Jump to content
Aero Precision provides OEM part support for military aircraft operators across more than 20 aircraft

High Oil Temp in Flight

Recommended Posts

Hi guys

I got a problem I need your thoughts with. Recently one of our birds twice turned back after oil temperature exceeded +-90°C at altitude.

History: The aircraft just completed ISO, and post-dock runs had oil temps getting up to 100°C at ground idle. OAT was in mid 40\'s. Two different oil coolers were fitted, and oil tank was drained and serviced with new oil. After this it was released.

The next mission off base the in-flight problem occurred, and the engine was removed for test cell evaluation.

In the Cell I started by draining all the oil from tank, RGB, ADH, oil cooler, and motoring the engine to empty all the oil hoses and tubes, and changed all filters.

I then serviced the tank, and motored the engine to refill hoses, tubes and filters, and then refilled the tank. I noticed that the quantity had gone down about 3 gallons during motoring. I then started up and proceeded to do a back-pressure run at 900°C TIT. Only the Fuel Heater back pressure was high at 15psi, but the temperature did go up to 93°C. Immediately after shutdown I checked the oil temperature in the tank with a digital thermometer, and confirmed 93°C. Also the oil qty had increased by 2gallons over the prestart qty.

The next day, I motored the engine again before starting, and noted the qty dropped by 2 gallons. I fitted all the panels, started up and accelerated to 900°C TIT. I ran the engine for 30mins and the temperature went up to 92°C. I throttled back to flight idle for 10 mins and the temperature dropped to 85°C. I then accellerated to 1000°C TIT. Oil temp after 30mins was 97°C, and I also noticed that the lighthouse pressure had increased slowly from 3\"Hg to 4\"Hg. Oil qty after shutdown was the same as before motoring.

I then changed the turbine scavenge pump, as it appeared to be a poor scavenge system causing retained oil to overheat. When I removed the pump, I found the \'O\'-ring on the spline shaft had completely carbonized. The oil temp dropped by 2°C, and the qty during the run was 1/2gal higher than before. I then changed the #2/#3 bearing scavenge pump, but only got 1°C improvement, and no qty improvement. I then decided to go for broke, and had the PS main oil pump and figure 8 seal changed, and had the turbine removed to inspect the oil tube seals inside the lighthouse. These seals were also carbonized.

Today, after motoring qty was only 1 gallon down - normal. I started up, went to 900°C TIT. After 30 mins, oil temp went 87°C. After 10 mins at flight idle, temp went down to 80°C. At 1000°C TIT, temp went up to 89°C. Also the qty during the run was only 1.25gal below prestart qty, and no increase in lighthouse pressure.

So, big improvement all round. But I still feel that the temperature should be somewhat lower. I have received a brand new, out-of-the-factory oil cooler, which I will install tomorrow, and give it the works .

Any ideas and/or suggestions from you would be welcome.

Thanx. Have a great day


Link to post
Share on other sites

Oil cooler bypass valve???

It thinks the oil is cold and does not require cooling.

Bypass valve is seperate from the cooler so you may have the same valve installed. The bypass valve determines if the oil needs to go into the oil cooler or not...it sounds like the oil is being sent to the tank without going through the cooler... If you start the engine when the oil is cold the valve will bypass all oil until it reaches approx 50C then oil will travel through the cooler...if your bypass vlave always thinks the oil is below aprox 50C it will always bypass the oil cooler.

The quantity issue may just be the expansion based on temperature and condition of the oil... I would focus on the temp first.

Link to post
Share on other sites

I believe NATOPS called it right. I fitted the new cooler today, and temperature maxed out at 84°C after 20mins at 1000°C TIT.

FYI engine has 1750hrs, vibes are 0.6, 0.5 & 0.7 resp. for turbine, compressor and RGB. No SOAP.

Being paranoid doesn\'t mean they aren\'t out to get you :P . I have been told I\'m very picky about what\'s acceptable and what\'s not in the cell. I just like to get it as close to perfect as I can. The oil system is acknowledged the most difficult system to troubleshoot, which is why we should go the extra mile when problems occur.

Thanx for your input guys. Really appreciated

Have a great day


Link to post
Share on other sites

I am glad you found the problem, good troubleshooting. I was going to add oil lines and rear scavenge pump to the list of troubleshooting items. I have had it happen once that the main oil supply line was collapsing and believe it or not, oil pressure was normal, but over-tempted. I have also seen where the aft scavenge pump was starting to fail, thus letting the oil cook back in the turbine before returning it to the system.

I bet your FE\'s appreciate how \"picky\" you are when troubleshooting your engines. Nothing is worse then shutting down a good engine because of oil temps.

Best Wishes,


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...