Interpretation ID: aiam2247
Engineering Manager
Motor Coach Industries
Inc.
Pembina
ND 58271;
Dear Mr. Hunt: This responds to Motor Coach Industries' February 20, 1976, lette asking whether Standard No. 121, *Air Brake Systems*, requires the installation of parking brakes on all non-steerable axles of a bus, including a lightly-loaded axle, in satisfaction of the emergency braking provisions of S5.7.1. Once parking brakes are applied on the non-driving, lightly-loaded axle on some Motor Coach Industries (MCI) buses in cold, wet weather, the linings can freeze to the drums and 'lock' the wheels so that they will not turn even after the parking brake is released. You suggest that parking brakes are inappropriate on a lightly-loaded axle, citing an interpretation of the standard that stated parking brakes are not required on an air-lift axle which lifts off the ground when the vehicle is parked.; MCI raised the same question of parking brake requirements fo lightly-loaded axles in an April 17, 1972, letter requesting interpretation of the provision of S5.7.1 that requires automatic application of parking brakes. At the time, the NHTSA had just issued S5.7.1, expanding the methods for meeting performance levels for emergency braking performance. The question of whether parking brakes should be required on all axles under S5.7.1 was left open pending formal rulemaking.; What was not raised in MCI's April 1972 letter was whether both method for meeting S5.7.1 performance levels necessitate parking brakes on all non-steerable axles. Section S5.7.1.2 permits reliance on retardation force capabilities of each non-steerable axle or, in the alternative, reliance on vehicle stopping capability using the vehicle's available parking brakes. In the second case, the NHTSA does not interpret S5.7.1 to require installation of parking brakes on an axle if it is not necessary to meet the stopping performance of S5.7.2.3 specified under S5.7.1.2. To the degree the language of S5.7.1 does not specifically address this method of satisfying the requirement, we regret that the agency's July 1972 response was not more clear.; An interpretative amendment of S5.7.1 would be appropriate in view o the difficulties that its misinterpretation has caused. However, in view of the short time remaining before the automatic application option will no longer be available, the NHTSA does not expect to undertake rulemaking to formalize this interpretation.; Yours truly, Frank Berndt, Acting Chief Counsel