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This week's topic:
NAVAIDs and Instrument Approaches
Last week, the Canada Air Pilot's Airport Diagrams were lightly explored. This was from a departure's point of view, primarily. Before we move on further in either our IFR Flight series, or in our Charts series, I want to look at Instrument Approach Aids and what to expect from each one.
Precision and Non-Precision Approaches
Non-Precision
NDB
ADF
Gauge
NDB
Approaches
NDB
as Aids to Other Approaches
VOR
VOR
Gauge
VOR
Approach Primer
TACAN
Localizers
Back-Course
Localizer
Quirks
Gauge
DME
DME on Approaches
Station
Passage
Reports
; Managing a Descent on a Non-Precision Approach
Minimum Descent Altitude
Precision
Decision Height
ILS
Glidepath
Cat II and III
PAR/GCA
GPS
Flexibility on Approach Design
LNAV
VNAV
LPV
RNAV (RNP)
RNAV (RNP)
Precision and Non-Precision Approaches
To begin, we should establish a few basics. There are two types of instrument approach procedures: Precision and Non-Precision approaches. Looking at them this way, the difference is that a precision approach has vertical guidance as well as course guidance. Non-precision approaches have only course guidance and the pilot must manage the vertical path.
We'll start with the Non-precision approaches. In this category, Canadians can fly approaches based on NDB, VOR, Localizers (the same NAVAID that provides the course guidance on an ILS, discussed below). For those who have heard of "Localizer Back Course" approaches, there are very few, if any, of these left. For the last couple of decades, newer-generation localizer antennas have been shielded to prevent radiation of a "back beam", thus this type of approach is unliekly to be encountered. Early GPS approaches were also non-precision approaches, but this has changed -- sort of. There are, in fact, LNAV RNAV approaches that only provide a pilot with lateral navigation guidance. Some will think, "But wait! GPS provides elevation, so why can't it calculate a glidepath?" You're not wrong, but wait for it.
An NDB, or Non-Directional Beacon, is a NAVAID that radiates a signal in all directions. Transmitting in the ranges of 190-415 kHz and 510-535 kHz, they are part of the Low Frequency (LF) and Medium Freuqency (MF) bands, and lie just below the AM radio band. In fact, the old Miramichi NDB (F9) had a frequency of 530 kHz and could be tuned in on the average AM radio. It was changed a few years ago to 520 kHz. Since the radiation emitted by an NDB lies so lowly in the electromagnetic spectrum, the waves have an ability, though limited, to follow terrain. Thus, a high-powered NDB may be received over the horizon. Just tune in your AM radio sometime early in the morning and see what radio stations you can receive from quite a distance away. That's another issue for NDBs. The radiation pattern and useable range can be affected by weather, either local phenomenon like thunderstorms or regional like fog and patterns in the ionoshpere. Bearings from NDBs are determined by reference to magnetic north unless otherwise specified. When in areas of compass unreliability, such as the Canadian far north, bearings on charts are used in reference to true north, and are noted by the addition of the letter "T". "090T" for example. NDB's were normally designated with two letters, though many have three-letter designators. To make things more confusing, some FMCs will list NDBs with their two- or three- letter designator and additional characters such as "NB" -- This may lead a pilot to be unable to find an NDB on a FMC by looking for the proper callsign -- for example the YZX NDB was labeled "YZXNB" in some systems. Some exceptions apply, including the use of some number-letter or letter-number designators. Previously, many low or medium power NDBs serving as approach facilities were designated with single-letter identifiers. Problems began to arise with the advent of FMS systems since the same letters were used across the country at various airports. How should the machine differentiate between the "G" NDB at Halifax and the "G" NDB at Charlottetown, or the "G" NDB at any of a dozen or so other fields across the country? The answer comes as each facility so designated gets it's own new identifier and a new name, too.
