Every aircraft is certified to operate with its center of gravity (CG) within a specific range defined in the Type Certificate Data Sheet (TCDS) and the aircraft's flight manual. When an aircraft's CG falls outside that approved envelope — either too far forward or too far aft — the aircraft is not airworthy and cannot legally be flown. Sometimes, after a major modification, equipment removal, or a reweighing, the CG ends up out of limits even when the gross weight is acceptable. In these situations, a qualified Aviation Maintenance Technician (AMT) must determine whether the condition can be corrected, and if so, how much ballast to add and exactly where to place it.
Ballast is temporary or permanent weight strategically installed in the aircraft to shift the CG back into the approved range. Computing ballast requirements is a precise mathematical process rooted in the lever-law principles that govern all weight-and-balance work. Getting it right is a fundamental AMT skill and a frequently tested subject on the FAA General knowledge exam.
Understanding the CG Problem
Before calculating ballast, you must clearly identify the direction and magnitude of the CG error. From a completed weight-and-balance computation, you will have three values in hand: the aircraft's actual empty weight, its total moment, and its computed CG location (usually expressed as an arm in inches from the datum). Compare that computed CG arm against the forward and aft limits published for the aircraft at the relevant weight.
If the computed CG arm is less than the forward limit, the aircraft is nose-heavy and requires ballast to be added aft of the current CG. If the computed CG arm is greater than the aft limit, the aircraft is tail-heavy and requires ballast to be added forward of the current CG. The goal is to shift the CG to a point that falls within, or exactly on the boundary of, the approved envelope.
The Core Formula
Ballast calculations use a straightforward algebraic rearrangement of the basic moment equation. Recall that moment equals weight multiplied by arm (M = W × A), and that CG equals total moment divided by total weight. When you add a weight at a known location, both the total weight and the total moment change, and therefore the CG shifts.
The standard formula for finding the required ballast weight is:
- Ballast Weight = (Actual Total Moment − Required Total Moment) ÷ Ballast Arm
This can be re-expressed in a form many technicians find easier to visualize:
- Ballast Weight = (Aircraft Weight × (Desired CG − Actual CG)) ÷ (Ballast Arm − Desired CG)
Both forms are mathematically equivalent. The second form is particularly useful when you know where you want to move the CG (for example, exactly to the aft limit) and you know where the ballast will be physically placed. Let's walk through each variable so the formula becomes intuitive.
The Variables Defined
- Aircraft Weight: The current total weight of the aircraft (empty weight plus any items already loaded) before ballast is added, in pounds.
- Actual CG: The computed CG arm you have already determined is out of limits, in inches from the datum.
- Desired CG: The target CG arm you want to achieve. For safety and to ensure a margin from the limit, it is common practice to aim for a point slightly inside the limit rather than exactly on the boundary, though the math works either way.
- Ballast Arm: The arm (in inches from the datum) of the location where the ballast will be installed. This location must be structurally approved to accept the weight.
A Worked Example
Suppose an aircraft's current empty-weight CG is computed to be at 84.0 inches aft of the datum. The aircraft's approved forward CG limit at this weight is 86.5 inches. The aircraft is therefore nose-heavy — the CG is forward of (numerically less than) the forward limit. The empty weight is 1,850 lb. You have identified an approved ballast shelf located at a station of 185.0 inches aft of the datum.
You decide to move the CG exactly to the forward limit of 86.5 inches. Plugging into the formula:
- Desired CG − Actual CG = 86.5 − 84.0 = 2.5 inches (the shift needed)
- Ballast Arm − Desired CG = 185.0 − 86.5 = 98.5 inches (the moment arm leverage of the ballast)
- Ballast Weight = (1,850 × 2.5) ÷ 98.5 = 4,625 ÷ 98.5 ≈ 46.95 lb
You would therefore need to install approximately 47 pounds of ballast at the 185.0-inch station to bring the CG to the forward limit. In practice, you should verify the answer by recomputing the full weight-and-balance using the new total weight (1,850 + 47 = 1,897 lb) and the new total moment, confirming the resulting CG arm falls within limits.
Verification Step
Never rely solely on the formula result. After computing the required ballast weight, always perform a complete recalculation of total weight and total moment to confirm the answer:
- New Total Weight = Original Weight + Ballast Weight
- New Total Moment = Original Moment + (Ballast Weight × Ballast Arm)
- New CG = New Total Moment ÷ New Total Weight
Compare this new CG against the limits for the new total weight (since CG limits sometimes vary with weight, as shown on an envelope graph). If the new CG is within limits, your ballast solution is mathematically valid. If it is still out of limits, you must recalculate using a different ballast arm or a higher ballast weight.
Ballast Types and Installation Requirements
Ballast can be classified as temporary or permanent. Temporary ballast is used when loading conditions may vary; it must be clearly labeled and secured so it cannot shift in flight. Permanent ballast, such as lead weights bolted to an approved structure, becomes part of the aircraft's licensed empty weight and must be documented on FAA Form 337 if it constitutes a major alteration, or through other approved data if minor. The installation location must be capable of bearing the load without structural damage, and the ballast must be secured in a manner that prevents movement under any flight load condition, including aerobatic stresses if applicable.
Ballast lead is the most common material because of its high density — a smaller physical volume is needed to achieve the required weight, making installation in confined spaces practical. The Weight and Balance Handbook (FAA-H-8083-1) provides guidance on ballast materials, marking requirements, and documentation.
Key Numbers and Rules
- The CG must fall within limits defined in the TCDS and approved flight manual at all loading conditions expected in service, not just at empty weight.
- Ballast arm leverage matters enormously: a ballast location farther from the existing CG requires less weight to achieve the same CG shift. Choosing the most advantageous arm reduces added weight and structural load.
- Always aim the desired CG at a point inside the approved envelope, not exactly at the limit, to provide a margin against measurement and loading variation.
- Any ballast installation that changes the aircraft's empty weight or datum location must be reflected in updated weight-and-balance records kept in the aircraft logbooks.
- Permanent ballast that constitutes a major alteration requires a Form 337 filed with the FAA.
- A CG that is too far aft is generally more dangerous than one too far forward, because aft CG reduces longitudinal stability and can result in loss of pitch control authority.
Common Test Traps
- Confusing the direction of ballast placement. A forward CG (nose-heavy) requires ballast placed AFT; an aft CG (tail-heavy) requires ballast placed FORWARD. The test often presents a scenario to see if you match the direction correctly.
- Using the wrong arm in the formula. The formula uses (Ballast Arm − Desired CG) in the denominator, not (Ballast Arm − Actual CG). Using the actual CG instead of the desired CG produces an incorrect answer.
- Forgetting to verify with a full recalculation. The formula gives an approximation based on original weight; because adding ballast increases total weight, the final CG after adding ballast must be confirmed with a complete moment summation.
- Assuming ballast can be placed anywhere. The ballast location must be structurally approved. The exam may present an attractive arm location that is not an approved station — always work from approved data.
- Neglecting to update aircraft records. Permanent ballast changes empty weight and must be documented. The exam tests whether you know this is a required maintenance record entry, not merely optional paperwork.