Weight and balance
In a glider the pilot is somewhere between a fifth and a third of the all-up weight, sitting a metre or more from the centre of gravity. That makes the pilot the biggest thing about the loading that changes from one flight to the next — which means the same aircraft can be perfectly in trim with one person in it and outside its envelope with another. This works out where the CG actually sits for a given loading, and then the question pilots need far more often: which pilot weights keep this aircraft legal at all?
Every figure comes from the boxes below, and every box is filled in from your own aircraft's weighing report and flight manual. There is no aircraft data built into this page, on purpose.
Switching converts everything on the page, the moment column included. Use whichever units your weighing report is written in.
These are made-up numbers. They belong to no aircraft that exists. They are here so you can see the page work before you go and find your paperwork.
The aircraft, empty
From the weighing report in this aircraft's papers — not the flight manual, not a type page, not the sister ship in the next trailer.
Arms increase aft. If the datum is behind a station — and on many gliders the datum is the wing root leading edge, with the pilot well in front of it — that arm is negative. Type the minus sign. Everything on this page handles it.
, derived: . Check it against the other column of your report.
What is going in it
Pilot weight as flown: boots, jacket, whatever is in your pockets. The parachute rides at the same arm as the person wearing it, and it is the load people leave out most often — a rig is 7–10 kg (15–22 lb), enough on its own to move a marginal CG across a limit.
Nose ballast goes in the trim row with a negative arm; tail ballast goes in with the fin's arm. Both arms are in the flight manual's loading section, and a tail arm is typically several times a cockpit arm — which is why a couple of kilograms back there is not a rounding error.
Your limits
From the flight manual and the placards for this airframe, at its current modification state. Nothing here is supplied for you.
Placarded cockpit load — optional, and the only one of these you can read without the weighing report
This is the one people get wrong, and it is worth 7–10 kg — easily enough to move somebody across a limit. Manuals differ and the placard usually does not say. Check the manual rather than guessing; if you cannot, try it both ways and take the stricter answer.
Before this can answer
Gross weight
Total moment
CG arm
Weight to spare
Margin to the forward limit
Margin to the aft limit
| Station | Weight () | Arm () | Moment () |
|---|---|---|---|
| Total |
The total row's arm is the centre of gravity: total moment divided by total weight. Stations with nothing in them are left out.
Legal front-seat pilot weight, with everything else exactly as entered
That is the pilot alone. With the parachute entered above, the seat carries in total.
The loading you entered — in the seat — sits inside that band. The loading you entered ( in the seat) is outside that band.
The light end is ; the heavy end is .
At the light end the CG lands on ; at the heavy end, . Both ends are legal — a CG exactly on a limit is inside the envelope — but neither leaves you anything for a heavier jacket or a full water bottle.
Nothing you have entered caps the heavy end, so this range has no top. Add the maximum gross weight from your flight manual and it will have one.
Why the light pilot is the dangerous one
Almost everyone arrives at this expecting the heavy end to be the problem, because that is how weight limits work everywhere else. In a glider it is usually the other way round, and the reason is geometry rather than strength.
The pilot sits well forward of the centre of gravity — typically a metre or more ahead of it — and makes up a fifth to a third of the all-up weight. Take weight out of that seat and you remove a large forward moment, so the CG moves aft. An aft CG shortens the tail arm the elevator is working against, so the aircraft becomes less longitudinally stable: it wants to keep pitching once it has started. The stall breaks more sharply, the wing drop at the stall is harder to catch, and — the part that matters — spin recovery degrades. Past a certain point the elevator no longer has the authority to unstall the wing, and a spin that would have stopped promptly does not stop.
That is the whole reason minimum cockpit loads exist. They are not a comfort limit or a seat-strength limit; they are the point at which the CG reaches the aft end of the envelope, converted into a weight a pilot can check on the scales. The forward limit is a real limit too — run out of up-elevator at low speed and you may not be able to hold the nose up for the round-out — but the two failures are not the same shape, and only one of them is spin recovery.
On the numbers you entered: take out of the front seat and the CG moves , which is of the entire envelope. One pilot swapping for a smaller one usually does more to the balance than anything else you can change about the loading.
The fix for an under-weight pilot is routine and clubs do it every weekend: flight-manual-approved ballast, in its approved position, properly secured. Not a bag of lead in the map pocket. If your aircraft carries a placarded minimum cockpit load, put it in the placard boxes above and it becomes a fourth bound on the window alongside the two CG limits and maximum gross — and the page will tell you which of the four is the one actually stopping you, which a placard on its own never can. It also covers the loadings a placard says nothing about: two pilots, water in the tail, ballast in the nose.
Tail ballast: a small weight on an enormous arm
A moment is a weight multiplied by a distance, and the distances in a glider are wildly unequal. The cockpit is perhaps a metre from the CG. The fin is four. So a kilogram in the fin does roughly four times the work of a kilogram in the cockpit, and it does it in the opposite direction. This is why fin tanks exist at all: they let a pilot trim the balance with a mass small enough to be worth carrying.
It is also why two-seaters catch people out. Filling a fin tank feels like nothing — it is a few litres, you can lift it — and it can move the CG further than swapping the pilot for a noticeably different one. Water in the fin is a balance decision that has to be made with the whole loading in front of you, and it must be checked again when the back seat empties, because dumping the wings without dumping the tail leaves the CG somewhere nobody planned.
On the numbers you entered: at the tail arm of moves the CG . To move it that far from the front seat instead you would have to — several times the mass, for the same effect. — a comparable mass, which means the tail arm you entered is not far behind the centre of gravity. Worth checking it against the loading section of your manual.
