Remote altimeter setting, use of baro vnav

[pstan]

I've been perusing approach plates. The Beckley WV, KBKW, RNAV 19 (with minimums for lpv lnav/vnav lnav and circling) allows use of an altimeter setting from Bluefield if local one is not avail. And the Jepp chart publishes lnav vnav minimums for this non local altimeter case. Lets say the temperature is ISA, so cold weather effects are not important. However, a note say baro vnav is N/A when using Bluefield altimeter setting.

Exactly what type of equipment in an aircraft will allow using vnav/lnav minimums without baro vnav? What sort of chance is there that waas units must resort to non lpv minimums?

thanks, Stan

 

[John Collins]

I've been perusing approach plates. The Beckley WV, KBKW, RNAV 19 (with minimums for lpv lnav/vnav lnav and circling) allows use of an altimeter setting from Bluefield if local one is not avail. And the Jepp chart publishes lnav vnav minimums for this non local altimeter case. Lets say the temperature is ISA, so cold weather effects are not important. However, a note say baro vnav is N/A when using Bluefield altimeter setting.

Exactly what type of equipment in an aircraft will allow using vnav/lnav minimums without baro vnav? What sort of chance is there that waas units must resort to non lpv minimums?

thanks, Stan

Stan,

Approaches with LNAV/VNAV minimums may be flown by two types of systems, one is a TSO C145A/146A WAAS GPS and the other is a Baro VNAV system. The LNAV/VNAV minimums are primarily available for the airlines to provide a stabilized approach option to the LNAV dive and drive approach.

There are not many of the Baro VNAV systems in GA, they are mostly a creature of the airlines. They build a glide path based on sensing the altitude and comparing the altitude with a calculated altitude for the aircraft position on the glidepath. Because the calculated glidepath is based on the reference altimeter setting, it requires a local altimeter setting and has high and low temperature limits.

A WAAS GPS determines a glidepath by its vertical and Horizontal GPS position. The glidepath is independent of the altimeter setting and there are no temperature limits. The only thing that is dependent on the altimeter setting is the DA or MDA, and it is adjusted on the approach chart if necessary as a result of using an alternate altimeter setting.

With a WAAS GPS, it will only downgrade from an LPV to an LNAV minimum, even if there are LPV, LNAV/VNAV, and LNAV minimums for a given approach. It the approach does not have LNAV minimums, the WAAS GPS will cancel the approach guidance if it does not meet the criteria and revert to Terminal mode for the missed approach.

The odds of an LPV downgrading are very low. In the CONUS, a typical day has 100 percent coverage over 90+ percent of the land mass, with only 99 percent coverage over the balance of the CONUS. The worst coverage is on the West Coast. 99 percent availability means that during the day, LPV is not available for about 15 minutes sometime during the day.

With a WAAS GPS, I see no advantage to ever using an LNAV/VNAV minimums when there are LPV minimums available. In some cases, when there are LNAV/VNAV minimums without LPV minimums for a given approach, they may offer an advantage over the LNAV, but even if the minimums are lower which is not always the case the visibility requirements are often higher and an approach to LNAV minimums can offer an advantage in the probability of successfully completing the approach.

 

[pstan]

Approaches with LNAV/VNAV minimums may be flown by two types of systems, one is a TSO C145A/146A WAAS GPS.... With a WAAS GPS, it will only downgrade from an LPV to an LNAV minimum, even if there are LPV, LNAV/VNAV, and LNAV minimums for a given approach. It the approach does not have LNAV minimums, the WAAS GPS will cancel the approach guidance if it does not meet the criteria and revert to Terminal mode for the missed approach.

Thanks John. I don't have any waas experience, so how does the unit differ once it is "downgraded" and lnav minimums are available? I understand that the accuracy of the position is not appropriate for LPV approaches, but does the unit still compute a vnav based on satellite ranging? And not through baro vnav? Or is there no vnav if the aircraft is not set up for baro vnav?

I note you said it will only downgrade to an LNAV minimum, but how does the unit know what minimums you fly to? Or is it just the pilot who has to respect the higher mda now that the satellites are not good enough for an LPV approach?

And finally, if the approach is indeed downgraded to lnav, and the lnav vnav minimums are lower than the lnav minimums, is it legal for a pilot to use the lnav/vnav minimums (assuming some form of vnav is available, eg baro vnav)?

