LNAV/VNAV Question

[gnvav8r]

I've just had my GNS430 upgraded to WAAS and am learning the new approaches. Can anybody explain to me why the plain vanilla LNAV minimum at Gainesville (KGNV RNAV 29 approach) is lower than the LNAV/VNAV minimum? I thought the LNAV/VNAV with a DA is more precise than LNAV with an MDA. If the required limits, RAIM, etc are in place, and you are shooting the LNAV/VNAV approach, why would anybody use the higher minimum?

 

[atldc9]

I will take a stab at this one. AS I recall, the reason you find LNAV min's being lower then LNAV/VNAV has to do with the obstacle clearance requirments from terps. Basically, when the approach is certified using LNAV only, the tolerances for obstacle clearance are based on a hard MDA, with no deviation below MDA. However, when LNAV/VNAV is used, the obstacle clearance must be calculated using the rules for an ILS and a DA, which allow for some normal bracketing of the glideslope. Hence, the obstacle clearance requirements are tougher. This usually results in increased visibility requirements for the LNAV/VNAV approach minimums when compared to LNAV only.

As to which minimum you actually fly, I would use the lower of the 2, along with the corresponding technique, eg dive and drive or follow a VNAV path, if the wx really necessitated it.

That is how I recall it.

FWIW

I also found this on page 5-23 of the Instrument Procedures Handbook:

"In some cases, the visibility minimums for LNAV/VNAV might be greater than those for LNAV only. This situation occurs because DA on the LNAV/VNAV vertical descent path is farther away from the runway
threshold than the LNAV MDA missed approach point."

 

[gnvav8r]

Thank you, atldc9. I had not known about the extra "cushion" they put in for glideslope bracketing. I did run across the sentences in IPH as you quoted, as well as this from AIM 5-4-21 "From AIM 5-4-21. ...

"Protected obstacle clearance areas for missed approach are predicated on the assumption that the missed approach is initiated at the decision height (DH) or at the missed approach point and not lower than minimum descent altitude (MDA)."

So I guess you use one or the other, at least on a check ride. But I bet you'd be safe (obstacle-wise) if you just continued at DH till you got to MAP before initiating the miss. After all, we fly thru that airspace when we land.

Pete

 

[DrewBlows]

So I guess you use one or the other, at least on a check ride. But I bet you'd be safe (obstacle-wise) if you just continued at DH till you got to MAP before initiating the miss. After all, we fly thru that airspace when we land.

Pete

I'm not sure exactly what you are saying here, but I think you are saying that you would stay at DH (assuming you are doing a VNAV approach) until you get to the MAP. You should do whatever the missed approach procedure tells you to do, ie if it tells you to climb, you should climb. If for some reason you decide to go missed on a Non-precision approach prior to the MAP, you should climb straight ahead until you get to the MAP then do the missed approach procedure. Continuing at MDA to "take a look" makes no sense. The airport will look exactly what it looks like on the taxiway diagram, whether you land or not. Climbing is the safest thing to do since there is no chance of obstacles being above you (unless they start building airports in caves, in which case the previous advice is wrong).

 

[gnvav8r]

I'm not sure exactly what you are saying here, but I think you are saying that you would stay at DH (assuming you are doing a VNAV approach)

Well no... change the word "would" to "could".

I understand the nicety of doing exactly what the procedure calls for. I was struck that we now have more than one miss procedure on the same page... DH starts an immediate climb to MAP before turning, if that is called for... MDA continues at same altitude to MAP before turning... And the altitudes are different. So I was speculating that if you did hold the higher DH to MAP you'd be safe, even though the procedure calls for an immediate climb.

A small point.

Why oh why would you ever be tempted? I was also speculating that if you had a 8,000 ft runway and the vis was blotto, if you got to MAP before climbing you might see the runway beneath you and land on the last half.

Yes,... for the record. Always do what the proceure calls for.

This thread started because I couldn't figger out why the precision LPV DA was higher than non-precision LNAV.

