Interpretation ID: aiam2420
Commissioner
Department of California Highway Patrol
P.O. Box 898
Sacramento
CA 95804;
Dear Commissioner Craig: Thank you for your August 12 and September 28, 1976, letters to ou Office of Standards Enforcement concerning possible noncompliance of certain air- braked school buses with Standard No. 121, *Air Brake Systems*. At issue is the functioning of a 'double check valve' between the two tanks of an air brake system designed to meet S5.7.2 of the standard (as it was in effect prior to September 1, 1976). The valve serves to provide air pressure from either supply reservoir to hold off the spring brakes while guarding the air brake system from air loss through either one of the reservoirs. The identified problem stems from construction or installation of the valve such that it does not necessarily operate to seal off the brake system from a loss of air in one of the tanks, permitting loss of air from both reservoirs unless the rate of air loss is substantial. Your letter advises that the State of California is permitting the continued operation of the school buses in question pending receipt of a response from the National Highway Traffic Safety Administration (NHTSA).; I would like to address the findings you raise in the order in whic they are listed on page two of your letter. I interpret your first finding to be that the 'split system' designs used in compliance with Standard No. 121 may be incapable of meeting the stopping distance requirements set forth in S5.7.2. You question whether S5.7.2 adequately specifies a 'back up' braking capability that is not cancelled out by a single failure of air pressure components, citing cases where a substantial rate of loss from one tank (introduced as a failure in accordance with S5.7.2.3) draws down the air pressure in both halves of a split system.; The general requirement of S5.7.2.3 (of the now superseded text) i that the vehicle, in one out of six attempts under specified conditions, must be capable of stopping from 60 mph within a distance of 613 feet 'with a single failure in the service brake system of a part designed to contain compressed air or brake fluid. . . .' To provide this performance (and meet the other requirements of S5.7.2) manufacturers have provided 'split systems' modulated by the service brake control. As a matter of general compliance with this requirement, you have asked if any single failure (other than failure of common components) can be introduced into the system as a test of its compliance. The answer to your question is yes. The NHTSA does not know the basis of Ford Motor Company's contention that the 'failure' introduced in the system must be at least 50 psi/min from an initial reservoir pressure of 120 psi.; A noncompliance with S5.7.2.3 occurs only if the vehicle, with th failure introduced, is incapable of stopping within the prescribed distance in one out of six attempts. Thus, an extremely small failure which is easily overcome by compressor pressure would be one way of testing for compliance with S5.7.2.3, but it would not, in all likelihood, result in the demonstration of a noncompliance. As you noted in the second paragraph of page 3 of your letter, a noncompliance with stopping distance requirements may depend (largely because of a compressor's ability to overcome air loss) on the rate of leakage introduced and the time allowed between introduction of the failure and application of the emergency brake. The standard does not (except for the provision of S6.1.14 for towing vehicles manufactured on or after September 1, 1976) provide detailed specification of procedures, particularly the time between failure and brake application. The agency therefore must utilize a reasonable procedure that does not unfairly test the system. In this case, the agency considers the introduction of any size leak, followed by brake application as soon as the low pressure warning activates, to be a reasonable procedure, approximating what would occur in the event of actual failure on the highway. The agency would find a noncompliance with S5.7.2.3 if the school buses are incapable of stopping within the required distance when the described procedure is followed.; Your second finding is that a particular double check valv installation can cause noncompliance with S5.7.2.3. Consistent with our general discussion of compliance with S5.7.2.3, stopping tests are the means to discover whether the buses in question comply. We are analyzing your data, and a noncompliance investigation has been opened. We have notified Ford Motor Company of its commencement. We have also forwarded data to our Office of Defects Investigation to see if the check valve problem constitutes a safety-related defect apart from the issue of compliance.; Your last three findings address the separate question of whether th concept of a split system is adequately interfaced with the parking brake requirements of Standard No. 121. First you make the general point that, if S5.7.2.3 permits a substantial loss of air from both sides of a split system, the benefit of redundant lines is negated and the split system requirement is unjustified. The agency understands your position and believes that a stricter control on the amount of permissible leakage (perhaps by means of a limitation on compressor replenishment or a longer period before emergency brake application) may be justified. Particularly important is your point that a compressor loses capability as it grows older, and that this is not accounted for in new-vehicle tests. At the same time, however, larger failures do occur and we continue to view the split system as extremely important for these cases. The split system provides extremely good protection against failures such as rupture of a line or brake chamber diaphragm.; With regard to your point that split systems do not guard agains failure of components common to the two systems as they are presently constructed (such as the compressor drive belt), I must agree with your point that no brake system can guard against every conceivable failure completely. It is for these cases, in fact, that the agency considers the automatic application of the parking brake system beneficial following loss of the other two capabilities.; You make the further point that, in cases of marginal compliance (i.e. the compressor can replace most but not all air loss from both sides of the split system), the gradual loss of system pressure permits parking brake application that interferes with modulation of the emergency capability. While S5.7.2 was designated as '*Modulated emergency braking system*', no specification for modulation was set forth in its requirements. The agency intended that a system conforming to S5.7.2 would be controlled by the driver instead of by automatic application. Our review of the systems you tested indicates that they are 'modulated systems' as contemplated by the agency.; The present emergency brake requirements only state that the system 'b applied and released, and be capable of modulation, by means of the service brake control.' While further specification of this requirement may be in order, I believe it is the early application of the parking brake that actually concerns you, as it affects the application and release of the emergency braking capability.; You make the observation that early application of the spring brakes i response to air pressure loss permits them to drag, become overheated, and fade, making them useless before they can be utilized. A related issue is that the spring brakes will apply shortly after emergency brake availability (even before activation of the low air pressure warning), immobilizing the vehicle with no capability to release the parking brakes. You suggest implementation of a parking brake arrangement that would keep the spring brakes off longer or provide an isolated source of air pressure to permit their release when they do lock up.; Analysis of your test reports leads me to agree that some specificatio to limit the early application of spring brakes would be in order. Of course, as you are well aware, notice to interested persons of any change in the standard is required, along with an opportunity for comment. I believe that such rulemaking could be joined with the earlier rulemaking undertaken in response to a California Highway Patrol (CHP) petition (Docket 75-16, Notice 04).; Your final point is that, as long as early application of the parkin brake is permissible, an isolated tank of air should be available to permit release of the parking brake from the driver's position. With regard to an isolated tank, the NHTSA continues to maintain its view set forth in our November 29, 1974, letter to Donald Gibson of the CHP that the second side of the split system provides more capability than the old systems (with an isolated tank) to avoid a lockup following a service brake failure. However, you have clearly demonstrated that the capability can be essentially negated by early application of the parking brake. I believe that limitations on early application of the parking brake would be a superior correction to the problem than the addition of more components (and complexity) to the existing systems.; Because school buses are involved in the problems you cite, you are n doubt aware that S 103(d) of the National Traffic and Motor Vehicle Safety Act, while preemptive of State regulations or laws of general applicability that are not identical to Federal standards on the same aspect of performance, does not prevent a State or political subdivision from specifying a higher level of brake performance in vehicles 'procured for its own use.' Thus, the State of California may wish to order school buses with the additional isolated reservoir that you have recommended. The addition of a third tank to a system that complies with the standard's requirements would not be prevented by S 103(d).; I appreciate the constructive approach being pursued by the State o California in enforcement of Standard No. 121. Our Office of Standards Enforcement will keep you advised of the results of its investigation.; Sincerely, Robert L. Carter, Associate Administrator, Motor Vehicl Programs;