In the last couple chapters we looked at navigation. This chapter also deals with navigation, namely how we navigate the last ten miles or so to the runway.
Fortunately today we have some very impressive technology that allows us to land even in conditions of very low clouds or very low visibility. Spend some time flying in Europe during the winter months and you'll come to appreciate this.
The simplest form of approach to landing is what's called a "visual approach". Just look at the runway and land. Simple enough. This is our preferred method of getting to the runway because it allows air traffic control to bring the planes in closer together. That gets more people into the airport in a shorter amount of time. That's a very big deal at a busy airport like O'Hare.
Unfortunately the weather doesn't always cooperate. Ever since the early days of aviation we've been inventing ways to at least get close to runway while in instrument conditions.
There are two main types of approaches, precision and non-precision. A precision approach gives glideslope (vertical) guidance while a non-precision approach gives only lateral (ground track) guidance. Some might argue that all of my approaches are "non precision".
Here's the general concept behind a non-precision approach.
At a predetermined point, called the Final Approach Fix, we will start a controlled descent down to what's called the Minimum Descent Altitude or MDA. Normally this is 500 feet or so above the runway. We'll level off at the MDA and continue on until we reach another predetermined point, called the Missed Approach Point. If we haven't seen the runway by then we'll have to go around (called a Missed Approach). We sometimes call this type of approach a "dive and drive".
Let's look at an NDB approach. I talked about NDBs a couple chapters ago. Basic Navigation
I'm not a big fan of NDB approaches because they're not very accurate. Still they're cheap to build and they work so they're out there. You'll mostly find them at smaller airports or in less developed parts of the world - like Billings.
I'm only going to talk about the "straight in" final approach portion of this. That's what we'd be flying if we were vectored to final by air traffic control. There's a way to fly this in a non-radar environment, called a "procedure turn" but that would be a whole chapter unto itself.
To fly this approach we would configure the aircraft for landing and fly a 098 degree bearing towards the NDB until we crossed the station.
When we cross the station, according to the chart we'll be 6.3 miles from the runway. Once we cross the station we're going to do three things:
1. We'll track 098 degrees outbound from the NDB.
2. We'll start a controlled descent (usually about 1000 feet per minute in a jet).
3. We'll hack a stop-watch. This is old-school stuff here.
We will descent to our MDA which in this case is 4300 feet (I've underlined it in blue). That will put us 716' above the runway. We'll level off at this altitude and continue straight and level towards the airport.
While all this is going on we're watching our clock. Take a look at the chart I've highlighted in green. Let's suppose our groundspeed today is 140 knots. According to the chart we've got 2 minutes and 42 seconds to work with.
Hopefully before that 2:42 is up, we've seen the runway and transitioned to a visual glide path. If not, we must go missed approach when the timing expires.
I think of NDB approaches as "airport finders". You'll probably find the runway but the approach may not line you up with it particularly well. You may have to do a last minute "jink" to line yourself up. These work best when there's a cloud deck but good visibility underneath the clouds.
OK, we got our steak in Billings. Time to head up to Providence for a lobster roll (I'm all about the food). Let's look at a VOR approach this time. These use the same basic concept but they're a little more accurate.
Here's a refresher in case you've forgotten what a VOR is Basic Navigation
To fly this approach we'll want to set up on the 232 degree course headed towards the VOR. We'll want to be level at 1100' and configured to land when we cross the Final Approach Fix at 4.0 DME. We'll start our controlled descent to our MDA of 440', which is 390' above the runway (Providence is almost at sea level).
The main difference is we'll be watching our DME instead of a clock. If we don't see the runway by the Missed Approach Point of .5 DME we must go missed approach.
Now take a look at the little "V" next to 1.7 DME on the lower picture. That's called the VDP or Visual Descent Point. This is where we should intersect a visual glidepath to the runway. If we don't see the runway by this point, we probably won't be in a position to land safely. We can go all the way to .5 DME but we'd be almost directly over the runway by then and not in a position to land on it, unless we happen to be flying a helicopter.
There are a couple other types of non-precision approach.
A TACAN approach is the military version of a VOR/DME but not quite as accurate. You'll never see one in the civilian world.
A surveillance approach is flown using ground based radar. The approach controller gives you headings to fly and then tells you when to descend and when you've reached the Missed Approach Point. I haven't flown one of these since I was in the military, so I'm not sure how common they are today.
Most accurate by far is a Localizer approach. We actually haven't talked about a Localizer yet. It uses a very narrow beam, with the transmitter placed at the departure end of the runway. At the approach end of the runway the beam is only as wide as the runway itself. These are obviously more expensive to build, because it requires a transmitter for each runway being served. Some smaller airports may only have a Localizer for the direction most commonly used for landings. Sometimes there is DME associated with the Localizer but not always.