Pete

 

[DrewBlows]

Why oh why would you ever be tempted? I was also speculating that if you had a 8,000 ft runway and the vis was blotto, if you got to MAP before climbing you might see the runway beneath you and land on the last half.

I fly mostly (99%) jets these days, so landing on the last half of the runway isn't an option (in fact landing past the TDZ isn't even an option). Even when I was flying pistons I rarely landed on the last half of the runway (sometimes I would land in the last half of a 10,000' runway for operational reasons) and never in IMC conditions.

As far as the LNAV/VNAV vs. VNAV only, if you use vertical guidance you must do the missed approach procedure listed. When you use the vertical guidance it is a DH vs. an MDA. Assuming the procedures are otherwise the same, I suppose, technically, you could use the vertical guidance as "advisory" information and level off prior to MDA and continue to the MAP, but I'd advise against this and certainly wouldn't do it on a check ride (why answer questions you don't have to?).

Incidentally, my company recently instituted Constant Angle Non-Precision Approach procedures (CANPA). We don't have vertical guidance other than "advisory", which is not approved for approach use, so the company has published descent rates based on ground speed which provide for a constant descent from the FAP to the runway. When you reach the Derived Decision Altitude (DDA) you initiate the missed approach procedure immediately. The DDA is figured by adding 50' to the MDA which allows for descent below without going below the MDA using normal missed approach procedures. When you go missed you climb along the final approach path until you reach the MAP then execute the procedure as published. This works very well because it doesn't require excess maneuvering to land like the "dive and drive" method. When you break out, you simply continue the constant descent to the runway just as you would for any visual landing.

You could do this on your own without descent rate charts by multiplying half your ground speed by 100 (e.g. 100 kts/2*10=500 fps). You still have to make sure you will not descend below any step-down fixes, but it's a nice way to do a non-precision approach.

 

[gnvav8r]

Your comments are appreciated, Drew

I became fascinated with CANPA a few months ago, and despite some negative commentary (from the likes of John Deakin at avweb.com) decided to explore what a private individual, without fancy equipment could do safely. I wrote a computer program (well excel really) that calculates the altitude of the 3 deg slope from FAF to MDA, which generally happens right around VDP, and prints out altitudes vs distance to go, so I can scotch tape it to approach plates. I've always used 5 x GS for my first cut at a descent speed when shooting an ILS.

Now I fly the approach as published, but instead of "dive and drive" I take a first cut at constant descent the same as an ILS. My wife (who is not a pilot) calls out "3 miles to go, you should be at 1500; 2 miles to go you should be at 1250" etc... so I can fine tune my descent. She also acts as the heads up "co-pilot" and if she does not see the runway, I miss even though I might have seen it. We rehearsed this in VFR conditions and I was pleased to see that when I got to MDA, generally right at VDP I had to do nothing at all. My descent was stablilized.

We done this IMC a couple of times and it seems to work well for us. Bottom line, of course, is I fly the published approach... I am not inventing a new procedure, just using guidance to make it work smoothly. Best of all we need no new equipment.

Pete

 

[DrewBlows]

CANPA is a very good approach to non-precision approaches although there are a few "gotchas". You have to make sure you make all crossing restrictions on step-down fixes. Generally a constant descent will put you above crossing restrictions but it's possible it won't. You still can't descend below MDA so you have to initiate the missed approach at an altitude above MDA, hence the DDA I wrote about above. And when you execute the missed approach you can't make any turns until you reach the MAP.

I'm guessing you have already considered these things, since you have obviously researched the subject.

Stay safe.

 

[JECKEL]

LNAV/VNAV (lateral navigation with vertical navigation) uses a barometric navigation capability. The FAA created LNAV/VNAV minimums for use by airlines that have baro-VNAV equipment. This is a navigational system that doesn't require GPS/WAAS. It creates a vertical glidepath between two waypoints or a descending angle from a single waypoint based on barometric altitude, as measured by a special barometric altimeter in the airplane.

