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For design of pilot seat, would the seat be considered as primary or secondary structure?

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    $\begingroup$ I wouldn't consider it structure as most are designed to be movable on some kind of rail system. $\endgroup$ – CrossRoads Jul 19 '18 at 14:36
  • $\begingroup$ How does that disqualify it from being a structure? It still needs to sustain loads, not mentioning that you would normally not want your seat to be moving during operation of the aircraft. $\endgroup$ – LuTze Jul 19 '18 at 14:42
  • $\begingroup$ I can see that. I would suggest Primary then, as the Pilot seat at least is usually in a fixed location prior to takeoff. (I never move mine, anyway). $\endgroup$ – CrossRoads Jul 19 '18 at 15:23
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    $\begingroup$ In what kind of context are primary and secondary structure defined? Please add a reference, as the definition of primary and secondary structure may greatly vary depending on the context. $\endgroup$ – DeltaLima Jul 19 '18 at 20:36
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Seats are not structure. They are equipment.

I believe you mixing system/sub-system definitions with design requirements. Primary and secondary structures are structural systems within the aircraft that have a primary or secondary purpose of transferring a load. Everything else on the aircraft is classified according to its primary function (air conditioning, flight controls, etc.)

Having said that, there are structural requirements that exist for all the components of these functional systems. That doesn't make them structure.

To illustrate the point, we can refer to ATA spec 2200 which provides a common structure for aircraft documentation. The documents use use a Chapter-Section-Subject numbering system that correlates to a System-Subsystem-Subject.

At the highest level, Chapters 20 through 50 cover Aircraft Systems, 50 - 59 cover structure, 60 - 67 cover Propeller/Rotor, and 71 - 84 cover Power Plant. Looking at the Chapter definitions we find (emphasis mine):

Chapter 25, EQUIPMENT/FURNISHINGS

Those removable items of equipment and furnishings contained in the flight and passenger compartments. Includes emergency, galley and lavatory equipment. Does not include structures or equipment assigned specifically to other chapters.

And under that is:

Section -10, Flight Compartment

The compartment above the floor and between the forward passenger partition and the forward pressure dome. Includes items such as flight crew seats, tables, pilot check lists and food containers, wardrobes, curtains, manuals, electronic equipment rack, spare bulbs, fuses, etc. Does not include cargo compartments.

Further along, we find:

Chapter 51, STANDARD PRACTICES AND STRUCTURES - GENERAL

Standard Practices, General Procedures and typical repairs applicable to more than one chapter and are not specifically covered under [Chapter (System) 52] thru [Chapter (System) 57] Sub-Sys/Sect breakdown.

Section -10, General

Airplane major structural breakdown diagram. Primary and secondary structure diagram. Principal area and dimensional data. Restricted area diagram. Zoning diagram. Access door and panel identification. Glossary.

The structural area of interest here is:

Chapter 53 FUSELAGE

Structural units and associated components and members which make up the compartments for equipment, passengers, crew, cargo, plus the structure of the envelope and gondola of airships. Includes skins, belt frames, stringers, floor beams, floor, pressure dome, scuppers, tail cone, fuselage-to-wing-and empennage fillets, attach/attached fittings, load curtains, cables, ballonets, etc.

You'll note that this specifies only physical structure, not systems or equipment installed in the fuselage.

To address the structural design requirements, we can refer to 14 CFR Part 25, Airworthiness Standards: Transport Category Airplanes.

Subpart C with the title Structure is broken down into the loads the aircraft must be designed to withstand (flight loads, gust loads, ground loads, etc.)

Subpart D covers Design and Construction which provides specifics for individual design elements of the aircraft. Therein we find:

§25.785 Seats, berths, safety belts, and harnesses.

Within this section is the necessary design requirements for seats, etc. which reference the applicable load specifications of Subpart C. A detailed reading of this section will reveal that the design loads for the seats are driven by the Emergency Landing Loads defined in Subpart C.

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That falls under the subject of Flight Deck and Furnishings. The Airline Transport Association (ATA) classifies that equipment under Chapter 25 Equipment and furnishings. OEMs as well separate that equipment from structural airframe components as well. Primary and secondary structures refers to structural components in an airframe.

For example, a fwd wing spar or a fwd pressure bulkhead are primary structures.

A Douglas clip might be an example of a secondary structure, etc.

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  • $\begingroup$ I disagree, it is not subject to flight deck (especially since pilot seats are in all aircraft that do not have flight decks), and it is not furnishing. The only way you can get out of the question through semantics, is by saying it is a safety equipment. But that would not help either. $\endgroup$ – LuTze Jul 19 '18 at 14:55
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    $\begingroup$ If you're going to disagree with everyone why ask the question? You haven't even given a context. Anyway, it seems pretty obvious that there's nothing intrinsic in your average pilots seat that relates to the flyability of the aircraft so I don't see how it could be considered structural. A gauge wouldn't be a Primary or Secondary structure either $\endgroup$ – Dan Jul 19 '18 at 15:03
  • $\begingroup$ "if I am going to disagree"? You are actually just avoiding the question. Right, so an example: An aerobatic aircraft goes into a manoeuvre, the back support bends, moving the pilot away from controls. Not being able to unbuckle, the aircraft crashes, as a result of structural failure. That would be my argument for it being primary structure. However, if i have to provide arguments myself, why the hell would I be here? $\endgroup$ – LuTze Jul 19 '18 at 15:09
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    $\begingroup$ Uh, no. That’s how Boeing and most OEMs classify that equipment. $\endgroup$ – Carlo Felicione Jul 19 '18 at 15:12
  • $\begingroup$ @LuTze Just because something is critical doesn't mean it's structural. The plane will crash if the Pilot has a heart attack, too - but it doesn't mean they've become structural! $\endgroup$ – Dan Jul 19 '18 at 16:12

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