RNAV approaches - How prepared are you? (Oct 2016)

After 50 years of comparative stability in IFR navigation and approaches, we are now in a time of unprecedented change. For the second half of the last century everything was pretty stable with ADF, VOR, DME, ILS and Radar, all of which were ground-based, with pretty much the same cockpit operation between different manufacturers and different installations.

We are now in a time of great change. ICAO resolved that there should be an approach with vertical guidance to all instrument runways, and the European Commission, Eurocontrol and EASA are strongly encouraging ANSPs and airports to achieve this goal by 2024. Furthermore, airspace constraints mean that aircraft have to be operated closer and closer together, and that means that they must be navigated and manoeuvred more accurately. Most of this will be achieved with RNAV technology.

What does this mean for us? Well, RNAV approaches are good news for light general aviation. We get high quality navigation guidance at reasonable cost and the lack of ground based equipment means more approaches at smaller airports. But we also have to make our contribution. By 2019 pilots and aircraft operating in controlled airspace will need to be PBN capable. That will entail Theoretical Knowledge and flight training. It will also require IFR aircraft used for skill tests and proficiency checks to have RNAV approach approval.

The growth of RNAV, particularly LPV, approaches is very rapid, over the past few years they have been most common in France, Benelux and Germany, but now what they are spreading all over Europe, particularly at smaller airfields which have traditionally only had non precision approaches.

Different words for the same thing?

These approaches have been called so many different things over the years, that there is a certain amount of confusion among pilots as to what each set of initials really means. Initially they were called GPS approaches. However, as other satellite systems, such as GLONASS and Galileo, were launched, the designation had to be changed to GNSS , which covers all the different constellations. But then it was agreed that other area navigation technologies could be used, provided they were accurate enough. Hence the next change to RNAV. But it was then recognised that technology used for approaches, particularly precision approaches, must have a degree of error recognition and handling. An RNAV system, in order to meet the standards of Required Navigation Performance, must have fault awareness. Hence the correct current name for these approaches is RNP.

But the letters we hear most bandied about are PBN. Performance Based Navigation has not only the requirements of precision and resilience provided by RNP, but also encompasses a wide range of other requirements, such as procedures, training, certification and much more. That is why we must become PBN qualified to be allowed to operate in PBN airspace.

RNAV/RNP/PBN Procedures

PBN brings with it a greater expectation of automation and very accurate track keeping, more reliance placed on aircraft, less on ATC, hence less vectoring and more following SIDs and STARs and more procedural (as opposed to radar vectored) approaches. PBN means that pilots need to understand their equipment, procedures and obligations such as:

  • Ensuring that GNSS databases and Plates are up to date
  • Ensuring that equipment is working
  • Checking satellite coverage and availability
  • Referring to up to date plates, charts and NOTAMs.

Augmentation principles

All certified GNSS equipment has one of two kinds of augmentation system.

Older (TSO129) units have Aircraft Based Augmentation System (ABAS), which means RAIM for GA. RAIM is fault aware, but not fault resilient (though some units go beyond the specification and provide a level of fault resilience.) This means that the pilot will be warned that the integrity of the system has been compromised. The most common of such units is the GNS430.

Newer (TSO146) boxes have Satellite Based Augmentation System (SBAS). SBAS increases reliability and accuracy and is fault resilient (it will exclude dubious signals from particular satellites and resolve a position based on those that remain). SBAS is called WAAS in USA and EGNOS in Europe. Confusingly, the ‘W’ in GNS430W stands for WAAS, although that is not the correct terminology in Europe.

SBAS integrity monitoring is required for vertical guidance. No SBAS:- no glideslope.

The SBAS system is based on a number of small satellite receiving stations called RIMS (Ranging and Integrity Monitoring Stations), which are fixed on concrete plinths all over and beyond the area of coverage. When they receive a GPS signal, a central system calculates the difference between that and their known position, horizontal and vertical. The difference signal is retransmitted to geo-stationary satellites. The onboard SBAS receiver combines the corrections with its own GPS reception to achieve very high accuracy of typically 1m horizontally and 2m vertically.

