Fuel flow enroute function behaviour

[YoanwM]

Hello !

I am a student working on flight planning and I noticed that the fuel flow (enroute) computed seems strange for cruising.
I tried to make a short case to illustrate it as you can see bellow. I have taken an A320 and computed the fuel flow in the same situation for different speed between a bit less than cruise speed (0,78) and the maximal cruise speed (0.82).
The fuel flow is always decreasing I think it should increase after the optimal mach 0.78.

Here is a plot and the code :

ac = aircraft(actype)
speeds = np.arange(start=ac["cruise"]["mach"]-0.04,stop=ac["limits"]["MMO"]+0.001,step=0.005)
res = []
masse = (ac["limits"]["MTOW"]+ac["limits"]["MLW"])//2

for speed in speeds:
    s = op.aero.mach2tas(speed,35_000*op.aero.ft)
    res.append(op.FuelFlow(actype).enroute(mass=masse , tas = s, alt=35_000))
    
fig = plt.Figure()
ax = plt.axes()
plt.xlabel("Mach")
plt.ylabel("Fuel flow computed at 35_000ft")
ax.plot(speeds, res)
plt.savefig("fflow")

Thanks in advance!