CS 25.1193 Cowling and nacelle skin

(a)PURPOSE This AMC provides guidance for showing compliance with the certification specifications relating to fire withstanding capability of engine cowlings and nacelles skins, and APU compartment external skins, in areas subject to flame if a fire starts in an engine or APU fire zone, in consideration of potential hazard levels associated to operating conditions (flight/ground).

(b)RELATED CERTIFICATION SPECIFICATIONS CS 25.1193(e), CS 25J1193(e)

(c)APPLICABILITY This AMC is applicable to engine cowlings and nacelles, and APU compartment external skins (fixed and/or removable).

(d)BACKGROUND CS 25.1193(e) and CS 25J1193(e) previously required the engine cowlings/nacelle skins and APU compartment external skins to be fireproof if a fire starts in the engine power or accessory sections or in the APU compartment. During past Type certification projects, it has been found that having non-fireproof engine cowlings/nacelle skins in some locations under some operating conditions do not adversely affect safety. Consequently, in practice, not all cowlings/skins ‘subject to flame if a fire starts in the engine power or accessory sections’ have been required to be fireproof under all operating conditions and, for instance, some portions were approved as fire-resistant only for ground operating conditions. As it represented a rule relaxation, such non-fireproof cowlings/skins were formally found to be ‘equivalently safe’ to comply with the rule. Over time, however, these equivalent safety findings became inherent within traditionally accepted design practices. Certification Review Item (CRI) released to cover the relaxation included also interpretations for zone definitions and operating conditions to be considered for fireproofness or fire-resistance compliance demonstration.

(e)FIRE WITHSTANDING REQUIREMENTS, OPERATING CONDITIONS AND POTENTIAL HAZARDS

(1)General The required level of ability to withstand the effects of fire varies with the potential hazard level associated with different flight and ground operating conditions, as follows.

(2)Flight Conditions For the purpose of CS 25.1193(e) and CS 25J1193(e), flight conditions are defined as aeroplane operation from airspeed above minimum V1 until minimum touchdown speed in approved normal or abnormal operations. Cowling and skin in areas subject to flame if a fire starts in an engine or APU fire zone must be demonstrated to be fireproof. For demonstrating the fireproof capabilities of the cowling/skin, the following apply:

(i)Credit from the external airflow on the cowling/skin can be considered.

(ii)The airflow levels and the engine/APU powers should be consistent with the operating conditions. These parameters should be examined and the most critical ones should be determined.

(iii)The engine/APU should be considered to be operative for the first 5 minutes, and during the remaining 10 minutes under windmilling conditions for engine and stopped conditions for the APU.

(3)Ground conditions For the purpose of CS 25.1193(e) and CS 25J1193(e), ground conditions are defined as aircraft operation not covered by the flight conditions provided in subparagraph (e)(2) of this AMC. It includes static, taxiing, take-off roll, and landing roll.

(i)Areas where fireproof skins are required — The portion of cowling and skin in areas subject to flames if a fire starts in an engine or APU fire zone, and located so that not containing the effects of the fire could result in serious hazards to the aircraft, injuries to crew, passengers or ground personnel, must be fireproof under all conditions. Serious hazards include, but are not limited to, events such as fuel tank explosion, hazardous spread of fire to flammable fluid sources outside the fire zone, fuselage penetration and flight control surface damages.

(A)Pod-mounted engines: The portion of the nacelle/cowling skin, which is required to be fireproof on ground, varies by installation. A design is considered acceptable when it is demonstrated that the fireproof area protects the pylon strut and other portions of the aircraft considered to be put at a serious hazard risk if a burn through occurs. Factors to consider within the analysis and to use when substantiating the design are: the engine location — wing or aft fuselage mounted, the coupling distance of the nacelle to the wing, the airflow characteristics, the fluid migration scheme and the fire plume patterns. After the initial analysis, similarity demonstration and in-service experience may be used as appropriate. Analyses have demonstrated that the typical area of concern ranges from 90° (± 45°) to 180° (± 90°) and is centred on the pylon centre line. This area may increase or decrease depending on the analysis results. For example, most wing mounted engines not closely coupled to the wing have been found acceptable with a ± 45° protection while more closely coupled installations and those with other unique design features have required ± 90° protection. The symmetry of the protection may also vary. Wing mounted engines usually have symmetrical protection while aft mounted engines may have non-symmetrical protection in order to cover more of the inboard area. [Figure or form omitted from this preview — available in the Avioverse workspace library.]

(B)Turbo-propellers, APUs and other non-pod-mounted engines: Due to the wide variations in installation configurations, each installation should be evaluated to determine if not containing the effects of a fire would cause a serious hazard such as the examples above. If so, the affected area of the fire zone skin should be fireproof.

