Why is it important?
It is a legal requirement and crucial to the safety and structural integrity of an aircraft that it is loaded in such a way that the specified maximum allowable weights are not exceeded and that the center of gravity as loaded will be within the permitted flight envelope and remain so for the rest of the flight.
It is also very important that aircraft baggage and freight load complies with the restrictions on the carriage of dangerous goods (DGR).
What is the purpose of trim on an aircraft?
Trim Systems are considered to be a “secondary” flight control system. By definition, to “trim” an aircraft is to adjust the aerodynamic forces on the control surfaces so that the aircraft maintains the set attitude without any control input.
And here comes the need for a ”load sheet” for an aircraft- An aircraft filled with passengers, baggage, cargo, and fuel could be too heavy for the engines to lift it off the ground. Therefore, the balance of the aircraft is crucial, and the location of the passengers, baggage, cargo, and fuel will affect the aircraft trim.
So where is trim located on an airplane and how do you achieve it?
Trim tabs have been used in airplanes for decades. Typically found on the outer edges of the ailerons, rudders, and elevator, they allow to control the airplane’s trim during the flight.
FUEL LOADING AND DISTRIBUTION
Many aircrafts use fuel distribution to optimize the center of gravity in cruise to reduce fuel burn. This is achieved by keeping the CG towards the aft limits of the envelope by utilizing outboard wing, aft body, or horizontal stabilizer fuel tanks. It is the aircraft commander’s responsibility to ensure that the fuel load before takeoff is correctly distributed and reflected on the load/trim sheet and maintained within the prescribed limits for the remainder of the flight.
DEPARTURE CONTROL SYSTEM (DCS)
Most Load and Trim Sheets used today in commercial air transport operations by multi-crew aircraft are produced by contracted Handling Agents who input flight-specific data into a proprietary DCS. There are several commercial DCS products available today.
It is to be noted that DCS will only produce output data as accurate as of the inputs, so it is important to guard against input errors.
MANUAL LOAD SHEETS
Manual Load sheets involve a calculation of Maximum Ramp Weight (MRW), Maximum Take-Off Weight (MTOW), and Maximum Landing Weight (MLW) while the Centre of Gravity is located by marking the requisite aircraft operating weight (vertical scale) on a ‘drop line’ located on a center of gravity ‘index’ scale which forms the horizontal axis. If the position so found is within the areas shown as the permitted safe flight envelope, (and remains within the safe area as fuel reduces to planned landing weight) then operation, as loaded, is possible.
- The aircraft commander must be given a copy of the completed load and trim sheet for the flight and should check and sign it, leaving a copy at the point of departure.
- In respect of Passenger Cabin Loading, the senior cabin crew member usually confirms the number of passengers actually on board using a headcount after boarding has been completed.
ELECTRONIC FLIGHT BAG GENERATION OF LOAD AND TRIM DATA
For some flights, especially but not only cargo flights, but the flight crew also have an electronic flight bag (EFB) which they use to calculate aircraft performance data, which takes account of the completed load and trim sheet.
CONSEQUENCES OF MISLOADING OR INCORRECT INPUT OF LOADING DATA
- An attempt is being made to operate the aircraft outside the AFM (aircraft flight management) limits
- Flight crew actions to control the aircraft are ineffective because the aircraft is unable to achieve the expected performance, whether manually selected or FMS (flight management system) generated safety speeds on the ASI (e.g Vr) or selected engine thrust parameters.
AIRCRAFT TAIL TIPPING
In the clip above, a Delta Airlines Boeing 737 tilts backward on its tail while parked at the gate. If you look closely, you can see bags being off-loaded from the forward cargo hold. What happened is that too many bags, and passengers, were offloaded from the front of the plane compared to the back. The result? This slow-motion lift-off at the gate, no doubt to the shock of the ground operations team and any passengers on the plane.
Incidents like these happen because an airplane is like a large seesaw with a fulcrum in the middle. The center of gravity of the aircraft is typically near the landing gear, by design. But the center of gravity is not a constant. Imagine a seesaw where the fulcrum can move. That’s what happens when you add fuel, passengers, or baggage at different weights for every flight. The center of gravity for the aircraft needs to be within published and tested limits created by the airplane manufacturer and approved by the regulators.
Hence again, load and trim/weight and balance becomes a very critical aspect of flight safety.
IMPORTANCE OF PROPER WEIGHT AND BALANCE
Weight and balance are so critical that the Federal Aviation Administration has issued a guide for operators, and it’s part and parcel for every pilot from the outset of their training.