Mr. Shane K. Lack
Mechanical Engineer
Human Factors Division
National Transportation Safety Board
490 L'Enfant Plaza East, SW
Washington, DC 20594

Dear Mr. Lack:

This responds to your request that we review your draft summaries and interpretations of Federal Motor Vehicle Safety Standards 205 (49 CFR 571.205, Glazing materials) and 217 (49 CFR 571.217, Bus emergency exits and window retention and release). We apologize for the delay in responding.

Because your documents are lengthy (36 pages) and contain so many statements, questions, and interpretations, we are unable to address each individual point in this letter. Instead, we will confirm that, in general, your summaries and interpretations of our standards are correct. We offer the following answers to your questions and corrections to a few of your interpretations, with reference to the page number and line number of your summary. For brevity, we have paraphrased the relevant portions of your letter in italics.

Federal Motor Vehicle Safety Standard No. 205, Glazing materials

Questions on page 1, lines 1 through 9, and lines 22 through 27: Will any material that meets the tests for a specific item of glazing be considered to be that kind of glazing, or do materials specifications (e.g., rigid plastic) restrict the choice of material?

The answer to the first part of your question is yes. The material of which the glazing is constructed is not specified. Both Standard 205 and the American National Standard, Safety Code for Safety Glazing Materials for Glazing Motor Vehicles Operating on Land Highways -- ANSI Z26.1-1977 (hereafter referred to as ANSI Z26.1, which is incorporated by reference in the CFR) may refer to specific kinds of glazing in headings. Examples of this are in paragraph S5.1.2.3 of the standard, which refers to "flexible plastics," and the column headings of Table 1 in ANSI Z26.1, which lists materials such as "laminated glass." These headings are for illustrative purposes only, to indicate the material and construction that typically is used to meet the enumerated tests. See note 1 to Table 1 in ANSI Z26.1, which specifically states that future materials that meet the enumerated tests may be used. Therefore, any material that meets all the tests for a particular item of glazing complies with the standard, regardless of composition or construction.

Question on line 28 through 33: What about paragraph S5.1.2.2, which in the text of the standard itself seems to restrict item 12 glazing to safety plastics?

Again, the mention of safety plastics is illustrative. The words "safety plastics" should not have been included in the standard. However, the performance-based method (i.e., whatever meets the tests) of determining compliance is maintained even for item 12 glazing.

Question on page 1, lines 34 and 35, and page 2, lines 1 and 2: Are the glazing materials approved for the side windows of buses the same regardless of the gross vehicle weight rating and whether the bus is a school bus or a non-school bus?

The answer to both questions is yes.

Question on page 2, line 8: Is the list on lines 9 through 22 showing approved glazing types for side windows of buses correct?

The list of approved glazing types on page 2 is correct.

Question on page 2, line 25: Does each piece of glazing material have to be stamped with "AS" followed by the item no. of the glazing?


Question on page 2, line 26: Does Standard No. 205 allow glazing materials that shatter to be placed in the side windows of buses, as long as those materials shatter into small pieces.


Question on page 2, line 28: Does Standard No. 205 require a measurement of the dynamic force deflection curve of glazing materials?

No, it does not.

Federal Motor Vehicle Safety Standard No. 217, Bus emergency exits and window retention and release

Interpretation at page 1, line 27: To be counted as an emergency exit, an exit must be in compliance with all emergency exit requirements for that type of exit.

We assume when you say "to be counted" you mean whether a particular exit would count for determining if the bus has the correct number of exits specified in Standard No. 217. When conducting a compliance inspection of a new vehicle, the National Highway Traffic Safety Administration (NHTSA) would normally have the manufacturer's certification data showing which exits are designated as emergency exits. If an exit did not comply in some way, this agency would not "uncount" that exit for meeting exit number and exit area requirements. Instead, the vehicle would be considered to have failed only the requirements that were not fulfilled. For example, if an otherwise compliant exit were not labeled, the bus would fail only the S5.1.1 labeling requirement. The area for that exit would still be counted for meeting the emergency exit number and area requirements.

Interpretation on page 1, line 31: The minimum size of an emergency exit window in a non-school bus is the area of an ellipsoid having a major axis parallel to the ground of 50 cm and a minor axis of 33 cm. This minimum total area of an emergency exit on a non-school bus is 510.5 square centimeters.

The first sentence of your interpretation is correct. Your computation in the second sentence is incorrect. The minimum emergency exit opening computes to 1296 sq. cm for a 33cm X 50cm ellipsoid. The area of an ellipse = 3.14 times the product of the major and minor semi-axes.

Interpretation on page 2, line 24: For non-school buses with a GVWR > 10,000 lb, there is no prohibition against placing side emergency exits vertically above one another.

While it is true that there is no prohibition explicitly stated in the standard, NHTSA interprets its regulations consistently with their purposes. The purpose behind the emergency exit requirements, which is clearly reflected paragraph S2 and in its preambles on the subject, is to provide readily accessible emergency egress. It is highly unlikely that a manufacturer would actually place emergency exits in such an unusual configuration. If it did, NHTSA would not regard the emergency exit placed on top of another as being accessible, and would not count it toward meeting the requirements.

Interpretation on page 2, lines 29 through 30: There are no guidelines on lateral or longitudinal placement of roof emergency hatches.

Paragraph S5.2.1 allows manufacturers of non-school buses to meet the specifications for non-school buses in S5.2.2 or the specifications for school buses in S5.2.3. If S5.2.2 is followed and a roof exit is needed because a rear exit cannot be provided, then paragraph S5.2.2.2 specifies that the roof exit is to be located in the rear half of the bus. If S5.2.3 is followed, then S5.3.2.2(b) specifies both the longitudinal and lateral position of all roof exits.

Interpretation on page 4, line 1: An emergency release mechanism is defined in previous NHTSA interpretations as a mechanism that prevents an exit from opening.

We do not know to which interpretation you are referring. A computerized search of all our previous interpretation letters did not reveal an interpretation with those words. It is true, however, that the provisions specifying that the release mechanisms can only be actuated by applications of a certain magnitude of force in certain directions are designed to prevent inadvertent opening of an exit.

I hope this is helpful. Again, my sincere apologies for the delay in our reply.  If you have any further questions, please feel free to contact us at (202) 366-2992.

Frank Seales, Jr.
Chief Counsel
ref:205# 217