Retention of Items of Mass

In this post, we will dig into the next regulation, 14 CFR Part 25.789. It is very broad and requires the retention of various sizes and shapes of items of mass.

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§ 25.789 Retention of items of mass in passenger and crew compartments and galleys.

(a) Means must be provided to prevent each item of mass (that is part of the airplane type design) in a passenger or crew compartment or galley from becoming a hazard by shifting under the appropriate maximum load factors corresponding to the specified flight and ground load conditions, and to the emergency landing conditions of § 25.561(b).

(b) Each interphone restraint system must be designed so that when subjected to the load factors specified in § 25.561(b)(3), the interphone will remain in its stowed position.

[Amdt. 25-32, 37 FR 3969, Feb. 24, 1972, as amended by Amdt. 25-46, 43 FR 50596, Oct. 30, 1978]

In the previous posts, we looked at:

  1. 14 CFR Subpart C Section 25-301: Loads
  2. 14 CFR Subpart C Section 25-303: Factor of Safety
  3. 14 CFR Subpart C Section 25-305: Strength and Deformation
  4. 14 CFR Subpart C Section 25-307: Proof of Structure
  5. 14 CFR Subpart C Section 25-365: Pressurized Compartment Loads
  6. 14 CFR Subpart C Section 25-561: General Emergency Landing Ultimate Loads
  7. 14 CFR Subpart C Section 25-562: Emergency Landing Dynamic Loads
  8. 14 CFR Subpart C Section 25-601: Hazardous Unreliable Design Features
  9. 14 CFR Subpart C Section 25-603: Materials
  10. 14 CFR Subpart C Section 25-605: Fabrication Methods
  11. 14 CFR Subpart C Section 25-607: Fasteners
  12. 14 CFR Subpart C Section 25-609: Structure Protection
  13. 14 CFR Subpart C Section 25-613: Strength of Materials
  14. 14 CFR Subpart C Section 25.619-625: Fitting Factor
  15. 14 CFR Subpart C Section 25.787: Stowage Compartments

(a) Retention of Items of Mass: Types of Mass

Throughout the cabin of a large aircraft, there are various items of mass. These items may be entire built up structures such as:

Or the items inside these larger items of mass such as:

  • Meal carts
  • Ovens
  • Standard units
  • Miscellaneous stowage
  • Emergency equipment
  • Overhead bin luggage
  • Even the passengers/crew and their seats can be considered items of mass

This regulation requires retention of each and every one of these items of mass under both limit and ultimate load conditions in such a way that if they do become loose, they do not become a hazard to the safety and exit of the passengers or crew.

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(a) Retention of Items of Mass: Load Conditions

The retention of these items of mass must be provided under limit (flight) as well as ultimate (emergency landing or crash) load conditions. For more information on these load conditions, click this link: 14 CFR Subpart C Section 25-561: General Emergency Landing Ultimate Loads.

Limit or Flight Load Conditions:

These are generally lower load factors for in flight load cases such as turbulence, gust loads, sudden maneuvers etc. Under such conditions, the yielding of the retention restraint devices is not allowed.

Ultimate or Emergency Crash Load Conditions:

Under these load conditions, the retention restraint devices are not allowed to fail or break loose, they are allowed to yield only as long as the permanent set does not impede the safe egress of the passengers or crew.

(a) Retention of Items of Mass: Restraint Devices

The restraint devices are the main components ensuring retention of the various items of mass.

Large Structures:

When we talk about retention of large structures such as the built up composite structures installed in the cabin and cargo areas of the aircraft attached to the aircraft structure, the restraint devices for these would be the attachment components such as floor fitting assemblies that include various parts of the joint, top attachment assemblies etc. This is at the macro level. Under the load conditions discussed above, the fittings must comply with the yielding and failure requirements mandated by this regulation.

Seats:

As far as seats are concerned, the retention restraint devices are the floor fittings and fasteners that attach the seat structure to the seat track in the aircraft floor.

Passengers and Crew:

Even though this sounds a bit odd, technically they are items of mass. And their retention restraint devices are seat belt assemblies. Since these are considered severe wear and tear items due to frequent engage disengage actions, they are generally certified to a higher 1.33 fitting factor.

Stowage Compartments:

These items of mass are at the micro scale, within larger items of mass such as galleys bins etc. Most common retention restraint devices for such items of mass are doors, door latches and hinges, quarter turns among others. Again, latches and quarter turns are also certified to a higher 1.33 wear and tear fitting factor.

(b) Retention of Items of Mass: Intercom

Last, but not least, part (b) of the regulation text above talks about the intercom. This is obviously very important, although not from a structural stand point but from a functional stand point under emergency landing conditions. The flight crew must be able to communicate during evacuation and it makes complete sense.

 

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So that is all for now folks, what did you think about this post? Comment below the post....

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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 a decade and a half of real world 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.