Unfortunately, as with all altimeters, it is affectd by extremes in temperture. Higher LNAV/VNAV minimums are partially the result of compensation for a full range of possible temperature errors. In addition, at the time of their creation, before WAAS, there was a requirement for stricter obstacle clearance criteria, resulting in high decision altitudes.

 

[BushwickBill]

I will take a stab at this one. AS I recall, the reason you find LNAV min's being lower then LNAV/VNAV has to do with the obstacle clearance requirments from terps. Basically, when the approach is certified using LNAV only, the tolerances for obstacle clearance are based on a hard MDA, with no deviation below MDA. However, when LNAV/VNAV is used, the obstacle clearance must be calculated using the rules for an ILS and a DA, which allow for some normal bracketing of the glideslope. Hence, the obstacle clearance requirements are tougher. This usually results in increased visibility requirements for the LNAV/VNAV approach minimums when compared to LNAV only.

As to which minimum you actually fly, I would use the lower of the 2, along with the corresponding technique, eg dive and drive or follow a VNAV path, if the wx really necessitated it.

That is how I recall it.

FWIW

I also found this on page 5-23 of the Instrument Procedures Handbook:

"In some cases, the visibility minimums for LNAV/VNAV might be greater than those for LNAV only. This situation occurs because DA on the LNAV/VNAV vertical descent path is farther away from the runway
threshold than the LNAV MDA missed approach point."

So can I just play devils advocate here for a moment?

There has been a lot of hype lately about LNAV/VNAV (WAAS) and how wonderful it is. I was finally able to see a VNAV approach first hand about a week ago. A pilot at the local airport just upgraded his 530/430 radios and had the VNAV included. We flew some approaches into APC. The minimums were ~1200 feet for VNAV. It was cool flying a glideslope that was purely based on satellite position and a database. However in terms of practicality it didn't mean anything.

1200 feet wont get you into APC when the fog is in. Usually the ceiling hovers around 500 to 700' all morning in the summer. The minimums for the LOC only approach to the same runway are 360'! I know which approach I will be using to get into APC on a low IFR day. Maybe if the ceiling was reported at 2000' I would use the WAAS approach. It would be a little more user friendly. No LOC freq to sort out and ID (530 does this for you anyway) no data change from GPS to LOC other than that I can't think of any advantage.

Why use an approach with such high minimums and risk not getting into the airport? It can be safely done by an IFR current pilot.

It costs 1500$ for each 530 box to upgrade it to WAAS. Plus from a practicality standpoint I have to update our Jep plates and I'm not sure how many new binders we are going to have to get to fit in all of the new (WAAS) approach plates. Why do all of this for no advantage?

Do any of you think that DA will be lowered in the future to at least get down to 500' at an airport like APC? After all the (WAAS) approach is a precision approach. It should have relatively low minimums. Shouldn't it? Or do the TERPS rules mean that this technology will always be hindered and we will never see any advantage to using it. Isn't this supposed to save us lots of money in terms of not having to build ground based precision approaches anymore?

Just curious please let me know if I'm wrong. Thanks for your time.

 

[atldc9]

So can I just play devils advocate here for a moment?

There has been a lot of hype lately about LNAV/VNAV (WAAS) and how wonderful it is. I was finally able to see a VNAV approach first hand about a week ago. A pilot at the local airport just upgraded his 530/430 radios and had the VNAV included. We flew some approaches into APC. The minimums were ~1200 feet for VNAV. It was cool flying a glideslope that was purely based on satellite position and a database. However in terms of practicality it didn't mean anything.

1200 feet wont get you into APC when the fog is in. Usually the ceiling hovers around 500 to 700' all morning in the summer. The minimums for the LOC only approach to the same runway are 360'! I know which approach I will be using to get into APC on a low IFR day. Maybe if the ceiling was reported at 2000' I would use the WAAS approach. It would be a little more user friendly. No LOC freq to sort out and ID (530 does this for you anyway) no data change from GPS to LOC other than that I can't think of any advantage.

