your vertical command bar will stay centered
Surely any HSI or EHSI would not display the GS bar unless an apparently valid GS is being received? My SN3500 suppresses the GS in such a case.
And if your VS happens to be set so it just happens to follow the 6deg or the 9deg GS then yes you will see the GS bar, centred, but you should still end up at the same MAP (albeit a lot faster).
Surely any HSI or EHSI would not display the GS bar unless an apparently valid GS is being received?
Ours (Honeywell Primus 1000) displays an identical GS bar (both in raw data and flight director mode and both in the EHSI and EADI) for VNAV and ILS mode. So if you descend in VNAV mode through the ILS, it will not capture the glideslope and you can’t tell from the display if it has or not. You must instead look at a little mode annunciator on top of the screen to actually see what is displayed. This is very easily overlooked, especially by inexperienced pilots!
I should know this but I have a feeling my KFC225 doesn’t intercept from above, and is looking for the bottom GS sidelobe first.
Pretty much all of the Bendix King autopilots will intercept from above or below. The algorithm is that the GS must transition thru center, high to low or low to high. Of course if you are in altitude hold mode on the normal GS, this will only occur on the 3 degree slope from below. To couple from above, you need to be in VS mode and at a greater rate than that required to hold the GS so it will intercept it. It can be a fast ride.
The DFC90 captured GS from above just fine and it’s actually described in the manual
The cross check of the altimeter with on GS indication is a good practice, primarily to verify the altimeter setting is correct.
That’s one one point and the other one is that it prevents a pilot from following a stuck GP needle into the ground (google knows everything about Alitalia flight 404)
I think my second observation covered that:
It can also point out a badly operating GS indication, more likely to be caused by own ship equipment
GPS substitution for DME is primarily a GA issue as the air carriers either have the mandated equipment or are permitted by their Opspecs. For many GA aircraft, particularly of N registry, DME or ADF are not installed. This is true both of older aircraft and newer ones that have G1000 systems installed. Modern GPS units such as the GNS, GTN, and G1000 systems have all the required DME locations in the database and if they don’t then they may need a DME to accomplish these procedures safely (I am not dealing with legality outside of the US). When the GPS has the ILS procedure in the database, this is an alternative way to determine DME fix locations using available ATD indications, but it requires some pre-planning to do the math and have that information available at the time of the approach.
