Performance Classes and Data

An aeroplane's flight manual contains the performance it achieved in flight test: how far it needs to take off and land, how steeply it climbs, how it behaves with an engine failed. How that data may be used for a commercial flight depends on the aeroplane's performance class. EASA's air operations rules sort aeroplanes into Class A, B or C by engine type, passenger seating and maximum take-off mass, and give each class its own set of requirements for take-off, en route and landing.

The data themselves come in two forms. Gross performance is what an average aeroplane of the type achieves; net performance is gross performance reduced by a safety margin. Knowing which one a chart shows, which conditions it assumes and which factors still have to be applied is most of the skill of performance planning, in the flight manual and in the CAP 698 examination manual alike.

Performance classes A, B and C

The classes are defined in Annex I to Regulation (EU) No 965/2012 and applied by the performance rules of Part-CAT (CAT.POL.A):

Seating is measured by the maximum operational passenger seating configuration (MOPSC), which the operator sets, so the same turboprop can be Class B with nine seats and Class A with more. Both Class B limits must be met. Over either one, a multi-engined turboprop is Class A and a piston aeroplane Class C, while a single-engined turboprop falls outside all three classes.

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Class A aeroplanes

Class A is the most demanding class, because its rules assume that an engine can fail at the most critical moment of every phase of flight. Most are certified to CS-25, the code for large aeroplanes, and they carry complete scheduled data: accelerate-stop and accelerate-go distances around a decision speed V1, take-off climb gradients with one engine inoperative, a net take-off flight path for obstacle clearance, en-route one-engine-inoperative data, and landing and go-around climb data.

The take-off distance is measured to a screen height of 35 ft, or 15 ft on a wet runway, and the maximum take-off mass is the lowest of several limits: the field length, climb, obstacle, brake energy, tyre speed, runway strength and maximum structural limits. At the other end of the flight, a turbojet must be able to land from 50 ft within 60 per cent of the landing distance available, a factor of 1.67 on the flight manual distance, and a turboprop within 70 per cent; a forecast wet runway adds 15 per cent to the required distance. Take-off is covered in take-off distances and field length and take-off climb segments.

Class B aeroplanes

Class B covers most commercial singles and light twins, certified to CS-23. Its take-off distance is measured to a 50 ft screen, and the flight manual figure must be factored:

• with no stopway or clearway, the take-off distance × 1.25 must not exceed the take-off run available (TORA);
• where a stopway or clearway is used, the unfactored distance must not exceed TORA, the distance × 1.15 must not exceed the take-off distance available, and the distance × 1.3 must not exceed the accelerate-stop distance available.

Non-commuter Class B twins have no accelerate-stop certification requirement, so the rules do not rely on one. A multi-engined Class B aeroplane must, however, meet climb gradients and clear obstacles by a vertical margin of 50 ft along a take-off flight path in which the all-engines gradient is taken as 77 per cent of its gross value until the engine is assumed to fail. At the destination the aeroplane must be able to land from 50 ft within 70 per cent of the landing distance available, a factor of 1.43, with a further 15 per cent if the runway may be wet. The en-route rules are in one-engine-inoperative en-route performance.

Class C aeroplanes

Class C is the class of large piston-engined aeroplanes. It has its own take-off, en-route and landing requirements in Part-CAT, but few such aircraft remain in commercial service, because modern large transports are turbojets or turboprops and therefore Class A.

Gross and net performance

Gross performance is the average performance that a fleet of aeroplanes of the type should achieve if properly maintained and flown to the techniques in the flight manual. It is neither a best case nor a guarantee: on a given day, an individual aeroplane may do slightly better or slightly worse.

Net performance is gross performance reduced by a margin that covers variations in individual aircraft, piloting technique and atmospheric conditions, so that it is very unlikely not to be achieved. Net data are used wherever the rules require a margin of safety, above all for obstacle and terrain clearance.

