Seat Pallet Assemblies - Structural Substantiation

INTRODUCTION

Static-strength substantiation of the Workstation and Observer Station seat-pallet assemblies and their installation on a DHC-8-100. The assessment covers governing flight and emergency loads, global FE response, seat-track strength, plate stresses, attachment reactions and fastener capacity.

The analysis compares calculated demand against aerospace material and joint allowables to demonstrate ultimate-strength compliance for the modified installations.

AircraftDHC-8-100Interior seat-pallet structural modification

InstallationsWSP + OSPPort / starboard mirrored arrangements

Assessment basisStarboard modelsOpposite-side installations covered by comparison

Substantiation routeFEM + classical checksSeat tracks, plates, attachments and fasteners

Technical figure from the Seat Pallet Assemblies structural substantiation. — Workstation and Observer Station seat-pallet installations on the starboard side of the DHC-8-100.

ENGINEERING SNAPSHOT

LOADSFlight + 6 g emergency

Station-specific limit loads screened against emergency conditions.

METHODFEMAP / NASTRAN + hand checks

Beam/plate idealization with classical strength and stability substantiation.

AUTOMATIONPython post-processing

Element geometry and plate resultants converted into fastener demand.

OUTCOMEPositive margins

Seat tracks, plates, attachments and local fasteners satisfy the source checks.

DESIGN ASSESSMENT

WORKSTATION PALLETX298.8 → X321.8

Port and starboard layouts are identical and see the same vertical environment.

OBSERVER PALLETX493.9 → X516.9

Port and starboard layouts are likewise identical with a common vertical load environment.

ANALYSIS SCOPEStarboard WSP + OSP

One model per pallet type captures the governing structural configuration without duplicating equivalent opposite-side analyses.

COMPARISON BASISGeometry + station symmetry

The corresponding port-side pallet is substantiated by comparison because its structural layout and applicable vertical loading are equivalent.

≡

Governing-model decision

Model only the unique structural configurations. Mirrored port installations are covered by comparison rather than duplicating equivalent finite-element models.

Seat Pallets Components

The pallets use machined/formed aluminum plates, thin-sheet end caps and extruded seat tracks. The table below retains the structural members, thicknesses and alloys used for substantiation.

Main structural components, thicknesses and materials.

Component Name	Assembly	Thickness (in)	Material
Base Plate	WSP	0.5	AL 7075-T651 Plate
Base Plate	OSP		AL 7075-T651 Plate
Two Side Plates	WSP & OSP		AL 2024-T351 Plate
Top Plate			AL 2024-T351 Plate
Two End Caps		0.05	AL 2024-T3 CLAD Sheet
Two Seat Tracks		0.38	AL 7075-T6 Extrusion

Weight Estimation

MASS IDEALIZATION

Structure + concentrated payload

Only load-carrying pallet structure is modeled explicitly. Seat and occupant inertia are introduced at a combined center of gravity so payload stiffness is not credited to the structure.

CONSERVATIVE MASS

Seat hardware allowance

The 55.9 lb seat weight is scaled by 1.15 to 64.3 lb to include omitted seat-related hardware; the occupant is modeled at 195 lb.

Weighted-Average CoG

The seat and occupant are combined at an equivalent center of gravity using weight-weighted coordinates.

\bar{x} = \frac{\sum W_{i} x_{i}}{\sum W_{i}}

\bar{y} = \frac{\sum W_{i} y_{i}}{\sum W_{i}}

\bar{z} = \frac{\sum W_{i} z_{i}}{\sum W_{i}}

Payload idealization used in the finite-element models.

Assembly	Total payload [lb]	CoG X [in]	CoG Y [in]	CoG Z [in]
WSP	259.30	15.97	14.54	26.21
OSP	259.30	9.94	14.34	26.21

Modeling rationale. Non-structural hardware is excluded from the structural mesh; its mass effect is represented through the conservative payload idealization above. This keeps the FEM focused on the actual load-carrying pallet structure.

Material Properties

The following table lists the mechanical properties of the materials used in the WSP and OSP installations [MMPDS-15-Table 3.7.10.0(g1), Table 3.7.10.0(b2), Table 3.2.4.0(c1), Table 3.2.4.0(b2)].

Material allowables used for the Workstation and Observer Station seat-pallet installations.

