fixed F-16 Pitching up and down

[TVC Pilot]

Twice now flying on an MP server, I have had the airplane start moving up and down. I look out at the wings and with an aim 9 on the tip, it was flexing up and down about 20 deg. On the second sortie, with no weapons, the wing was flexing up and down. I looked at the elevators and they were going + / - 30 deg at about a 2 cycles a second. I was at about 12,000 feet and around 400 kts.  Has only happened on MP server, now when flying my own missions.  Any idea what causes this?

 

[mtd2811]

Lag?


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[Terzi]

Same thing happened to all of us while flying on our MP server. The air temp was -8, no winds, snowing, mild turbulence. Almost half of us experienced the jet is violently pitching up/down, which would never happen in real life. It was totally extraordinary shaking.

[Deano87]

Make sure your probe heat is selected to on just before takeoff. Nevertheless there seems to be a pitot probe heat issue with it causing FLCS issues in low temperatures. 

[taco3rd]

I've experinced it. Backing off the throttle and pitching down or up usually helps. I'm not sure what it is.

[Deano87]

4 minutes ago, taco3rd said:

I've experinced it. Backing off the throttle and pitching down or up usually helps. I'm not sure what it is.

Depends what its related to. There is an issue if you go too fast at low level that can cause it, but you run into that well beyond the aircrafts airspeed limit so you shouldn't reallybe going that fast anyway.

[taco3rd]

Just now, Deano87 said:

Depends what its related to. There is an issue if you go too fast at low level that can cause it, but you run into that well beyond the aircrafts airspeed limit so you shouldn't reallybe going that fast anyway.

It's an uncontrollable oscillation for about 10 seconds. Happened to me recently on a training server and probably a half dozen times overall. I always switch my probe heat on. It's fairly violent. I'm pretty sure it wasn't a speed issue.

[Deano87]

Just now, taco3rd said:

It's an uncontrollable oscillation for about 10 seconds. Happened to me recently on a training server and probably a half dozen times overall. I always switch my probe heat on. It's fairly violent. I'm pretty sure it wasn't a speed issue.

Cold air temp?

[taco3rd]

1 minute ago, Deano87 said:

Cold air temp?

Not sure. I doubt I do anything different than anyone else. I've turned into a fairly good sim pilot and have startup procedures down and don't have any other issues. The only common denominator I can think of is that I use the 162nd vipers training server just about daily.

[CoRBan]

I've faced a similar problem, but with yaw axis. The rudder deflection causes repeatable oscillations. I haven't tracked the conditions that cause this issue yet. Checked the rudder axis in settings, it's completely normal.

[Alpiinoo]

+1 Same problem. And nose up when the landing gear retract.

 

[SCPanda]

+1 Same problem, and the nose up and down behavior is extremely violent, it can gets to 30 degrees ish up and down for 2 or 3 times in just one second. So yeah this is clearly a bug. Also, your indicated air speed in the jet will become highly inaccurate after this takes place and it is impossible to recover. All you can do is to eject and choose a new jet, which is annoying...

11 hours ago, Deano87 said:

Depends what its related to. There is an issue if you go too fast at low level that can cause it, but you run into that well beyond the aircrafts airspeed limit so you shouldn't reallybe going that fast anyway.

Seems like you have not experienced this issue, what you are saying sounds like this is a realistic issue but if you have seen this bug occurring, you will know this is totally a bug... 

[Deano87]

5 minutes ago, SCPanda said:

+1 Same problem, and the nose up and down behavior is extremely violent, it can gets to 30 degrees ish up and down for 2 or 3 times in just one second. So yeah this is clearly a bug. Also, your indicated air speed in the jet will become highly inaccurate after this takes place and it is impossible to recover. All you can do is to eject and choose a new jet, which is annoying...

 

Again, does this usually happen in cold missions? There seems to be something up with the probe heat not working on occasions, which messes up the FLCS and results in the oscilations.

[SCPanda]

13 minutes ago, Deano87 said:

 

Again, does this usually happen in cold missions? There seems to be something up with the probe heat not working on occasions, which messes up the FLCS and results in the oscilations.

Yes, cold missions. I guess you are right, could be probe heat issue messing up FLCS, because IAS displays wrong when this happens. Even if you recover from this, IAS will still be incorrect which leads to this behavior happening again. BTW I turn on my probe heat every time I start my jet. 

Edited January 6, 2021 by SCPanda

[TVC Pilot]

On 1/5/2021 at 2:43 PM, Deano87 said:

Make sure your probe heat is selected to on just before takeoff. Nevertheless there seems to be a pitot probe heat issue with it causing FLCS issues in low temperatures. 

I did turn it on, but that might be a clue, the MP server I'm on is set to winter right now. Not sure the temp. 

