Weight and balance
Enter the figures from your flight manual and see the loaded point on the envelope, at the ramp and after the fuel burn.
This is a cross-check, not an authority. It re-does your arithmetic so a transposed digit shows up. Every figure below must come from the flight manual and weighing record for this aircraft. No aircraft data is built into this tool, deliberately.
The arithmetic is trivial. That is the problem.
Weight and balance is a column of multiplications and one division. Nobody gets it wrong because they misunderstand it. They get it wrong because they typed 1,470 as 1,740.
Every item aboard has a weight and an arm, the arm being its distance from an arbitrary reference point the manufacturer picked, called the datum. Multiply the two and you have a moment. Add up all the weights, add up all the moments, divide one by the other, and the answer is the centre of gravity: the single point the whole aircraft balances about.
That number then has to sit inside a shape published in the flight manual, the envelope, whose horizontal axis is centre of gravity and whose vertical axis is weight. Inside it the aircraft handles as certificated. Outside it, which way it is out decides what goes wrong: too far aft the aeroplane is unstable and hard to recover from a stall, and a spin can go flat with recovery difficult or impossible. Too far forward there may not be enough elevator authority to rotate on takeoff or to flare on landing, and both runs lengthen. Those are the FAA's words, cited in the notes at the foot of the page.
Why a second pass is worth doing
Because the failure is mechanical, an independent recalculation catches it. That is all this is: the same figures, arithmetic done again, and the result plotted rather than described. If the two answers differ, one is wrong, and it is much better to discover that on the ground.
The case a single calculation misses
Fuel is weight at an arm, so burning it shifts the balance of everything left aboard. If the tanks sit forward of the loaded centre of gravity, burning fuel moves the centre of gravity aft — towards the aft limit, which is the limit that matters for stall recovery.
A load can therefore be comfortably legal at the ramp and outside the envelope by the time you arrive. Nothing in a takeoff-only calculation reveals it. Enter the planned burn and the tank arm and both points appear on the chart, with a line showing the path between them.
What this tool refuses to do
It ships no aircraft data at all: no presets, no example airframes, no envelope library. Two aircraft of the same type have different empty weights, and a repaint or an avionics change moves them again. The current figures are on the weighing record for your tail number, and a plausible-looking number from somewhere else is worse than a blank field.
There is one Load synthetic example button, and what it loads is invented: round numbers in the classic envelope shape, there so the chart has something to draw while you read this. It is not any aircraft, real or typical. While those figures are on the form the page says so in a banner above the tool, and the banner goes when you change them.
It also does not remember anything between visits, which means re-typing the aircraft figures. That is a deliberate trade against storing an empty weight that quietly goes out of date.
Terminology
- Datum
- An arbitrary reference point chosen by the manufacturer. All arms are measured from it, and it need not be anywhere on the aircraft.
- Arm
- Distance of a station from the datum. Negative arms are normal and legitimate.
- Moment
- Weight multiplied by arm. Some manuals divide it by 100 or 1,000 to shorten the figure and call the result a moment index. This tool works in arms and whole moments, so enter the arm from the manual, never an index.
- Centre of gravity
- Total moment divided by total weight. The point the aircraft balances about.
- Envelope
- The certificated region of weight against centre of gravity, published as a chart in the flight manual.
- Ramp weight
- Everything aboard before moving. What most of this calculation is about.
Questions pilots actually ask
Why does this not have my aircraft built in?
Because the numbers would be wrong for your aeroplane, and a wrong number that looks authoritative is more dangerous than a blank field.
Two airframes of the same type do not share an empty weight or an empty moment. A repaint, an avionics upgrade, a new interior, a wing-tip modification — every one of them changes it, and the current figures live on the weighing record for your tail number, not in a database. The envelope corners come from the flight manual for the same reason.
So this tool is deliberately empty until you fill it in. It is a calculator, not a reference.
What is this actually good for, then?
Catching arithmetic mistakes. Weight and balance is a column of multiplications and one division, done by hand, usually in a hurry, often on a kneeboard. The failure mode is not conceptual — it is a transposed digit or a mis-multiplied moment.
Enter the same figures here and you get a second, independent pass, with the point plotted so it is obvious rather than inferred. If the two answers disagree, one of them is wrong and it is worth finding out which before you fly.
Why does the centre of gravity move as fuel burns off?
Because fuel is weight at an arm, and taking weight away from one arm shifts the balance of everything that is left.
Which way it moves depends on whether the tanks are forward or aft of the loaded centre of gravity. If the fuel sits forward of it, burning fuel moves the centre of gravity aft, towards the aft limit. That is the case worth checking, because it means a load can be perfectly legal at the ramp and outside the envelope on arrival — and a takeoff-only calculation never shows it.
