Small Volumes Decompression #1 - Understanding the Problem

Introduction

This article is the first one of a series dedicated to Aircrafts Small Volumes Decompression.

We will introduce the Small Volume Decompression (or Closed Compartment Decompression, or yet Cabin Monument Compartment Decompression) subject and the different issues one need to face when discussing this topic.

The following posts will describe our investigations about this subject. These investigations are based on decompression analysis performed with Esonix.


NOTE: This article and the following may use indefferently the terms like small volume, closed compartments, cabin monument compartments. All those terms are equivalent and refer to a small enclosed volume subjected to differential pressure with its surrounding environment.


Small Volume - Differential Pressure applied on it

In case of cabin modification, installed bulkheads, floor structure and partitions in pressurised compartments must be designed to withstand maximum differential pressure generated during the most critical decompression event, as specified in CS25.365(e).

Additionally any enclosed Cabinet (also called Small volume, closed compartment, cabin monument compartment) should be built such as none of its constitutive part will become detached and could potentially injured occupants while in their seats in case of maximum differential pressure, as specified in CS26.365(f).

For instance, imagine a fully sealed box installed inside the Main Cabin of an aircraft. Before any aircraft depressurization problem occurs, the sealed box and the Aircraft Main Cabin have the same internal pressure — let’s call it \(P_c\).

Now, if for any reason an opening hole appears in the Main Cabin fuselage, the Cabin Air will be sucked out the aircraft. After a short time, the pressure inside the Aircraft will drop down to the external ambient pressure — let’s call it \(P_a\).

If the box was perfectly sealed, the pressure difference between the box and the Aircraft cabin will start to increase from the initial moment (when box and aircraft had same pressure) until it achieves the maximum differential pressure of \(P_c\) - \(P_a\) (when box pressure is \(P_c\) and Aircraft pressure is \(P_a\)).

In other words, this hypothetical fully sealed box would have to sustain the Maximum Aircraft Differential Pressure (\(P_c\) - \(P_a\)) in case of opening in the aircraft fuselage (depressurization of the aircraft).

Translating this hypothetical situation to reality, the box represents the Small Volumes (or Closed Compartment, or yet Cabin Monument Compartment). In reality those Cabinets (or Compartments) are not sealed and have vents to the Aircraft cabin (as for example gap around the Cabinet’s door). So, in case of opening in the Aircraft fuselage, not only the Cabin Air will be sucked out from the Aircraft to the atmosphere but also the Cabinet Air (or Compartment Air) will be sucked out from the Cabinet (or Compartment) to the Aircraft Cabin. It means that the differential pressure sustained by the Cabinet (or Compartment) - Small Volume Decompression - will be always lower than the Aircraft Maximum Differential Pressure (\(P_c\) - \(P_a\)).

Small Volume Decompression X Aircraft Configuration

In case of an opening in the fuselage and consequently depressurization of the cabin, the differential pressure sustained by the Cabinet/Compartment (Small Volume Decompression) would depend of several factors related to the Aircraft type on one side (flight altitude, hole size, Main Cabin/Room Volume, etc) and the Cabinet/Compartment itself on the other side (Cabinet Volume, Vent Area between it and the room where it is installed, cabinet stiffness, etc).

The aircraft manufacturers (OEM) usually provide guidance regarding the pressure load cabinets/compartments should withstand in case of an explosive decompression. Those guidances documents are:

  • Boeing: D655441 “Galley/Interior Furnishing Structural Requirements
  • Airbus: 2520M1F010100Stowage and Stowage Installation Frame Specification

This load is usually based on the cabinet volume and its vent area for a cabinet installed in a standard PAX configuration (see figure below).

Small Volume (Cabinet) installed in airline configuration \label{SVD_airline_conf}

In case of aircraft interior refurbishment (airline modification or VIP outfitting), the Aircraft Main Cabin is frequently divided in smaller rooms (Volumes), see example in the figure below.

In this case, the differential pressure provided by the manufacturers may not cover the actual differential pressure sustained by the Cabinet/Compartment enclosed in resized rooms (smaller rooms).

Small Volume (Cabinet) installed in refurbished configuration \label{SVD_refurb_conf}

In case of aircraft interior modification, Esonix is an easier and efficient tool to determine accurate Decompression sustained by Small Volumes (Cabinets/Compartments) enclosed in resized rooms.


NEXT STEP

In the next Article we will investigate the influence of aircraft configurations on the pressure sustained by the Cabinets/Compartments (Small Volume Decompression).


See here the list of Articles referring to the study of Small Volume Decompression:

  • Article #1: Small Volumes Decompression #1 - Understanding the Problem

  • Article #2: Small Volumes Decompression #2 - Airline/Pax, VIP configurations and Aircraft Parameters

  • Article #3: Small Volumes Decompression #3 - Analysis of closed compartment in the Cabin Airline/PAX Configuration

  • Article #4: comming soon. Stay tuned!

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