AV-265 Antenna Installation - Structural Provision

Introduction

Description of the Modification

AircraftDHC-8-315Ventral fuselage installation

ModificationTwo AV-265 antennasForward and aft structural provisions

Primary reinforcement2024-T3 external doublers0.8 in feed-through cutouts

AssessmentAerodynamic + pressure + joint loadsHand substantiation and Swift load distribution

Two AV-265 antennas are installed on the lower fuselage: the forward antenna is aft of X488.5 between STR 30S and STR 31S, while the aft antenna is forward of X589.00 between STR 32P and STR 33.

FEED-THROUGHØ 0.8 in fuselage cutout

Each antenna requires a local cable penetration through the fuselage skin.

REINFORCEMENTExternal aluminum doubler

The doubler restores local load-carrying capability around the cutout and distributes skin loads into the surrounding rivet pattern.

DOUBLER ATTACHMENTNAS1097AD flush rivets

The forward pattern uses 74 rivets and the aft pattern uses 58, reflecting the local geometry and required load-transfer path.

ANTENNA ATTACHMENT4 × NAS8604-6 screws

Each antenna is mounted with 100° countersunk alloy-steel screws to retain the external aerodynamic profile.

Load-Path Rationale

I separated the direct antenna-attachment path from the fuselage pressure-load redistribution path because they answer different structural questions. Inertial and aerodynamic blade loads enter through the four NAS8604-6 mounting screws and are checked directly against the attachment hardware and local substrate capacity. Cabin pressure acts as far-field hoop and longitudinal membrane loading in the skin; the external doubler then restores load-carrying capability around the feed-through, and the NAS1097AD rivet rows transfer part of that load into the doubler. Swift strain-compatibility DCLR/LDF calculations are used to quantify how much load reaches the doubler center and how much enters through the first rivet row before skin, doubler, rivet, and corner-bearing margins are checked separately. This keeps the load path explicit and avoids mixing direct antenna loads with pressure-driven skin redistribution.

AERODYNAMIC LOADSDive-speed blade loading

Conservative NACA 0012 idealization is used to develop lift and drag at V_D before checking the four-screw antenna interface.

PRESSURE LOAD PATHFuselage membrane-load redistribution

Cabin-pressure hoop and longitudinal loads are redistributed around the feed-through cutout through the skin, doubler and surrounding fastener rows.

JOINT ANALYSISSwift strain-compatibility method

DCLR and first-row LDF values capture progressive shear transfer instead of assuming uniform rivet loading.

STRENGTH CHECKSNet section · rivet shear · bearing

Adjusted skin/doubler stresses, corner-rivet resultants and local bearing are checked against the source allowables with positive margins.

Technical figure from the AV-265 Antenna Installation structural provision report. — Fuselage stations diagram for DHC-8-315 aircraft showing the FWD and AFT antennae installation locations

Mechanical Properties and Material Specifications for Aircraft Antenna Installation Components.

Description	Location	Size	Material	S_tu	S_su	S_bru	E	µ
Description	Location	Size	Material	ksi				-
Fuselage Skin	AFT	0.032	Alclad 2024−T3/T42	60	37	121	10.5 x10³	0.33
Fuselage Waffle		0.028	Alclad 2024−T3/T42	60	37	121
Doubler		0.071	Alclad 2024−T3	62	38	125
Fuselage Skin	FWD	0.032		60	37	121
Fuselage Waffle		0.028
Doubler		0.05
Blanking plate	-	0.09		62	38	125	-	-

Dimensions and Parameter Specifications for Forward and Aft Antenna Installations on Aircraft.

		Unit	FWD Antenna	AFT Antenna
Fuselage	Thickness	in	0.06*	0.06*
	Cutout area	in²	0.5027	0.5027
	Cutout Diameter	in	0.8	0.8
	Fuselage maximum radius (R)	in	62	62
Doubler	Thickness	in	0.05	0.071
	Dim, hoop direction (W)	in	5.4	5
	Dim, longitudinal direction (L)	in	10.7	8.8
AV-265	Weight	lb_f	0.4	0.4
	Dim, longitudinal direction (L)	in	5.25	5.25
	Dim, hoop direction (W)	in	1.8	1.8
	Height	in	3.25	3.25
Cabin	Maximum pressure differential (∆P)	psi	5.95

t_eff

Effective fuselage thickness = 0.060 in

The bonded 0.032 in skin and 0.028 in waffle are treated as one load-carrying section. The combined thickness is used for pressure stress and local load-transfer calculations at both antenna provisions.

