ILS Approach – Altitude between Intermediate Fix and the glideslope intersection point

4 Answers

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   Mark Kolber on Jan 20, 2019

   The problem with the ILS glideslope is false glideslope. The false indications are at multiples, 6, 9, 12, and 15 degrees in the case of the typical 3 degree glideslope. That’s the reason for the standard technique to go down to the charted altitude and avoid interception from above.

   In your example, it’s probably fine to wait. We’re talking about 200 feet and you are some distance away, so the glideslope you intercept is probably the right one.

   I said “probably” for a reason – It can be calculated to know for sure, but I’m not going to do the cockpit math to figure to figure it out when it much simpler to follow a uniform SOP to go down.

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2. Best Answer
   
   

   
   

   John D Collins on Jan 20, 2019

   The altitude shown between the IF and FAF (the intermediate leg) is a minimum altitude and once established on the localizer and cleared for the approach, you may descend to the minimum. In my aircraft, I don’t usually descend to the minimum altitude when the descent requires 500 feet or less and prefer to intercept the glidepath a little further out than one would if descending to the IF leg minimum. It requires fewer configuration changes. But this is a pilot decision.

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   KDS on Jan 21, 2019

   Although I’ve heard about false glideslopes since way back when, I’ve never encountered one. But Mark’s answer prompted me to see if I could find more information from such an event and I came across this article from Boeing:

   http://www.boeing.com/commercial/aeromagazine/aero_21/glidescope_story.html

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4. 
   

   
   

   Warren Webb Jr on Jan 21, 2019

   KDS – Thanks so much for the link – very interesting. During vectoring I’ve seen the needles move erratically many times, but when given the approach clearance fairly close to the localizer and 3-4 out from the FAF, the needles have always been steady from there in. In Charles’ situation of intercepting further out, I think the possibility of a false GS signal is realistic. It’s surprising the crew descended as far as they did. An immediate descent upon selecting approach mode is unusual. Normally there’s verification of approach mode and glide slope arm mode, a stable track on the localizer, the glide slope needle high and descending at a normal rate, then glide slope capture and change to GS mode (from ARM mode) with descent starting, a time-frame that was completely missing but didn’t raise too much suspicion. I can’t imagine really not having some sort of cross-check at the FAF – could be DME, a crossing radial, or even a request to ATC to verify crossing the FAF. I agree with John generally about the altitudes and pilot preference, but I wouldn’t go below in this case 1800′ without a cross-check.

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