GP capture failure

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Boatguy
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Re: GP capture failure

Post by Boatguy »

On further reading, I realize Wayne and I went down a rabbit hole and did not answer the OPs question which was "will the AP intercept vertical guidance before it intercepts lateral guidance". The OP was below the GP, had not captured lateral guidance, and the AP did not capture vertical guidance.

As Rich said, and Greg acknowledged, no capture of vertical guidance without lateral guidance because vertical guidance isn't necessarily safe if not on lateral guidance.

But I also noticed in the capture of the panel that while the HSI shows an intercept of about 30˚ the MFD seems to show the lateral intercept angle was more like 120˚, and the AP will not capture lateral guidance at more than 90˚. I'm confused by the difference between the HSI and MFD.
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Re: GP capture failure

Post by Rich »

Boatguy wrote: Mon Jun 05, 2023 5:04 pm But I also noticed in the capture of the panel that while the HSI shows an intercept of about 30˚ the MFD seems to show the lateral intercept angle was more like 120˚, and the AP will not capture lateral guidance at more than 90˚. I'm confused by the difference between the HSI and MFD.
Look again. They are in synch.
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Boatguy
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Re: GP capture failure

Post by Boatguy »

Rich wrote: Mon Jun 05, 2023 5:12 pm
Boatguy wrote: Mon Jun 05, 2023 5:04 pm But I also noticed in the capture of the panel that while the HSI shows an intercept of about 30˚ the MFD seems to show the lateral intercept angle was more like 120˚, and the AP will not capture lateral guidance at more than 90˚. I'm confused by the difference between the HSI and MFD.
Look again. They are in synch.
OK, I see that now. I thought that was a HILPT but it was the missed hold. It's runway 14.

And I think I see the problem. Greg was holding altitude at 2,500' while 1.5nm from the FAF where the crossing altitude / intercept was 2,000'. Presumably ATC gave him something like "cross ALPAT at or above 2,500'", setting him up to be high of the GP. Exactly what they do at my home airport at the IF.

As you noted Rich, the AP performed as expected.
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gcampbe2
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Re: GP capture failure

Post by gcampbe2 »

Rich wrote: Mon Jun 05, 2023 2:55 pm What I do is set VS to -800 FPM and put T/O flaps in. Pull power as necessary to keep speed under control. I've also used the CWS method, but recognize that this disengages ALL servos, so you're hand-flying all dimensions during that period. And be hair-trigger on releasing it upon GS capture.
That was precisely my approach, and for the same reasoning. Forcing a VS descent in excess of 500 fpm allowed me to keep the AP engaged, and all I had was to do was focus on maintaining control of the speed during the abnormally steep descent, easy to do with the flaps at T/O.

Thanks for all the comments and confirming my understanding of the AP behaviour.
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Re: GP capture failure

Post by gcampbe2 »

Boatguy wrote: Mon Jun 05, 2023 5:42 pm And I think I see the problem. Greg was holding altitude at 2,500' while 1.5nm from the FAF where the crossing altitude / intercept was 2,000'. Presumably ATC gave him something like "cross ALPAT at or above 2,500'", setting him up to be high of the GP. Exactly what they do at my home airport at the IF.
Actually in my case, there was a lot of fast moving traffic ahead of me, including an airliner that went missed about a minute before I intercepted the FAC. As a result of this traffic I was vectored in tight to the FAF and told to hold 2500, which resulted in me flying through the GP before intercepting the FAC. This is very unusually at my home airport where they always set us small guys up to intercept the GP from below, and typically have us on the FAC at least a few miles from the FAF. I think it was just controller overload in this case, and that he was expediting getting me down and out of the way. Next time I'll anticipate the need for a manual descent to capture the GP from above, whereas this time it caught me by surprise.
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Re: GP capture failure

Post by Ed McDonald »

In the "bad old days" one could capture an ILS glideslope before capturing the localizer. This was with analogue equipment, not the nice, digital software driven devices we have today.

The practice of descending a glideslope without being on the localizer is bad news; from a procedure design perspective, you are in no man's land as there is no obstacle protection while descending on the glideslope as the obstacle assessment only occurs within the localizer protected area. Fortunately, the modern avionics are now designed with logic that will prevent this from happening/

In the airline world, the practice of capturing the glideslope from above due to bad vectoring or any other cause is called "capturing from above" - very imaginative! The way this is managed is to set the altitude in the altitude reminder to 1,000 ft above the runway or field elevation. Next, dirty up as much as possible including all the way to landing configuration if necessary to help control the aircraft speed. After that, it is a matter of creating a descent rate (or flight path angle) greater than the glideslope - a 3 degree or greater FPA or a rate of descent greater than what a 3 degree glideslope creates ( 5 times the ground speed). For example, a 1,200 ft/min rate of descent for a 100 kt ground speed would net a 700 ft/nm convergence on the glideslope.

If, by an altitude of 1,000 ft above the aerodrome, the glideslope is not captured, the aircraft will level off either with the autopilot or manually (with or without a flight director) and a missed approach is commenced. The "stabilized IFR approach criteria" used by airlines and many others dictates that if, by 1,000 ft above the airport, the aircraft is not stabilized a go around is mandatory. The stabilized approach criteria are:
1. aircraft in proper landing configuration
2. briefings and checklists complete
3. within 1/2 scale laterally and vertically of the lateral and vertical path
4. maximum sink rate of 1,000 fpm
5. speed within +20/0 of reference speed
6. only small heading and pitch changes required
7. in IMC, stable by 1,000 ft AGL
8. in VMC, stable by 500 ft AGL

The stabilized approach has been one of many safety improvements that have prevented hard landings (too high of a sink rate), runway excursions (landing too far down the runway and too fast) and other undesireable outcmoes.
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Re: GP capture failure

Post by CFIDave »

And just to add to this discussion, a similar "capture from above" can be used to capture a VNAV descent path. Typically this is because the pilot forgot to reset the altitude bug to a lower bottom of descent (BOD) altitude when they programmed the G1000 for VNAV -- thus causing the autopilot to ignore the VNAV magenta caret/chevron when arriving at TOD (top of descent) and maintaining the current altitude. Since a VNAV descent rate is usually set to -2.5 or -3 degrees, usually all the pilot has to do is set a 1000 fpm descent rate to "catch it" from above.

Unlike capturing an ILS approach glideslope or LPV glidepath from above, capturing a VNAV path from above may not be as risky because the aircraft is usually not as close to the ground.
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mfdutra
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Re: GP capture failure

Post by mfdutra »

Or you can just use VNAV-Direct and reset the profile.

I always descend at 2.0 by the way. I find 3.0 too aggressive without pressurization.
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