Large Aeroplane Certification (CS-25 / Part 25)

Every airliner that carries passengers in Europe or the United States has been shown to meet CS-25, EASA's certification specification for large aeroplanes, or 14 CFR Part 25, the FAA's airworthiness standards for transport category airplanes. The two codes are largely harmonised, and most of what an airline pilot learns as a figure or a limitation starts in one of their paragraphs: the take-off and landing speeds, the stall warning margin, the approval to fly in icing, the number of passengers a cabin may hold.

This article covers the parts of the codes that pilots meet most often: their scope, the icing envelopes and the approval to fly in icing, the stall warning margin, and the exit and evacuation standards. How a design is certified against the code, with special conditions, equivalent safety findings and type certificate data sheets, is explained in type certification. The system safety rule, 25.1309, has its own article: failure conditions and system safety.

Scope of CS-25 and Part 25

CS-25 covers large aeroplanes and Part 25 transport category airplanes. In practice they take over where the light aeroplane codes stop: an aeroplane with more than 19 passenger seats or a maximum take-off mass above 8,618 kg (19,000 lb) is beyond CS-23 and Part 23 (see aircraft categories and CS-23 / Part 23).

The European code descends from JAR-25, which kept the paragraph numbering of the US FAR 25, so CS 25.207 and 14 CFR 25.207 deal with the same subject. The paragraphs are grouped in subparts: B for flight (performance, controllability, stalls), C for structure, D for design and construction, E for powerplant, F for equipment and G for operating limitations and information, followed by appendices that define conditions such as icing envelopes. EASA publishes its acceptable means of compliance (AMC) in the same book as the specifications; the FAA publishes its guidance in advisory circulars.

Some of the figures the codes set are the basis of other Library articles:

• Performance: V2 at least 1.13 VSR, VREF at least 1.23 VSR0 (see take-off speeds).
• Structure: a positive manoeuvring limit of +2.5 g for aeroplanes above 50,000 lb and -1.0 g, with +2.0 g with flaps extended (see load factor and the flight envelope).
• Bird strike: continued safe flight and landing after striking a 4 lb (1.8 kg) bird at VC at sea level or 0.85 VC at 8,000 ft, whichever is more critical.
• Airspeed indication: an aural overspeed warning at VMO + 6 kt or MMO + 0.01.

Codes are amended continually, but a design keeps the amendment in force when it was applied for, so aircraft of one type can meet different standards depending on when and how they were certified.

The icing envelope (Appendix C)

Approval to fly in icing is not automatic. If the applicant seeks it, CS 25.1419 and 14 CFR 25.1419 require the aeroplane to operate safely in the icing conditions of Appendix C. The appendix defines two families of conditions:

• Continuous maximum conditions, typical of layer cloud: moderate liquid water content over long distances.
• Intermittent maximum conditions, typical of cumuliform cloud: higher water content over shorter distances.

Both are cloud droplet conditions, with mean effective droplet diameters of up to 50 µm. For each, the appendix tabulates liquid water content against temperature, altitude and cloud extent, and a second part defines the ice accretions the applicant must assume for each phase of flight when showing performance and handling qualities with ice on the unprotected surfaces. The ice protection systems, the stall warning settings in icing and the ice speed additives in the flight manual all come from this work. The A320, for example, is fully certified in Appendix C conditions with its anti-ice systems on.

SLD icing (Appendix O)

Drops larger than 50 µm, the supercooled large drops (SLD) of freezing drizzle and freezing rain, lie outside Appendix C. They strike farther aft than cloud droplets and can build a ridge of ice behind de-icing boots or heated leading edges, where nothing can remove it.

That mechanism caused the loss of an ATR 72 near Roselawn, Indiana, in October 1994, when a ridge of ice behind the boots reversed the aileron hinge moments and the aeroplane rolled out of control (see airframe icing). The response was Appendix O, an SLD envelope covering freezing drizzle, with drops of about 50 to 500 µm, and freezing rain, with larger drops. The FAA added it to Part 25 in 2014 and EASA to CS-25 at Amendment 16 in 2015, together with an Appendix P for mixed-phase and ice crystal conditions in deep convective cloud.

