Understanding Instrument Approaches

Introduction to Instrument Approaches

An instrument approach is a way to get lined up on a long final approach to a runway, and to descend to just a few hundred feet above the ground in low visibility or low ceilings.  Each instrument approach ends at a point very near the end of the runway, and at an altitude between 200 and 800 feet above the ground.  If you are unable to see the runway at this time, then you are required to “go missed,” or climb away from the airport to either try again or go someplace else.

Under VFR weather conditions, we are able to see and avoid obstacles as we descend to the runway, and we typically remain within about 1 mile of the runway during our entire approach.  An instrument approach changes this by guaranteeing an area that will be free of obstacles along a long straight-in path, perhaps 10 miles long, in which the pilot can safely descend to the (very low) altitudes published on an instrument approach procedure.

Plan View & Profile View

An IAP (instrument approach procedure) consists of an overhead map of the area around the airport (plan view), and a vertical approach path (profile view) that shows appropriate altitudes as you fly along the final approach course.

The plan view shows one or more possible ways to get lined up on the final approach course.  Each plan view will show the airport and its runways, as well as a final approach course line that one must follow along to the runway or airport.

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The profile view, then shows the same named “fixes,” as the plan view, but adds information about the altitudes you can fly as you get closer to the airport.

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These named fixes are IFR waypoints that are searchable in any aviation GPS, and are sometimes listed on low altitude IFR enroute charts.  For example, if you wish to land at the Lakeland Airport, you can see the final approach path that you will follow for runway 27, as shown on a low altitude IFR enroute chart.

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Therefore, as you are flying this approach, once you are past GILDE, you may descend to 1700 feet, and once past ECADE, you may descend to 780 feet.  The final altitude is omitted from the profile view because it depends on the type of approach being flown.

Precision and Non-Precision Approaches

Each IAP represents multiple types of approaches that all follow the same lateral path, but may have different vertical paths, and different final altitudes (minimums).

There are 2 types of approaches that are each flown in a different way.  Again, they both follow the same lateral path, but the manner in which the pilot descends along each approach is different.

Precision Approaches are flown in a steady descent – following a glideslope down to the minimum altitude, which is in this case called a Decision Altitude.  This is what is represented by the slanted line in the profile view of our example.  One could, at GILDE, begin to descend steadily, following the guidance of the glideslope needle, all the way down to minimums.

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One could also choose to descend quickly at GILDE to the minimum altitude for that segment (1700), and then proceed to follow the guidance of the glideslope needle down to minimums.  You must be on the glideslope by what is called the Final Approach Fix.  This will be discussed in more detail later.

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Precision Approaches make use of both needles on the VOR Indicator to show your position relative to both the final approach course (seen on the plan view) and the glideslope (seen on the profile view).  On this instrument, the middle circle represents your aircraft, and the needles represent the line you are supposed to be following.  If the localizer needle (the vertical needle that moves left and right) is to the right, then the course you are supposed to be on is somewhere to your right.  If the glideslope needle (the horizontal needle that moves up and down) has moved down, then the glideslope you are supposed to be following is below you – you are too high.

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Non-Precision Approaches, on the other hand, are flown in a “step down” fashion.  After crossing each fix, the pilot will initiate a relatively rapid descent to the next allowable altitude, until he reaches the lowest allowable altitude, called the MDA (Minimum Descent Altitude).  Continue to fly along at the MDA until you either have the runway in sight, or reach the Missed Approach Point.

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Non-Precision approaches are flown with the localizer needle only (the needle that moves left and right).  Altitude changes are made upon reaching each of the named fixes, and the final altitude is held until either the runway is in sight, or until reaching the Missed Approach Point (usually, the beginning of the runway).

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How do you know which type of approach to fly – precision or non-precision?  The GPS 27 approach allows either one.  The decision will depend on the equipment in the plane, or simply the pilot’s choice.  The types of approaches applicable are listed in the minimums box at the bottom of each IAP.  You must learn to recognize the listed approaches, and memorize whether they are precision or non-precision.

Examples of Non-Precision Approaches

  • LP (Localizer Precision)
  • LNAV (Lateral Navigation)
  • S-xx (VOR Approach straight-in to a runway, where XX refers to the runway number)
  • S-LOC (Straight-in Localizer)

Examples of Precision Approaches

  • LPV (Localizer Precision with Vertical Guidance)
  • S-ILS (Straight-in Instrument Landing System)

Because both LPV and LNAV are listed in the minimums box, the GPS 27 approach can be flown either way:

Flying the approach

Ultimately, we must find a way to line up with the final approach course.  In a VFR landing, you fly a traffic pattern and largely select your own headings and altitudes as you descend toward the runway.  Under IFR, there must be set rules so that ATC knows where you will be at all times.  Additionally, all turns made under IFR are slow – standard rate or less.  For these reasons, there are standardized ways to navigate to the final approach course that allow the pilot to make easy, slow turns, in a predictable fashion.

