For transport category airplanes, a 16g seat is one that meets the 9g requirements of Sec. 25.561 and the dynamic requirements of Sec. 25.562. A 16g seat is tested in a manner that simulates the loads that could be expected in an impact-survivable accident. Two separate dynamic tests are conducted to simulate two different accident scenarios: one in which the forces are predominantly in the vertical downward direction and one in which the forces are predominantly in the longitudinal forward direction.The highest load factor is in the forward direction at 16 g's. This is why these seats are commonly referred to as 16g seats. The test procedure requires "crash testing" the seat (i.e., rapidly decelerating the seat in accordance with the criteria in Sec. 25.562(b)). For the 16g test, this means deceleration must go from a minimum of 44ft/sec to 0 ft/sec in not more than 0.09 seconds with a peak deceleration of at least 16g's. The seats are tested with seat floor tracks that are representative of those that will be used in the airplane installation. The seats are also tested with test dummies in each seat position. The reaction of the test dummies during the dynamic test imparts loads into the seat restraints and seat structure more accurately than the cable and winch system used in the 9g seat static pull test. The test dummies are instrumented to measure data like forces and accelerations that are then used for evaluating occupant protection criteria. As an example, accelerometers in the heads of the test dummies measure accelerations that are used in calculating the Head Injury Criteria (HIC). Limitations on an acceptable level of HIC serve to protect the occupant from serious head injury where head contact with seats or other structure can occur. 16g seats also: (1) Protect the occupant from debilitating leg and spine injuries; (2) Improve the attachment to the airframe; (3) Protect crewmembers from serious chest injury when upper torso restraints are used; and (4) Ensure occupants do not become trapped in their seats due to excessive seat deformation. Amendment 121-315, effective October 27, 2005, requires that transport category airplanes in part 121 operations, certificated after January 1, 1958 and manufactured on or after October 27, 2009, must comply with the 16g dynamic standard. Regulations and TSO for 16G seats: In 1988, the emergency landing conditions were revised to include dynamic landing conditions to improve occupant protection. Four years later, TSO-C127 ("Rotorcraft, Transport Airplane, Normal and Utility Airplane Seating Systems") was issued and included guidance on dynamic testing of 16 g's for seats that would be used in transport category airplanes. TSO seat approval is not installation approval. Although TSO C127 is the basis for getting most 16g seats approved for use in transport category airplanes, installation approval is not as easy as it is for a 9g seat. The 16g seat installation approval process is more complicated than the 9g seat installation approval process because the dynamic standard includes several occupant protection criteria not required for the 9g seat. These occupant protection criteria can only be completely evaluated when the seat is considered in relationship to how and where it is installed in the airplane. For example, the dynamic test will cause a test dummy's upper torso and head to swing forward in an arcing motion since the test dummy is constrained only at the pelvis by the safety belt. A record of the motion of the test dummy's head through the arc, called a headpath trace, can be recorded during the testing for the TSO approval. The headpath trace is used during the installation approval process to ensure there is enough clearance from objects, like bulkheads or equipment mounted to partitions, to reduce the possibility of a head strike. Because airplane interior arrangements differ by airplane model-and even from operator to operator for the same airplane model-the headpath trace must be evaluated for each unique installation. This illustrates one reason why installation approval cannot rely solely on the TSO approval. 3. 16g "Compatible" Seats Transport category airplanes designed between 1952 and 1988 were required to have seats that met the 9g emergency landing conditions in Sec. 25.561. These standards were met by the static testing described above in the section entitled "Regulations and TSO for 9g seats". Typically, the seats approved in those airplanes were also approved to TSO-C39. When Amendment 25-64 went into effect in 1988, any transport category airplane design submitted for approval was required to have seats that met both the 9g static standard in Sec. 25.561 and the 16g dynamic standard in Sec. 25.562.However, Amendment 25-64 applied only to new airplane designs like the Boeing B-777. Airframe manufacturers occasionally redesign an existing airplane design to meet marketing demands rather than develop a new design from scratch. These redesigned airplanes are referred to as derivative models, since they are based largely on a previously approved airplane design. An example of this is the Boeing B-737NG models (737-600, -700, -800, -900), which are based on the previously approved B-737 airplane design. The basis for a derivative model design approval is the regulations in place at the time of the original design approval. However, for a variety of reasons, the derivative model design will be approved to regulations more current than those in existence when the original design was approved, but not quite to the level of the regulations current at the time of application for a derivative model design approval. There are numerous derivative transport category airplane models approved after 1988 whose original design was approved before 1988. These airplane models' seats do not meet all the requirements of Sec. 25.562 (16g seats), but meet more than the requirements of Sec. 25.561 (9g seats).The dynamic standard in Sec. 25.562 includes criteria to evaluate the seat's structural integrity and occupant protection during dynamic testing. Most of the derivative models meet the seat structural integrity requirements in Sec. 25.562 but none or only a few of the occupant protection requirements in Sec. 25.562. Seats that have been approved to meet the 9g requirements in Sec. 25.561 and the seat structural integrity requirements in Sec. 25.562 are commonly called 16g "compatible" seats