 

[TMetzinger]

Most units will tell you what sort of guidance they're giving, and you should find the corresponding minimums The WAAS G1000 system will annunciate:

LNAV - LNAV only, use LNAV Minimums
LNAV+V - LNAV with advisory vertical guidance, a stabilized descent to the LNAV MDA, use LNAV minimums, and be aware that you might prefer to "dive and drive".
LNAV/VNAV - Use LNAV/VNAV minimums.
LPV - use LPV minimums.

 

[bobmrg]

Thanks John. I don't have any waas experience, so how does the unit differ once it is "downgraded" and lnav minimums are available? I understand that the accuracy of the position is not appropriate for LPV approaches, but does the unit still compute a vnav based on satellite ranging? And not through baro vnav? Or is there no vnav if the aircraft is not set up for baro vnav?

I note you said it will only downgrade to an LNAV minimum, but how does the unit know what minimums you fly to? Or is it just the pilot who has to respect the higher mda now that the satellites are not good enough for an LPV approach?

And finally, if the approach is indeed downgraded to lnav, and the lnav vnav minimums are lower than the lnav minimums, is it legal for a pilot to use the lnav/vnav minimums (assuming some form of vnav is available, eg baro vnav)?

Unless baro nav gets a lot less expensive and trickles down to general aviation, don't worry about it.

Bob Gardner

 

[Alice R]

I note you said it will only downgrade to an LNAV minimum, but how does the unit know what minimums you fly to? Or is it just the pilot who has to respect the higher mda now that the satellites are not good enough for an LPV approach?

And finally, if the approach is indeed downgraded to lnav, and the lnav vnav minimums are lower than the lnav minimums, is it legal for a pilot to use the lnav/vnav minimums (assuming some form of vnav is available, eg baro vnav)?

John gave a good answer to your original question . Im not exactly sure what your follow up question is . You either have verticle guidance or you use LNAV mins . The unit doesn't know what mins you bug . Baro VNAV , or what is refered to as Uncompensated Baro VNAV is not available on most GA planes . What are you flying .

 

[Areeda]

I think Stan may be unfamiliar with WAAS units. One thing that might not have been spelled out is that the unit itself estimates how precisely it knows its position. And based on this it will select the appropriate approach type.

If LPV, and LNAV minimums are available for the approach and precision is good enough it will annunciate LPV and provide vertical guidance, if not it will annunciate LNAV and no vertical guidance. You have no input into the process. On the 530W/430W, you could ignore the vertical guidance and fly it to LNAV minimums but no way to force vertical guidance if it doesn't want to do it.

My confusion is if LPV and LNAV/VNAV minimum are available, will it ever choose LNAV/VNAV?

Joe

 

[John Collins]

Thanks John. I don't have any waas experience, so how does the unit differ once it is "downgraded" and lnav minimums are available? I understand that the accuracy of the position is not appropriate for LPV approaches, but does the unit still compute a vnav based on satellite ranging? And not through baro vnav? Or is there no vnav if the aircraft is not set up for baro vnav?

When the GPS downgrades the approach to LNAV, no, repeat no, vertical guidance is provided. The pilot is expected to fly the approach according to the published LNAV procedure and MDA. The GPS does not compute a glidepath, period, so following an LNAV/VNAV to a DA is not supported. GPS WAAS does not have Baro VNAV equipment but is authorized to fly LNAV/VNAV approaches using the GPS WAAS equipment, providing the GPS signal meets the criteria for integrity. Baro VNAV is a totally different piece of equipment and the odds of you having it in your aircraft are very close to zero. My understanding is that this equipment often can cost in the hundreds of thousands of dollars.

I note you said it will only downgrade to an LNAV minimum, but how does the unit know what minimums you fly to? Or is it just the pilot who has to respect the higher mda now that the satellites are not good enough for an LPV approach?

The WAAS GPS doesn't know or care what minimums you fly to. It only tells you what the highest in the hierarchy of minimums (LPV, LNAV/VNAV, LNAV+V, LNAV)you are authorized to fly and that it will provide guidance for. It is the responsibility of the pilot to follow the rules.

And finally, if the approach is indeed downgraded to lnav, and the lnav vnav minimums are lower than the lnav minimums, is it legal for a pilot to use the lnav/vnav minimums (assuming some form of vnav is available, eg baro vnav)?

Since you don't have a Baro VNAV system, it is not legal to fly to LNAV/VNAV minimums if the WAAS GPS downgrades to LNAV. As a side note, often the LNAV/VNAV minimums are higher than the LNAV minimums, particularly with respect to visibility, but often the DA is higher than the MDA. The whole purpose for LNAV/VNAV minimums was to provide for a stabilized approach for the airlines even if it meant a reduction in minimums.

