TFE731 Summer 2003

TFE731 High ITT
2003 Intelli-Conferences

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TFE731 High ITT

— Mike Healzer

With summertime and warmer days, the likelihood of high ITT complaints also increases. The TFE731 is designed to automatically change the maximum RPM of the engine in response to ambient temperature and pressure altitude. When hot day conditions prevail, the electronic engine control (EEC or DEEC) varies maximum RPM by reducing fuel to the engine. The example chart below shows 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 authenticate 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, the approach is to determine whether the problem originates from the airframe systems or the engine. Two predominant airframe systems that cause high ITT squawks are instrumentation and bleed air, the most common being bleed air. A review of recent maintenance activities will sometimes point to the most probable cause.

The next step is a ground run to duplicate the squawk. After authenticating the problem, troubleshooting the bleed air system is in order. Various airframes have their own idiosyncrasies. Look for either leaks or malfunctioning valves. Anti-ice valves could be stuck open, mixing valves could be faulty (usually the mixing valve on the cool engine is the problem), pre-coolers may be leaking, or leaks can be coming from other sources.

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. After airframe systems have been ruled out, trouble-shooting 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.

Please call our toll-free number in Lincoln, Nebraska, for free advice with your troubleshooting of this or any other TFE731 issues. Ask for Lanny Renshaw at 800.228.4277.