15 hours ago, Claudius_ said:

I'm not satisfied with your explanation because I think that you are not assuming the lack of power of the engines at higher altitudes, the mach number you are talking about is varying with the altitude: more altitude, more higher the mach number because the air density, temperature, etc. But this has nothing to do with the vertical speed, where you can see a very strange climbing jump where the engines are struggling to push the mass of a full loaded airplane when closing to the max planned altitude. Secondly, but not least important, this Ifly 737 is just the only one sim airplane behaving in this way, maybe the other devs are all wildly wrong on this aspect?

Stearmandriver flies the real aircraft and knows what he is talking about.

The Mach number does not vary - the autopilot/autothrottle is programmed to hold a constant Mach number in climb, set by the pilot during performance initialization on the VNAV climb page or automatically set based on the cost index, and that is why you see the effect you describe.

The relationship between Mach and IAS is absolutely fixed. At a given pressure altitude, a specific IAS will always equal a specific Mach number regardless of air temperature. The higher the aircraft climbs, the lower the IAS will be for a given Mach number. True airspeed on the other hand does vary with air temperature, but autopilots do not calculate climb performance based on on TAS - they use IAS or Mach.

In the initial climb, the aircraft is targeting a specific IAS, using a calculated power setting and varying the pitch to hold the selected IAS. There will come a point where the selected IAS will equal the target Mach number set in the VNAV climb page. That is called the “crossover altitude”. It usually occurs somewhere between 26,000 to 30,000 feet.

Most aircraft will automatically switch from holding a specific IAS to holding a specific Mach at the crossover altitude.

Because the required IAS to hold a constant Mach number decreases as the aircraft climbs, the autopilot will pitch up to decrease IAS in favor of Mach,  which has the effect of increasing vertical speed when the switch between IAS and Mach occurs.

As the aircraft climbs higher, available thrust does decrease due to lower air density. Air density is also very dependent on air temperature which (in the summer months) can be quite high (relatively speaking) at higher altitudes. There will come a point where vertical speed decreases even holding a fixed Mach number, but the increase in vertical speed at the IAS/Mach crossover point is often quite noticeable.

You say that no other sim aircraft does this but I disagree. The Fenix A320 does it, as does the Aerosoft CRJ. Left to its own devices the real CRJ normally switches to Mach hold at a fixed altitude of 31,600 feet, but pilots can (and do) switch to Mach hold mode manually in the CRJ once current desired Mach for further climb is reached.

And the absolute “gold standard” for real world accuracy of sim aircraft, the Aerowinx PSX 747-400, definitely does it.

I am not an airline pilot but I am am avionics and air data specialist, who has worked on many types of airliner autopilot and auto thrust systems. I am intimately familiar with the mathematical computations that an autopilot does (based on current air data) to achieve specific climb performance, and I can assure you that an increase in vertical speed when switching from IAS hold to Mach hold is not a bug. It often happens in r/w aircraft. 

26 minutes ago, JRBarrett said:

Stearmandriver flies the real aircraft and knows what he is talking about.

The Mach number does not vary - the autopilot/autothrottle is programmed to hold a constant Mach number in climb, set by the pilot during performance initialization on the VNAV climb page or automatically set based on the cost index, and that is why you see the effect you describe.

The relationship between Mach and IAS is absolutely fixed. At a given pressure altitude, a specific IAS will always equal a specific Mach number regardless of air temperature. The higher the aircraft climbs, the lower the IAS will be for a given Mach number. True airspeed on the other hand does vary with air temperature, but autopilots do not calculate climb performance based on on TAS - they use IAS or Mach.

In the initial climb, the aircraft is targeting a specific IAS, using a calculated power setting and varying the pitch to hold the selected IAS. There will come a point where the selected IAS will equal the target Mach number set in the VNAV climb page. That is called the “crossover altitude”. It usually occurs somewhere between 26,000 to 30,000 feet.

Most aircraft will automatically switch from holding a specific IAS to holding a specific Mach at the crossover altitude.

Because the required IAS to hold a constant Mach number decreases as the aircraft climbs, the autopilot will pitch up to decrease IAS in favor of Mach,  which has the effect of increasing vertical speed when the switch between IAS and Mach occurs.

As the aircraft climbs higher, available thrust does decrease due to lower air density. Air density is also very dependent on air temperature which (in the summer months) can be quite high (relatively speaking) at higher altitudes. There will come a point where vertical speed decreases even holding a fixed Mach number, but the increase in vertical speed at the IAS/Mach crossover point is often quite noticeable.

You say that no other sim aircraft does this but I disagree. The Fenix A320 does it, as does the Aerosoft CRJ. Left to its own devices the real CRJ normally switches to Mach hold at a fixed altitude of 31,600 feet, but pilots can (and do) switch to Mach hold mode manually in the CRJ once current desired Mach for further climb is reached.

And the absolute “gold standard” for real world accuracy of sim aircraft, the Aerowinx PSX 747-400, definitely does it.

I am not an airline pilot but I am am avionics and air data specialist, who has worked on many types of airliner autopilot and auto thrust systems. I am intimately familiar with the mathematical computations that an autopilot does (based on current air data) to achieve specific climb performance, and I can assure you that an increase in vertical speed when switching from IAS hold to Mach hold is not a bug. It often happens in r/w aircraft. 

Thank you for your dissertation about the Mach number and the IAS, but if you my posts carefully my doubt is regarding the engine envelope (see power variation with altitude), and how the Ifly sim plane V/S is varying close to the max altitude. The IFly is just the one that behaves in that way, and Stearmandriver told me that the other devs/airplanes are wrong and that I'm a "little too accustomed sim aircraft". In my opinion Ifly is wrong on this aspect, not the other devs.