Enter the tail tank or tail ballast arm above and this becomes a sum on your own aircraft rather than a general statement about gliders.
Water in the wings is a performance decision, not a balance one
Wing tanks sit close to the centre of gravity, near the spar, and roughly symmetrically about it. That is deliberate: a hundred kilograms of water can raise the all-up weight by a third and barely touch the balance. Ballast changes the wing loading, which moves the whole polar — better glide at speed, worse climb in weak thermals — and it changes the stalling speed, the approach speed and the energy you arrive with. What it mostly does not change is where the CG sits.
On the numbers you entered: of wing water at moves the CG , which is of the envelope. in the fin moves it further than that, on a small fraction of the mass.
Whether to carry it is a soaring question, not a balance one. It still counts against maximum gross weight, and against maximum mass of non-lifting parts if your manual sets one.
Enter the wing tank arm above and this becomes a sum on your own aircraft. It is usually close to the empty CG, which is the whole point of putting the water there.
Two cautions the arithmetic above does not cover. Water that will not come out symmetrically is a handling problem long before it is a balance one, so a frozen or partly blocked tank matters more than any number here. And a manual that sets a separate maximum mass of non-lifting parts is limiting the fuselage, not the whole aircraft; filling the wings does not relieve it.
The empty weight in the manual is not your empty weight
The figure printed in a flight manual is the one the factory recorded for a prototype or an early production aircraft, sometimes decades ago. Yours has since been repainted, possibly refinished completely, repaired, re-instrumented, fitted with a different battery in a different place, given a flight computer and an aerial, and had its seat pan padding replaced. Every one of those moves the empty weight, and moving weight at an arm moves the empty CG as well. Gliders generally gain weight over their lives, and the gain is rarely evenly distributed.
The only valid source is the weighing report in that specific aircraft's papers, and it has a date on it for a reason. Most national regimes require a re-weigh at an interval, and always after work that changes the mass or its distribution: a repair, a refinish, an instrument fit. A weighing report that has expired is not a slightly stale number, it is no number at all — you cannot know what the intervening work did. If you cannot find the report, the answer is not to use the manual's figure. It is to find the report.
Two further traps worth knowing. A club aircraft may carry a placarded loading chart worked out from an older weighing — check its date against the current report. And on any aircraft you have not flown before, check what the empty weight is defined to include: fixed ballast, the battery, the tow-out gear and the parachute are all things one manual counts and another does not.
What this assumes, and what it can't know
This page ships no aircraft data at all, and that is the design. No empty weight, no CG envelope, no maximum gross mass, no seat arm, for any type. All of those are specific to one airframe on one day — a repair, a repaint, a different battery or a new instrument panel moves them — and the only valid source is the weighing report in your own aircraft's papers. Every number above came out of the boxes you filled in. If a box is wrong, the answer is wrong, and the page has no way to know.
The arithmetic itself is exact. Moment is weight times arm; the CG is the total moment divided by the total weight. There is nothing estimated, no correlation and no rule of thumb in the gross weight, the moment or the CG. The pilot weight window is the algebraic solution of that same equation, not a search: wherever a CG limit is what bounds an end, that end lands exactly on the limit you entered, and the page says which of the three limits is doing the work at each end, because an end set by maximum gross weight calls for a different fix from one set by the CG.
Arms increase aft, and arms forward of the datum are negative. Your datum is wherever your paperwork puts it, and it is frequently the wing root leading edge, which puts the whole cockpit on the negative side. Getting a sign wrong is the single most likely way to make this page confidently wrong, and it will not look wrong — it will look like a CG in a slightly different place. Sanity-check the answer: with a pilot aboard and nothing in the tail, the loaded CG sits forward of the empty CG, because the pilot is forward of it. If yours has come out the other way, look at the signs before you believe it.
It checks three limits, and your flight manual may set more. Maximum gross weight, forward CG and aft CG. It does not check maximum mass of non-lifting parts, a maximum mass for a lower water ballast configuration, a placarded maximum or minimum cockpit load, a baggage compartment limit, a separate limit for aerobatic or cloud-flying categories, or wing loading. Read the loading section of your manual, not just the numbers this page asks for.
The parachute and the pilot are treated as one load at the seat arm. That is right for a worn rig. If yours sits somewhere else, or you carry something bulky at a different station, add it as baggage with its own arm instead.
A calculation is not a substitute for the weighing report, and neither is a substitute for asking. If your aircraft's loading is anywhere near a limit, or you are not certain about your datum, ask an instructor or the club's inspector and check it against the aircraft's own paperwork before you fly it.
Not yet checked by anyone but me. If you instruct, or one of these assumptions is wrong, I would genuinely rather hear it than not — tell me and I will credit you here. This is ground school, not a flight computer, and not an authority on your aircraft. Fly the numbers in your own flight manual and the instruments in front of you.
If all you have is the placard
You do not need the weighing report to answer "can I fly it today?". A placarded minimum and maximum cockpit load has already had the balance done for you, for the standard loading — fill in just those two boxes and the parachute convention, and leave the rest empty. What the weighing report adds is everything the placard is silent about: two pilots, water anywhere, ballast fitted, an aircraft just back from a re-weigh, and which limit is the one binding your window rather than simply that one of them is.
If you have not flown yet and have no placard either, the weight section of the learning-to-fly guide answers the question without needing anything from an aircraft, and tells you what to ask a club.