Troubleshooting TFE731 Low Performance
By Craig Bohling
Author's Note: This article is an excerpt from a TFE731 engine troubleshooting class created by Duncan Aviation engine specialists at the request of aircraft operators. The class provides very practical and useful information about how to troubleshoot some of the more common TFE731 discrepancies. The content of the presentation was created by combining years of Duncan troubleshooting experience with the excellent troubleshooting information contained in the Honeywell engine maintenance manuals. The actual class, which covers six TFE731 troubleshooting topics in detail, is presented with audio/visual aids and the support of a Duncan technical panel that offers real life tips and answers to questions. This article will address the topic of troubleshooting low performance problems.
This troubleshooting guide will begin with a typical scenario to help everyone identify with a real world situation. The scenario will be followed by some definitions and background information that are foundational to understanding the detailed steps that will follow. Next, the general troubleshooting logic or 'big picture' will explain how the problem will be approached. This will be followed by some suggested questions for the crew, the actual troubleshooting steps, and finally some technical hints. IMPORTANT: Because of limited space and time, this article will skip over some of the basic maintenance practices used while troubleshooting. It is assumed that the maintenance technician understands proper techniques for preparing the aircraft, applying power, connecting and disconnecting components. As always, the technician should consult the proper approved maintenance manuals for specific steps and information during troubleshooting or any other maintenance work.
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?
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
When troubleshooting a problem, it is important to learn as much about the situation as possible. Asking the right questions of the crew can help the technician be more efficient and possibly eliminate several troubleshooting steps. Here are some suggested questions to ask that may reveal clues to the source of a performance problem.
1. What was the N1 target for the day? What was the pressure altitude and outside air temperature? (Maybe the N1 target for the day was misread.)
2. Did the problem happen gradually or quickly? (A problem that comes up quickly may be related to a bleed air component failure or to a serious problem in the engine.)
3. How did the N1 and ITT compare to the good engine? (The good engine may be running cooler than normal, which could be a sign that the warm engine is handling more of the bleed air load than normal.)
4. Was the anti-ice on? (Anti-ice systems typically use bleed air, which will make an engine run hotter than normal.)
5. Has the aircraft had other maintenance recently?
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.
To get started, several steps can be accomplished during an engine ground run. Start by opening all necessary access panels and removing the engine cowling. After arriving at the run up area, carefully determine the required takeoff N1 setting for the day per the aircraft flight manual. Advance the throttle and attempt to achieve the N1 for the day being careful not to exceed the ITT limit during the engine run. This will confirm that there is a problem.
The next step could be to swap N1 and ITT gauges to see if the problem follows a faulty indicator.
• Record N1 and ITT readings at the takeoff power setting for the day.
• Shut down the engine and swap N1 and ITT indicators, then record N1 and ITT readings at takeoff power for the day again with the gauges swapped.
• Compare the recorded data looking for any changes. If the problem follows one of the indicators, have the unit repaired and verify that the problem is corrected before continuing.
Also, during the engine run, while the aircraft is at idle, have another technician check for bleed air leaks at the engine. Any bleed air leaks will cause an engine to run warmer than normal. Check for leaks in the aircraft pylon and in the tail area to ensure there are no blown seals, cracked or broken bleed air pipes. After completion of the leak check, have the technician move to a safe area so all the engine and aircraft anti-ice and bleed air systems can be checked. Advance the engine throttles and, in accordance with the airframe maintenance manual, check for proper operation. Operate the engine anti-ice switch to verify that there is an engine ITT temperature change. No temperature change could signal a faulty anti-ice valve stuck in the open position. Next, cycle the engine bleed air open and closed on each engine and verify an engine ITT temperature change. When working properly, these systems should have a predictable effect on the engines. For instance, if the right-hand bleed air system is turned on, at certain N1 power settings there should be a predictable rise in ITT (as bleed air is extracted from the RH engine). If there is no ITT increase, the airframe system may be malfunctioning. In this case the left-hand system (and engine) may be carrying more of the bleed air load resulting in what seems to be a performance problem with the left engine. If these checks do not produce anything conclusive, continue troubleshooting. If a fault is found, make repairs and verify that the performance problem is corrected before continuing. Also note that the pilots can accomplish these bleed air system checks while in flight, if conditions permit. This could help save valuable troubleshooting time.
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.
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.