Real Answers from Real Pilots

Backcrouse ILS Approach

Can someone enlighten me more on Backcourse ILS approaches. I’ve come across it a few times in the instrument written prep but I can’t seem to grasp the concept. I also get that they are very rare and aren’t many of them around.

Tucker,

You’re right there are fewer and fewer ILS BCs around but you should be familiar with the concept and of course you never know. Anyway here’s the quick and dirty. Many runways will have an ILS associated with them and often the same piece of pavement will have 2 separate ILS transmitters going in opposite directions (ie, ILS RWY 9 and ILS RWY 27, same piece of pavement, 2 ILSs). HOWEVER, back in the day ILS equipment was expensive to install, maintain etc. and the FAA knew that 1 ILS was capable of sending it’s signal out in the opposite direction and under certain conditions 1 transmitter could be used to service 2 rwys (1 pavement). The only problem was because an ILS transmits on 2 “frequency lobes” (1 on the left, 1 on the right). when the signal goes out the other way (the backcourse) the lobes are reversed. What that means is your ILS indicator will respond backwards when it comes to deflection (aka REVERSE SENSING, ie. If you’re right of course it’ll show left and visa versa). Now a common “technique” (and this is what messes up most people) is if you dial in the FRONT COURSE heading it will not reverse sense (flying the ILS BC RWY 9, if you set your HSI for 270 it will show correct deflection). Make sense? Or did I make it worse?

Adam

No that makes more sense. The biggest confusion was one specific question that asks which ILS equipment is showing where plane x is located and the correct answer was exactly like you said, it was backwards. That clears it up a bit. Do you have experiance actually flying back course approaches?

I do but it’s been a while and I can probably count on one hand the times I’ve done them. Nowadays most non-precision approaches have GPS overlays so we just fly that. MUCH easier.

Adam

Tucker,

I have flown very few backcourse approaches, probably less than ten in my entire career.

Chris

So know it and save that knowledge for a rainy day, sound about right? I feel like I was just overthinking it, the actual concept makes more sense than what I was piecing together in my head.

Yes Tucker,

Know it and save it. Regionals these days fly everywhere, major airports and some pretty small ones. Also there’s always the chance you get a sadistic DPE who wants to see what you got! Never know when you might have to bust out that BC knowledge.

Adam

The name just sounds cool, I like it. And I will make sure I do that!

Thank you guys!