Odd "Speed" restriction

[Daniel Churchman 1050104]

Problem

 

While in cruise configuration, typically in the ballpark of FL350, I have this strange situation will the autopilot refuses to hold a "reasonable" speed setting. This takes some careful explanation to prevent misunderstanding.

 

Mach hold set for 0.78 on a B738. Prior to adding ISG1 gauges, FSCopilot, FSInn, FSUIPC4, logging in to VATSIM, and using VAFS3 to log the flights for my VA, this is a situation I had done many times before. I would expect to see the power at about 90% N1, maybe a little higher, to maintain this configuration.

 

However, when running all of the above named programs, I find that the autopilot seemingly "ignores" the speed setting, and settles in at around Mach 0.70, give or take a couple of hundredths, with a power setting of about 80 - 83% N1.

 

I believe I have ruled out the ISG1 gauges, because if I start a flight without using any other software, or logging into VATSIM, the aircraft will cruise at the speed I set. I believe that the FSCopilot and FSInn programs, plus FSUIPC4 are running automatically, and if true this suggests they are not responsible either.

 

I see 3 possibilities

 

1. Logging into VATSIM is either directly, or indirectly causing the problem. I had found a curious correlation between the Flight Plan default TAS of 470, and the fact that the max TAS my B738 would do was within +/- 5Kn of the flight plan default value.

 

2. Loading VAFS3 is either directly or indirectly causing the problem.

 

3. Either the ISG1 gauges or FSCopilot/FSInn are being affected by the presence of one or more other programs, and this behavior is thus triggered.

 

To reiterate, it is an apparent malfunction of the autopilot, not an issue of available power. The autopilot says it is set for 0.78, but does not "try" to achieve that speed.

 

Any takers on this one?

[Ivan Kovacevic 920456]

Well, I can [Mod - Happy Thoughts]ure you it's not issue #1 that you brought up. The other two might be the cause. Maybe you should inquire with your VA staff if they have some sort of limits set up that would inhibit your speed setting if above a certain pre-determined value.

[Aaron Fraher 1026851]

Do you use VNAV with the ISG gauges? If so then i presume you know that the VNAV is uses the Autothrottle. Check in the FMS if you have the Cost index/CRZ Mach set at .70. I recall the ISG gauges using very low airspeed values, so i dont use the ISG VNAV.