Why does this approach have higher LNAV/VNAV minimums than just LNAV?

[photofly]

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Does anyone know why the LNAV/VNAV minimums are the same as the circling minimums and a lot higher than the LNAV ones? There's no NOTAM for an error.

[Rockie]

Probably because at the lowest temperature the approach can be flown (-33) there is an obstacle that penetrates the depressed obstacle clearance plane necessitating the higher minimums. At that temperature the LNAV approach would have a corrected FAF crossing altitude whereas the LNAV/VNAV does not. Without seeing the actual data it's impossible to say for sure though.

[ant_321]

There is one of these in Milwaukee as well. I haven't been able to figure it out.

[photofly]

Probably because at the lowest temperature the approach can be flown (-33) there is an obstacle that penetrates the depressed obstacle clearance plane necessitating the higher minimums. At that temperature the LNAV approach would have a corrected FAF crossing altitude whereas the LNAV/VNAV does not. Without seeing the actual data it's impossible to say for sure though.

I can follow the technical "we crank the handle on the TERPS machine and this is the sausage that drops out at the end" explanation.

Am I right in thinking that this means a pilot who has the choice of either can pick:

A)LNAV/VNAV - with vertical guidance presentation and a DA, but higher minimums - OR

B)LNAV - roll your own vertical guidance from the stabilized descent table, or chop and drop, and an MDA in return for lower minimums

Are there operations which approve *only* LNAV/VNAV? Why would a pilot choose A over B? And why would he or she not choose A initially, following the vertical guidance until the higher minima and then continue descent to LNAV minima anyway?

[ahramin]

Are there operations which approve *only* LNAV/VNAV?

I don't know of anyone who uses LNAV/VNAV only.

On the heavies we add 50' to the LNAV MDA and use the FMSs VNAV down to that MDA.

I find these silly LNAV/VNAV minimums all the time on NavCanada created GNSS approaches. I'm guessing that in a misguided effort to make the procedure useful in as wide a range of temperatures as possible for non temperature correcting FMSs, they make the procedure completely useless for anyone at any temperature.

[photofly]

Somebody sent me this, which explains the technical reasons in more detail:

http://cfiruss.blogspot.ca/2015/10/why- ... igher.html

[gonnabeapilot]

I don't know of anyone who uses LNAV/VNAV only. On the heavies we add 50' to the LNAV MDA and use the FMSs VNAV down to that MDA.

I find these silly LNAV/VNAV minimums all the time on NavCanada created GNSS approaches. I'm guessing that in a misguided effort to make the procedure useful in as wide a range of temperatures as possible for non temperature correcting FMSs, they make the procedure completely useless for anyone at any temperature.

I'm not sure if you're implying that nobody makes use of LNAV/VNAV limitations or that no operator exclusively uses LNAV/VNAV limits like photofly was suggesting but I can tell you that we use them all the time at Sunwing and I'm sure there are other Canadian operators that make use of LNAV/VNAV as well. Using LNAV/VNAV requires specific lateral and vertical RNP limits in the FMS but typically get you lower than LNAV only minimums so they definitely prove quite useful although the example in this thread is the obvious exception. LNAV/VNAV also doesn't require the addition of 50 ft to the LNAV MDA as it is DA. We still have the option to use LNAV only minimums instead of the LNAV/VNAV minimums and frequently do if weather isn't a deciding factor as there is a lot less work required for LNAV only approaches.

[Rockie]

Am I right in thinking that this means a pilot who has the choice of either can pick:

A)LNAV/VNAV - with vertical guidance presentation and a DA, but higher minimums - OR

B)LNAV - roll your own vertical guidance from the stabilized descent table, or chop and drop, and an MDA in return for lower minimums

You're correct, an operator can choose either of the minimums to use. However with a non-temperature compensated system the LNAV/VNAV cannot be used below the TLIM (-33 degrees in this case), and below 0 degrees the operator must correct the FAF crossing altitude and probably the selected flight path angle for the LNAV. Airlines do not use the "chop and drop" method anymore and must use constant descent angle procedures without leveling out at MDA.

[Rockie]

I don't know of anyone who uses LNAV/VNAV only. On the heavies we add 50' to the LNAV MDA and use the FMSs VNAV down to that MDA.

I find these silly LNAV/VNAV minimums all the time on NavCanada created GNSS approaches. I'm guessing that in a misguided effort to make the procedure useful in as wide a range of temperatures as possible for non temperature correcting FMSs, they make the procedure completely useless for anyone at any temperature.

LNAV/VNAV is not just different limits than LNAV on the same GNSS approach, they are in fact two entirely different approaches with completely different obstacle clearance criteria. As was mentioned they are usually lower than LNAV minima, and as it has a DA rather than an MDA there is no requirement to add 50 feet to comply with CAR 602.128 (2)(b). LNAV/VNAV approaches aren't limited to NavCanada either, they are worldwide.

[Rockie]

Are there operations which approve *only* LNAV/VNAV? Why would a pilot choose A over B? And why would he or she not choose A initially, following the vertical guidance until the higher minima and then continue descent to LNAV minima anyway?

Missed this part. There might be operators who only approve LNAV/VNAV but I don't know why they would give up the capability to do LNAV if the LNAV/VNAV was unavailable due to temperature. The LNAV can be flown at any temperature.

