Aircraft Categories and CS-23 / Part 23

CS-23 in Europe and 14 CFR Part 23 in the United States are the airworthiness codes for light aeroplanes, from two-seat trainers to small twin turboprops and light jets. For decades they certified each design in one or more categories: normal, utility, aerobatic and commuter. The category decided how strong the structure had to be, which manoeuvres the aeroplane could fly and, for the commuter category, how demanding its performance standards were.

In 2017 both codes were rewritten as performance-based rules, with a single normal category divided into certification levels. Yet almost every light aeroplane now flying was certified under the old codes, and it keeps its old category for life. That is why the flight manual of a Cessna or a Piper still speaks of normal and utility categories, and why the PPL, CPL and ATPL syllabi still teach the traditional figures. This article covers both, marking which is which. How codes, type certificates and certification bases work in general is described in type certification.

Scope of CS-23 and Part 23

Until 2017 CS-23 was titled Normal, Utility, Aerobatic and Commuter Aeroplanes, and Part 23 carried the equivalent US title. Their scope was set by seats and mass:

• Normal, utility and aerobatic categories: aeroplanes with nine or fewer passenger seats, not counting the pilot seats, and a maximum take-off mass of 5,670 kg (12,500 lb) or less.
• Commuter category: propeller-driven twin-engined aeroplanes (multi-engined under Part 23) with 19 or fewer passenger seats, excluding pilot seats, and a maximum take-off mass of 8,618 kg (19,000 lb) or less.

Anything larger is certified to the large aeroplane code, CS-25 or Part 25, described in large aeroplane certification.

The certification thresholds are close to, but not the same as, the thresholds of the operating rules. EASA's performance Class B, for example, covers propeller aeroplanes with nine passenger seats or fewer and a maximum take-off mass of 5,700 kg or less (see performance classes), while the FAA calls an aircraft of more than 12,500 lb "large" in 14 CFR 1.1 and requires a type rating to fly it.

Normal category

The normal category is the category of touring and training aeroplanes. It is limited to non-aerobatic operation, meaning any manoeuvre incidental to normal flying, stalls except whip stalls, lazy eights, chandelles and steep turns in which the angle of bank does not exceed 60°. Intentional spins are prohibited.

The structure is designed for a positive limit load factor that depends on the design maximum weight, reaching +3.8 g for light types, with a negative limit of -1.52 g. With flaps extended the positive limit falls to +2.0 g, which is why manoeuvring with flap out is restricted.

Utility category

The utility category allows limited aerobatic operation: spins, where the type has been approved for them, lazy eights, chandelles, and steep turns with more than 60° but not more than 90° of bank. The structure is designed for +4.4 g and -1.76 g.

Many trainers are certified in both the normal and the utility categories. The utility approval then applies only inside a smaller envelope given in the flight manual, which commonly limits the mass, allows only the front seats to be occupied and narrows the centre of gravity range. Being below the normal category maximum mass is not enough: a spin is permitted only when the aeroplane is loaded within the utility limits. A type certified in two categories must meet the requirements of each, and the flight manual and placards state which manoeuvres are approved in which configuration (see spins and spiral dives).

Aerobatic category

The aerobatic category (the FAA writes acrobatic) has no restrictions on manoeuvres other than those found necessary in the certification flight tests. Aerobatic aeroplanes are designed for at least +6.0 g and -3.0 g, the negative limit being half of the positive one rather than 0.4 times it as in the other categories. Dedicated aerobatic types are generally cleared for spins, and many are built to considerably higher load factors than the minimum the code demands.

Commuter category

The commuter category was added to cover propeller-driven multi-engined aeroplanes of up to 19 passenger seats, the size of a small regional turboprop. Its manoeuvres are restricted to those incidental to normal flying, stalls except whip stalls, and steep turns of not more than 60° of bank. Its performance standards are close to those of the large aeroplane code, and ATPL performance questions test the difference:

For a single-engined aeroplane the rule is simpler still: VR may not be less than VS1. The missing accelerate-stop requirement matters in practice. On many runways, a light twin that loses an engine near lift-off can neither stop nor be sure to climb away, and the pilot has to plan for that.