(C)For the purpose of the demonstration: No credit from external airflow on the cowling/skin should be considered in conjunction with the assumption that the aircraft may be static. The engine/APU should be considered to be operative for the first 5 minutes and stopped for the remaining 10 minutes. Engine/APU operation — Requirements for ability of cowling/skin in areas subject to flames if a fire starts in an engine or APU fire zone to withstand the effects of fire in ground operating conditions apply with either the engine operating or not operating, whichever is the more critical. The Engine/APU operating conditions shall be justified by the applicant.

(ii)Other areas: For the remaining portions of cowling/skin in areas subject to flames, if a fire starts in an engine or APU fire zone, the degree of fire resistance can be lower than ‘fireproof’ due to less serious or less probable hazard to the aircraft, crew, passengers and ground personnel under the critical operating conditions. Any burn through of the APU compartment external skin should consider hazards associated with combustion product and possible outgassing and re-ingestion of toxic air into cabin air system.

(A)Fire-resistant cowlings/skins provide adequate fire protection for those areas as they provide sufficient time to stop the aeroplane and evacuate it.

(B)A lower than ‘fire-resistant’ degree of fire protection may be considered; the following conditions should then be analysed and submitted to the Agency for approval: Cowling/skin should have the ability to withstand fire at least equivalent to the ability of a 1 mm (0.040 inch) aluminium sheet in the worst aircraft and engine/APU ground conditions anticipated; Applicants must substantiate that this lower fire protection level will not lead to hazardous effects including but not limited to: Upon burn through of the lower than ‘fire-resistant’ area, both the fire-resistant and/or fire-proof areas shall not have their fire withstanding capability affected, Liberation of parts that would affect the aeroplane evacuation procedure or reduce the efficiency of fire protection means, Reduction in flammable fluid drainage capability such that fire severity would be increased (magnitude, residual presence, propagation to surrounding area), Reduction in aeroplane evacuation capability due to proximity to evacuation paths or due to the visibility of the fire hindering the ability of the passengers to evacuate the aeroplane in a rapid and orderly manner, Note: There is some hazard involving aeroplane evacuation even in the absence of burn through due to such concerns as smoke and flaming liquids exiting from openings. Burn through of nacelle skin should not significantly increase these hazards. Reduction in fire detection capability such that the flight crew would not be aware of the fire, especially in a situation involving taxiing prior to take-off, Reduction in fire extinguishing capability which could cause or aggravate one of the potential hazards listed above. Flammable fluid and/or fire spreading on the aeroplane evacuation path

(f)SPECIFIC CONFIGURATION CONSIDERATIONS

(1)Multiple skin layers: For some specific fire zones, a fire originating in that zone will have to pass through several layers of cowling or skin before burning through the external skin. This may be the case, for example, for the core zone of some turbofan installations. In such cases, credit may be taken for multiple layers, having regard to the location of the fire source and the likely direction of propagation from that location, providing burn through of the inner layer does not produce other hazardous effects and it does not invalidate other certification specifications such as fire extinguishing capability. The corresponding compliance substantiation should take into account particular geometrical configuration with respect to the risk of flame propagation, as well as critical systems or structures.

(2)Inlet skins: For external inlet skins, which enclose fire zones, the guidance provided above for multiple skin layers applies. Inlet ducts should meet CS 25.1103/CS 25J1103 specifications.

(3)Openings: The following considerations are applicable to openings in a fire zone skin whether the openings are of fixed size, variable or controllable size, or normally closed, such as access or inspection doors, or pressure relief doors.

(i)Openings should be located such that flame exiting the opening would not enter any other region where it could cause a hazard in flight or a serious hazard on the ground as per subparagraph (e)(3). Exception is made for covered openings which meet the same criteria for ability to withstand the effects of fire as the surrounding cowl skin, and which are not expected to become open under fire conditions. Since pressure relief doors may open during some fire conditions, they should be located such that flames exiting the door will not cause a hazard. However, doors that will remain closed during most fire conditions, or will tend to re-close following initial opening, have traditionally been assumed to be closed for the purposes of evaluating fire detection and extinguishing.

(ii)Openings should have the same ability to withstand the effects of fire as the adjacent skin with respect to becoming enlarged under fire conditions. Some enlargement, such as burning away of louvers or doublers surrounding the opening or gapping of covered openings, is acceptable provided that the hazard is not significantly increased by a reduction in fire extinguishing or detection capability, increased airflow causing increase in fire size or intensity, or increase in probability of a hazardous spread of fire to other regions.

(4)Hinges, Fittings and Latches: These attaching means maintaining the nacelle/cowlings between them or to the aircraft/engine/APU structure may need to have a greater ability to withstand the effect of fire than the surrounding skin. Loss of attaching means may create more severe hazards such as cowling liberation in comparison to a skin burn through. The applicant must justify the required level of fire withstanding capability by test and/or analysis.

(5)Seals: Where seals are used part of the external engine nacelle/cowling or APU compartment boundaries, they should at least comply with the same fire integrity standard as the surrounding cowling/skin.