Readers will also come across a system called GBAS. This provides CATIII-like accuracy for CAT and is outside the scope of this article.

Augmentation and regulation

The Aircraft Flight Manual must state what approaches airframe is approved for. If the AFM does not specifically allow for a particular approach and then you are not permitted to fly it. We have heard of ramp checks being made on the aircraft which have arrived using an RNAV approach.

Two dimensional (2D) approaches require RAIM for fault detection. Loss or unavailability of RAIM, which generally means five usable satellites in view, means that the approach cannot be started or continued. RAIM must be checked, online or on the receiver, before the approach is made. RAIM availability forecast can be checked in advance on http://augur2.ecacnav.com/augur/app/npa. If RAIM is not forecast to be available, the approach must not be started. If a RAIM or integrity warning appears during the approach, it must be immediately discontinued and a missed approach started.

Three dimensional (3D) approaches require SBAS. 3D has generally replaced the expression “precision”, i.e. those with vertical guidance. SBAS supersedes RAIM, so RAIM no longer needs be checked. As I mentioned above, in order to perform a 3D approach you need both suitable equipment and approval. The most common light aircraft certified equipment capable of 3D approaches are the Garmin GTN series, the GNS W series, later G1000s and, more recently, the Avidyne IFD540. If you have an older GNS unit, it can be upgraded relatively cheaply to W standards, and, thanks to PPL/IR Europe, can be a certified on most light aircraft very cheaply.

The different types of approach

There is, quite justifiably, quite a lot of confusion between the different types of approach available.

LNAV and LNAV+V

Non-SBAS boxes are only capable of LNAV approaches. They are annunciated as APR, because, when they were designed, they were the only approach available. Unlike conventional approaches (ILS, VOR, NDB) the horizontal guidance provided by a non-SBAS box does not increase in sensitivity as you approach the runway. It only provides a pair of tram lines 0.3 nautical miles either side of the final approach track. That makes it rather inaccurate as you approach the runway, which is one of the reasons why LNAV minima tend to be rather high.

An SBAS box performing the same LNAV approach annunciates LNAV. However, in the case of Garmin boxes (and I don’t know if this is also true of other manufacturers) the accuracy and sensitivity of the horizontal guidance is considerably greater than in non-SBAS boxes, similar to that of an ILS localiser. There is, however, no certified vertical guidance. But, although the accuracy of an LNAV approach using an SBAS receiver is greater than on a non-SBAS box, no advantage can be taken in terms of minima, because the approach designers have to assume the worst case of a non-SBAS receiver being in use.

Further confusion is added by the LNAV+V approach. From a regulatory and design point of view, the LNAV+V approach is exactly the same as an LNAV approach. However, Garmin provides an advisory glideslope from the Final Approach Fix (FAF) to the Missed Approach Point (MAP). This glideslope assists the pilot in ensuring that the aircraft follows the non-precision descent profile, and passes through all the check altitudes. However it is not a certified to glideslope and it remains the responsibility of the pilot to check his descent using conventional means, usually GPS range against altitude. Unlike certified 3D approaches, the glide slope is not guaranteed to provide terrain separation after the MAP. In a few places this can lead to distinct danger as the synthesised glide slope penetrates terrain.

Pilots using LNAV+V must use LNAV minima, even though the display, sensitivity and accuracy are much the same as for the ILS.

LNAV/VNAV

LNAV/VNAV approaches were an early attempt to replicate barometry based glideslopes, which are only found in large commercial aircraft. They are comparatively rarer than other kinds of approach but there are still plenty of them about.

On Garmin boxes the sensitivity of both needles is similar to an ILS, but that is not required by the specification, hence different minima. So if you are making an LNAV/VNAV approach you should use of the LNAV/VNAV minima. It annunciates as L/VNAV on the receiver. The glideslope is certified and should provide safe terrain and obstacle clearance all the way to the threshold.

