What MACTOW to use?

[Aleks]

Any tips on what MACTOW to select for flights, or is it just random selection I should do between 17 and 26?

What is the typical range in reallife?

Thanks.

[Mawea]

Hi Aleks,

that depends on the loading of the plane and will result in a trim setting corresponding to the load. So it is not random and has an effect on performance. We use aft CGs to get out of fields which are critical performance wise, high elevation with high temperatures MEX for example.

But to answer your question if you have a setting around 18-22 that will be a good average for normal ops.

Cheers Jay

PS
Sorry, if I misinterpret your question. I was thinking of what numbers to put into the T/O REF page for TO CG!
OR is it where to place the slider CG %MAC in the situation/aerodynamics menu on instructor page?

[Aleks]

Thank you, Jay.

I was thinking of the slider in the situation/aerodynamics page. Sorry for not being clear in my question.

[Will]

Hi Aleks,

The CG limits of the actual aircraft vary by aircraft version, but in one instance (based on online resources), the forward limit is 11%-13% depending on weight, and the aft limit is 33%. This is a wide variance, and in real life operations that range (11-33%) is probably more typical of a cargo aircraft that has fresh flowers in one end and stacks of gold bullion in the other--than of a passenger variant, where the weight is generally more equally distributed. The slider in PSX lets you experience these extremes, from 8.5% MAC (forward) to 33.0% (aft).

You can set the slider in PSX to whatever you want to simulate. For "normal" operations, especially with passengers, you should follow the advice of Mawea above and set something in the range of 18-22%. Anything in that range will be within the 747's typical profile.

However, if you want to simulate an extreme scenario, like a freighter with lead ingots in the front and pallets of tulips in the back, then you can set 13% and see how it flies. Or move the lead ingots to the back and set 33%. Where you put the actual slider in PSX depends on what you are trying to do.

I typically set it randomly (meaning, I take my hand off the mouse without paying attention to the exact point) somewhere typically in the range of 18-24% and go from there.

[Cbf]

Hi Aleks,

Here are data found in accident reports (freighters) available on the web:

- B747-4B5F_Reg HL7467
  TOW 787,143lbs_CG 28.0%
  Estimated LW 563,143lbs_CG30.5%

- B747-428BCF_reg N949CA
  TOW 675,296lbs_CG 30.4%

When you choose a T/O CG, be sure that it will remain within limits during landing.

[Will]

Some additional information about N949CA is informative... The cause is believed to be improperly secured cargo (namely five 12-15 ton armored combat vehicles) that shifted backwards during the takeoff sequence and damaged hydraulic lines which run in proximity to the aft pressure bulkhead.

Assuming maximal motion of the cargo to the rear (with the rearmost cargo striking the rear pressure bulkhead, and all other cargo shifting backwards in turn to occupy available space), the "worst-case scenario" CG was calculated to be 56% MAC, which is far outside of the limits that PSX can simulate.

However, in the absence of additional failures, 56% MAC was considered to be recoverable using a combination of forward control pressure and trim. (In my opinion, this demonstrates that the 747 is a robustly designed aircraft.)

What rendered the accident unrecoverable was the damage to the hydraulic lines, which degraded performance of the flight controls. The combination of reduced flight control authority, plus aft CG created an unflyable situation.

Ref: https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR1501.pdf