	AL 7075-T6 Extrusion t=0.25” – 0.499”	AL 7075-T651 Plate t=0.5” – 1.0”	AL 2024-T3 CLAD Sheet t=0.010” – 0.062”	AL 2024-T351 Plate t=0.5” – 1”	Unit
F_tu	81	77	60	63	ksi
F_ty	73	70	44	48	ksi
F_cy	73	68	36	39	ksi
F_su	43	44	37	37	ksi
F_bru	146	145	121	117	ksi
F_bry	113	117	82	90	ksi
E x10³	10.4	10.3	10.50	10.7	ksi
E_c x10³	10.7	10.6	10.70	10.9	ksi
μ	0.33	0.33	0.33	0.33	-
ρ	0.101	0.101	0.1	0.1	lb_m/in³
G x10³	4.0	3.9	-	4	ksi
e	7	7	12 or 15	8	%

Fasteners Allowables

Joint allowable philosophy

Use the weakest applicable failure path for the actual fastener / sheet stack-not the isolated fastener strength.

P_{allow} = \min (P_{single-shear}, P_{CSK-joint}, P_{bearing})

Sheet thickness and material can govern the joint.Countersunk geometry is checked with the applicable joint-strength reduction.Rivet tension is evaluated only where meaningful tensile reaction exists.

LOAD PATH

Fastener + sheet act together

Joint strength is controlled by compatibility between fastener strength, bearing resistance and local sheet geometry.

COUNTERSUNK JOINTS

Local geometry matters

The countersink changes the effective load path and can reduce the usable static joint strength relative to an isolated fastener.

BEARING CHECK

Critical thin layer

Where the non-countersunk sheet is thinner, its bearing capacity is compared directly against fastener/joint shear strength.

DESIGN INTENT

Shear-dominant rivet loading

Rivet tensile failure is not treated as governing unless the extracted reactions indicate a meaningful tensile component.

MS24694 Screws

MS24694 · Countersunk screws

Thread standardMIL-S-7742 · UNC-3A MaterialCadmium-plated low-alloy steel Material strengthF_tu = 125 ksi · F_su = 75 ksi #8-32d = 0.164 in · P_tu = 1,750 lb_f #10-32d = 0.190 in · P_tu = 2,500 lb_f Design ruleUse the weakest applicable fastener / bearing path

Governing MS24694 shear/joint allowables retained from the source assessment.

Screw	Application	Single-shear allowable [lb_f]	Sheet-bearing path [lb_f]	Governing allowable [lb_f]
#8-32 MS24694-S11	End caps to 2024/7075 plates	1,580	992	992 · bearing
#10-32 MS24694-S51 / -S55	Seat track to 7075-T651 base plate	2,125	10,402	2,125 · screw shear

Allowable basis. The #8 joint is governed by the thin 2024-T3 CLAD bearing path; the #10 seat-track joint is governed by screw single shear. Source values are based on the cited MMPDS and MS24694 data retained in the references.

LOAD CASES FORMULATION

REGULATORY SOURCE BASISFAR Part 25 structural-load criteria

Flight-load framework

FAR 25.321 and 25.331–25.351 are used with the aircraft load-case data to establish the flight envelope at each installation station.

Emergency landing

Emergency inertial factors are compared direction-by-direction with the extracted flight factors; the larger absolute limit value is retained for the FEM.

Load-Case Selection Rationale

WSP loads are screened at X312.35 and OSP loads at X508.17. Forward emergency loading governs longitudinally at 6.0 g; flight loads govern the vertical up/down cases. Aft loading is conservatively covered by the forward case.

Governing limit-load factors applied to the WSP and OSP finite-element models.

Direction	WSP applied [g]	WSP basis	OSP applied [g]	OSP basis
Upward	2.90	Flight	5.27	Flight
Downward	4.68	Flight	5.89	Flight
Outboard	2.00	Emergency	2.00	Emergency
Inboard	2.00	Emergency	2.00	Emergency
Forward	6.00	Emergency	6.00	Emergency
Aft	Covered by Forward		Covered by Forward

Ultimate-strength treatment. The source methodology applies the required 1.5 ultimate factor in the component margin calculations rather than multiplying the FEM limit loads before solution.

FINITE ELEMENT ANALYSIS (FEA)

MODEL

Load-Path Rationale

Seat/occupant inertia is introduced at the combined CoG through RBE3 distribution to the seat pins, then carried through the seat tracks and pallet plates into the ANCRA/NAS8604 interfaces and the aircraft OEM seat tracks.