The associated rule, 25.1420 on supercooled large drop icing conditions, does not require every aeroplane to fly through freezing rain: a manufacturer may certify for none, part or all of the Appendix O envelope. Aeroplanes certified before Appendix O carry flight manual limitations instead, with visual cues that call for leaving the conditions at once: ice aft of the protected areas, ice on the side windows, or ice on the propeller spinner.

Flight into known icing approval

Flight into known icing (FIKI) is the usual name, especially in general aviation, for an aircraft's approval to fly in the icing conditions of Appendix C. Light aircraft are certified against the same envelope through the Part 23 and CS-23 icing requirements. A light aeroplane with FIKI approval typically has wing and tail de-icing or anti-icing, propeller de-icing, heated pitot and stall warning sensors, windshield protection and an ice inspection light for night flight, and the approval holds only with all of it working. Aircraft with equipment for inadvertent icing only, such as a heated pitot, are not approved and must leave icing immediately.

The operating rules turn the approval into a limit. EU rules such as CAT.OP.MPA.255 allow flight into expected or actual icing only in an aircraft certified and equipped for it. The FAA frames the same idea as flight into known icing conditions, which its 2009 legal interpretation defines from everything a prudent pilot would know: temperature, visible moisture, forecasts such as G-AIRMET Zulu and pilot reports.

Stall warning margin

A stall must never come as a surprise. Under CS 25.207 and 14 CFR 25.207, which are harmonised, a clear and distinctive stall warning must begin at a speed that exceeds the stall identification speed by at least 5 kt or 5 % CAS, whichever is greater, when speed is reduced at no more than 1 kt per second. Once started, the warning must continue until the angle of attack is reduced to about the value at which it began. The warning may be natural buffet or a device such as a stick shaker, but a visual indication that needs attention inside the flight deck is not acceptable on its own.

The margin works together with the reference stall speed VSR, the 1 g stall speed on which the operating speeds are based. Where a stick pusher is fitted, VSR must be at least 2 kt or 2 % above the pusher speed. Light aeroplanes have no equivalent fixed margin, and their stall warners typically sound about 5 to 10 kt above the stall.

Ice lowers the angle of attack at which the wing stalls, so a warning set for the clean wing may come late or not at all. Certification in icing therefore checks stall warning and stall characteristics with the Appendix C ice shapes, and manufacturers add procedures such as the 737's VREF ICE. The details of warning and protection systems are in angle of attack, stall warning and stall protection.

Emergency exit types

CS 25.807 and 14 CFR 25.807 classify emergency exits by size and position, and the number and type of exits on each side of the fuselage set the maximum passenger seating capacity. At CS-25 Amendment 13 the seats permitted for each exit of a type on each side are:

The Type III exit is the overwing hatch of narrow-body airliners: an opening at least 20 in wide and 36 in high, reached over a step and leading onto the wing. Type III and Type IV exits are self-help exits, opened by the passengers seated beside them rather than by cabin crew, which is why exit row passengers are briefed and must be able to operate them. On the ground, every non-overwing exit whose sill is more than 1.83 m (6 ft) above the ground needs an assisting means, normally an escape slide.

The A320 shows how the table works. It has a floor-level door at each end and two overwing Type III exits between them on each side, written C-III-III-C in its type certificate data sheet: 55 + 35 + 35 + 55 = 180 passengers. Its floor-level doors were certified as oversized Type I exits performing as Type C. Higher figures on later models come from modifications that credit over-performing exits with more seats after comparative evacuation tests, and every cabin layout still needs its own approval.

90-second evacuation demonstration

The exit arrangement has to work in practice. Under CS 25.803 and 14 CFR 25.803, an aeroplane with more than 44 passenger seats must show that its maximum passenger load, together with the crew required by the operating rules, can be evacuated to the ground within 90 seconds under simulated emergency conditions, with only half of the exits usable. The conditions of the demonstration are set out in Appendix J to the code.

For a derivative of a tested type the authority may accept a combination of analysis and tests instead of a new full-scale demonstration. Airbus's approvals for over-performing A320 exits, for example, combined comparative tests on a mock-up and partial evacuation tests with an analysis based on the original A320 full-scale demonstration.

Real evacuations can be slower than the demonstration, with smoke, damage, slopes, blocked exits and passengers taking bags. The standard is a design benchmark, not a prediction, and the procedures that make it achievable, from door arming to evacuation commands, are covered in emergency evacuation.

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.