%MAC B737NG
Thread Starter
Joined: Feb 2025
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From: Dublin
%MAC B737NG
Who calculates %MAC? How does a pilot know CofG, %MAC of the aircraft they are flying ,is there a chart or is specific to the aircraft? Does it change with speed or is it a set value for that particular aircraft weight and configuration?
Thanks in advance.
Thanks in advance.
Joined: Feb 2025
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From: HUNTINGTON BEACh
Great question! For pilots, %MAC is typically provided rather than calculated manually (especially in commercial ops). However, understanding the concept is crucial, especially for weight & balance adjustments, fuel burn considerations, and aircraft handling characteristics. If you’re flying GA, using the aircraft POH and W&B tables will help you determine the CofG and %MAC before flight.
Thread Starter
Joined: Feb 2025
Posts: 4
Likes: 0
From: Dublin
Great question! For pilots, %MAC is typically provided rather than calculated manually (especially in commercial ops). However, understanding the concept is crucial, especially for weight & balance adjustments, fuel burn considerations, and aircraft handling characteristics. If you’re flying GA, using the aircraft POH and W&B tables will help you determine the CofG and %MAC before flight.
Joined: May 2006
Aviation Qualifications: ATPL
Posts: 231
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From: Here, there and everywhere
In commercial operations, this is basically dependent on the airline.
Some airlines have Load Control departments/agents that plan and issue loading instructions to the ramp agents (to load freight/mail and baggage in the different holds/loading positions). Passenger figures are derived from the check-in system, with seats per cabin zone easily determined and considered. Close to departure time, the flight is closed (roughly 30 min before ETD) and the system calculates the load sheet, which is then given to the crew (in person or sent via ACARS).
This document has all the flight's load information, actual weights, underload information and also the % MAC CG for ZFW, TO and LDG weights. During the final checks before start, the crew will input the % MAC CG into the appropriate page in the FMC and will get the stab trim setting.
Some other airlines have a more streamlined operation and do not have a Load Control agent to perform all these tasks, so upon flight closure, the crew is given a simplified load sheet with only the passengers and freight/baggage distribution and the crew has to calculate the % MAC CG and stab trim setting. This can be done using a software made by the aircraft's OEM, i.e. Boeing OPT, or if you are brave enough (or it is your lucky day) using a good and old manual load and balance sheet, with the help of tables and graphs. The end result is the same: you get the MAC CG and input it into the FMC.
BF
Some airlines have Load Control departments/agents that plan and issue loading instructions to the ramp agents (to load freight/mail and baggage in the different holds/loading positions). Passenger figures are derived from the check-in system, with seats per cabin zone easily determined and considered. Close to departure time, the flight is closed (roughly 30 min before ETD) and the system calculates the load sheet, which is then given to the crew (in person or sent via ACARS).
This document has all the flight's load information, actual weights, underload information and also the % MAC CG for ZFW, TO and LDG weights. During the final checks before start, the crew will input the % MAC CG into the appropriate page in the FMC and will get the stab trim setting.
Some other airlines have a more streamlined operation and do not have a Load Control agent to perform all these tasks, so upon flight closure, the crew is given a simplified load sheet with only the passengers and freight/baggage distribution and the crew has to calculate the % MAC CG and stab trim setting. This can be done using a software made by the aircraft's OEM, i.e. Boeing OPT, or if you are brave enough (or it is your lucky day) using a good and old manual load and balance sheet, with the help of tables and graphs. The end result is the same: you get the MAC CG and input it into the FMC.
BF
Joined: Jan 2024
Posts: 156
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From: Finland
Each individual aircraft is weighted at specified intervals (several years). The weighting includes determining the Center of Gravity and it’s results are included in the documents of an aircraft. For each flight actual(*) mass and position of crew, fuel, pax etc. are added to this basic figure to determine the actual mass and CofG (expressed in %MAC) of the aircraft.
* In some cases an aproved standard mass can be used instead of the actual mass. That’s why every pax isn’t weigted before flight.
* In some cases an aproved standard mass can be used instead of the actual mass. That’s why every pax isn’t weigted before flight.
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
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From: various places .....
CG and %MAC are the same thing looked at from either the left side perspective, or the right. For the line pilot, they are the same, albeit that the numbers are different.
Then, why ?
Traditionally, the aerodynamicists get to work up the AFM. Aerodynamicists and FT folk are very interested in CG expressed as %MAC as that measure correlates with likely aircraft handling characteristics. If the aerodynamicist is going to run up an AFM, is he/she likely to rework all the CG data from %MAC to distances aft/forward of the datum ? Of course not. The line pilot gets the data as %MAC and that's just tough.
It follows, then, that the line pilot needs to be able to convert %MAC to FS and FS to %MAC. Minor arithmetic sums which are just one extra minor nuisance for the line pilot to do to pass the Regulator's exams and so forth.
Generally, the small aircraft OEM will avoid %MAC in the POH (not sure why) while, as the size goes up, you tend to get it all in %MAC.
Not worth worrying about to any extent .....
Who calculates %MAC?
Anyone who has the relevant data and an interest/need to do so.
How does a pilot know CofG, %MAC of the aircraft they are flying
It is an essential calculation pre-flight so the pilot knows the value pre-flight every time ... or, should know.
Does it change with speed or is it a set value for that particular aircraft weight and configuration?
Varies only with aircraft loading flight to flight and, as fuel burns, during flight.
Then, why ?
Traditionally, the aerodynamicists get to work up the AFM. Aerodynamicists and FT folk are very interested in CG expressed as %MAC as that measure correlates with likely aircraft handling characteristics. If the aerodynamicist is going to run up an AFM, is he/she likely to rework all the CG data from %MAC to distances aft/forward of the datum ? Of course not. The line pilot gets the data as %MAC and that's just tough.
It follows, then, that the line pilot needs to be able to convert %MAC to FS and FS to %MAC. Minor arithmetic sums which are just one extra minor nuisance for the line pilot to do to pass the Regulator's exams and so forth.
Generally, the small aircraft OEM will avoid %MAC in the POH (not sure why) while, as the size goes up, you tend to get it all in %MAC.
Not worth worrying about to any extent .....
Who calculates %MAC?
Anyone who has the relevant data and an interest/need to do so.
How does a pilot know CofG, %MAC of the aircraft they are flying
It is an essential calculation pre-flight so the pilot knows the value pre-flight every time ... or, should know.
Does it change with speed or is it a set value for that particular aircraft weight and configuration?
Varies only with aircraft loading flight to flight and, as fuel burns, during flight.