The gauge in the aircraft, when the ADF (Automatic Direction Finder) radio is tuned to an NDB's frequency, will show the direction of the NDB relative to the aircraft's nose, with straight up on the gauge being straight ahead of the aircraft. Considering the aircraft's heading when the gauge is read, the pilot can determine the aircraft's bearing from the NDB, thereby using this facility for track guidance. It isn't terribly precise, so the weather limits are relatively high for both ceiling and visibility that use NDBs as approach guidance.
It's interesting to note, at this point, that the ADF gauge hasn't been part of "stnadard IFR equipment" for several years now. As such, NDB approaches may be published for an aerodrome, but a pilot may have no way to fly them. With all of this in mind, NDBs are being decommissioned across Canada and many other jurisdictions either have already largely done away with them or will be in the near future.
For an approach based on an NDB, the facility can be located on the airport itself, or out on final somewhere. They are named by the runway they serve. For example, the "NDB RWY 10" approach at Bathurst, NB (CZBF). For an airport with the facility on the field, the pilot essentially flies toward the beacon throughout the approach. For an approach with the NDB out on final, the pilot flies inbound to the NDB initially, passes over the facility, then flies away from the beacon as the aircraft continues inbound to the field. See Station Passage below. Additionally, many NDB approaches are based on bearings from facilities that are not lined up with the extended centerline of the runway. Such approaches are named with letters instead of runway numbers to indicate that they are not lined up on final, starting with "NDB A" read as "NDB Alpha". If there is more than one approach at an aerodrome based on this type of procedure, they are lettered sequentially. NDB B, NDB C, and so on, should there be that many.
NDB approaches are often based on a pilot overflying the NDB and completing a Procedure Turn to line the aircraft up on the Final Approach Course (FAC). Some NDB approaches will have fixes established, often RNAV fixes as opposed to fixes that use ground-based NAVAIDs to locate them, such that the pilot can fly "straight in" to the FAC from a variety of directions without overflying the NDB first. Such fixes will not have other fixes to provide a "base leg", so these would only be useful from about 90 degrees to the FAC itself.
At one point, NDBs were typically part of other types of approaches. ILS approaches, discussed below, commonly had low-power NDBs as Final Approach Fixes (FAF) have low power NDBs as final approach fixes, and these facilities themselves could often serve as an approach aid to the same runway when the Localizer fails. While some still do exist, most are slated for decommissioning.
VOR approaches are (surprise!) based on VORs. Operating the Very High Frequency band (VHF), VHF Omnidirectional Ranges (VOR), normally radiate in the 112-118 MHz range, just above localizers and just below VHF Comm frequencies. Where the signals are VHF, they are line-of-sight only and reception range depends on the height of the aircraft. There are actually two signals radiated by VORs, and these can be pictured like an AC current wave. The signal starts out at zero, rises to a crest and then falls below zero to drop as far below zero as it goes above, and then it rises back to zero to complete its cycle. The phase difference of the two signals depends on where the aircraft is in relation to the station. If the aircraft is due north of the station, the two signals are in sync, showing zero displacement. This is considered the 360 radial. If the aircraft is due south of the facility, the signals are received by the aircraft as being 180 degrees out of sync, and therefore the aircraft is on the 180 radial. East is 90 degrees out of sync and west is 270. In theory, there are an infinite number of radials radiated by a VOR, but in practice only integer values are used, so there are 360 useable courses, one for each degree of phase shift.
In Canada, VORs are normally designated with a "Y" followed by the last two letters of the airport they serve, or a "V" followed by two letters if they are located away from an aerodrome. They are also aligned with magnetic north unless otherwise noted with "T" on their radials on charts, in which case the facility is oriented on True North, such as in the high Arctic where compass readings are unreliable close to the magnetic north pole.
The gauge in the aircraft has an Omni Bearing Selector, or OBS, which allows the pilot to determine what radial the aircraft is on and which radial to use the gauge to track. If the aircraft is due north of the facility, it is on the 360 radial and if the pilot sets the OBS to 360, the track bar will be centered. That would be a bearing of 360 from the VOR, and many older gauges have a "to" and "from" flag which would show "from" in this case. If the pilot were to rotate the OBS to read 180, the gauge's track bar will also show centered. The to/from flag would now show "to", indicating the pilot that the course to fly is 180, but the aircraft is still on the 360 radial.
