I once read an article that said one of the best feelings in flying is taking off into thick clouds only to see it get lighter and lighter and lighter until....poof! Out of the tops of the clouds you come and into the clear blue sky above! While I will admit that is an incredible feeling, it a close second to just the opposite: descending into the clouds below only to pop out of the murk (or "soup" as pilots like to call it) and have a big, beautiful runway sitting right there in front of you. After all, getting up into the sky is only half the battle and, as an old saying goes: "Aviation has a perfect record. We haven't left one up there yet!"
To passengers, however, there is perhaps no greater mystery in flying (other than how you are rocketing thru the air 7 miles above the earth in a metal tube) than how do they get you safely on the ground when fog thicker than pea soup keeps you from seeing anything until seconds before you land. The answer is using what's called an "Instrument Approach Procedure" or IAP. Many people assume that GPS has taken all the guesswork out of IAPs. But, consider this: many airlines have not approved GPS - based IAPs and are using the venerable Instrument Landing System or ILS. Amazingly, the first airliner landed using an ILS approach in 1938!! Imagine that! Technology that was invented in the 1920s and first used in the 1930s is still used to reliably land the majority of airliners in 2012! Pretty impressive record. So, how does it work? Let's delve into it.
The inflight movie just ended and your seat back and tray table are in their upright and locked position. The plane slowly descends into the fluffy overcast layer below, the sun fades away and slowly your plane is engulfed in nothing but grey. Air traffic control (ATC) issues your pilots a series of turns and descents (vectors) until they are lined up with and using the first component of the ILS approach called the "localizer". Think of this as an imaginary runway centerline that originates from a radio transmitter at the far end of the runway. There is a needle in the cockpit that represents the runway centerline that floats to the left and right. If it's to the left, you need to fly left to get back on the runway centerline. If it floats to the right, you fly to the right....just like you were looking at the centerline of the runway itself!
ATC will keep your pilots flying toward the runway on the localizer at a certain altitude. When they reach a certain point called the "Outer Marker", a blue light that says "O" illuminates and a distinct series of tones is heard in the cockpit which lets the pilots know what point they are over. This point, known as the "Final Approach Fix" (FAF), is where a second needle known as the "glideslope or glidepath" comes alive. The radio signal that moves this needle comes from a second radio transmitter that is on the side of the runway about 1,000 feet from the runway threshold. Think of this one as a tractor beam that is pulling the plane diagonally onto the runway. This needle floats up and down. If it floats up you are too low. If it floats down you are too high. Just fly toward the needle. The localizer and glideslope create a set of crosshairs. Your pilots (or the autoflight system) keep the crosshairs centered so that your plane descends down to the runway and lands roughly 1,000 feet onto the runway! Easy, right? Well, sort of. At some point, the pilots have to actually see the runway or else they have to execute what is called a "missed approach procedure" (go missed) which means they fly back up into the air as specified by their charts and try the approach again or divert to another airport. How do they know when to go missed? Simple. At either the "Middle Marker" (Amber cockpit light and some tones) or the "Inner Marker" (White cockpit light and some tones) and not below a charted altitude known as the "Decision Altitude" (DA), either your pilots see the runway or they go missed. For a standard ILS, that DA or "minimum" comes at only 200 feet above the ground at the Middle Marker which is about 1/2 to 3/4 of a mile from the runway threshold. By the way, seeing those crazy flashing lights at the end of the runway counts as seeing the runway.
What about "autoland" that I mentioned in my previous post on autopilots? Well, if your airplane is so equipped, the airport you are flying to has a specially certified ILS installation and the pilots are specially certified, the need for actually seeing the runway is almost eliminated and the pilots can use the autoflight system to fly the airplane right onto the runway.
Pretty amazing for 1930s technology, huh? Hey, it works!
If you are so inclined, head out to YouTube and search on "ILS minimums" and check out what flying an ILS aproach looks like from the "front office". You'll see why I say it's one of the best feelings in the flying!
By the way, there are many other types of instrument approaches. If you're so inclined, you can Google: RNAV, GPS, VOR, NDB, RNP (these are new and particularly cool), and PAR approaches to name a few.
Blue Skies,
Jeff
Full credit to http://en.wikipedia.org/wiki/Instrument_landing_system for some of this data. Check it out for the real nuts and bolts of how an ILS works!
