When one thinks of what an instrument rating entails, two things are likely come to mind:
- Flying in the clouds
- Instrument approaches
An approach is the “fun part” of an instrument rating. It is, after all, one of the most tangible new topics that an instrument student learns as part of their training. The student learns how to read, brief, and interpret an approach plate, and determines from that plate what course to fly, what altitudes to hold, what ground speed to maintain, and what to do if the approach needs to be called off (essentially the equivalent of a go-around, called the “missed approach”). The pilot workload is high during an instrument approach, but with training and a strong basic attitude instrument flying skillset, they can be done confidently and safely.
CDIs and Glideslopes: Understanding the different types of approaches
Pilots will frequently refer to two different sorts of approaches: non-precision approaches and precision approaches. A non-precision approach refers to one that provides lateral guidance only. Lateral course correction can be provided in a variety of ways — VOR or NDB transmitters, localizers, and certain RNAV (GPS) approaches all provide this capability. The way that an aircraft descends to a landing is through the use of predefined stepdown altitudes at waypoints along the approach. These waypoints can be identified through a GPS navigator, a DME readout, or a supporting VOR.
A precision approach, on the other hand, is an instrument approach that provides both lateral and vertical guidance. This is done through the addition of a glideslope on the CDI, and the objective is to keep both needles centered. An ILS (instrument landing system) is one such example of a precision approach. A PAR approach is another such approach, where a controller guides the aircraft down using radio callouts. These approaches are rare, but do exist.
If you’d like some trivia to impress (or annoy) your friends at the airport, there is a third distinct category of approach, as defined by the FAA: an APV approach, or Approach Procedure with Vertical guidance. A GPS approach with a glideslope (ie an LPV approach) is NOT a precision approach but rather an APV approach. The reason for this distinction is because ICAO defines a precision approach as a very specific thing, and it was ultimately easier for the FAA to create a third category of approach rather than try to retrofit RNAV approaches to fit into the legacy category of “precision”.
If you have read the ACS, you may recall that an applicant must accomplish at least two non-precision approaches and one precision approach, and “acceptable instrument approaches for this [precision] part of the practical test are the ILS and GLS [approaches].” Thankfully, the FAA allows for an LPV approach as well by stating: “..if the installed equipment and database is current and qualified for IFR flight and approaches to LPV minima, an LPV minima approach can be flown to demonstrate precision approach proficiency if the LPV DA is equal to or less than 300 feet HAT [height above terrain].”
So for all intents and purposes, LPV approaches can be treated much in the same way as an ILS is: a very precise approach that gets you down low.
Let’s look at the VOR-B approach into KSFZ for a reference. We won’t go over absolutely everything, but cover the important parts the apply to civilian aircraft. The way we brief an approach plate like this is top to bottom, right to left.
- The top briefing strips tells us that the VOR used for navigation is the VOR/DME at Putnam, channel 117.4.
- The approach course to be dialed into the CDI and flown is the 111 degree radial FROM the PUT VOR.
- The “T” and “A” embedded in a triangle refers to the fact that non-standard takeoff and alternate requirements exists. The local altimeter setting is required to fly down to 980 feet MSL after the final approach fix, otherwise the Providence altimeter setting must be used and the minimum descent altitude must be increased to 1080 feet MSL. Furthermore, if the approach is made during night, landing on Runway 33 or 15 is not authorized.
- The missed approach instructions are to make a climbing right turn to 2500 feet MSL via the Norwich 057 degree VOR, and hold at the FOSTY intersection.
- The plan view shows “the big picture” of the approach. We start the approach at the initial approach fix at the Putnam VOR and it terminates with the missed approach point at JOPVO. Each of the waypoints can be identified in a variety of ways: DME distance from the PUT VOR, identification of VOR radials from the Providence transmitter along the approach course, or using a GPS navigator.
- The inverted “V” with a dot in the middle represents obstructions. There aren’t a ton on this approach, but one in particular jumps out at us: right near the MAP near the airport! The altitude listed is 573 — thankfully, this is in MSL, not AGL, which means that if we remain at or above our MDA of 980 MSL, we are assured 500 feet of clearance.
- The circle in the top right corner of the plan view shows the Minimum Sector Altitude in a 25 NM radius around the Putnam VOR. This is a reference that provides us the minimum altitude to avoid all obstacle clearance in the areas depicted.