Design Assessment

Critical Loading Condition

REGULATORY SOURCE BASISEmergency and maneuver load criteria used for the installation

FAR 23.561 · Emergency landing

Upward: 3.0 g
Forward: 9.0 g
Sideward: 1.5 g
Downward: 3.0 g

Governing downward case

FAR 23.337(a)(1) maneuvering factor:

3.8 \times 1.5 = 5.7 g

Because 5.7 g exceeds the 3.0 g emergency downward value, 5.7 g governs downward.

Load-Case Selection Rationale

I treated the installation with three bounding load families because each acts through a different mechanism. For direct antenna inertia, the FAR 23.561 emergency conditions are retained, except the downward direction where the 3.8 g positive maneuver factor multiplied by 1.5 gives 5.7 g ultimate and therefore exceeds the 3.0 g emergency value. For the exposed antenna blade, the aerodynamic check is bounded at dive speed, VD = 1.25 VMO = 303.75 kn, rather than normal operating speed. For the fuselage reinforcement, cabin pressure is treated separately with the source ultimate pressure differential of 11.87 psi. This prevents unrelated load paths from being combined while retaining the governing demand for each failure mode.

Selected governing load conditions used in the structural assessment.

Case	Condition	Source / candidate value	Selected design value	Governing basis
1	Upward inertia	3.0 g	3.0 g	Emergency landing
2	Forward inertia	9.0 g	9.0 g	Emergency landing
3	Sideward inertia	1.5 g	1.5 g	Emergency landing
4	Downward inertia	3.0 g emergency / 5.7 g maneuver	5.7 g	Maneuver load governs
5	Aerodynamic envelope	V_MO = 243 kn	V_D = 303.75 kn	Dive-speed aerodynamic bound
6	Cabin pressure	ΔP = 5.95 psi	P_ult = 11.87 psi	Ultimate pressurization

Antenna Blanking Plate Installations

✓

Blanking plates - passed by inspection

Each previously approved provision is closed with a 0.090 in AL 2024-T3 plate secured by four NAS8604-6 #10-32 screws. With negligible applied loading and an existing approved structural provision, no separate detailed strength calculation is required by the source assessment.

Inertial Loading Considerations

Antenna weight0.4 lb_f

9 g forward inertia3.6 lb_f

NAS8604-6 shear allowable720 lb_f

NAS8604-6 tensile allowable1,200 lb_f

Substrate bearing strength121–125 ksi

Why inertia is non-governing. The 3.6 lb_f emergency forward load is orders of magnitude below the screw shear/tension capacities, and the supporting aluminum layers have high bearing allowables. The source therefore passes the direct antenna inertia path by inspection.

Aerodynamic Loading Considerations

Lift Force

LOAD ORIENTATION

Side load normal to the flight path

The aerodynamic side force is treated perpendicular to the antenna’s forward trajectory to capture a conservative cross-flow condition.

AIRFOIL IDEALIZATION

Symmetric section

The blade is idealized as a symmetric airfoil so established lift/drag relations can be used with the scaled antenna geometry.

DESIGN SPEED

Dive speed, not V_MO

The analysis uses V_D = 1.25 V_MO per the cited FAR criterion, increasing dynamic pressure relative to normal operating speed.

CONSERVATIVE PROFILE

NACA 0012 envelopes the scaled section

The scaled thickness ratio is about 8.63%, closer to a thinner NACA 0004-type profile; NACA 0012 is selected because its higher lift coefficient produces a conservative aerodynamic load.

NACA

NACA 0012 selected as the conservative aerodynamic surrogate

The measured mean thickness ratio is 8.63%. Using the thicker symmetric NACA 0012 profile intentionally biases the lift estimate upward relative to the thinner profile suggested by the scaled geometry.

