Alasdair Campbell wrote: > I wonder if I have rediscovered an old problem, or is there really a > difficulty in flying an ILS approach where a runway has locators at > each end which share the same frequency? > > Here is a test scenario, a 10 minute flight from my local airport, > Glasgow international(EGPF) to Edinburgh (EGPH). Set up your flight so: > fgfs --aircraft=c172p --airport=egpf --runway=05 --nav1=63:108.90 > --nav2=100:115.40 > You will need the UK scenery (w010n50) to duplicate this flight. > > Fly outbound radial on nav2 (GOW), until you intercept the the EGPH > runway 06 ILS on nav1, about 10 nm south of the airport. Track the ILS > on nav1 down to the runway. Never mind the beautiful bridges to your > left; just keep those needles centred. I find that as I approach the > runway threshhold, my nav1 radio goes berserk. The autopilot doesn't > stand a chance. It will dump you on the ground somewhere between > Dundee and the Normandy beaches. > > If I temporalily remove the southern locator from nav.dat.gz > (4 55.94144200 -003.39767200 110 10890 18 238.785 ITH EGPH > 24 ILS-cat-I), I can approach and fly right down the centre line. > > My question for you avionics gurus is: How do real nav radios deal > with directional radio beams as the aircraft approaches (and more > importantly) passes the closest locator at such airports? > I do not have the UK scenery. But I have flown into at least 2 US airports in the real pa24-250 that had similar setups. In general, I do not expect to be able to trust a LOC when I am actually over the runway. In the situation you describe, the role of a conventional LOC needle reverses when you acquire the new LOC so that if you are slightly to the right of the runway at the approach end, the needle will be to the left. As you acquire the other LOC for the reciprocal runway, it will reverse indicate as you are now out bound on the new LOC, so the needle will now be deflected to the right indicating you need to move to the left to center the needle.
Usually at the middle marker, I disengage the autopilot, even if I am going to do a missed approach. It is especially important to do this in the situation you describe. Or you could go to HDG mode until you are set up to switch to LOC REV mode as you fly the 2nd LOC outbound. In reality, approaching the threshold of the runway is not the dangerous trap in this situation. The confusion can occur in the transition from an airway to the LOC as you begin the approach. You see the LOC and GS come alive and "assume" it is the LOC you are expecting. But depending on where you are from the field, the LOC GS may be tracking the "other" ILS. IFR training teaches that you always check the code before you trust the needles. Even though both ILS have the same frequency, they do have different codes. Regards, Dave Perry ------------------------------------------------------------------------- SF.Net email is sponsored by: Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://ad.doubleclick.net/clk;164216239;13503038;w?http://sf.net/marketplace _______________________________________________ Flightgear-users mailing list Flightgear-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-users
