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Are you flying with a tailwind? Remember that the posted cruise speed for any aircraft is its calm-wind speed, so if you're running at .74 mach with a tailwind you can easily overspeed, even if your throtte is only at 80%. I try to keep my '37 speeds between 290 and 310 KIAS depending on the winds aloft. I use the Shift-Z key combo (first press) to show lat-long, heading, wind speed/direction and current indicated airspeed. It lets me know if I'm flying in a headwind, tailwind, or quartering wind. :)
I'm sorry but this misleading and simply inaccurate. Typically airline pilots use the autothrust system to manage airspeed control, rather than I suppose a typical FSX player using his/her throttle. I say this as a RL A320 guy that sits in the left seat.
The aircraft cruise airpseed performance is not calm wind specific. If it takes me 88% N1 to cruise at .78 westbound (typically into the wind), it will take me 88% N1 to cruise eastbound at .78 with a tailwind. The difference will be in my ground speed, not my true airspeed.
To the OPs original question, the OVERSPEED may have something to do with your Landing Gear or Flap configuration rather than something related to your engines. A clean (gear and flaps up) airplane should not overspeed at .75 mach. I've got zero time in the 737, but I had nine years in the other Boeing workhorse, the 757.
For my airplane, 275 KIAS is the turbulence penetration speed above FL200 until that KIAS intersects .76 mach. This usually happens around FL310. So between FL200 and FL310 it's 275 KIAS and then it's .76 above FL310.
So unless we know what triggered the OP's overspeed, we're all just guessing.
HTH
Second - discussions about how well FSX models the real thing aside it does appear that the configuration in the (default) aircraft.cfg file is wrong for this and several other default aircraft. See: https://sites.google.com/site/flightsimulatorxtra/Home/fsx-improvements/correct-premature-overspeed-warnings-in-fsx
For a little bit of reading on very high altitude flight you might be interested in this: http://code7700.com/coffin_corner.html
Editing the aircraft.cfg file is easy. Open it in Notepad. Go to the section you want to edit (in this case the information in the section [Reference Speeds] and adjust the Vmo and Mmo to match the real thing. Note the use of the double forward slash ( // ) after the max_mach and Max_indicated_ speed parameters. This is good practice - the '//' comments out anything that follows it so you can, as this web site shows, keep the original number or other parameter in case you want to go back and reset it to the default. You can also add other information (like noting that speeds are in knots TAS).
If you try this and the changes won't save that is a permissions problem with Windows. The ultimate solution is to change the permissions for the folder but the easy way around this for a one-time change (and hopefully once you get into making modifications in the aircraft.cfg files you will start fixing many of the flight dynamics flaws in FSX) is to save the docuement to some other folder (like your desktop) and then copy it back into the FSX | Sim Objects | Airplanes | [whatever airplane you are fixing] folder.
Dan
www.elitepremairvirtual.com
FWIW, you are correct that IAS (indicated airspeed) is displayed on the Primary Flight Display, pilots also can read the TAS (true airspeed) on the Nav Display. The only reason I know TAS is there is I often compare the GS (ground speed) which is displayed next to it. TAS is a curiousity, not really all that useful to me like the ground speed. But your point is well made, we fly in reference to IAS rather than TAS.
True, we're all guessing, but you're applying real world data to explain a phenomenon in FSX that is not occurring under real world conditions. So am I truly misleading the OP, or posting a simple hypothesis pertaining to the behavior of an unrealistic aircraft moving through unrealistic air? ;)
APUtech
I cannot tell you why this happens in the 737, but others have had it, and it seems to be fixed by setting IAS and not TAS.
There was a time when TAS was required to be calculated to then calcualte gs (or to be used for gs if winds aloft where unknown) for navigation. Now that we have radar/gps/navaids aircraft know where they are, TAS is not really important, as IAS is what a pilot needs to know regarding maneuverability and stall speeds. V-speeds[en.wikipedia.org] these days are IAS for this exact reason.
Well yea, what does the code do?
But we can relate this to real world situations, which is what the similator is trying to simulate.
