Mass definitions and maximum payload

Two questions

Before any flight, the captain signs for two things: how heavy the aeroplane is, and where that weight sits.

Too heavy, and it may never leave the runway or may break in the air. Weight in the wrong place, and no one can control it. This chapter is the first question. The next is the second.

A goods truck has the same two limits. Too much load and the axle cracks. All of it at the back and the front wheels lift.

In this subject, mass and weight mean the same thing.

Strictly, mass is how much stuff there is and weight is gravity pulling on it. The exam uses the two words freely, so we will too.

The ladder

Start with something you know: your car on a weighbridge.

Just the car, with its oil and its spare wheel.

Add the driver and what the car always carries.

Ready to drive. No one else yet.

Now the family and their luggage: the reason for the trip.

Last comes petrol. It is weight too, but it burns away on the road.

An aeroplane is the same ladder, with stricter names.

Slice it open and we will fill it, one rung at a time.

Rung one: Basic Empty Mass. The aeroplane and everything fixed to it.

That includes unusable fuel and fluids, engine oil, fire extinguishers, pyrotechnics, emergency oxygen and supplementary electronics.

Rung two: Dry Operating Mass. Ready for this flight, but no usable fuel and no traffic load.

Add crew and their bags, catering, removable passenger-service equipment, drinking water and toilet chemicals. Other names you will meet: BOM, or APS (aircraft prepared for service).

Crew are in the DOM. They are never part of the traffic load.

Rung three: Zero Fuel Mass. Add the people and things that pay.

Passengers, their baggage and cargo, including any non-revenue load. Together they are the traffic load, also called payload.

Rung four: Take-Off Mass. Fuel goes in last.

What is in the tanks at brake release is the fuel on board, FOB: trip, contingency, alternate and reserve. Everything and everyone, at the start of the take-off run.

On the stand there is a little more: start and taxi fuel.

It is burned before the take-off run, so it is not part of take-off mass. Ramp mass, or block mass, is take-off mass plus taxi fuel.

Then fly the trip. Fuel burns off the top, and you land lighter.

Landing mass is take-off mass minus trip fuel. Whatever fuel was not used lands with you.

That is the whole ladder. Learn it in this order and the definitions answer themselves.

DOM + traffic load = ZFM ZFM + FOB = TOM TOM − trip = LM TOM + taxi = Ramp

Three words

This lesson is part of the Navigation plan. See plans

The ceilings

This lesson is part of the Navigation plan. See plans

Three columns

This lesson is part of the Navigation plan. See plans

Litres and kilograms

This lesson is part of the Navigation plan. See plans