NACA 0012 source data

c_l,max 1.1744c_d,max 0.017821α_stall 12.18°

Dive speed, V_D303.75 kn

Mean thickness ratio8.63%

Reynolds number8.25 × 10⁵

3D C_L,max0.46444

3D C_D,max0.08193

\begin{aligned} {L_{antenna}}&={\,\frac{1}{2}\rho V_{D}^{2}SC_{L_{\max}}}\\ {}&={\frac{1}{2}\overbrace{0.002378}^{\binom{\text{Air}\,\text{Density}}{\text{@Sea}\,\,\text{Level}}}\,\,\,\,\,\overbrace{512.672244 ^{2}}^{\binom{\text{Diving}}{\text{Speed}}}\,\,\,\,\overbrace{0.068487}^{\binom{\text{Antenna's}\,}{\,\text{Surface}\,\text{Area}}}\,\,\,\overbrace{0.464436911}^{\binom{\text{Antenna's}\,\text{Maximum}\,}{\text{Lift}\,\text{Coefficient}}}\,} \end{aligned}

Drag Force

\begin{aligned} {C_{D_{\text{max}}}}&={\overbrace{0.017821}^{c_{d_{\text{max}}}}\,+\frac{\overbrace{0.464436911^{2}}^{C_{L_{\text{max}}}^{2}} }{1.071016642 \pi }} \end{aligned}

\begin{aligned} {D_{antenna}}&={\,\frac{1}{2}\rho V_{D}^{2}SC_{D_{\max}}}\\ {}&={\frac{1}{2}\overbrace{0.002378}^{\binom{\text{Air}\,\text{Density}}{\text{@Sea}\,\,\text{Level}}}\,\,\,\,\,\overbrace{512.672244 ^{2}}^{\binom{\text{Diving}}{\text{Speed}}}\,\,\,\,\overbrace{0.068487}^{\binom{\text{Antenna's}\,}{\,\text{Surface}\,\text{Area}}}\,\,\,\overbrace{0.081928282 }^{\binom{\text{Antenna's}\,\text{Maximum}\,}{\text{Drag}\,\text{Coefficient}}}\,} \end{aligned}

Ultimate side force9.940 lb_f

Lift load / screw2.485 lb_f

Ultimate drag1.753 lb_f

Drag load / screw0.438 lb_f

Why aerodynamic attachment loads are non-governing. Both per-screw demands are very small relative to the NAS8604-6 ultimate shear allowable, so the source passes this direct attachment path by inspection.

Cabin Pressurization Loads

PRESSURE BASISMaximum cabin differential = 5.95 psi

The source applies FAR 25.303 and 25.365(d), including the 1.5 strength factor and 1.33 pressure multiplier, to obtain the ultimate pressure differential used below.

\begin{aligned} {P_{p_{\text{,ult}.}}}&={ \left(1.5\right)\left(1.33\right)\left(5.95\right)} \end{aligned}

\begin{aligned} {P_{p_{\text{,ult}.}}}&={ 11.87\,\,\text{psi}} \end{aligned}

Conservative load combination. At the ventral fuselage, flight bending places the skin in compression and therefore relieves pressure-induced tensile stress. The bending contribution is omitted so the pressure-only tensile state remains conservative for this local check.

Total Far-Field Hoop and Longitudinal Stresses in the Skin

Using the bottom-fuselage radius $R_{bottom}$ = 62 in and the effective 0.060 in skin-plus-waffle thickness, the pressure-induced far-field stresses are:

\sigma_{hoop}=\frac{P_{p_{\text{,ult.}}}\,R_{\text{bottom}}}{t_{\text{skin(s)}}}

\sigma_{long}=\frac{1}{2}\sigma_{hoop}

\sigma_{hoop}^{\text{fwd}}=\sigma_{hoop}^{\text{aft}}=\sigma_{hoop}=\frac{\overbrace{11.87}^{\binom{\text{Max.}\,\text{Cabin}}{\text{Pressure}}}\,\overbrace{62}^{\binom{\text{Max.}\,\text{Fuselage}}{\text{Radius@Bottom}}}}{\underbrace{0.032}_{\text{SkinThickness}}+\underbrace{0.028}_{\text{WaffleThickness}}}

\sigma_{long}^{\text{fwd}}=\sigma_{long}^{\text{aft}}=\sigma_{long}=\frac{1}{2}\sigma_{hoop}

\Rightarrow

\sigma_{long}=6.1328 \,\text{ksi}

FWD hoop load7,874.6 lb_f

FWD longitudinal load1,987.1 lb_f

AFT hoop load6,476.3 lb_f

AFT longitudinal load1,839.9 lb_f

Doubler Central Load Ratio (DCLR) and First-Row Load Distribution Factors (LDF)

DCLR

Load reaching the doubler center

The Doubler Central Load Ratio represents the fraction of the far-field hoop or longitudinal load accumulated at the doubler center after progressive transfer through the rivet rows.

