Aircraft W&B - Part 1 - A key to flight safety

Author: Kristjan Röx

W&B = Weight & Balance (same as mass & balance)

Content

Weight and Balance

We are all familiar with flying as passengers. We know what it entails; we book our flight, select a seat, and decide how many bags we want to carry with us. We check-in online and use the airport bag drop - or we just queue up for a one-stop shop kind of experience in check-in. Security checks, duty free….and waiting, lots of waiting.

Whilst we wait - a hidden world is in full swing behind the scenes to make sure our aircraft and ourselves get to our destination safely.

One thing must occur before our flight can leave. Our aircraft’s weight and balance must be determined.

Why is that?

An aircraft can only weigh so much - the manufacturer specifies maximum weight limits.

The aircraft's balance must also be determined - i.e. the location of its Center of Gravity (CG). Why is that?

The location of the CG affects the aircraft’s stability and handling characteristics in the air - in the same way as a car handles on the road.

For the aircraft to handle as designed - the manufacturer specifies limits on the location of the CG, i.e. how far forward and aft it can be at a certain weight. It is also important for pilots to know, before flight, where the CG is located, in order to know what kind of handling characteristics to expect.

Together, the limits on weight and balance form what is called the weight & balance envelope of the aircraft. Being within the limits is referred to as being inside the envelope or synonymously as being in trim. Analogously, an aircraft outside the limits is outside the envelope or out of trim.

So simply stated, it’s a matter of safety to establish weight and balance before each flight to verify the aircraft is inside its weight and balance envelope.

Before going further, feel free to enjoy Bruce Dickinson of Iron Maiden demonstrating the details and importance of aircraft weight and balance.

Nothing is done without a reason

Since determining weight and balance is a safety related matter - it’s required by regulations.

In Europe, under EASA Air Operating (Air Ops) rules, it is required specifically by regulation paragraphs CAT.MAB.POL.100 and 105.

Together, these paragraphs establish the requirements for an operator's weight & balance program, i.e. how the operator intends to develop, implement, and maintain processes and procedures to ensure control of aircraft weight and balance in its operations.

These regulation paragraphs are quite dense, but provide a few things;

  • they specify WHAT an operator must do,

  • provides Acceptable Means of Compliance (AMC) to do that,

  • and provides other Guidance Material (GM) related to it.

Although the AMCs provide acceptable means of complying with the regulation, they are not the only means. An operator can propose a different way of compliance and get it approved by its regulatory authority, or what EASA calls a “competent authority”. Competent Authorities are typically each country’s Civil Aviation Administration (CAA) which issues Air Operator’s Certifications (AOCs) to operators/airlines.

For countries outside Europe, e.g. in the USA under FAA rules, or elsewhere, there will be similar regulations requiring weight and balance to be established before each flight.

To summarize, the regulations state WHAT needs to be done, but not HOW. As long as the WHAT is satisfied in a safe and reliable manner, the operator is in compliance with the regulations.

How to comply?

So, how do we determine that both an aircraft’s weight and CG are within the limits specified by the manufacturer before each and every flight?

For this, we need three things:

  1. Certified and operational weight & balance data for our aircraft.

  2. Quantity and weight of each item or group of items on board.

  3. Location of each item on board and associated effect on balance.

Let's look into each item in more detail.

Certified and operational weight & balance data

The aircraft manufacturer, i.e. Airbus, Boeing, Embraer, etc., is required to publish certified weight and balance data and make it available to operators.

This data consists of information, instructions, and limitations that allows the operator to calculate weight and balance of the aircraft. It also includes the certified limits to weight and CG. This data is published as part of the Aircraft Flight Manual (AFM), most commonly in a document called the Weight & Balance Manual (WBM).

The first requirement of the Operating Regulations, CAT.MAB.POL.100(a), requires that:

During any phase of operation, the loading, mass and centre of gravity (CG) of the aircraft shall comply with the limitations in the AFM, or the operations manual if more restrictive.

In aviation regulations, no word or phrase is there without intent.

In CAT.MAB.POL.100(a), the phrase any phase of operation serves an important purpose. What it requires is for the weight & balance of an aircraft at all times during operation to be within the certified limits set by the manufacturer. This applies to the aircraft on the ground, during taxi, at takeoff, in cruise, on approach, during landing, and again on the ground at the destination.

