JT15D Engines Winter 2004
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JT15D Performance After Hot Section Inspections
— Dan Arrick
Many operators throughout the years have questioned the performance of their JT15Ds following Hot Section Inspections. After a week of downtime and thousands of dollars, the engines perform the same or even run a little warmer. While there are many ways to determine the health of an engine, the old ITT billboard in the cockpit usually gets the most attention, especially following a major event. During a Hot Section Inspection, components are inspected and repaired if necessary to maintain the integrity of the engine until overhaul. One of the components that gets the most attention is the Small Exit Duct Assembly. This component contains the HT Stator and the HT Disk Shroud Segments. The clearance between the segments and the HT Disk play a big role in the performance of the engine. During the Hot Section Inspection, the segments are usually replaced and ground along with the HT Disk to regain HT Disk blade tip clearances lost during normal operation of the engine for 1750 hours. Once this is accomplished, operators assume that their engine will perform as well or better. What isn't taken into consideration is that during the process of regaining proper tip clearances, the HT Disk diameter is reduced. Just like a small chain sprocket on a bicycle, a smaller HT Disk has to work harder to maintain performance levels. This requires additional fuel and the result is more heat or ITT. The engine is still healthy, just a little warmer.
For more information please contact Dan Arrick at 800.525.2376.
Throttle Split Troubleshooting
— Dan Arrick
Throttle splits on JT15D powered aircraft are not uncommon. Operators usually get accustomed to the splits and compensate for them without doing any rigging adjustments. However, most throttle splits can be corrected with a minimal amount of troubleshooting. Most often there are no parts involved in a repair and downtime can be just a few hours.
To get an accurate idea of the throttle split in your aircraft, push the throttles up while the aircraft is running to about 80% N1. Be sure to move the throttles only toward the take-off position. If you pass 80% N1 on one throttle, just move the other one up to match the first. The 80% target is not critical. Do not reduce either of the throttles to match N1 speeds. Pulling back on the throttles will give an inaccurate indication of the actual split. Once you have determined the actual spilt, ensure that the engines can achieve take-off N1 for the day and idles properly. With this information, technicians can determine how to correct the problem and whether there is a component contributing to the split. Be sure to stay within all engine operating parameters while checking for the split and consult appropriate maintenance manuals for rigging procedures.
Helpful Hint: Throttle splits usually don't suddenly show up. If there is an increase in the throttle split over a short period of time, there may be other problems not associated with normal rigging discrepancies.
Trivia Question
— Dan Arrick
The previous question was: What discrepancy would a P3 leak cause during engine start?
The answer: Generally, a P3 leak will cause a slow start. In some cases the engine will only accelerate to min-flow.
This edition's question is: On JT15D-1 through -4 series engines, what is the purpose of the Step Modulator?
The first 10 callers with the correct answer will receive a small prize.
Do you have a trivia question suggestion? Please respond to Dan Arrick by phone by calling 800.525.2376.
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