TFE731 Summer 2006

• Smart High ITT Troubleshooting

Smart High ITT Troubleshooting

— Mike Healzer

Summer is here and experience tells us that high ITT complaints increase with outside air temperature. The TFE731 is designed to automatically change the maximum RPM of the engine in response to ambient temperature and pressure altitude. When the weather gets hot, the electronic engine control (EEC or DEEC) varies maximum RPM by reducing fuel to the engine. The chart below shows an example of the relationship of N1 and OAT. As the temperature increases beyond the engine flat rate (EFR) point, N1 decreases.

If you’re having complaints of not achieving target N1 because the maximum ITT limit would be exceeded, here are a few things to consider.

First, ask some questions to help verify the squawk. Has this condition come on gradually or quickly? What was the target N1? What were the ambient conditions at the time? Double-check for the correct target N1 using FAA-approved charts in the airplane flight manual (AFM).

After verifying the complaint in the AFM charts, your next job is to determine whether the problem originates from the airframe systems or the engine. Two airframe systems which may cause high ITT discrepancies are instrumentation and bleed air. A review of recent maintenance actions will sometimes point to the right place.

The next step is a ground run to duplicate the squawk. After authenticating the problem, troubleshooting the bleed air system is the next logical step. Things to look for include leaks, malfunctioning valves, anti-ice valves that are stuck open, faulty mixing valves and leaking pre-coolers.

To confirm if a bleed air leak exists, consider isolating the engine from the airframe by blocking off the bleed air ports at the engine (for ground operation troubleshooting only). Blocking the bleed air ports at the engine can be a significant undertaking, requiring removal of the thrust reverser or afterbody and other related hardware. After blocking the bleed air ports, run the engine again and note changes between blocked and unblocked ground runs. A 5-point run should be performed subsequent to blocking the bleed air.

Cockpit gauge accuracy can be verified by connecting a J2 test box or swapping indicators. For aircraft with the ECTM N1 DEEC’s installed, you can use the ECTM program and then go to the DEEC HHT COMMUNICATION menu and use the SHV (Show Value) commands to see what the DEEC is seeing and compare this to your cockpit indicators. After airframe systems have been ruled out, troubleshooting of the engine is next. A review of JetCare trend monitoring of your engine is always a good place to start. Other checks include a functional test of the surge bleed valve, checking the ITT circuit in accordance with 72-00-00 Inspection Check instructions, and a 5-point run.

We can help you every step of the way, just contact us for free advice with this or any other TFE731 issues. Ask for Lanny Renshaw at 800.228.4277 or Ken Kuchenreuther at 800.525.2376.

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