the threshold, they are designed with intermediate altitude steps vs distance. Therefore, no avionics manufacturer can build a vertical glidepath capability that can be solely relied upon as the data is not designed into the approach, and therefore not coded into any database, hence the advisory nomenclature.
The FAA say no different to CASA in relation to use of advisory VNAV.
Thread Starter
Alphacenturi, fascinating. You state
Let’s look at the approach plate for Goulburn RNAV-Z (GNSS) RWY 04. 
How does the LNAV+V know when to advise with the glideslope indicator to leave 4,700 feet? Presumably it has a GPS position to start, or it could interpolate back at 3 degrees from the 4,510 feet at the 7 mile distance from the missed approach waypoint to 4,700 feet. This still means it is operating from a GPS position to start the descent.
Are you saying this is not what happens? What do you believe happens, and why wouldn’t it happen in the way that I have explained?
I point out whether the LNAV+V descent starts at an actual GPS position, or is interpolated back from another GPS position, it has exactly the same result – that is, a dramatic improvement in safety.
“The CAAP makes not mention of it because it is untrue.”
How does the LNAV+V know when to advise with the glideslope indicator to leave 4,700 feet? Presumably it has a GPS position to start, or it could interpolate back at 3 degrees from the 4,510 feet at the 7 mile distance from the missed approach waypoint to 4,700 feet. This still means it is operating from a GPS position to start the descent.
Are you saying this is not what happens? What do you believe happens, and why wouldn’t it happen in the way that I have explained?
I point out whether the LNAV+V descent starts at an actual GPS position, or is interpolated back from another GPS position, it has exactly the same result – that is, a dramatic improvement in safety.
Thread Starter
Ftrplt. I am clearly not the risk taker you and your CASA friends are
I would never rely solely on the glide slope - even on a baro VNAV approach- once had the indicator stick!
I would never rely solely on the glide slope - even on a baro VNAV approach- once had the indicator stick!
it has exactly the same result – that is, a dramatic improvement in safety.
Ftrplt. I am clearly not the risk taker you and your CASA friends are
Folks,
You all really know what Dick is getting at, that "CASA" actively discourage the use of some of the features of GARMIN (and some others) that internally enables the generation of a VNAV path, by whatever name.
Of course, we (I hope) understand that you have to monitor and fly the printed letdown, as designed. Which I hope you would (monitor, that is) be doing even if you had a baro-VNAV or SBAS/GBAS VNAV procedure available.
What gets me ( I don't know if Dick is even aware) but a coterie of FOIs "very very actively" discourage, all on the basis of "unapproved", but in reality, in my opinion, because they have nil/little actual real time knowledge and experience with what is on the market, and what so many are using in the real world.
Tootle pip!!
You all really know what Dick is getting at, that "CASA" actively discourage the use of some of the features of GARMIN (and some others) that internally enables the generation of a VNAV path, by whatever name.
Of course, we (I hope) understand that you have to monitor and fly the printed letdown, as designed. Which I hope you would (monitor, that is) be doing even if you had a baro-VNAV or SBAS/GBAS VNAV procedure available.
What gets me ( I don't know if Dick is even aware) but a coterie of FOIs "very very actively" discourage, all on the basis of "unapproved", but in reality, in my opinion, because they have nil/little actual real time knowledge and experience with what is on the market, and what so many are using in the real world.
Tootle pip!!