Visit Captain Sim web site  
  Welcome, Guest. Please Login or Register

 

Page Index Toggle Pages: 1 [2]  Send TopicPrint
 25 767 Overspeed issue. Please help! (Read 4644 times)
Alex T
Senior Member
*
Offline



Posts: 415
Location: Boston, MA
Joined: Jun 21st, 2015
Gender: Male
Re: 767 Overspeed issue. Please help!
Reply #15 - Apr 20th, 2021 at 2:52am
Print Post  
Just noticed in your CDU that the first pseudo waypoint of 800ft MSL was missing, I reselected the SID to get it back in there. Not sure if you deleted it or what, but it was causing a confusion to the VNAV.
  
Back to top
YouTube  
IP Logged
 
youngie767
Full Member
*
Offline



Posts: 9
Joined: Apr 14th, 2021
Re: 767 Overspeed issue. Please help!
Reply #16 - Apr 20th, 2021 at 3:34am
Print Post  
Thanks Alex, haven't tried a de-rate departure as yet and may be the key as you say. Other issues l've found is the FMC calculated climb and descent speeds - way over the top so having to adjust those (i'm using 280/.78 as my default). Also getting a massive overspeed during the descent (around FL200) when using VNAV. I'll try the next one using FLCH.
Thanks for the help.

Cheers Steve
  

FMC_Climb.jpg ( 310 KB | 78 Downloads )
FMC_Climb.jpg
Back to top
 
IP Logged
 
Alex T
Senior Member
*
Offline



Posts: 415
Location: Boston, MA
Joined: Jun 21st, 2015
Gender: Male
Re: 767 Overspeed issue. Please help!
Reply #17 - Apr 20th, 2021 at 3:51am
Print Post  
I noticed that as well! You do have a pretty high cost index (they need to fix this too) I saw the weird speed in the CDU LEGS page so I reselected the SID the climb speeds went back to normal.
  
Back to top
YouTube  
IP Logged
 
fowler
Full Member
*
Offline



Posts: 72
Joined: Oct 31st, 2009
Re: 767 Overspeed issue. Please help!
Reply #18 - Oct 22nd, 2022 at 3:28am
Print Post  
I have the same issue. I can't get the A/T to follow the profile. Normally you can set your cruise target speed either by entering the cruise mach speed in the FMS and use VNAV, or bypass the FMS cruise speed by using the speed mach mode of the A/T.

Neither method work, and what happen is that the A/T is chasing it's displayed EPR (in green) and not the target speed. I tried switching on/off/on various automation (VNAV, A/T, Speed mode, FMS entry etc.) but the A/T always chase the EPR limit and not the target speed.


I also tried to put the trottle either in min/max and hitting F1 or F4 to syncronize, put less sensivity and more deadband in the throttle calibration, but it didnt change nothing. If the A/T is on it will always put the aircraft in overspeed

I am at loste here...
  
Back to top
 
IP Logged
 
fowler
Full Member
*
Offline



Posts: 72
Joined: Oct 31st, 2009
Re: 767 Overspeed issue. Please help!
Reply #19 - Oct 22nd, 2022 at 3:49am
Print Post  
Another weird thing is how the aircraft react to thust change when A/T is off. In cruise when you do a very slight power up adjustment you can see the aircraft constantly accelerate for 10' minutes long and more. I got from 0.82 mach to 0.86 mach in more than 15 minutes with a very limited power change when I was stable at 0.82 mach.

This is really unrealistic because the air resistance is not a linear function but a square function of speed. And that means that for a slight power change you should see a slight speed change rapidly that get amortized quickly and settle in the next equilibrium state of power/drag ratio. There should't be a large change of speed at a low rate how it occurs here.
  
Back to top
 
IP Logged
 
Alex T
Senior Member
*
Offline



Posts: 415
Location: Boston, MA
Joined: Jun 21st, 2015
Gender: Male
Re: 767 Overspeed issue. Please help!
Reply #20 - Oct 23rd, 2022 at 11:49pm
Print Post  
fowler wrote on Oct 22nd, 2022 at 3:28am:
I have the same issue. I can't get the A/T to follow the profile. Normally you can set your cruise target speed either by entering the cruise mach speed in the FMS and use VNAV, or bypass the FMS cruise speed by using the speed mach mode of the A/T.

Neither method work, and what happen is that the A/T is chasing it's displayed EPR (in green) and not the target speed. I tried switching on/off/on various automation (VNAV, A/T, Speed mode, FMS entry etc.) but the A/T always chase the EPR limit and not the target speed.