There has historically been a regulatory problem with approaches which are coded for LNAV/VNAV, where the regulatory authority is not satisfied that the obstacle surface has been appropriately considered. In these cases, vertical guidance has been switched off in the database and therefore, somewhat ironically, neither LNAV/VNAV nor LNAV+V vertical guidance is available. If you make an LNAV/VNAV approach you must be prepared for the reasonable expectation that there will be no glideslope.

LPV

The Localiser Precision with Vertical guidance (LPV) approach is the big daddy of them all. It has the same sensitivity and precision as an ILS, and operates to similar minima. The actual cone of sensitivity is slightly adjusted from the ILS model. The localiser component of the ILS is based on an aerial at the far end of the runway, whereas in the LPV the end point is at the threshold, ie the length of the runway closer to the MAP. If the sensitivity were based on that, it would be impossibly sensitive near the MAP, so instead it reaches a maximum sensitivity shortly before the MAP and then remains constant. The glideslope follows a similar pattern. Most people find the LVP rather easier to fly that the ILS, both because of these adjustments and its comparative stability and reduced twitchiness.

The LPV has coded into the database a Final Approach Segment data block, which defines with great precision, reliability and repeatability the exact position in space of the final approach path, particularly aspects like height over the threshold, with redundancy checks. If a redundancy check fails, the approach will not be available and the receiver may fall back to a less accurate approach, such as LNAV.

When flying the LPV to LPV minima, it is important to monitor the LPV annunciation on the receiver. There are circumstances under which the annunciation will change to LNAV. Provided you are above 1000’ when this happens, you can continue, but only to LNAV minima. Below 1000’ it’s a go around.

FAF Checks

The following vital actions should precede the final descent at the Final Approach Fix:

  1. The Final Approach is shown in Magenta
  2. The Annunciation is correct for the approach (eg LPV, LNAV)
  3. HSI is Centred (or very nearly so)
  4. The HSI To Flag is showing

Fly over/Fly by waypoints

It is an important part of PBN procedure that you know whether a waypoint must be flown over or flown by. Typically a PBN track must be flown within 1nm and that includes the expected turn. Most waypoints are fly-by, and in some AIP procedure drawings the arc is shown, as it is on SBAS receivers. But some procedures demand fly over, usually to ensure terrain or obstacle avoidance.

Fly-over waypoints are represented by a circle around the symbol and must be crossed before the turn is made. Fly-by waypoints have no circle and should be anticipated before the turn is made. The anticipation is calculated by SBAS receivers based on groundspeed and wind. It can be hand flown but is designed to be followed by GPSS/Roll Steering.

Approach Design

It is easiest to understand approach design by working backwards through it.

Its purpose is to place the aircraft at the Missed Approach Point (MAP) in a stable descent. That is achieved by ensuring that it is at the Final Approach Fix (FAF) at the correct altitude and speed and pointing in the right direction. To ensure that that happens, an Intermediate Fix (IF) is created before the FAF, lined up with the final approach. The distance from the IF to the FAF must be at least 3.3nm, to ensure both stability and that autopilots have time to activate the glideslope (many need to be on the final approach for 20 seconds before they will do so).

But it is a design condition of the IF that no turns may be made at it which exceed 90°. In order to ensure that this happens, Initial Approach Fixes (IAFs) are created such that an aircraft arriving from any direction can go to an IAF and then turn towards the IF without exceeding the limitation. That is the reason that so many procedures fall into the “T”, “Y” or Trident shapes. These formats are not the only option, but they do provide the most efficient way to ensure that all the specifications are met with the minimum flight distance from any direction. In “T” or “Y” procedures the IF also serves as an IAF for aircraft approaching from within 90° of the final approach track. In trident shaped procedures there is an additional leg from an IAF before the IF, which is normally there for terrain or airspace separation purposes.