Furthermore, the loading of the aircraft - i.e. the physical placement and restraint of each item of mass in the aircraft - shall be in accordance with the required load factors and within the structural strength limits of the aircraft as provided by the manufacturer.

This sounds reasonable. But how do we ensure this?

Weight and balance are state specific properties of an aircraft, i.e. they vary with time during operation.

  • The weight of an aircraft reduces in flight through fuel burn, which also affects the balance.

  • Passengers, crew, and various items of mass may shift or move about the aircraft in flight, which affects its balance.

  • Therefore, determining weight and balance in one state (one point in time) as being inside limits does not ensure that for another point in time.

It is difficult to track the weight and balance in real time to ensure compliance at all times, i.e. during all phases of operation.

What we would like to do instead, is to determine a minimum required number of weight and balance states as being inside limits, which then automatically ensures that limits are satisfied for all other possible states during operation.

Luckily, the regulation provides an Acceptable Means of Compliance (AMC) in AMC 1 to CAT.MAB.POL.100(a), with an acceptable way of doing this. This AMC1 is dense, but in simple terms is allows the operator to adjust the certified limits set by the manufacturer by applying operational margins to the limits, to account for any variation from the calculated states during flight. Therefore, complying with the adjusted limits will ensure compliance with the certified limits.

The operational margins applied to the certified limits are, in industry speak, referred to as curtailments to the weight and balance envelope.

The adjusted certified limits are referred to as operational limits and are the limits we need for compliance. Typically, this is just done once for a specific aircraft and a specific type of operation, i.e. the same data is used again and again. If any changes are made to the aircraft or operational profile, the data may have to be updated.

Quantity and weight of all items on board

The following quantity and weight data is needed:

  1. Weight of the empty aircraft.

  2. Quantity and weight of all operational items on board, e.g. crew, crew bags, catering etc.

  3. Quantity and weight of all passengers and cabin luggage.

  4. Quantity and weight of passengers’ checked bags.

  5. Weight of fuel.

  6. Quantity and weight of any cargo or mail carried on board.

  7. Any other type of weight.

This allows us to establish, or more accurately to estimate, the total weight of the aircraft. More on that to come.

Location of each item on board

For balance we first need to know the CG of an empty aircraft.

Then, we need to know where each of the above weights are located on board the aircraft. Knowing that, we can estimate by calculation the location of the aircraft’s center of gravity when everything is on board.

In the end - if both final total weight and CG are within the limits, the flight can legally depart.

Is this so difficult?

Conceptually, determining an aircraft's weight and balance is quite simple - based only on what is needed to do it.

However, in practice, there are some challenges.

Primarily, there is a lot of information that needs to be gathered and important decisions have to be made.

Two key questions come up:

  1. How do we determine the weight of each item?

  2. How do we determine the balance effect of each item?

Question #1 - how to determine weights

The regulation CAT.MAB.POL.100 provides an answer to Question 1.

However, the acceptable ways of determining weight have variable levels of certainty. Items can be weighed, or standard weights can be used instead if they accurately represent the weight of the specific items.

  1. Aircraft are weighed at regular intervals. This determines an empty weight and CG for the aircraft. There is some uncertainty associated with this weighing, since the aircraft configuration at the time of weighing may not be completely representative of an empty operational aircraft.

  2. However, the weighing procedures permit adjustments to be made to the weighing results to account for any variation in configuration to yield a result that sufficiently represents an empty aircraft. These adjustments themselves are also subject to uncertainty, but will bring the nominal results closer to the real values.

  3. Typically, operators use standard weights for operational items such as crew, crew bags, and catering. The standard weights have to be determined to accurately reflect the standardized nature of these items, within an acceptable tolerance.

  4. Passengers’ checked bags are weighed at check-in. This yields a total weight for all bags. Although each bag has a unique bag tag (identifier) with a certain weight, the industry commonly does not have the means to track where each bag is loaded on board an airplane. Therefore, what is obtained is a total quantity of bags (pieces) and a total weight, allowing for an average weight to be determined.

    • Recent developments in aviation include systems such as BRS - Baggage Reconciliation Systems, which track a bag from check-in to aircraft. This includes automated and handheld scanners that track the location of the bag throughout the airport, primarily with safety and security in mind. This is analogous to shipment tracking by logistics companies such as FedEx, DHL, UPS, etc.

