reported landing during flare FCS WOW mode after FM update

[Ahmed]

Hi,

Looking for the cause behind the takeoff/landing issues noted after the FM update, I think that I have found an issue with the new FCS code. It looks like the transition to WOW mode, at least in pitch command, is not happening, or that WOW mode logic has been changed to a wrong one.

According to documentation, for stabilator command in WOW operation, aircraft sensor feedbacks are set to fixed values or do not exist. This was how it seemed to work in DCS prior to the FM update, but now it seems to receive the same feedbacks than in "Not auto flaps up operation".

I did the following tests:

- taxi.trk: noted that the stabilator reacts to changes in pitch rate/aoa while taxiing (like it would on a viper, but should not in a hornet, as seen in any YT video)

- takeoff_12.trk: take off with 12º ANU stabilator. Rotation speed for this configuration is about 145kt. At approximately 100 knots the FCS starts commanding ANU stabilator, as in seeking to capture an AOA. The aircraft lifts off by itself. FCS reverses the ANU stabilator command and captures 6º AOA.

- takeoff_16.trk: take off with 16º ANU stabilator. Same observations than above, just the FCS captures 10º AOA.

- landing.trk: notice that immediately after touchdown, the FCS commands 24º ANU trying to recapture the trimmed AOA and holds it throughout the landing rollout until the speed reduces to near zero.

It seems that the FCS logic after the update disregards WOW mode and just remains in "Not auto flaps up operation" logic, trying to capture the command trim AOA. This explains all the weird things observed after the update, such as:

- Aircraft taking off by itself prior to rotation speed (due to FCS commanding unrequested ANU stabilator)

- Nose strut staying extended after nose gear touchdown (due to FCS commanding 24º ANU stabilator on touchdown)

- Possibly also explains the uncontrolled pitch up issues that people have reported if taking off with flaps auto

The correct behavior would be as documented in the WOW operation description (and as it seemed to work prior to the update)

Aside from the quoted documentation reference to "WOW operation" logic, I link below the following videos:

- Taxi out (little-to-nil stabilator motion)

- Landing (no such thing as sudden command of permanent 24º ANU on touchdown.

- Another example (notice that the pilot commands aft stick later on during the rollout to aid in braking, but there is no such thing as a FCS 24º ANU command on touchdown)

 

This was working well before the update, so it must be an issue with the new code and WOW logic.

Probably some of the SMEs in the forum can amplify on this.

landing.trk takeoff_12.trk takeoff_16.trk taxi.trk

Edited March 8 by Ahmed

[Sarix]

Thanks for the investigation Ahmed!

I'd like to extend on the "weird things" part, specifically taking off with AUTO flaps. The Hornet would aggressively command 22-degrees ANU for the stabilators starting at approximately 100kts and does not attempt to correct itself after weights are off wheels, leading to... amusing behaviors (continued pitch-up).

However the FCS will correct itself and gradually revert stabs to 0 degrees if flaps are put to either HALF or FULL and then put back into AUTO.

Tracks attached - hopefully it would be of use.

Cheers.

Hornet flapsup takeoff then flapsdown.trk Hornet flapsup takeoff.trk

[Special K]

First - the F/A-18 does not reset trim on WoW but on gear up, according to the documentation. Second, it does that only, when flaps are on half. Taking off with flaps on auto will not reset the trim, thus you stay with 12 which you can easily monitor on your FCS page.

I get the report of Ahmed though and from the looks of it, it is already being looked into.

 

[Ahmed]

I'd advise though to keep anything that is not directly related to the WOW issue in a different thread, or we may get everything marked as "correct as is", if both reports get intermixed. My initial report is intended to address the apparent lack of WOW logic since the last update, and the associated unexpected FCS stab commands during the take-off/landing rolls.

[Sarix]

Moved my extension on "weird stuff" to a separate thread.

Cheers.

[BIGNEWY]

Hi all, 

first some points to share. 

Above 100 knots FCS will start to command stabilator which is correct, essentially when the aircraft gets to 103knots the aircraft is no longer considered WOW. 

Pitch rate feedback is available while on the ground during taxi according to the schemes, so stabilator movement can be present during hard breaking for example. 

About stub command 24 NU at touchdown, it looks correct when a normal landing procedure is followed, with -800 fpm landing, like the first landing video, because of big pitch rate feedback at touchdown.

But during landing with flare it looks wrong, and we will tweak it, I am creating a report for the team this morning. 

thank you