Sign in to follow this  
Ken.Pascoe

Bug Report : IAS/Mach Changeover

Recommended Posts

Howdy All

 

When climbing in FLCH or VS/FPA (or in VNAV with speed intervention selected) at an IAS, the speed window (and the selected value on the PFD) should change to MACH as the IAS reaches the equivalent of M.84 - this doesn't seem to be happening.

 

FCOM 4.10.05/06 refers:

 

IAS-Mach-Changeover-FCOM.jpg

 

Instead, what I'm seeing is:

 

- Ops normal FLCH SPD climb at 320 KIAS until the equivalent Mach reaches about .835

- Suddenly the IAS jumps 6 knots or so (to 326 IAS); the equivalent Mach jumps to .851; the selected IAS remains 320.

- Then the speed windows changes across to M.84 (as well as selected magenta speed on the PFD)

- Aircraft pitches to reduce speed back to selected M.84

 

The changeover should be much smoother that this. The only change noticeable really (apart from the visual indications) is that the aircraft pitches up slightly as it maintains constant mach and reducing IAS. At least a couple of times, the IAS hasn't changed to Mach - but I can't reliably reproduce the problem yet.

 

... just a bit odd.

 

IAS-MACH-1-large.jpg

 

IAS-MACH-2-large.jpg

IAS-MACH-3-large.jpg

 

 

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

 

 


Howdy All

 

Ken,

 

As I've stated in your other posts, submit these as tickets, otherwise they aren't guaranteed to be seen.

 

Additionally, it's a forum rule to sign your name to the body of your posts. While I'm guessing your user name is your real name, please configure a signature to be automatically added to your posts. Top right of the screen, click on your user name, click My Settings, click Signature over on the left.

Share this post


Link to post
Share on other sites

I BELIEVE that's altitude based and the speed converts to mach after FL240 or FL250. Note please that i said BELIEVE, as I'm about as far from being an expert in the PMDG 777 as one can be.

 

 

 

 

.

Share this post


Link to post
Share on other sites

Hey Kyle

 

- Apologies, after I was allowed to post I went in to add a signature - but wasn't allowed to for some arbitrary period and forgot to go back and do it.

- I've raised support tickets at PMDG for these and the others.

 

Hi Dave

 

When climbing at an IAS, the changeover occurs when the Mach equivalent of your indicated airspeed equals .84 - at this point the MCP speed selector changes to Mach (.84) and climb continues. On descent, the reverse happens at 310 knots (that is, descent at .84 will be continued until indicated mach equals 310 knots, at which point the MCP speed selector will change to 310 knots and continue the descent). The exact altitude of the changeover will depend on OAT.

Share this post


Link to post
Share on other sites

I don't see this happening. I get a perfectly smooth switch from IAS to Mach. I wonder if something external is causing the airspeed jump (weather related maybe) coincidentally at the point where the changeover takes place.

 

Airspeed jumps due to a disturbance, which the AP will reduce with pitch. Also the AP sees this airspeed as being above the equivalent to Mach .84 and switches to Mach mode. Because the speed blip happens near the changeover point it makes it look like the two things are related.

Share this post


Link to post
Share on other sites

 

 


Hi Dave



When climbing at an IAS,

 

Thanks Ken, good to know information. I spend the majority of my time with indepth research and operations in the MD-80 and Dash 8/Q400 (given that they are shared cockpit (human pilot and co-pilot) capable, and shortly I'll be working on the development end of the B717 systems and possibly the MD-88, so I've not even had time to scratch the surface in the 772.

Share this post


Link to post
Share on other sites

Just tried this with no wind and the changeover was at about 30 000 feet and was really smooth, I think a jump in airspeed could be caused by a wind shift or an extremely light payload where the Mach is decreasing quickly due to the high rate of climb.

Share this post


Link to post
Share on other sites

As allready mentioned, the jump from IAS to Mach will occur when M.84 is reached.

Since Mach changes with temperature, if you have a weather addon that causes sudden temp jumps in the Atmosfere, then the changeover will not be smooth.

 

So check if your OAT jumped when you see that speed jump.

And then try without a weather engine.....just use FSX own weather interface to set a weather theme.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this