I also tried to put the trottle either in min/max and hitting F1 or F4 to syncronize, put less sensivity and more deadband in the throttle calibration, but it didnt change nothing. If the A/T is on it will always put the aircraft in overspeed

I am at loste here...


Can you make a video or screenshots of this here?
  
Back to top
YouTube  
IP Logged
 
Alex T
Senior Member
*
Offline



Posts: 415
Location: Boston, MA
Joined: Jun 21st, 2015
Gender: Male
Re: 767 Overspeed issue. Please help!
Reply #21 - Oct 23rd, 2022 at 11:59pm
Print Post  
fowler wrote on Oct 22nd, 2022 at 3:49am:
Another weird thing is how the aircraft react to thust change when A/T is off. In cruise when you do a very slight power up adjustment you can see the aircraft constantly accelerate for 10' minutes long and more. I got from 0.82 mach to 0.86 mach in more than 15 minutes with a very limited power change when I was stable at 0.82 mach.

This is really unrealistic because the air resistance is not a linear function but a square function of speed. And that means that for a slight power change you should see a slight speed change rapidly that get amortized quickly and settle in the next equilibrium state of power/drag ratio. There should't be a large change of speed at a low rate how it occurs here.



I'll have to admit that I don't agree with this.

In real life, I fly an airliner which weighs a bit less than the Boeing 767, but has a very similar thrust to weight ratio. This is one of the scenarios we do in training to show the lack of excess thrust in high altitudes. In the another company simulator, we start at 250KT/0.75M at FL350 more or less, then we set the thrust to max power and see how long it takes to get to 270KT/0.80M, and it generally takes 90-120 seconds. If this is at MAX power, it almost makes sense that a slight power change would cause a 0.04M increase in 10 minutes. This issue isn't as much air resistance but lack of excess thrust. If you have excess thrust, yes it will equalize at a particular speed because of the exponential increase in drag, but how do you know how that is with a PW4060 at FL350 at 300,000lb, for example?
  
Back to top
YouTube  
IP Logged
 
Jaehang
New Member
Offline



Posts: 3
Joined: Jun 15th, 2019
Re: 767 Overspeed issue. Please help!
Reply #22 - Oct 24th, 2022 at 2:49pm
Print Post  
That's wierd
  
Back to top
 
IP Logged
 
fowler
Full Member
*
Offline



Posts: 72
Joined: Oct 31st, 2009
Re: 767 Overspeed issue. Please help!
Reply #23 - Oct 31st, 2022 at 5:33am
Print Post  
I only did two flight with this new B767 and couldn't lock the A/T to either follow the speed mode (in mach as I'm talking about the cruise portion) or the VNAV mode. (Both modes set to M082.

In fact the reason of the "infinite acceleration and thus overspeed" comes from the fact that the A/T constantly goes for max cruise setting (displayed in green) which is way to much and will result in a overspeed.

BTW I tried it with the B757 on the same flight and I have the exact same issue.

Does it work for you Alex ?

  
Back to top
 
IP Logged
 
fowler
Full Member
*
Offline



Posts: 72
Joined: Oct 31st, 2009
Re: 767 Overspeed issue. Please help!
Reply #24 - Oct 31st, 2022 at 5:46am
Print Post  
Alex in your real sim scenario you talk about applying max power (is it max CONT ?) from a low speed situation at altitude. I talked about starting from the same situation but with applying very minimal increase of thust (let's say from 1.20 to 1.21 EPR) and seeing the aircraft accelerate far too long. It's not only the duration of acceleration which is the issue it is also the increase of acceleration. It seems that the rate of acceleration goes higher which is the part I find unrealistic. For any excess thrust applied in level flight there is an acceleration in speed that must be dampened. i.e. the rate of this acceleration is diminishing) due to squared function of air resistance. I have no idea of what the final accel will be on any scenario this must be shown on aircraft perf. manual or engine data.

  
Back to top
 
IP Logged
 
fowler
Full Member
*
Offline



Posts: 72
Joined: Oct 31st, 2009
Re: 767 Overspeed issue. Please help!
Reply #25 - Oct 31st, 2022 at 6:38am
Print Post  
OK I think I found the issue I have, it has to do with my joystick. Once in cruise I unplugged my joystick and immediately the A/T was nailing the commanded speed. I also tried some speed change (M082 to M080 to M082) and the A/T responded immediately and nailed the new commanded speed) Strange I calibrated my joystick but I suspect a "drifting" button somehwere interfering with the A/T.
  
Back to top
 
IP Logged
 
Page Index Toggle Pages: 1 [2] 
Send TopicPrint
 
  « Board Index ‹ Board  ^Top