777-300ER MMS logic

[Alpha Sierra]

Can someone shed a little more info on the logic behind the MMS jumping to approx 300kts on the 777-300ER when climbing through FL200.

My understanding:
Top of the MMS provides
-1.3g to stick shaker with flaps down
-1.3g to stick shaker or Vref +80 (whichever is less) with flaps up below 20,000FT

-1.3g to low speed buffet with flaps up above 20,000FT. Would this be affected by weight of the aircraft?

Tks

[Alpha Sierra]

Anyone??

.....................Old Smokey?

[Old Smokey]

Working on it, I have observed the phenomenom, but to not as high a speed as you quote, more like a jump to 280 KIAS...... close to you:)

Flying the 300ER tomorrow, will have a closer look and take a video (if work load permits). You have my interest:ok:

Weight, Pressure Height, and Mach Number would DEFINATELY cause increase in MMS, but the jump at 20,000 ft, that's got me wondering. Do remember that for Mr. Boeing, many things change at 20,000 ft, Flap extension etc.

Regards,

Old Smokey

[King on a Wing]

MMS increasing to 300 kts...??? Sounds quite unlikely.Could be a software glitch too though.Having said that,the MMS band DOES definitely jump by about 20-25 kts at 20,000 ft.Simply because you cannot use your high lift devices above that altitude.
Now assuming a MTOW take off at say 350T.The MMS with flaps up would be in the region of say 260-270 kts( i can't remember the exact figures ).Now bump this up by another 20-25 kts( at 20,000 ft) and voila,you have your magic 300 kts...:)
Of course I could be completely out of order here.But its my 2 krunos worth.Mebbe more....:cool:

[BuzzBox]

Below 20,000ft, MMS is limited to the 1.3G manoeuvring speed or Vref30+80, whichever is LESS. Above 20,000ft the flaps aren't available, so the system logic limits MMS to the 1.3G manoeuvring speed ONLY.

At heavier weights (greater than approx 285T), Vref30+80 is less than the 1.3G manoeuvring speed. Below 20,000ft the MMS will therefore be limited to Vref30+80. As the aircraft climbs through 20,000ft the Vref30+80 restriction on MMS is lifted and the MMS 'jumps' to the 1.3G value of approximately 280 knots.

Makes your eyes water if you're climbing out at 270 knots due to an ATC speed restriction!

[Old Smokey]

Alpha Sierra,

Here it is, straight from Boeing to you. The words are Boeing's, the Italics and EMBOLDENED CAPITALS are mine, to highlight the differences. Boeing speak of MMS in 2 modes, Minimum Manoeuvering Speed, and Maximum Manoeuvering Speed. Minimum first:-


Minimum Maneuvering Speed

Indicates maneuver speed margin to stick shaker or low speed buffet. Top of the bar is the airspeed that provides:

• 1.3 g manuever capability to STICK SHAKER with flaps down.
• 1.3 g maneuver capability to STICK SHAKER or VREF + 80 , whichever is less, with flaps up below 20,200 FT.
• 1.3 g manuever capability to LOW SPEED BUFFET(or an alternative approved maneuver capability as preset by maintenance), with flaps up above20,200 FT.

Displayed with first flap retraction after takeoff.

Note: 1.3 g maneuver capability occurs at 40 degrees of bank in level flight.

MY NOTE - Thus, below 20,200 ft, MMS (MINIMUM) is basically referenced to STICK SHAKER, above 20,200 ft, MMS is basically referenced to LOW SPEED BUFFET. (Low Speed Buffet occurs substantially above Stick Shaker).

Maximum Maneuvering Speed

When displayed, indicates maneuver margin to high speed buffet. Bottom of the bar is the airspeed that provides 1.3 g maneuver capability (or an alternative approved maneuver capability as preset by maintenance). Bar may be displayed when operating at high altitude at relatively high gross weights.

Note: 1.3 g maneuver capability occurs at 40 degrees of bank in level flight.

MY NOTE - Thus, above 20,200 ft, MMS (Maximum) is basically referenced to MACH NUMBER.

Therein lies the Seattle logic.:ok:

A bit of Old Smokey logic - Additional low speed protection may be achieved at approximately 7 Kts IAS per 0.1G. Thus, to provide protection against Moderate Turbulence (1.5g), fly at no less than MMS+14. (Does that sound familiar to Boeing's recommendation to fly at not less than MMS+15 in turbulence?). Where I come from, Moderate Turbulence was defines as 1.6g (John_T may still have some of Ansett's old manuals). Thus I fly at not less than MMS (VLS)+20 in turbulence, or M0.82, whichever is the greater.

WARNING! FMC recommended level only 'knows' the 1.3g limit, and in favourable wind gradient will recommend you all the way up to where you only have 1.3g protection, far less than adequate should things turn bumpy!:eek:

I hope that this reply, and the 2 fore-going posters have answered many of your questions.

Regards,

Old Smokey