I believe very little of what I read on the internet.

 

Check out Ken's website: http://www.infinidim.org/

 

He is a RW 777 pilot.

Howdy All

 

I might be reading across posts badly, but I'm not sure that's correct ...

 

NB : I used the following site for the calculations:

- www.hochwarth.com/misc/AviationCalculator.html

 

B772

 

Broadly speaking - for the B772 VMO is 330 knots and M.87 (87% the local speed of sound LSS) at any altitude, limited by the most limiting of either of these restrictions. The red dots (gear and flap notwithstanding) will always indicate the more limiting of the VMO or MMO limitation.

 

At sea level an indicated (calibrated) airspeed on the PFD of 330 KIAS in ISA translates to a TAS of 330 knots as well. At this point the speed of sound is 661 knots and so the calculated MACH of 330 TAS is M.49 - since this is well below M.87 it's the VMO limit that you see.

 

As you increase altitude, temperature and pressure decrease. As such (at least) two things now affect the calculations. Because of decreasing temperature, the local speed of sound reduces because of temperature, until at 27,000 it's about 596 knots. Meanwhile with the aircraft flying a constant 330 knots CAS, the air is less dense (temperature and pressure) and so 330 CAS is 485 kts TAS (note this calculation will vary depending on the deviation from standard temperature at 27,000 ft). Therefore with a TAS of 485 and the LSS of 596 the equivalent aircraft Mach is M.81 - since this is less than M.87 - the 330 knot limit shows on the PFD.

 

At 35,000 ft the LSS is now 576 knots. 330 knots CAS at 35,000 becomes a TAS of 547 knots, which is a Mach of M.95 - and in a 772 we now have a problem. We're above M.87 and so the PFD will not limit speed to 330 knots - but to M.87 (red dots). At 35,000 M.87 is TAS of 501 knots and a PFD CAS of 298. Hence at high altitude MMO is limiting (rather than VMO) and hence the automatic changeover to commanded airspeed (by the FMC and by the MCP) in Mach Numbers rather than indicated airspeed. Just re-iterate - the M.89 limit at 35,000 ft shows as a CAS limit of 298 on the PFD (red dots).

 

In between the last two is a crossover point where 330 Kts CAS equals M.87 in the standard atmosphere. This is 30,476 ft. A more relevant point however is the altitude where 320 kts (normal climb speed CAS) changes to M.84 (normal "high speed" climb Mach) which 30,097 ft.

 

B777-300ER (B77W)

 

The 300ER has a different VMO/MMO to most of the rest of the 777's in that it's VMO is 330 knots at sea level, but increases with altitude until it's 350 knots at 28,900 ft. Meanwhile the Mach limit is M.89. As such you'll see the PFD red dots slowly increase as the aircraft climbs until they are driven by the Mach Limit at about 29,000 ft - at which point the dots will reduce (representing a constant M.89 but a reducing CAS on the PFD) with further climb.

 

It's this last point that the PMDG 777-300ER simulation has incorrect as it reflects a constant VMO from sea level on up, instead of the increasing VMO limit - see the post below for details including a diagram from the AFM.

 

http://forum.avsim.net/topic/458015-bug-report-b777-300er-vmo/

 

Other 777's

 

I can't recall now whether the -200LR/LRF and -300 has the increased VMO/Mach limit. From memory they do, but I would have to go look them up to be sure ...

The 300ER, 200LR and 200F VMO is 330 KEAS/M0.89. As it is defined in terms of equivalent airspeed, the corresponding IAS increases with altitude.

 

For other 777s the VMO is a constant 330 KIAS until it reaches the MMO limit of 0.87 (0.89 for 777-300).

There ya have it, thx for the info.

 

Quite interesting (honestly), but even if that means the PMDG777 LR/F Vmo/Mmo is not correct since it has a fixed 330kt Vmo........I can live with that.

 

Sure, it is a good find amd if they update it that would be good, but for the quality of the simulation, or for how immersive and real everything looks, I could care less.

Also have a look at www.flight.org

Sorry to bring up such an old thread but is there any new information from PMDG about this? Is it something that's being worked on or is it something that can't be fixed?

 

Kind regards,

Sorry to bring up such an old thread but is there any new information from PMDG about this? Is it something that's being worked on or is it something that can't be fixed?

 

Kind regards,

If you have valid information that contradicts the behavior of the PMDG simulation, the best thing to do is to submit that information on a trouble ticket to get it documented in their bug tracking system.  For example, the work going on right now on the NGX incorporates several years of accumulated improvements that are not the result of forum posts but validated information that is accululated in the bug tracking system.

 

Bring it up every month in the forum will not help, but thanks in advance for submitting the information to Product Support.

Ok I made a ticket and since no one else has done it I will post the response so other people don't start threads about it in the future.

 

 

Thanks for the info.  This has already been reported to the team and is being verified by the Tech team.  If confirmed it should be implemented in a future update for the 777

 

Kind Regards,