Enter the planned burn and the tank arm and both points are plotted, with a line between them showing the path.
The point is outside the envelope. What now?
Look at which limit first, because the fixes are opposite.
Over maximum weight: nothing else matters until that is fixed. No redistribution makes an overweight aircraft legal — weight has to come off.
Aft of the aft limit: move weight forward, or take it out of an aft station. Forward of the forward limit: the reverse. The tool works both out for you, from the station furthest in the offending direction: how much to move, and how much to leave behind. It only offers an amount that station is actually carrying, and it checks each answer by running the whole calculation again on the changed load, so a fix that would take you off the bottom of the envelope is not offered at all. It never proposes unloading the empty weight, which is the aeroplane.
Aim for inside the envelope, not for the limit itself. Solving exactly to a limit leaves no margin for an error in any of the figures you typed.
It says the load cannot be checked as entered. What is wrong?
Something was typed in that could not be used, and the tool will not give a verdict over a hole in the arithmetic.
Almost always it is a row with a weight and no arm. Without both there is no moment, and assuming an arm of zero would move the centre of gravity to somewhere nobody asked for. So the row is left out, and that makes the total come out lighter than the aeroplane actually is. Since that is the direction that hurts, the tool says which rows it could not read and withholds the verdict until they are filled in or cleared, rather than showing a total that looks reassuring.
An envelope corner missing one of its two figures does the same thing to the shape being tested, and a weight typed with a minus sign in front of it is treated the same way.
Is being exactly on a limit legal?
Yes — a published limit is inclusive, and the tool reports that case as within limits rather than rounding it to one side or the other. It also says outright that there is no margin, which is the part that matters.
It is worth knowing that at exactly maximum weight there is no vertical margin at all, because the top of the envelope is a horizontal edge: every permitted centre of gravity at that weight sits on the boundary by definition.
Can I use kilograms and millimetres?
Yes. The arithmetic only requires that all the weights share one unit and all the arms share another, so pounds and inches or kilograms and millimetres both work. The unit selectors change the labels on the output.
Nothing converts between systems, on purpose. A conversion you did not ask for is exactly the kind of silent transformation that has no place in this calculation — if your manual is in kilograms, work in kilograms.
Does anything I type get uploaded?
No. This site is static files with no server. Everything is calculated in your browser and nothing is stored or transmitted.
Nothing is saved between visits either, which does mean re-entering the aircraft figures each time. That is a real inconvenience and a deliberate trade: the alternative is a stored empty weight that silently goes stale after the next weighing.
Can I rely on this for a flight?
No. Rely on the flight manual and your own calculation. This is a cross-check, and it can only be as right as the figures you type into it.
It cannot know that an arm was mis-typed, that the empty weight predates the last equipment change, or that your aircraft has a limitation the envelope does not capture — a baggage compartment maximum, a zero-fuel weight limit, or a landing weight lower than the takeoff weight. Those are in the manual and the manual is the authority.
Notes
This is a cross-check and not an authority. The flight manual for your aircraft is the authority, and it can only be as right as the figures entered here. It cannot detect a mis-typed arm, an empty weight that predates the last equipment change, or a limitation that is not a simple envelope — a baggage compartment maximum, a zero-fuel weight limit, or a maximum landing weight below the maximum takeoff weight.
No aircraft data is built in. Every weight, arm and envelope corner comes from you. This is the same decision as the BS 7671 circuit check on this site: where the authoritative figures live in a document the user already has, asking for them beats shipping a copy that goes stale.
The arithmetic is unit-agnostic and nothing converts between systems. Weights must share a unit and arms must share a unit; pounds and inches or kilograms and millimetres both work.
Being exactly on a published limit is reported as within limits, because a limit is inclusive. The envelope test distinguishes inside, on the boundary and outside rather than rounding the boundary case, and it says when there is no margin left.
What is quoted from a source, and where it comes from. The consequences of being out of limits described above are the FAA's, from chapter 1 of the Aircraft Weight and Balance Handbook(FAA-H-8083-1B): a CG too far aft leaves the aeroplane unstable and difficult to recover from a stall, and a spin entered from there "could become flat making recovery difficult or impossible", while a CG too far forward can leave too little elevator authority to rotate at takeoff speed or to flare on landing. The moment index mentioned in the terminology, a moment divided by 100 or 1,000, is from chapter 5 of the same handbook. Nothing else on this page is quoted from anywhere: the arithmetic is arithmetic, and every figure in the tool is yours.
If something here disagrees with your manual, the manual is right and I would like to know: tell me.