For climb, the margin is a subtraction from the gradient. A gross climb gradient is the gradient the average aeroplane achieves; a net climb gradient is that gradient less a fixed decrement:

A Class A twin that must show a gross second-segment gradient of 2.4 per cent therefore has a net gradient of 1.6 per cent to clear obstacles with. For distances the margin is a factor instead: the Class B landing factor of 1.43, applied to the gross landing distance, gives the net, or required, landing distance.

Performance planning factors

Planning factors are multipliers applied to flight manual data. They are of two kinds:

• Regulatory factors, the dispatch margins required by the rules, such as the Class B take-off factors of 1.25, 1.15 and 1.3 and the landing factors of 1.43 and 1.15.
• Correction factors that adjust the data to conditions the chart does not cover. For Class B, when the flight manual gives no better data, take-off distance is multiplied by 1.2 on dry grass up to 20 cm long on firm soil and by 1.3 on wet grass, and increased by 5 per cent for each 1 per cent of upslope; landing distance is multiplied by 1.15 on dry grass and increased by 5 per cent for each 1 per cent of downslope.

Factors multiply; they are never added. A 400 m take-off distance from short dry grass (1.2) with a 2 per cent upslope (1.1) becomes 400 × 1.2 × 1.1 = 528 m, and a commercial flight then applies the 1.25 dispatch factor on top. Reported wind is not a factor on distance but an input: no more than 50 per cent of a headwind component and no less than 150 per cent of a tailwind component may be used. The corrections are described in factors affecting take-off and landing performance.

Associated conditions

Every performance chart is valid only for its associated conditions, the assumptions printed with it. A typical Class B take-off graph assumes full take-off power, a stated flap setting, a paved, level and dry runway, and the rotation and screen speeds given in its inset box. A climb graph assumes a power setting, mixture, flap and gear positions, cowl flaps and a single climb speed. Flying a different speed or configuration means the chart no longer applies. The data were also obtained with a new aeroplane and engine, flown by a test pilot using exactly the stated technique, which is one reason the regulatory margins exist.

Reading charts and interpolation

Flight manual data come as graphs, tables or both. The CAP 698 Aeroplane Performance Manual, published by the UK Civil Aviation Authority for the European theory examinations, uses the same reference aircraft as the flight planning manual CAP 697: a single-engine piston, a multi-engine piston and a medium-range jet transport.

Many of its graphs are drawn as a series of grids, or carpets, each handling one variable, and are known as carpet plots. A Class B take-off graph, read from left to right, typically has a carpet for pressure altitude and temperature, then one for mass, then one for wind, and finally one for obstacle height, or the 50 ft screen if there is no obstacle. At each carpet the reading line is carried from its reference line parallel to the nearest guide line before moving on. The wind carpet already contains the 50 per cent headwind and 150 per cent tailwind factoring, which is why its tailwind lines are steeper; the actual reported wind is entered directly. Climb graphs give an air gradient in still air, which must be corrected for wind before it is used against obstacles on the ground.

Tables require interpolation. It is linear and in proportion to where the actual value lies. If a table gives 420 m at 1,000 ft and 500 m at 3,000 ft, the distance at 2,500 ft is 420 + 0.75 × 80 = 480 m; taking the midpoint, 460 m, is the common error. Where a value cannot be read with confidence, the conservative practice is to use the more demanding adjacent entry, never the more favourable one.

Short-field and maximum-effort take-off

Published take-off distances assume a specific technique, and a short-field take-off chart assumes the most demanding one: the stated flap setting, full power set before brake release, rotation at the stated speed and a climb at the best angle of climb speed Vx to the obstacle height. A pilot who uses a rolling start, less flap, a late rotation or a climb at Vy will need more distance than the chart shows.

For multi-engined Class B aeroplanes, CAP 698 contains both a normal take-off graph and a maximum-effort take-off graph. The maximum-effort graph uses the short-field flap setting and technique and gives a shorter distance, but only when that technique is flown exactly. After lift-off the aeroplane climbs at Vx until the obstacle is cleared, then accelerates to Vy or the normal climb speed.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.