 

 

Edited January 7, 2021 by TVC Pilot

[_SteelFalcon_]

Also only had this happen on winter maps a couple times, never in summer

dont know the temp tho

[4NGRY]

Try recycling FCS CAT 1 to 3 and back, or vice versa depending on your config

[pbishop]

To separate the known from the unknown, ED would need to answer if it is normal the aircraft starts pitching when pitot static icing is present.  I did a few tests and posted a while back about this where I performed tests with pitot heat "OFF" on purpose to see the results.  I found that once the pitot system ices over, descending in altitude causes the airspeed to cross 0 kts into what seemed like negative values which therefore causes the pitch oscillations.  Increasing the altitude caused the airspeed to increase and re-cross the 0 kts and stop oscillations.  There may be an issue as Deano states with the pitot heat system that would inadvertently cause icing, but the question remains that if the pitot system is subject to icing, is the behavior normal.  If it is normal behavior for the aircraft to pitch when frozen and airspeed crosses 0, then the flight manual may need to state this or it be communicated.  For those experiencing it with pitot heat on, the resulting pitching would be a result to the fact the system has icing due to the heating system  not working correctly.  However, these are two uniquely different issues only tied by the fact one bug may be leading to the discovery of the second (which still has not been confirmed as being normal or a bug).

 

1. Pitot heat system bug (which is a cause)

 

 

Which would inadvertently lead to the second issue:

 

2. Pitching as a result of negative airspeed/icing.

 

 

[Deano87]

Something like a violent pitch oscilation could certainly be the result from a frozen over pitot system on a FBW aircraft like the F-16. The FLCS needs airspeed data to know what part of the flight envilope its in, and what gains to use, in order to be able to control the aircraft.

 

This was the crash of the X-31 caused by the exact same issue.

 

Great video which is worth watching in its entirity, but skip on to 15:45 for particularly relevent bit.

 

 

Although In all likelyhood the F-16 would have some kind of failure mode and reversion to a more conservative FBW state if it sensed a pitotstatic issue.

The other thing I'd like to mention is that I thought the pitot heat system in the Blk 50 was always active once the WOW switch was released on takeoff and that the cockpit switch is mearly for pitot heat on the ground prior to takeoff and pitot system testing. I've been flipping it to on just before takeoff anyway because it seems to help avoid issues. But my knowledge of the real jet from reading various -1s was that as soon as WOW was released then the pitot heat system was switched on, but I don't know if that applies to the USAF Blk50s

Perhapse @FoghornF16 could chime in on this?

Edited January 8, 2021 by Deano87

[pbishop]

Repeated again today, this time I made a track file.

 

File is just over 5MB, so had to throw it on the google drive:

https://drive.google.com/file/d/1Z_c-flmTTHo6ie56Ck8TaWEoFk7ttMBZ/view?usp=sharing

 

[Donglr]

Unrelated to all this pitot talk, another possibility could be active autopilots. Have not watched the track file (PC is too far away from my couch where I am currently located ), so I do not really know what the phenomenon in discussion looks like, but when I flew yesterday I had my jet pulling down all the time. Turns out my ALT HLD was on because my HOTAS switch was in that position from flying another jet earlier.

[Deano87]

Autopilot wasn’t a factor with the times I’ve seen it.

[BIGNEWY]

On 1/9/2021 at 7:36 PM, pbishop said:

Repeated again today, this time I made a track file.

 

File is just over 5MB, so had to throw it on the google drive:

https://drive.google.com/file/d/1Z_c-flmTTHo6ie56Ck8TaWEoFk7ttMBZ/view?usp=sharing

 

Thanks for the track I have reported it to the team, it looks like an icing issue to me, but the team will know for sure. 

[Deano87]

TY BIGNEWY!

[KalAbaddon]

On 1/5/2021 at 11:32 AM, TVC Pilot said:

Twice now flying on an MP server, I have had the airplane start moving up and down. I look out at the wings and with an aim 9 on the tip, it was flexing up and down about 20 deg. On the second sortie, with no weapons, the wing was flexing up and down. I looked at the elevators and they were going + / - 30 deg at about a 2 cycles a second. I was at about 12,000 feet and around 400 kts.  Has only happened on MP server, now when flying my own missions.  Any idea what causes this?

 

Not sure if related but iirc the reason the F-16 puts the 120's on the wingtip is cause the aim 9's ( or nothing ) can make it flutter or something of the sort.  The 120's provide some stability to the wings.  Maybe this is partially related since you mentioned it happening with aim 9 or nothing.

 

 

 

 

On 1/8/2021 at 10:55 AM, Deano87 said:

Something like a violent pitch oscilation could certainly be the result from a frozen over pitot system on a FBW aircraft like the F-16. The FLCS needs airspeed data to know what part of the flight envilope its in, and what gains to use, in order to be able to control the aircraft.

 

This was the crash of the X-31 caused by the exact same issue.

 

Great video which is worth watching in its entirity, but skip on to 15:45 for particularly relevent bit.

 

 

Although In all likelyhood the F-16 would have some kind of failure mode and reversion to a more conservative FBW state if it sensed a pitotstatic issue.

The other thing I'd like to mention is that I thought the pitot heat system in the Blk 50 was always active once the WOW switch was released on takeoff and that the cockpit switch is mearly for pitot heat on the ground prior to takeoff and pitot system testing. I've been flipping it to on just before takeoff anyway because it seems to help avoid issues. But my knowledge of the real jet from reading various -1s was that as soon as WOW was released then the pitot heat system was switched on, but I don't know if that applies to the USAF Blk50s

Perhapse @FoghornF16 could chime in on this?

 

 

 

IIRC you are correct.  We would have issue doing ground power and someone left the switch on or the wow switch was faulty and we would learn it cause the pitot cover would start to melt.  Also had a few people burn thier hands on the intake deicer if they tried to check it to quickly after shutdown or doing checks while someone is turned on the battery for another issue iirc.