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**1) Someone help! I’ve noticed this problem on my last flight KUL-LHR(Casual server). As you can see on the 1st screenshot the IAS is 234KTS (M0.85) at FL380 and literally seconds later the IAS drops to 212KTS still M0.85 and the AC starts rolling from side to side and almost stalling! Is this a bug or something? I’m not too heavy or anything for that altitude. I’ve always planned my flights with simbrief and never had a problem like this.
Side note, I was reading the temperatures in Fahrenheit (typical Westerner 😂) and thought we had entered a period of global cooling, then I realized that was displaying in centigrade.
Hi. From your first to second screen shot it’s true your wind increased in size by only 1kt.
However, the wind direction shifted abruptly left by 9 degrees (91-82).
That direction shift makes for several knots of wind increase from right to left (and several kts of this is robbed from IAS that goes into that sideways speed).
Your aircraft takes time to catch up with the change in sideways wind drift, and you can demonstrate in solo mode that this causes some moments of increased instability (yaw oscillations etc.), which gets more intense when close to your weight limits for a given altitude (you’re at FL380 and angle of attack of a few degrees shows the aircraft exertion to generate sufficient lift at this altitude).
To summarize, the cause looks like a combination of weight for the altitude (though you may have been under the limit), combined with an abrupt wind speed direction.
edit: note that the aircraft tends more to instability with an abrupt sideways shift in wind rather than a shift in wind along the direction of travel. I tested with more safety margin in flight profile (weight, altitude, speed etc.), and could only barely see the effects of a sudden change in wind direction.
I don’t know why it would suddenly dump 20 knots though. Changing 10 degrees on a wind that is 22 knots physically cannot shave off 20 knots instantly (you’d have to change close to 90 degrees to do that). In addition, it seems the overspeed tape also moved down while altitude hasn’t changed, normally pointing to an OAT change. However, it’s shown the OAT is only different by 1 degree which doesn’t seem like a problem.
My theory (which I came across during fuel burn testing) is that OAT doesn’t scale linearly from ground to cruise, but cruise speeds are based on ground OAT. For example, while testing, I noticed if I dumped 10 degrees of surface air temperature, my airspeed scale would mysteriously change (like is shown in the pictures) but my OAT (at cruise) would barely change at all. With this said, it’s possible that OP flew into a zone of higher or lower OAT at 0 MSL (which, it shouldn’t change that rapidly but perhaps it’s a modeling problem) and that dropped his airspeed and overspeed tape.
Although, during my testing I believe altering this OAT also significantly altered GS which seems relatively untouched in OP’s photos, so in conclusion I have no idea what I’m talking about and I’m just trying to provide justification for what is probably an extraneous set of circumstances.
I failed to pay attention to that before a night’s sleep. Good point. It’s a mystery to me. What would make the overspeed tape move that much with such stable temp(?).
And the amount of IAS change, as you said. Something doesn’t fit…
I over-anchored on the heading and wind direction change.
edit: The OP supplied confusing information when describing the two screen shots:
1st image:
2nd image:
13 minutes difference.
So for one thing, we’re not comparing apples with apples when looking at the two images.
The first image was actually taken 13 minutes later than the SECOND image!? Are these even the same flight?
You’re right, maybe we should wait for some OP clarification because none of this lines up.
If you look at the map location, the screenshot with 214 IAS is taken before the one with 234 IAS (and also the image number of the 234 is after the 214 one). This is directly contradictory to this:
We can clearly see the order is wrong and the photos are 13 minutes apart, not “literally seconds”. I have no idea why you would lie about something like this so I shall just sit and wait for OP to explain themselves.
The problem I’m trying to illustrate is that how is it possible to loose 20kts of airspeed while going at the same Mach and minimal change to the OAT and wind?
Honestly, I have no idea. Perhaps a mod or something could answer this because as far as I can tell, that 1 degree change in OAT accounts for maybe 2-3 knots of airspeed. Also, when I put in that OAT, my overspeed bar is set at 263 IAS and M 0.85 is around 250. In order to get to the 214 IAS with the overspeed bar that low I need to have the OAT be -1C (in contrast to the -48 shown). Perhaps there is a hidden game mechanic that I don’t know about but as far as I’m concerned this is just a bug.
With no change in wind or OAT, it’s not logically possible to suddenly lose 20kts of airspeed.
For one thing, if IAS changed by 20kts, it would be accompanied by a noticeable change in Mach.
Mach number is a measure of TAS. And IAS is TAS reduced for altitude and temperature. So under constant conditions of altitude and temperature, IAS and TAS move in tandem (separated by a constant gap).
As mentioned before, the screen shots you provided were taken at widely different times and, it looks like, not with the same aircraft (red overspeed tape is too different).
If you’re still claiming your above comment to be true, you need to show convincing visual evidence for it to be considered as a bug report.