Now let's look at how a VOR approach can be done. The approach plate will specify which radial to fly on final. The pilot will maneuver the aircraft to final as described in the approach plate, and will set the OBS to read the radial so the track bar can be used as guidance on final. If the facility is on the airport, it's simple: Fly straight in to the approach minimums. If the facility is located on final, the pilot have to fly inbound on one side, overfly the VOR, then fly outbound to get to the runway. The pilot will get an indication of station passage which lets the pilot know when to "change gears" mentally and track outbound from the facility instead of inbound to it. As with NDBs, the approach need not be aligned with the runway itself. Higher weather minima will result, as it will with an NDB approach not aligned with the runway, since the aircraft must be maneuvered visually to final approach once clear of cloud and in sight of the runway. VOR approaches not aligned to the runway within a certain tolerance are designated the same way NDB approaches are: with the use of letters instead of runway numbers. For example, a "VOR A" instead of "VOR RWY 09".
The TACAN is another type of facility that can provide for a non-precision approach. The TACAN, short for Tactical Air Navigation, is essentially a VOR with DME (see below) for military use. The frequency range in use is within the UHF band, so the radial and distance information is also line-of-sight. Civilian aircraft don't have TACAN receivers, but the military aircraft can make use of TACAN just the same as VOR for approaches. The one benefit civilian aircraft can receive from a TACAN facility is DME. By selecting the appropriate frequency in either Nav radio, the DME can be received, though without the benefit of bearing information. Some DME receivers are stand-alone (that is, not associated with a Nav radio) and in such a case, the pilot can select the DME channel from the charts instead. Another note: VOR and TACAN facilities are often combined into something called a VORTAC. Both civilian and military aircraft can receive radial and DME information from this kind of facility, and approaches can be designed for either TACAN or VOR, and usually for both, though separate approach plates are drawn for each. TACANs are normally designated as are VORs, except a "U" is used as the first of the three letters if they serve an aerodrome directly.
The Localizer is the next category of Non-Precision approach aid. The localizer is an antenna array that radiates a signal that is received by the aircraft and the radio interprets the displacement of the aircraft with respect to the track's centerline and displays this to the pilot. To receive localizer signals, the pilot tunes the frequency indicated on the approach plate into the Nav 1 radio. Localizers normally transmit in the range of 108.0 to 112.0 MHz. The antenna array itself is located on the far end (departure end) of the runway it intends to serve and the beam's "centerline" is normally lined up quite accurately with the runway's centerline. Needle deflection on the gauge indicates an angle relative to the final approach course: If paralleling the localizer centerline, the closer you get to the localizer antenna, the wider the deflection of the track bar needle, even though the aircraft's disctance from the actual centerline of the runway remains unchanged. The width of the beam ranges from 3.5 to 6 degrees, but is normally set at 5 degrees. This provides a pilot with full track bar deflection at 2.5 degrees. Some localizers are not lined up with the runway. If the course difference is more than 3 degrees, a cautionary note will be placed on the approach plate and the localizer is designated with an "X" in its three-letter ident, rather than the standard "I".
Older localizers radiated a back course or back beam which allowed for an approach to be designed for the opposite end of the runway. Newer generation ILS and localizer antennas are shielded to prevent radiating a back beam, so these approaches are becoming increasingly rare. Some aircraft avionics are not programmed to fly them. I've included them in this article more as a historical curiosity than anything else.
The limits were a little higher for two reasons. First, there is no useable glidepath indications on a back course approach. I understand that some back courses in Flight Sim behave differently, but the previous statement is actually correct. Second, with the localizer antenna located off the far end of the "front course", this means an aircraft would actually have to fly over the localizer antenna before reaching the end of the "back course" approach. Since the closer you get to the antenna the wilder the needle movement is, the minimum visibility requirement for a localizer back course approach is higher than for an ILS, or a localizer only approach which is based on a "front beam".
