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Question about "inertial virtual glislope" in case of invalid GP signal

Started by tango4, Tue, 25 Apr 2017 18:20

tango4

Good evening,


I just read an article in Aviation Week about the crash of ACT Airlines 747-400 that happenned on Jan 16 in Kyrgyztan where there was false glideslope interception (the 9° one).
At one point in the report they say:
"At Bishkek, the IAC said an "FMA [Flight Mode Annunciator] Fault 2" event that occurred 15 sec. after capturing the false glideslope sent the autopilot into inertial mode in addition to notifying the pilots with generic master caution textual and aural alerts. Fault 2 indicated that the aircraft autopilot could no longer track the glideslope; however the autopilot remained engaged, as designed."


In PSX, if I am correct, this inertial reversion is modeled, but without any warning happening ?
Do you know what this "fault 2" is, and what warning to expect ?
Perhaps they are mistakenly referring to a LAND2 reversion, as in a diagram in this article you can see Glide capture at 01:15:52, LAND3 at 01:15:58, transfert to inertial mode at 01:16:07, LAND 2 at 01:16:18.


Hardy, I would be curious to get your opinion on this.


Charles

Hardy Heinlin

Good evening,

yes, inertial G/S guidance is modeled; it works for short term radio problems. When there is a long term signal loss, the mode words get a yellow line (mode fault status).


|-|ardy

Hardy Heinlin

Example with mode fault:

Load: 04 Approach 005 - Real-world weather - Frankfurt.situ

At ca. 1000 ft RA enter the ILS identifier IFCW in the ILS box under "Failed radio stations" on Instrcutor > Situation > Malfunctions > General.

Watch the G/S pointer. It goes down and stays there; this is a permanent radio failure. The G/S pitch mode keeps following the inertial 3° path for ca. 10 seconds, then the G/S mode fails and the FD pitch bar disappears.

(The error simulation is randomized; on some runways only the LOC will fail, on other runways only the G/S will fail, e.g. on EDDF Rwy 25C.)


Example without mode fault:

Delete IFCW in the ILS box under "Failed radio stations". (This permanent radio failure feature is not included in the DVD version, hence the DVD situ files don't store the radio failure status. So if you reload this situ, the radio failure status will not be reset. It will only reset when you save a situ file within PSX version 10.0.2 or higher.)

Reload: 04 Approach 005 - Real-world weather - Frankfurt.situ

On Instrcutor > Situation > Malfunctions > Nav: Activate "Ground station error - G/S" (severe or non-severe).

Watch the G/S pointer. It sometimes fluctuates for a short moment. The G/S pitch mode keeps following the inertial 3° path during the error, and the G/S mode remains operative because these errors only last for a couple of seconds.


|-|ardy

tango4

Many thanks, I'll try that when I have time.
I missed the difference between the long and short signal disruptions.


Charles

tango4

Good morning Hardy,
I just ran the tests and there is still something bugging me where PSX does not behave in the way I understand the report.


When I trigger the IFCW failure, I see the yelow line appear after ca 10secs, but then, the autopilot commands a level off., the horizontal FD bar disappears, and the AP stays in LAND 3.


When I read the report I understand that AFTER getting the yellow bar, the AP KEEPS following the inertial 3° Glideslope AND AP reverts to LAND2 status. If the AP had commanded a leveloff, the crash might have been avoided.


Would you like me to send you a copy of the Aviation Week report ?


Charles

Hardy Heinlin

I can't analyse that Aviation Week report, sorry. Too busy :-)

LAND 2 is not related to station errors anyway. It can only relate to errors in one of the three receivers.


|-|ardy

tango4

Don't sweat it.
It would be nice if you could just have a look at this accident in the coming months when your schedule allows, but no rush at all.
I know you have your hands full.


Charles

Hardy Heinlin

Note that I'm not an accident expert. Feel free to discuss this report with the pros on this forum. Don't wait for me :-)


|-|

tango4

Actually what I meant is that I have the feeling that PSX does not behave as it should here which is why I wanted YOUR oponion.
Just in case anyone with real life knowledge reads this thread, opinions welcome.
Please beware, there are frequently some small inaccuracies in those reports. And there are lots of inaccuracies in my brain !

