FMC performance

[Cbf]

I have a question about FMC computed performance. I was flying from EDDM to LEBL, VNAV mode, with a GE aircraft, Cost index 120. Initial FL was FL400. FMC recommands to climb to FL420 at the beginning of cuise.
At 340NM to destination:
- Weight 235.8T
- FL 420 ( OPT 414 / MAX 441)
- FMC ECON SPEED .834 (IAS 241Kt). for information, FMC LRC SPEED .834.
- TAT -16°C / SAT -47°C
I checked the L/D max speed on VNAV CLB page (MAX ANGLE): 243Kt. It is strange that the FMC recommands to fly in the reversed command region!
Moreover, the speed is very close to the minimum maneuvering speed (Approx 239Kt).
I simulated an engine failure. Descent must be initiated rapidly. If not, stick shaker speed is reached after 35".

Any idea?

[FlyItLikeYouStoleIt]

The FMC said you could, but you needed to think whether you should. Different philosophy Boeing v Airbus.

[United744]

How long was the total flight? If you're making FL400 initial with a step immediately to FL420 this suggests the flight is short/aircraft is very light.

For any flight length less than 400 NM, I limit my cruise altitude to the flight distance, so 350 NM would be limited to FL350 for example.

I ignore the FMC econ/LRC speeds and use Mach 0.85 for LRC. It seems to work the best.

[Cbf]

EDDM to LEBL: 600NM. ZFW 219T + FUEL 26T. I usually fly shorter (350 to 400NM & 150 to 200NM). During those flights, there is no step climb available.
During my last long flight 0f 4400NM:
- 80NM to TOD, Cost index 100, 244T
- FL 410 (OPT 406 / RCMD 410 / MAX 434)
- .848/ 252Kt ECON
- L/D max speed 247 Kt
So 5Kt margin from reversed command region and 10Kt from minimum maneuvering speed.

[simonijs]

Hi,

For your first flight, and using my formulas, the minimum maneuvering speed should be 233 KIAS (at 10% CG) or 225 (at 30 % CG). These numbers are confirmed by a Buffet Onset Boundary chart in KLM's AOM. For your second flight, they would be 237 (at 10 %) and 229 (at 30 % CG).

L/DMAX would also be used while holding. From the AOM/FCOM (for a freighter aircraft - GE engines), at 240 MT and at 40.000 ft, the listed KIAS for a Flaps Up holding = 245.

The first available FL after FL410 would be FL430 (FL420 is not used).
M0.85 is not LRC, I think this is only true when the aircraft (ERF, GE-engines) is flying at its OPT ALT for the actual aircraft mass.For the PAX/COMBI that would be M0,858 (GE).

Regards,
Simon

[Cbf]

Here is my understanding concerning cruise speeds (highest to lowest):
- Maximum speed (VMO)
- Maximum maneuvering speed
- LRC speed
- Maxi range speed (CI=0)
- L/D max speed (holding & boundary reversed/normal command regions)
- Minimum maneuvering speed
- Minimum speed (Stick Shaker)
ECON speed should be above Maxi range speed with CI=120?

[simonijs]

ECON speed should be above Max range speed with CI=120?

For the 747-400 CI-230 approximates LRC.

In my other post I should have mentioned that ICAO has set standards for holding speeds, dependent on altitude. For altitudes above 34000 ft/10000 mtr, that is M0,83 - which would result in an IAS quite a bit above L/DMAX speed.

Regards,
Simon