Quick discussion of approach segments

There are 4 segments of an approach:

  • Initial Approach Segment
  • Intermediate Approach Segment
  • Final Approach Segment
  • Missed Approach

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Beginning the approach: From an IAF

Initial Approach Fixes are identified with the letters IAF next to a certain fix on the plan view.  You can begin the approach by flying directly to one of these fixes from any direction, and then by following the lines that lead you to the final approach course.

There are three IAFs listed on our example approach:

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Since the profile view only shows the final approach course, sometimes there are altitudes listed in the plan view on the initial and intermediate approach segments.  For example, if starting the approach at KONDE, after crossing KONDE, you would fly along the 184 degree course and descend to 2700 feet until reaching GILDE.  Then, after crossing GILDE, turn to the final approach course and descend to 1700 feet according to the vertical guidance of the plan view.

But what about starting the approach at GILDE?  It is also an IAF, but a special kind with a circle that surrounds it, called a “Hold in lieu of a procedure turn.”  The logic is that again, you could approach GILDE from any direction.  If approaching from the west, an immediate 180 degree turn would be required to become established on the final approach course.  Since this is impractical under IFR, we are provided with a standard way to turn around and get lined up with our 274 degree inbound course.

The procedure involves flying directly to GILDE, and then using standard holding pattern entries (Direct, Teardrop, or Parallel) to get turned
around.  You are not required (nor expected!) to continue flying around the hold.  As soon as you are established on the inbound leg, continue to fly the approach toward the airport.appr_iaf_nopt

It must be noted that if approaching from the East, this “Hold in lieu of a procedure turn” seems unnecessary, since you’re already lined up with the final approach course.  And, in fact, this approach has a special note, “NoPT,” which means “No Procedure Turn.”  It waives the requirement for a turn around the hold if approaching from anywhere between 184 degrees and 004 degrees (See figure).  Be careful, though because some holds should be flown once around regardless of the direction from which they are approached.  If you are approaching from a direction from which the hold seems unnecessary, look for the note “NoPT,” and perhaps contact ATC if it seems unclear.

Finally, if starting at GILDE, what altitudes are appropriate?  Since GILDE is lined up on the final approach course, this information can be found in the profile view.

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Notice the words, “4NM Holding Pattern” to the right of the GILDE fix?  This tells us that after crossing GILDE from any direction, and while flying the Hold in Lieu of a Procedure turn, it is appropriate to descend to 2700 feet.  And finally, only after crossing GILDE inbound, we can descend to 1700 feet.

Beginning the approach: Vectors

For most every approach, the pilot has the option of requesting “Vectors to Final” from ATC.  This is often the fastest and simplest way to fly the approach.  ATC will issue you headings to fly and altitudes to hold, until you are in a position to easily turn onto the final approach course.  ATC is required to set you up for a reasonable intercept angle – usually about 30 degrees off of the final approach course, and at an altitude that is low enough to allow you to easily continue your descent once established.

At some point, ATC will issue you the instruction, “Fly heading XXX, maintain XXX (some altitude) until established.  Cleared for the Approach.”  This very important statement means that you should hold the specified heading and altitude until you are about to cross over the final approach course, at which time, you must turn and fly the approach as published, abiding by all headings and altitudes as printed on the IAP.

ATC will always vector you to a position that is before the Final Approach Segment (usually to somewhere on the Intermediate Approach Segment.)  You must fly the Final Approach Segment on your own, using the information published on the chart.  Therefore, when being vectored to final, you should plan to comply with the altitude and heading information for the Intermediate Approach Segment, immediately upon becoming established on the final approach course.  In our example, you would plan to turn to heading 274 and descend to 1700 upon being both Cleared for the Approach, and Established on the final approach course.

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What does it mean to be established?

Remembering that an Instrument Approach Procedure exists in an area that the FAA has ensured is clear of obstacles, we logically don’t want to descend until we are sure that we are in that protected area.  Obviously, being right over the course line, with your localizer needle centered means we are in the correct place, however there is some flexibility as to the exact position.  The charting office has not only ensured that the course lines, as depicted on the chart, are clear of obstacles at the published altitudes, but they have given us a small margin to either side of the line as well.