1st-row LDF

Critical first-row fastener demand

The first-row Load Distribution Factor represents the fraction of the applied directional load entering the doubler through the first rivet row-the row used to establish the critical individual fastener load.

Swift

Swift strain-compatibility method

The source uses the Fastener Shear Load Distribution Analysis tool based on Swift’s strain-compatibility formulation to capture progressive shear transfer through the joint rather than assuming uniform load per rivet. [Damage Tolerance Assessment Handbook-Volume II: Airframe Damage Tolerance Evaluation]

E_p, E_s, E_fPlate, doubler and fastener elastic moduli

D / materialRivet diameter and material

t_p, t_sSkin and doubler thicknesses

W_p, W_sFastener pitch across the evaluated row

LDistance between adjacent rivet rows

Why multiple cases are required. Rivet pitch and row spacing vary around the doubler, so both minimum and maximum geometric combinations are evaluated. The aft provision is also solved for both 1/8 in and 5/32 in rivets. This brackets the stiffness/load-transfer sensitivity instead of relying on one nominal joint geometry.

Doubler Central Load Ratio (DCLR) and the 1^st row Load Distribution Factors (LDF) for various case-scenarios for both the FWD and AFT antenna installation locations.

	AFT antenna installation location
Load Direction	Hoop		Longitudinal		Hoop		Longitudinal
Rivet Size (in)	1/8				5/32
Parameter (%)	$\alpha_{\text{LDF}}^{\text{H}}$	$\beta_{\text{DCLR}}^{\text{H}}$	$\alpha_{\text{LDF}}^{\text{L}}$	$\beta_{\text{DCLR}}^{\text{L}}$	$\alpha_{\text{LDF}}^{\text{H}}$	$\beta_{\text{DCLR}}^{\text{H}}$	$\alpha_{\text{LDF}}^{\text{L}}$	$\beta_{\text{DCLR}}^{\text{L}}$
W_max, L_max	25.19	36.85	28.11	43.41	26.64	38.60	29.31	44.68
W_min, L_min	26.22	39.44	29.85	46.70	27.57	41.03	30.93	47.66
	FWD antenna installation location
W_max, L_max	20.78	31.9	23.42	39.05
W_min, L_min	24.44	36.07	24.66	41.27

Representative load-transfer result. For the 1/8 in rivet / maximum-spacing case, Swift DCLR transfers 2,512.00 lb_f hoop and 775.96 lb_f longitudinal load into the FWD doubler center; the remaining 5,362.60 lb_f and 1,211.14 lb_f remain in the skin. The AFT representative case transfers 2,386.52 lb_f hoop and 798.70 lb_f longitudinal load into the doubler.

Adjusted Hoop and Longitudinal Stresses at the Doubler and Skin Center

Net-section basis. The critical section passes through the feed-through and fastener holes, so the effective width and length are reduced for removed material before the center loads are converted into adjusted stresses. The AFT calculation conservatively uses the 5/32 in rivet diameter when defining the net section.

\left[W_{net}\right]_{\text{doubler}}=\left[W\right]_{\text{doubler}}-D_{\text{cut}}-n_{\text{r,max}}D_{\text{r,max}}

\left[L_{net}\right]_{\text{doubler}}=\left[L\right]_{\text{doubler}}-D_{\text{cut}}-n_{\text{r,max}}L_{\text{r,max}}

FWD net width3.85 in

FWD net length8.15 in

AFT net width3.10625 in

AFT net length6.28125 in

FWD governing skin hoop10.9665 ksi

AFT governing skin hoop10.8518 ksi

FWD governing doubler hoop6.9702 ksi

AFT governing doubler hoop5.9583 ksi

Adjusted far-field hoop and longitudinal stresses (ksi) in the skin for the evaluated FWD and AFT joint cases.

	AFT antenna installation location
Rivet Size (in)	1/8		5/32
Load Direction	Hoop	Longitudinal	Hoop	Longitudinal
MAX (W, L) values	10.8518	5.5866	10.5511	5.4612
MIN (W, L) values	10.4068	5.2618	10.1335	5.1670
	FWD antenna installation location
MAX (W, L) values	10.9665	5.2430
MIN (W, L) values	10.2949	5.0520

Adjusted far-field hoop and longitudinal stresses (ksi) in the doubler for the evaluated FWD and AFT joint cases.

