Ever wonder how pilots land when they can’t see the runway? This article will review how to read an ILS instrument approach chart and review some of the differences between precision and non-precision approaches. This is the ILS/Localizer approach at runway 32 left at Boeing Field in Seattle. See image 1 below. ILS stands for instrument landing system and is a precision approach which provides pilots with horizontal and vertical guidance to the runway. Instrument approaches are used to help pilots land safely in conditions of reduced visibility.
Precision vs. Non-Precision Approaches
A precision approach provides both lateral guidance (to the left and right of the runway) and vertical guidance (helping keep pilots on the proper glideslope) to help an aircraft descend safely to the runway. Get too far of course to the right or left and the pilot must fly back towards the needle to get back on course. Fly too high or low and the pilot must get back on the proper glideslope. A non-precision approach such as a VOR or LNAV approach only provides lateral guidance. This means the pilot needs to manage the descent using step-down altitudes. Common examples include VOR, NDB, and some RNAV GPS approaches without vertical guidance.
Some RNAV approaches such as this RNAV GPS approach into runway one six right at Paine Field (see image 2 below) in Everett are not technically precision approaches but provide a high level of accuracy similar to ILS. Notice how this approach has LPV (which stands for Localizer Performance with Vertical Guidance). While technically non-precision, LPV approaches provide both lateral and vertical guidance, similar to an ILS. This has made them increasingly popular, especially at airports without ILS systems. Compared to VOR or RNAV GPS approaches without vertical guidance, precision approaches such as an ILS are more accurate and typically allow for lower decision altitudes (DA) and visibility requirements.
How to Find the Localizer Frequency, Approach Course, Runway Landing Distance and Airport Elevation
Let’s review exactly how to read and brief and ILS approach plate. The top briefing strip (image 3 below) is the box in the upper left corner of the chart and for an ILS approach includes the localizer frequency, approach course, runway landing distance, touchdown zone elevation and airport elevation.
Non-Standard Takeoff Minimums
The notes box in the middle briefing strip (image 4 below) includes pertinent information about the approach to aid pilots. In this case, distance measuring equipment (abbreviated DME) and radar are required. The letter “T” in the upside-down triangle indicates that non-standard takeoff minimums exist. The letter “A” in the triangle indicates that non-standard IFR minimums exist.
How to Find the Communication Frequencies
All the relevant communication frequencies are included in the communications briefing strip as shown below. Typically, the ATIS, approach control, tower, ground control and clearance delivery or UNICOM frequencies will be shown in the communications briefing strip.
Missed Approach Procedure
The missed approach procedure (MAP) is illustrated in the upper right corner of the middle briefing strip. In this missed approach procedure shown below, the pilot would climb on a heading of three one five to cross the Seattle 8 DME at or below 1,700 feet, then make a climbing left turn to 6,000 on a heading of two six five then continue to the holding procedure as shown.
Minimum Safe Altitude
The minimum safe altitude, or MSA is shown in the plan view section of the chart. According to the FAA Chart users guide, minimum safe altitudes are published for emergency use on instrument approach charts. Minimum safe altitudes appear in the plan view of all instrument approach charts except on approaches for which a Terminal Arrival Area is used. Minimum safe altitudes are expressed in feet above mean sea level and normally have a 25 nautical mile radius. Minimum safe altitudes provide 1,000 feet clearance over all obstructions but do not necessarily assure acceptable navigation signal coverage. In this example, the minimum safe altitude when tuned into the Seattle VOR and on headings between 190 degrees and 340 degrees, the minimum safe altitude is 6,400 feet.
Use the Planview Section to Find Relevant Course Headings, Navaid Frequencies, Approach Fixes, and Terrain/Obstacle Heights
In the plan view section, the approach is illustrated with the relevant course headings, primary navaid frequencies, approach fixes and terrain or obstacle heights in the vicinity of the approach area.
Profile View
As shown in image 9 above, a profile diagram is shown to illustrate the approach and descent path to the landing runway. In the upper left, the missed approach procedure is shown with the pertinent heading, altitude, DME distance, VOR radial and waypoints. The glideslope altitude for this approach is at or above 1,800 feet and is at the DUANE fix. The localizer/DME antenna is located at the orange arrow and is 0.3 nautical miles from the approach end of runway three two left. The numbers highlighted by the blue boxes are the DME distances from the DME antenna. The initial approach fix of LACKR is located 9.8 miles from the DME and is 10.1 nautical miles from the approach end of runway three two left as illustrated by the distances below highlighted in orange. The glideslope interception altitude is shown by the zigzag line and the “X” in the diagram.
Airport Sketch
The airport sketch will give the pilot further details on runway lighting, runway length, and displaced thresholds. MIRL stands for medium intensity runway lights. HIRL stands for high intensity runway lights and REIL stands for runway end identifier lights.
Final Approach Fix to Missed Approach Point
Below the airport sketch, you will see the time required from the Final approach fix to the missed approach point which in this case is 5.3 nautical miles. For instance, if we were flying a Cessna 172 and our final approach speed was 60 knots, it would take five minutes and 18 seconds to get from the final approach fix at Duane to the missed approach point.
Landing Minimums
The landing minimums for the approach is below the profile view of the approach chart and shows the decision altitude (if it’s a precision approach such as this ILS approach at Boeing Field) or the minimum descent altitude if it’s a non-precision approach such as a GPS LNAV approach without vertical guidance.
There are four categories – A, B, C, and D in the landing minimums section. Per the instrument procedures handbook, category A aircraft are categorized as having approach speed of less than 91 knots. Category B is speeds of 91 knots or more but less than 121. Category C is 121 knots or more but less than 141 knots. Category D is 141 knots or more but less than 166 knots.
Decision Altitude vs. Minimum Descent Altitude
Before we proceed any further, this would be a good time to mention the difference between decision altitude and minimum descent altitude. These are two critical terms every instrument and commercial pilot must know when flying instrument approaches. But what’s the difference?
On a precision approach, like this ILS at Boeing Field, we use a decision altitude (or DA for short). This is the altitude where the pilot must decide whether to continue descending based on whether the runway environment is visible. If not? Execute the missed approach. On the other hand, a non-precision approach, like a VOR or RNAV approach without vertical guidance, uses a minimum descent altitude (also known as MDA). Here’s the key difference: the pilot levels off at the minimum descent altitude and continue flying until reaching the missed approach point. If the pilot cannot see the runway environment by then, it’s time to climb out and go missed. And what about decision height (DH)? The decision height is the height above the runway threshold instead of mean sea level. Think of it as a more specific way to measure your decision point.
To sum it up: decision altitude is for precision approaches, minimum descent altitude is for non-precision approaches. On final approach, maintaining the correct glide path is critical for a safe landing. This is where a Visual Glide Slope Indicator, or VGSI, comes in. You’ve likely seen systems like the PAPI or VASI at the runway. These provide visual cues to keep you on the correct descent angle, typically around 3 degrees.
Some approaches, however, use a glide slope steeper than 3 degrees—like this one at 3.1 degrees.
The threshold crossing height, or TCH, is the altitude you’re expected to cross the runway threshold while on the glide slope. For a typical PAPI or ILS, this is often around 50 feet, ensuring you clear obstacles while setting up for a safe landing. Thanks for reading. We hope you now have a better understanding of how to read an instrument approach chart and some of the differences between precision and non-precision approaches. Please share this article with your fellow pilots or aviation enthusiasts if you found this information useful.