To inform flight crews of operational effects of PW4000 engines equipped with Ring-Case Compressor (RCC) section.
Boeing and Pratt & Whitney are introducing the Ring-Case Compressor (RCC) modification to correct high power surge problems experienced with the PW4000 series engine. High power surging is the subject of Boeing Service Related Problem (SRP) 72-0024. The modification includes replacement of the basic segmented high-pressure compressor case with a new ring-style case designed to improve compressor blade tip clearance. Boeing estimates RCC-equipped engines will be intermixed with non-RCC engines in the 767 fleet for a period of four years.
Electronic Engine Control (EEC) software version SCN13 is required to optimize the engine operating characteristics of the new RCC design. The SCN13 EEC software incorporates new control logic that varies stator vane schedules and acceleration rates, but only when installed on an RCC-equipped engine. The new control logic will not be activated when SCN13 EEC software is installed on non-RCC engines and is not present in previous EEC software versions.
Due to the SCN13 EEC software control logic, noticeable differences in acceleration time between RCC and non-RCC engines can occur. Thrust asymmetry levels can be reached, which can create low airspeed directional control difficulties. However, the problem is only significant if engines are accelerated directly to the takeoff thrust setting from idle. Per the Boeing recommended normal takeoff procedure, if engines are stabilized at approximately 1.10 EPR prior to setting takeoff thrust, asymmetry levels will be minimized and directional control problems avoided.
In addition, engines equipped with SCN13 EEC software installed exhibits reduced bleed air supply pressure. This reduction in bleed air pressure does not affect airplane systems operation with exception of responding to a forward or aft cargo compartment fire warning above 35,000 feet. If a cargo compartment fire warning occurs at airplane altitudes above 35,000 feet, the CARGO FIRE checklist contained in the QRH is revised by this Operations Manual Bulletin to include flight crew guidance to accomplish a descent to 35,000 feet or below. This will ensure sufficient bleed air supply to comply with cargo compartment smoke penetration requirements set forth under Federal Aviation Regulations.
Flight crews are reminded to follow the takeoff thrust setting practice contained in the normal Takeoff Procedure as published in Volume I of the Boeing 767 Operations Manual:
Advance thrust levers to approximately 1.10 EPR. Push EPR (THR, as installed) switch.
Stabilization at 1.10 EPR will minimize thrust asymmetry and directional control problems, which might occur during operation with an airplane intermix of RCC and non-RCC equipped engines.
This Operations Manual Bulletin will be canceled upon operator notification of pure-fleet incorporation of Boeing Service Bulletin 767-71-0108.