	AFT antenna installation location
Rivet Size (in)	1/8		5/32
Load Direction	Hoop	Longitudinal	Hoop	Longitudinal
MAX (W, L) values	5.3513	3.6215	5.6054	3.7275
MIN (W, L) values	5.7274	3.8960	5.9583	3.9761
	FWD antenna installation location
MAX (W, L) values	6.1644	4.0310
MIN (W, L) values	6.9702	4.2602

Skin Minimum Margin of Safety

GOVERNING SKIN CASEMaximum adjusted hoop stress controls.

For both antenna locations, the critical skin stress occurs with maximum pitch/row spacing and the 1/8 in rivet case. The corresponding minimum margins are calculated below.

\begin{aligned} {\text{M.S}_{skin}}&={\frac{S_{tu_{skin}}}{\sigma_{hoop_{\text{,adj}}}}-1} \end{aligned}

Doubler Minimum Margin of Safety

GOVERNING DOUBLER CASEMaximum adjusted hoop stress controls.

The doubler is governed by the minimum pitch/row-spacing case; the aft 5/32 in rivet case produces the highest local doubler hoop stress. The resulting margins are calculated below.

Skin MS · FWD+4.47

Skin MS · AFT+4.53

Doubler MS · FWD+7.61

Doubler MS · AFT+9.41

Individual Hoop and Longitudinal Load per Rivet

\left[P\right]_{\text{r}}=\alpha_{\text{LDF}}\,\,\frac{P_{\text{applied}}}{n_{r}}

First-row hoop/longitudinal load per rivet (lb_f), ${[P]}_{r}$ , for the evaluated FWD and AFT joint cases.

	AFT antenna installation location
Rivet Size (in)	1/8		5/32
Load Direction	Hoop	Longitudinal	Hoop	Longitudinal
MAX (W, L) values	148.31	86.20	156.84	89.88
MIN (W, L) values	154.37	91.54	162.32	94.85
	FWD antenna installation location
MAX (W, L) values	116.88	77.56
MIN (W, L) values	137.47	81.67

FWD max rivet shear166.71 lb_f

AFT max · 5/32 in188.02 lb_f

AFT max · 1/8 in179.47 lb_f

Rivet MS · FWD 1/8 in+1.039

Rivet MS · AFT 5/32 in+1.883

Rivet MS · AFT 1/8 in+1.078

Bearing Stress at Corner Holes

BEARING CHECKEvaluate the thinnest layer at the corner rivets.

FWD: 0.050 in doubler. AFT: 0.060 in effective fuselage skin/waffle section. Using the thinner participating layer maximizes bearing stress for the local joint check.

\sigma_{bearing}=\frac{\left[P_{\text{net}}\right]_{r}}{t_{layer}D_{h}}

FWD bearing stress26.674 ksi

AFT bearing · 5/32 in20.055 ksi

AFT bearing · 1/8 in23.929 ksi

FWD bearing MS+3.663

AFT bearing MS · 5/32 in+5.139

AFT bearing MS · 1/8 in+4.198

The opposite companion layers are thicker than the layers checked above, so their bearing stresses are lower and are covered by comparison.

Substantiation Outcome

SKINPositive margins

Minimum pressure/load-transfer margins: FWD 4.47, AFT 4.53.

DOUBLERPositive margins

Minimum hoop-stress margins: FWD 7.61, AFT 9.41.

RIVETSCorner demand governs

Maximum resultant shear: 166.71 lb_f FWD and 188.02 lb_f AFT.

BEARINGPositive margins

Critical corner bearing margins remain positive; the thinner companion layers are passed by comparison.

PASS

Source substantiation supports the AV-265 structural provision.

Direct inertial and aerodynamic antenna loads are non-governing by inspection, while cabin-pressure load redistribution, skin/doubler stresses, rivet loads and corner-hole bearing are substantiated with positive margins.

REFERENCES

Regulatory & Aircraft Load Basis

Federal Aviation Regulations - FAR 23.335, FAR 23.337 and FAR 23.561
Federal Aviation Regulations - FAR 25.303 and FAR 25.365

Aerodynamic Methods

Airfoil Design and Data - Richard Eppler
Fundamentals of Aerodynamics, 3rd Edition - John D. Anderson

Load Distribution & Structural Methods

Damage Tolerance Assessment Handbook - Volume II: Airframe Damage Tolerance Evaluation

Fasteners & Hardware Data

NAS1097AD flush shear rivet standard
NAS8604-6 countersunk screw standard