AMC 25.801 primarily provides guidance and acceptable means of compliance with the ditching certification specifications of CS 25.801 (Ditching).
Definitions and guidance material provided in AMC 25.563 can also be used when showing compliance with CS 25.801.
1.CS 25.801(a) — Evacuation after an unplanned ditching
Although there are many possible scenarios that could result in an unplanned ditching, the following assumptions may be used for addressing compliance with the unplanned ditching specifications in CS 25.801(a).
(1)To simplify compliance determinations for an unplanned ditching scenario, no aeroplane damage should be considered and calm water states may be assumed. For this reason, the dynamics of entry into the water should not be considered, including analysis of dynamic pressures resulting from the aeroplane coming to rest; it may be assumed that the aeroplane has come to rest in the water immediately after an unplanned ditching.
(2)Because an unplanned ditching immediately after a failed or aborted take-off could occur at high aeroplane weights, for the purpose of developing a flotation analysis, the worst-case combination of aeroplane weight and centre of gravity should be considered (typically expected to be maximum take-off weight with the centre of gravity at the aft limit).
(3)All sources of water leakage into the aeroplane should be considered.
(4)Since not all aeroplanes are required to carry ditching equipment associated with overwater flights, it is not necessary to account for the time to retrieve and launch rafts.
(5)For the purpose of developing a flotation and evacuation analysis, an exit should be conservatively considered unusable when water comes in over the top of the door sill.
(6)Aeroplane flotation should be assumed to end when the first ditching exit goes below the waterline or the attitude of the aeroplane is such that it would require extraordinary effort to move through the cabin (e.g. 20 degrees). However, if it can be shown to 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.
Note regarding points (5) and (6) above: 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.
(7)In order to be credited with a full passenger seat-to-exit ratio, each ditching exit should either remain above the waterline during the entire evacuation or be available for use long enough to allow the number of evacuees equal to its seat-to-ditching exit ratio to use the exit (e.g. a ditching exit with a 35-passenger seat-to-exit ratio should either remain usable for the entire evacuation or long enough to allow at least 35 evacuees to exit the aeroplane through that exit in order to receive the full 35 passenger ratio). A lower passenger seat-to-exit ratio may be sought provided the exit remains above the waterline for the majority (greater than 50 %) of the total aeroplane evacuation time. No passenger seat credit should be allowed for a ditching exit that does not remain above the waterline for the majority of the total aeroplane evacuation time.
(8)In the case of non-overwing ditching exits, it is acceptable to assume that passengers will exit the aeroplane by entering the slide/raft (if provided) or by jumping into the water and swimming away from the exit. In the case of overwing exits, it is acceptable to assume that passengers will exit onto the wing and, depending on the circumstances, either remain on the wing or jump into the water. No credit should be taken for aeroplane weight reduction resulting from evacuees exiting the aeroplane through overwing exits.
(9)For the purpose of preparing an evacuation timeline, the longest full-scale evacuation demonstration (FSED) exit preparation time for an exit of that type, for that aeroplane, or 15 seconds, whichever is greater, should be assumed prior to the initial occupant evacuation from the aeroplane.
(10)For the purpose of preparing an evacuation timeline, evacuation rates obtained from the aeroplane FSED are acceptable for preparing a ditching evacuation analysis if the evacuees exit in the same or a similar manner as during the FSED and the assisting means (if deployed) do not block the emergency exit opening. Alternatively, data developed by testing and analysis for demonstrating compliance with CS 25.803 land evacuation specifications are also acceptable. However, the aisle flow rate may determine the evacuation rate at a pair of exits if it is fed by passengers from only one direction and the combined exit pair flow rate is greater than the available aisle rate.
Note: The evacuation rate for slides/rafts deployed from representative sill heights should not exceed 60 persons per minute per lane for a duration of 70 seconds.
(11)For the purpose of preparing an evacuation timeline, it is acceptable to assume that the flow of evacuees to the emergency exits is not diminished by the retrieval or the donning of life vests.
2.CS 25.801(b) — Certification with ditching provisions
CS 25.801(b) requires an evaluation of the probable behaviour of the aeroplane at ditching and the hydrodynamic characteristics. This evaluation can be performed in conjunction with the variation of parameters or the loads development if using numerical techniques and simulations.
Section 4 of AMC 25.563 provides accepted methods for evaluating hydrodynamic behaviour, which can be used for showing compliance with CS 25.801(b)(1) and (2).
CS 25.801(b)(3): since ditching events can occur with varying degrees of aeroplane and passenger preparedness, the following assumptions are appropriate for assessing the flotation of the aeroplane and evacuation of the occupants following a planned ditching.
(1)It should be assumed, in accordance with AFM ditching procedures, that the aeroplane enters the water at the maximum design landing weight, with the most adverse aeroplane centre of gravity. For the flotation analysis, the aeroplane weight may be reduced to account for items of mass in non-pressurised sections of the aeroplane that are shown to separate from the aeroplane as a result of the planned landing on water.
(2)All sources of water leakage into the aeroplane should be considered, including leakage from damage resulting from the conditions prescribed in CS 25.563.
(3)For the purpose of developing a flotation and evacuation analysis, an exit should be conservatively considered unusable when water comes in over the top of the door sill.
(4)Aeroplane flotation should be assumed to end when the first ditching exit goes below the waterline or the attitude of the aeroplane is such that it would require extraordinary effort to move through the cabin (e.g. 20 degrees). However, if it can be shown to 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.
Note regarding points 3 and 4 above: If an exit can be shown to still be conservative, it 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.
(5)To receive its full passenger seat-to-exit ratio, each ditching exit should either remain above the waterline during the entire evacuation or be available for use long enough to allow the number of evacuees as per the seat-to-ditching exit ratio to use the exit (e.g. a ditching exit with a 35-passenger seat-to-exit ratio should either remain usable during the entire evacuation or be available long enough to allow at least 35 evacuees to exit the aeroplane through that exit in order to achieve the full 35 passenger ratio). A lower passenger seat-to-exit ratio may be sought provided that the exit remains above the waterline for the majority (greater than 50 %) of the total aeroplane evacuation time. No passenger seat credit should be allowed for a ditching exit that does not remain above the waterline for the majority of the total aeroplane evacuation time.
(6)For the purpose of preparing an evacuation timeline, the longest FSED exit preparation time for an exit of that type, for that aeroplane, or 15 seconds, whichever is greater, should be assumed prior to the initial occupant evacuation from the aeroplane.
(7)For the purpose of preparing the evacuation timeline, it should be assumed that the aeroplane has ditching equipment required for overwater flights. Therefore, it is necessary to account for the time to retrieve and launch life rafts and board the rafts.
(8)For the purpose of preparing an evacuation timeline, evacuation rates obtained from the aeroplane FSED are acceptable for preparing a ditching evacuation analysis if the evacuees exit in the same or a similar manner as the FSED and the assisting means (if deployed) do not block the emergency exit opening. Alternatively, data developed by testing and analysis for demonstrating compliance with CS 25.803 emergency evacuation requirements are also acceptable. However, the aisle flow rate may determine the evacuation rate at a pair of exits if it is fed with passengers from only one direction and the combined exit pair flow rate is greater than the available aisle rate.
Note: The evacuation rate for slides/rafts deployed from representative sill heights should not exceed 60 persons per minute per lane for a duration of 70 seconds.
(9)For the purpose of preparing an evacuation timeline, it is acceptable to assume that the flow of evacuees to the emergency exits is not diminished by the retrieval or the donning of life vests.
[Amdt No: 25/27]