Edited June 14, 2025Jun 14 by Claudius_

28 minutes ago, Claudius_ said:

Thank you for your dissertation about the Mach number and the IAS, but if you my posts carefully my doubt is regarding the engine envelope (see power variation with altitude), and how the Ifly sim plane V/S is varying close to the max altitude. The IFly is just the one that behaves in that way, and Stearmandriver told me that the other devs/airplanes are wrong and that I'm a "little too accustomed sim aircraft". In my opinion Ifly is wrong on this aspect, not the other devs.

I just purchased the IFly last night for MSFS 2024 and will fly it for the first time later today. I will see what it does…

Haven't been able to get the panel to even function or come alive. No switches even move.

1 hour ago, lenny777 said:

Haven't been able to get the panel to even function or come alive. No switches even move.

Exactly which Panel are you talking about? 

5 hours ago, Farlis said:

Another issue I have is that this thing will not obey the FL100 speed restrictions in VNAV descent. If you are above profile it continues to stay in VNAV PATH instead of going into VNAV SPD and thus blows the restriction.

I don't remember the NG's doing that.

It's dependent on location and configuration.  Inside the IF or after you've extended any flaps, it'll remain in VNAV Path.  This is correct behavior, and you're right that the NGs didn't do it - they were modeling an older version of VNAV.  

There are times when I think the iFly takes a little longer than it should to change to VNAV Spd if outside of the above conditions, but it will do it - when it's supposed to.

3 minutes ago, Stearmandriver said:

Inside the IF or after you've extended any flaps, it'll remain in VNAV Path. 

Well 35 Miles out at FL120 is not exactly meeting these conditions. It should slow down by pitching up when desending below FL110 and it just doesn't do that.

2 hours ago, Farlis said:

Well 35 Miles out at FL120 is not exactly meeting these conditions. It should slow down by pitching up when desending below FL110 and it just doesn't do that.

It definitely does - if it's supposed to.  You well could have been inside an intermediate fix on a long approach at that point.  It's not impossible that something on that procedure confused it, or the box wasn't set up properly (if you were flying a non-US livery for instance, the decel point could be set differently by default).  Without seeing the specific case, hard to say.... But in general, yes of course VNAV respects 10k.  It shouldn't need to revert to VNAV speed to do it, either.  

Anyone having issues capturing the ILS with the iFly in 2024? Never had this issue in 2020. Haven't seen this issue on every flight, but I experienced it at Flytampa KBOS and BMWorld & Amsim KMIA.

I had already two flights that the plane started rocking around from left to right on HDG mode in autopilot

9 hours ago, Claudius_ said:

Thank you for your dissertation about the Mach number and the IAS, but if you my posts carefully my doubt is regarding the engine envelope (see power variation with altitude), and how the Ifly sim plane V/S is varying close to the max altitude. The IFly is just the one that behaves in that way, and Stearmandriver told me that the other devs/airplanes are wrong and that I'm a "little too accustomed sim aircraft". In my opinion Ifly is wrong on this aspect, not the other devs.

I am flying from KELM to KBNA at FL360. Target IAS in climb was 285 knots and target Mach number was .791 The crossover was at FL319. Prior to the IAS/Mach crossover, the vertical speed was about 1000 FPM. At the crossover the IAS target started dropping (absolutely correct) and the aircraft started to pitch up to follow the IAS target. Even when Mach hold is the active speed mode, the autopilot is still tracking IAS at all times. This resulted an increase of vertical speed from 1000 FPM to 1400 FPM.

The important thing to understand is that an airliner autopilot, operating in VNAV SPD mode in climb, is using PITCH to control airspeed, with engine thrust being secondary. If the autopilot needs to track a steadily dropping IAS, it will do so by pitching up, which has the result of increasing vertical vertical speed.

That is 100 percent correct behavior for either the sim or real aircraft

Engine N1 at this point was about 92 percent. When it leveled out at FL360 the IAS had dropped to 262 knots which is the IAS equivalent of Mach .791 at the pressure altitude of FL360. Engine power rolled back to 84 percent N1.

Optimal altitude is shown as FL380 but I am sticking to FL360 because the headwind is less.

Air temperature right now is -49C which is ISA +6. Perfectly normal for summer.

By the way, I flew the PMDG 777F on this very same route earlier today. Despite your insistence that “no other developer’s aircraft” show an increase in vertical speed when switching to Mach mode, the 777 certainly did. The speeds were different: 310 knots IAS climb up to the crossover and a target Mach of .83, but the increase in vertical speed at the IAS/Mach crossover was very evident. 

I hand fly literally every flight, because I do only short flights, so I haven't had any issues related to autopilot. If I wasn't so lazy, I probably would have tried it by now, but that is odd that it causing rocking. I actually flew the bird into KBOS about an hour ago, and it was fantastic.

Cheers, Pete

4 minutes ago, JRBarrett said:

In my opinion Ifly is wrong on this aspect, not the other devs.

Well in my opinion you can't admit when you are wrong.

Yes, the handflying is great, really like this plane.

Maybe it is the Sim Beta I am trying which is causing problems.

Also when pointing the mouse to another screen, the generators turned off and autopilot like five times in a row and the engines when to 0 percent. But then I could turn it on again but when going to another screen it happened again.

Edited June 14, 2025Jun 14 by rob0203

1 hour ago, Farlis said:

Well in my opinion you can't admit when you are wrong.

Did you quote the wrong person.. or are you saying @JRBarrett is wrong? I think you didn't mean that.