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Asked by: david tobar
LPV Flight Instructor, Instrument Rating
i Have waas capability in my GPS wich One should I use the lpv or lnav/vnav
i think the lpv but why????
Timothy Broadwater on Feb 28, 2013
You need to check the annunciation on the GPS prior to the approach and it will tell you which minimums to use. LPV, LP, L/VNAV, LNAV+V or LNAV all depending on the conditions at the time.
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Koehn on Feb 28, 2013
Neither until you’ve gone over them with a CFII familiar with your GPS.
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John D. Collins on Feb 28, 2013
The WAAS GPS annunciates the level of service available at the time of the approach. In general, if there are LPV, LNAV/VNAV, and LNAV minimums available on an approach, usually the LPV will offer the lowest minimums, but not always. Most WAAS GPS units are authorized to fly the LPV or LNAV/VNAV, or LNAV minimums and in most cases, you can ignore the LNAV/VNAV minimums as they don’t often offer an advantage if there are LPV minimums. There are a few exceptions, most notable is the case where LPV minimums are not published or in extremely rare situations, the signal from the GPS constellation is adequate for LNAV/VNAV but not for LPV. In these cases, L/VNAV will be annunciated. In many more cases, the LNAV option may be a good option when it has lower visibility minimums and the conditions prohibit seeing the runway at the DA for the LPV. This is because the DA for the LPV will almost always put the aircraft further from the runway than the LNAV which typically takes you all the way to the threshold. So when you review the minimums available and compare it to the prevailing conditions, you may decide to fly the procedure which provides you the greatest chance of completing the approach and landing safely.
Understanding the meaning of the annunciations is important to understanding your options. When the FAF is the active waypoint on the approach, the approach annunciation will be displayed. The annunciations indicate a level of service that is supported for the approach. The pilot is authorized to fly any of the procedures on the same approach chart as long as the level of service is the same or higher. That means that if the annunciation is LPV, the pilot may fly the LPV or LNAV/VNAV or LNAV procedure. If the annunciation is L/VNAV, the pilot may fly the LNAV/VNAV or LNAV procedure, and if the annunciation is LNAV, then the pilot may only fly the LNAV procedure. Although the annunciation is displayed as soon as the FAF becomes the active waypoint on the approach, it is possible for the approach to be downgraded to LNAV when you are one minute flying time to the FAF waypoint. If there is a downgrade at this point, the only option available is the LNAV procedure and no vertical guidance will be provided (no GS). The pilot should always be ready for this eventuality.
There is a relatively new procedure type which will annunciate as LP if you are permitted to fly it. Not all WAAS GPS units have this capability, so if LP is on the approach chart, you may not use the LP procedure minimums unless LP is specifically annunciated. LP has no vertical guidance and is sort of like a localizer only procedure. A LP procedure will only be charted with a LNAV procedure and never with a LPV or LNAV/VNAV. LP is a higher level of accuracy than the LNAV, so it also has the possibility of downgrading to a LNAV. A pilot with a GPS (WAAS or not) can always fly the LNAV procedure, but the GPS must be WAAS to fly the LPV, LNAV/VNAV, or LP procedure.
There is more nuance to this topic, but this post is long enough.
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