The choice of IAF is usually determined by the direction of arrival of the aircraft, and the plate will often have acceptable segments for each IAF drawn on it together with a Terminal Arrival Altitude (TAA) for each sector of arrival. The TAA is effectively the same as a sectorised MSA.

However, in some cases there is only one IAF and a reversal turn has to be made at it, in others there is a choice of IAF which may be exercised by the controller or the pilot depending on circumstances.

It is important to note whether each of the fixes in the procedure is fly-over or fly-by, as this can be of safety critical on some approaches. The receiver will already be coded with this information, so the HSI must be precisely followed.

Transitions

Transitions are the section of instrument flight procedures which link the standard arrival to the approach. In Garmin receivers the IAF is known as the Transition. In some Garmin boxes, such as the GNS series, the pilot is prompted to enter the transition, and it is difficult to overlook; however, in the GTN series radar vectors are assumed unless you specifically select the IAF you want. Pilots must then be careful to discipline themselves to ensure that the appropriate IAF or transition is entered.

Y and Z Approaches

Sometimes there is more than one approach of the same type to the same runway. When this happens they are designated with a letter after the name of the approach. The choice of approach will sometimes be made by the controller, but sometimes it is a decision that the pilot must make, for example by category of aircraft or equipment carried. For example EGMD Lydd has different approaches for Category A & B aircraft and Category C aircraft. In other cases it may be a question of whether the aircraft carries an ADF or DME.

Approach Speed Categories

Aircraft are categorised according to their final approach speed. Many private pilots seem to assume that all at light aircraft are Category A, but this is far from the case. Most GA IFR aircraft are in category B (i.e. with an approach speed of between 91 and 120 knots). As this affects minima as well as choice of procedure, it is important to know what category you are flying.

Trombones

The flightplan in the receiver always proceeds top to bottom without fail and without exception. It might seem obvious but it does help explain some of the odd things that we sometimes see happening.

In some receivers, and under some circumstances, if you put in an instrument approach procedure or a STAR without removing the latter part of the flight plan, such as the arrival waypoint and the airfield itself, you can find yourself proceeding to the overhead of the airfield and then all the way back to the beginning of the STAR or the procedure.

It is important to check, therefore, that the sequence of waypoints and legs in the flight plan makes sense and represents what you want to do. Usually, the easiest way of doing this is to look at the map view.

“Cleared for the Approach” and Activate Approach

You will normally hear the expression “cleared for the approach” when the controller is transferring to you the responsibility for flying the instrument approach procedure. If you are on radar vectors, it tells you that the vectoring is finished and that it is your responsibility to establish on the final approach track and to descend with the procedure.

However, if it is said before the start of the procedure it means that you should proceed directly to the IAF, arriving there at the procedure altitude, ensuring that you do not descend below the TAA until you reach the IAF.

On the receiver this can be achieved in any of three ways: selecting “Activate Approach”, DCT IAF or removing the waypoints before the IAF.

Activate Approach means the same thing as “cleared for the approach”, proceed DCT to the IAF then follow the waypoints from there, from top to bottom to the MAP. There is no other “magic” involved.

The Missed Approach and SUSP

At the MAP sequencing goes into SUSP mode, which means that waypoint sequencing is suspended. While in SUSP mode, guidance is straight ahead indefinitely. It does not follow the Missed Approach Procedure. This is initially safe to follow as you concentrate on go-around, focussing on aviation as opposed to navigation or communication.

But, on an SBAS receiver, as soon as your hands and brain are free, deactivate SUSP mode. The Missed Approach is now activated. This can all be done on autopilot with GPSS/roll-steering, but you may need to change mode from APPR to HDG, depending upon the autopilot.

But if you are flying with a non-SBAS receiver, such as a GNS430, it is important that you only press SUSP once you are above the first height restriction. Doing so early risks turning towards terrain and obstacles.