While newer generation localizers don't radiate a useable back beam, some older localizers were shielded to prevent radiation of a back beam for safety reasons. This may include terrain on the path, restricted airspace, or some other reason as to why a pilot shouldn't use a back beam. Yet another quirk is where one runway has an ILS localizer on each end. In such a case, the systems are interlocked so only one system is radiating a signal at any given time. Typically, it takes a few seconds to "switch" the localizer as one has to shut down while the other powers up, completes some self-tests and gives the signal it's good to go. Normally, such a switch is controlled at the airport level, so a Tower or FSS would have access to the switch, not the controller in an Area Control Centre (ACC).
The gauge in the cockpit varies depending on the equipment installed. The basic gauge that most will learn to fly on is the very same VOR gauge. It operates independantly of the OBS when a localizer is tuned up, regardless of where the aircraft is. With an east-west runway, the gauge will show the needle to the right if the aircraft is south of the localizer whether the aircraft is east or west of the airport and regardless of whether it is heading east or westbound. Mind you, the localizer's signal is not likely to be received all the way around. The standard is 35 degrees off localizer alignment out to 10 NM and only 10 degrees out to 18 NM. Many aircraft have reported receiving localizers as far out as 80 NM, in my experience. This pattern of needle deflection has the effect of having the pilot fly toward the needle on a front course, and away from the needle on a back course. This means that, on a front course, if the needle is to the left, the localizer centerline is to the left, and a pilot must alter course slightly to the left to center it. On a back course, if the needle is left, the pilot must alter course to the right to center it. The exception would be a Horiztonal Situation Indicator, or HSI, which may have the ability to set the reverse course for display, thereby also reversing the directions of the track bar's deflection.
While DME isn't useful as an approach aid itself, it is often used in conjunction with another NAVAID listed above. DME, or Distance Measuring Equipment, is a method of reporting distance from a DME facility. There are two parts: the aircraft's DME equipment, which sends out pairs of pulses on one frequency, and a DME facility on the ground which replies with pairs of pulses on a different frequency with the same timing as the originating signal. The difference in time between the transmission of the pulse from the aircraft and the reply from the ground station determines the distance in nautical miles. One quirk is that the distance is the direct distance between the aircraft and the facility, not the actual distance over the ground. If an aircraft were to fly straight up without moving closer to the facility in terms of latitude and longitude, the DME reading would increase, as this distance reported on the gauge is slant range. Similarly, an aircraft at 12,000 feet directly above a DME facility will show 2.0 NM from it (6,076 feet per nautical mile). DME is either a stand-alone unit where a DME channel can be selected, or it can be present in the usual format, a gauge, often implemented within another gauge like a VOR or HSI, and the channel for the DME is automatically tuned with the frequency selected by the Nav radio in the aircraft. As with VOR and TACAN, the signals are line-of-sight.
The use of DME allows approach designers to place fixes on final to allow pilots better descent profiles to exclude terrain and obstructions from various segments of a non-precision approach. If a pilot can accurately determine the aircraft is beyond a certain point such as a tower or a hilltop, the weather limits can be lowered slightly to allow a better chance of success on the approach. It can also allow design of a straight-in approach transition by use of having the pilot fly a "DME arc", or a circular segment maintaining the same distance from the facility. DME is often collocated with a VOR or Localizer installation, but may be a stand-alone facility of its own. As mentioned above, aircraft equipped with DME receivers can receive DME from TACAN facilities. If a facility happens to be a VORTAC, radial and distance information is available to civilian aircraft. DME facilities collocated with VOR or Localizer installations have no identifier of their own. Independant DME installations, or those collocated with an NDB, normally have a two-letter or a letter-number designator.
