Loss of Glide Slope on ILS approach.

13 Answers

1. 
   

   
   

   Gary Moore on Mar 13, 2012

   You aren’t REQUIRED to fly the LOC – you could always execute the missed approach.

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2. 
   

   
   

   Brian on Mar 13, 2012

   This is why answer #2 was …”So you have the option…”
    I realize that it isn’t required. But that doesn’t help me with my question. 🙁

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3. 
   

   
   

   John D. Collins on Mar 13, 2012

   Some instructors (not including myself) have suggested that timing for an ILS is so that if the GS fails during the approach that the pilot has the option of continuing the approach to the localizer minimums. If the ILS is an ILS LOC/DME approach, a timing chart will not be provided as the pilot determines the localizer MAP by using the DME distance.  If the MAP can only be determined by timing, then, obviously one would need to know the relative time passing the FAF, the groundspeed, and the expected time. Depending on the approach, there may be a turn after the MAP.  You can’t start the turn until after reaching the MAP, as the obstacle protection is premised on starting the turn after the MAP.  For me, I don’t brief the approach for both a GS approach and a lost GS with a Localizer MDA. Once I start on the approach, I am not going to switch to a Localizer only variation and will miss the approach, re-brief for the Localizer only alternative and then shoot that approach.  However, I still note the time at the FAF so that I can determine the MAP location if the GS fails during the approach. As long as the GS is available, I don’t need the time to determine the MAP as it is when I reach the DA on the GS. I have seen instructors offer contrary opinions, but that is all they are, opinion. My opinion is that once I have started the approach and something as significant as a navigation signal fails, I am going to miss the approach, as I believe that there is a significant possibility that the high workload and confusion that can set in is not conducive to trying to salvage the approach. Also, it is not part of the Instrument PTS precision approach task.

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4. 
   

   
   

   Kris Kortokrax on Mar 13, 2012

   The answers to your questions only apply if the glideslope fails before reaching the MDA.  What are the answers if the GS fails below LOC MDA?
    
   I’m always intrigued by this scenario.  If I were in charge of procurement for the FAA, I would have the company that manufactures the localizer transmitter make the glideslope transmitter (the localizer transmitter never fails, or at least nobody ever considers this possibility).
    
   I’m with John.  If anything goes wrong during the ILS, execute the miss, regroup and pick a viable alternative.
    
   I’m sure there are pilots who are still flying with only a CDI and clock for guidance on the ILS, however in this day and age, I would suspect that a vast majority are flying with either an installed or handheld GPS.  I would trust the depiction on the GPS to identify the MAP over the ability to correctly guess my groundspeed and interpolate (in my head while concentrating on flying) using the time tables on the approach plate to determine the location of the MAP.
    
   To answer your question, I see nothing in the regs, AIM, Instrument Flying Handbook or PTS that either suggests or requires timing on a precision approach.

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5. 
   

   
   

   Bill Trussell on Mar 14, 2012

   After looking through all the documentation over the last couple of days I have come to the conclusion that the FAA has outlined normal instrument operations for the ILS approach but has left out abnormal operations and as a result has effectively excluded continuing the ILS approach as a LOC only.
   I teach and practice the concept that any flag or warning light during an approach is not a good thing and should result in a very careful execution from that point on, including strong consideration for a missed approach.  A good example of this is a gear warning light.  You would never consider continuing an approach with a gear warning light, rather you would execute a missed approach and proceed to a location where the problem could be diagnosed.
   Another teaching point is that when granted approach clearance you would receive “cleared for ILS xx” and technically would need a revision of the clearance to “cleared for LOC xx” before executing it.  This would be too much work that low to the ground on the first pass.

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6. 
   

   
   

   Nathan Parker on Mar 14, 2012

   “Does anyone have a reference, preferably regulatory or government advisory (such as the AIM), where these answers can be found?”
    
   There is no such reference.  I’m with the others, I think that switching to a LOC if the GS failed is poor human factors.  You probably haven’t briefed the LOC approach and trying to identify new minimum altitudes and fixes while on final is a potentially hazardous activity.  Go missed.

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7. 
   

   
   

   John D. Collins on Mar 14, 2012

   Bill,

    

   I would interpret a clearance for the ILS the same as a clearance for the localizer only alternative of the procedure if it exists.  ATC doesn’t know if your aircraft is equipped with a GS receiver or not and would issue the same clearance.

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8. 
   

   
   

   Bill Trussell on Mar 15, 2012

   John,
   I would have to very respectfully disagree with your assumption that the clearances would be the same.  In some cases there are step down fixes that would show to ATC that you were below the GP for the ILS but would be on target for a LOC approach.  I would contend that ATC would want to know and understand what approach you would be executing.  Extending the logic a step further, one would never assume that ATC would be ok with using a VOR approach that overlays with the LOC/ ILS as a backup without being recleared, even though they were along the same path as the ILS?
   Perhaps it would be a good guess that you would not be denied a reclearance for whatever you needed under the conditions outlined but that would add to the workload that you would be trying to avoid in this instance.
   In the case of an equipment limitation on the original clearance one would assume that this would be communicated with ATC upon the initial approach request that that the appropriate clearance would be granted initially.

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9. 
   

