14 CFR Subpart C Section 25 561 – Ultimate Loads

Emergency Landing Ultimate Loads:

In this post we will try to dig into the next regulation, 14 CFR Subpart C Section 2-561, and try to understand what it means.

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§ 25.561 - General

(a) The airplane, although it may be damaged in emergency landing conditions on land or water, must be designed as prescribed in this section to protect each occupant under those conditions.
(b) The structure must be designed to give each occupant every reasonable chance of escaping serious injury in a minor crash landing when—
(1) Proper use is made of seats, belts, and all other safety design provisions;
(2) The wheels are retracted (where applicable); and
(3) The occupant experiences the following ultimate inertia forces acting separately relative to the surrounding structure:
(i) Upward, 3.0g
(ii) Forward, 9.0g
(iii) Sideward, 3.0g on the airframe; and 4.0g on the seats and their attachments.
(iv) Downward, 6.0g
(v) Rearward, 1.5g
(c) For equipment, cargo in the passenger compartments and any other large masses, the following apply:
(1) Except as provided in paragraph (c)(2) of this section, these items must be positioned so that if they break loose they will be unlikely to:
(i) Cause direct injury to occupants;
(ii) Penetrate fuel tanks or lines or cause fire or explosion hazard by damage to adjacent systems; or
(iii) Nullify any of the escape facilities provided for use after an emergency landing.
(2) When such positioning is not practical (e.g. fuselage mounted engines or auxiliary power units) each such item of mass shall be restrained under all loads up to those specified in paragraph (b)(3) of this section. The local attachments for these items should be designed to withstand 1.33 times the specified loads if these items are subject to severe wear and tear through frequent removal (e.g. quick change interior items).
(d) Seats and items of mass (and their supporting structure) must not deform under any loads up to those specified in paragraph (b)(3) of this section in any manner that would impede subsequent rapid evacuation of occupants.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR 5673, Apr. 8, 1970; Amdt. 25-64, 53 FR 17646, May 17, 1988; Amdt. 25-91, 62 FR 40706, July 29, 1997]

In the previous posts, we looked at:

14 CFR Subpart C Section 25-561 provides guidelines on:

  • Emergency landing ultimate loads and load factors
  • Certification requirements for safe egress of the passengers and crew (occupants)
  • Positioning
  • Wear and tear requirements

14 CFR Subpart C Section 25 561

(b) Minimum Ultimate Loads Requirements for Occupant Safe Egress

Note the language used in parts (a) and (b) of this regulation. The interpretation of that language is important here in terms of minimum ultimate loads.

If an airplane crashes head on into a concrete structure or a mountain or maybe it free falls onto land or water, it would be catastrophic. The loads will be well over any ultimate loads the aircraft would ever see under normal operating conditions in its entire life span.

There is very little anyone can do with current design practices and standards to protect the occupants in such a crash.

Although certain radical design features may help mitigate a severe crash to some extent, it would either be cost prohibitive or too heavy. Also, it would most likely be unsuitable for mass transport on an economical scale.

Therefore, the FAA mandates certain minimum ultimate loads or ultimate load factors that the aircraft and cabin interiors must be designed to withstand, based on past aviation history. These ultimate loads or load factors are limited to minor crash landings that may be survivable, provided the conditions in b(1)(2) are met.

As the regulation says, these minimum ultimate loads are 9.0G FWD, 3.0G UP, 6.0G DOWN, 3.0G SIDE (LHS and RHS), and 1.5G AFT.

But what exactly is a minor crash landing? How do we make sense of these ultimate loads? The ultimate loads presented in b(3) above are relative load factors.

But what the heck is relative? To understand that, see the illustration below.

14 CFR Subpart C Section 25 561 Ultimate Loads
14 CFR Subpart C Section 25 561: Ultimate Loads, 9.0G FWD Inertia Load Factor

Looking at the image above, let us say the pilots are able to land this aircraft with wheels up in an emergency situation. They land it on its belly and on a stretch of flat land (can be a body of water as well). As the plane slides along, the ground (or water) underneath exerts a drag force due to friction.

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But the plane has a lot of inertia (weight moving FWD at high speed), it wants to keep moving forward (FWD). The relative acceleration FWD is what the FAA mandates as the 9.0G FWD ultimate loads condition. This is a minimum ultimate loads condition (in this case the FWD direction) that the aircraft and its cabin structures must be designed to withstand for at least 3.0 seconds. During this process, the equations of static equilibrium at an instant produce these relative accelerations or body loads with respect to the surrounding structure.

It is very much like you slamming on the breaks of your car while driving at 70 miles per hour (similar to the drag force on the plane). You plunge forward with your seat belt on. In the beginning, your chest will feel an immense force as you push FWD on it. But as you slow down and spring back occurs, the G force you experience on your chest decreases.