Why use an approach with such high minimums and risk not getting into the airport? It can be safely done by an IFR current pilot.

It costs 1500$ for each 530 box to upgrade it to WAAS. Plus from a practicality standpoint I have to update our Jep plates and I'm not sure how many new binders we are going to have to get to fit in all of the new (WAAS) approach plates. Why do all of this for no advantage?

Do any of you think that DA will be lowered in the future to at least get down to 500' at an airport like APC? After all the (WAAS) approach is a precision approach. It should have relatively low minimums. Shouldn't it? Or do the TERPS rules mean that this technology will always be hindered and we will never see any advantage to using it. Isn't this supposed to save us lots of money in terms of not having to build ground based precision approaches anymore?

Just curious please let me know if I'm wrong. Thanks for your time.

For the record, I don't know squat about WAAS. The LNAV/VNAV question refers to barometric Vertical navigation and the differences between LNAV only and VNAV/LNAV. I believe the LPV minimums utilize WAAS. The B-717 I fly is not WAAS equipped and therefore we don't use LPV minimums. We use barometric VNAV/LNAV.

 

[gnvav8r]

That's curious. Wonder why the LPV minimum is so high at APC? Have a look at AVL. The LPV gets you down to within 50 of the ILS DH. And Asheville is in the mountains! You'd think they could do at least as well at APC?? (and airports without an ILS or Loc approach ~ that's the whole idea of WAAS, isn't it?)

 

[BushwickBill]

For the record, I don't know squat about WAAS. The LNAV/VNAV question refers to barometric Vertical navigation and the differences between LNAV only and VNAV/LNAV. I believe the LPV minimums utilize WAAS. The B-717 I fly is not WAAS equipped and therefore we don't use LPV minimums. We use barometric VNAV/LNAV.

My first post should have read the LPV minimums at APC are 1200'. Until now I have been confused about the definitions of VNAV/LNAV and WAAS (LPV) approaches. I learn something everyday.

 

[BushwickBill]

That's curious. Wonder why the LPV minimum is so high at APC? Have a look at AVL. The LPV gets you down to within 50 of the ILS DH. And Asheville is in the mountains! You'd think they could do at least as well at APC?? (and airports without an ILS or Loc approach ~ that's the whole idea of WAAS, isn't it?)

I'm not sure why the APC minimums are so high for the WAAS approach. There is an existing LOC only approach to 36 L that will get you down to 360'. After looking at the AVL approach I can't figure out why the APC WAAS approach wouldn't be lower than 1200'. Seems as if there is plenty of terrain around AVL.

If the FAA could get the minimums as low as AVL at airports that didn't have an existing precision approach I could see what all the hype is all about.

Anyone have any ideas?

 

[DrewBlows]

Rod Machado's take on it...

