AMC3 27.307 Proof of structure

SEAT ADAPTER PLATES

(a)     Purpose

This AMC provides an acceptable means of compliance for seat adapter plates. A seat adapter plate includes any other forms of new interface structure installed between the seat and the rotorcraft floor.

(b)     Related Certification Specifications

—                   CS 27.307 ‘Proof of structure’

—                   CS 27.561 ‘General’

—                   CS 27.562 'Emergency landing dynamic conditions'

—                   CS 27.785 ‘Seats, berths, safety belts, and harnesses’

(c)      Explanation

The requirements for seats under emergency landing dynamic conditions have been developed to prevent detachment of the seat under floor deformation and for the seat to help absorb the energy developed in crash conditions. This dynamic condition has been addressed with the 10° roll and 10° pitch deformation required by CS 27.562(b)(3) to ensure that the seat and the floor attachments will be designed to accommodate deformation. This objective should be maintained when a seat adapter plate is installed between the seat and the floor.

Introducing an adapter plate can move the problems created by floor deformation from the seat-to-track interface to the adapter-to-floor interface. The same level of safety is appropriate for the occupant of the seat whether it is installed in the rotorcraft with or without an adapter plate. The floor structure itself is not subject to the dynamic requirements of CS 27.562, therefore when additional structure such as an adapter plate is introduced to fix the seat to the floor, it is very important to determine whether that structure should be considered to be part of the seat or part of the floor. The installation of any interface between the existing floor and the seat should not create a weak element between the seat and the existing airframe. This has successfully been assured by testing the adapter with the seat according to the requirements of CS 27.562.

This AMC provides further guidance and acceptable means of compliance for classification of seat adapters, such as plinths or pallets, and supplements FAA § AC 25.562.

Plinths are subject to CS 27.562 compliance whereas pallets (traditionally defined as large adapters) are not, except for the attachment of the seat to the pallet.

FAA Policy Memo PS-ANM100-2000-00123 (which is applicable to CS-25 and can be extended to CS-27) suggests that it may also be possible to classify some smaller adapters as an integral part of the floor as follows:

‘Generally speaking, adapters of the size that contain a single row of seats (whether they are individual seat places or a common assembly), and mount into seat tracks, should be treated as part of the seat for purposes of certification in accordance with § 27/29.562. Larger, or more integrally mounted adapters, should be assessed to determine whether they should be treated as part of the floor for purposes of certification in accordance with § 27/29.561.’

To treat an adapter or other new interface structure as part of the floor when it does not appear to be similar to conventional floor structure, the applicant must substantiate that the adapter plate or any other structure installed between the existing floor and the seat attachment will not constitute a weak element under emergency landing conditions. The issue is whether the critical interface is between the seat and the adapter or between the adapter and the rotorcraft. No further detailed guidance is available to assist with the assessment required to make the classification of an adapter as part of the floor.

Where the proposed floor design utilises a plate above the existing floor or otherwise significantly differs in concept from the type design’s existing methods of floor construction, geometries and utilisation of load paths, it is not adequate to rely on compliance with CS 27.561 alone to determine whether the adapter plate may be considered to be part of the floor. This guidance does not intend to request a complete crash scenario evaluation, but asks for evidence that the adapter plate and associated new under floor structure will not degrade the level of protection compared to that offered by the seat if it were installed directly on the helicopter existing floor seat track and floor construction. For an adapter plate to be considered sufficiently integrated to be part of the floor, the adapter plate should be capable of accommodating floor deformation and be able to safely react and distribute the seat loads into the rotorcraft.

(d)     Seat adapter plate definition and classification

(1)     Definitions

The definitions of plinth and pallet that are available in AC 25.562(b) are valid.

In general, swivelling seat adapter plate systems are by definition considered to be plinths.

(2)     Classification

There are three possible options for the seat-to-floor interface with corresponding means of compliance. In each case, the applicant is requested to show that any interface between the existing floor and the seat will not create a weaker element between the seat and the existing airframe than that that would exist for a CS 27.562-compliant seat attached directly to the standard floor, e.g. seat track.

Acceptable means of assessing seat installations using adapter plates:

Option 1

—                  The adapter is classified as a plinth following AC 25.562-1B.

—                  Compliance with CS 27.561 and CS 27.562 must be shown.

—                  The plinth must be tested as part of the seat according to CS 27.562(b)(1) and (b)(2) unless alternative compliance is agreed as per CS 27.562(d).

—                  The guidance of AC 25.562-1B and AMC 27.307 may be used to reduce the number of tests based on design similarity.

Option 2

—                  The adapter is classified as a pallet due to its size following AC 25.562-1B.

—                  The seat and its attachments to the pallet only are tested according to CS 27.562 and CS 27.561.

—                  The pallet is justified against CS 27.561 only.

Option 3

—                  If neither Option 1 nor 2 clearly apply, seat-to-floor interface structure may be proposed to be classified as an integral part of the floor based on one of the methods described below.

—                  If classification as part of the floor is agreed with the Agency, the seat and its attachments to the structure are tested according to CS 27.562 and compliance with CS 27.561 is shown for the whole installation.

Acceptable methods to be used in support of Option 3, allowing classification of the new seat-to-floor interface structure as an integral part of the floor structure:

Method 1

A design review showing the floor design for seat installation uses the same or an equivalent design principle as the current floor provided in the type design. If the pre-existing floor design used seats directly attached to seat track independently of the floor panel, then the introduction of a structural floor panel to which a seat is attached would represent a change in design philosophy, and a different method (e.g. Method 2) would need to be used to support Option 3.

Method 2

A detailed design review showing the level of integration of the plate to the floor, including the redundancy and strength of the attachments, that is acceptable to the Agency based on the experience of the applicant and the Agency with similar designs.

Any other alternative methods have to be agreed with the Agency.

Note:

When assessing the design, the following points should be considered by the applicant and the Agency, in particular for design change certification:

—                  The modified structure may be evaluated using AMC1 27.307 to categorise the structural elements as new, similar-new or similar. Comparison can be made with the existing type floor design (Method 1) or with designs that the applicant has previously substantiated according to Method 2.

—                  An adequate number of appropriately distributed attachments between the adapter plate and the rotorcraft floor structure should be provided to assure that the additional structure behaves as an integral part of the rotorcraft floor. The appropriate number, strength and degree of redundancy of the attachments will depend on the design of the adapter plate and positioning of the seats on the plate.

—                  A considerable degree of engineering judgement is required when making the classification of the structure; when there is any doubt about the capability of the proposed adapter design to act as an integral part of the floor, it will be classified as a plinth under Option1.

[Amdt 27/10]