BEAM IDEALIZATION

Seat tracks

Beam elements retain the section properties governing axial and bending response.

PLATE IDEALIZATION

Pallet structure

Base, side, top and end-cap panels use plate elements for membrane, bending and shear response.

CONNECTORS

CBUSH + merged joints

MS24694 screws use high-stiffness CBUSH translation with free rotations; selected NAS8604 plate joints use merged connectivity where local fastener stress is checked separately.

PAYLOAD / CONTACT

RBE3 + non-penetrating contact

RBE3s distribute payload without artificial stiffness; end-cap contact prevents penetration while allowing separation/sliding.

Finite-element models for the Workstation and Observer Station seat pallets. — WSP (left) and OSP (right) finite-element models with the principal structural load path retained.

Idealization strategy

Preserve stiffness and load-path physics that govern strength; suppress non-critical holes/fillets and other detail that would add mesh cost without improving the global substantiation.

LOADS AND CONSTRAINTS

Five body-load directions are evaluated in FEMAP using the governing pallet-specific load factors.

BOUNDARY CONDITIONSInboard OEM track interface

T_x, T_y, T_z restrained.

Outboard OEM track interface

T_x, T_y, T_z restrained.

New seat-track ends

T_z restrained to represent vertical load transfer through base-plate contact.

Engineering rationale

Constraints are applied only at physical attachment/load-transfer locations. Full translational restraint at the OEM track interfaces prevents rigid-body motion, while the added vertical restraint at the new track ends represents contact-supported transfer into the base plate and surrounding aircraft floor structure.

ANALYSIS

SOLVERSimcenter NASTRAN · SESTATIC / SOL 101

Linear static solution is appropriate for the global strength response under the prescribed inertial load cases.

UNIT SYSTEMlb_m · in · s

Mass and geometry are defined in a consistent English engineering unit system.

MASS CONVERSION

WTMASS enabled

Mass is entered in lb_m, density in lb_m/in³ and acceleration in in/s². WTMASS provides consistent mass-to-force conversion so solver forces are recovered in lb_f and stresses in psi.

101

Analysis choice

The objective is static strength and load redistribution, not dynamic response. SOL 101 therefore provides the required global stress/reaction solution without adding unnecessary nonlinear or transient complexity.

FEM Results

Total Translation Contours

the associated figures illustrate the Total Translation Contours (in inches) for the WSP and OSP installations, respectively.

Seat Tracks

Seat tracks are modeled as beam elements using the actual section properties. Axial force and biaxial bending are combined element-by-element so extrema from unrelated locations are not artificially combined.

Seat-track beam cross-section used in the FE model.

Seat-track section properties used in the FE model.

Property	Value	Unit
Area, A	0.490593	in²
I_zz	0.091901	in⁴
I_yy	0.010491	in⁴
Extreme fiber, +y / −y	+0.670 / −0.670	in
Extreme fiber, +z / −z	+0.287 / −0.213	in

Element-by-element stress recovery

Beam-end axial and bending terms are combined at the same physical element/end. This preserves the real load path and avoids pairing unrelated global maxima.

Combined beam stress

σ_{comb} = \frac{f_{a}}{A} \pm \frac{M_{1} c_{y}}{I_{zz}} \pm \frac{M_{2} c_{z}}{I_{yy}}

Axial-force and bending-moment contours for the Workstation Seat Pallet seat tracks. — WSP seat-track axial-force and bending-moment contours under the forward case.

Axial-force and bending-moment contours for the Observer Station Seat Pallet seat tracks. — OSP seat-track axial-force and bending-moment contours under the forward case.

Seat-track combined-stress calculation workflow exported from the supporting spreadsheet. — Representative combined-stress calculation workflow used to recover beam stresses.

Governing seat-track demands and recovered combined stresses.

Assembly	Governing element	Load case	Axial force [lb_f]	Largest listed bending moment [in-lb_f]	Max tension [ksi]	Max compression [ksi]
WSP	61	Forward	−3,068.74	615.71	12.78	−23.11
OSP	8047	Forward	−1,611	316.07	12.90	−12.01

For AL 7075-T6 extrusion, F_tu = 81 ksi and F_cy = 73 ksi. With the 1.5 ultimate factor, the governing seat-track margin is compression.

