Mass and balance basics

An aircraft can be under its maximum weight and still be unsafe to fly, because where the weight sits matters as much as how much of it there is.

This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.

Two questions, not one

Loading an aircraft asks two separate questions. Is it too heavy? And is the weight in the right place? Exceed the maximum mass and the aircraft may not climb or stop in the distance available; put the centre of gravity outside its limits and the aircraft can become unstable, hard to recover from a stall, or simply uncontrollable in pitch. Both have to be answered before every flight, and the method is the same arithmetic either way. The reference works are the FAA Aircraft Weight and Balance Handbook (FAA-H-8083-1B) and Chapter 10 of the FAA Pilot's Handbook of Aeronautical Knowledge.

The vocabulary

A handful of terms do all the work:

• Datum. A reference plane chosen by the manufacturer, often at the nose, firewall, or leading edge. Everything is measured from it.
• Arm. The horizontal distance from the datum to an item of mass. Arms behind the datum are positive; arms ahead of it are negative.
• Station. A location along the aircraft expressed as its arm, so "station 2.30" means 2.30 m aft of the datum.
• Moment. Mass multiplied by arm. It is the turning effect of that mass about the datum, and it is what you actually add up.
• Basic empty mass. The mass of the aircraft itself with its standard equipment and unusable fuel.
• Useful load. What you can add: crew, passengers, baggage, and usable fuel, up to the maximum.
• Centre of gravity (CG). The balance point of the whole loaded aircraft: the total moment divided by the total mass.
• CG envelope. The range, between a forward and an aft limit, within which the CG must stay.

A worked example

These are illustrative figures, not data for any real aircraft. Work each item's moment as mass times arm:

• Basic empty mass 700 kg at arm 2.30 m gives a moment of 1610 kg m.
• Pilot and front passenger 160 kg at 2.05 m gives 328 kg m.
• Rear passengers 80 kg at 3.00 m gives 240 kg m.
• Fuel 110 kg at 2.40 m gives 264 kg m.
• Baggage 20 kg at 3.60 m gives 72 kg m.

Add the masses: 700 + 160 + 80 + 110 + 20 = 1070 kg. Add the moments: 1610 + 328 + 240 + 264 + 72 = 2514 kg m. Then divide:

CG = 2514 / 1070 = 2.35 m aft of datum

If the published envelope for this loaded mass runs from a forward limit of 2.20 m to an aft limit of 2.45 m, the CG at 2.35 m is inside it, and the total mass of 1070 kg is under the maximum. The aircraft is legal to load this way. Change the baggage or move passengers and you must work it again.

What the authorities require

The arithmetic is universal, but the limits and the documentation come from the certification of the aircraft and the operating rules. The CG envelope and mass limits are set when the type is certified, under standards such as EASA CS-23 for smaller aeroplanes and CS-25 for large ones, with the FAA's equivalent parts in the United States, and they appear in the approved flight manual. For commercial operations, EASA's Air Operations rules (CAT.POL.MAB) require the operator to establish the mass and balance of each flight, all under the international standard of ICAO Annex 6. One terminology note: the FAA handbooks speak of "empty weight" and "useful load", while EASA and ICAO use "dry operating mass"; the concepts line up, but keep the framework consistent within one calculation.

Common pitfalls

• Forgetting that fuel shifts the CG. As fuel burns off, the CG moves, so the aircraft must stay in the envelope at take-off, in the cruise, and at landing, not just at the start.
• Using the wrong arm for baggage. A heavy bag in an aft locker has a long arm and a large moment, and is a common cause of an aft-CG problem.
• Assuming "under max weight" is enough. Being within the mass limit says nothing about whether the CG is in the envelope.
• Mixing units or frameworks. Keep to one set of units and one definition of empty mass throughout.

In Pilot EFB

Pilot EFB does not calculate mass and balance, and it is not a certified Electronic Flight Bag. It is a study and planning aid that keeps your weather, NOTAMs, flight time, and logbook in one offline-first place. Work your loading from the approved aircraft flight manual or pilot's operating handbook and your operator's loading procedures, which carry the real arms, limits, and CG envelope for your aircraft. Saved data stays readable offline; pulling fresh data needs a connection.