Avoiding False Glideslope

6 Answers

1. 
   

   
   

   John D Collins on Aug 10, 2022

   The GS is not to be used outside of the IF as any step downs are based on MSL altitude and on a very hot day on some ILS systems, the GS may be below the step down minimum. The GS is fixed in space, but MSL indications move up/down with temperature. On hot days, the altimeter indicates lower than the true altitude and on cold days it indicates above the true altitude. The standard service volume for the GS is 10 NM and the altitude on the intermediate leg that ends at the PFAF is typically close to 1600 feet above the runway and about 5 to 6 NM from the threshold. A typical IF minimum altitude will be 500 feet higher, so under nominal conditions, if the GS crosses the PFAF at 1600 AGL, it will cross the intermediate leg at 2100 AGL about 1.7 NM further out from the PFAF. So crossing the IF at 2100 AGL and remaining at that altitude until you intercept the GS will put you at 6.7 NM from the threshold, well inside the 10 NM GS SV.

   Intercepting the GS at a higher altitude outside of the IF on a step down is not authorized, mainly because of the effects of temperature can put the MSL altitude below the minimum altitude for the segment and it is likely to be outside the GS SV. So when there are multiple step downs prior to the IF, one should not use the GS, but abide by the MSL minimum altitudes until crossing the IF. However, once beyond the IF and when the PFAF is the next fix, it is reasonable to not make the last step down to the GS intercept altitude, because you will intercept the GS inside the IF about 1.7 NM from the PFAF and can follow the GS from that point. In some aircraft, this makes the power and configuration management easier. In my Bonanza, I just lower the gear when I intercept the GS and don’t need to make any power changes. If instead, after crossing the IF, I did the last step down, it would require at least one or two power/configuration changes to intercept the GS.

   See the UZA ILS Y Rwy 2 as an example of an approach. The IF is CONEL with a minimum crossing altitude of 3000 feet. AZAKA is the PFAF with the GS intercept altitude of 2500 MSL. In these circumstances, I would cross CONEL at 3000 with my power set to 16 inches and remain at that altitude until intercepting the GS outside of AZAKA by roughly 1.7 NM, lower my gear, and track the GS and Localizer to the DA. I would make no power changes. I would intercept the GS well inside the standard GS SV.

   False GS are 2 to 3 times the angle (6 degrees and 9 degrees), and the first false GS is reverse indicating, so if you are crossing the IF at the minimum charted altitude, there is no risk of encountering a false GS. Of course with an LPV, there are no false GS to deal with.

   This is not a complete discussion of the topic, but in summary, you can’t use the GS outside of the IF for stepdowns, but can intercept at a higher altitude than the minimum altitude on the “IF to PFAF” intermediate segment.

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2. 
   

   
   

   htf17 on Aug 10, 2022

   For an approach such as the ILS 04R at KBOS, there are multiple step down fixes before the PFAF and the IF is not the fix before the PFAF. Therefore, do you have to use 1700 as the altitude to intercept the glideslope?

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3. 
   

   
   

   Mark Kolber on Aug 11, 2022

   No. if you intercepted the GS way out at GOSHI in this approach, you could follow it from there, so long as you monitored to ensure you crossed the stepdowns along the way.

   You can do the math but my quick guestimate glancing at the chart indicates you are below the GS at GOSHI and would keep you above the stepdowns.

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4. 
   

   
   

   Russ Roslewski on Aug 11, 2022

   In addition to the good answers provided by John and Mark:

   By the design criteria, the IF and any fixes between it and the FAF must be below the glidepath. Any exceptions to this would require a waiver during the development stage. So it does happen, but is pretty rare. However, this does not account for high-temperature induced altimeter errors, so you still need to make sure you don’t go below any published altitudes, even on glideslope, on a hot day. LAX was a famous example of this several years ago, with multiple airliners getting busting minimum altitudes and getting violations (made worse since there could have been crossing traffic underneath).

   On the BOS example, GOSHI is an IAF, not an IF. The Initial segment from the (IAF to the IF) does NOT have the same limitation that the fixes have to be below glideslope. So an IAF could very well be above glideslope.

   In this case it’s not, although due to the distances involved it’s not far off. A quick calculation gives me a glideslope altitude of about 6470 at GOSHI. So it’s possible on a hot day that you could be above glideslope at GOSHI already.

   Also note that GOSHI is about 22 nm from the runway, and as previous stated the standard service volume on a glideslope is 10 nm. Now, this is at BOS, and the glideslope is probably still receivable at 22 nm, but there is no guarantee about that or lack of interference from other radio sources. The only guarantee is that it’s receivable at 10 nm OR at glideslope intercept if it happens to be further than 10 nm.

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5. 
   

   
   

   Russ Roslewski on Aug 11, 2022

   Oh, another thing. As John mentioned, false glideslopes are at multiples of the real glideslope. So while they exist, they’re really pretty high. To be on a false glideslope, you’d have to be at roughly double the height you should be. For the BOS approach this means at MILLT (the FAF), you’d have to be at about 3400 instead of 1700 to be on a false glideslope. With modern avionics, if you’re that high that close in you have already seriously lost situational awareness (or are on a really bad vector) and need to be going missed anyway.

   If you’re trying to intercept the glideslope around GOSHI, you’d have to be at almost 13,000 to be on a false glideslope.

   So if you’re anywhere near the altitudes published for the IF or in the intermediate segment, you shouldn’t get a false glideslope.

   I’m not saying it doesn’t happen, but with modern avionics you should be getting a lot of clues for a while before it’s a problem. I imagine false glideslopes were more of a real concern back in the days 1) before radar vectoring most places, and 2) when we didn’t usually know exactly where we were (no distance information or moving maps, etc).

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6. 
   

   
   

   John D Collins on Aug 11, 2022

   I would agree that a GS, if it was adequate to provide an acceptable signal, could be used from GOSHI, but at 20.2 NM, it is double the distance for the SV of a standard GS. Because of the minimum altitudes for the stepdown fixes and the nominal height of the GS over them, it is not likely that a high temperature would put you below the minimums at the step downs if you did follow a GS. Regardless, as Mark pointed out, one must cross these fixes at or above the minimum altitude regardless of what the GS indicates.

   You are also correct that the IF is not the fix before the PFAF on this approach, the stepdown fix NABBO is the fix that connects to the PFAF. Nominally the GS could be intercepted a little over 1 NM past NABBO at 3000 MSL and inside the standard GS SV or you could descend to 1700 MSL and intercept the GS at the PFAF.

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