{M.S.}_{T} = \frac{81}{12.90 \times 1.5} - 1 = 3.18

{M.S.}_{C} = \frac{73}{23.11 \times 1.5} - 1 = 1.11 \to PASS

Flange Crippling

The thin-walled seat-track section is checked for local crippling using the Needham/Gerard angle-element method. The cross-section is decomposed into angle elements and their crippling capacities are summed using the source methodology.

Angle	A_n	C_e	F_ccn [ksi]	A_nF_ccn
1	0.047	0.342	104.46	4.882
2	0.234	0.366	178.22	41.677
3	0.234	0.366	178.22	41.677
4	0.047	0.342	104.46	4.882

Seat-track cross-section idealization used for the crippling calculation.

$F_{ccn} = \frac{C_{e} \sqrt{F_{cy} E}}{{(\frac{h + b}{2 t_{\min}})}^{0.75}}$

The resulting seat-track crippling allowable is 165.93 ksi. Using the governing 23.11 ksi compressive demand:

{M.S.}_{crippling} = \frac{165.93}{23.11 \times 1.5} - 1 = 3.79 \to PASS

Column Buckling

Column stability is checked using the source Euler-type formulation:

$P_{cr} = \frac{n^{2} K π^{2} E_{c} I}{L^{2}}$

Effective column definition. The seat tracks are attached to the base plate through 10 screws on the WSP and 8 on the OSP. The source assessment uses the pinned-pinned case with K = 1 and n = 9 / 7, respectively.

Seat-track column-buckling results.

Assembly	Critical buckling stress [ksi]	Governing compressive demand [ksi]
WSP	178.64	23.11
OSP	209.186	23.11

{M.S.}_{buckling} = \frac{178.64}{23.11 \times 1.5} - 1 = 4.15 \to PASS

Structural Sheet Metals

Plate strength is screened using major/minor principal stresses on both shell faces. Tensile demand is compared with F_tu and compressive demand with F_cy. The compact table below retains the governing source values rather than every intermediate contour extraction.

Governing principal-stress magnitudes recovered from the WSP and OSP plate models.

Assembly	Material / thickness	Max tension [ksi]	Max compression [ksi]	Governing case
WSP	AL 7075-T651 · 0.5 in	18.51	17.78	Forward
	AL 2024-T3 CLAD · 0.05 in	14.02	11.82	Forward
	AL 2024-T351 · 0.5 in	22.14	17.11	Forward
OSP	AL 7075-T651 · 0.5 in	9.66	9.26	Forward
	AL 2024-T3 CLAD · 0.05 in	1.34	2.22	Mixed
	AL 2024-T351 · 0.5 in	7.43	4.97	Forward

AL 7075-T651 Plate

The AL 7075-T651 base/top plate check uses F_tu = 77 ksi and F_cy = 68 ksi. The WSP forward case governs this material family with 18.51 ksi tension and 17.78 ksi compression.

{M.S.}_{T} = \frac{77}{18.51 \times 1.5} - 1 = 1.77 \to PASS

{M.S.}_{C} = \frac{68}{17.78 \times 1.5} - 1 = 1.55 \to PASS

The Attachment Points

NAS8604-7 attachment reactions are resolved into resultant shear and tensile demand. The recruiter version retains the governing source check instead of the full node-by-node reaction tables.

Governing attachment-bolt demand used in the source substantiation.

Joint	Load case	Shear demand [lb_f]	Tensile demand [lb_f]	Shear allowable [lb_f]	Tensile allowable [lb_f]
WSP NAS8604-7 attachment	Forward	1,110.15	254.65	4,660	2,250

The source joint check includes the 1.5 ultimate factor and 1.15 fitting factor:

{M.S.}_{shear} = \frac{4660}{1110.15 \times 1.5 \times 1.15} - 1 = 1.43 \to PASS

{M.S.}_{tension} = \frac{2250}{254.65 \times 1.5 \times 1.15} - 1 = 4.12 \to PASS

Seat Pallets’ Fasteners

LOCAL FASTENER CHECK

NAS8604-17 top-plate screws

Plate resultants are converted to element-edge forces using extracted mesh geometry, then resolved into screw shear/tension demand.

CONSERVATIVE DISTRIBUTION

Single-fastener screening

Where a local maximum is compared against one screw capacity, the assumption intentionally avoids crediting redistribution across the full fastener group.