Hardy Heinlin

Quote"At Bishkek, the IAC said an "FMA [Flight Mode Annunciator] Fault 2" event that occurred 15 sec. after capturing the false glideslope sent the autopilot into inertial mode in addition to notifying the pilots with generic master caution textual and aural alerts. Fault 2 indicated that the aircraft autopilot could no longer track the glideslope; however the autopilot remained engaged, as designed."

In my first PSX example above, all these reported effects happen in PSX as well (despite the slightly shorter delay):

1. Erroneous radio signal

2. Inertial mode activated

3. G/S mode fault (yellow line) after 10 sec.

4. Inertial mode deactivated (level-off)

5. Caution message and aural alert

6. A/P remains engaged (pitch protection mode)

I don't know why their aircraft continued the descent. Maybe there were aerodynamic problems etc.


|-|

tango4

Thanks a lot Hardy.
The question indeed is why did it continue to descend.
The way I understood the report, the journalist implied that it was a normal behaviour of the system under such conditions (meaning the yellow line across the glide mode would not imply a level off).
But I have trouble finding good documentation on this specific submode behaviour.
So if any of the gurus of this forum have documentation or could help me understand the report, that would be nice !

Have a nice day Hardy and thanks again for your time.

Charles

John H Watson

QuoteWhen I read the report I understand that AFTER getting the yellow bar, the AP KEEPS following the inertial 3° Glideslope AND AP reverts to LAND2 status. If the AP had commanded a leveloff, the crash might have been avoided.

That was my understanding, too. Here's the relevant extract from the accident report:

"As per the Boeing Company explanation, FMA FAULT 2 record means that that the AFDS identified pitch mode failure i.e. the aircraft could not longer be tracking the glideslope beam (see FCT 747 5.19 - 5.20). The mode failure results in the following:

-the pitch flight director bars are removed from the PFDs

- a yellow line is displayed through the G/S mode annunciation on the PFDs (FMA)

-both Master Caution lights are illuminated;

- a Master Caution aural is activated;

– a Master Caution Aural is activated;

- an amber AUTOPILOT Caution message is annunciated on EICAS;

Meanwhile, the A/P will not disengage. In the pitch channel, the A/P will maintain an inertial path which tracks a 3 deg glideslope regardless of the actual glideslope at a certain aerodrom. The path will be maintained until the crew intervenes by disengaging the A/P or initiating a go-around (TOGA switch is pushed). Without crew intervention, the A/P will maintain the inertial path until the FLARE mode is engaged. The Autoland status LAND 3 (or LAND 2) will also continue being annunciated.

According to the manufacturer's information, the inertial path generation is a feature in all airplane models 737, 747-400/-8. 757, 767, 777, 787 that allows the A/P to continue the approach for disruptions of either G/S or LOC ground station signals."

Cheers
JHW

Hardy Heinlin

Note that my examples above are station errors of a beam. If that beam is momentarily wrong due to interference or something, the yellow line will not promptly occur. Instead the AFDS will follow the inertial path for some seconds. Only if the error lasts longer, the caution status will activate and the descent will be stopped.

Is that accident the one with the beam?

Maybe the caution status also activates if the detected IRS path angle is greater than ca. 4°, and maybe it takes 15 seconds to detect this error. This is not modeled anyway.

QuoteWhen I read the report I understand that AFTER getting the yellow bar, the AP KEEPS following the inertial 3°

I don't think the yellow line appears on every short term error. On an intermittent radio problem you would get the caution beeper every few seconds. So if that yellow line appears prompty, it can only do this due to a completely unreasonable path angle. This implies that this story is about a false 6° or 9° beam.


|-|


OK, here's a theory (and I just re-read the first post; yes, it was a 9° beam):

1. The AFDS captures the beam, thinks it's a 3°, and follows it based on IRS data and radio data (this data mix is always in use for stabilization; the IRS is a fantastic assistant for all kinds of control loop functions).

