Boeing 737NG VMO/MMO (barberpole) moving up with increasing airspeed

Flutter speed · 26th Jun 2012, 08:25

I noted on the PFD of a 737NG level-D training simulator, that accelerating the plane in level flight slightly changes the red/black squared VMO/MMO (maximum operating speed) indication on the speed-tape.
My precise observation was that for example flying at FL350, accelerating the aircraft from 250 to 270 kts, moved the barberpole up several knots.
I thought the VMO/MMO should be independent of airspeed itself? Anyone who can shed light on the way VMO/MMO is calculated and how this is in the real aircraft?

Many thanks!

ps: I am talking about a clean aircraft. I know that flaps, gear can have effects on the VMO/MMO.

InSoMnIaC · 26th Jun 2012, 10:11

try the same experiment at FL200 and I bet the barber pole will stay at your VMO.

at FL350 you Fly MN not IAS. The barber pole readjusted as you increased speed to give you a more accurate representation of where your MMO is. the MMO value doesn't change

OK465 · 26th Jun 2012, 16:09

NG Level D, Standard atmosphere, FL350, no turb, acceleration from 250 KIAS to 270 KIAS...

The VMO/MMO indication remains absolutely steady at 280 KIAS.

You're not talking about the 0.3G "maneuver" margin amber hook are you?

(one of us may have a potential sim problem

)

root · 26th Jun 2012, 16:29

Shitty sim software.

Flutter speed · 28th Jun 2012, 08:23

Thanks a lot for the replies!

@InSoMnIaC: at flight level 200 the effect is still there, though less prone (the effect decreases with altitude)

@OK465: thanks, that is helpful

I am not talking about the amber 'hook'. It is really the barberpole moving. I attached a picture. I just have so many opinions from an equal number of pilots... My opinion is that it should not move because it is confusing. Though, I also know that the maximum selectable speed (e.g. MCP SPD)comes from the EDFCS, while the maximum displayed speed (barberpole) comes from the ADIRU and is calculated dynamically with MMO as a basis (0.82 for FL 350).

@root

InSoMnIaC · 28th Jun 2012, 08:46

Quote:

at flight level 200 the effect is still there, though less prone (the effect decreases with altitude)

How much less prone can it be at lower altitude when at FL350 it is only 3 kts.

with gear up and flaps up, doesn't the barber pole just sit at 350kts below the IAS/MACH transistion altitude?

Flutter speed · 28th Jun 2012, 08:58

I am able to read out decimal values, and there I can clearly see the difference. Plus, 3 kts is simply a bigger jump on the PFD than 1 kts. But maybe prone is not the right word here.

As for your other comment, below transition altitude.. I have to test that actually. Hopefully I can do that today.

john_tullamarine · 28th Jun 2012, 23:50

Sounds more like a sim/software specific artefact. Have you discussed it with the facility's sim tech folk ? I wouldn't be surprised to find their answer along the lines of "yeah, it's one of those known problems but we all live with it .."

.. and none of us would ever expect that any sim models a real aeroplane better than approximately ?

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