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Glide slope intercept altitude

Asked by: 8110 views FAA Regulations, Instrument Rating

In my IR training, I was taught two different ways to shoot a precision approach. One was to follow all the step-down fixes and then intercept the glide slope at the altitude marked with the light bolt symbol in the chart, and the other was to stay at the last assigned altitude for the approach clearance (when getting vectors) or the altitude you descend to for the procedure turn (when shooting full procedure) until intercepting the glide slope.

Instructors who prefer the second way argue that it's the better way because you can stabilize the approach easier and earlier. I agree with them on that point, but I'm not sure it's really the "right" way to shoot the approach, because (1) all precision approach charts specifically mark the intercept altitude with a clear symbol and I think there's a good reason for that; (2) there's a possibility you can intercept the glide slope from above and get a false signal from the system. This is not a problem when I shoot the approach in a familiar airport because I know I'll intercept the glide slope from below at those altitudes, yet if you have to shoot an approach you've never practiced before, how can you be assured that the current altitude you're maintaining will allow you to intercept the glide slope from below? 

So my questions are:

(1) Is the second way to shoot a precision approach officially approved by the FAA or ICAO?

(2) Which way is usually preferred among the airline pilots in the US?

3 Answers

  1. Best Answer


    Dave M on Jun 22, 2017

    Either way is fine. Unless told otherwise, step down altitudes are “at or above”. There is nothing saying you have to step down.

    As you say, you must be careful not intercept the glide slope from above, but ATC is usually pretty good at having you intercept it from below. They should clear you to final when you are at an altitude that will put you below the glide slope when you intercept the localizer.

    As for question 2: I would guess that most airline pilots prefer the second method because it’s less work.

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  2. John D Collins on Jun 23, 2017

    The step down altitudes are minimums and are not necessarily below the GS, so flying the GS can put you below the minimums. This has to do with the fact that step down altitudes are MSL and display on the altimeter, so on a hot day the MSL altitude can be substantially higher and move to be above the GS which is fixed in space. There is no issue under these circumstances with terrain, but there is an issue with vertical separation standards, for example an ILS overflies or is near another airport and traffic at that airport is assigned altitudes below the step down.

    There were many airports in the US, where this would occur regularly and pilots would be violated for following the GS and being below step down fix minimums. LAX was one such airport. The step downs were moved to provide more distance and in effect temperature compensate them. Regardless, the GS is not to be used to clear step down fixes outside of the segment with the GS intercept, as it is the step down altitudes which must be adhered to. If the pilot adheres to the step down fix altitude minimums and the GS does not violate the minimums, then no harm is done, but it is not good piloting form in my opinion. Modern FMS systems provide vertical navigation and have no issue with performing the step downs automatically. If you are hand flying, you are expected to descend to the step down minimums in the appropriate segment, even though they are minimums and higher altitudes are not prohibited.

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  3. Wes Beard on Jul 13, 2017

    John is correct about the glide slope protruding the step down altitudes on warm days. See InFO 11009 for more info. https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/info/all_infos/media/2011/InFO11009.pdf

    The glide slope intercept altitude is also the precision final approach fix. Basically weather has to be above minimums till passing that point. If ATC gives you a lower altitude, the PFAF move to where the glide slope intercepts that altitude.

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