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Old 17th Dec 2002, 17:04   #1 (permalink)
 
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Seat occupancy limits

I've been banging my head up against seat occupancy limits over the last few days, and I'd be very interested in anybody else's experience of the subject.

In most of the codes, seat occupancy limits are in three places. Para 25 for W&CG purposes, para 562 for test purposes, and para 785 for design purposes. When comparing the various codes, I get the following:-

Parts 25, 27, 29, 31
77 kg / 170 lb for all purposes.

Part 23
(W&CG purposes) 77kg (170 lb) for norm/comm, 86kg (190lb) for util/aero
(test purposes) 77kg (170 lb)
(design purposes) 77kg (170lb) for utility, 98kg (225lb) for all other cats.

Part VLA
86kg / 170 lb for all purposes.

Part 22
110kg / 242lb for all purposes.

The reason for one inconsistency is clear, which is part 22 - it's a glider code and the 110kg allows for a parachute. That doesn't worry me at-least.

What does worry me is...

- Why is part 23 internally consistent?
- Has it not occurred to anybody that very few adults these days weigh under 77kg?
- Why is is appropriate for a VLA non-aerobatic aeroplane to use 86kg, yet a 23 non-aerobatic 98kg? They fly the same role, both in the public transport (OK, training) role.
- Is there a good reason why training helicopters and aeroplanes use different crash limits?
- Why is there no form of disclaimer or notification used anywhere that seats - either crew or pax - are designed for a specific weight which is certainly only in the mid-high percentile range?

One can find more and more questions the more one looks into this.

Does anybody have any thoughts or experiences that might help make any kind of sense out of this?

G
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Old 17th Dec 2002, 19:28   #2 (permalink)
 
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Make sense out of regulations??
Lie down for a while, and it will pass.
Seriously, though the typical weight thing is often surpassed by operations manuals and common sense.
One helicopter operator in the Gulf of Mexico has weigh scales at the loading site to weigh each passenger and baggage.
Real weights should always be used if you can possibly do it.
Fixed and Rotary wing
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Old 17th Dec 2002, 20:03   #3 (permalink)
 
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Sure, but if the minimum standard was applied by the manufacturer, and they only used 77kg in the design of the harness attachments (for example) no amount of common-sense in the ops manual is going to help when a 120kg pilot bends an aeroplane.

G
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Old 17th Dec 2002, 22:06   #4 (permalink)
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The matter comes down to putting a line in the design sand pit.

170 lb hails from a US military demographic study dating back to the days of Pontius the Pilate (I have some relevant files in the archives but wouldn't like to try and track them down). If my recollection is correct, 170 lb represented the 50th percentile military chappie from the sample population in question. Like so many of the early rules, the regulatory guys had to pick somewhere to start ..... So, for instance, the 70mph max stall speed started out as a finger in the wind good idea without any rational background ... the 50ft screen dates back to a very early US military demo by (I think) the Curtis Jenny (?) wherein the aircraft was demonstrated into a parade ground somewhere which was surrounded by a tree line of 50ft nominal height .. again as good a place to start with rule making as any ...

77 kg is, of course, only a poorly metricated backwoods cousin ... I still have a good feel for real units ... and have to convert mentally to have any feel for strange units .... everybody is comfortable with slugs and poundals are they not ?

The CAB then adopted the figure for seat design although, without delving into some research activity, it is possible that they just pinched it from another design standard or wherever which had adopted it first .... I must dig out my old seat design files and refresh my memory from another engineering life....

Util/acro increment allows for the 170lb chappie to be inserted into a parachute.

Talking US rules, the seats are designed to the relevant static crashloads or Type design loads whichever is the higher in each direction .... often the longer aircraft have higher vertical loads due to pitching considerations, for instance. For more recent designs, the seat also has to pass the crash sled tests to check out the situation regarding seat flexibility and energy attentuating qualities (spinal loads, attach pop out etc) and surrounding headstrike hazards (HIC assessments).

Many people seem unaware of this latter consideration and that

(a) a current seat is certificated for a particular installation (headstrike considerations mainly), and

(b) one has to be very careful with any mods either structural or trim/upholstery due to the potential for adverse dynamic changes re the sled test environment.

I wouldn't worry too much about the real population variation in mass from the point of view of the seat design as the inertial load factors to be applied to the design provide a pretty reasonable restraint capability overall. Mind you, I clearly recall a flight over the trench to NZ a year or two ago ... seated with two amply fleshed Islanders of pleasant disposition .... I don't know that I would have liked to put the crashloads to the test ... visions of the three of us spinning down the cabin to say hi to the flight deck crew come to mind ...

