Performance is the subject that turns “can we fly today?” into arithmetic. Every dispatch rests on proof, worked out before engine start, that the aeroplane at this mass can accelerate and stop or fly away on this runway, clear whatever stands beyond it, and land where it intends to. This opening lesson maps the legal machinery behind that proof: which certification rules an aeroplane is built under, which operational performance class it flies in, and how raw flight-test measurements become the cautious numbers the charts actually show. Two layers of law share the work:
Certification specifications (CSs) — the airworthiness layer. CS-23 and CS-25 tell the manufacturer what performance must be demonstrated in flight test and published in the aeroplane flight manual.
Operational requirements — the operator's layer. The air operations rules tell the operator how those published figures must be used, factored and respected on the day of flight.
Two rulebooks: CS-23 and CS-25
Which certification specification applies depends on the kind of aeroplane:
CS-23 — the light end: normal, utility, aerobatic and commuter category aeroplanes. Broadly the propeller-driven types up to 5,700 kg, with the commuter category stretching to propeller twins of up to 19 passenger seats and 8,618 kg.
CS-25 — large aeroplanes: turbine-powered aeroplanes with a maximum take-off mass above 5,700 kg. Every airliner you will fly under an ATPL is certified here.
The difference between the two codes is not paperwork, it is stringency. CS-25 assumes fare-paying passengers who did not choose their aeroplane, so it demands engine-failure accountability at the most critical point of every stage of flight: take-off, climb-out, en route and landing must all still work with an engine gone. Its published data also carry built-in regulatory margins. CS-23 asks far less of small aeroplanes: performance is demonstrated and published, but mostly without an engine-failure case for the singles and with lighter margins, on the logic that these aeroplanes fly fewer people who accept more risk.
The performance classes
On the operational side, the requirements a commercial air transport flight must meet are graded to match the aeroplane's capability rather than written once for everyone. The basic idea is simple: the more capable the aeroplane, the more the rules demand of it. The applicable operational requirements sort aeroplanes into three performance classes:
Performance class A — all multi-engine turbojet aeroplanes, plus multi-engine turboprops with more than 9 passenger seats or a maximum take-off mass above 5,700 kg. Most are CS-25 aeroplanes, flown to the most demanding rules; a CS-23 commuter turboprop with more than 9 seats lands in class A too.
Performance class B — propeller-driven aeroplanes with 9 or fewer passenger seats and a maximum take-off mass of 5,700 kg or less, single-engine types included.
Performance class C — piston-engined aeroplanes with more than 9 passenger seats or a maximum take-off mass above 5,700 kg: the old piston airliners, now vanishingly rare in European commercial air transport.
Note the asymmetry the exam loves: every multi-engine jet is class A, regardless of how small it is or how few seats it has. A six-seat business jet flown commercially is class A; a six-seat turboprop twin of the same size is class B. The seat and mass thresholds only ever decide the fate of propeller aeroplanes.
One level of safety, two layers of rules
What do the operational rules actually do with the certified figures? Their most visible product is a mass limit: for commercial air transport, the performance required on the day may limit the mass at which the aeroplane dispatches. If the runway is short, the air hot or an obstacle awkward, the payload comes off until the required performance is achievable. Mass is the one variable the crew can always trade for margin.
The minimum level of safety itself is delivered by the two layers working together: airworthiness requirements fix what the aeroplane demonstrably can do, and operational limitations fix how much of that capability a flight may plan to use. The layers also trade against each other. Because CS-25 certification already accounts for an engine failure at every stage of flight, class A aeroplanes are permitted a wider range of operating conditions than lighter types: they may fly the schedules, the weather and the runways that class B operations must refuse. The stricter code buys the wider franchise, and the combined safety level comes out the same.
Measured, gross and net
Where do the numbers in the flight manual come from? Every performance figure climbs a three-rung ladder, each rung more cautious than the last:
Measured performance — what certification flight testing recorded: a new airframe and new engines, flown by test pilots to precise techniques, over many repeated runs, under conditions line flying rarely matches, so it flatters what an average crew will achieve.
Gross performance — the average of the measured data: the performance an average aeroplane of the type, flown by a crew of average skill, has a 50 per cent chance of achieving on any given day.
Net performance — gross performance diminished by safety factors, down to a level so poor that the fleet is very unlikely ever to fall short of it. Net is the performance the law lets you count on: obstacle clearance and performance scheduling are worked in net figures.