--- title: "CS 25.563 Structural ditching provisions — CS-25 · Avioverse" description: "(See AMC 25.563) If certification with ditching provisions is requested, those parts of the airframe structure that are necessary to maintain flotation…" url: "https://avioverse.aero/regulations/easa/cs-25/subpart-c/cs-25-563" --- # CS 25.563 Structural ditching provisions Large Aeroplanes (CS-25) · CS-25 · EAR revision 26 Jan 2023 IR Implementing rule ## CS 25.563 Structural ditching provisions (See AMC 25.563) If certification with ditching provisions is requested, those parts of the airframe structure that are necessary to maintain flotation of the aeroplane must withstand ditching loads, considered as ultimate, associated with a planned emergency landing on water. Damages may occur provided that these are accounted for in the assessments required by CS 25.801(b), and that the airframe structural integrity is maintained. The airframe loads must account for reasonable variations in the flight parameters when the aeroplane enters the water. IR · CS 25.563 — CS-25 · ED Decision 2003/2/RM · CS-25 Easy Access Rules · EAR revision 26 Jan 2023 AMC Acceptable means of compliance ## AMC 25.563 Structural ditching provisions 1\. Purpose AMC 25.563 primarily provides guidance and acceptable means of compliance with the ditching certification specifications of CS 25.563 (Structural ditching provisions). It also includes guidance that may be used when showing compliance with the following certification specifications that are applicable to ditching certification: CS 25.801 (Ditching), CS 25.809(g) (Emergency exit arrangement), CS 25.1581 (Aeroplane flight manual — General). Note: The following certification specifications are also related to ditching certification: CS 25.807(i) (Emergency exits), CS 25.1411 (Safety equipment — General) and CS 25.1415 (Ditching equipment). 2\. Definitions 2.1. Buoyancy. On aeroplanes, buoyancy features allow the aeroplane to float and include, but are not limited to, the portion of the following features that displace water: fuel tanks, pressure vessels and any other items that can be shown to remain intact after the ditching event and to displace water (e.g. structure and systems of the aeroplane, landing gear, bell jar volume of the landing gear wheel wells). 2.2. Ditching. An emergency landing on water, either planned or unplanned (see definitions below). In principle, the phases of a ditching event are as follows: (a) the ‘approach’ phase concerns what happens before the initial contact with the water; (b) the ‘impact’ phase concerns what happens from the first water contact to immersion in the water; (c) the ‘deceleration’ phase concerns what happens while the aeroplane is gliding until it comes to rest in the water; (d) the ‘flotation’ phase concerns the depth and attitude of the aeroplane in the water over time; (e) the ‘evacuation’ phase concerns the time it takes to fully evacuate the aeroplane. Note: some of these phases overlap. 2.3. Ditching exit. To qualify as a ditching exit, the exit sill must be initially above the waterline, and it should remain above the waterline for the duration of the evacuation during a planned or unplanned ditching. Note: If it can be shown to still be conservative, an exit may qualify as a ditching exit even if it does not remain above the waterline for the full duration of the evacuation. The substantiation of conservatism should include an assessment of how long the ditching exit remains above the waterline, the number of persons expected to remain in the aeroplane when the ditching exit sill(s) go(es) below the waterline and the number of other ditching exits remaining above the waterline. 2.4. Evacuation time. The time it takes for all occupants to exit the aeroplane. The evacuation is assumed to start when the aeroplane comes to rest in the water. In the case of a planned ditching, the evacuation time ends when the last aeroplane occupant leaves the aeroplane and enters a raft. In the case of an unplanned ditching, the evacuation time ends when the last aeroplane occupant leaves the aeroplane and enters a slide/raft, enters the water or steps on the wing. 2.5. Flotation time. The time from when the aeroplane comes to rest in the water to when the aeroplane condition is such that occupants can no longer safely evacuate. Note: For certification purposes, the flotation time is generally considered to be the time from when the aeroplane comes to rest in the water to when the first ditching exit sill goes below the waterline, or when the attitude of the aeroplane is such that it would require extraordinary effort to move through the cabin to reach available ditching exits. However, if it can be shown to still be conservative, the flotation time may be extended. Evidence of conservatism should include an assessment of the number of persons expected to remain in the aeroplane when the ditching exit sill(s) go(es) below the waterline, the number of ditching exits remaining above the waterline and the attitude of the aeroplane. 2.6. Inadvertent water entry. Runway overshoot (at take-off or landing) or runway undershoot (at landing) that results in the aeroplane alighting on water. This type of event is considered to be a minor crash, where the aeroplane inadvertently ends up in water where it is supported or partially supported by land. It is possible that during the departure from land to water the aeroplane encounters varying terrain such as berms, rocks etc. It is not uncommon for aeroplanes to be severely damaged. However, these events rarely include scenario where the aeroplane is floating after it comes to rest. It is more typical for the aeroplane to be resting on the lake or sea bed or partially supported on land. This is not considered to be a ditching event, and it is not addressed by the ditching requirements or this AMC. Rather, this type of event is addressed by other crashworthiness specifications such as CS 25.561, CS 25.721 and CS 25.963. 