In 1985, operational guidelines were created to be used in determining possible burner can distress on JT8D-1 through -17AR engines (DC-9). This was in response to an accident involving a Boeing 737 aircraft in Manchester, England. Subsequently, we added similar guidelines to the JTD8D-200 series Operating Instructions. Earlier this year, the entire issue was reevaluated. The purpose of this All Operators Letter is to emphasize the operational guidelines used on JT8D-1 through -17AR series engines and to redefine recommendations for JT8D-200 series engines.

The following guidelines were established to be used by flight crews as checks in the early detection of burner can distress on JT8D-1 through -17AR series engines. The possibility of combustion chamber separation or sever burn through remains a real possibility and we feel these checks are important in identifying failures. The following operational guidelines for the -1 through -17AR model engines remain in place, unchanged from the original recommendation. Flight crews should check that:

a) Engine start time (spool-up from first indication of EGT rise to idle) does not exceed 30 seconds. For clarification purposes, idle is intended to represent the point during the start process when the engine reaches a normal idle N2 parameter, the flight crew observes an appreciable decrease in the rate of N2 acceleration, and the engine start process is considered to be essentially complete. A starting time in excess of 30 seconds should not be considered normal and should be written up by the flight crew for appropriate maintenance action.

b) Acceleration from idle to about 1.4 EPR while setting takeoff thrust does not vary more than 3 seconds between engines. Normal procedure for establishing takeoff thrust on the JT8D engine is to initially adjust the aircraft throttles to obtain a thrust setting of approximately 1.4 EPR, then smoothly apply takeoff thrust. Acceleration time from idle to 1.4 EPR should be monitored. Acceleration times can vary among engines and variations up to 3 seconds are considered acceptable. However, differential acceleration greater than 3 seconds could be indicative of abnormal operation and should be written up the flight crew for appropriate maintenance action. Upon advancing throttles from the intermediate power setting of 1.4 EPR to takeoff thrust, all engines should accelerate equally to takeoff thrust.

c) All engines throttles are closely aligned. Excessive throttle stagger can be attributed to various factors, one possible factor being combustion chamber distress. Flight crews should be aware that reasonable throttle misalignment or stagger at constant EPR will occur when engines are intermixed. Operators should contact the airframe manufacturer for information when determining what is deemed reasonable for the specific intermix being used. When an unacceptable throttle stagger has been identified, it should be written up by the flight crew for appropriate maintenance action.

d) Engines exhibiting low N2 RPM at idle power settings also indicate a corresponding decrease in fuel flow. This decrease in RPM and fuel flow can result from a shift in fuel control trim.
Conversely, engines that indicate low N2 RPM along with a discernible increase in normal fuel flow readings may be operating with combustion chamber distress. When this latter condition is observed by the flight crew, it should be written up for appropriate maintenance action.

In 1993, we issued an All Operators Letter, emphasizing that differential acceleration from idle to 1.4 EPR in excess of 3 seconds on JT8D-200 series engines was indicative of abnormal operation. This AOL is now rescinded. Experience in complying with this guideline has shown that the check is not important in identifying combustion chamber stress and need not be performed on the -200 series engine. In order to remain consistent with other engine models, however, pilots should squawk differences in acceleration between engines based on how aircraft handling is affected.

The only check we now recommend be performed on JT8D-200 series engines is a revised check “a)”. Production acceptance testing indicates that on -200 series engines, a start to idle time of 30 seconds is too short, therefore, perform check “a)” using a start to idle time of 40 seconds. Although the combustion chamber on -200 series engines has shown different and less frequent distress compared to JT8D-1 through -17AR series engines, this check is considered good practice, is consistent with most other engine models, and will provide flight crews with a guideline for time to idle.

Although checks “c)” and “d)” are valid operational concerns and have their own place in the troubleshooting process, failures of these checks on -200 series engines should not be considered indicative of combustion chamber distress, and can be used at the discretion of the pilot.

We must emphasize that the intent of these guidelines is to trigger maintenance action when necessary and should not be interpreted as operational limits. The pilot in charge should use his/her discretion in deciding if a flight should continue if an engine does not fall within the guidelines set forth in this letter. Please provide this information to all appropriate Flight Operations Personnel.