Single-engine climb was not guaranteed either. Under the former CS-23.67, a twin of 2,722 kg (6,000 lb) or less whose VS0 exceeded 61 kt had to show a one-engine-inoperative climb gradient of 1.5 % at 5,000 ft pressure altitude, but a lighter or slower twin only had to have its gradient determined, and that gradient could be negative (see one-engine-inoperative en-route performance).

Load factor limits by category

The load factor limits are the figures most often examined. They come from the former CS 23.337 and the equivalent FAR 23 section:

W is the design maximum take-off weight in pounds, and n need not exceed 3.8, so every normal category aeroplane of up to about 4,100 lb is designed for +3.8 g, and heavier ones may be designed for slightly less. Under the former FAR 23.337 commuter category aeroplanes use the same weight formula.

The limit load is the greatest load expected in service, and the structure must carry it without permanent deformation. The ultimate load is the limit load multiplied by a safety factor of 1.5, and the structure must support it for at least three seconds without failure. The margin between the two is not a reserve for pilots: a load beyond the limit may bend the structure permanently, and an overstress must be reported and inspected even if nothing broke.

Gusts are covered too. At low altitude the gust envelope of a normal category aeroplane assumes vertical gusts of 50 ft/s at the design cruising speed VC and 25 ft/s at the design diving speed VD. How these figures shape the V-n diagram and the manoeuvring speed is explained in load factor and the flight envelope.

Other light aeroplane standards

Several figures that pilots meet in the pilot's operating handbook also come from the certification code rather than from the aeroplane's real capability:

• Maximum demonstrated crosswind component: Part 23 requires satisfactory controllability to be shown in a 90° crosswind of at least 0.2 VS0. The published figure is what was demonstrated, not a structural or control limit, unless the limitations section makes it one.
• Fuel quantity indicators need be accurate only when reading empty: the gauge must read zero in level flight when only the unusable fuel remains. Fuel is therefore checked visually and managed by time and consumption.
• Stall warning: light aeroplane stall warners typically sound about 5 to 10 kt above the stall, from a vane or reed on the leading edge (see angle of attack, stall warning and stall protection).

Performance-based Part 23 and CS-23

The old codes described designs in detail. That made the certification of conventional aeroplanes predictable, but new technology such as electronic engine controls, new materials, electric propulsion or simplified flight controls often fitted none of the paragraphs and needed special conditions. The FAA's Part 23 Amendment 23-64, published in December 2016 and in force from August 2017, and EASA's CS-23 Amendment 5 of 2017 turned the codes into performance-based rules. They state the safety objective to be met, and the detailed ways of meeting it are moved into accepted means of compliance, many of them industry consensus standards. The detailed figures of the old code, such as the load factors above, survive only where they are used as such means of compliance. Part 23 is now titled Normal Category Airplanes, and CS-23 Normal-Category Aeroplanes.

The new codes have one category, normal, for aeroplanes of up to 19 passenger seats and a maximum take-off mass of 8,618 kg (19,000 lb). The utility, aerobatic and commuter categories no longer exist for new designs; aerobatic capability is instead an approval within the normal category. The requirements are scaled by two measures:

The performance level is low speed when VNO and VMO do not exceed 250 kt CAS and MMO does not exceed 0.6, and high speed otherwise. Requirements are scaled to these levels, so a two-seat trainer is spared requirements meant for a 19-seat turboprop or a fast jet.

An aeroplane not certified for aerobatics may still fly the manoeuvres incidental to normal flying, including stalls other than whip stalls, and lazy eights, chandelles and steep turns of up to 60° of bank. One certified for aerobatics may be flown without manoeuvre restrictions other than the operating limitations established in its certification.

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.