Differences Between LNAV/VNAV and LNAV Minima on a GPS Instrument Approach
Charles, a 2000+ hours ATP asked the following:
“Hi, I have a question about GPS approaches. Why are LNAV and LNAV/VNAV minima different on RNAV GPS Approaches?”
Max Trescott - Author and 2008 National CFI of the Year
Great question Charles. Things are changing so fast in the GPS world that it’s hard for instrument pilots to keep up with all of the new acronyms and just what they mean. But you want to understand them before you’re stuck in the clouds, furiously twisting knobs, while hoping to avoid an encounter with a cumulo-granite cloud (Ouch!).
I’m going to be self-serving–since it will save me a lot of typing–and quote directly from two of my books and CDs. Many readers may not be familiar with LNAV/VNAV minimums for a GPS approach, so let me quote from my new Max Trescott’s GPS and WAAS Instrument Flying Handbook (which I might add would be a great Christmas gift for pilots to buy now for themselves!):
“As previously mentioned, airliners with special equipment fly to LNAV/VNAV minimums and now you can too with a WAAS-capable receiver. From a practical standpoint, however, you’ll probably use these minimums only on the handful of approaches for which no LPV minimums are designated, since LNAV/VNAV minimums are almost always higher than LPV minimums. In very rare cases, LNAV/VNAV minimums can be lower than LPV minimums, due to differences in how obstacles are evaluated for these approaches. So surprisingly, a GA pilot’s WAAS-capable receiver that can fly to LPV minimums is far more versatile than many airliners’ equipment, which cannot go lower than LNAV/VNAV minimums. Like LPV minimums, LNAV/VNAV minimums are specified with a DA or decision altitude.”
Charles, you wanted me to contrast that with LNAV minimums. Let me briefly explain to readers that LNAV, which stands for lateral navigation, is the tradition minimums to which we’ve always flown GPS approaches. Originally there was only one minimums type for a GPS approach, so those minimums didn’t need a name. Now that there are 5 different minimum types available for GPS approaches, the original ones needed a name so they’re called LNAV. LNAV minimums are basic, non-precision minimums. Just fly to the MDA (minimum descent altitude) specified and don’t go an inch lower than that altitude!
Now we need to know how what pilots call the “protected area” differs for LNAV/VNAV and LNAV minimums. This time let me quote from the audio track of one of my computer courses, Max Trescott’s WAAS and GPS CD-ROM Course (another great gift idea for a pilot to get him or herself for Christmas).
“Let’s talk briefly about the protected areas for LNAV/VNAV approaches. First, the horizontal protected area is the same as that used for constructing a LNAV approach, so the equipment required to fly these approaches only has to meet the TSO-C129a, which is the standard for non-WAAS capable receivers. Since the navigation will therefore be less accurate, larger protected areas are needed than for an LPV approach.”
Now we know that the horizontal protection areas are the same for LNAV/VNAV and LNAV minimums. So essentially your question would be identical to “Why are the minimums different between an ILS and a localizer approach.” The answer is of course the glide path (or in the case of an ILS, the glide slope). Non-precision approaches resemble the steps in a flight of stairs and pilots can immediately descend down to the next altitude at each step. But the glide path of a RNAV (GPS) approach more close resembles the handrail for a flight of stairs. It provides a smooth, continuous descent path. The glide path can be designed to avoid obstacles that might force a higher MDA for a non-precision approach. So the answer to your question is that LNAV/VNAV minimums are usually lower than minimums LNAV minimums since it incorporates a glide path, helping it avoid some obstacles.
Jim on Dec 20, 2009
Great timing, I was just reading that chapter last night. Thanx again for a great resource.
Patrick Flannigan on Dec 21, 2009
So now you’ve got me curious about LNAV/VNAV versus LPV. Are receivers capable of flying to LPV minimums simply more accurate, thus requiring less obstacle clearance, and therefore leading to lower minimums (generally)?
Max Trescott on Dec 21, 2009
In a word, Yes.
A WAAS receiver is required for LPVs but not for LNAV/VNAV.
TreeHouseForRent on Dec 24, 2009
We’ve all heard about “negative transfer” in the past. An easy example of it is how it pertains to many new student pilots who have to unlearn what the steering wheel is really about in a primary airplane.
Well, that negative transfer can also occur during instrument chart interpretation. It happened to me. As I was flying a Hawker Jet some 100 miles from our destination, my co-pilot and I chose the lowest possible MDA for our arrival. As we we’re well conditioned to always look at the bottom left of the approach chart for the lowest available MDA, we found that we could go 550 ft above the ground at the lowest point for our category. As the ASOS was reporting 700 feet, we knew it would be close, but OK.
