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Mach Disagree


isaac bogarat
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9 hours ago, Hans Hartmann said:

You're flying Mach 0.817 and the speed bug is below that at Mach 0.77. This doesn't look like a disagreement to me. Neither on the PFD nor the HGS. Or am I looking at the wrong things?

The 2 pictures below are a good representation 

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First full flight today.  I use Weather Force so no sim weather.  Climb to cruise was without issue.  Speeds and Mach were in agreement.  During cruise I started to notice that they did not agree by about 1-2 Mach, however it always cleared up.  It seemed to happen when local pressure changed and the place climbed or descended to match 29.92.  Other than that no problems.

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i believe that happens because it's synched with KIAS at the transitionpoint to mach. So when you keep climbing an offset will occur. if you turn the speedknob one notch and back again it sync again with the KIAS at that moment/altitude.

 

edit: it has done that since day one, i believe it was reported back then.

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I’ve written about this over Here.


The issue isn’t with the Mach Number or the position of the Mach bug on the speed tape; it seems that the IAS indication is incorrect. If you compare your TAS against your Mach number for a given pressure Altitude and temperature, they agree. However, the IAS will disagree.

 

EDIT: I actually wonder how this was programmed the more I think about it. The IAS, Pressure Altitude, OAT, and Groundspeed are the only things that are measured by the aircraft, so Mach Number and TAS should be “inferred” or calculated variables. I find it a bit weird that the IAS indication is the one that’s wrong, but the values calculated from it (TAS and Mach Number) are correct…

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Thanks for all of you proving I'm not totally crazy, and amahran you are smart as a whip, never would've come up with that but it makes perfect sense 

3 hours ago, amahran said:

I’ve written about this over Here.


The issue isn’t with the Mach Number or the position of the Mach bug on the speed tape; it seems that the IAS indication is incorrect. If you compare your TAS against your Mach number for a given pressure Altitude and temperature, they agree. However, the IAS will disagree.

 

EDIT: I actually wonder how this was programmed the more I think about it. The IAS, Pressure Altitude, OAT, and Groundspeed are the only things that are measured by the aircraft, so Mach Number and TAS should be “inferred” or calculated variables. I find it a bit weird that the IAS indication is the one that’s wrong, but the values calculated from it (TAS and Mach Number) are correct…

 

Microsoft Flight Simulator 8_14_2021 12_28_29 PM.png

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15 hours ago, StanTGM said:

Indicated airspeed decreases with altitude, because the pitot measures dynamic pressure (and only that).

 

Okay, go cruising at 35000’ pressure altitude with live weather. Using your TAS, OAT, and Pressure Altitude, calculate what your IAS should be.

 

Now compare that to the actual IAS value shown on the airspeed indicator. You’ll find that there is a significant difference between the two values.

 

I’m familiar with the topic of IAS and that it decreases with altitude due to the thinning of the air; I’m an aerospace engineer and licensed pilot. That’s not what this topic is about, however; this topic is about the fact that the speed tape will show you above a certain Mach number and simultaneously show that your Mach number is higher (or vice versa).

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vor 21 Stunden , amahran sagte:

this topic is about the fact that the speed tape will show you above a certain Mach number and simultaneously show that your Mach number is higher (or vice versa)

 

You’re right - got it now!

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  • 3 weeks later...
On 8/18/2021 at 6:32 AM, Danny Vincken said:

Stupid question maybe but could'nt this be related to the altitude bug from msfs with live weather?

This was an issue with the CRJ since it was released, well before SU5. SU5 made it more prominent, but the speed tape was always off.

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  • 5 weeks later...

I have found in my recent flights that the bug needs a refresh. If it shows (wrongly) 0.77 for example when the actual speed is 0.80, clicking on Mach-to-knots and back knots-to-Mach refresh the bug and it agrees again with the indicated 0.80 Mach speed.

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Hi,

I was about to open a post for this problem when I just saw this one.

 

Well, I noticed the same problem. Setting the bug at Mach 0,74 I will be way less if I match the bug. Of course if I really fly at mach 0,74 then I am above the bug. There is a 0,2 - 0,15 mach difference it seems.

 

In the picture, I matched the bug with my current airspeed of mach 0,744 and the bug ended at mach 0,76.

 

E79A993E-EE7A-4A01-836F-952D8D41492A.thumb.jpeg.6508ea68c5d87dd651b277ef3857c12a.jpeg

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vor 40 Minuten, Sweet19blue sagte:

There is a 0,2 - 0,15 mach difference it seems.

In my observation, the difference actually increases when climbing higher.

 

Btw, does someone know which indication is correct, the mach number or the number next to the speed bug?

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11 hours ago, SinusJayCee said:

In my observation, the difference actually increases when climbing higher.

 

Btw, does someone know which indication is correct, the mach number or the number next to the speed bug?

Did you try to set the bug to knots and back to Mach? If you do, the bug will get updated to current ALT, temp, pressure and the tape Mach and the bug Mach will agree to the actual speed. I believe the tape measurements (above it) is the correct one

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vor 10 Stunden , Gilbnx sagte:

Did you try to set the bug to knots and back to Mach? If you do, the bug will get updated to current ALT, temp, pressure and the tape Mach and the bug Mach will agree to the actual speed. I believe the tape measurements (above it) is the correct one

I will try the next time! This suggests that the bug value is indeed wrong and gets set to a fixed knots value (or whatever) when switching to Mach.

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I wonder if this has anything to do with the changes in the Sim with the Atmospheric Pressure changes ....   They updated the Sim to model an accurate Pressure atmosphere (hence the issues on Vatsim where the plane is not at the altitude of other sims).

 

Since it no longer basis the the model on a standard Lapse Rate it is constantly updating and changing .... hmm... neat that if you flight back to IAS and then Back to MACH it will trigger it to "check" the calculation again...

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6 hours ago, LesOReilly said:

I wonder if this has anything to do with the changes in the Sim with the Atmospheric Pressure changes ....   They updated the Sim to model an accurate Pressure atmosphere (hence the issues on Vatsim where the plane is not at the altitude of other sims).

 

Since it no longer basis the the model on a standard Lapse Rate it is constantly updating and changing .... hmm... neat that if you flight back to IAS and then Back to MACH it will trigger it to "check" the calculation again...

I believe it's exactly what you have described. I've seen VATSIM maps keep reporting me flying FL380 (MSFS reporting 38,200ft) where my alt is set to FL370. Other aircrafts (guessing xPlane, FSX? based on aircrafts models) were reporting close to FL370. Luckily controllers get the transponder alt reporting which will still show FL370 in this case.

And yes, I checked yesterday - on a straight and level flight, at FL400, I saw that "refreshing" the bug value (switching to knots and back to Mach) kept getting different values throughout the cruising period due to change in local atmospheric pressure

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