Some more information:

When you load the approach into the WAAS GPS, it will identify one and only one of the following in the hierarchy of minimums: LPV, LNAV/VNAV, LNAV+V, or LNAV. LNAV+V wont show up as a minimum choice on the approach plate as it is only an LNAV minimum approach with an advisory glidepath. Any particular approach may offer more than one in the list above, but only the first in the hierarchy is listed in the database. The GPS doesn't care what minimums you fly an approach to, that is totally the pilot's choice. Just because an approach has vertical guidance available, doesn't mean you need to use it if LNAV minimums are available for the approach and you choose to fly the approach to the LNAV minimums.

One minute prior to the final approach fix, the WAAS GPS will determine if the GPS integrity is sufficient to support vertical and horizontal guidance on the approach of the highest in the hierarchy of minimums. If it fails this check, the approach is downgraded to LNAV minimums and no vertical guidance is provided. So if the approach has LPV minimums as the highest in the hierarchy, it will downgrade to LNAV minimums, even if the approach also supports LNAV/VNAV or LNAV/+V. If a particular approach has LNAV/VNAV as the highest, it still downgrades to LNAV. In very rare cases, the integrity is bad enough that even LNAV horizontal integrity isn't satisfied, in which case you are waived off by the GPS. In some other cases, there are stand alone LPV approaches that do not have an associated LNAV procedure, and if the integrity doesn't meet the criteria for the approach, you are waived off as well, since there is no LNAV minimums to downgrade to.

 

[John Collins]

My confusion is if LPV and LNAV/VNAV minimum are available, will it ever choose LNAV/VNAV?

Joe

No! This is true even if it could meet the LNAV/VNAV integrity requirements.

An example where this could happen would be an LPV with a DH of 200 feet requires HPL of 40 meters and VPL of 35 meters, whereas an LNAV/VNAV requires an HPL of 556 meters and a VPL of 50 meters.

An LPV with a DH equal to or greater than 250 ft requires a HPL of 40 meters and a VPL of 50 meters. Even so, normally it is the VPL value that exceeds the limit when it occurs. That is because vertical position of the GPS is inherently less accurate than the horizontal position. The reason is that for horizontal position, the geometry makes better use of all satellites in view that are above 5 degrees of the horizon. To get equivalent vertical precision, the GPS would have to see the satellites below the horizon, which are blocked by the earth. So the vertical inaccuracy is often 50% greater than the horizontal.

The bottom line is that when vertical or horizontal integrity is exceeded, no vertical guidance is provided. Remember that both vertical and horizontal are required to fix the position of a glidepath.

 

[flyingcheesehead]

Most units will tell you what sort of guidance they're giving, and you should find the corresponding minimums The WAAS G1000 system will annunciate:

LNAV - LNAV only, use LNAV Minimums
LNAV+V - LNAV with advisory vertical guidance, a stabilized descent to the LNAV MDA, use LNAV minimums, and be aware that you might prefer to "dive and drive".
LNAV/VNAV - Use LNAV/VNAV minimums.
LPV - use LPV minimums.

I think that at least on the 430, LNAV/VNAV is annunciated as L/VNAV.

 

[bobmrg]

When the GPS downgrades the approach to LNAV, no, repeat no, vertical guidance is provided. The pilot is expected to fly the approach according to the published LNAV procedure and MDA. The GPS does not compute a glidepath, period, so following an LNAV/VNAV to a DA is not supported. GPS WAAS does not have Baro VNAV equipment but is authorized to fly LNAV/VNAV approaches using the GPS WAAS equipment, providing the GPS signal meets the criteria for integrity. Baro VNAV is a totally different piece of equipment and the odds of you having it in your aircraft are very close to zero. My understanding is that this equipment often can cost in the hundreds of thousands of dollars.



The WAAS GPS doesn't know or care what minimums you fly to. It only tells you what the highest in the hierarchy of minimums (LPV, LNAV/VNAV, LNAV+V, LNAV)you are authorized to fly and that it will provide guidance for. It is the responsibility of the pilot to follow the rules.



Since you don't have a Baro VNAV system, it is not legal to fly to LNAV/VNAV minimums if the WAAS GPS downgrades to LNAV. As a side note, often the LNAV/VNAV minimums are higher than the LNAV minimums, particularly with respect to visibility, but often the DA is higher than the MDA. The whole purpose for LNAV/VNAV minimums was to provide for a stabilized approach for the airlines even if it meant a reduction in minimums.

