IAS/Mach Transition

[CRJay 434]

I noticed some weird behavior in the IAS/Mach transition. This normally occurs automatically at the same altitude as 1/2 Bank (31600ft). In the sim version, it happens twice. Once at M.74 and then again at 31600ft. I was climbing, it switched by itself to M.74 around 28800ft, I manually set M.77 instead, it jumped back to M.74 a few times. Once I finally got M.77 firmly set again, the speed was not being kept by the AP with pitch adjustment. Passed through 31600ft at M.79 and the selected speed jumped to M.79 by itself. 

 

I am guessing that it was assumed that having a climb schedule in the FMS will set those speeds, but this is not correct. If the climb schedule is 250/290/.74 the pilot needs to monitor the speed in IAS, and once he/she sees the Mach number above the speedtape reaching M.74, press the IAS/Mach button. If this is not done, the plane will switch automatically at 31600ft. I will try to back this up with video of some kind next time I fly, as it is not described in the FCOM, but probably in one of the technical/avionics manuals.

 

One sort of neat rule of thumb is to climb with 280kts IAS, at 31600ft this will most of the time translate to M.77. And that speed will get you to your ceiling. 

[JRBarrett 675]

8 hours ago, CRJay said:

I noticed some weird behavior in the IAS/Mach transition. This normally occurs automatically at the same altitude as 1/2 Bank (31600ft). In the sim version, it happens twice. Once at M.74 and then again at 31600ft. I was climbing, it switched by itself to M.74 around 28800ft, I manually set M.77 instead, it jumped back to M.74 a few times. Once I finally got M.77 firmly set again, the speed was not being kept by the AP with pitch adjustment. Passed through 31600ft at M.79 and the selected speed jumped to M.79 by itself. 

 

I am guessing that it was assumed that having a climb schedule in the FMS will set those speeds, but this is not correct. If the climb schedule is 250/290/.74 the pilot needs to monitor the speed in IAS, and once he/she sees the Mach number above the speedtape reaching M.74, press the IAS/Mach button. If this is not done, the plane will switch automatically at 31600ft. I will try to back this up with video of some kind next time I fly, as it is not described in the FCOM, but probably in one of the technical/avionics manuals.

 

One sort of neat rule of thumb is to climb with 280kts IAS, at 31600ft this will most of the time translate to M.77. And that speed will get you to your ceiling. 

I just posted a discussion on this very topic in our internal forum, and I believe you are correct - the transition appears to be happening at two points. Interestingly, the avionics system manuals do not discuss this topic either - (I have looked extensively). It appears that (as you noted) the system in the sim is coded to do the transition at two points - 31,600 feet and also at the IAS/Mach climb schedule set in the FMS which may be creating a conflict.
 

I work for an operator of 3 CRJ-200s and asked one our pilots if 31,600 feet is indeed the auto transition point,  but he was not sure, since our SOP climb procedure is 290/.74 and the crews do the switch manually which is usually between FL280 and FL290, so the switchover has already been done before they reach 31.6

 

If there is indeed a bug in this area, it may be too late to include a fix in the upcoming service pack, but we would certainly look at adding it to the next one.

[CRJay 434]

I will not be able to provide a video until a week or two from now, as I currently have a vacation. But our entire fleet has the same behavior as far as I know, both 700s and 900s. I just can't guarantee it is a hard limit at 31600ft, I'm about 90% sure only, haha.

 

Our SOP is generally also 290/.74 but I often opt to fly 280/.77 all the way up to cruise since it is a bit smoother and more comfortable for pax (switching to M.74 usually brings a spike in climb rate and cabin climb rate to keep up). With the bonus that 280kts is also turbulence penetration speed so you are covered for some unexpected turbulence. 

 

I'll report back here if there is no news before I fly again.

[Chris Arndt 4]

40 minutes ago, JRBarrett said:

I just posted a discussion on this very topic in our internal forum, and I believe you are correct - the transition appears to be happening at two points. Interestingly, the avionics system manuals do not discuss this topic either - (I have looked extensively). It appears that (as you noted) the system in the sim is coded to do the transition at two points - 31,600 feet and also at the IAS/Mach climb schedule set in the FMS which may be creating a conflict.
 

I work for an operator of 3 CRJ-200s and asked one our pilots if 31,600 feet is indeed the auto transition point,  but he was not sure, since our SOP climb procedure is 290/.74 and the crews do the switch manually which is usually between FL280 and FL290, so the switchover has already been done before they reach 31.6

 

If there is indeed a bug in this area, it may be too late to include a fix in the upcoming service pack, but we would certainly look at adding it to the next one.

The CRJ550s I fly do go to half bank at 31,600 (10,000 meters). In the FMS per page, the transition altitude and transition speed/mach numbers is how the FMS calculates fuel and climb performance, with IAS being used to calculate vertical flight path below the transition altitude and mach used to calculate vertical flight path above the set transition altitude. 

The IAS won't switch to mach automatically with our software on the way up, but it does switch from mach to IAS on the way down when mach intercepts 300KIAS. 

[JRBarrett 675]

3 minutes ago, Chris Arndt said:

The CRJ550s I fly do go to half bank at 31,600 (10,000 meters). In the FMS per page, the transition altitude and transition speed/mach numbers is how the FMS calculates fuel and climb performance, with IAS being used to calculate vertical flight path below the transition altitude and mach used to calculate vertical flight path above the set transition altitude. 

The IAS won't switch to mach automatically with our software on the way up, but it does switch from mach to IAS on the way down when mach intercepts 300KIAS. 

“Transition” from IAS to Mach, rather than transition altitude. Sorry for the confusing wording. The sim does appear to be auto switching IAS to Mach in climb at both a fixed and variable altitude, which is probably a bug. We will investigate further. Thanks for the report - input from r/w pilots is always helpful.

[Matthew2312 95]

4 hours ago, JRBarrett said:

I just posted a discussion on this very topic in our internal forum, and I believe you are correct - the transition appears to be happening at two points. Interestingly, the avionics system manuals do not discuss this topic either - (I have looked extensively). It appears that (as you noted) the system in the sim is coded to do the transition at two points - 31,600 feet and also at the IAS/Mach climb schedule set in the FMS which may be creating a conflict.
 

I work for an operator of 3 CRJ-200s and asked one our pilots if 31,600 feet is indeed the auto transition point,  but he was not sure, since our SOP climb procedure is 290/.74 and the crews do the switch manually which is usually between FL280 and FL290, so the switchover has already been done before they reach 31.6

 

If there is indeed a bug in this area, it may be too late to include a fix in the upcoming service pack, but we would certainly look at adding it to the next one.

The plane autotransitions from IAS to MACH at 31600ft (and vice versa) only. What you put in the FMS VNAV page has no effect on it. At least for the birds I fly.

[JRBarrett 675]

2 minutes ago, Matthew2312 said:

The plane autotransitions from IAS to MACH at 31600ft (and vice versa) only. What you put in the FMS VNAV page has no effect on it. At least for the birds I fly.

Yes. It appears that the logic is double-coded to work both ways at the moment. We will address this.

[JRBarrett 675]

11 hours ago, Matthew2312 said:

The plane autotransitions from IAS to MACH at 31600ft (and vice versa) only. What you put in the FMS VNAV page has no effect on it. At least for the birds I fly.

This has been fixed, and the fix made it into the new update. Thanks for the report and background info.

[Aerosoft Team [Inactive Account] 51558]

Solved with 1.0.0.1 that has just been released.

[Secondator 643]

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