The Instrument Approach Procedure Chart (sometimes referred to as an 'approach plate') is one of the critical publications necessary for operating an aircraft under instrument flight rules. The most difficult phase of flight is landing, even when flying VFR; in IFR conditions, it becomes the reason instrument pilots, and especially air transport pilots, train so hard. Remember, we're talking about the process, procedures and materials necessary to bring (potentially) a 40-ton airliner with 350 or more people on board from several miles in the sky moving at around four-fifths the speed of sound to a safe landing, back on the ground, halted at the gate. If necessary, without the pilots being able to see anything out of their windows until the aircraft is anywhere from a couple of hundred feet up to, if they're trained properly and the aircraft certified, never seeing anything out of the windows at all until it comes time to turn off the runway to taxi.
You need a lot of information to do this.
The approach plate is either a paper or electronic publication which contains the information necessary for an aircraft and its crew to safely land at a given airport using a given approach method, or procedure. This writeup is about those in use in the United States, which correspond to those standard around the world but in particular those published by the U.S. FAA.
In any case, an airport may have several - up to a dozen or two - different approach procedures. These will differ by which runway they are intended to place the aircraft on at landing and the approach method used for that particular procedure. This makes sense; to land of different runways the aircraft will always be following different paths over the ground to ensure it ends up at the right place facing the right direction.
One reason the approach plate exists is to help ground air traffic controllers. All aircraft utilizing the approach plates will always be under positive ground control, as it is a requirement for IFR operations. The approach procedures are so that ground does not have to explain to each aircraft precisely where to go, and what altitude to be at. Instead, ground can direct aircraft to use a particular published approach, and then only need worry if that aircraft deviates from the expected track and behavior, minimizing communication and information transfer during the landing. Remember, there may be dozens of aircraft on approach to a particular runway at the same time, at different points along the published approach procedure!
Let's finally look at an approach plate. Here is an example of one, courtesy of the FAA. Call it up in another tab or window, or the rest of the writeup won't make much sense.
The plate has several distinct sections, which will always be present (although not necessarily arranged exactly the same way). Let's start at the top.
The top of the plate has a table of information which is called the Pilot Briefing or Notes section. This includes a great deal of important information which is designed to be used quickly while in the air. First, at the top left, is the name of the city nearest to the airport the chart covers. These publications are organized by city name, not airport name or identifier, because it's more likely you'll remember that you're headed for San Jose than that the airport there is named Mineta International, or even KSJC. The top right corner of the briefing area contains more chart identifiers: most importantly, the name of the approach procedure that this chart covers. In our example, the chart depicts both the ILS and LOC/DME approach to Runway Three Zero Left (30L). This means that our aircraft will be traveling on a heading of approximately three hundred degrees magnetic when we land, and that this is the left of two parallel runways which face that direction.
The top left gives us the frequency for the I-SJC Localizer and DME station - 110.9 and Channel 46, respectively. The Approach Course, which we will need to be flying as we approach the runway, is 303° magnetic, which jibes with 'Runway 30'. Next we have several important measurements. The length of each runway which faces the approach is given here in feet, along with the elevations above sea level of their touchdown zone and the official airport elevation (taken from somewhere central).
Below this, there is a section for notes. To the right of that, the type of runway approach lighting installed on Rwy 30L is given (this helps pilots identify the correct runway at night). Then a short version of the missed approach procedure, to be followed if the pilot is forced to abort the landing. Next there is a row of communications frequencies we will likely need. Here we are given ATIS, then the freq for Northern California Approach, the SJC Tower, Ground Control (who we will need to talk to after landing, if the Tower tells us to) and the Clearance Delivery frequency for KSJC.
After the Briefing area comes the Planview. This is the bit that looks like a relief map with a bunch of arcane stuff on it, because that's what it is. Essentially, it's a map of all significant features you might need to fly this approach. One of the most important features on it, though, is the circle in the lower left of the map which reads 'MSA SJC 25 NM'. This stands for 'Minimum Safe Altitude, KSJC, 25 nautical mile radius' and is meant for VFR and emergency use. The circle tells the pilot that for 25 miles from the named VOR (SJC, whose location is shown on the planview map with the same symbol) remaining at a minimum altitude of 5600 feet above sea level is the minimum required for safe clearance with no other information. If, for example, you are a VFR pilot lost in cloud in the area, you know you had better remain above that height to avoid all obstacles. Actually, it tends to contain a roughly 500 foot 'safety pad' but better safe!
The rest of the planview involves obstacles and their heights, waypoints and navigation beacons, and the local terrain.
On the lower left of the diagram (it may be on the lower right of some charts) is an airport diagram. This is handy both for identifying the airport and the intended runway - airports from the air tend to look a lot like their diagrams, for some reason. It also tells us about obstacles in the close vicinity of the airport (there is a tower of some sort which reaches to 144 ft above sea level just northeast of the runways, e.g.)It also describes the runway lighting installed on each and identifies standard intersections, thresholds and hold points.
To the right of that is two boxes. The top contains the Profile, which is a side view of the approach course. It instructs the pilot at what altitudes (minimum) the aircraft must be at over each of the approach procedure waypoints. While it is acceptable to be higher than those altitudes, it is dangerous and against the rules to be below those altitudes. So for example, when crossing over the KLIDE INTERSECTION, the aircraft must be at 4000 feet ASL or above. By the next waypoint, ZULUP, the lower limit is 3100 feet. At HIVAK, where the X and gray cone are, the glideslope and/or localizer must be captured at or above 2700 feet, and so on until landing. The numbers just below the waypoint names show the number of ground miles to the terminal localizer (I-SJC 11.4 in the case of ZULUP, e.g.) and the length of each segment is show at the bottom. At the top left of the profile is the missed approach procedure; in this case, climb straight ahead to 1900 feet along the 303 outbound radial from the SJC VOR, and then hold at the SUNNE Intersection (a hold is a standard maneuver and procedure).
Finally, at the bottom, there is a table of Minimums. These are various conditions that must be met before the aircraft can land. The 'Category' indicates the minimum approach speed of your aircraft (slower aircraft are lower, so A is the slowest minimum speed and D or E the highest - A might be a Cessna 172 and D a Boeing 767). The left column indicates which runway and approach the minimum is for (S=ILS 30L, or S-Localizer 30L). 'Sidestep RWY 29' means that the aircraft, while using the Runway 30 approach, expects at the last minute to 'sidestep' and instead land on Runway 29. This allows a single set of runway instrumentation to serve two or three runways (in this case, three). The numbers are the minimum decision altitude and the required RVR or runway visual range. As an example, an aircraft of any category using S-ILS 30L approach must be able to see the runway visually at or above 257 feet altitude, and must have a Runway visual range (i.e. be able to see directly ahead) of at least 2400 feet at the runway threshold. If the pilots cannot see the runway at 257 feet, or if they cannot see enough of the runway, they must immediately execute the MAP or Missed Approach Procedure. In this case, remember, immediately climb to 1900 feet while continuing on course straight ahead, following the SJC 303 radial outbound, and then hold at the SUNNE intersection.
That's an approach plate, or instrument approach procedure chart. There's a ton of information on it, more even than we discussed. It may be read these days on an electronic display, either built into the airplane or on a tablet computer such as on iPad or Android tablet - but the information is the same. (Note that to use a device such as that for primary navigation, you'd better make sure you're approved! Private pilots can go ahead, but corporate and transport pilots had better have official permission!)