Some more information:

When you load the approach into the WAAS GPS, it will identify one and only one of the following in the hierarchy of minimums: LPV, LNAV/VNAV, LNAV+V, or LNAV. LNAV+V wont show up as a minimum choice on the approach plate as it is only an LNAV minimum approach with an advisory glidepath. Any particular approach may offer more than one in the list above, but only the first in the hierarchy is listed in the database. The GPS doesn't care what minimums you fly an approach to, that is totally the pilot's choice. Just because an approach has vertical guidance available, doesn't mean you need to use it if LNAV minimums are available for the approach and you choose to fly the approach to the LNAV minimums.

One minute prior to the final approach fix, the WAAS GPS will determine if the GPS integrity is sufficient to support vertical and horizontal guidance on the approach of the highest in the hierarchy of minimums. If it fails this check, the approach is downgraded to LNAV minimums and no vertical guidance is provided. So if the approach has LPV minimums as the highest in the hierarchy, it will downgrade to LNAV minimums, even if the approach also supports LNAV/VNAV or LNAV/+V. If a particular approach has LNAV/VNAV as the highest, it still downgrades to LNAV. In very rare cases, the integrity is bad enough that even LNAV horizontal integrity isn't satisfied, in which case you are waived off by the GPS. In some other cases, there are stand alone LPV approaches that do not have an associated LNAV procedure, and if the integrity doesn't meet the criteria for the approach, you are waived off as well, since there is no LNAV minimums to downgrade to.

Hang around here, John....your level of knowledge is badly needed.

Bob Gardner

 

[pstan]

Actually, I'm in business aviation and fly a lear. It has baro vnav, but no waas.

And I've no waas experience.

Those last few posts seem to have cleared it all up though.

Could someone clarify one further situation? For an aircraft using waas, if one approach has lpv minimums, and a second has no lpv minimums but has lnav/vnav minimums, will it take the same degradation in gps signals to downgrade both to Lnav minimums?

Agree with Bob 100%

 

[Everskyward]

Actually, I'm in business aviation and fly a lear. It has baro vnav, but no waas.

WAAS hasn't trickled up to business aviation yet.

 

[John Collins]

Actually, I'm in business aviation and fly a lear. It has baro vnav, but no waas.

And I've no waas experience.

Those last few posts seem to have cleared it all up though.

Could someone clarify one further situation? For an aircraft using waas, if one approach has lpv minimums, and a second has no lpv minimums but has lnav/vnav minimums, will it take the same degradation in gps signals to downgrade both to Lnav minimums?

Agree with Bob 100%

The parameters that are used by a WAAS GPS to determine if the position integrity (both horizontal and vertical) are HPL (Horizontal Protection Limit) and VPL (Vertical Protection Limit). They are 99.999% probability limits that the actual position will be within the calculated values.

Each type of approach has its own alarm limits, HAL (Horizontal Alarm Limit) and VAL (Vertical Alarm limit). If either HPL exceeds the required HAL or VPL exceeds the required VAL, then the approach will either be downgraded or you will be waved off. Below is a list of HAL and VAL values for various types of GPS WAAS approaches, all values are in meters:

Approach Type -----HAL---VAL
LPV (DH>= 250 ft) ---40 ---50
LPV (DH< 250 ft) ----40 ---35
LP ------------------40 ---NA
LNAV/VNAV --------556 ---50
LNAV/+V -----------556 ---50
LNAV --------------556 ---NA

There are also HAL values for other modes:
Enroute ----3707
Terminal ---1853
RNAV RNP 1 1853
RNAV RNP 2 3707

Typically the vertical integrity is around 150% of the horizontal integrity, so in most cases, the vertical is likely to cause a limit to be exceeded before the horizonatal. Regardles of which is exceeded, vertical guidance will no longer be supported as both are required to define a glidepath in space.

 

[poadeleted20]

I think that at least on the 430, LNAV/VNAV is annunciated as L/VNAV.

That's correct. Ditto the 530.

 

[pstan]

I'm see a reference to waas units annunciating "LNAV+V", Tim for example in post #4.

In what case would one see this?

John tells us there is no vnav if a unit downgrades, so I'm thinking a downgrade would NOT be the case

Could this be from a straight LP approach, and if so what airport has one?

Or can this be from a straight lnav approach (in which case one should never see LNAV annunciated, but LNAV/+V)?

Just trying to know what to expect....

 

[John Collins]

I'm see a reference to waas units annunciating "LNAV+V", Tim for example in post #4.

In what case would one see this?

John tells us there is no vnav if a unit downgrades, so I'm thinking a downgrade would NOT be the case

Could this be from a straight LP approach, and if so what airport has one?