Therefore, the term “established,” has been defined as being within a ½ scale deflection of the localizer needle.  The needle can be either to the left or to the right, but when it is halfway from the edge to the center, you are considered established and should begin to descend to the altitude appropriate for the current segment of the approach.

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In fact, whether being vectored to final, or if beginning from an IAF, the words “Cleared for the approach” are very important.  ATC is expecting you to comply with the heading and altitudes you are given until you are established on a published segment of the approach, and they have issued the words, “Cleared for the approach.”  ATC must clear you for the approach before the Final Approach Segment, but it is possible that they will withhold this instruction for a time in order to keep you at a different altitude, even while flying on the Initial or Intermediate Approach Segments.  As soon as you hear those words, though, and if you are established on a published segment of the approach, ATC expects you to adhere to altitudes as published on the chart.

Final Approach Segment

The initial and intermediate segments are the paths that lead you up to the Final Approach Fix – the point from which the Final Approach Segment begins.  The Final Approach Fix is identified on the plan view, with the letters “FAF,” and on the profile view with either a Maltese cross (if flying as a non-precision approach), or a lightning bolt (if flying as a precision approach).

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In this example, the Final Approach Fix for the non-precision version of this approach is at ECADE.  The Final Approach Fix for the precision version is an altitude – the location at which you are both on the glideslope and at an altitude of 1700 feet.  In this case, both Final Approach Fixes happen to be in the exact same location.

Regardless of how you begin the approach, you must be on the final approach course prior to the Final Approach Fix.

The missed approach segment begins at the Missed Approach Point and consists of instructions telling you what to do in the event that you are unable to see the runway at the Missed Approach Point.  This will be discussed in more detail later.

Minimums

We now now how to fly the approach all the way to the Final Approach Segment.  But what about the final altitude?  The final altitude is called your minimums, and is not listed in the profile view.  It is not listed because it changes depending on the way the approach is being flown.

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When flying a precision approach, you may follow the glideslope down to 342 feet.  This is called the Decision Altitude (DA) because it also defines your Missed Approach Point, the end of this approach, at which point you must either be able to see the runway or go missed.  Remember this by knowing that on a precision approach if you reach this altitude, you must make the decision to land or go missed.

If flying as non-precision, the final altitude (past CUVNA) is 520 feet.  Because non-precision approaches are flown in step down fashion, this altitude must be held for a time while you continue toward the airport.  For this reason it is called the Minimum Descent Altitude (MDA).

Ending the approach – Landing or Going Missed

The goal of the approach is to get lined up with a runway and land the aircraft.  However, one must always be prepared to deal with the possibility of not seeing the runway, and being forced to go missed – climbing back away from the airport to either try again, or go someplace else.  The charting office has established a protected area around the approach course, but also around a course that leads you back away from the airport to a safe altitude and location.  There are missed approach instructions for each instrument approach that define a course to follow and altitudes to climb to as you make your way back away from the airport.  These instructions are found in multiple places on the IAP.  They are listed in text form, on the plan view, on the profile view, and are depicted graphically in boxes near the plan view.

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A pilot can elect to go missed at any time during an approach.  However you still need to remain within the protected area for the approach, until you reach the spot where the missed approach protected area begins.  This spot is called the Missed Approach Point (MAP).  A pilot can elect to go missed at any time, but is required to go missed upon reaching the Missed Approach Point.  If he goes missed early, he still should fly to the Missed Approach Point, at or above the published altitudes, until beginning to follow the missed approach instructions.

Missed Approach Points

As discussed, a single IAP can represent multiple ways to fly the approach (Precision or Non-Precision, i.e. LPV, LNAV.)  The Missed Approach Point is generally the point at which the lines on the IAP change from solid to dashed.  The Missed Approach Point for a precision approach is an altitude.  The pilot will fly the glideslope down to a certain altitude, and no lower.  This is the altitude that, once reached, requires the pilot to make the decision to continue to land or go missed.  That is why the final altitude for the LPV approach is called the “DA” (Decision Altitude).

For a non-precision approach, the Missed Approach Point is the last depicted waypoint in the profile view.  In our example, the last named point on the profile view is RW27, or the beginning of runway 27.

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If the runway environment is not in sight upon reaching the RW27 fix (if flying non-precision,) or upon reaching 342 feet (if flying precision), the pilot is required to follow the missed approach instructions as presented on the chart (unless otherwise instructed by ATC.)  In this example, the pilot would climb straight ahead to 1000 feet, then turn right and climb to 2000 feet, and finally proceed direct to the PLUMY fix and hold.