2.7. Planned ditching. An event where the flight crew knowingly makes an emergency landing on water. In ideal cases, the flight and cabin crews have sufficient time to fully prepare the aeroplane and the passengers, and execute the ditching in accordance with the aeroplane flight manual (AFM) procedures. It is recognised that some circumstances may degrade the ability of the flight crew to execute the ditching exactly as per the AFM procedures. Therefore, an assessment should address variations in the aeroplane assumptions (e.g. attitude (pitch) and descent velocity) to account for potential degraded conditions. All phases of ditching (defined above) should be evaluated when showing compliance with ditching certification specifications. 2.8. Reduced engine power or thrust / no engine power or thrust ditching conditions. An event where the flight crew knowingly makes an emergency landing on water but with reduced engine power or thrust, or no engine power or thrust available. The flight and cabin crews may or may not have sufficient time or opportunity to fully prepare the aeroplane and passengers for ditching. The flight crew is able to perform the emergency landing in accordance with the AFM procedures for a reduced/no power or thrust landing on water. It has been shown that for this condition the amount of control the flight crew has over the high-lift devices is the dominant factor in maintaining water impact loads within the structural capability of the aeroplane. This condition is addressed by AFM procedures (see Section 9). For such an event, the applicant may focus on the approach phase of the ditching event (defined above) when showing compliance with ditching certification specifications. Other ditching phases, as well as the definition of the structural impact loads and the structural capability assessment, need not be considered. 2.9. Unplanned ditching. An emergency landing on water that is typically associated with a failed or aborted take-off or landing overrun at an airport adjacent to a large body of water where the aeroplane is in water deep enough to float (i.e. the aeroplane is not supported by land). The flight and cabin crews do not have sufficient time or opportunity to prepare the aeroplane and passengers for this type of ditching event. Typically no actions are taken before the ditching to improve the flotation characteristics of the aeroplane (e.g. closing the Environmental Control System (ECS) outflow valves). For such an event, the applicant may focus on the flotation and evacuation phases of the ditching event (as defined above) when showing compliance with ditching certification specifications. 3\. General Successful emergency landings on water depend on several crucial factors. The aeroplane should possess good hydrodynamic characteristics, the ditching procedures should be attainable, and the airframe should be intact enough for orderly evacuation. The natural variability of potential ditching events and the inherent difficulties of an emergency water landing do not support a precise definition of a design ditching condition. For these reasons, the following structural and aeroplane features should be such as to ensure a level of structural performance that provides a reasonable chance of a successful ditching. Therefore, structural substantiation of ditching capability as per CS 25.563 necessitates consideration of the following aspects: (a) hydrodynamic behaviour during a planned ditching event should be predictable and consistent; (b) the predicted hydrodynamic, aerodynamic and inertial loads experienced by the aeroplane during the ditching should be based on methods shown to be reliable or conservative; (c) reasonable variations of flight parameters should be considered to ensure that the execution of a successful ditching does not require exceptional pilot skills or strength and that the inherent uncertainties associated with a water impact do not jeopardise the ditching structural performance; (d) the airframe assessment of the ditching loads should demonstrate the requisite strength and deformation to maintain the required flotation characteristics. 4\. Accepted methods for evaluating hydrodynamic behaviour To show acceptable hydrodynamic behaviour, testing and/or numerical simulation should be used. Testing need not be on the configuration under consideration if sufficient similarity can be shown, and it need not be performed by the applicant if performed by a suitable organisation (e.g. the applicant may use the content of document NACA-TN-2929, Experimental investigation of the effect of rear-fuselage shape on ditching behavior, dated April 1953). Numerical simulation of water impacts may be acceptable if validated and may be appropriate for unusual design features such as large cut-outs, open bays, scoops and projections. While the occurrence of emergency landing on water is rare, there have been instances of water entry during approach, ditching due to fuel exhaustion or engine power or thrust loss from ingestion damage etc. Hydrodynamic and structural performance in these incidents has generally been satisfactory