Generally my greater concerns have always been the two very real problems of

(a) poorly engineered seat to track anchorage .. and, having done a squillion static pullout tests and been involved with a few dynamic tests, the standard sort of button is not a good design unless each button has an integral or adjacent shear peg fitting to minimise misalignment with the track under load .. the ease with which buttons otherwise can jump out of the track under load is quite frightening.

(b) head strike .. especially with lap belt installations... many is the safety audit where I have drawn this to the client's attention with the proviso that the design and operational standards permit such installations.... sometimes the day is won on corporate risk management grounds ....

I have little background in gliders and VLA so I can't offer any comment there other than to suggest that the glider case is probably a very critical consideration for crashworthiness due to the absence of energy attentuating structure in the keel ... it is this consideration which led to the variation in dynamic deceleration factors for the different classes of machine .. and, in the case of rotorcraft, the higher typical impact angle leading to the same sort of injury magnitude consideration.

The weight control considerations certainly present a problem with small aircraft if one uses the good old 170lb standard weight. I don't know the situation elsewhere but Australia discarded standard loading weights some years ago and went for a range of options including a variable standard load dependent on the number of people in the beast .. to account for a reasonable statistical level of confidence in minimising the potential use of understated loads. Interestingly, on the occasions where I have had the experience of having to weigh the passengers in operations on F27 in the 70s in Australia (fuel critical etc) 170lb (at that stage) still worked out to be pretty good as an overall average with a reasonable number of pax .... I guess with the increasing consumption of that dreadful plastic takeaway food and the increasing level of obesity (takes a bow and exits stage left) the rational statistical assessment process makes much more sense. The Australian regulator published a very detailed report of the earlier study done by John K (in the 80s as I recall) and it is probably still available through CASA. I have a very early copy in the files although I don't know if the final version contained anything in the way of material changes.

Last edited by john_tullamarine; 17th Dec 2002 at 22:57.
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Old 22nd Dec 2002, 02:38   #5 (permalink)
 
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Requirements now are for 16G for pax seats in corporate aircraft...seems a bit excessive to me.
Comments please, and especially as we are considering seats for a Lockheed JetStar...and it is NOT designed for a moon landing.
Good grief, where has common sense gone?!?
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Old 22nd Dec 2002, 10:24   #6 (permalink)
 
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Is that 16g longitudinal or normal, and at what occupant mass?

G
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Old 22nd Dec 2002, 16:54   #7 (permalink)
 
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My understanding is 16G longitudinal with a 200 pound pax.
At least this was the information supplied.
These seats are heavy....and expensive. Of course the seat tracks require beefing up/redesign as well....seems a bit OTT to me.
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Old 22nd Dec 2002, 17:06   #8 (permalink)
 
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As presented, it does seem a bit OTT, as you say. Most codes call for a 9g forward case, with the more enlightened taking a 95th percentile adult male as 215 lbs.

That said, 16g would be a survivable load if retained in the seat, and 200lb is around 90th percentile; whilst a seat that failed at, say 9g, would potentially not be survivable in some circumstances.

Do you know where this 200lb/16g case is recorded, because it doesn't sound like any of the published codes - is it perhaps a company spec?

G
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Old 22nd Dec 2002, 20:01   #9 (permalink)
 
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Have been told that it is now an FAA requirement for "new design" seats. We plan to get around this by re-upholstering the older seats. Don't know if this is for corporate only, or for airline as well, but suspect both. Read about this in Aviation Week about six months ago.
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Old 22nd Dec 2002, 20:39   #10 (permalink)
 
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This doesn't make sense to me. Looking up FAR-23 at http://www.access.gpo.gov/nara/cfr/c...4cfr23_00.html which claims to be up to date for this month.

para 785 gives a 215lb mass (or 170lb in utility), para 561 gives a 9g forward load for seats and their occupants.

The only thing I can think is a player is para 562 which has a whole stack of requirements for maximum shock loads in various collisions using a 170lb dummy - but these are rather more to do with energy absorption than static retention, which is what 561 and 785 are about.

G
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Old 22nd Dec 2002, 22:35   #11 (permalink)
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Times have changed, chaps.

The old static load requirements (6G if you go back to Pontius, and 9G in more recent times - but do keep in mind that these static loads also involve a 33 percent overload capability for the attachments - some seat manufacturers just went straight to 12G seats for simplicity in testing) have been added to and somewhat superseded by dynamic test requirements based on motor vehicle protocols.

It is not a case of the loads having magically increased ... quite different in that the static load is basically a 3 second application without failure while the dynamic load is a very short lived triangular collision impact pulse based on motor vehicle test standards. There is a set of acceleration loads ... best to read the regs to pick up the subtleties.