2. The radio beam drifts away because the aircraft aims for a path of ca. 2.5° to 3.5°

3. As the radio beam drifts away, the AFDS thinks there is a station interference, hence it disregards the radio signal and follows the IRS path only (max 3.5° or something).

4. 15 second timeout: Caution status. Now the big question: Is the AFDS designed to autoland the aircraft in IRS mode? I don't think so. I think the AFDS will command a level-off, and I don't know why it didn't work in that accident.


tango4

Thanks John !


Hardy, I think you are misunderstanding what I said.
It is indeed the accident with the 9° beam.
I totally agree with you that the yellow line should not appear on every small deviation, it is not what I said.
Apparently, in this accident, pilots captured momentarily the 9° glideslope.
What I understand is that upon capture, all modes went to LAND 3 , LOC GS, as per normal landing. But they were almost in level flight, quite close to the runway, closing in fast(see the picture here: https://www.dropbox.com/s/ltmxm3nbxpw05e9/2017-04-26%2016.44.47.jpg?dl=0) so the GS signal almost immediately went full down and was deemed invalid. So the aircraft reacted by flying the 3° Inertial glide without warning initially (as in PSX). Only 10 seconds later, as in PSX once again did the warning appear.
But in PSX, when the yellow line appears, it also commands a level off.
What I understand from this report is that it should keep flying the 3° inertial path (and perhaps revert to LAND2, not sure if this one is related). But that is the ONLY part where I FEEL PSX disagrees with what happened.


Hardy Heinlin

I was editing my post while you wrote your reply.

One more thought in addition to my edit above: The IRS mode is also used shortly before the flare as the G/S beam is not useable at very low heights. So I can imagine that autoland will continue on pure IRS if the error was detected at a very low height. There must be a height under which this kind of caution status is inhibited. Obviously, this caution status never appears during a normal flare.

Will

I flew this four times last night and here's what I saw:

Two times, the aircraft leveled off and flew straight ahead.

To other times, the aircraft went all the way down into an autoland, and touched down smoothly, although very far short of the runway.

Will /Chicago /USA

tango4


John H Watson

QuoteTwo times, the aircraft leveled off and flew straight ahead.

Is this because the aircraft didn't recognise G/S as a valid mode and levelled off at the preselected GA altitude? (which may have been below the aircraft altitude when signal loss occurred)

Will

I didn't have time to troubleshoot. I won't have time to look at it again until Saturday. It seemed fairly low to level off, certainly below 1000'.
Will /Chicago /USA

Will

Okay, I can reproduce this now.

When I fail IFCW at 1000' when starting with a fresh run of PSX, or in any circumstance when the IFCW has not previously been failed or when the failure has been reset, then:

1. The glideslope fails.
2. The glideslope indicator goes full-scale down.
3. The FD pitch bar stays put and the aircraft descends on the inertial glideslope.
4. About 7 seconds later, the yellow line appears though G/S, the master caution activates, and we get > AUTOPILOT.
5. As that happens, the aircraft, still in LAND 3, pulls up slowly to fly level, with flaps and gear down, tracking the localizer but staying roughly at the altitude the aircraft had reached at the time > AUTOPILOT. It stays in LAND 3.

When I load the situ again with IFCW already failed (from the previous situ), the following happens (and hang on, it's a wild ride):

1. The glideslope has already failed or fails immediately at situ start.
2. The FD pitches down to 12 degrees nose low. The aircraft follows the FD.
3. The aircraft descends steeply at about 2800 fpm, stabilizing at about 10º nose low pitch.
5. The yellow line through G/S does not appear, neither does > AUTOPILOT. There is no master caution.
6. GPWS starts whoop whooping, PULL UP! PULL UP!
7. At about 200 feet AGL, the autopilot starts to raise the nose and lower the descent rate.
8. FLARE mode engages on schedule.
9. The aircraft performs a smooth autoland, about 1.5 miles short of the runway.

Creating a new situ that saves the status of the IFCW failure doesn't fix the problem, although reloading a situ in which IFCW is originally not failed will clear the failure, as Hardy says. But any situ that starts with IFCW already failed will be in for the steep descent and the exciting landing 1.5 miles before the runway threshold.
Will /Chicago /USA