SEAT LOAD PATH

Seat pins → track → base plate

Rigid-body reactions at the four seat pins provide the input for the MS24694 seat-track attachment check.

SUPPLIER HARDWARE

Certified seat pins

Seat pins supplied with the certified seat are accepted for the emergency-load application; the pallet-side fasteners remain explicitly checked.

NAS8604 Bolts

NAS8604 Bolts were utilised in three locations within the WSP and OSP:

Connecting the Base plate to the OEM Seat Track
Connecting the Side Plates to the Top Plate.
Connecting the Side Plates to the Base Plate.

NAS8604 Bolts utilized in Side Plates – Top Plate Joint.

The top plate is attached to the side plates with 20 NAS8604-17 screws. FEMAP plate resultants are converted to element-edge forces and screened across every load case using a Python post-processing workflow.

NAS8604-17 screw pattern attaching the top plate to the side plates.

Python Workflow

Python automates FEMAP plate-resultant post-processing so every valid element and load case is screened with the same calculation sequence.

Read FEMAP-exported nodal coordinates and plate resultants.
Reconstruct quadrilateral element geometry and local dimensions.
Convert membrane/shear resultants into element-edge forces.
Screen all load cases and export the governing shear/tension demand.

N_{x} = d_{x} n_{x}, N_{y} = d_{y} n_{y}, N_{xy, x} = d_{x} n_{xy}

F_{s, \max} = \sqrt{{(N_{x} + N_{xy, y})}^{2} + {(N_{y} + N_{xy, x})}^{2}}

Plate element force-resultant convention used by the Python post-processing workflow.

Why automate?The governing fastener demand can occur in an element that does not contain the largest individual plate-force component. Automated screening preserves a repeatable, traceable search across the complete result set.

Governing Python-recovered demand for the top-plate fasteners.

Assembly	Load case	Max screw-direction shear [lb_f]	Max tensile demand [lb_f]
WSP	Forward	881.80	189.41
OSP	Forward	290.96	61.06

Fastener screening. NAS8604-17 ultimate capacities are 4,660 lb_f in shear and 4,480 lb_f in tension. Even the conservative single-fastener WSP demand remains below both capacities.

MS24694 Screws utilized in Seat Track – Base Plate Joint.

The new seat tracks are attached to the base plate with MS24694-S51/-S55 screws. Seat-pin reactions transfer the seat/occupant inertia into the track; the source assessment conservatively screens the maximum resultant pin shear and axial demand against the screw/joint allowables.

Seat attachment points for the Workstation and Observer Station seat pallets. — Seat attachment points used to recover the governing pin reactions.

Governing seat-pin demand retained from the source assessment.

Assembly	Governing case	Max shear [lb_f]	Max axial/tension magnitude [lb_f]
WSP	6 g Forward	389.23	1,415.67
OSP	6 g Forward	389.23	1,418.71

Load-path treatment. OEM seat pins are accepted as part of the certified seat. The structural installation check focuses on the new track/base-plate screws carrying the recovered seat reactions.

{M.S.}_{S} = \frac{2125}{389.23 \times 1.5 \times 1.15} - 1 = 2.16 \to PASS

{M.S.}_{T} = \frac{2500}{1418.71 \times 1.5 \times 1.15} - 1 = 0.02 \to PASS

REFERENCES

Structural Methods & Allowables

MMPDS-15 - Metallic Materials Properties Development and Standardization
Stress Analysis Manual - Air Force Flight Dynamics Laboratory, Wright-Patterson AFB

Regulatory & Aircraft Load Basis

Federal Aviation Regulations - 14 CFR Part 25
FAA Advisory Circular AC 120-27D - Aircraft Weight and Balance Control
DHC-8-100 Load Cases and Applied Loads

Fasteners & Hardware Data

Fastener Design Manual - NASA Reference Publication 1228
MS24694 screw technical data

Track & Fitting Data

ANCRA Aircraft Track technical data
42182-10 ANCRA flush seat-track fitting data

SUBSTANTIATION OUTCOME

CONFIGURATIONSWSP + OSP

Starboard models substantiate the mirrored port installations by comparison.

SEAT TRACKSPASS

Combined stress, crippling and column-buckling checks satisfy the source assessment.

PLATES + ATTACHMENTSPASS

Principal-stress and attachment reaction checks remain within the applicable allowables.

FASTENERSPASS

NAS8604 and MS24694 fastener demands are below the governing shear/tension or joint allowables.