The same thing happens to all the attachments of the interior structures and the entire primary structure load path all the way to the ground. The drag on the belly balances the FWD inertia forces. At some point the drag is too high and the plane has to come to a stop, assuming things are all still intact.

Until the plane comes to a stop, the aircraft and its cabin structures must be designed to not break free from their supports due to a peak 9.0G FWD ultimate loads condition. This is considered a minimum to give the occupants a fair chance of escape.

The same applies to all other ultimate loads directions, applied independently from each other. The reason for independent loading is to ensure sizing for maximum ultimate loads in each specific direction.

OEM or Integrator Specific Loading:

In addition to the minimum FAA mandated ultimate loads mentioned above, there may be additional load cases required by either the completion center, the aircraft integrator, or the OEM (Original Equipment Manufacturer, example Boeing).

Boeing requires higher load factors than those mandated by this regulation in some cases. It also requires various combination load cases (also analyzed as ultimate loads) that combine different ultimate loads in different directions and magnitudes based on the aircraft type.

For example, if you are sizing a structure to be installed in a Boeing 737-600, Boeing requires a 3.5G UP and 6.5G DOWN load case (higher than the FAA mandated cases). An additional 3.0G SIDE (LHS and RHS) + 1.5G DOWN combination load case is also required.

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Flight Ultimate Loads:

There may be interior items that aren't required to be substantiated to ultimate loads as they are only used inflight.

Examples are flight attendant jump seats or lower lobe crew rests. For such items, a different set of 'Flight' load factors may be used, but these are also analyzed as ultimate loads.

It is important to note that some of these flight load cases, especially for structures at the FWD and AFT ends of the aircraft, could have higher ultimate loads than the FAA mandated ultimate loads.

The reason for this is simple, inflight turbulence will cause the FWD and Aft ends of the aircraft to experience greater G loads due to the longer moment arms from the CG of the aircraft. So these cases could very well end up being critical sizing cases for some components.

After all, the aircraft is a simply supported beam, with the total lift at the wings of this beam, the overall weight acting down at the forward side of this beam, which is balanced by the down force on the horizontal stabilizers at the aft side of this beam. This type of a spread is important to obtain a good tradeoff between efficiency and stability.

Ultimate Loads Simply Supported Beam
Ultimate Loads: Simply Supported Beam, Forces (Video)

The gross take off weight (GTOW) distribution of an aircraft is roughly 50% Wings, Landing Gear and Fuel, and the rest is spread out approximately evenly along the length of the aircraft. However, the overall CG is generally FWD of the center of pressure under the wings as shown in the image and video link above.

So we can see how an aircraft can be treated as a simply supported beam and how the inflight turbulence static equivalent load cases would tend to be more severe at the FWD and Aft ends of this simply supported beam.

14 CFR Subpart C Section 25 561

(c) Positioning of Items of Mass

This section sort of ties into another regulation on retention of items of mass. But we will discuss that later.

The bottom line for interior items is that almost all loose contents of the interior equipment or cabin structures are restrained using various restraint devices such as latches and hinges on doors, quarter turn retainers, the walls of the structures made from sandwich panels etc.

Generally, there are no items allowed to be completely unrestrained in any direction that could fly out into the cabin and either insure the occupants or impede their safe egress after an emergency landing condition. Thus all these restraint devices are sized for ultimate loads.

Restraint devices are considered quick change or frequent wear and tear items. Therefore, a 1.33 safety factor is required on quarter turn retainers for example, multiplied with maximum ultimate loads.

As far as positioning is concerned, an example may be positioning of these restraint devices in such a way that they are not forward facing, or designing the structure to restrain the contents using the panel walls of the structure, installing inserts along a less critical load path etc.

14 CFR Subpart C Section 25 561

(d) Permanent Deformation

Seat structures, and any other interior items are allowed to deform under ultimate loads, however, not so much that the safe egress of the occupants is impeded. Complying with this requirement is a little subjective in nature and probably depends on the acceptance of the DER or ODA Unit Member approving the test.

Note that it is not quite as straight forward to satisfy, if certification is done by analysis instead of static test. But we know that ultimate loads are 1.5 times limit loads. In other words, limit loads are 2/3 rds the ultimate loads. For most Aluminum alloys, the material's yield strength is also close to 2/3rds of the ultimate strength as well.

Therefore, if we are able to show a decent positive safety margin under ultimate loads, then it is also reasonable to demonstrate no permanent deformation under limit loads and acceptable permanent deformation under ultimate loads. With composite materials, there is no appreciable yielding before failure and therefore a positive safety margin is sufficient to comply with this part of the regulation.

There you have it, if you know of any other important aspects related to Ultimate Loads and this regulation, share it, make sure you comment below 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 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.