747-400 Buffet margin versus turbulence speed

[Pitch Up Authority]

Hello,

Looking at the AFM Cruise Maneuvering Capability graph I see that when flying at Mach 0.86 I have the highest aerodynamic ceiling however the turbulence penetration speed is 290-310 kts or M 0.82- M 0.84 whichever is lower.

I am puzzled since I would expect that M 0.86 would give me the biggest margin to buffet in case of turbulence?

Can anyone explain this to me?

[Old Smokey]

M 0.86 will give you a greater protection against LOW speed buffet, but degrade your margins to HIGH speed buffet.

Boeing's leter versions of the B777 provide both a low speed and a high speed awareness yellow 'bar' on the ASI (Not retro-fitted to earlier models). the lower bar provides 1.3G awareness of LOW speed buffet, the upper bar provides 1.3G awareness of HIGH speed buffet.

Fly the 'happy medium', i.e. the recommended Turbulence Penetration Speed:ok:

Regards,

Old Smokey

[Feather #3]

The turbulence penetration speed isn't about buffet, it's about gust loading.

If you never fly >1,000ft below the MAX ALT in the FMS, there won't be a problem. Note, this assumes your protection limit in the FMC database is set at 1.3g; some aren't!!

G'day ;)

[Avenger]

Correct, Turbulence penetration speeds are based on the manufacturers load certıfıcatıons and structural strength requıred under constant gust condıtıons. There are many refs to thıs on the net and Boeıng support sıtes. Hıgh speed buffet margıns vary wıth CofG, weıght etc and are not related.
If you want a rough ıdea of the buffet margıns they are ın the QRH under the LRC sectıon. Max ALT mınus 1000 or Optımum plus 1000 ıs usually safe,

[Pugilistic Animus]

The turbulence penetration speed isn't about buffet, it's about gust loading.

O_S definitely knows that...read carefully what he said ...:cool:


:):):)

[Pitch Up Authority]

Thanks for the feedback. From all this I understand that M 0.86 will give me the greatest margin to buffet in case of g-load increase due to turn or turbulence but will provide less margin in case of speed excursions due to turbulence. The combination of g-load and speed excursion at M 0.86 will put me on the limit, hence reduction of M 0.86 to M 0.82-0.85.

We have 1.3 set as a margin and since we have the have the CF680C2B5F our max will almost always be 1.3 limited (forward CofG)

However when using cruise MAC 29 after center tank runs empty our max goes up by +/- 1000 ft getting close to or even higher then the thrust limit alt (defined by R/C 100 ft/min at CLB thrust and wings level)

Does anybody know if the FMC LNAV limits the bank angle when flying at MAX Thrust Lim Alt as a function of available CRZ thrust? (CRZ is about 2 % lower than CLB on this engine eroding the 100 ft/min extra with CLB)

I know the 737 has that functionality but have not seen it written for the 747-400.

Same question when using HDG SEL in AUTO (Uses V2, TAS and Config) to set bank but what about: bank = (f) (thrust) in AUTO

Could be usefull when flying at ISA +20 while avoiding TS (gust during banked turn)

Thanks PuA

[Intruder]

Remember that aft CG increases fuel efficiency but reduces stability. The increased max altitude reflects that.

Your post seems to indicate that you manually insert a cruise CG. I have never seen that practice, and do not know why you would do so. If so, why do you do it and how do you calculate it?

the .82-.85 range tends to place you in the center of the available cruise speed window at high altitude, between the yellow and red arcs, thereby giving you the best compromise between Mach buffet and stall buffet limits. If you search through the old versions of Boeing AERO magazine, you will find the discussion of turbulence speeds, and the recommendation to err on the high side of the limit if at all.

[SMOC]

FAA and JAA -400s have different FMC C of G default values, JAA is 8.5%, FAA is 20%, So FAA -400s can show a higher performance limited Max CRZ ALT.

We occasionally update the FMC using the current C of G to show a new max alt closer to the FAA types, usually 1000' or less improvement. We could I suppose also just put in the FAA default value.

Only used when you're limited by the FMC Max CRZ ALT and you want a level that is for example only a couple of hundred feet above it, as opposed to say being sent down several thousand feet by ATC.

The times I've used the procedure can be counted on one hand.

Calculated by using the MACZFW and a fuel load correction.

Edit : Correct FAA value added.

[BBK]

Intruder

The default on our a/c is 8.5% as SMOC says and sometimes it is useful to know the most accurate prediction of max alt, eg if trying for a flight level in Africa. There is a table in the QRH that provides a more accurate value normally around 20%. Use with caution of course as we all know that the loadsheet figures themselves could underestimate the true weight.

rgds

BBK

[Pitch Up Authority]

Cruise CofG is calculated using a table reflecting the index change due to fuel load. The table is set up for various fuel densities. This correction is added to the ZFW Index to obtain a new index for current weight.

With this index and the actual weight you enter the Index-Weight graph (on the loadsheet) to obtain your Cruise-MAC. Weight and balance computers are used on ground to check loadsheet versus calculated weight.

Once center wing and R2/R3 are empty changes are minor.

This way you obtain a new Max Alt from the FMC. Only problem is that you do not know if it is thrust or 1.3g limited. With our engines it must be 1.3g unless we change it to 1.2g.

Max Cruise thrust is available till ISA + 30 °C.

This technique is useful outside RVSM (4000 ft steps) and in areas where RVSM is imposing you steps that are greater than 2000 ft like Russia and China.

Since Yellow manoeuvering band is calculated by FMC based on weight and speed and red tape stall margin comes from the MAWEA based on AoA you have two independent sources of information. Only problem here is that the AoA is not corrected for Mach unless you ave a software update.