Troubleshooting TFE731 Low Performance
By Craig Bohling
September 2000
Typical Scenario
Shortly after dispatching the company aircraft
for a trip on a warm, summer afternoon, the flight crew returns complaining that the left engine 'temps out' before reaching the target N1 for the day. After some discussion, they admit that the engine has been running warmer than the right side for some time. The crew quickly departs leaving the problem in the hands of the maintenance technician.
Where does the technician start to resolve this problem? What steps need to be taken to get the aircraft back on line?
Background Information
To get started on troubleshooting this problem, it is important to understand several terms that will be used and some background information.
N1: Fan speed indication. The low-pressure rotor spool speed. The N1 fan speed is an indication of the thrust output of the engine. The pilots reference the N1 fan speed to determine the engine power settings.
Take-off N1 for the day: The N1 setting where the engine produces the required takeoff power for the particular temperature and pressure altitude. This setting is found in the Aircraft Flight Manual.
N2: High-pressure rotor spool speed. This is the speed indication for the power producing, core spool of the engine.
ITT: Interstage turbine temperature. The engine temperature taken between the high-pressure turbine outlet and the low-pressure turbine inlet. (Also known as T5 or temperature station 5.)
P3: High-pressure compressor discharge pressure. This is an air pressure reading taken at the exit of the high-pressure compressor and the entrance of the combustion plenum (at pressure station 3).
P2.3: Low-pressure compressor discharge pressure. This is a pressure reading taken between the low-pressure compressor outlet and the high-pressure compressor inlet. (Also known as pressure station 2.3)
Surge Valve: The surge valve is located between the low-pressure compressor outlet and the high-pressure compressor inlet. The valve modulates open and closed to prevent engine surges and stalls.
Computer mode: This is the normal engine-operating mode when the electronic engine control (EEC) is performing governing, limiting and fuel scheduling functions.
Manual mode: This is an abnormal engine-operating mode when the governing, limiting and fuel scheduling functions are performed mechanically by the fuel control and the pilot.
JEDA: Jet Engine Data Acquisition. The JEDA test box is used to acquire engine data during performance runs.
5-point performance run: A 5-point engine performance run is the process of accumulating engine data taken at five different power settings, always ensuring that the ITT limit is not exceeded. At each power setting two sets of data are taken, one manually from the cockpit engine gauges and the other by the JEDA test box plus redundant instrumentation installed on the engine. This data is then input into the MEDRA computer program for reduction to standard day conditions. The reduced information will show if the engine performance margins are within acceptable manufacturer flat rate limits and will also verify calibration of the cockpit instrumentation. The data acquired during the 5-point run is N1, N2, ITT, P3, P2.3 and Fuel Flow.
Questions for the Crew
Troubleshooting Approach
Now that the crew has provided more information about the performance problem, a logical plan is needed to solve the problem. Here is a general overview or 'big picture' of the troubleshooting logic.
• Verify that there is a problem.
• Check to see if the problem is simply an indication error.
• Check to see if an airframe bleed air system is causing the performance loss.
• Check the line replaceable units on the engine that may be malfunctioning.
• Prove that the problem is an internal engine problem that will need to be handled by an approved service center.
Look at line replaceable units
At this point, the performance issue has not been related to indicators or bleed air problems. The line replaceable units on the engine should be the next place to search. The most suspect component is the engine surge valve. Again, while doing the engine run, comply with the surge valve operational check per the maintenance manual. This check is done to verify the proper operation of the surge valve and to ensure that the valve is not stuck open. A valve that is stuck open or malfunctioning would dump compressor air, robbing the engine of performance. This check will require a full-power engine run at a proper run up area.
Switch the electronic engine control (EEC) to the OFF or 'manual mode' position.
• Advance the engine throttle carefully to the required N1 for the day or ITT limit, whichever occurs first, and let the engine stabilize approximately three minutes (not to exceed five minutes at take-off N1).