Or can this be from a straight lnav approach (in which case one should never see LNAV annunciated, but LNAV/+V)?

Just trying to know what to expect....

LNAV+V is what is displayed on the WAAS GPS for an LNAV approach with "Advisory Vertical Guidance". Most, but not all LNAV approaches are provided with advisory vertical guidance. This means that you will see a glidepath indication when flying these approaches. The glidepath is advisory only and doesn't in any way change the approach, it still has a MDA.

You can ignore the glidepath, but you can't ignore the MDA, which means you can't fly it to minimums as if it were a DA. An approach with a DA allows you to make your decision at the DA, which can result in the aircraft momentum allowing the aircraft to sink below the DA before the go around is commenced. In the case of a MDA, no sink thru is allowed as the MDA is a hard Minimum altitude until you meet the requirements of 91.175.

If you follow the glidepath, it should keep you above any step down minimum altitudes, but you should verify this when you are on the approach.

You can not consistently determine from the approach chart if it will be LNAV+V or just LNAV, but approach charts with a GS or a GS and a TCH depicted usually are good indications. Approaches that are not a straight in such as GPS-A won't have an advisory glidepath.

 

[poadeleted20]

If you follow the glidepath, it should keep you above any step down minimum altitudes,

That is not at all guaranteed, so it's quite important that you...

verify this when you are on the approach.

 

[pstan]

John, I'm presuming the slope produced when in "advisory" vnav is generated by the waas unit using all the satellites available, including the geo ones. So...can one "normally" expect the accuracy of this GS to be just as good as in lpv (assuming vertical integrity is acceptable for lpv)? Not to say "always", but in most cases? And of course I'm not saying one can rely on it, as I presume loss on lpv integrity would NOT be annunciated.

Do we have any LP approaches that do not have vertical component? Where?

Thanks for help. Stan

 

[pstan]

That is not at all guaranteed

Ron, with our UNAC IE, IC, and IM units, whenever a step down fix penetrated the slope, the FAF crossing altitude is raised to clear it. One can see this altitude opposite the waypoint on the flightplan page. One can tell when an approach had this change as the FAF crossing altitude was no longer the charted one, but was raised by the amount necessary to clear, eg 1483 feet instead of 1400 feet. So...if one was at the minimum altitude for the intermediate segment of 1400 feet, the glideslope was not intercepted until about .25 nm past. This then met the step down requirement.

Question for the group. Does the Garmin 1000 and other units do this too?

Or will you in fact bust the step down fix altitude? And for what airport/approach please.

 

[TMetzinger]

Ron, with our UNAC IE, IC, and IM units, whenever a step down fix penetrated the slope, the FAF crossing altitude is raised to clear it. One can see this altitude opposite the waypoint on the flightplan page. One can tell when an approach had this change as the FAF crossing altitude was no longer the charted one, but was raised by the amount necessary to clear, eg 1483 feet instead of 1400 feet. So...if one was at the minimum altitude for the intermediate segment of 1400 feet, the glideslope was not intercepted until about .25 nm past. This then met the step down requirement.

Question for the group. Does the Garmin 1000 and other units do this too?

Or will you in fact bust the step down fix altitude? And for what airport/approach please.

I've not yet had the G1000 guide me below a stepdown fix. That's not in anyway an authoritative answer, though, it may be possible. I have to look at some approaches that have a lower MDA if you "have" a fix inside the FAF (these are usually VOR or VOR/DME overlays) and see what (if any) vertical guidance is given and how it behaves. For the authoritative answer, I think a Garmin PHG (pointy headed geek) will have to answer, unless there's TSO or other guidance that specifies that an advisory GP cannot bust a stepdown fix altitude.

On LNAV+V approaches, particularly the ones that aren't aligned with a runway, I will often descend as rapidly as I can from the FAF to the MDA so that I have more time to look for the airport. Straight-in approaches aren't as hard so I may track the advisory GP on those.

 

[poadeleted20]

Question for the group. Does the Garmin 1000 and other units do this too? Or will you in fact bust the step down fix altitude? And for what airport/approach please.

To my knowledge, no such computation is performed by the Garmin panel-mounts (400/500-series, G1000, etc) in the LNAV+V mode. You get a straight line from FAF at published crossing altitude down to the runway surface in the touchdown zone. The system does not have the crossing altitudes for the stepdown fixes in its database, so it can't alter the vertical guidance to prevent busting a step-down altitude.

 

[John Collins]

That is not at all guaranteed, so it's quite important that you...

