14 CFR Subpart C Section 25-365

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Alright then, time to dig into the next regulation, 14 CFR Subpart C Section 25-365.

§ 25.365 - Pressurized Compartment Loads

For airplanes with one or more pressurized compartments the following apply:
(a) The airplane structure must be strong enough to withstand the flight loads combined with pressure differential loads from zero up to the maximum relief valve setting.

(b) The external pressure distribution in flight, and stress concentrations and fatigue effects must be accounted for.

(c) If landings may be made with the compartment pressurized, landing loads must be combined with pressure differential loads from zero up to the maximum allowed during landing.

(d) The airplane structure must be designed to be able to withstand the pressure differential loads corresponding to the maximum relief valve setting multiplied by a factor of 1.33 for airplanes to be approved for operation to 45,000 feet or by a factor of 1.67 for airplanes to be approved for operation above 45,000 feet, omitting other loads.

(e) Any structure, component or part, inside or outside a pressurized compartment, the failure of which could interfere with continued safe flight and landing, must be designed to withstand the effects of a sudden release of pressure through an opening in any compartment at any operating altitude resulting from each of the following conditions:

(1) The penetration of the compartment by a portion of an engine following an engine disintegration;

(2) Any opening in any pressurized compartment up to the size Ho in square feet; however, small compartments may be combined with an adjacent pressurized compartment and both considered as a single compartment for openings that cannot reasonably be expected to be confined to the small compartment. The size Ho must be computed by the following formula:

Ho=Maximum opening in square feet, need not exceed 20 square feet.
P=(As/6240) .024
As=Maximum cross-sectional area of the pressurized shell normal to the longitudinal axis, in square feet; and

§ 25.365 - Pressurized Compartment Loads (continued)

(3) The maximum opening caused by airplane or equipment failures not shown to be extremely improbable.

(f) In complying with paragraph (e) of this section, the fail-safe features of the design may be considered in determining the probability of failure or penetration and probable size of openings, provided that possible improper operation of closure devices and inadvertent door openings are also considered.

Furthermore, the resulting differential pressure loads must be combined in a rational and conservative manner with 1-g level flight loads and any loads arising from emergency depressurization conditions.

These loads may be considered as ultimate conditions; however, any deformations associated with these conditions must not interfere with continued safe flight and landing. The pressure relief provided by intercompartment venting may also be considered.

(g) Bulkheads, floors, and partitions in pressurized compartments for occupants must be designed to withstand the conditions specified in paragraph (e) of this section. In addition, reasonable design precautions must be taken to minimize the probability of parts becoming detached and injuring occupants while in their seats.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-54, 45 FR 60172, Sept. 11, 1980; Amdt. 25-71, 55 FR 13477, Apr. 10, 1990; Amdt. 25-72, 55 FR 29776, July 20, 1990; Amdt. 25-87, 61 FR 28695, June 5, 1996]

In the previous posts, we looked at:

The next regulation, 14 CFR Subpart C Section 25-365, provides guidelines on pressurized compartment loads and compliance. We will focus on the paragraphs (e), (f) and (g), the most significant ones for cabin structures.

14 CFR Subpart C Section 25-365

(e) Rapid Decompression for Cabin Structures

First of all, passenger and crew safety is paramount in aviation. Therefore, the primary focus is on "safety during" a rapid decompression event. Secondly, the "safe landing and safe egress" after the event.

In general, pilots or the system brings down or turns off cabin pressurization well before landing. The human body tolerates pressure levels up to 8000 ft ASL (above sea level). Many people actually live at those elevation levels or higher on the planet. Read more about it at this link for a discussion on cabin pressure variation, check this link for sample charts.

Various aircraft OEMs, their vendors, contractors and sub-contractors have their own internal tools and algorithms. They are used to determine rapid decompression pressure differentials on the walls of interior structures.

Here is another forum discussion link on some technical aspects of 14 CFR Subpart C Section 25-365: Rapid Decompression Analysis

As noted in this forum discussion, various "Discharge Coefficients (CDs)" play a major role in the calculation of the pressure differentials across structures in the entire lay out of the cabin.

We are generally at the user end of these pressure differentials, in other words, the data is given to us to use in stress analysis. So the more important numbers for us would be typically the FWD or AFT pressure differential values that need to be applied as separate load cases. Sizing of the interior structure components for this load case is done for an ultimate condition.

An example situation is shown below. We can see three volumes P1 thru P3, a blowout indicated with the red hatch box, and the resulting pressure differentials dP1 and dP2, in this example scenario.

14 CFR Subpart C Section 25-365 Rapid Decompression Pressures
14 CFR Subpart C Section 25-365: Rapid Decompression Pressures, Img. Greenpoint

However varied the tools may be, the results need to be proven to be consistent with the expectations of the FAA, and that serves as validation for using a particular analysis tool to carry out the rapid decompression analysis. This approval process is apparently quite rigorous.

14 CFR Subpart C Section 25-365

(f) Loading Requirements

Another approach to 14 CFR Subpart C Section 25-365 decompression analysis would be to assume that due to an operator error, a door has opened inadvertently in flight. But in practice, this operator error is basically eliminated using fail safe design practices, hence this condition is highly improbable. Therefore, other possible blow out areas may be considered that could be smaller in size.

Also, self weight in the DOWN direction must be applied in combination with any pressure loading. This is done by defining a 1.0 body load factor in the -Z direction.

The rapid decompression load case is an ultimate load case. Meaning, permanent deformation is allowed as long as it does not detrimentally impact adjacent structures or impede safe egress out of the aircraft. Various cutouts and blow out features are included in different interiors structures to balance compliance and ventilation requirements with customer requirements.

14 CFR Subpart C Section 25-365

(g) Design Features and Requirements

Critical cabin structures subject to high decompression loading per 14 CFR Subpart C Section 25-365 (g) are large Structures. For example, partitions or bulkheads, full width galleys, aircraft floor panels and ceiling panels, among others. The design pressures (prior to actual decompression analysis pressures are available) can be in the range of 0.75 to 1.5 psi.

For instance, for a full height full width bulkhead installation weighing roughly 400lb, the 9.0G FWD load would be 3600lb.

With a total surface area of roughly 9000 in^2, and 1.0psi pressure loading, the decompression pressure load would be 9000lb. Clearly this is a very critical load case.

Certain design features are typically included to control the decompression pressure differentials such as:

  • Blow out panel installations in partitions or galleys
  • Floor panel cutouts
  • Blow out hinges on doors
  • Drop down decompression ceiling panel assemblies etc.

Any such elements of the structure are designed in such a way that they do not pose a safety hazard to occupants even after the blow out happens. Examples would be special hinge installations or lanyard installations that keep the blow out parts in place, so as to not get dislodged and injure occupants.

There you have it, if you know of any other important aspects pertaining to 14 CFR Subpart C Section 25-365, make sure to comment and let us know...

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Surya Batchu
Surya Batchu

Surya Batchu is the founder of Stress Ebook LLC. A senior stress engineer specializing in aerospace stress analysis and finite element analysis, Surya has close to two decades of real world aerospace industry experience. He shares his expertise with you on this blog and the website via paid courses, so you can benefit from it and get ahead in your own career.