Tuesday, March 30, 2010

Closing the Back Course?

When I saw a notice in the latest Aviation Information Circular about replacement of outdated localizers and glide path units with new state-of-the-art equipment, I wasn't too excited. Nav aids get shut down for maintenance all the time and you just use another approach if you happen to need to go there in bad weather. It's rare, in my experience, that the weather is in between the precision and non-precision minima, so I almost browsed right past it, but then I saw this line:

As well, new ILS systems do not generate a useable back-course signal; consequently, localizer back-course procedures will be replaced with area navigation (RNAV) approaches, where applicable.

I did not know that. ILS stands for Instrument Landing System, but that's a really useless abbreviation because there are half a dozen types of instrument landing systems, of which only one is called ILS, a.k.a. precision approach. An ILS provides both lateral and vertical guidance to a runway. It's not as simple as riding a single beam down to the runway threshold, but that's the image that works for me. The part of the transmission that the pilot follows to align with the runway is called the localizer and the part that lets the pilot know if she is too high or too low is called the glideslope. The signals are usable within ten degrees of the runway centreline, but the needle that indicates position on the localizer reaches full deflection 2.5 degrees from the centreline, so to make an acceptable approach, the airplane must remain within about one degree of the centre of the beam. Although the beam is associated with one runway (that is one end of a particular piece of pavement) the signals leak out in the other direction to the opposite runway. If there are no obstacles or terrain that interfere with a safe approach, it may be possible to design a backcourse approach using the same nav aid on the opposite runway. A pilot flying a backcourse approach follows the localizer signals in the opposite direction in every sense. If the front course approach is on a track of 090 to runway 09, then the back course approach will be on a track of 270 to runway 27, and when the instrument signals that the pilot is left of the localizer, the pilot should fly left and when the instruments show the pilot is too far to the right, the pilot should fly right! (There are ways to set some instruments up backwards so you don't always have to use reverse sensing, but that's the traditional way). The glideslope signals may be received on the back course, but they are not usable at all and the pilot should ignore fly up or fly down indications, not do the opposite. Also, you get to fly the airplane pointy end first and right side up.

So this AIC is telling me that all my long ago work learning how to fly with reverse sensing is going out the window because new technology localizers don't have a back course? Cool. I wish I could tell you more, but the link to more information in the bulletin 404s. Someday people will look at the LOC REV switch on old autopilots and wonder what the heck that was for. I wonder what else I'll see fall by the wayside during my aviation career. I hope a lot of those old autopilots get replaced, but I know some of them will never go away, even if they don't ever work again.

12 comments:

Blake said...

I've only been flying for three years, but the following have disappeared in my "career":

- VOTs
- DF Steers
- (and now) LOC BC approaches.

Bob said...

This appears to be the correct
link for the ILS schedule.

Aluwings said...

An old instructor's test: "You're flying the backcourse localizer when suddenly the left engine quits and the aircraft rolls upside down and the LOC needle goes full right. Which rudder do you push? Quick!"

When you mention the difference between non-precision and precision approach limits, it's true that seldom does this make the difference between getting in or not (but when it does, it pays for itself I guess.) The more common dilemma is choosing non-precision into wind with poor touchdown and approach lights and short runway, vs. precision limits with a significant (but legal) crosswind on a long runway with excellent approach and touchdown lights. Sometimes that's a tough call.

At any rate - good riddence to BC LOC approachs!

zb said...

To the outside observer that I am, it sounds like the LOC BC was only good for test questions and was hardly ever used in reality. Is that true?

My reasoning is that if it was used more often, you wouldn't sound as cheerful about its disappearance.

Echojuliet said...

I never saw too many LOC BC approaches until I started flying in Alaska, where they and NDBs are still the norm. It will be interesting to see what happens. Any ideas on how long it will take for these to be put in?

Jimmy said...

A former co-worker of mine used 'shooting the back course' as a euphemism for something best not shared here lol. My mind is forever tainted by that guy.

Still a BC at a couple of airports near me. Used one as a common departure running freight. Arrive on the ILS and depart on the BC. Always fun to try and climb out at a perfect three degrees. I wonder how long they will last...

Sarah said...

zb - the back course is not popular ( no, not that one Jimmy ) because it's supposed to be harder to fly. Being the opposite direction to the ILS to a given runway, there generally is no glide-slope, just the reverse-indicating localizer. Worse, the localizer antenna will be on the nearer approach end of the BC runway, so the indications will be that much more sensitive. And it's generally more squirrelly and short-range because the localizer is just "accidental" radiation from the front course ILS.

Aviatrix said...

It's a common metaphor, Jimmy. I had to play with the blog entry title to compensate.

Paul said...

Blake,

Add LORAN-C and NDB's to the list of the disappeared.

How many pilots know that the shading of the colors on the very old VOR instruments were there to help you run the back-course?

--paul

A Squared said...

the following have disappeared in my "career":

- VOTs


I just tuned 2 different VOT frequencies today. they don't seem to have disappeared.

Jimmy said...

Please accept my sincerest apologies ladies...

The embarrassment is twofold. The fact that I felt the need to share that information is primary. Secondary is the fact I thought I was dealing with a singularly disturbed individual in relating the anecdote. If I had known it was a common metaphor I would have exercised better judgment.

Then again if I had good judgment to start with I probably would not be flying for a living ;P

I'll shut up now...

Aviatrix said...

Jimmy, it's okay, really. No offence taken. You don't fly very long without meeting one of those. I shared company housing with one. He would never take off his muddy boots in the house, either. But he wasn't evil, just had never learned to think about others' feelings and had incredibly poor impulse control.