Understanding and equipping for different GPS-based approaches
What you need in the cockpit along with training to take advantage of the latest precision space-based landing systems.
By Mike Venables
Approach plate for MDT (Intl, Harrisburg PA).
Since the first deployment of the instrument landing system (ILS) in 1938 in Pittsburgh PA, no new radio-based precision approach aids were introduced until GPS came along.
Purists will point to the microwave landing system (MLS) as a newer alternative to ILS but, while this is true, it was never really deployed in North America. Ground controlled approach (GCA), or precision approach radar (PAR) as it is sometimes called, doesn't really count either it was not autonomous.
Now, there is a bewildering variety of GPS-based approach types available with differing degrees of precision and utility, including LNAV, LNAV/VNAV and LPV. There is another type of procedure called localizer precision (LP) but there are very few, if any, in existence, so they will not be addressed here.
It is clearly understood that lateral navigation (LNAV) approaches, without vertical guidance, are nonprecision approaches (NPAs) and that localizer performance with vertical guidance (LPV) approaches are considered precision approaches with limits similar to ILS.
What is not so well understood by many pilots is the role played by lateral navigation/vertical navigation (LNAV/VNAV approaches). The common assumption is that they are better than LNAV-only approaches but not as useful as LPV (in terms of minima).
In most cases, this assumption holds true, but there are situations where LNAV alone provides lower minima than LNAV/ VNAV and these exceptions have confused more than one pilot and led to busted minimums.
The RNAV (GPS) Rwy 13 approach to MDT (Intl, Harrisburg PA), shown in the figure below, illustrates this anomaly. The LPV procedure gets you down to 300 and 1/2 (height above touchdown and visibility, respectively), the LNAV minima for Category B aircraft are 900 and 3/4, while the LNAV/VNAV minima are way higher, at 1300 and 5! Further adding to the confusion is that the circling minima, at 900 and 1-1/4, are also significantly lower than the LNAV/ VNAV minima.
All approaches are designed in accordance with the instrument procedure design criteria—a complex set of rules that provide a minimum clearance between the aircraft and the ground and man-made obstructions. (The current version, applicable to GPS—FAA Order 8260.54A—is some 350 pages.)
The minimum clearance from terrain and obstructions depends on the phase of the procedure. An LNAV approach provides no altitude guidance (other than the altimeter), while the LNAV/VNAV approach provides vertical guidance. However, this vertical guidance is only available during the final approach phase prior to reaching the decision altitude (DA).
There is no vertical guidance from then on if a missed approach is necessary.
The required clearance during the final approach phase of an LNAV/ VNAV procedure is typically lower than that of the LNAV alone, but the clearance for the missed approach phase is no better than that of the LNAV-only missed approach phase. Then why are the LNAV/VNAV minima in this case worse than the LNAV alone?
John Ainsworth, president of Air Navigation Data, a company specializing in instrument procedure design services and software, provides the answer. He says, "The missed approach point for the LNAV procedure is usually at the runway threshold, while the point where the aircraft reaches the DA on the LNAV/VNAV procedure is typically a mile short of the runway.
There's probably an obstruction that is behind the aircraft when it's at the threshold but still in front (albeit off to one side) when the aircraft is at the DA for the LNAV/VNAV procedure.
The LNAV procedure takes the aircraft safely past this obstruction before the missed approach needs to be initiated. The missed approach for the LNAV/VNAV procedure, on the other hand, needs to be initiated before reaching this obstruction and, as vertical guidance is no longer available, greater clearance from the obstruction must be
Ainsworth ran the MDT approach through "Final Approach"—Air Navigation Data's procedure design tool—and confirmed that the 1079-ft obstacle, seen on the approach plate immediately southwest of the runway threshold, is the culprit.
Circling MDA lower than LNAV/VNAV DA
Honeywell Primus 2000 after LPV upgrade.
Another area for confusion is that the circling minima are much lower than the LNAV/VNAV minima. According to the FAA definitions, both the LNAV/VNAV and the LPV procedures are considered approaches with vertical guidance (APVs), while (as stated above) the LNAV procedure, lacking vertical guidance, is an NPA.
The charting rules state that the circling minima can be no lower than the associated NPA. Neither the LPV nor the LNAV/VNAV procedures are considered in the determination of circling minimums in this case.
These 2 anomalies can cause confusion and have done so. Confusion in the cockpit, needless to say, is not a good thing.
This issue has been addressed by the US Aeronautical Charting Forum—a government and industry group that makes recommendations to FAA. The group has recommended that, at the least, whenever the LNAV/ VNAV DA is higher than the LNAV MDA, the LNAV/VNAV procedure be published as a separate procedure with its own "plate" showing circling minima no lower than the LNAV/VNAV DA.
They have further recommended that, if the LNAV/ VNAV minima are more than 60 ft higher than the associated LNAV procedure, the entire LNAV/ VNAV procedure be eliminated. This makes sense when we look at the origins of LNAV/VNAV procedures.