Terminal Airspace Navigation
Terminal Airspace Navigation
Approach
Historically there were two types of approach:
The pilot manually flies the vertical profile for an NPA, descending at step-down fixes until reaching minimum altitude for this phase of the approach. This is commonly referred to as ‘Dive and Drive’.
With an NPA, the pilot can lose along track situational awareness. This is particularly true of NPAs based on VOR or NDB only where distance information is not available. Loss of longitudinal situational awareness leads to a risk of descending too early, and the very real danger of controlled flight into terrain (CFIT).
Conventional NPA
As an initial step towards improving operational safety, a move away from ‘Dive and Drive’ to a Continuous Descent Final Approach (CDFA) is being made. This is a technique for flying the non-precision approach procedure as a stabilised constant descent without level-off, whilst respecting the minimum altitudes along the approach that have been calculated by the Procedure Designer.
PBN offers alternatives to PA and Conventional NPA, including approaches with or without vertical guidance.
All PBN approaches are RNP procedures (requiring on-board performance monitoring and alerting functionality).
Even without vertical guidance the improved situational awareness provided by these procedures increases safety.
The Approach phase of flight is covered by the following Navigation Applications:
RNP APCH
The standard approach specification that all aircraft certified for that operation can use.
RNP APCH can be implemented with or without vertical guidance however procedures designed with vertical guidance normally provide lower minima than those designed with just a lateral path.
The RNP APCH is loaded from the navigation database.
GPS with RAIM provides the lateral positioning to support +/-0.3NM track accuracy on final approach.
The RNP APCH to LNAV minima is expected to be flown as a Continuous Descent Final Approach (CDFA) to a Minimum Descent Altitude/Height (MDA/MDH).
RNP APCH with CDFA
If the pilots can't see the runway as they approach the minimum altitude they will initiate the missed approach procedure.
RNP APCH with Vertical guidance (APV)
The vertical path of an RNP APCH final approach is loaded into the navigation database.
There are 2 types of APV, both of which are flown to a Decision Altitude/Height (DA/DH):
APV BARO VNAV
It uses GPS with RAIM for lateral guidance and Barometric altimetry for vertical guidance,
It is flown to an LNAV/VNAV minima.
Ensuring the correct QNH is set is critical for these procedures.
An incorrect QNH setting will result in the aircraft being either higher or lower than the desired vertical path. For example, a 10 millibar error results in + or - 280ft position error vertically.
For Baro VNAV, accurate vertical guidance is based solely on the correct QNH being set. Therefore, to avoid errors:
APV SBAS
It uses SBAS augmented GPS for both the lateral and vertical guidance.
SBAS provides better performance than GPS with RAIM (Horizontal and Vertical Alarm Limits of +/-40m and +/-50m respectively for APV SBAS procedures based on APV 1 procedure design criteria),
It is flown to a Localiser Performance with Vertical guidance (LPV) minima,
Under certain circumstances APV SBAS can offer minima similar to ILS CAT I minima (200ft),
The vertical profile of an APV SBAS is coded in the aircraft's database in the form of a ‘Final Approach Segment (FAS) datablock’ which provides higher data integrity.
NP APCH with vertical guidance (APV) - SBAS
RNP AR APCH (Approval Required)
The video and chart below shows the kind of ‘rich’ terrain that RNP (AR) APCH was originally designed. The approach is into Queenstown, NZ for Runway 05:
Note: This approach has now been superseded. With thanks to Qantas and Naverus for permission to use the chart.
It is a complex approach requiring specific functionality and special authorisation.
Operators will need to demonstrate the appropriate capabilities and the regulatory authorities of the State publishing the procedure will certify the aircraft and crew.
An RNP AR APCH is used where terrain or obstacles will not permit a normal RNP APCH to be used.
This type of approach requires specific approval from the State's Regulator and may necessitate additional aircraft equipage i.e. dual GNSS systems and Inertial platform, aircrew procedures and pilot training.
The lateral track accuracy required can be as low as 0.1NM and aircraft functionality may be used in the final and missed approach segments.
From a procedure design perspective, the track can be placed as little as two times the required track accuracy from obstacles or terrain.
The procedure design benefits from a closely defined vertical error budget.
SBAS systems, if approved, may be used on RNP AR APCHs.
AR procedures may also be employed for ATM purposes (e.g. closely spaced tracks, or environmental mitigation).