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 21:57.