Basic IAPs

Basic IAPs
By Tom Seeley

PURPOSE: To provide a basic understanding of the elements of an Instrument Approach Procedure, and provide the pilot with an understanding of the techniques to fly the approach to a successful conclusion.

OBJECTIVE: Upon completion of this segment, pilots should possess an understanding of the different IAP's available and the proper techniques required to conduct them.

PREREQUISITES: Pilots should have a preliminary understanding of instrument flight, holding, procedure turns, speed control, VOR, DME, GPS, ILS, LOC and other basic elements of flying instrument approaches.

DISCUSSION: At the conclusion of every flight it becomes necessary to locate an airport, a particular runway at that airport, and a landing on that runway. In visual conditions, this is normally not a big problem. But when weather conditions at the airport of intended landing are such that a visual approach and landing cannot be safely accomplished, an instrument approach procedure, if available, is used. The type of instrument approach procedure may vary from among many.

ILS Approach: Probably the best-known of all instrument approaches, the ILS or Instrument Landing System is considered a "precision" approach, and provides both lateral and vertical guidance to the aircraft, normally enabling the aircraft to safely maneuver to a height of 200' above the runway (or higher), from which point the landing is continued visually. This is a category I ILS, and in addition to the navigational resources, it requires certain runway lighting and markings. There are also CAT II and CAT II I approaches with lower minimums, beyond the scope of this segment. To conduct an ILS approach, a "localizer" frequency is tuned in the navigation receiver, and the aircraft is flown, either by "pilot nav" meaning the pilot is responsible for navigating to the Initial Approach Fix, or by "vector" meaning ATC will provide guidance to the initial approach fix (IAF). As the aircraft approaches the IAF, the "localizer" begins to display guidance by moving either from the left or right toward the center of the instrument. (See NavigationAids) When the aircraft is established on the inbound localizer course and centered on the localizer "beam", the display in the aircraft will be dead center. If the aircraft continues precisely on this course, it will proceed directly to the runway associated with the localizer. 

At the same time, for a full ILS approach, the system is sending a vertical beam similar to the localizer, and the aircraft receiver interprets this as the "glide slope" which will provide vertical navigation to the runway. Most ILS approaches are set up so that the glide slope centers on the navigation display just as the aircraft crosses the Final Approach Fix (FAF) inbound. If the aircraft descends from that point with the glide slope indicator centered on the display, it will arrive at the 200' (or other height as appropriate) in position to land, provided the runway or its environment is visible.

Any published instrument approach requires certain visibility to exist in order to begin the approach. There are variations of this rule in several forms, but in general, the visibility must be ? mile or greater. This requirement is visible on the approach plate for every approach depicted. Looking at the profile view in the third graphic below, you can see the notation "218/18 (200- ½ )". The "18" is "runway visual range" or RVR, and the "½" refers to forward visibility. If RVR is available, it is controlling. If the required visibility does not exist, another option must be chosen, such as holding in case the weather improves, or going to an alternate airport.

Localizer Approach: This is a "non-precision" approach which is basically one half of an ILS approach. That is, the glide slope is either unavailable or not present; a localizer approach can be either a stand-alone, meaning it was developed without a glide slope, or it can be an ILS without vertical guidance, meaning only the localizer portion of the approach is available. This means higher "minimums" to the pilot; the aircraft cannot descent as low as the full ILS permits because the precision vertical guidance is not available. To conduct a localizer approach, the aircraft is navigated or vectored just as it is for the full ILS, with the difference being that the pilot is responsible for descending in a series of "step down" levels to a point at which either the runway environment is visible or not. If not, a missed approach procedure is initiated. It should be noted that because the localizer approach is a non-precision approach with no vertical guidance, the minimums, that is the limit of the descent on the approach is higher than for a precision or ILS approach.

This image shows the top portion of an approach plate as it is seen on the flight deck. You can see that this approach is either a full ILS or a localizer approach. In the left-most area you can see the localizer frequency, listed here as "LOC/DME I-BOS" and the frequency 110.3 is provided. To the right of that is the inbound course, listed as "APP CRS". The inbound heading for this approach is 036°

The next box also contains important information. It shows the runway length, 8851' and the touchdown zone elevation, 18'. In an area like Boston the TDZE is not terribly important, but in an area like Aspen, CO or the Swiss Alps, it is vital to know what the runway touchdown zone elevation is. The lower panel in this portion of the approach plate contains communications frequencies. Boston Logan has two ATIS frequencies, one for arrivals and one for departures. This is not uncommon for busy Class B airports. The ATIS is important for a number of reasons, but it is important for this discussion because it will tell you what approach to expect upon arrival. This allows you to accomplish much of the approach setup when time permits, before it gets real busy on the actual approach.

In the next section of the approach plate, pictured at right, you can see a "plan view" of the approach, from a top-down perspective. This view shows the navigational guidance provided by the localizer, wider at the outer end and narrowing toward the runway. Where the bold circle intercepts the localizer, you can see a fix called WINNI with the label IAF above it. This denotes the fix WINNI as the Initial Approach Fix. The aircraft at this point may be on or below the glide slope, depending upon how it was vectored to the approach. Further inbound on the localizer you can see a fix called MILTT with the label LOM above it. This stands for "locater outer marker" and it is the Final Approach Fix (FAF) for this approach. At this point, as the aircraft reaches the LOM or FAF, the glide slope indicator will center if the aircraft is at the appropriate altitude. The aircraft starts descending while keeping the needle centered. If at any point the glide slope indicator shows full deflection in either direction, the aircraft should execute a missed approach. As it continues on the localizer and glide slope, both guidance systems become increasingly sensitive.

