[RESOLVED] Autopilot logic and low speed gain.

[pierrewind]

Hi razbam.

I have noticed for a long time the autopilot being overly agressive at low speed. (gains too high). This is very clear in heavy configurations.
Due to this high gain, the autopilot enters an oscillation. Depending on speed this oscilation can be divergent leading to a loss of speed/control. 

Unfortunately this renders the autopilot unusable in these configurations.

There is also a logic issue when capturing an ILS.
When the localiser is captured, the glide slope also captures even if not on glide. This is wrong.

From alt mode -> arm ILS -> Loc captured + alt; gs armed -> glide captured -> loc + gs; alt gone.

In the current configuration the autopilot would loose altitude on approach. Correcting it requires a trim up input wich in turns disables the gs mode (undesired). 

Seeing these corrected would be a great step towards the flying through autopilot philosophy of the M2000.

PS: I am aware there might be some overlap with previously reported bugs and some grey areas in the various operating modes.

MIRAGE 2000 AUTOPILOT ISSUES.trk

[jojo]

It's a good reminder, thanks

[RAZBAM_ELMO]

Is this information based on data publicly available as to the autopilots limits/ abilities in a heavy, low speed configuration or simply just an opinion on what you feel it should handle like? Genuinely curious but I can press the issue to the SME to get their feedback.

[jojo]

On 12/15/2020 at 1:56 AM, pierrewind said:

In the current configuration the autopilot would loose altitude on approach. Correcting it requires a trim up input wich in turns disables the gs mode (undesired). 

MIRAGE 2000 AUTOPILOT ISSUES.trk 665.74 kB · 5 downloads

 

I missed the bold part the first time.

When LOC + GLIDE are captured:
- pitch trim disengage GLIDE
- roll trim disengage LOC
This is normal behavior...

But the system must capture LOC before GLIDE.
So if you un-couple AP LOC + GLIDE with trim input and you want to re-engage LOC + GLIDE, you must unselect L|G on AP panel then re-arm L|G.

@RAZBAM_ELMO
That behavior is described on the French Mirage 2000C RDM floating on the web for more than 15 years...

Edited January 28, 2021 by jojo

[RAZBAM_ELMO]

SME is gathering his available intel on this as it is not typically used at low speed- heavy load situations. Reported.

[pierrewind]

It was also my understanding that this is the normal expected behavior in regards to the trim inputs. 

However when arming the approach mode, the autopilot disengages the alt hold mode and  doesn't maintain the altitude correctly. 

This is to be corrected by a trim up input. Which in turn will disable the glide. This is expected, but undesired in this situation. 

The issue is not in regards to the trim up input. But it is about being forced to use the trim up input to correct for the autopilot unable to maintain the altitude. 


Then low speed gain. Yes 200kts or 18 AOA from the widely available manual.
This used to be a significant issue for me at speeds even close to 220kts. I haven't seen anything on the changelogs but the behaviour may now be a bit less agressive. 

My experience would tend to dictate this limitation was introduced to prevent a speed decay to "stall".

But, as can be seen in this video, it likely has been cleared to lower speeds since then,  particularily in approach mode. 

Mirage 2000 Djibouti

Edited January 29, 2021 by pierrewind

[pierrewind]

Found this extract from the mirage F1CZ flight manual (using a similar autopilot) available here: https://www.avialogs.com/aircraft-d/dassault/item/2764-1f-f1k50cz-1-1-mirage-f1-flight-manual-aircraft-operating-instructions