(g)COMPLIANCE DEMONSTRATION Compliance should be substantiated per CS 25.1207. Substantiation involving airflow patterns may include analytical methods such as Computational Fluid Dynamics, test methods or other flow visualisation methods or a combination of these methods. Fire testing should be accomplished according to the guidance of ISO 2685 with considerations of applications of representative conditions (airflow, loads, vibrations) and establishment of appropriate pass/fail criteria (burn through, elongation, dislocation).

[Amdt 25/13]

a. Purpose and scope CS 25.1193(e)(4) requires design precautions to be taken to minimise the risk of any in-flight opening or loss of an engine cowling that could prevent continued safe flight and landing. CS 25.1193(f) requires the retention system of each removable or openable cowling to have a means, which is demonstrated to be reliable and effective, to verify that the cowling is closed and latched prior to each take-off. Reported occurrences of engine cowling separations revealed that features like latch handles hanging down, cowling gaps, and detection capabilities offered by walk-arounds and/or checks at the completion stage of maintenance activities, had not been reliable or effective in preventing aeroplanes from taking off with unclosed/unlatched cowlings. For turbofan engines, these occurrences have concerned fan cowls only. Thrust reverser cowls have shown satisfactory in-service experience with regard to the risk of a cowling separation. Therefore, specifications CS 25.1193(e)(4) and (f) are intended to be applicable to engine fan cowls only. All dispatch configurations, as permitted by the master minimum equipment list (MMEL) and the configuration deviation list (CDL), should be considered when showing compliance with CS 25.1193(e)(4) and (f). b. Selection of appropriate design features The following guidelines are provided to help the applicant in selecting design features appropriate to the engine/nacelle characteristics, and in showing compliance with CS 25.1193(e)(4) and (f). Human factors In determining the most appropriate design feature, or combination of design features, to cope with the human-factor aspects that contribute to the risk of an aeroplane being released with unclosed or unlatched cowlings, attention should be placed on the following aspects of cowling latched/unlatched indications: Their verification by personnel should not necessitate unusual physical effort (e.g. bending down or kneeling on the ground); Their verification by personnel should take into account the variability in the physical capabilities of personnel; The provision of these indications should take into account a possible lack of diligence of personnel in conducting walk-arounds and in completing their maintenance activities; The combination of indications should draw the attention of personnel without ambiguity (e.g. by paint effects) and should not be rendered ineffective by lighting conditions (night/day), weather conditions, or the operational environment. Design considerations The following considerations should be taken into account when selecting design features to mitigate the risk of a cowling separation: A wing-mounted engine/nacelle presents a higher risk than a rear-mounted engine/nacelle, therefore it requires more noticeable cowling latched/unlatched indications and/or a combination of them; An engine/nacelle with a small ground clearance presents a higher risk than one with a large ground clearance, therefore it requires more noticeable indications and/or a combination of them; A hanging heavy/large piece or part on an engine/nacelle with a large ground clearance may draw the attention of personnel; A unique indication on the lower part of an engine/nacelle that has a small ground clearance may not be sufficient to draw attention to it; The noticeability of a forced gap between the fan cowl and the surrounding structure may be adversely affected by its environment, such as the ambient lighting conditions, external painting or the condition of the surrounding structure, and may not be individually sufficient to draw attention to it; A flashing light in an open gap or outside the nacelle skin may draw the attention of personnel. In such cases, the reliability of the flashing light should be investigated and substantiated, taking into account the effects of the engine/nacelle environment; A mechanical flag on the outside of the nacelle skin may draw the attention of personnel; A latch which is locked by a key equipped with a red flag may draw the attention of personnel, however a duplicate key without a flag could be used, and therefore the use of a flag may not be sufficient; A design with a remote indication (i.e. on the flight deck) of the unlatched/unclosed fan cowl condition may effectively draw the attention of the flight crew. Other guidelines Furthermore, the following guidelines related to the use of some of the design features should be taken into account by the applicant: Procedural control measures may not always be followed as a result of the pressure to dispatch the aeroplane, and because of routine issues; Improper Instructions for Continuing Airworthiness may be issued, which may lead to: Improper rigging of the cowls and the associated latches; Poor maintenance of design features intended to prevent aeroplane dispatch with unlatched cowlings, such as bright paint fading over time (or becoming soaked with the dirt accumulated at the bottom of the nacelle), hold-open cowl devices not performing their intended function, etc.; Some nacelle painting can defeat the design precautions: Red or orange nacelle colours may negate the visibility of red/dayglow latches; A dark nacelle colour may reduce the noticeability of gaps. Specific tools may be improperly defined and maintained (e.g. keys required to open cowls, normally fitted with a red flag, being used without a flag). In order to address the human factors that contribute to the risk, it might be necessary to conduct an in-service and practical evaluation of the proposed design.

[Amdt 25/21]