- The text in the top left corner of the plan view states that the VOR-B approach is not authorized for aircraft arriving at the PUT VOR via the V146-151 airway. Why is this? It’s due to a national policy called the United States Standard for Terminal Instrument Procedures, also known as “TERPS”. TERPS 2-4-1.a states: “When the IAF is part of the en route structure, the angle of intersection between the en route structure and a ground-based initial approach segment course must not exceed 120 degrees. For RNAV routes, apply Order 8260.58.” What this means is that the turn from V146-151 northwest-bound to the approach course is too sharp, and thus not authorized. What this effectively means is that pilots wishing to execute the VOR-B approach should do so via radar vectors.
- In the bottom left corner of the plan view, an alternate missed approach fix exists. Instead of holding at the FOSTY intersection, a pilot may elect to hold back at the PUT VOR, allowing for positioning to reattempt the approach.
- The bottom left corner of the approach plate shows a zoomed in view of the airport, and where the approach terminates. Assuming the approach is flown correctly, the pilot should find themselves approximately midfield of the 5/23 runway. The distance between the final approach fix and the missed approach point is 4.9 NM, and if flown at 90 knots groundspeed, should take three minutes and sixteen seconds.
- The bottom right corner of the plate shows the profile view. This view shows the approach course to be flown (111 degrees FROM PUT) and the stepdown altitudes. The line underneath the altitude shows the minimum descent altitude for that segment — beginning the approach and until intercepting the BIRDS intersection, the pilot must not descend below 2500 feet. After BIRDS, the pilot may descend no lower than 1800 feet. At the final approach fix, identified on the profile view with a Maltese Cross, the pilot may descend no lower than 980 feet MSL (539 feet AGL) with 1 mile of visibility. Even when the circling approach may be executed, the aircraft may not descend lower than the MDA until three conditions are met:
- The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers.
- The flight visibility (or enhanced flight visibility, if so equipped) is not less than the visibility prescribed in the standard instrument approach being used.
- At least one authorized visual reference for the intended runway is distinctly visible and identifiable to the pilot, such as the threshold markings, lights, or threshold itself, the runway lights, the touchdown zone, etc. If the approach light system is in view, the pilot may descend to 100 feet above the touchdown zone, only if the the red terminating bars or the red side row bars are also visible and identifiable.
- Finally, a short reminder of the missed approach procedure is listed in the profile view.
As mentioned earlier, this particular type of approach doesn’t provide the pilot with any vertical guidance, hence the “non-precision” nature of the approach. Let’s look at another plate, this time being a precision approach.
Introducing the Glideslope and DAs
With an ILS approach, the glideslope is introduced to the pilot to guide them down from the final approach fix. We brief the approach in much the same manner as we did the VOR-B into KSFZ by reviewing the briefing strips, the plan view, and the profile view of the plate. Some key differences between the VOR-B and this approach:
- A procedure turn exists if intercepting the localizer between CUTSI and KENTE intersection. However, if we are on radar vectors to the FAF (which will almost certainly will be), we will not fly the procedure turn.
- The glideslope should be intercepted and alive at CUTSI. At this point, we may fly the glideslope, but we may not descend below 740 feet until we intercept FEXUX. After FEXUX, we may descend to an altitude of 253 feet MSL. At this time, we MUST execute a missed approach if not able to meet the three criteria listed previously (position to land normally, visibility adequate, at least one visual reference). This is why the DA is called a “decision height”.
- There is a V denoted on the profile view 2.8NM DME out from the localizer. This is the Visual Descent Point. This would apply if we were flying the LOC approach, but doesn’t apply to the ILS approach. A VDP is a point on the final approach course of a non-precision straight-in approach from which normal descent from the MDA to the runway touchdown point may begin provided adequate visual reference is established. We’ll go over VDPs in a separate post in the future, as there is a lot of confusion surrounding what they are and what they do.
The main advantage of a precision approach versus a non-precision approach is that with a precision approach, you have an established, stabilized approach for most of the approach. The Instrument Procedures Handbook states:
“A constant-rate descent has many safety advantages over non-precision approaches that require multiple level-offs at stepdown fixes or manually calculating rates of descent. A stabilized approach can be maintained from the FAF to the landing when a constant-rate descent is used. Additionally, the use of an electronic vertical path produced by onboard avionics can serve to reduce CFIT, and minimize the effects of visual illusions on approach and landing. Some countries even mandate the use of continuous descent final approaches (CDFAs) on non-precision approaches.”
There is a lot of information on approach plates, but the information is always presented in a logical, standardized manner. It is always wise to brief approaches before takeoff and during cruise so that we are not rushing to familiarize ourselves with this critical and information-rich chart. Given enough practice, an instrument student will find themselves able to read plates quickly and effectively so that any approach will make sense to them.