Glideslopes galore
By Rod Machado



Aviation writer and humorist Rod Machado knows WAAS up — most of the time.
There are a few folks out there who believe that the Wide Area Augmentation System (WAAS) involves liposuction to help manage a pilot's waistline. I hope you're not one of them. Why? Because if you're WAAS-wise, IFR-rated, and properly equipped, the chart minimums section will have a lot more meaning for you.
What is WAAS? It's a means of continuously correcting a GPS signal to account for tiny oddities in satellite orbits, satellite clock drift, and transmission delays caused by atmospheric conditions, using information transmitted from ground-based stations. This additional degree of accuracy is demanded by the FAA in order to permit a GPS-guided precision (i.e., with a glideslope) approach (see "On Display: Going the WAAS Way," page 139).
With WAAS, your average GPS approach typically offers one of three different varieties of software-generated glideslopes for you to fly. It's glideslopes galore for those who know WAAS up.
The figure below is the minimums section for the NACO (National Aeronautical Charting Office) RNAV (GPS) Runway 24 approach to Carlsbad, California's Carlsbad/McClellan-Palomar Airport. At first glance, the far left column of the horizontal minimums equipment section (i.e., LPV, LNAV/VNAV, LNAV) is a little different from the standard format to which you've become accustomed. Let's examine each line of minimums from the bottom up.
Line by line
The bottom line shows the circling minimums for this approach. This is the minimum descent altitude (MDA) and required visibility for an approach to an airport instead of a specific runway. There's nothing new here. I'm sure you're already very familiar with this line.
The next line up is the LNAV MDA. This is the MDA for lateral navigation. In other words, these are the minimums for flying an ordinary nonprecision GPS approach, just like the ones you've probably been flying with your non-WAAS GPS receiver for some time. If you were cleared for the approach and crossed JABAL at 3,100 feet, then you could descend to 2,060 feet until reaching GUGEC, 1,240 feet until reaching ZUXAX, then 860 feet to the missed approach point (the intersections are all named after the aliens that crashed near Roswell, New Mexico.
If, on the other hand, you have a WAAS receiver (assume this to mean a WAAS-capable, IFR-certified GPS receiver), the possibility exists for you to fly a precision approach (one with lateral navigation and a glideslope) to a runway. This will be the first of three possible software-generated glideslopes you might fly.
Precision approach minimums are found in the next lineup, next to the letters LNAV/VNAV, meaning, lateral navigation with vertical navigation (a glideslope). If you flew this as a precision approach, your decision altitude (DA) would be 1,000 feet and the required visibility would be one and three-quarter miles. If LNAV/VNAV minimums were allowed for this approach (based on the GPS receiver meeting stringent horizontal and vertical accuracy and integrity requirements), the letters L/VNAV would automatically appear in your WAAS receiver's approach annunciator window. You'd now see a software-generated glideslope on your omni display (your VOR or horizontal situation indicator [HSI] display, which is slaved to your GPS). The glideslope would typically become active before (or near) the intermediate fix (KANEC). The slope of the descent path would be 3.20 degrees, with a threshold crossing height of 52 feet. In short (or in pants), you'd fly a glideslope just as you would on an ILS, except this glideslope is generated by your WAAS receiver.
In this instance, you'd probably intersect the glideslope somewhere before JABAL, at a minimum of 3,100 feet, and fly it down to a DA of 1,000 feet. At DA, if you didn't have the visibility and the runway environment in sight, you'd execute the missed approach.
Take notice that the LNAV/VNAV DA is 140 feet higher than the LNAV MDA, and the required visibility for landing is one and three-quarter miles. Isn't a precision approach supposed to get you down lower and put you in a good position for landing in low visibility? WAAS up here?
Baro VNAV
The answer lies in something called baro VNAV, or barometric vertical navigation capability.
When the FAA began designing RNAV (GPS) approaches and before WAAS was online, the airlines wanted a piece of the approach action. So, in addition to the LNAV minimums, the FAA created LNAV/VNAV minimums for use by airlines that have baro-VNAV equipment. This is a navigational system that doesn't require GPS/WAAS. It creates a vertical glidepath between two waypoints (or a descending angle from a single waypoint) based on barometric altitude, as measured by a special barometric altimeter in the airplane.
Unfortunately, baro VNAV uses an altimeter, and as with all barometric altimeters, it's affected by extremes in temperature. Higher LNAV/VNAV minimums are partially the result of compensating for a full range of possible temperature errors. Also, LNAV/VNAV approach minimums were created under TSO-c129 (FAA technical standard order) specs (before WAAS was in use) and, therefore, require stricter obstacle clearance criteria, often resulting in unusually high LNAV/VNAV decision altitudes.
Much of the airline fleet is stuck with baro VNAV, and that's probably because the airlines didn't keep their receipt when they bought it. As a result, they can use only LNAV or LNAV/VNAV minimums. You'll be hard-pressed to find even one small airplane that has baro-VNAV equipment. You'll be hard-pressed to find a small airplane pilot who'd want it, either. That's because with a WAAS receiver, you can fly to something known as LPV approach minimums that offer near-ILS performance. This is the second of three possible WAAS software-generated glideslopes.
The very top line in the Carlsbad minimums section shows the letters LPV DA. LPV means "localizer performance with vertical guidance." LPV minimums require that you have a WAAS receiver on board (not at home or on your Christmas wish list). Just to be clear here, you need a WAAS receiver to fly any software-generated glideslope (except for those of you with baro VNAV, who can fly to crummy LNAV/VNAV minimums). An ordinary GPS unit won't do. That's because an ordinary GPS unit can't generate a glideslope, leaving the glideslope needle on your HSI inactive, or off.
If you were cleared for the RNAV Runway 24 approach at Carlsbad, you'd know that LPV minimums were allowed if you saw the letters LPV appear in your WAAS receiver's approach annunciator window. If you saw the letters LPV, then you could descend to a DA of 700 feet if you wanted. The visibility minimums for this approach are 4,000 feet or three-quarter-mile visibility. This approach brings you 300 feet lower and a lot closer to the threshold than the LNAV/VNAV minimums do.
This shows how much improvement WAAS was over baro VNAV in terms of lower minimums. So, see if your chief pilot can find that baro-VNAV receipt. As an aside, most airliners can't take advantage of these lower minimums since they don't have GPS- or WAAS-capable receivers on board. Of course, you probably can't carry 400 passengers.
Keep in mind that for an RNAV precision approach to have minimums as low as 200 above the touchdown zone and one-half-mile visibility, there must be approach lighting available. Not all RNAV approaches are made to runways (or airports) with approach lights (MALSR, SSALR, ALSF), despite the fact that these same approaches may have LNAV/VNAV or LPV minimums. Even without approach lighting, it's possible to have LNAV/VNAV or LPV minimums of 200 feet above the touchdown zone and three-quarter-mile visibility, and that's not bad at all. Without approach lights, however, it's a lot more difficult to actually identify the runway environment for landing.