• After the engine has stabilized, record the manual mode N1, N2 and ITT indications.
• Return the engine to idle.
Next, cycle the EEC switch to ON or 'computer mode' position. Advance the engine throttle to the recorded manual mode N1 taken in the previous step. After the engine has stabilized record the N1, N2 and ITT indications. Return the engine to idle. Compare the recorded computer and manual mode data. The N1 should be the same, N2 should show a slight speed increase in manual mode and the ITT should be warmer by approximately 15 to 30 degrees in manual mode. If there is not an increase in ITT during the manual mode check, the surge valve system should be investigated by checking the surge valve control solenoids, the associated wiring and P3 lines for breaks or leaks. If nothing is
found, it will be necessary to gain access to the surge valve on the engine and check for cracked or broken P3 lines, functional check the surge valve or replace the surge valve.
Engine health check
If at this point the source of the problem
still has not been found, then actual, internal low engine performance is suspected. It will be necessary to perform a 5-point engine run to determine the health of the engine. Because of the special equipment involved, it may be necessary to contact one of the many Honeywell authorized facilities to have this check performed. In many cases, a TFE731 engine service center might go directly to this first when troubleshooting this problem because all of the steps discussed previously can be accomplished during this one engine run.
Prior to performing the 5-point run, install the JEDA test box, block all bleed air, install any remaining test equipment and nose the aircraft into the wind. When performing a 5-point run, it is necessary to acquire data as accurately as possible. Remember that accurate test results require careful, accurate data collection. When performing the 5-point run, record redundant outside air temperature (OAT) and pressure altitude from the altimeter set to 29.92 inches Hg. Using this information, calculate the proper takeoff N1 for the day from the
aircraft flight manual and record it. After verifying the aircraft is still nosed into the wind, advance the throttle to the takeoff N1 for the day and let the engine stabilize for three minutes. After the engine has stabilized record the cockpit instrument readings and capture the JEDA data. (The JEDA test box will take an electronic snapshot all the necessary parameters.) When complete, retard the throttle to the next power setting, 1.5 percent below takeoff N1 for the day, and let stabilize for the 3-minute time period. Continue this process at 3 percent, 6 percent, and 9 percent below takeoff N1 for the day and when complete retard the throttle to idle. A surge valve operational check should be performed at this time if not previously done or if a recheck is desired.
The data acquired during the 5-point run will be input into the MEDRA computer program for processing to determine the performance health of the engine. A close review of the MEDRA output information, showing the engine performance margins, will confirm the low performance and help determine if the engine can be repaired by entering the hot section or if a compressor repair is necessary.
Hints and Experiences
The following are some tips and reminders learned over the years.
• If the pilot is using the cockpit OAT gage to determine the N1 of the day, ensure that it is properly calibrated.
•Check and see if the aircraft has just come out of maintenance. The problem may have inadvertently been induced at the last maintenance event.
• When taking OAT for 5-point runs, use a redundant thermometer outside in the shade, not in the sun, not on the wing and not on the ground.
• When recording pressure altitude, verify that the altimeter is set to 29.92 inches Hg and check a second altimeter in the aircraft to be sure both are reading the same.
• When letting your engine stabilize for three minutes, during the 5-point runs, if the engine drifts slightly off from the set N1, do not move the throttle to adjust it. It is better
to leave it and record the data than to try and re-adjust it because it will make that point unusable for data reduction, causing inaccurate MEDRA information to be output. If you do need to adjust your throttle, the engine will need to be stabilized again. Keep in mind the five-minute takeoff power limit is in effect.
Final Test
Hopefully, after working through the previous
steps, the cause of the performance problem will have been found and fixed. Of course, the final test is when the pilot takes off in the aircraft and climbs out safely and smoothly. For any aircraft technician, there is great satisfaction in resolving a problem quickly and efficiently. Hopefully, these troubleshooting tips, along with the use of the proper approved manuals, will help bring troubleshooting success.