Station Passage for NDB, VOR, TACAN, and DME
What is station passage? It's an indication to the pilot that the aircraft has passed over a ground-based NAVAID. For the NDB, VOR and TACAN/VORTAC, the track guidance is all based on angular measurements. When the pilot is directly overhead the facility, there are two aspects which make this difficult to measure. First off, the signal strength may not be as strongly radiated directly upward, since the antenna is designed to radiate horizontally. Secondly, the slightest change in position when direcltly overhead the facility could result in a very large angular difference, especially if the pilot passes from one side to the other. Together, these make up what is called the "Cone of Ambiguity" and it is larger at higher altitudes than lower ones. In the case of the VOR, you'll see the track bar swing to one side or the other, slowly at first then quite rapidly, then center itself as the "OFF" flag appears. After passing overhead, the track bar will deflect to the side again and slowly come back near center (unless the aircraft flies directly and exactly overhead, in which case it will center again) as the track guidance is received again after passage. For the NDB, the needle will slowly start to move to one side, then progress rapidly, maybe even swing rapidly from one side to the other, and then eventually settle out and point straight down (assuming direct overhead passage with the aircraft aligned straight-on before station passage).
For DME, there is a different slant. Slant being the key word here. As mentioned above, DME reports slant range. This means that if an aircraft were to fly at 6,076 feet directly over a DME facility, which is a distance of one nautical mile, the aircraft's DME receiver would report a distance of 1.0 NM. GPS, RNAV, INS, etc, would all report the distance from the fix as 0.0 NM, but the DME looks at the slant range, or the physical distance between the facility and the aircraft, not the horizontal distance between the aircraft's position and the facility's position. Normally, DME station passage is not reported. ATC may ask for reports passing by fixes marked on charts as DME fixes, or may ask for a report at a certain distance. Years ago, DME and RNAV-based distances were not interchangable, but these days, ATC has approval to use either interchangeably when establishing separation in a non-surveillence environment.
If flying an airway, a pilot confirming station passage should make a position report to ATC unless previously having been told the aircraft was "identified". Whether the airway is based on an NDB or a VOR, this is applicable. If the facility is a final approach fix, the pilot should over the facility on final if inbound to the airport, or by the facility outbound if over the facility and proceding away from the airport, such as being outbound during a procedure turn. If a race-track procedure turn is used, similar to that of a hold, the outbound report should be made when you are abeam the facility heading away from the airport. A pilot is "abeam a facility" when it is 90 degrees off the nose on the left or right and the aircraft is established on the outbound heading.
Managing a Descent on a Non-Precision Approach
All of these options for approach procedure design but not a single one of them provides a vertical descent path. How does a pilot cope with this?
As with all instrument approach procedures, there are minimums the pilots must adhere to for the sake of safety. In the case of a non-precision approach, there will be a number called the Mininmum Descent Altitude, or MDA. This is the lowest altitude above sea level the aircraft may descend on the approach procedure. If the pilot doesn't see the runway enviroment (approach lights, runway lighting, the runway itself), upon reaching the Missed Approach Point (MAP), the pilot must "go around", executing the published missed approach procedure. This MDA is determined by criteria, taking a wide variety of items into account, such as nearby terrain, the approach path relative to the runway itself, and much more.
A pilot has a few means by which to manage a descent on a non-precision approach procedure. First, seeing the altitude at which the aircraft will join the final approach segment (or perhaps further back on the intermediate segment), the pilot may look at the MDA, consider the distance and the estimated groundspeed of the aircraft on final, and calculate the rate of descent at which to make a conitnuous descent. Alternatively, the lazy or aggressive pilot may simply drop to the minimum altitude for each segment as published, perhaps "stepping down" and levelling off at each altitude, and hope to see the runway. Some avionics squites are capable of managing this descent for the pilot.
See below with RNAV approaches for more on vertical paths as they apply to those approaches.
Precision Approaches are approaches which not only give lateral guidance, as all of the above do, but they also provide vertical guidance. This menas the pilot no longer has to time the descent or program a computer to manage it -- hte pilot simply flies a presentation much like the localizer that is organized for a vertical presentation. An indicator (either a physical needle on older gauges or a virtual one on electronic displays) similar to that used to show course guidance is presented to show whether the aircraft is on, above, or below the glidepath. Such a presentation now means a pilot need only cross-check the altitude of the aircraft rather than using it directly. On precision approaches, a Decision Height or DH is indicated, and, unlike the MDA, this is an actual height above ground, not above sea level. A radio altimeter is useful for this, if the aircraft has one.