Although I have never seen a case where the GS did not provide clearance for all step downs, I agree that it is important to trust, but verify.

 

[John Collins]

John, I'm presuming the slope produced when in "advisory" vnav is generated by the waas unit using all the satellites available, including the geo ones. So...can one "normally" expect the accuracy of this GS to be just as good as in lpv (assuming vertical integrity is acceptable for lpv)? Not to say "always", but in most cases? And of course I'm not saying one can rely on it, as I presume loss on lpv integrity would NOT be annunciated.

Do we have any LP approaches that do not have vertical component? Where?

Thanks for help. Stan

The vertical guidance is the same for LPV DH>=250 and LNAV/VNAV and LNAV/+V. The WAAS Geostationary satellites also transmit a ranging signal (same as a regular GPS) in addition to the WAAS corrections. For some time, the accuracy of the Geo WAAS satellites ranging transmission has been qualified for precision approach (WAAS term, not actual ICAO precision approach) is included in the calculations for the GS.

I suspect, but have not confirmed that if you are flying an approach with LNAV+V and the signal integrity doesn't meet the criteria one minute before the FAF, it will downgrade to just LNAV. [edit: this is confirmed by the GNS530W Pilot Guide]

As of this writing, there are no LP approaches in the USA or the world for that matter, but I understand that early in 2010 Watsonville, CA is one of the airports which will get one.

 

[John Collins]

To my knowledge, no such computation is performed by the Garmin panel-mounts (400/500-series, G1000, etc) in the LNAV+V mode. You get a straight line from FAF at published crossing altitude down to the runway surface in the touchdown zone. The system does not have the crossing altitudes for the stepdown fixes in its database, so it can't alter the vertical guidance to prevent busting a step-down altitude.

I agree with your analysis. I guess that Garmin or Jeppesen decides which LNAV approaches are coded as LNAV/+V and, therefore, they have the opportunity to confirm that they clear all step down fixes. They use two points to define the approach GP, the intercept altitude at the FAF and the TCH at the threshold.

You can calculate the GP at any point from the threshold, by adding the threshold to the TCH as a base, and then add 318 feet per nautical mile. These figures need to be adjusted upwards by the curvature of the earth. A good approximation for the curvature of the earth is to square the distance in NM at the point you wish to correct for and the result is feet to add. So at 5 NM from the threshold, add 5 x 5 = 25 ft. This will get you the altitude of the GP over the fix, and will be within 20 ft out to about 15 NM from the threshold.

 

[gismo]

To my knowledge, no such computation is performed by the Garmin panel-mounts (400/500-series, G1000, etc) in the LNAV+V mode. You get a straight line from FAF at published crossing altitude down to the runway surface in the touchdown zone. The system does not have the crossing altitudes for the stepdown fixes in its database, so it can't alter the vertical guidance to prevent busting a step-down altitude.

FWIW I finally found the statement from Garmin that claims the advisory vertical guidance on LNAV approaches keeps you above any step downs. It's in the 6th "GNS480 Newsletter":

"Interim step down altitude constraints will be met if you remain on the vertical path."

 

[pstan]

You get a straight line from FAF at published crossing altitude down to the runway surface in the touchdown zone. The system does not have the crossing altitudes for the stepdown fixes in its database.

Ron, just want to make sure I understand. So in some cases this advisory GS may be well less than 3 degrees? The reason I'm asking is the UNAC units will default to a standard 3 degrees (or greater if step down fixes require it). And if so, then the pilot flies level past the FAF until he picks up the 3 (or >) degree GS. But never less than 3 degrees.

Funny that these older non waas units of mine have the step down fixes, and their altitudes, and will calculate a glides slope (or perhaps pull one from the database?) to ensure any step down fix altitude restriction is met. While newer garmin waas units don't have this capability.

 

[poadeleted20]

FWIW I finally found the statement from Garmin that claims the advisory vertical guidance on LNAV approaches keeps you above any step downs. It's in the 6th "GNS480 Newsletter":

"Interim step down altitude constraints will be met if you remain on the vertical path."

Keep in mind that the GNS480 is totally different software written by UPSAT than that written by Garmin for the 430/530 and G1000 families.

 

[poadeleted20]

Ron, just want to make sure I understand. So in some cases this advisory GS may be well less than 3 degrees?