for large aeroplanes. Consequently, applicable fleet history may also be used by the applicant to supplement test and simulation data if acceptable to EASA. Note: These test or simulation methods, supplemented by applicable fleet history, may also be used when showing compliance with CS 25.801(b)(1) and (2). The following should be considered. (a) Test methods A model test should define the approach conditions and describe the hydrodynamic behaviour until the aeroplane comes to rest after ditching. For typical model tests, 1:10 or smaller-scale models are ‘ditched’ in a water tank. These models may be comparatively rigid or structurally similar (i.e. scaled strength parts such as fuselage joints, flaps attachments and lower fuselage skins) to a full-scale aeroplane, with the aim being to understand the dynamic behaviour after impact on water by examining a variety of parameters. The models may be equipped with pressure transducers and linear and angular accelerometers, to assess the pitch, roll and yaw of the model. The motion of the aeroplane may be observed by high-speed imaging systems. (b) Numerical methods Numerical simulation techniques may be applied to determine the hydrodynamic behaviour of the aeroplane when it is in contact with water. This may be achieved by using commercially available software or in-house tools specifically developed and validated. Some of these tools may be used for pressure (loads) generation during the impact phase only, but typically the complete period between initial contact with water and the aeroplane coming to rest is simulated. 5\. Accepted methods for developing ditching pressures and loads Ditching loads may be developed by analysis or by test. Analysis methods should be validated by applicable testing. The guidance in this section concentrates on the approach conditions and the impact analyses. Furthermore, CS 25.563 provides the relevant conditions: a planned ditching is the water entry of a controlled aeroplane; reasonable variations must be accounted for, as described in Section 6 of this AMC. Ditching loads, considered as ultimate, are to be applied to the airframe taking into account the hydrodynamic effects resulting from a water landing, with accompanying aerodynamic and inertia loading. The hydrodynamic loads act directly on the lower skins of the fuselage and/or on the lower wing structure. The methods that follow are acceptable methods for developing loads when showing compliance with CS 25.563. (a) Test methods Water pressures, in terms of magnitude and (fore-aft, lateral) distribution, that occur during the impact phase may be determined based on ditching model testing, with a model equipped with a sufficient number of properly distributed pressure transducers. Typically, accelerometers are also installed to measure accelerations. Ditching model test results need to be properly scaled to aeroplane size. It would be conservative to envelope the measured (scaled) peak water pressures and use these directly to design the bottom structure of the aeroplane. If, on the other hand, the measured data is modified (e.g. smoothing of peak pressures), this should be further substantiated. Document NASA-TM-X-2445 (Ditching investigation of a 1/30-scale dynamic model of a heavy jet transport airplane), dated February 1972, provides a table of scaling coefficients as applied in published aeroplane model tests, as shown below. \ \\\\ \\\\ \\\\ \\\\ \\\\ \\\\ \\\\ \\\\ \\\\ \\\\ \
Quantity\Scale factor\
Length\λ\
Force\ \
Moment of inertia\ \
Mass\ \
Time\ \
Speed\ \
Linear acceleration\1\
Angular acceleration\ \
Pressure\λ\
The underlying physics are based on similitude of Froude’s law, which allows use of the scale (linear or non-linear) as a transfer function from measurement to the real aeroplane. The model scale, λ, is the ratio of the model dimension to the full-scale aeroplane dimension. Quantity (model) = Quantity(full-scale aeroplane) x Scale factor \[Figure or form omitted from this preview — available in the Avioverse workspace library.] (b) Analysis methods (i) In order to quantify the structural capacity of aeroplane structures under hydrodynamic loading, the prediction of global and local structural loads and resulting deformations is of fundamental importance. The analysis, however, is very challenging, as ditching is a time-dependent, highly non-linear multiphysics problem with different length and time scales resulting in complex loading conditions and coupled fluid– structure interaction. Hydrodynamic phenomena affect the fluid–structure interaction, and their occurrence may therefore influence the global aeroplane motion during the landing phase. To circumvent some of these complexities, an uncoupled analysis is often performed. In uncoupled computational approaches, the fluid solution is obtained independent of the structural solution, and the computations are run separately. The aeroplane structure is typically represented by a Finite Element Model. This model represents the global aeroplane structural stiffness and mass distribution, whereas the applied hydrodynamic models are generally based on the momentum theory and the concept of added mass developed by von Kármán and Wagner. Whatever analysis technique is used, either coupled or uncoupled, validation of the analysis by model ditching test results is necessary, as is an assessment of how each of the hydrodynamic phenomena described above is addressed. (ii) Suitable analytical methods may include a comparison with aeroplanes of a similar configuration for which