Loss of ALT on ILS Approach and Other ILS Problems

[pdmike]

This is a constantly recurring problem for me, which I have yet been unable to solve.

I am on approach for an ILS landing in the 737. I am 35 miles or so out, at 8000 ft, on runway heading, with the ILS frequency for the runway tuned to Nav 1. I am on AP with HDG, SPD and ALT activated.

At 27 miles out, the localizer activates. I hit NAV on the AP and the plane begins to seek (and finds) the localizer. At 23 miles out, I begin a descent to 1200 ft AGL. Early in the descent, the glide slope indicator (GSI) activates. When it does, I hit APP on the AP.

As I continue the descent to 1200 feet, all of a sudden and for no apparent reason, the ALT hold on AP deactivates. The ALT light just goes out and the plane begins to immediately lose altitude from its descent path. I have to click on ALT to reactivate it and get the plane back, on the proper descent path.

That's the first part of the problem. The second part takes place when the plane reaches the glide slope. I will have been level at 1200 ft. for a short period of time. The GSI karat begins to come down as the plane approaches the glide slope. I am "flying into" the glide slope from underneath, as I am supposed to be doing.

When the karat reaches the midpoint on the GSI, the plane is supposed to begin down the glide slope - only it doesn't. It just keeps flying level. If I want to land it, I have to switch of AP and do it manually.

That's not my idea of an ILS/GS landing.

And, to further complicate matters, it doesn't happen all the time. I can have it happen, reset the flight, do it again, and it works fine. Then it won't. Then it will. GRRRRRRR!!!!!

When I tried to discuss this problem before, all I got was a huge chorus of "I don't have that problem - what are you talking about?" Well, I have this problem, and it's driving me nuts.

If you don't have this problem, don't respond to this thread - unless you used to have the problem and fixed it. Then I would like to know how you fixed it, of course.

So - any thoughts from anyone who might have experienced this issue?

[kingnorris]

As for the first part of your question,I noticed the alt light goes off as well.But, if I maintain my speed to around 170 knots or so,the plane does a nice gradual descent down onto the runway. About 400 ft above, I turn off AP and land. I believe this is normal(?)and I've been landing the jets this way for quite some time now.

[Amtran618]

I fly for a fractional jet operator, little tiny small jets but I was always taught to put the missed approach altitude in the alerter at glideslope intercept. Don't think that's going to help you though, not sure why your autopilot is acting up, the jet I fly has honeywell avionics and once we select approach mode and it captures it flys the glide and loc until we stop it. Take it to maintenance.

[pdmike]

>As for the first part of your question,I noticed the alt
>light goes off as well.But, if I maintain my speed to around
>170 knots or so,the plane does a nice gradual descent down
>onto the runway. About 400 ft above, I turn off AP and land. I
>believe this is normal(?)and I've been landing the jets this
>way for quite some time now.

I am talking about the ALT decactivating BEFORE the plane starts down the glide slope. ALT is SUPPOSED to deactivate once the plane starts down the glide slope (for the same reason that HDG will decactivate once you hit NAV for the plane to start seeking the localizer).

The reason your plane is making a "nice, gradual descent" with ALT deactivated is, it is flying the glide slope.

My problem concerns the ALT hold deactivating long BEFORE the plane ever gets to the glide slope. It happens during the final descent from 8000 ft. to 1200 ft. AGL. During this descent, the plane is nowhere near the glide slope yet. The ALT hold is supposed to remain on and active - it is what is bringing the plane down from 8000 ft. to 1200 ft.

For some reason, ALT deactivates (on its own) during this portion of the descent - and that just should not happen.

[pdmike]

>I fly for a fractional jet operator, little tiny small jets
>but I was always taught to put the missed approach altitude in
>the alerter at glideslope intercept. Don't think that's going
>to help you though, not sure why your autopilot is acting up,
>the jet I fly has honeywell avionics and once we select
>approach mode and it captures it flys the glide and loc until
>we stop it. Take it to maintenance.

The problem I am describing happens to me only in Flight Simulator. I am not a real pilot, nor do I play one on t.v. . . .

