Thomas Built Buses, Inc.
P.O. Box 2450
1408 Courtesy Rd.
High Point, N.C. 27261
Dear Ms. Dawson:
This responds to your letter asking about the meaning of "high-force" and "low-force access regions" in S18.104.22.168 of Federal Motor Vehicle Safety Standard No. 217 for school bus emergency exit windows, and the reason for the differing force requirements in the two access regions. I regret the delay in responding.
* * * [E]ach school bus emergency exit window shall allow manual release of the exit by a single person, from inside the passenger compartment, using not more than two release mechanisms located in specified low-force or high-force regions (at the option of the manufacturer) with force applications and types of motions that conform to either S22.214.171.124(a) or (b) of this section. * * * * *
- Emergency exit windows--Low-force application.
- Location: Within the low-force access regions shown in Figures 1 and 3 for an emergency exit window.
- Type of motion: Rotary or straight.
- Magnitude: Not more than 89 [N]ewtons.
Emergency exit windows--High-force application.
- Location: Within the high-force access regions shown in Figures 2 and 3 for an emergency exit window.
- Type of motion: Straight and perpendicular to the undisturbed exit surface.
- Magnitude: Not more than 178 [N]ewtons.
The terms "high force" and "low force access regions" have been used in Standard 217 since 1972. S126.96.36.199 limits where release mechanisms for a window exit may be placed, and the force and motion needed to release the exit. The high force and low force access regions depicted in the figures show where release mechanisms may be located, and show, depending on where the release mechanism is located, which force and type of motion requirements apply. The idea underlying the exit release requirements is that if the direction of motion necessary to operate a release mechanism makes operating the mechanism relatively easy, and if the mechanism is within relatively easy reach, the force level necessary to operate the mechanism may be relatively high. Otherwise the force must be relatively low.
You ask: "Does [the reference to the access regions] mean that release mechanisms located in the low-force region are limited to utilizing a rotary or straight motion and that release mechanisms located in the high-force region are limited to straight and perpendicular motion?" You are correct that among the requirements of S188.8.131.52 are that rotary or straight type motions must be used for release mechanisms located in regions of low force application and that a straight motion must be used for release mechanisms located in regions of high force application.
If you have other questions, please call us at (202) 366-2992.
Acting Chief Counsel