Wouldn't a gear up be more vertical impulse force than forward? Those may or may not handle 16g of deceleration
16g for a 250 pound passenger would be two tons of forward force. No way would those seats withstand such a force.
The g-loading regulations are based on crash tests, circa 1950. This led to ratings of 9 G forward, 2 G upward, 4.5 G downward, and 1.5 G sideward. The 9 G forward rating was later upgraded to 16 G for passenger seats, but not cargo.
Some things about the ratings and the crash tests aren't very clear, like whether the crashes were attempting to simulate a gear-up landing or something more violent. It is pretty clear that the aircraft speeds were about what you would expect for a landing, though.
Here's some info:
16G seats in airliners:
http://aviationglossary.com/airline-definition/16g-seats/
The history of the ratings (with some interesting comments about how nobody knows exactly where some of them came from, and whether they are all that reliable):
http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA055343
excerpts from the latter:
The Civil Air Regulations, Part 4a, "Air Plane Airworthiness", 7 April 1950,
required seats be installed to withstand 6Gs and the only reference to cargo
stated "suitable means shall be provided to prevent the contents of mail and
baggage compartments from shifting." Therefore no real criteria existed. By
1953 this document had undergone a major change. The seat criteria was chang-
ed to:
Upward 2.OG Downward 4.5G
Forward 9.OG Sideward 1.5G
... We now enter a new phase, that of crash testing aircraft to determine
the physics of a crash. It appears that the first crash tests related to
crash dynamics occurred in 1953. ...
The next effort is a study entitled "Seat Design for Crash Worthiness".
This report provides an in-depth review of aircraft seat design relative to
human survival and aircraft crashes. An interesting part of this report is
a discussion of longitudinal deceleration of transport (C-46) and cargo (C-82)
aircraft. This data most probably was developed as part of the fire tests con-
ducted in 1949 as discussed in this section as no other crash tests had been
conducted since those tests....
The aircraft seats are the structural links between the passengers and
crew anJ fuselage floor. If a person were fastened rigidly to the seat and
the seat rigidly to the floor, then the person, seat, floor and aircraft
would move as a unit. However, this is not the case. The person is in
reality a free moving body within the restraint system of the seat. Further,
the seat is made of flexible members with some movement above the floor. The
actual condition of the events are best explained by the model developed for
the study which is quoted in Appendix C. Basically the human would undergo a
deceleration after the aircraft deceleration when he impacts his restraint.
"The peak passenger deceleration can be nearly 1.8 times the peak airplane
deceleration". The more rigidly the person is fastened into the seat the low-
er this ratio. The reason for this ratio is dynamic overshoot of the person
against the seat restraint resulting in an increasing delta velocity as the
aircraft stops and the person continues forward until he impacts his restraint.
This raises the question of relationship between fixed equipment, cargo and
the seat in the aircraft. This could be the reason behind a 9G cargo system
and 16G seat system (9 X 1.8 = '16.2)