Considering this, as you would expect, the older standard seats don't fare too badly when looked at in respect of the basic deceleration test requirements (FAA CAMI did some fullscale fuselage tests on older seats which supported this contention) but the static standards are quite deficient in some survivability aspects -

(a) seat/track retention.

The typical round button attachment fitted into the usual MS profile track is especially suspect with button assemblies not incorporating an integral or adjacent shear peg .. very, very easily do they pop out (and away the seat flies) due to minor deformations under tension loads. More importantly, if there is any floor disruption/distortion, then forget it with a static seat .... this is one of the major improvements incorporated in the dynamic standards.

(b) occupant acceleration.

One of the problems with older seats is that they are too rigid with the result that accelerations and peak loads sustained by the occupant are too high (in many a typical accident scenario) for a reasonable chance of low injury or survivability. The dynamic standards impose requirements for maximum spinal compression load (around 1500lb) which may require some crushable volume or restrained mechanical articulation to keep the accelerations down.. in the case of the much higher helo decelerations, this is a must have and, typically, one sees 5-6 inches of stroking action to bring the peak loads down to a manageable level. In addition, some of the standards impose maximum belt loads.

(c) head strike requirements.

A very significant deficiency in the older standards was that flailing impact injuries were largely ignored and head strike was "covered" by using seat back breakforward articulation and delethalisation padding. Problem is, though, that padding foams exhibit impact rate dependent force-deflection characteristics ... which is why alloy foil padding structures are used in some applications as they don't have this problem. The HIC requirement now imposes a maximum head impact level intended to keep brain damage down to a more controlled level relating to demonstrated survivability in road accident statistics. It is VERY illustrative to watch a sled test in slow motion and just see the extent of arm/leg flailing articulation and, more importantly, the extent of head/torso articulation ... the upper body straps aren't there to stop the head impact .. they just reduce the impact velocity .. which is why we have explosive bag supplementary impact systems in motor vehicles.. which bring their own impact injury and hazardous materials risks to the occupant ... I guess you pays your money and takes your chances.

(d) no longer a seat standard, but a system standard

Previously the seat was addressed as a TSO exercise in isolation from the aircraft. Now the head strike and spinal load requirements, in particular, require that the seat be certificated in respect of a particular airframe installation.

As a result, you may not change upholstery padding (the flammability requirements also present a problem here) or install a seat in an airframe without consideration of the current standards .. these considerations only apply to aircraft for which the TC incorporates the new rules .. for older aircraft, the older standards apply in general.

Nor can you reposition the seat or alter the surrounding airframe structure etc without some serious head scratching.

All in all ... a very, very different ballgame now ....


The present standards are -

Small Aircraft

Go to FAR 23 and hyperlink to 23.561 for static load and 23.562 for dynamic load requirements respectively. General seating requirements are at 23.785.

(The situation is a little complicated in the case of some seats developed during the period that the revised FAR was introduced as there was an interim document used during that period until it became clear where the FAA rule was headed).

Large Aircraft

Go to FAR 25 and, likewise, hyperlink to 25.561, 25.562 and 25.785.

Helos

Similarly for ...
FAR 27 - Normal Cat and FAR 29 - Transport Cat.

Last edited by john_tullamarine; 22nd Dec 2002 at 23:13.
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Old 23rd Dec 2002, 01:12   #12 (permalink)
 
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Certainly will agree that the older Brownline tracks were poorly designed, and often resulted in seat separation.
Recall years ago in a DC-6 sitting down in a double and having the whole assembly wobble sideways. In the early 707's, the economy seats were also poorly designed, and tended to separate from the tracks, especially if three hefty folks moved the same way...at the same time, sideways. Perhaps it was the case of...if the tracks/mountings are redesigned, the inherent seat frame shortcomings became apparent.
Have noticed that in corporate jets, the seats do seem to be more firmly attached, especially in the JetStar, an old design, to be sure.
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Old 23rd Dec 2002, 03:58   #13 (permalink)
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The problem with the MS profile, of which the example you quote is typical, relates to

(a) asymmetric loading, causing failure of the button or the track lip. An Australian design engineer, Rudi Paspa, many years ago patented an interesting and quite effective way of overcoming this problem by a redesign of the attach button but it never took off for whatever reason. The present dynamic standards, requiring a test simulating floor disruption, have generated a standard method of overcoming this problem to some extent, by incorporating a flexible link in the lower leg to permit the button some degree of rotation so that the button flanges continue to load both track lip faces.

(b) minor deflection of the seat structure, permitting the button to move into the cutout space and leave the track. This can be overcome by having individual shear pegs (to prevent movement by locking the leg into the track) or by having a reasonably rigid seat keel to lock a button to a remote shear peg.
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