I am in the cruise FL340 over Poland and notice traffic on TCAS about 10 miles crossing left to right at the same level, ATC transmissions suggest that is a Swiss AC however keep advising that it is 400 below its allocated FL and to climb and maintain FL340.
As I pass through the contrail I notice that it looks like this AC is climbing and descending in a "Rollercoaster" type fashion, anybody else experience this strange occurrance.
I am wondering if it something to do with QNH not being set at 1013mb on certain AI.
I have listened to this happen on numerous flights and whilst annoying didn't pay much attention, but this is the first time I have actually witnessed it first hand :o
It only seems to happen on a very small percentage of my installed AI.
Once again I am appealing to this forums abundance of knowledge
Jonny
"Never in the field of Aircraft Handling have so many been dispatched by so few"
I don't use M$ ATC (or any other for that matter) but my guess is that the model/s concerned is/are probably not suitable for AI and this is causing what you're seeing (basically.. the aircraft is going into and out of a stall). Try using Mike Stones Bristol Freighter and you'll see much the same thing happening as it tries to land :roll: DM's 2002 Trident couldn't be left to it's own devices using the default flightplanner and AP as this would stall too. Fly it manually or use something like FSNav which keeps a closer eye on speed and sacrifices RoC and it's fine. Looks to me like this is the same thing happening only higher! Thinking about it.. Mike Stones Tristar does it too
I have looked at this many times in the past and found it's down to to much drag in the air/aircraft.cfg files, the ai in cruise loose speed and descend to regain speed climb back to the correct height till it slows again and descends in a unending porpoise fashion, the porpoise approach is flap drag as they extend, dive to overcome the drag then climb to the glide slope till the next flap extension and the proccess is repeated.
I'm sure that drag (or too much of) can be a cause of this but I don't think it's the whole story.. certainly not on DM's Trident (2002). The default sim/flightplanner assumes the max V/S (RoC) right up to it's cruise alt and AI will operate in the same way. This of course is neither practical or possible in the majority of cases. Reducing the drag is one way of combating it but it won't perform as it should (though this doesn't really matter as AI doesn't perform as it should anyway) Reducing the models rate of climb/descent should have the same effect
I must admit I have yet to even try many 'set' flights in FS9 - but the whole AI in FS8 was as Dave and Co point out - unrealstic to say the least. The AI stuff must have had some power!!
In fact, for a good while I went off FS due to the inabilty to fly the way ATC wanted me to - 'Please expedite your climb to FL370' - while I am trying to coax a fat plane upward!
forthbridge wrote:In fact, for a good while I went off FS due to the inabilty to fly the way ATC wanted me to - 'Please expedite your climb to FL370' - while I am trying to coax a fat plane upward!
That is why I tend to fly VFR, to avoid the ATC instructions - especially the multiple turn commands on approach.
This whole porpoising with flaps going up and down in an AI aircraft cruise at altitude is all caused by one simple error in the Aircraft.cfg...it is in the Reference Speeds section and it is the "cruise_speed="...the error is in thinking that this is IAS. This is incorrect as this figure should be TAS. Take this example:-
You can see my edit there...The original was 280 knots and as AI, this aircraft when passing a certain altitude was lowering flap to try to stay in the air at a very low TAS of 280 knots. The need for flap was induced by the very low IAS that he had been forced to reduce to in order to cruise at 280 knots TAS. In fact this aircraft was absolutely unable to reach its proper cruise altitude when used as AI. My edit to make this Cruise TAS 440 knots completely fixed this aircraft's AI performance and it now reaches it's proper flightplanned cruise altitude and with no sign of any flap selection
Thanks Peter, good to know what to check when that problem crops up. May or not be related but is this perhaps the cause of hearing ATC tell an aircraft it is 400 feet below assigned flight level, turnn right heading xxx, climb and maintain, etc? Seems to happen a lot with 747's but since I usually am not close enough to see them, I don't know if they deploying flaps and porpoising.
Thanks for that explanation. I've set up a number of AI flights for formation flying and noticed the same effect on approach where the AI plane takes a dive on finals only to rise up miraculously at the same (slow) speed just before the threshold to then descend to land.
Do you have a "rule of thumb" relationship between IAS and TAS that we could use generically to amend the CFG data you quoted ?
Also, is there any simple way of amending the final approach speed of AI planes ? When I use the same plane (= same FD files) for AI as the one I fly in pursuit, the AI plane always seems to hold final approach speeds that I am incapable of holding with any combination of flaps, power, spoiler and pitch. I end up having to do a "hold" loop to keep a good distance between the two planes. :think:
Reducing the models rate of climb/descent should have the same effect 
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