The first precision approach was the PAR (Precision Approach Radar), which is a ground controlled approach. These use two very accurate radars, one for course and the other for glideslope. The PAR controller essentially talks you down to the runway.
You'll hear something like:
"Fly heading 210"
"Begin your descent"
"On course, on glidepath"
"Going slightly left of course, come right heading 212"
"Going slightly below glidepath"
"Correcting to glidepath"
"On course, on glidepath"
These can work exceedingly well but they require a very skilled radar operator. The pilot has no indication in the cockpit other than what the controller is saying. I haven't seen one of these since I was in the military, although they're still widely used by the Navy.
The most common precision approach by far is the Instrument Landing System or ILS. I didn't realize just how long these had been around until I read the wiki article. Apparently they started working on them in 1929 and the first one was in use in 1938!
An ILS uses two transmitters. One is the Localizer, which we've already looked at. In addition, a second antenna for glideslope is located near the approach end of the runway. Now we can accurately position ourselves in 3 dimensions. In addition, either DME or a series of Marker Beacons (Outer, Middle, Inner) can help us determine our distance from the runway.
1 - Localizer
2 - Marker Beacon
3 - Glideslope
This aircraft is on glideslope and slightly left of course
Let's take a look at the ILS for runway 09 at Stewart/Newburgh in upstate New York.
This approach has a decision height of 681 feet, or 200 feet about the runway. That's pretty standard for what's called a "Category 1" ILS. This approach can be flown in visibility as low as 1800 feet or 3/8 of a mile.
The Category 1 is pretty much the "standard" ILS. There are special ILS approaches called "Category 2" or "Category 3" that can be flown in conditions of very low visibility. It varies from company to company, but where I work we can go down to 300 feet visibility - not much at all.
These special approaches are only found at certain airports and require both the aircraft and the crew to be certified to fly them. They normally require an autoland capable aircraft plus a large degree of redundancy in the aircraft systems (multiple autopilots, multiple ILS receivers). Pretty much everything has to be "full up" before you can fly a Cat 2 or Cat 3 approach.
Flying one of these approaches down to minimums is a real eye-opener. In Europe in the winter it's pretty common to have to do these. With a decision height of 50 feet in some cases, you're basically over the runway threshold when you're making the decision to land or not.
Procedures vary from company to company. We normally have the First Officer fly the approach while the Captain is looking out the window. At decision height, if the Captain likes what they see they'll take the aircraft and land (actually the plane is landing itself). If not, the First Officer will press the go-around switch and the autopilot will execute the missed approach.
Here's a video of an ILS approach in low visibility 767 Approach to Santiago Chile
Finally, let's take a look at GPS (RNAV) approaches. These are kind of cool because you don't need any navigational aids at the airport. These are technically non-precision approaches, but the GPS can give you an artificial glideslope (vertical navigation) so you can fly them like a precision approach. In fact, rather than fly an NDB or VOR approach, we'll "overlay" the GPS and use it to fly the same ground track more accurately. I find this takes a lot of the effort out of flying a non-precision approach. No more "dive and drive".
GPS approaches will become more common because they it allows for some new capabilities like curved approaches or approaches that zig-zag around terrain.
The last thing I'll talk about today is what's called a "circling approach". This involves shooting an instrument approach to one runway but actually circling around to land on a different runway. You might see this at an airport that only has an approach to one runway but the winds are not favoring that runway. Once you broke out of the weather you would (keeping the airport in sight) circle to land on the other runway.
I've done these in the military but not in the civilian world (my airline doesn't permit them). It's somewhat of a dangerous maneuver. You're flying a few hundred feet off the ground at a slow speed while trying to stay out of the weather and maneuver around to line up visually on a different runway.
There are some other types of approaches like LDA but they're not that common. I think I've covered most of the bases here.
I'm sure people will want to know what I think happened to Asiana Flight 214. I've been working on this diary for over a week now and suddenly it become very relevant.
I don't know exactly what happened to them. What we do know is that the glideslope was out of service for that runway because of construction. In addition the PAPI (visual glideslope indicator) was also out of service.
It was a long flight but there were 4 pilots on board so fatigue may or may not have been an issue.
The weather was clear and they were cleared for a visual approach. They could have used their GPS to give them vertical guidance but we don't know if they did. They could also have used what we call the "3 to 1 rule" to monitor their glideslope. Basically, at 1 mile from the runway you should be 300 feet above the runway. At 2 miles you should be 600 feet up and so on.
I don't know if they did any of those things. For whatever reason, it seems that they got low and struck the tail just prior to the runway threshold. Note that the "touchdown zone" is the first 3000 feet of runway and the "sweet spot" for an airliner landing is around 1500 feet past the threshold. To hit short of the threshold would be quite low indeed.
That's all we'll know until the crew is interviewed and the data recorders are analyzed.
Until then fly safe.