Impact on takeoff performance

3.2.1. Optimized takeoff distance or takeoff weight The aircraft operating speeds are referenced to the stalling speed.

At takeoff: V₂ ≥1.2Vs (1.13Vs_1g)

Now, the V₂ value directly impacts the takeoff distance (or the takeoff weight).

Vs V₂ TOD

So for a given TOD: the lower the stall speed (Vs) value, the greater the takeoff weight (TOW).

The further aft the CG, the smaller the TOD (the greater the TOW).

• Influence on TOD

A340 CONF 3, AC OFF, AI OFF, OAT 15°C, Zp 0ft TOW = 250 000 kg

Forward

CG CG at

26%MAC TOD (m) 3241.5 3164.8

% 42 . 2 7 . 76 =

=

∆TOD m

!This example shows that, for a given TOW, the aircraft will need a longer runway if its CG is forward.

3.2.2. Takeoff roll

An aft CG position gives the aircraft a nose-up attitude that helps the rotation. On the contrary, a forward CG position leads to a nose-heavy situation and a difficult rotation.

Taking into account no other limitation (gear strength, VMU…),

The further aft the CG, the better the takeoff roll performances.

3.2.3. Takeoff climb

An aft CG position gives the aircraft a nose-up attitude that helps the aircraft climb. On the contrary, a forward CG position leads to a nose-heavy situation and a difficult climb.

Taking into account no other limitation (gear strength, VMU…),

The further aft the CG, the better the takeoff climb performances.

• Influence on Climb Gradient

A340 CONF 3, AC OFF, AI OFF, OAT 15°C, Zp 0ft TOW = 250 000 kg

Forward CG

CG at 26%MAC 2^nd segment

gradient 5.569% 5.689%

!The aircraft climb is facilitated with an aft CG position.

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• Influence on TOW

A340 CONF 3, AC OFF, AI OFF, OAT 15°C, Zp 0ft

The difference in 2^nd segment gradient value may not seem significant.

So instead of calculating the difference in gradient for the same take-off weight, it is interesting to determine the maximum take off weight for a given climb performance.

A second segment climb gradient of 5% is imposed (i.e. obstacle in take-off path).

Forward CG

CG at 26%MAC TOW (kg) 256215.2 257581.3

kg TOW =1366.1

∆

!For a given second segment climb gradient, the allowed TOW is increased when the CG is aft.

3.2.4. Basic and alternate CG positions From the 3 previous paragraphs it can be concluded that :

The best takeoff performances correspond to an aft CG position.

Now, when determining the aircraft takeoff performance (TOW, TOD), the calculation is always performed at the most forward-certified CG position. Therefore, for aircraft having a naturally aft CG position (tail heavy aircraft), everyday operations might be penalized using this very forward value.

The A320 and the A340 usually have a DOW CG of around 30%MAC and their most forward certified CG limit is around 17%MAC.

These “tail-heavy” aircraft usually have an aft takeoff CG position. This means that their everyday performance is better than the certified performance for the most forward CG position.

So a second CG position forward limit has been certified for these two aircraft.

− at 25%MAC for the A320

− at 26%MAC for the A340.

All takeoff performances in the certified Airplane Flight Manual are given for both these CG positions.

However, to take into account the errors in the determination of the CG position, a margin has to be applied on the calculated takeoff CG.

I.e., for an A320, in order to be able to use the performance calculated for a 25% CG, the calculated takeoff CG must be of at least 27%.

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a) A320

A320 Basic CG limits A320 Alternate CG limit

Therefore, for the A320, the basic certified CG is the aft CG (25%RC) and the alternate CG is the forward CG.

Takeoff charts and performances published in the FCOM are usually established for the basic CG position (25%)

- Takeoff chart for an A320-232 -

If the CG position is forward to the 27% RC point, and if the operator does not have the take-off charts for a forward position, performance corrections have to be applied. These are given in the FCOM 2.02.10.

However, should an A320 be operated with an alternate CG position, it is preferable to establish the takeoff charts for that CG position, as the corrections given in the FCOM are conservative.

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b) A340

A340 Basic and Alternate CG

For the A340, the basic CG is the most forward CG and the alternate CG is the aft CG (26%).

Take off charts and performances published in the FCOM are established for the basic CG position.

No corrections are given in the FCOM because the alternate CG performances are better than the basic CG performances. Therefore, an operator that wishes to benefit from the advantages of the alternate CG must establish the appropriate takeoff charts.

Influence on TOW

Note : The following figures come from the PEP for Windows Take Off and Landing application. It takes into account all the limiting factors for take-off (runway length, climb…).

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The following graphs show ∆TOW(%) as a function of the TORA length.

100

(%) ×



 



 −

=

∆

BasicCG

BasicCG G

AlternateC

TOW

TOW

TOW TOW represents the difference in term of TOW when

using the alternate CG for performance calculation instead of the basic CG.

Calculations have been performed for a range of TORA and for different airfield altitude.

A340-313 CONF 1+F

We can notice that the difference in take off weights is all the more important with short runways and an elevated terrain. For example:

OAT 20°C

Pressure Altitude (Zp) 0ft Runway length 3000m CG position Basic Alternate Takeoff Weight (kg) 266013.9 264207.8

kg TOW =1806.1

∆

• Conclusion

Therefore, by using the performances calculated for the alternate CG (A340) or for the basic CG (A320), the operator can increase the aircraft takeoff weight, thus the payload or the range of the aircraft.

On the other hand, the envelope is narrower, thus leading to more constraints on the loading aspect.

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