View Full Version : Flight below optimum altitude,

12th Jul 2002, 06:37
Flight carried out below optimum altitude, would result in:

a. less time,less fuel
b. less time,more fuel
c. more time, more fuel

For me answer should be c.
but in the book answer is b.
Anybody knows why? :confused:

Many thanks.

12th Jul 2002, 07:57
Hello Moon 11,

My guess is that by being at a lower altitude, you'll have a slightly greater TAS(flying faster therefore less time) which would equate to a greater F/F resulting in more fuel burn !?

While we're on the subject, here are a couple more questions to make you fall asleep:
1) What is higher on a B-747-400, Vmcg or Vmca!?
2) If TOW is limited by an obstacle in second segment,what does that mean !?
3) If you were loading an A/C to obtain max range,would you load it with a fwd or aft CG !?
and finally,
4) During what phase of flight is lift the greatest and why!?

Anyone willing to take a run at these are more than welcome.

See ya.:confused: :cool:

12th Jul 2002, 08:01
Moon 11,

The publication that you are refering to has a few,shall we say,"discrepancies"..........so if you want to compare notes, you know where to find me................


12th Jul 2002, 08:03
I meant ShakeSlick.............must be the time !? Or those damned questions!?!?

12th Jul 2002, 13:55

Ill have a go at those questions of yours.

1) Vmcg is higher, as the aircraft can not bank into the dead side, and must maintain direction by using rudder only (tiller can not be taken into account).

2) TOW limited by second segment, means that the aircraft must maintain certain climb gradient (diffrent between 2, 3, or 4 engined aircraft) with takeoff power, gear up, takeoff flaps, and an engine failure. These parameters make up the second segment, and is almost always limiting factor in twins. In the second segment a twin needs to do 2.4% gradient, and the net gradient needs to clear all obstacles by 35'. If the obstacle is in the second segment, then the gradient must be higher than above mentioned 2.4%. The only way to acheave this is by making the aircraft lighter, which in turn limits the TOW.

3) For max range, one loads the aircraft so the CG is as aft as possible. This is, so the vertical stabilizer needs to produce as little negative lift as possible.

4) Lift is greatest at takeoff. Than the aircraft is heaviest.

12th Jul 2002, 15:34

Thanks for the timely response. Didn't realize that someone would be interested in that stuff !?:confused:

Now that my curiosity is up, for quest#2, would increasing your V1 speed not help you achieve a better climb gradient for the second segment as opposed to reducing your TOW !?:confused: Even with the engine failure, presuming that the remaining engine(s) are at max thrust !?
Just wonderin'........


12th Jul 2002, 16:14
I believe so Shakeslick, called improved climb.

It is used at least on the B737-400 (not know the NG737), then the TOW is limited by the climb gradient. Then the V1, Vr and V2 are increased which gives a better gradient.

However, it is not always feasible to use that procedure because then the accelerate stop distance increases, and one does not always have the luxury of a very long runway.

However, assuming a long runway with good B/A then it can be done. Another option is to use lesser flap setting for T/O, which also gives a better gradient.

If all this has been done and that is not enough, few extra pounds can be squeezed out of the engines, by using "bleeds off" takeoff; either using the APU for pressuratation or simply by taking off unpressureized if the APU is U/S.

I don't know however, how much the weight can be increased by any of those procedures, as I haven't flown this type for a couple of years, and I don't have the performace graphs anymore.

13th Jul 2002, 01:29

1.Vmca is higher,
1 eng out Vmca=124 KTS
2 eng out Vmca=161 KTS

2.We should climb best angle speed.
3. With aft CG(Why; because I read that many times on loadsheet)
4.During climb in second segment.


I have first one soon,stay in touch.

13th Jul 2002, 08:36
So what's the definition of optimum altitude then?

13th Jul 2002, 13:50
The altitude at which best fuel mileage will occur for a given speed schedule, fixed M, LRC etc.

Sometimes operating cosiderably off optimum altitude can work out better if better winds are at a different altitude, hence the wind altitude trade table (Boeing) which looks at the increase or decrease in wind speed that is required to opperate off optimum altitude and break even. So the question above I suppoe assumes nil wind and a fixed M no? in which case yes 'b'

Interstingly enough the B738 fuel milage penilty for opperating 2k either side of optimum at LRC is the same, its only when you are 3k and below optimum that fuel milage penilty really starts to increase.

Best Rgds

13th Jul 2002, 15:29

1: Where did you get your figures? And what conditions are they based on? As far as I remember our B744 have SL/15/B5F engine VMCGs of around 135 kts

2: Fresca gave the perfect answer for this. Best angle speed really doesnt equate to a B747, they will be flying V2-V2+10 in the 2nd segment with an engine failure.


16th Jul 2002, 22:06
My figures in regard of Vmcg and Vmca are based on 744 performance:
CF6 engine

Full thrust(TO):
OAT 15 C/Airport pressure alt=0
Vmcg=125 KIAS

Derate One(TO1):
Same OAT/Press. alt.
Vmcg=123 KIAS

Derate two(TO2):
Vmcg=117 KIAS as I remember,because now I don't have that chart available.

Vmca figures are based on information recived during ground school.That kind of data Boing don't publish in AFM (I don't know why) .

17th Jul 2002, 07:46

Ouch, I didnt realize that the 6000 lbs extra takeoff thrust offered by the CF6-80C2B5F increased VMCGs by so much. For comparison.

Full thrust(TO):
OAT 15 C/Airport pressure alt=0
Vmcg=133 KIAS (135 is the 2000 feet PA speed)

Derate One(TO1):
Same OAT/Press. alt.
Vmcg=127 KIAS

Derate two(TO2):
Vmcg=121 KIAS

VMCA data is available in our AFM, for the SL/15C case, the VMCA is 132 kts.

Boeing dont give VMCA data to crews, the operations manual has a VRMIN, as this must be equal to or greater than 1.05 VMCA and V1, it protects you for VMCA.