Required Equipment

As discussed, a single IAP can represent multiple ways to fly an approach.  It is up to the pilot and the capabilities of the airplane to determine whether to fly it as a precision or non-precision approach.  Normally, the pilot would choose to fly the approach in the manner that will give him the lowest minimums, in this example, the LPV approach.  However, not all GPS units are capable of flying LPV approaches, which would restrict the pilot to flying the approach as an LNAV only.  The pilot should start by looking at the name of the approach, as well as the details box that includes information on what equipment is required to fly it.

Can I fly this approach?

Most airplanes are not equipped to fly every approach at every airport.  Consider a few examples:

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After determining if you are able to fly the approach at all, next, determine the minimums that are applicable to your airplane.  If we have a WAAS equipped, IFR GPS, we can fly the glideslope to LPV minimums.  If we have a non-WAAS GPS, we must fly this as a non-precision approach, and only down to LNAV minimums.  A minimums box is also broken down by aircraft Categories A through D.  In this example, when flying either an LPV or an LNAV approach, the minimums for categories A through D are the same, but it is not the case for the circling approach.  The circling approach minimums change depending on aircraft category.  Circling approaches will be discussed later, but suffice it to say that we must determine in which category our aircraft falls in order to know which minimum altitude to use.

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Aircraft Categories

Aircraft categories depend either on the aircraft’s published approach speed, or 1.3 times VS0 (stalling speed in landing configuration), whichever is greater.

  • Category A: Speed less than 91 knots.
  • Category B: Speed 91 knots or more but less than 121 knots.
  • Category C: Speed 121 knots or more but less than 141 knots.
  • Category D: Speed 141 knots or more but less than 166 knots.
  • Category E: Speed 166 knots or more.

Many light aircraft do not have a published approach speed, so we must calculate 1.3 times VS0.  A Cessna 172 may stall at around 40 knots, which, multiplied by 1.3 would equal 52 knots, and as long as we are planning to actually fly the approach less than 91 knots, it would fall in Category A.  If the pilot elects to fly the approach at a higher speed than the maximum for that category, then a higher category of minimums should be used.

Circling approach

A circling approach is used in the case that the pilot wishes to fly an approach to one runway, but then land on another.  The circling approach can be flown as either a precision or non-precision approach, however the final altitude is much higher in either case, because after breaking out of the clouds, or spotting the airport in low visibility, we must then maneuver the airplane for landing on a different runway.  Cross referencing our Category A aircraft column with the row of minimums for a Circling approach, we determine that our minimums are 600 feet.

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appr_circling_nonprecision

Circling approaches have higher minimums, however you will notice that these are still much lower than the typical traffic pattern altitude.  For this reason, aircraft must stay relatively close to the airport while maneuvering for a different runway to ensure obstacle clearance.  For category A aircraft, the pilot must remain at or above circling minimums (600 feet), and within a radius of 1.3nm from the end of any runway while maneuvering to land on the runway of choice.  The radius (and minimums) change depend on the caterogy of aircraft being flown.  In the example below, the pilot must stay within the light blue shaded area at all times while circling.

appr_circling_radius

The pilot may choose to use most any method to get lined up with the runway of choice for landing.  As long as you stay within the protected area and at or above circling minimums, any of the following patterns would be appropriate.

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How do you select a good circling entry?  Remembering that you are low and slow as you break out of the clouds on a circling approach, avoid any kind of steep turns or drastic descents.  It is often more appropriate to fly around the airport until you are in a better position to land using normal maneuvers.  For example, Pattern B may be appropriate if you find yourself too high or at an awkward angle to the runway.  The best thing to do would be to overfly the runway and enter a downwind for a more normal approach.  Pattern C would be appropriate if the approach you chose to fly brings you in downwind, and you choose to land instead with a headwind on the opposite runway.

Missed Approach while Circling

There are two possibilities that may require a pilot to go missed while executing a circling approach.  The first is the chance that the pilot does not see the runway after descending to circling minimums and reaching the Missed Approach Point.  This would require the pilot to continue flying straight ahead, and following the missed approach instructions as usual.  The second is the chance that the pilot loses sight of the runway environment while trying to get lined up with the runway of choice.  Perhaps visibility gets worse, or the pilot flies into a stray cloud.  In this case, the AIM recommends immediately climbing and turning toward the center of the airport, and then reestablishing on the original final approach heading, and flying the missed as published.  Remember, the approach you came in on is the approach ATC is expecting you to fly, so follow the missed approach instructions as published for the original approach, not some approach for the runway you were circling to.

 

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