The code trys to mimic real world. In FSX--------------
KIAS - Knots Indicated Air Speed - basically how fast your aircraft is moving in relation to the air around it
KTAS - Knots Total Air Speed - is approximatly the speed over the ground. I am posting about KTAS and not True Airspeed (TAS = The actual speed of an aircraft through the air ) since it as been said tailwinds cause overspeeds of the airframe.
As the aircraft climbs higher, the KTAS becomes much faster than the KIAS. An airplane flies or stalls or overspeeds by KIAS - not by it's relationship to how much ground it is covering. Weather/winds have a big impact on how much ground is covered how fast.
In real world and FSX it's possible to fly at 100 kts KIAS and not move forward if you are flying into a 100 kts head wind.
It's also possible to fly at 150 kts over the ground and stall because the plane is flying too slow relative to the air around it with a 100 kts tail wind.
Tail winds in FSX DO NOT cause overspeeds of the airframe but only get you where you are going faster. If tail wind causes overspeed of the airframe there are a lot of planes out there coming apart in mid air.
The GPS is the easiest way to check how fast you are covering the distance between flight legs (GS in the lower corner). If flying the default B737-800 there is a wind direction indicator inside the MFD gauge. It also includes the TRUE AIR SPEED (TAS) and the ground Speed (GS)
East bound in a Jet? Then fly the Jet Stream. Tailwinds of 150-200 MPH can add a lot of speed to the TAS giving you a GS of more then 750 MPH.
APUtech
Ground speed (GS) is the sum of the true air speed (TAS) plus the wind factor - the component causing a resultant headwind or tailwind and the component requiring heading deviation ("crab angle") to maintain the desired track - this also costs your forward motion - do the calculation for a pure crosswind, you will see that your forward velocity is reduced to the amount equalt to your TAS times the cos(deviation). First Law of Thermodynamics (conservation of energy) still holds - There is not free lunch. Not much of a cut in most cases - maybe a percent or two - but still affects the groundspeed. In reality most of the time this can be ignored and the tailwind or headwind component only applied to the TAS to get GS. Also, some aircraft have GS on the MFD and, as pointed out earlier, this can also be seen on the GPS.
IAS is just that - what is indicated on the speed tape, airspeed indicator, whatever shows you the speed in the particular aircrraft you are flying. Most of the commercial carrier turbofan and larger turboprop aircraft also display the TAS (typically on the MFD - note that the default CRJ-700's TAS is not displayed correctly but there is a fix for this on Fly Away Simulation's site). One rule of thumb for TAS is to take half your Flight Level and add it to your IAS. So, flying at 28,000 feet (FL 280) with an IAS of 280 knots, 1/2 of 280 = 140 so 280 + 140 = 420 knots. There are other factors that affect TAS but this rule of thumb does give a fair approximation.
For mspbusdriver the rule of thumb of increasing your IAS by 2% per 1000 feet of altitude (and, if you really want to be accurate, 1000 ft. of density altitude) - so for an IAS of 150 knots and an altitude of 10,000 ft the TAS would be 20% greater (2% for every 1,000 ft of altitude) or 180 knots. The 2% rule is an easier and a bit more accurate one for lower altitudes.
Higher altitudes mean lower air density and lower temperature. IAS is based on the ambient conditions so at high altitude IAS is lower than TAS due to the lower density of air. Forces on the aircraft are also dictated by air density so we fly using IAS - hence Vne, Vs, Vo etc. remain the same - always based on IAS.
I agree with what you say. I should have used different letters for total airspeed instead of KTAS.
My point is, the only guessing is we don't know what the OP was doing and how the aircraft was confiqured to create the airframe overspeed at .75 mach. In FSX it is not the GS due to tail winds as stated by others.
This picture speaks for itself in the way FSX works when there is a 199 kts tail wind at .75
mach
FL310
winds 227 @ 199
Mach .75
IAS 283
TAS 447
GS 646
MPH 742.9 over the ground
http://s3.postimg.org/45qablgqb/f10.jpg