continued...

 

[DrewBlows]

...continued.


Type three
The third type of WAAS software-generated glideslope you might encounter is called LNAV+V. This is an LNAV approach with advisory vertical guidance. It's essentially a nonprecision approach (LNAV) with an advisory glideslope (+V) that provides vertical guidance so you don't have to worry about things like the step-down fix between the final approach fix and the missed approach point. You can fly the glideslope down to the lowest allowable MDA and, if the requirements for landing aren't met, level off and fly to the missed approach point. If you do meet the requirements for landing, you could remain on the glideslope and ride it all the way to touchdown if it pleases you (and I hope it does).
RNAV (GPS)-generated glideslopes take you to a runway intersection point approximately 1,000 feet past the landing threshold. Of course, you may ignore the vertical guidance if you want and just descend to the MDA, as you would during any nonprecision approach. You won't see "LNAV+V" in the minimums section of an approach chart because these letters don't represent minimums. In your WAAS receiver's approach annunciator window, if the advisory glideslope is available you should see the letters "LNAV+V."
The important point to notice here is that although you may want to fly an approach to LPV minimums or LNAV/VNAV minimums, or use the advisory glideslope (LNAV+V), you can do only what your WAAS receiver allows. And it will allow only the lowest minimums (LPV, L/VNAV, LNAV, or the LNAV+V advisory glideslope) for which it can meet specific accuracy/integrity requirements at the time you are flying the approach. Said another way, you can do only what your WAAS receiver's approach annunciator window shows. If LPV is shown, then you're good for a DA of 700 feet. If LNAV+V is shown, then you're good for an MDA of 860 feet, but can stay on the glideslope to help with step-down fixes.
It appears that the future is not what it used to be, because the future is here. WAAS is what's happening. It's glideslopes galore with WAAS. If you'd like to try a few LPV approaches for yourself, download Garmin's free 500 WAAS simulator online. Visit AOPA's Airport Directory Online to download NACO charts for a specific airport.