ILS (Instrument Landing System) is the primary, non-visual precision approach aid approved by ICAO, the International Civil Aviation Organization, and is expected to be supported into the future, at the time of writing. The ILS has subcategories of its own, too, and they are designated by Roman Numerals. The Category I ILS is the basic ILS installation, and it consists of a Localizer which provides the track guidance as described above and a Glidepath, which provides vertical guidance. The runway must also meet criteria with high intensity approach and runway lighting, along with a method of determining visibility along the runway, called RVR, or Runway Visual Range.
The Glidepath antenna radiates vertical guidance in a similar fashion to the way the localizer works. The transmissions are in the range of 329.3 to 335.0 MHz, in the UHF band. Glidepath signals are automatically received based on the frequency tuned up in the Nav 1 radio, so there is no need for the pilot to dial up a glidepath signal separately. The antenna for the glidepath transmitter is normally located around 1,000 feet from the runway landing threshold, and about 400 feet to one side of the runway. This has the effect of guiding the aircraft to the "touch down area" of the runway, or a certain distance from the landing threshold. In the case of a back course, the pilot may receive erratic glidepath indications, but these must be ignored. For one reason, the glidepath signal originates from the far end of the runway, so the pilot would, if the signal were useable, touch down with only 1,000 feet or so, of runway left. Secondly, the signals are simply reflections and will likely be very erratic at best, so trying to chase them down would be impractical, if not impossible. There are notes in approach plates reminding pilots to ignore glidepath indications while flying a back course. You know, should you ever actually encouter one these days.
Category II and III ILS systems involve more equipment than just a standard ILS. The following is a general list of what is required equipment:
Also, the aircraft equipment has to meet more stringent specifications for Cat II and III operations, and the flight crew must be trained and certified for the lower weather operations. Cat II and Cat III both require use of "coupling" meaning the autopilot is flying the aircraft. Increased ATC separation on final is required during Cat II and Cat III operations as well, since any metal (including preceding aircraft on rollout, aircraft taxiing or departing in front of an arrival, or even a large vehicle) in the "sensitive areas" of the localizer or glidepath antennas could cause fluctuations in signal radiation leading to trouble in the cockpit of an aircraft doing a "coupled" approach.
Another form of precision approach that doesn't rely on any equipment on board the aircraft is called Precision Approach Radar (PAR), also known as Ground Controlled Approach (GCA). It is primarily a military procedure these days. ILS took over from the GCA days in the civilian world, although at some aerodromes, a military presence provides for the possibility of a PAR for a civilian aircraft. The radar equipment in a PAR installation is a high-definition radar in the 9 GHz range with two antennas providing for (separate) azimuth and elevation detection, as well as range. This provides three dimensional position information at a high "refresh rate" allowing a controller to detect an aircraft's movement, and any deviation from centerline or glidepath, relatively quickly. At regular intervals, he controller issues position information and instructions to the aircraft in the form of headings and instructions to adjust rate of descent to allow the aircraft to fly a precision approach.
To a civilian aircraft, the primary benefit or PAR is during an emergency. The cool voice of a good PAR controller can help calm the pilot in a nasty situation such as when where most of the aircraft's instrumentation is out. There is no reliance on aircraft-borne equipment -- not even a transponder is required -- since reflected radiation is used. The PAR controller can provide more than just "cloud breaking" to any aircraft, so long as it's visible on radar and as long as the pilot can receive radio transmissions, either by VHF Comm or even by voice reception by cell phone (or from NDBs in some places in the "old days"). These approaches can be accomplished in low IFR weather to low minimums.
