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FAR 25.561 Applicability Interpretation Help

FAR 25.561 Applicability Interpretation Help

FAR 25.561 Applicability Interpretation Help

I had a quick question about FAR 25.561. My understanding is that 25.561 applies to mainly cabin interiors and any structure which has the risk of explosion (Auxiliary Fuel Tanks if not primary ones). Primary structures in the fuselage & wing etc. are not subjected to the above regulation? Is my understanding correct? What about engine pylon & related mounting structures? I am assuming they need to be complied with emergency inertial loading conditions as well.

Here is what it says from FAA website:

" §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"

RE: FAR 25.561 Applicability Interpretation Help

I've understood 561 as items of mass in the cabin need to be restrained to prevent injury to passengers in the event of a crash.

I'd've thought fuel tanks were covered by their own requirements (ie not to release fuel in event of a crash).

Applying these loads to external items would be typical practice, but Required maybe not.

but it'll be an interesting discussion !

another day in paradise, or is paradise one day closer ?

RE: FAR 25.561 Applicability Interpretation Help

Hi RB1957,
I think you are right. So far, I've seen 25.561 being applied to mainly cabin interior monuments and one of our product is Auxiliary Fuel Systems and we used to apply 561 for them as well.

But I would assume for engine pylon mounting structures and fitting, perhaps 561 could be applicable as well. In the event of an emergency landing, to show that engine does not dislodge and hurl towards fuselage.

Would love to hear opinions of other experienced engineers about applicability of 25.561 on primary structures.

RE: FAR 25.561 Applicability Interpretation Help

See if you can quickly go back and EDIT your posting if you meant to write "561" instead of "571". Those are two different animals!


RE: FAR 25.561 Applicability Interpretation Help

AFAIK, FAR 25.561 is not intended to cover engine mounts. Look into requirements specific to engines starting at FAR 25.901.

The key condition when deciding that 561 is applicable is this phrase: "...designed to give each occupant every reasonable chance of escaping serious injury in a minor crash..."

If you are considering an item of mass, but it cannot threaten the occupants of the aircraft, then 561 does not apply. Equipment aft of the rear pressure bulkhead comes to mind. Generally, every object weighing more than a couple of pounds mounted in the cabin must be show to be secured against all of these load conditions.

FYI, 9g forward and 6g downward was a "first pass" estimate of the tolerance of the typical human body to abrupt accelerations worked out in the 1950's or so, and codified by this regulation ever since. Later (1970's and 1980's) it became apparent that people were frequently surviving these kinds of crashes, except that their seats failed so they died anyway. Accident investigators are very thorough and they don't just look for causes of accidents, they also examine effects. As a result of these findings, a body of evidence was built up showing that a different design of seats would allow many more people to survive those marginally survivable accidents. This led to the development of the "dynamic seat" which has built-in energy absorption features, in addition to the typical 9g forward/6g downward strength requirements.


RE: FAR 25.561 Applicability Interpretation Help

check out the engine load conditions 25.36x. After that I'd design the engine structure for the inertial loads from flight (and landing) cases. It makes sense that there'd be some requirement to prevent the engine from detaching and becoming a missile in the event of a crash, as much as there's a requirement to keep fuel tanks intact, but i couldn't see it on a quick scan.

How does 9g fwd compare to the thrust generated by the engine ? I'd've thought that engine thrust exceeded 9g fwd by a margin.

another day in paradise, or is paradise one day closer ?

RE: FAR 25.561 Applicability Interpretation Help

There's a recent "publication" from TAD, "Lists of Part 25/26 Rules and Their Associated Advisory Circulars and Policy Statements" that lists each regulation:


Best part is, there are hyperlinks to the various AC's and Policy Memos. Knock yourself out.

One document that has some clues, Advisory Circular AC 25-21 "Certification of Transport Airplane Structure", was quite surprisingly cancelled two weeks ago, "because it's 17 years old". Sheesh. As if us Certification folks don't already have our favorite old/new regulatory references and Amendment levels memorized.

AC 25-21 frequently "punts" and says "see the Preamble" to the rule. That is a good tool, either the hyperlinks in RGL or directly from the Federal Register. Another good tool is the CAM's (Civil Aeronautics Manuals, also available in RGL). Goes to show you 50 year old information still has value.

RE: FAR 25.561 Applicability Interpretation Help

DER8110, thanks for that link. Would be extremely useful I reckon.

SparWeb, thanks for bring the errors to my attention. Have corrected them. Also thanks for penning down your opinion.

RE: FAR 25.561 Applicability Interpretation Help

re-reading 561, it looks like c(2) could address engines and APUs, although not really clear ...

another day in paradise, or is paradise one day closer ?

RE: FAR 25.561 Applicability Interpretation Help

Hi VN1981,

I reckon to some extend you correct in your understanding that FAR 25.561 mainly focuses on monuments and interiors which could go loose at the time of crash landing. For primary structures and PSE like fuselage and wing (for that matter even cross beams and stanchions) there is another regulation (FAR 25.801, ditching) which should take care of it.

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