the characteristics during the ditching event (such as aeroplane attitude, movement of centre of gravity and vertical and horizontal speeds/accelerations) are known. This approach addresses generating loads and the structural assessment. Reference data for this technique can, for example, be found in document NACA-TN-2929 (Experimental investigation of the effect of rear-fuselage shape on ditching behaviour), dated April 1953. (iii) Analysis using seaplane float pressures per § 25.533 of the US Code of Federal Regulations Title 14, Part 25. FAR Part 25 contains a set of regulations for water loads for seaplane designs that can also be used for conventional large aeroplanes. These methods, however, may not be applicable to large aeroplane configurations with flat or essentially flat impact areas. Seaplane design methods may be used if these are shown to be appropriate for the specific aeroplane configuration to be certified. This involves the determination of seaplane design parameter equivalency based on ditching model testing and establishing similarity of the product to the ditching model(s) used. Design parameter equivalency should be established by analysis based on test data, and product similarity should be established by consideration of geometric (dimensions and shape) characteristics, number of engines and their placement, wing configuration and mass properties. Per FAR Part 25, § 25.533(b) local pressures are to be developed for use in the design of local stringers and skins and their attachments to the supporting structure. These pressures are intended to simulate pressures occurring during high localised impacts on the hull but are not required to be extended over areas that would induce critical loading in frames or the overall structure. Note that, for derivation of local pressures, FAR Part 25, § 25.533(b)(1) for unflared bottoms, is considered to be more appropriate for conventional large aeroplanes. With FAR Part 25, § 25.533(c), distributed pressures are given with a distribution along the fuselage length, for the design of the frames. In addition, FAR Part 25, § 25.527, allows calculation of water reaction load factors along the fuselage. When applying these FAR Part 25 seaplane requirements to conventional large aeroplanes, some of the seaplane design parameters cannot be applied directly and may need some adjustment. For example, on a seaplane, the so-called step defines the forebody and afterbody of the hull, but this design feature is not present on conventional large aeroplanes. In addition, seaplanes have a flared or unflared bottom structure, with a physical chine line, whereas conventional large aeroplanes are (semi)circular in shape. As a result, applicants typically do not apply the local and distributed pressure distributions in the fore-aft and lateral (transverse) directions exactly as prescribed in the FAR Part 25 seaplane requirements, but derive a more rational pressure distribution based on ditching model testing data. Similarly, the definition of a chine line (defining the wetted area where water pressures are applied) and an equivalent deadrise angle as applicable to a (semi)circular-shaped fuselage need to be derived from ditching model test data. When using the local or distributed pressure equations contained in FAR Part 25, § 25.533, the aeroplane speed and weight at impact should be as defined in FAR Part 25, § 25.125(b)(2)(i)), established for the aeroplane assessment weight and corresponding to the flap setting established under the preferred AFM ditching procedure. 6\. Variation of flight parameters Considering the inherent complexity of the ditching event, the following flight parameters and characteristics should be used to define the structural loads with prescribed variations in certain impact parameters and approach configurations. In general, the ditching condition should consider the certified design ranges of aeroplane weight, centre of gravity and allowable configurations. Variation in certain flight parameters may be reduced if the aeroplane has reliable design features that control the variability. Typically, the following flight parameters have to be considered and appropriately defined: pitch attitude, forward speed, sink rate, mass configuration (mass, centre of gravity, moments of inertia), flap setting, landing gear extended/retracted, engine power or thrust setting, rupture of engines, flaps or fairings. This list of flight parameters may not be complete, depending on the aeroplane design. Model tests or simulations may deliver time histories of all investigated flight parameters plus pressure distributions, which can be integrated to obtain global loads. In this case, no further pressure calculation is necessary. The objective is to find conditions that show: smooth (hydro)dynamic behaviour (no nosedive or rebounce); accelerations less than or equal to the inertia forces specified in CS 25.561(b) (higher load factors may be acceptable provided the structural components are designed for the higher loads and also provided it can be shown that the occupants are protected from serious injury under these loads). As an example, this assessment may result in the following: sink rate of less than 5 feet/second, forward speed of 100 knots at maximum landing weight and flaps fully extended, assuming that the fuel is dumped or burned off, pitch attitude between 7 degrees and 9 degrees, landing gear retracted, landing parallel to waves. These conditions may be directly used for an AFM procedure and should be reviewed by pilots