In the original article from nearly 20 years ago, RNAV approaches (then known as GPS approaches since GPS was the only satellite system acceptable at the time) were just starting to gain ground. They were originally lumped into non-precision approaches because that's all they were approved for. In today's world, they can be non-precision or precision, and they have evolved quite a bit since then. I felt that they deserved their own section when revising this article. This is by no means the complete picture, but it's a really good primer.
GPS has been legal for usage by IFR aircraft in navigation since the mid-1990s. Unlike the ground-based NAVAIDs mentioned above, it has an accurate and repeatable performance but also has yet to see a single actual, system-wide "outage", meaning availability is effectively 100%. Localized phenomena affect it, including such things as jammers (use of which is illegal under ISED regulations), and even occasional limitations due to one satellite having issues while a spare is moved to replace it. Either way, it hasn't let us down yet. Despite all of this, Global Naviation Satellite Systems (including GPS, GLONASS, and Galileo) took a long time to really be adopted for approach procedures. But that time is here.
Air Naviation System (ANS) providers such as NAV CANADA are fully embracing the technology. And why wouldn't they? Waypoints are no longer dependent on NAVAIDs and can be placed anywhere. Courses can be optimized for traffic flow, restricted airspaces, and obstacles/terrain as needed. Plus, less ground-based infrastructure to be maintained. Aircraft operators were quicker on the draw, as navigation benefits are immediate and, despite the mammoth costs of equipping an airliner, the cost savings associated with regularly flying fewer miles on more efficient routes add up fast.
RNAV approach procedures benefit pilots and controllers alike. From the pilot's side of things, the traditional "T" design with fixes lining up for left and right base legs (the initial approach segments, between the Initial Approach Waypoints, or IAWP, and the Intermediate Waypoint, or IWP) set the aircraft up for a turn onto the intermdiate segment to join final at the Final Approach Waypoint (FAWP). Use of these waypoints can set an aircraft up on final from virtually any direction. Since the aircraft's navigation gear can be used to guide the pilots all the way from the enroute portion of the flight through the initial descent and onto final means less work for the pilot and for the Air Traffic Controllers -- vectors are often no longer required unless other traffic is in play. With pilots doing self-navigation, they can better monitor the aircraft's progress, set up for "idle descents" and avoid "flat segments" where the aircraft reaches an assigned altitude and must level off, mostly due to not knowing how far the aircraft will be vectored during the descent. The benefits for both sides of the radio are huge.
The first thing to talk about in RNAV Approaches is course guidance. LNAV, or Lateral Navigation, is provided in a way that appears to the pilot much like the localizer course guidance on an ILS as discussed above. Of course, the actual appearance will depend on the system being used. RNAV units using GNSS may be stand-alone units, or may be tied into full-blown FMS on the pilot's Primary Flight Display (PFD). In any case, with LNAV only, a RNAV (GNSS) approach is a non-precision approach procedure.
What? No vertical guidance? Can't GPS provide that? Yes! VNAV stands for Vertical Navigation, of course. This may not be accomplished the way you think it is, though. Barometric VNAV, or Baro VNAV uses an Air Data Computer coupled with an aircraft's FMS to calculate a glidepath from current conditions around the aircraft, including corrections for cold weather altimeter errors in more advanced FMCs. I wrote two topics on Cold Weather Altimeter Errors and they can be found here and here.
LPV stands for Localizer Precision with Vertical guidance. This is considered a Precision Approach in most jurisdictions, and with ILS-style lighting standards mentioned above, the weather limits are often as low as ILS allows. This can, effectively, provide aircraft with Cat I ILS precision to any runway without the requirement for localizer and glidepath transmitter arrays.
Not all RNAV approach procedures have all three of the above modes permitted. In the box containing the minimums for the approach, one line will be designated for each LNAV, LNAV/VNAV, and LPV minima, and those minima will include ceiling and visibility for each.