On the RNAV approaches, the MAP is at the threshold, not where you go around. This is necessary because otherwise there would be no guidance beyond the MAP, which could be dangerous in poor visibility. For this reason, there is likely to be an extended period between your decision to go around and reaching the point at which the receiver goes into SUSP mode. At a ground speed of 80 knots from a 300ft decision altitude, it is a full 45 seconds to the MAP.

Many missed approach procedures still demand navigation to an NDB. PPL/IR Europe is trying to persuade the authorities that it is acceptable to use GPS fix substitution and that therefore an ADF is not required. So far, no such agreement has been reached; therefore, if you do not have an ADF you should to negotiate with ATC an alternative missed approach procedure.

Flying the hold on arrival

Because the receiver will always sequence flight plan waypoints from top to bottom, it is not possible to insert the hold in the correct arrival sequence. The hold is therefore placed at the end of the MA Procedure. To enter the hold on arrival, you must highlight the hold and press DCT to activate it. Normally, when the hold is cancelled, you can simply activate the approach or activate the outbound leg of the procedure in order to continue the approach.

Error modes

As described above, augmentation is needed for all GNSS approaches. Therefore if augmentation is lost, the pilot must initiate a missed approach. Indications of lost augmentation include a RAIM warning, an INTEG or LOI flag, the annunciator does not show the correct mode (ie APR for non SBAS systems or LNAV, L/VNAV, LNAV+V, or LPV for SBAS systems) or that the track is not magenta.

If other aids are tuned and idented, they can be used to their minima. If a lesser approach is indicated (eg LNAV rather than LPV) above 1000’ the approach can be continued to appropriate minima.

FPL equipment codes

To fly an RNAV approach you must specify the capability in the FPL. To enter codes in the FPL, the right approvals must be in AFM.

It is important to get the codes right; a typical GA aircraft may be SBDFGRY/B1 with PBN/B2D2S1 in item 18. If you do not have S1 you may not be permitted to do an RNAV approach. Commercial software, eg SkyDemon, AeroPlus, RocketRoute, Autorouter etc, can help get this right, but if in doubt, ask on the PPL/IR Europe forum about your particular equipment fit.

Training

To fly an RNAV approach you must be trained. This means different things in different states. But in all states, from August 2018 RNAV approaches become mandatory on all IR initials and renewals.

Also, all IFR pilots, current and new, will need to pass a PBN theoretical knowledge exam by August 2018. We do not yet know what format this will take, and it will probably vary from state to state; some states may take the view that an oral examination by the IRE is sufficient. But anyway, the examiner may ask theoretical, procedural and practical questions at initial test or revalidation as he sees fit.

Equipment diversity

TSO 129 (non SBAS) and TSO 146 (SBAS) set technical standards, but the user interface can be very different across receivers. Peculiarities can cause confusion or disorientation at a critical moment. You must make sure you are properly trained on the equipment you are going to use, being particularly clear on alert and error annunciations.

Overlay approaches

While this article is about RNAV approaches, I must also mentioned overlay approaches, which have not been specifically designed for RNAV, but follow the tracks of existing, conventional approaches. These are either advisory or approved as an RNAV approach, though the vast majority in Europe are advisory. The receiver will identify which it is and, if the overlay is advisory, will give a warning notice.

If it is advisory, you are obliged to use underlying aids such as ILS, NDB, VOR and DME. The overlay is only assisting you in doing so.

Be sure to set and fly the right category. Garmin is poor at labelling, with no differentiation between them; generally “Slowest is Lowest” ie Category A is at the bottom of the list and category D at the top.

The most common in error is to fly the final approach with HSI or OBS still set to the GPS. Therefore make use of the ILS CDI Auto Selection, but still visually check that it has successfully switched to VLOC. ILS should auto select between IF and FAF but it must be checked, and CDI pressed if necessary. The autopilot will not engage the glideslope if ILS is selected too late (typically 20 seconds before the FAF).

Ask the Forum

This has been a very short taster for the subject. If you have any queries, ask on the forum . Someone will know!

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