"Well less than 3 degrees?" No, it can't be, as there are limits to how shallow the approach path can be above the 40:1 obstacle clearance plane. There's also the maximum 400 ft/nm gradient to consider. However, it would not be a violation of TERPS for there to be a 300 ft/nm gradient from the FAF to the step-down, and then a 350 ft/nm gradient from the step-down to the TDZ. In that case, flying a straight line from the FAF to the TDZ would be a legal gradient (very close to 3 degrees) but would still bust the step-down fix.

The reason I'm asking is the UNAC units will default to a standard 3 degrees (or greater if step down fixes require it). And if so, then the pilot flies level past the FAF until he picks up the 3 (or >) degree GS. But never less than 3 degrees.

Your software works differently than Garmin's LNAV+V, which draws a straight line up from the TDZ to the FAF. Garmin's line may be slightly more or less than 3 degrees, and is the same as the gradient shown in the profile view on the approach chart.

Funny that these older non waas units of mine have the step down fixes, and their altitudes, and will calculate a glides slope (or perhaps pull one from the database?) to ensure any step down fix altitude restriction is met. While newer garmin waas units don't have this capability.

I've flown with a lot of different light plane GPS's dating back to the Trimble 2000, but I've never seen one which includes step-down fix altitudes.

 

[John Collins]

Keep in mind that the GNS480 is totally different software written by UPSAT than that written by Garmin for the 430/530 and G1000 families.

That is true regarding the GNS480 and the GNS430, but the Garmin AT folks that developed the software for the CNX80/GNS480 were enlisted to totally rewrite the software for the GNS430W/530W. I don't know if they also wrote the WAAS upgrade software for the G1000 or not.

 

[pstan]

"Well less than 3 degrees?" No, it can't be, as there are limits to how shallow the approach path can be above the 40:1 obstacle clearance plane. There's also the maximum 400 ft/nm gradient to consider. However, it would not be a violation of TERPS for there to be a 300 ft/nm gradient from the FAF to the step-down, and then a 350 ft/nm gradient from the step-down to the TDZ. In that case, flying a straight line from the FAF to the TDZ would be a legal gradient (very close to 3 degrees) but would still bust the step-down fix.
Your software works differently than Garmin's LNAV+V, which draws a straight line up from the TDZ to the FAF. Garmin's line may be slightly more or less than 3 degrees, and is the same as the gradient shown in the profile view on the approach chart.
I've flown with a lot of different light plane GPS's dating back to the Trimble 2000, but I've never seen one which includes step-down fix altitudes.

What 40:1 plane? this is an advisory glideslope, to an mda.

Consider a ficticious airport with the FAF crossing is at 2000 feet, no step down fix, and 5nm to the threshhold. Consider the TDZE and airport elevation is 950 feet. Then flying to 50 ft TCH, there is a 1000 ft loss in 5 miles. Thats 200 feet per nautical mile, about 1.9 degrees. Is that what the advisory vna would give, 1.9 degrees? Or does it default to 3.0 degrees?

 

[poadeleted20]

What 40:1 plane?

The plane through which no obstacle can penetrate on the final approach course. TERPS requires a minimum clearance above that plane, and that restricts how low an angle an FAF-TDZ path can take, on both precision and nonprecision approaches.

Consider a ficticious airport with the FAF crossing is at 2000 feet, no step down fix, and 5nm to the threshhold. Consider the TDZE and airport elevation is 950 feet. Then flying to 50 ft TCH, there is a 1000 ft loss in 5 miles. Thats 200 feet per nautical mile, about 1.9 degrees. Is that what the advisory vna would give, 1.9 degrees? Or does it default to 3.0 degrees?

I'm not sure you can have enough clearance above the 40:1 plane for this to be TERPS-legal, but if it were, you'd see 1.9 degrees as the descent gradient on the chart, and the Garmin 430/530 GPS's would set the LNAV+V glide slope accordingly. You would not see a delay after passing the FAF at 2000 MSL for the aircraft to reach any arbitrary 3-degree limit before the GS needle started down from its centered position at the FAF/2000 MSL the way it seems to work in the "UNAC" system pstan's talking about.

 

[John Collins]

The following is an excerpt from the TERPS for LNAV approach regarding step down fixes:

"For step-down fixes published in conjunction with vertically-guided minimums, the published altitude at the fix must be equal to or less than the computed glidepath altitude at the fix."

I don't know if this would apply to an LNAV+V as that is not defined by the TERPS, but would apply to an approach with an LPV and or an LNAV/VNAV along with LNAV minimums. Inside the FAF, the step-down fixes only apply to the LNAV minimums as the GP defines the vertical path for approaches with a DA.