and flight physics specialists in order to confirm that they are within the capability of the flight crew and aeroplane. The AFM procedure is then completed by defining the preferred ditching technique from level flight to the water surface. The following criteria apply for the assessment of the variation of flight parameters: (1) an aeroplane vertical descent rate not less than 1.52 m/s (5 feet/second) relative to the mean water surface, unless a lower value is justified that fully accounts for likely variation over the value established under the preferred AFM ditching procedure; (2) a forward aeroplane speed along the flight path not less than VREF (as defined in CS 25.125(b)(2)(i)) established for the aeroplane assessment weight and corresponding to the flap setting established under the preferred AFM ditching procedure, unless a lower value is justified that fully accounts for likely variation over the value established under the preferred AFM ditching procedure; (3) an increase in aeroplane attitude of at least 1 degree (nose up), as compared with the attitude established under the preferred AFM ditching procedure, and, separately, a decrease in aeroplane attitude of at least 1 degree (nose down) as compared with the attitude established under the preferred planned AFM ditching procedure. The following criteria apply for planned and unplanned ditching evaluation for all aeroplanes. (1) For planned ditching, aeroplane weights may take into account fuel jettisoning provisions or fuel burn-off but may not be less than the maximum design landing weight at the moment of water impact. Unplanned ditching flotation is to be based on maximum take-off weight. Structural damage need not be considered for the unplanned ditching condition. (2) Calm water states may be assumed. (3) Fresh water is assumed for flotation calculations. (4) Any leakage should be accounted for in the flotation analysis. (5) For planned ditching, withstanding ditching loads implies an airframe assessment that needs to account for local loads (skin, stringers) and load factors for the fuselage and establish distributed pressures. Local damage may occur, but the airframe structural integrity should be maintained. Any leakage should be accounted for in the flotation analysis. Additionally, breakaway or loss of large items (e.g. gear doors, belly fairing, flaps and engines) and its effect on flotation and hydrodynamic behaviour should be considered. (6) For planned ditching, only symmetrical conditions need to be considered, and the resulting pressures can be considered ultimate loads. A rational distribution may be used to develop the pressure distribution along the side of the fuselage. 7\. Fidelity of loads analysis The fidelity of the loads analysis depends on the process of structural substantiation, which follows the loads generation. The greatest fidelity achievable by analyses is when, for each calculation point on the airframe, a pressure value is defined for each time step of the simulation. This can be used to develop a static design case to be loaded on a Finite Element Model or to be used by any other method for evaluating stresses. In addition, a pressure time history can be built, which can be used in a structural substantiation. For simplification, pressures can be averaged in time or space, or they may be integrated in local forces for frame or skin stress evaluation. This depends on stress methods used by the applicant. 8\. Airframe assessment Fuselage structure should be assessed to verify that the frames can carry the distributed pressure loads, that, consistent with flotation assumptions, the skins and stringers can withstand local pressures and that items of mass are retained (load factors). Global aeroplane integrity (i.e. not exceeding the certification strength envelopes) for the planned ditching event should also be ensured. In addition, as per CS 25.809(g), there must be provisions to minimise the probability of jamming of the emergency exits resulting from fuselage deformation in a planned emergency landing on water. 9\. Aeroplane flight manual procedures AMC 25.1581 states that the AFM should include emergency procedures, including those for ditching. For ditching, the AFM should include, as a minimum, procedures for a planned emergency landing on water and procedures for a reduced/no engine power or thrust emergency landing on water. The AFM ditching procedures should be consistent with the assumptions made in the ditching analysis and in the definition of the ditching conditions. For example, if the ditching conditions assume that the high-lift devices are to be extended during an emergency landing on water, then the AFM procedures should include the necessary steps to extend the high-lift devices. The AFM procedures should be verified for practicality and effectiveness, as required by AMC 25.1309 Chapter 9, paragraph b(5). 10\. Cabin crew training The applicant should provide appropriate information to support cabin crew training for planned and unplanned ditching scenarios in relation to the design precautions (e.g. ditching exits, lifelines, raft portability, etc.). AMC · AMC 25.563 — CS-25 · CS-25 Easy Access Rules · EAR revision 26 Jan 2023 [All rules in SUBPART C – STRUCTURE](https://avioverse.aero/regulations/easa/cs-25/subpart-c) Consolidated from the EASA Easy Access Rules (revision 26 Jan 2023, extracted 13 Oct 2026) for convenience. Not the official publication — verify against the Official Journal of the European Union and the EASA publications before operational use.