Given a choice though we would always choose the LNAV/VNAV because it's easier, and it provides vertical guidance rather than just selecting a flight path angle that may or may not be in the right place.

You would NEVER fly the LNAV/VNAV to minimums and then continue down to LVAV minimums in this case. As I said they are two separate approaches with separate obstacle clearance criteria and you run the risk of hitting something. The minimums are what they are because of an obstacle. It would be like flying a VOR approach then continuing down to ILS minimums.

[rxl]

I always thought that it was simply due to the fact that a DA makes allowance for a slight dip below that altitude when commencing a missed approach whereas an MDA does not need to make any such allowance. But then I have never encountered an approach with a difference as large as the OP's example.

[photofly]

You would NEVER fly the LNAV/VNAV to minimums and then continue down to LVAV minimums in this case. As I said they are two separate approaches with separate obstacle clearance criteria and you run the risk of hitting something.

If you (try to) fly a 3 degree approach path using the LNAV/VNAV presentation or using the CDA data on the plate, what's the actual difference?

[ahramin]

The LNAV/VNAV may not be 3°. Plus the LNAV/VNAV gives you vertical guidance that your FD/AP can follow.

[Rockie]

You would NEVER fly the LNAV/VNAV to minimums and then continue down to LVAV minimums in this case. As I said they are two separate approaches with separate obstacle clearance criteria and you run the risk of hitting something.

If you (try to) fly a 3 degree approach path using the LNAV/VNAV presentation or using the CDA data on the plate, what's the actual difference?

If you're on a true 3 degree flight path in both cases there is no difference, but the LNAV/VNAV in this case can be flown down to -33C and as the temperature decreases the flight path angle also decreases. The published minima is there for a reason and is not just some number plucked out of thin air, don't start making up your own procedures.

[FICU]

I always thought that it was simply due to the fact that a DA makes allowance for a slight dip below that altitude when commencing a missed approach whereas an MDA does not need to make any such allowance. But then I have never encountered an approach with a difference as large as the OP's example.

Bingo... it's based on the missed approach and the fact the LNAV/VNAV is designed having a missed initiated from a constant descent.

[lazyeight]

Airlines do not use the "chop and drop" method anymore and must use constant descent angle procedures without leveling out at MDA.

Are we talking only Airlines? Or legally speaking? Because you most definitely CAN level off at the MDA if you like and past the FAF you're good down to whatever the MDA is provided it depicts that on the plate. You're not required to maintain a constant rate of descent, however it is recommended. Now if were talking SOPS then OK, gotcha.

[lazyeight]

Airlines do not use the "chop and drop" method anymore and must use constant descent angle procedures without leveling out at MDA.

Are we talking only Airlines? Or legally speaking? Because you most definitely CAN level off at the MDA if you like and past the FAF you're good down to whatever the MDA is provided it depicts that on the plate. You're not required to maintain a constant rate of descent, however it is recommended. Now if were talking SOPS then OK, gotcha.

[lazyeight]

I always thought that it was simply due to the fact that a DA makes allowance for a slight dip below that altitude when commencing a missed approach whereas an MDA does not need to make any such allowance. But then I have never encountered an approach with a difference as large as the OP's example.

You're basically right. A DA is a decision altitude, as in that is the point you add power to go around and start the missed or continue if visual. On a large aircraft that can inherently mean a second or two for power to fully apply and the aircraft to start a climb. Hence the depicted dip on a plate. An MDA is exactly that, minimum descent altitude. You CAN NOT go below under any circumstances unless visual. You may also notice on an approach with an MDA it is depicted on the plate where you start the missed approach, usually abeam the threshold or over a DME or something. So you fly along at MDA until that point, then commence the missed procedure. DA's the missed is started at the DA, when you add power.

[Rockie]

Are we talking only Airlines? Or legally speaking?

It's procedural - not legal. Years ago the industry recognized that unstabilized approaches were a major contributor to CFIT accidents, and the chop and drop technique is by definition an unstabilized approach. We don't even train for them anymore. Stabilized basically means you are in the landing configuration and on the correct path to landing by 1000', and by 500' the airspeed is on target, thrust is stabilized above idle, rate of descent not above 1000 fpm and the landing checklist is complete. If any of those parameters are not met by 1000/500 feet, or the approach becomes unstable at any time then a go-around is mandatory.

These airplanes have a lot of inertia. Leveling them off at MDA then starting down again when visual with the runway takes time and large thrust changes, and the big fan engines do not respond as fast as piston or turboprops either. The automation also greatly increases the workload of the crew when trying to do the chop and drop method whereas SCDA's are much simpler.

You CAN NOT go below under any circumstances unless visual.

Normally you're right. However TC and other regulators will give a waiver to an operator allowing them to go below MDA while conducting a go-around (using the MDA as a DA), but there are specific operational and training requirements and 4 conditions under which it cannot be used (overweight, system failure effecting the go-around etc.)
http://www.tc.gc.ca/CivilAviation/Regse ... n/2509.htm

[valleyboy]

So all this banter and why not just do the LPV approach, OK let the "what if's" begin, double unrelated failures are not allowed in aviation -- LMFAOOoooooo