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.