It was only after we landed that I noticed the LNAV/VNAV tab directly above the published MDA!
We never would have even seen the airport with the MDA that was actually available to us. I had not done a GPS approach in awhile. Oops!
Max Trescott on Jan 14, 2010
TreeHouseFor Rent, Great story! Just shows how with the addition of new minimums we have to pay even more attention to the labels associated with each minimum. The lateral obstacle clearance area is the same for LNAV/VNAV and LNAV, so you were probably safe there. However it was a dice roll as to whether the vertical protection would work out in your situation. Glad you’re around to tell the story!
Patrick Vieux on Oct 28, 2010
Hi Max,
You said that a WAAS receiver is required for LPVs but not for LNAV/VNAV.
Morever the accuracy level required by LNAV/VNAV , according to DO229 standard (Class Beta 3) is only reachable by a GPS receiver using WAAS corrections (Fast, long term and ionospheric).
It seems to me that the difference between LPV and LNAV/VNAV is about the using of Final Approach Segment
Sorry for my english (I’m French)
Max Trescott on Oct 28, 2010
Patrick, The airlines flew RNAV/VNAV approaches for years before WAAS became operational. They did it with baro-VNAV equipment in combination with a TSO-C129a GPS receiver, which is a non-WAAS GPS. I’m not familiar with the DO229 standard, but I assure you that RNAV/VNAV approaches were flown long before the first WAAS GPS receiver became available.
Kel on Apr 25, 2011
Max,
Please take a look at RNAV (GPS) RWY 22L (LFT)
We have there a LNAV/VNAV DA of 511′ but an LNAV MDA of 480′
What’s tha deal?
Kel.
Dan on Apr 27, 2011
Kel- During a normal LNAV approach, the aircraft levels off at minimums and should never go below the LNAV minimums until the runway is in sight. When shooting an approach with VNAV, you are descending during the approach. Thus your minimums are slightly higher to account for the altitude you will lose during the process of going missed. Hope this helps.
Dan
CAPT. SAAD on Jun 15, 2011
Max, i’m ATP in Brazil and fliyng ERJ-145 (by EMBRAER on PASSAREDO airlines) that have FMZ 2000 (FMS) Honeywell NZ5.2 FMS software version and this kind of airplane present by FGC LNAV button and only VNAV guidance because FGC do not have VNAV button. I’d like to know if I can do LNAV/VNAV (DA) procedures or only LNAV procedures (MDA). If possible send me some docs that present specific criteria.
Best regards
Congratulations for your Blog.
Dan on Jun 28, 2011
My understanding of the rules are LNAV/VNAV approaches may only be flown if the equipment is certified for these approaches and the autopilot can couple to the approach. The FMZ 2000 is certified, but since the autopilot will not couple with the VNAV info, you may only fly the approach as LNAV. It’s the same way in our Falcon with a Universal FMS.
Mike on Aug 01, 2011
I am a new pilot, got my privite pilot certificate Oct. 2010 and have scheduled my instrument check ride for 8/11/2011. How do I know if the KLN94 is Baro aided non-wass is approved for LNAV/VNAV? The users guide says it is for Non-precision approaches but it does couple with the autopilot and will desend at the proper way points. The GPS approaches I have trained on have not had LNAV/VNAV DA’s but the examiner told me to expect a gps approach at an airport that has LPV, LNAV/VNAV, and LNAV. I realize I can’t use the LPV but I am unsure about the LNAV/VNAV.
Curtis Johnson on Apr 22, 2012
What does the acronym LPV actually stand for?
Max T on Apr 22, 2012
Curtis, LPV stands for Localizer type Performance, with Vertical guidance. At one time, different FAA sources simultaneously said that: 1) LPV didn’t mean anything, 2) means Localizer Performance with Vertical guidance 3) another definition that I can no longer find.
Christos Flagkakis on Nov 03, 2012
LPV = Localizer Performance with Vertical guidance but that goes to Localizer approaches.
In the RNAV/GPS approach world the LPV simply means Lateral Precision Performance with vertical guidance.
You may also want to know, APV = Approache with vertical guidance, which is associated.
The hierarchy when it comes to GPS minimums goes as follow,
1. LNAV (MDA)
2. LNAV+V (MDA)
3. LNAV/VNAV (DA)
4. LPV (DA)
There are always exceptions.
