Instrument approaches

[slick1537]

Can anyone please explain the differences between

 

LPV

LNAV/VNAV

LNAV

 

Also the differences between a

 

RNAV GPS approach and

GPS stand alone approach

 

thanks for the help!

[PhotoFlyer]

LPV and LNAV/VNAV are precision GPS approaches. They use WAAS or a barometric pressure sensor in the GPS to provide precision glideslope information to the pilot. (Someone please correct me if I'm wrong!)

 

LNAV is a standard non-precision GPS approach.

 

In a practical sense (how they are flown), there is no difference between a RNAV GPS approach and a GPS standalone approach. I think (not certain) that there is a difference in the way the two types of approaches are certified, but I would have to get back to you on that.

 

Check out this link. The instrument procedures handbook is a plain language (well, as plain as instrument flying can be) version of the TERPS. It explains everything you ask and more. I don't have time to look these approaches up now, or I would.

 

Edit:

From the Instrument Procedures Handbook

 

Stand-alone GPS procedures are not based on any other

procedures, but they may replace other procedures. The

naming convention used for stand-alone GPS

approaches is “GPS RWY XX.” The coding for the

approach in the database does not accommodate multisensor

FMSs because these procedures are designed

only to accommodate aircraft using GPS equipment.

These procedures will eventually be converted to

RNAV (GPS) approaches. [Figure 5-10 on page 5-12]

RNAV (GPS) approach procedures have been developed

in an effort to accommodate all RNAV systems,

including multi-sensor FMSs used by airlines and

corporate operators. RNAV (GPS) IAPs are authorized

as stand-alone approaches for aircraft equipped

with RNAV systems that contain an airborne navigation

database and are certified for instrument

approaches.

Edited July 25, 2008 by PhotoFlyer

[KrisRamJ]

Many of those confusing designation systems like LNAV/VNAV etc are to do with how rapidly the GPS infrastructure has grown and developed over the years.

 

Early GPS precision approaches were LNAV/VNAV with lateral guidance from GPS and barometric pressure for glideslope. When WAAS came along that could be substituted for VNAV, but a new class of approaches was also developed to take advantage of WAAS - the LPV approaches - with lower DA's.

 

It's similar with RNAV GPS and standalone GPS approaches. There were systems before GPS that were RNAV systems such as TACAN etc, so an RNAV approach didn't necessarily have to just use GPS.

[Jeff]

Can anyone please explain the differences between

 

LPV

LNAV/VNAV

LNAV

 

Also the differences between a

 

RNAV GPS approach and

GPS stand alone approach

 

thanks for the help!

 

There are a lot of subtleties between the approaches, but the biggest difference in the obstacle clearance plane. LPV typically has lower minimums because the RNP decreases as you get closer to the MAP (much like a localizer). There is also a vertical path to follow. LNAV/VNAV usually has higher minimums than the LPV because when designing the approach, the designers look further from the approach course for obstacles. LPV and LNAV/VNAV approaches are both with WAAS. LNAV is your standard GPS approach with no vertical guidance, except the pilot looking at the altimeter.

 

Garmin has an LNAV +V on their WAAS machines. This is exactly the same as an LNAV approach, except Garmin provides the pilot with an advisory glidepath indication. LNAV +V is proprietary to Garmin. Hope that helps.

 

~Jeff

[Collective Down!]

LPV = Localizer Performance, with Vertical Guidance

LNAV/VNAV = Lateral and Vertical Navigation

LNAV = Lateral Navigation

LNAV+V = Lateral Navigation with advisory vertical guidance

 

If you don't have a WAAS enabled GPS, the only one minimums that you can use is LNAV. There will be no glideslope information at all, and the GPS will only give you left/right guidance.

 

If you're using a WAAS (Wide Area Augmentation System) GPS, you have to pay close attention to the GPS screen when you're just outside of the final approach fix, inbound on the final approach course... It will annunciate which charted minimums you can use. In the lower left corner, it will either say "LPV" , "LNAV/VNAV" , "LNAV+V" , or "LNAV." LPV requires the best satellite geometry and integrity and such. If those requirements are not met, it gets bumped down a step (in the order I listed them).

 

LPV and LNAV/VNAV are nearly identical from the pilot's standpoint. They sometimes have different decision altitudes and visibilities because the design criteria for each of them is slightly different. The LPV will narrow down as you near the runway, similar to an old-fashioned localizer and glideslope. The LNAV/VNAV lateral and vertical guidance has the same sensitivity from the final approach fix, all the way to the missed approach point (arrival at decision altitude), so it doesn't get too twitchy near the runway.

 

The one you need to be rrrreally careful with is LNAV+V (Lateral plus advisory vertical guidance). You will never see "LNAV+V" minimums on the chart, you just use the LNAV minimums. The advisory vertical guidance is only intended to stabilize your approach so you have a more steady rate of descent from the FAF to the MAP, in hopes of preventing controlled flight into terrain I presume. However, the advisory vertical guidance does not ensure that you'll cross step-down fixes at the appropriate altitudes, and that's the big gotchya. You have to ignore the glideslope to level off at the step-down fixes if you reach the crossing-altitude before you pass the fix.

 

You can download the Garmin 430 WAAS simulator and 430W pilot's guide for free online... Just search for it on Google. I'm not sure how well the sim models the operation, but I've flown with the 530 WAAS, and it's incredible! The screen update is lightning fast, and the lateral and vertical needles are rock solid on LPV and LNAV/VNAV approaches.

 

By the way, none of the WAAS GPS approaches are "precision approaches," they are considered "APV's".. Approaches with vertical guidance. A precision approach needs an electronic glideslope (ie a radio signal, not just a calculated glideslope). It's definition difference is not a big deal for part 91 helicopter instrument pilots, but alternate airport criteria for airplanes are different depending on the alternate having a precision or non-precision approach available.

 

Check out "Instrument Flying Update" by John C. Eckalbar. His book goes into deep detail about everything related to GPS, particularly WAAS.