Advanced RNAV systems use Required Navigation Performance (RNP) specifications to ensure accuracy throughout the planned procedure. Remember the DME arcs mentioned above? For the most part, those are disappearing from approach procedures, but they were often 10-20 NM arcs, meaning a long distance to fly. RNAV approaches using RNP allows similar arced segments, but these can now be much smaller arcs, typically with a 2.5 NM radius on an approach. In its simplest manifestation, consider an aircraft approaching a runway that needs to make a 180 degree turn from the inbound direction to the final approach course. Even a typical RNAV approach procedure generally has the aircraft fly downwind 10 NM past the airport, fly an initial approach segment that's typically around 5 NM in length before joining the final approach course and having to fly the 10 NM return leg -- 25 NM in total. With RNAV (RNP) approaches, the initial approach segment can be as short as 2 NM, joining an arc with a typical radius of 2.5 NM to do the course reversal before rolling out on a 3 NM final approach segment. Using simple math, this means an arced segment just under 8 NM (7.8), put with the 2 NM inital nad the 3 NM final, meaning the total distance flown on this approach would be around 13 NM -- 12 NM shorter than the example of the RNAV "T" design. Save that distance traveled on 3 flights a day every day for a year and just think about how much fuel is saved.
RNAV RNP is not something available on all aircraft, and even if the aircraft has it, the flight crews must be certified nad the operator (airline) must also have approval from the regulator (Transport Canada). But the costs of all this are quickly offset.
It's common to have multiple Instrument Approach Procedures for the same runway that are slightly different but still based on RNAV. For example, a traditional RNAV "T" design with only LNAV minima may be published to a particular runway, but there may also be a separate RNAV-based procedure using RNP requirements for that same runway. Since they're both to the same runway and using the same base equipment (RNAV), they must be identified separately. Enter the letters. In a way similar to NDB and VOR approaches being designated "VOR A" and "VOR B", multiple RNAV approaches will start at the end of the alphabet and work their way forward. The subtype of RNAV being required for the approach will be denoted in parentheses to show the pilot what is needed, the most common being (GNSS) indicating that it's RNAV and must be flown with a GNSS-based RNAV unit (the label will be "RNAV (GNSS)"). RNP approaches will be labeled "RNAV (RNP)". The US differs in their naming convention, but in Canada, the general method is to give the RNAV-based procedure that is flyable by most aircraft the letter "Z". These are usually the long-time standard RNAV "T" design, sometimes only having LNAV minima. The next most-useable procedure will carry "Y", and so forth. Note that this is not the only means of designating them. CYYT, St. John's, NL has three RNAV procedures to each of their runways: One RNAV (GNSS), one RNAV (RNP) and a second RNAV (GNSS) that is for helicopters only. CYFC has a criteria-related anomaly due to the proximtiy of CYR724 on runway 15. The RNAV (GNSS) LNAV and LNAV/VNAV minima are on the "RNAV (GNSS) Z RWY 15", the RNP approach is on the "RNAV (RNP) Y RWY 15", and the "RNAV (GNSS) X RWY 15" is the GNSS-based approach with the LPV minima. In this case, the LPV had to be separated from the LNAV and LNAV/VNAV due to procedure coding requirements and it was decided to keep the naming convention for the RNAV RNP the same so that all RNP procedures would carry the "Y" designation.
As mentioned, RNAV approaches don't rely on designing the approach on ground-based NAVAIDs, either singular or a set of them. Basically, all the previous approach aids are one or two transmitters, thereby only really allowing a narrow range of geometry. With RNAV, waypoints can be placed wherever desired, thus allowing off-angle approaches to final, step-down fixes where desired, the design of initial approach segments (base legs) to allow a straight-in from whatever desired direction, and even feeder routes which can be used to get around restricted airspaces or obstacles/terrain that only affect the approach from one direction, all without requiring a single ground-based NAVAID or an antiquated procedure turn.
This was meant to be an overview of the types of IFR approaches available, rather than a tutorial on how to fly each type. There is a lot more background information on each type listed above, but I figured I already spouted off more than you need to know to accomplish each -- other than how to fly it. I'm not a flight instructor, just trying to introduce new pilots to the stuff and bridge the gap between pilots and controllers a little. By all means, contact me if you like (or don't like) what I've said by e-mail at mo@xlii.ca .