    • With BRS systems, the exact location of each bag on board an aircraft can be determined. In that case, a more accurate distribution of bags in an aircraft can be obtained, and weight distribution also. If BRS is not used, then only quantities can be tracked, and average bag weight needs to be used.

    • The weight of passengers is difficult to determine. The only way to do it is to weigh each and every passenger. Although that is practically possible, there are obstacles to that from a privacy and societal standpoint. Therefore, this is in the vast majority of cases not done.

      Due to this, regulators have provided as part of their Acceptable Means of Compliance, a set of acceptable average passenger weights. These can be used as standard weights for passengers.

      Typically, average weight values are provided for males, females, and children. Infants have zero weight, which is acceptable as infants have a marginal effect on total passenger weight from a statistical point of view. Using these standard weights, operators only have to tally up how many males, females, and children are on board and where they sit to determine their effect on weight and balance.

    • Recently, Finnair, the national carrier of Finland (although privately owned), apparently sparked outrage for implementing anonymous weighing of passengers. Since Finnair’s efforts were anonymous, they could only be used to establish bespoke average passenger weights for Finnair. The reason for this weighing is to obtain more accurate average values to reduce the uncertainty in Finnair’s weight and balance calculations.

      Finnair’s objective with this process is improved safety and operational efficiency, but is instead blamed for discrimination and body shaming.

      Finnair operates under a Finnish CAA (Traficom) operating certificate, which itself, as a member of EASA, requires compliance with European EASA regulations. As discussed above, EASA permits use of its acceptable standard passenger weights. However, Finnair proposed a different way of complying with the regulation, and got it approved by Traficom.

    • In the USA, the FAA recently changed its AMCs related to passenger weights. Previously, as part of its Advisory Circular (AC) no. AC 120-27F, now requires each operator to set up, operate, and maintain a process to determine applicable standard weights for its passengers and cabin luggage. This only applies if an operator elects to use standard weights. Previous versions of AC 120-27F provided similar standardized weights as EASA still publishes as acceptable.

  5. The fuel required by an aircraft for a given flight is specified by its weight, not its volume. Fuel’s energy is in its mass (weight), which needs to be converted into mechanical energy of the aircraft to travel from origin to destination. As part of flight planning, the required weight of fuel is determined and can be used for weight and balance calculations.

    Adjustments to the final fuel weight can be done by flight crews, so the final weight used for weight and balance calculations needs to be verified before departure.

  6. Cargo and mail is weighed in warehouses and recorded on cargo manifests. That information can be obtained and used for weight and balance calculations.

Question #2 - how to determine balance

The regulations do not provide a clear path towards Question 2.

Determining balance requires each item of mass to be assigned a location on board the aircraft. Each location has a certain balance arm and a certain effect on the aircraft balance for a unit mass being located there.

To simplify this process, the aircraft is broken down into specific locations or zones. The location of passengers can either be done by each seat row or in specific cabin zones. The lower holds of an aircraft, where bags and cargo are loaded, is typically broken down into compartments or bays. Each seat, cabin zone, compartment, or bay, is assigned a balance arm and a per unit mass effect on balance.

Tracking the location of each mass and summing up the effects on balance, and combining with the balance of the empty aircraft, yields the final balance state.

Once this is complete, regulation paragraph CAT.MAB.POL.105 requires the results of this process and calculations must be documented and provided to the airplane captain.

Important decisions to be made

Since the final balance state of the aircraft must be within limits, there is a decision process that has to occur to go from an empty aircraft to a fully loaded aircraft that is properly in balance, or using industry terminology, an aircraft that is “in trim”.

Here is where things get a bit sensitive. 

We can’t afford to load an aircraft in a certain way, only to find out that it is outside its weight and/or balance limits when fully loaded and 5 minutes from its departure time. An “out of trim” aircraft cannot legally depart. We only get one shot at this; otherwise, we’re hit with significant cost penalties.

It is therefore imperative to load an aircraft in a way that will guarantee a fully compliant aircraft, that is within all limits, and that can depart.

To ensure this, operators use a process called Load Control. Those who work in this process and are responsible for it are called Load Controllers.

✈️ Coming up next: Part 2 - A closer look at the role of weight and balance in efficiency

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