Weight and balance

The datum and the sign convention

The datum is a reference plane chosen by the manufacturer. It need not be on the aircraft — it may sit ahead of the nose, so that every arm is positive. Your RFM's weight and balance section defines it and lists the arm of each seat, hold and tank.

Lateral arms are measured from the centreline, positive on one side and negative on the other. Which side is positive is the RFM's convention; get it wrong and a load on the right looks like a load on the left.

Sources 1, 2

A worked example (illustrative numbers — not AS350 data)

The same flight on landing (illustrative — not AS350 data)

The longitudinal envelope

The RFM plots the approved CG range against mass: a forward limit and an aft limit, which may change with mass. Plot both the take-off and the landing point; each must be inside.

At the forward limit, aft cyclic is what runs short — for a flare, a quick stop, a tailwind hover. At the aft limit, it is forward cyclic — for accelerating, or holding position in a headwind. Either way the shortfall shows up in exactly the manoeuvre you need.

Sources 1, 3

The lateral envelope

The lateral limits form a narrow band either side of the centreline — far narrower, in distance, than the longitudinal range. A modest mass well out to one side can use a large part of it.

Check it whenever the cabin is loaded unevenly: one heavy passenger, a pilot alone, cargo on one side, a hoist, a side-mounted basket or a bubble-window seat used for vertical reference.

Source 1

Lateral CG, worked (illustrative — not AS350 data; right = +)

Single pilot in the right seat

The Astar is flown from the right seat, so a pilot alone puts their whole mass on one side. Light, with little fuel and nothing else aboard, that can be a noticeable share of the lateral range.

Load the first passenger or the heaviest cargo on the left where you can. Check your RFM for any restriction on solo flight or on loading the right side.

Source 1

Fuel burn and the landing CG

As fuel burns, the CG moves. Which way and how far depends on where the tank sits relative to the loaded CG. Your RFM's loading tables give the tank's arm; the Fuel system chapter shows where it sits.

Check the CG for take-off and for landing with minimum fuel. A load that is in limits at departure can be out of them on arrival.

Sources 1, 4

External load and the CG

A load on the hook hangs from a point close to the aircraft's CG range, so it adds mass without moving the CG much — while it hangs straight down. That is why external loads can be large.

A load that swings, or a long line dragged off the vertical, pulls sideways and fore-and-aft through the hook. Meanwhile the aircraft's own lateral CG — pilot right, fuel burning — still applies, and the External load chapter's right-seat, bubble-window posture adds to it.

Sources 1, 5

Hoists and doors-off

A hoist hangs a load off one side of the aircraft at the end of its boom — about the worst place for lateral CG. As the load comes up and in, the moment changes, and so does the cyclic you need.

Doors-off work moves people too: a crew member leaning out, or a photographer moving across the cabin, shifts the lateral CG in flight. The hoist supplement and the operator's procedures set where people sit and how much can be lifted.

Sources 1, 6

Useful load arithmetic

Useful load is maximum mass minus empty mass: crew, passengers, cargo and fuel all come out of it. Every kilogram of fuel is a kilogram of payload you can't carry.

Start from the aircraft's own empty mass — the weight and balance record, including fitted equipment such as a hook, floats or air conditioning — not the type's brochure. Two aircraft of the same model rarely weigh the same.

Sources 1, 4