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Two ILS glide paths

Postby dilbert » Sun Dec 31, 2017 5:35 pm

Yes, there are two, with the second 1500 ft above the normal.

I tested using the F104 (it has both ILS and TACAN, as well as VOR). Also, has a great panel and excellent flight dynamics.
Initial setup was at KCQF (Fairhope), which lies directly beneath the KBFM (Mobile Downtown ILS flight path), 13 miles distant.
Tuned NAV 1 to 108.5 (Mobile Downtown ILS), set direction 320, and TACAN to 75X (KBFM VORTAC).
SET flaps full. With brake set, gave about 3/4 throttle.
Then set air location for 3000ft and 220 Knots.
Once Aloft turned to intersect ILS. At 13 miles, direction indicates, but glide path doesn't, until TACAN registers 9. Maintained 3000 until activation of glide path bar at 9 miles. Flew normal glide path to runway.

Then repeated process, but initially set and maintained altitude at 4500. Glide path bar activated at 9 miles. Flew down glide path, which terminated 1500 ft above threshold, with bar then fluttering up and down.

Found similar situation at KNPA, but it's a little harder to check without the VORTAC.

Suspect same may be present elsewhere, as well.

Not a problem CAVU or with a known elevation above an outer marker. but otherwise dicey in actual instrument conditions.

Happy New Year and Best Regards.
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Re: Two ILS glide paths

Postby AndersG » Mon Jan 01, 2018 1:07 am

Is this not a question of side lobes? A problem that affects real radios since it is not possible to construct a perfect directional antenna and that, IIRC, is simulated in FG too.
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Re: Two ILS glide paths

Postby Thorsten » Mon Jan 01, 2018 6:40 am

Not a problem CAVU or with a known elevation above an outer marker. but otherwise dicey in actual instrument conditions.


Which is the reason procedures published on approach charts are used when flying IFR, so that you always intercept the correct lobe of the signal and don't crash following a spurious side lobe.
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Re: Two ILS glide paths  

Postby dilbert » Mon Jan 01, 2018 1:06 pm

Thanks for the explanation. Flew Pensacola, launching from KJKA. Encountered a lobe at 4500 at Alabama Pass. Also encountered a higher lobe
at 6000. When I flew Navy 50 odd years ago, we commenced at high station with a specific approach plate altitude, were clock timed out to and around procedure turn legs, then descended inbound to a specific low station altitude, before commencing final. Also, many approaches were by ear, employing low frequency range stations instead of VOR. During training, flying the SNB, we also did MDF approaches-you had to hand crank the loop antenna in the overhead to find the null, in order to locate the station; and then fly what today would be a TACAN or ADF approach. Generally, the instructor would give you a single engine at an inconvenient time. Had one who pulled the mixture back with a death grip and conked. the engine, requiring a real single engine approach. Fortunately, the P&W RN 985 would fire right back up with full mixture for wave-off on final, as it was hard enough to wheel land the SNB with both, and not porpoise down the runway.
Thanks again for the clarification, and Best Regards. Dilbert :)
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