QFI 93 on Jan 03, 2013
Interesting discussion since it mirrors one I’ve beem having with my coworkers lately. The issue comes down simply a lack of clarity being provided by both national regulators and system manufacturers. Regulations often contradict or are outdated. Certification is confusing and often buried under layers of tech talk. (Similar ti lawyer speak). Here is my understanding.
Equipment certified TSO C145 & C146 are WAAS certified and can fly RNAVS to any published RNAVS minimums.
Equipment that is TSO C129 is only able to descend to published LNAV minimums.
In order to descend to LNAV/VNAV minimums you need a few additional things in addition to TSO C129. You also need to be BARO VNAV certified as well as being a multi sensor FMS. These last two points cause the most confusion. BARO VNAV requires a barometric input in order to produce a VNAV solution. The equipment manuals and supplements should indicate where this input is received from. The second term, which causes confusion is the whole multi sensor FMS. Contrary to many people’s understanding, GPS’s used in aviation are not considered sole source unless they are installed with a completely separate and parallel system. (Required for aircraft using GPS as sole sorce navigation eg: oceanic). Multi sensor in this case refers to a Best Computed Position (BCP) which is derived by a GPS system augmented by either DME, VOR, TACAN or inertial system. In the case of the system I use, it takes its GPS position and refines it by scanning up to 15 DME sources within approx 300NM. It also takes a limited amount of VOR information. Both come from the #1 VHF reciever. Failure of this reciever means I’d no longer be multi sensor and could not shoot an approach to LNAV/VNAV minimums but COULD still go to LNAV minimums.
The question I have I have for people here is this. Where LNAV/VNAV DH’s are significantly higher than the LNAV MDA’s what is stopping me from using my VNAV all the way past the LNAV/VNAV DH, and ensuring I don’t descend below my LNAV MDA. I’ve seen differences exceeding 140′ between these two minimums where the DH exceeds the MDA. I assure you it does not take me 140′ to hit a DH, make a decision, apply power and execute a go-around. I’ve also seen DH’ significantly lower than LNAV MDA’s. What operational benefit is there then to a LNAV/VNAV if the weather minimums are higher than if I just used the LNAV MDA?
Dave Affinito on Jan 30, 2013
I believe the LPV approach assisted by WAAS is similar to the ILS in that the guidance courses (lateral and vertical) get more sensitive as you approach DA. In other words, the course and vertical guidance gets “narrower” as you get closer to the airport. I do not believe the same is true with a VNAV approach. There, I believe the course keeps the same width and you are still bound by an MDA. I recall this from Rod Machado’s Instrument flying handbood, but I haven’t confirmed it recently.
Max Trescott on Jan 30, 2013
Dave,
I totally agree with your comments about LPVs becoming increasingly more narrow versus LNAV approaches, which remain at a fixed width (plus and minus 0.3 NM from centerline) on the final approach segment.
Gugu Mnguni on Jun 01, 2013
Hi Mr Trescott, I am a new flight inspector and sooner we will start with the testing of GNSS(RNAV) procedures. I would like to find out if there is a software or internet simulator that shows how an aircraft approaches an airport runway. Thanks
Brent on Jul 08, 2013
The author glossed over a very important point that the AIM clarifies:
In section 5-4-20:
“5. In rare cases the LNAV minima may have a lower HAT than minima with a glide path due to the location of the obstacles. This should be a clear indication to the pilot that obstacles exist below the MDA which the pilot must see in order to ensure adequate clearance. “
When the minimums for the more precise approach are higher than the less precise on the GPS plate, it is a *warning* to the pilot. This is similar to the implied warning/caution when the visibility minimums on an ILS plate are greater than 1/2 mile.
Patrick on Oct 29, 2013
HI everyone!
It still takes me looooong time to get an answer. I’m still not sure to clearly understand what’s going on. About minimums in CYFB (Iqualuit Airfield): VNAV minimums for RNAV GNSS RWY 17 are 1020ft. Same RWY, LNAV minimums are 740ft only. Why such a huge gap? Obstacle calculations? Thanks!
Peter on Apr 25, 2014
The lower minimum for LNAV only compared to LNAV /VNAV is not due to the different obstacle situation. VNAV rather has a built in temperature correction, usually to -15 degrees C. LNAV does not and requires a manual temperature correction for any ISA deviation e.g. 4% per 10 degrees C
Max on Apr 25, 2014
Peter, agreed, the areas evaluated for obstacles are the same for LNAV and LNAV/VNAV. LPV’s has smaller areas and hence lower minimums. Regarding the temperature compensation formula, I believe the full formula is: 4% per 10 degrees C per every thousand feet above the elevation of the reporting station.