   
   

   John D. Collins on Mar 15, 2012

   Bill,

    

   If the approach procedure is a combined ILS OR LOC, you will be cleared for the procedure using terminology such as Cleared ILS runway 2.  This assumes that both the full ILS and the Localizer only option are using the same procedure outside of the FAF, the same FAF, following the same lateral localizer course and the same missed approach procedure.  What is different is that with the full ILS you descend to a DA following the glidepath, whereas with the Localizer, you descend to the MDA and the MAP is defined by a fix or by time. In other words, in every respect you fly the same procedure at the same altitudes excepting the final approach segment. You determine the missed approach point differently, but follow the same missed approach procedure.  If ATC was required to know whether the pilot is flying the full ILS or the Localizer alternative, the controller manual would require a different clearance for each and would have specific guidance on the subject in their manual 7110.65U, but such guidance does not exist. Neither is there any guidance I can find in the AIM for the pilot to declare that they are doing the localizer approach 

    

   A low altitude alert would occur regardless of the type of procedure being used.  The MSAW alert altitude is based on 100 feet below the lowest non-precision MDA and covers the distance between the FAF and 1 NM to the threshold. If an aircraft is at or is projected to be below the MSAW altitude for two consecutive sweeps within the next 15 seconds, the alert is generated. It might occur more frequently for an aircraft performing a localizer approach than the full ILS as the pilot may chose a higher descent rate for the localizer, but regardless if the pilot had announced that they were doing a localizer approach, the alert would get generated and the controller would be obligated to forward it to the pilot. 

    

   It doesn’t hurt to inform the controller that you don’t have glideslope capability, but it is not required.  In the same way, you don’t need to tell the controller when you are doing a RNAV approach that has a LPV, LNAV/VNAV, and a LNAV option, which one you have chosen or which one your aircraft is capable of performing. Your clearance will be the same, cleared RNAV runway 2 approach.

    

   Regards,

    

   John Collins

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10. 
    

    
    

    Brian on Mar 15, 2012

    Thank you all for the replies. Bill, upon rereading my question and your first reply I can see how you may have come to your interpretation. That said, I’m sorry for impromptly snapping without first thinking!
     
    To the topic, I was taught (the question posted here is from my college’s IFR quiz), and believe it to be perfectly safe, to brief an ILS to include the localizer MDA and time when conditions warrent it. I adjust my airspeed to to avoid the need for interpolation and subsequently note the corresponding time. Upon discussing this with our most senior instructor, a 25,000 hour ex military/airline captain/check pilot with experience in over 100 aircraft, I find us in agreement. Of course this doesn’t mean that it’s law, just a differing preference, but with an experienced pilot’s (I’m not, nor am I a II or even instrument current) comendation. Followed with a scenario:
     
    The Scenario: You’re cleared for the ILS into your local airport, weather is 700 broken and your ILS takes you to 200, your LOC only takes you to 400. You’ve got VIP on board who will likely be late to their meeting if you go missed. (Note: Of course this shouldn’t make you risk safety, but it should make you maximize utility). At 800′ your GS craps out.
     
    Questions: What is the harm in descending to, assuming you’ve diligently briefed the LOC only MDA and have your timer running, the LOC approach MDA and continueing till your time is up before going missed?
     
    My thoughts: You’ll need to continue the approach till your MAP before going missed regardless of what altitude you fly to it at. You know the weather to be above LOC only minimums. You know that same approach gives you safety to MDA with LOC only. And you’ve briefed all this when you briefed your ILS.
     
    I guess my point is, on and ILS approach you should have time running to identify the missed as an absolute backup regardless. And in the business of aviation, utility should always be considered a top priority while maintaining safety. I don’t find it one bit unsafe to brief a second minimum altitude and fly to it before going missed. A GS failure isn’t an emergency for the prepared pilot, it’s a minor inconvinience.
     
    **PS Keep in mind my standpoint is from one of a passenger based operation with well practiced/prepared pilots. Things may certainly be different for the average joe taking a trip a month to his/her weekend vacation spot.

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11. 
    

    
    

    n on Mar 15, 2012

     I always time a ILS, I figure chit happens (not only possible with the ground station, but I’ve also had a needle stick in my VOR head), sometimes you are not shooting an approach for training or fun, there are times you need to get down and are not looking to burn time, this gives me yet another OUT, I can go missed OR fly the LOC.
     The more outs/options I have the more likely my plane will be undamaged, my mission will be completed, my schedule intact and my arse will be in one piece.
     
     

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12. 
    

    
    

    tsehnoutka on Apr 09, 2022

    How is a glide-slope failure indicated in the aircraft? If I remember correctly in steam gauges you get a flag and full needle deflection. Doesn’t the procedure say if you get full needle deflection, you initiate a go around ? So isn’t that what we should be doing here ?

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13. 
    

    
    

    John D Collins on Apr 09, 2022

    tsehnoutka,

    Different GS indicators have different means of indicating a GS failure. For example some CDI with a VDI will display a red flag such as a KI209, while others such as a KCS55A will remove the VDI indication, but don’t display a flag. If you get an indication of a GS failure, then you can continue the approach if you are above the MDA using the localizer option if it is available, otherwise you are expected to initiate the missed approach.

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