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Is it recommended to intercept the ILS glideslope from above? Are there any special considerations like unstable approaches or GPWS warnings?

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Assuming that you're asking about intercepting the glideslope from above rather than the localizer, the answer is that it is definitely NOT recommended.

There are at least two significant problems if you do this. First, is the fact that at high vertical angles there can be false glideslopes. Looking at the diagram below, if you're coming at the glideslope from above and the needle starts to move, are you intercepting the real 3° glideslope or the false 9 ° glideslope?

False glideslope image

The diagram above is the best I could find, but it has one error. It shows a "Flag no signal" at 6°. Actually what you'll have there is another false glideslope, but one which has reverse sensing.

You can, of course, do a reasonability check if you know your distance from the runway, or you could compare your vertical speed if following it with what you know your vertical speed should be, or on the 6° line a command to go up when you know you should go down would be a hint, but it's unwise in my opinion to put yourself in a situation that increases your work load that much during an instrument approach.

You asked about GPWS warnings. None of the aircraft I flew that had GPWS would sound a GPWS warning just from receiving a false glide slope. However, all of them would have sounded the GPWS if you followed a false glideslope and were close to the ground because doing so would give you an excessive sink rate.

A second problem with approaching from above is that the vertical angular width of the glideslope as measured from needle full down to needle full up is very small. As I remember only about one degree. Whatever it is, though, the point is that if you descend into the glideslope with a vertical speed of only a few hundred feet per minute greater than the glideslope itself is descending, the needle is going to go from full down to full up very quickly. Without working it out, I'd guess that if you're inside the outer marker you might well miss its movement if your instrument scan had you away from the needle more than a second.

In the late 1970s a friend of mine was flying a charter in a Cessna 310. He unwisely decided to intercept the glideslope at Salem, Oregon from above. The aircraft hit the ground in the vicinity of the outer marker, killing all aboard. During the accident investigation, the NTSB found from radar data that he had a descent rate greater than 1,000 fpm going through the glideslope.

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  • $\begingroup$ What causes false gbdislopes? $\endgroup$ – TomMcW Aug 24 '15 at 23:28
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    $\begingroup$ @TomMcW I don't know. However, your comment induced me to include a false glideslope diagram in my answer. Let me encourage you to ask that as a proper question. I'm sure there are others here who can answer that if it hasn't already been answered, in which case when you start to ask the question the site may show you that the question has already been asked and answered. $\endgroup$ – Terry Aug 25 '15 at 1:51
  • $\begingroup$ That question is asked here but doesnt get a real in-depth answer as to the causes. I can't find a good explanation as to the causes on the net anywhere. BTW sorry to hear about your friend. $\endgroup$ – TomMcW Aug 25 '15 at 2:44
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    $\begingroup$ Approaching the glideslope from above was also cited as a contributing factor to the crash of Turkish Airlines Flight 1951 in Amsterdam. $\endgroup$ – TomMcW Aug 25 '15 at 2:55
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    $\begingroup$ @JanHudec I agree, the diagram is incorrect in not showing the reversed false glideslope at 6°. It also shows the angles as wildly exaggerated, but that's okay I think. I'll look for a better diagram or at least put a note on the existing diagram. And I think a separate question is a good idea. $\endgroup$ – Terry Aug 25 '15 at 20:56
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If you mean the glide slope (vertical guidance) rather than the localiser (horizontal guidance), then you should capture from below.

The reason is because if you capture from above, you must be descending and there is a risk, particularly if you capture the glide slope late, that you will descend below it. This is a Bad Thing™.

The approach, if followed, or ATC vectoring will bring you onto the localizer (horizontal guidance) at an angle, typically 30 degrees or so. You capture the localizer signal first, then then fly at a fixed altitude until you capture the glide slope, at which point you start descending.

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Another important factor in intercepting a glideslope from above is speed. This is particularly relevant for faster jet aircraft. Jets will typically be able to descend or slow down, but will have a hard time doing both at the same time.

If the jet needs to intercept the glideslope from above before slowing down enough to extend flaps and landing gear, the spoilers may not provide enough drag to slow down much. If the aircraft does manage to intercept the correct glideslope, the aircraft may be traveling faster than usual while still descending on the glideslope. Going too fast at this point can qualify as an unstable approach, and increases the risk of a late touchdown and a runway overrun due to the excess speed.

If the aircraft is intercepting from above with flaps and gear extended, and engines at idle, the pilots should take care that when the glideslope is intercepted or a go-around is performed, the aircraft is not pitched up too far before the engines are allowed to develop sufficient power to overcome the high drag of this configuration.

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There's never a reason to do this. Proper Planning. If too high at the FAF request vectors back around and set it up correctly.

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    $\begingroup$ It isn't common, but there ARE real-world situations where it is necessary to do exactly this. A bad vector from ATC, you're 12 miles out & 1 dot high, "can you make it from there?" Yes, absolutely you can in a case like that, and getting vectors is a waste of everyone's time. It's a reasonable tool to have in your bag of tricks. $\endgroup$ – Ralph J May 18 at 16:53

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