[wdscobie]

>My problem concerns the ALT hold deactivating long BEFORE the
>plane ever gets to the glide slope. It happens during the
>final descent from 8000 ft. to 1200 ft. AGL. During this
>descent, the plane is nowhere near the glide slope yet. The
>ALT hold is supposed to remain on and active - it is what is
>bringing the plane down from 8000 ft. to 1200 ft.

which airport are you catching the glidslope at such a high altitude? in addition how far out from the runway are you activating approach?

in general one only follows the ILS down the last 2000-2500 feet and 8nm-6nm from the runway. example: KSEA 16C cat II&III you would be catching the decent at 1900ft (ksea is at 430') after a stepped approach - ERYKA (5000', D18 ISZI), MGNUM (4000', D14 ISZI), ANVIL(3200', D11 ISZI), and then catching the ILS a bit before SZ/LOM (1900', D5.8 ISZI).

--

[brion77]

pdmike...

#1. Does the ALT hold deactivate intermittently at "any" airport?

#2. When ALT does deactivate, is APP still armed?

#3. When the GSI is midpoint, and you fail to descend, are both APP & ALT illuminated?

#4. Is this problem only with the 737?

#5. Have you tried the same ILS approach with the King Air 350?

[skylab]

PDMIKE:

Without being there and seeing exactly what you're doing, it's a little hard to say what your problem might be.

One comment tho:

What are you doing descending to 1200 AGL 23 miles from the airport? MSAs could be as high as 4000 feet within 25 miles of the FAF.

Assuming an airport elevation of SL, one would not descend below about 2000 before reaching the Outer Marker and then, normally, only when on the Localizer. And, you wouldn't be doing that so far out. Good "rule of thumb" would be about 2000 feet or more above field elevation. Try descending to about 2500 to 3000; intercept the Localizer (NAV Mode); proceed inbound (with your ALT Hold ON) and when the Glide Slope becomes active, place the Autopilot in APP (or ILS) Mode. Try different approaches on different runways. Is this happening at ALL runways? Some ILS systems act differently in FS.

The above numbers are average only (and not for real world use.....my "disclaimer") and many approaches are different, obviously. One must check the Approach Plates for the different approaches to determine the correct altitudes.

[pdmike]

>which airport are you catching the glidslope at such a high
>altitude? in addition how far out from the runway are you
>activating approach?

I guess I was not making myself clear. I am at 1200 ft when I catch the glide slope.

The way I run my approaches is to descend from cruise altitude (usually 335 or 330) to 8000 ft. I "time" the descent so that I hit 8000 ft at around 35 miles out from the airport. I then fly level at 8000 ft until the plane is around 23 miles from the airport. I then descend to 1200 ft and fly level until intercepting the glide slope.
I generally reach 1200 ft around 9 miles out.

The localizer generally kicks in around 27 miles out - while I am still flying level at 8000 ft. When the localizer kicks in, I immediately activate the NAV hold in AP. I wait until the GSI indicator kicks in on the HSI before switching from NAV to APP in the AP - this is usually at a maximum of 20 miles out; I will generally wait until I am 14 miles out before switching from NAV to APP.

>in general one only follows the ILS down the last
>2000-2500 feet and 8nm-6nm from the runway. example: KSEA 16C
>cat II&III you would be catching the decent at 1900ft (ksea is
>at 430') after a stepped approach - ERYKA (5000', D18 ISZI),
>MGNUM (4000', D14 ISZI), ANVIL(3200', D11 ISZI), and then
>catching the ILS a bit before SZ/LOM (1900', D5.8 ISZI).

My understaning is that you can intercept the glide slope anywhere from 3000 ft down to 1000 ft. The higher you are flying, the sooner you will intercept it; the lower you are flying, the later you will intercept it. At 1200 ft (assuming the airport is at or near zero elevation), you will interceplt the glide slope around 4 miles out.

I hope this serves to clarify things.

[pdmike]

>pdmike...
>
>#1. Does the ALT hold deactivate intermittently at "any"
>airport?

No. Only at certain airports.


>#2. When ALT does deactivate, is APP still armed?

Yes.

>#3. When the GSI is midpoint, and you fail to descend, are
>both APP & ALT illuminated?

Yes.

>#4. Is this problem only with the 737?

No. I generally fly a download, A321 from WestWind Airlines (my VA). The problem happens with both this plane and the default, FS 737. I am using the 737 (alias) panel with the A321.

>#5. Have you tried the same ILS approach with the King Air
>350?

No.