 

[pstan]

That's not my understanding at all Ron, for a non precision approach. Once you go past the FAF, you can descent immediately to the mda, no worrying about a 40:1 plane clear of obstacles. The criteria is based on type of non prec approach. For eg, LNAV mda must be 250 feet above all obstacles from the faf to the map. NBD off the field is 350 feet. No 40:1 plane is considered for non precision. Once visual and descending below the mda, there are slopes the terpster has to consider when he designs the approach.

But ok...1.9 degrees, I understand. Way back 5 years ago or so, UNAC changed to the 3 degree (or more) standard. Seems to make sense!!

 

[TMetzinger]

Hmm. Perhaps you guys (Stan, Ron), you guys are talking past each other? I know of quite a few approaches with stepdowns PAST the FAF, where you can't descend to MDA until you pass the stepdown, but I think they may all be VOR based. Perhaps on standalone LNAV approaches there aren't stepdowns past/inside the FAF?

 

[John Collins]

The plane through which no obstacle can penetrate on the final approach course. TERPS requires a minimum clearance above that plane, and that restricts how low an angle an FAF-TDZ path can take, on both precision and nonprecision approaches.

The 40:1 plane is used for approaches with a glidepath and is not used for an LNAV approach obstacle clearance. The LNAV ROC is defined by a 7:1 slope at the FAF and any step-downs, with a 250 ft clearance requirement in the primary area.

I'm not sure you can have enough clearance above the 40:1 plane for this to be TERPS-legal, but if it were, you'd see 1.9 degrees as the descent gradient on the chart, and the Garmin 430/530 GPS's would set the LNAV+V glide slope accordingly. You would not see a delay after passing the FAF at 2000 MSL for the aircraft to reach any arbitrary 3-degree limit before the GS needle started down from its centered position at the FAF/2000 MSL the way it seems to work in the "UNAC" system pstan's talking about.

My understanding is that a lower than a 3 degree slope may be permitted, but that it requires special permission. Normally 3 degrees is the minimum.
The 40:1 slope is about 1.4 degrees and I would agree it does not provide any significant protection for a 1.9 degree slope. My guess if this was part of an LNAV/+V, the glidepath would be well above the FAF and probably a minimum of 3 degrees, but definitely sufficient angle to clear the step-down fix. If a particular geometry wouldn't allow the step down fixes to be cleared, the approach would not have an advisory glidepath.

 

[pstan]

Tim, of course step down fixes must still be considered. And yes, they do exist for LNAV only approaches. I'm wondering if Ron means a 40:1 slope back from the threshhold, but that would still only be 1.5 degrees.

 

[John Collins]

Hmm. Perhaps you guys (Stan, Ron), you guys are talking past each other? I know of quite a few approaches with stepdowns PAST the FAF, where you can't descend to MDA until you pass the stepdown, but I think they may all be VOR based. Perhaps on standalone LNAV approaches there aren't stepdowns past/inside the FAF?

Tim,

There are step-downs past the FAF on LNAV approaches. The step down allows the MDA to be lowered once you are past the controlling obstacle. A step down wont be used unless it can at least lower the MDA by 60 ft. This is one reason why you will sometimes see an LNAV with a lower MDA than a LNAV/VNAV DH.

 

[TMetzinger]

That's not my understanding at all Ron, for a non precision approach. Once you go past the FAF, you can descent immediately to the mda, no worrying about a 40:1 plane clear of obstacles.

This is the part I was having trouble with - it's not true...

 

[John Collins]

That's not my understanding at all Ron, for a non precision approach. Once you go past the FAF, you can descent immediately to the mda, no worrying about a 40:1 plane clear of obstacles. The criteria is based on type of non prec approach. For eg, LNAV mda must be 250 feet above all obstacles from the faf to the map. NBD off the field is 350 feet. No 40:1 plane is considered for non precision. Once visual and descending below the mda, there are slopes the terpster has to consider when he designs the approach.

But ok...1.9 degrees, I understand. Way back 5 years ago or so, UNAC changed to the 3 degree (or more) standard. Seems to make sense!!

Stan, the 40:1 doesn't apply to an LNAV approach. The 250 ft obstacle clearance at the MDA or the step-down altitude applies. When you leave one altitude at a fix for a lower authorized altitude, there is a 7:1 gradient that obstacles must not penetrate, so although it is steep, you literally can't descend immediately to the next altitude. If 600 ft per minute is used on a 3 degree glideslope (A normal 3 degree slope is 19 to 1 as a reference), then a 7 to 1 would mean to stay above 250 ft above it, you couldn't descend any faster than about 1600 ft/min, shouldn't be a problem.