In this section (left) of the approach plate, known as "profile view", you see the approach from a horizontal perspective. The information discussed in the earlier section is visible here as well, and you can get a better understanding of how the aircraft descends along the glide slope, indicated by the bold line. You can see that the aircraft must be at 1800' as it crosses MILTT in order to be on the glide slope. If this were a localizer approach only, the aircraft would navigate inbound on the localizer, crossing WINNI at 4000' and descending to 3000' to cross NABBO, then 1800' at MILTT. However, looking at the table below the profile view, next to the label "S-LOC 4R" you see the number 440/24. This is the "MDA" or "bottom" of a localizer approach; the aircraft may not descend below 440' unless the runway environment is visible. If not, a missed approach is executed. The "24" indicates that 2400' or a half-mile forward visibility is required. Also in this section is a graphic representation of the missed approach procedure. It is placed here for quick reference. You can see the description of the missed approach in more detail in the first panel above.

The last section of the approach plate front view (right) is an airport diagram. Although small, there is much information of importance here, including a graphic of the airport and runway layout. This can be crucial if the airport is unfamiliar and the aircraft "breaks out" of the weather at a low altitude. If you have already studied the aircraft layout, it will help to orient yourself when it becomes visible. The table in the bottom of this panel is used to time an approach. An aircraft with a ground speed of 120 knots will cross the missed approach point 2:33 after leaving the FAF.

The Instrument Approach Plate pictured above is the ILS RWY 8R approach for Charles De Gaulle Airport in Paris. Looking at this plate you can see that all of the ingredients discussed earlier are present, although some of the symbology is a bit different. Even if you don't speak the language, (although the plate is bi-lingual) an understanding of basic instrument approaches will permit you to safely conduct this approach to a successful conclusion by following the approach plate.

GPS RNAV Approach: A relatively recent development in Instrument Approach Procedures, the GPS RNAV has slowly superceded a great many other non-precision approaches, including VOR, NDB, LDS, SDF and others. Some of these approaches still exist, and some are "overlaid" by GPS approaches. There are additional GPS RNAV approaches being developed, many with precision-like vertical guidance, but again they are beyond the scope of this discussion. 

An approved GPS with WAAS (Wide Area Augmentation Service) can reliably fix an aircraft's position to with a very small area, less than seven meters. In this case, the GPS is "coupled" to an indicator in the aircraft, normally the HSI or VOR display. Lateral guidance is the same as discussed previously, and is separated into terminal and approach sensitivities, which simply means that as the aircraft approaches the airport the guidance is more sensitive.

In the full approach plate pictured above for the RNAV GPS RWY 26 approach into Ft. Lauderdale Executive Airport, you see a very basic GPS approach with lateral guidance only. This approach is flown in the same manner as the localizer approach, but with different navigational inputs. A feature of this approach not previously discussed is the PROCEDURE TURN. A procedure turn is also known as a course reversal, and is used to transition the aircraft onto the approach by navigating to a fix, then flying the procedure as pictured to arrive on the inbound course at the proper designated altitude. A procedure turn is very similar to a Holding Pattern∞ in the way it is entered and flown. In this case, using GPS navigation, the aircraft might receive a clearance similar to: "N1234 cleared present position direct FORSU. Cross FORSU at or above 2000', cleared for the RNAV GPS 26 approach". Depending upon what direction the aircraft is coming from, it would navigate to FORSU, and fly the procedure turn to arrive back at FORSU on the inbound course of 267°. After crossing FORSU inbound, the aircraft would descend to cross KOKEY at 1500' and continue down to the minimum altitude (MDA) of 620'. In addition, there is a light (not bold) arrow leaving the FLL VOR with a 052° heading. This is known as a "feeder" route and in this case, the clearance could be, "N1234 cleared direct to the FLL VOR for the RNAV GPS RWY 26 approach". In that case the aircraft would navigate to the VOR, depart on the 052° heading to FORSU, cross at or above 2000', and continue as above.

Visual Approach: It's fairly obvious that a visual approach is an approach and landing under visual conditions. A visual approach is given to an aircraft when the airport or runway is in sight, and the pilot accepts responsibility for continuing the approach on his own. It is NOT a VFR operation, but is still conducted under instrument flight rules, with the flight plan being automatically cancelled upon landing (in the U.S.). 

Missed Approach: This discussion would not be complete without taking a look at the Missed Approach, an essential and important part of the IAP. Every approach necessarily includes studying the approach plate for the ingredients and mechanics of the approach, but this should always include paying attention to the missed approach segment. You may feel that only really bad weather would indicate a need for this knowledge, but there are other developments which can lead to initiating the MAP, including loss of separation, an incident on the runway, or a problem with the aircraft's navigation reception. Most missed approach procedures include a climb to an intermediate altitude followed by navigation to a fix or navaid and a hold. Without any instructions to the contrary, the published missed approach is flown and the aircraft enters the hold as depicted. However it is possible to receive different instructions from ATC which can consist of simple vectors to begin another approach, or a hold somewhere else. In any event, it is possible and even probable to be quite low when the MAP is initiated, and it is extremely important to add power and begin the climb without any delay. Without adequate concentration, it is very easy to become absorbed in raising the gear and flaps and looking at the approach plate while the aircraft, already low and slow, continues to descend into terrain.

In the RNAV GPS RWY 26 approach above, the MDA of 620' puts the aircraft very close to office buildings located right off the approach end of the runway. In reduced visibility, failing to initiate an immediate climb to 2200' as depicted in the missed approach instructions could result in an unwelcome visit to some offices in these buildings with suitably disastrous consequences.

Where do I obtain IAP charts? There are numerous sites available on the internet where approach plates can be downloaded, including Airnav∞and FlightAware∞ which are U.S. sites. Most, if not all VATSIM facilities include links to approach charts for use at airports within their boundaries. For example, VATFRANCE∞ provides this link: SIA Civil Aviation∞.