Mixing MIL-PRF-23699 and MIL-PRF-7808

[FieldTeam]

My question concerns the mixing of MIL-PRF-23699 and MIL-PRF-7808 in an aircraft turbine engine (JT3D-7).

On one side I have an engineer who states that we can unconditionally mix these oils in any quantity/ratio, no documentation required.

Printed data I have found states that these oils can be mixed in emergency situations, but under the condition not to exceed more than 50% of the oil tank capacity and at the first opportunity the oil tank shall be drained and reserviced with the correct oil (42B2-1-1 dated July 2005).

P&W has a service bulletin advising I can change from 7808 to 23699, but have to stencil the tank with the new oil number and document this change. Nowhere does the SB state I can service without condition between 7808 and 23699.

Does anyone have any thoughts or input?

 

[chief]

Go with the P&W bulletin, it is documented. The engineers advice is not backed by documentation.

Offshore Engineering&Design

 

[FieldTeam]

We always follow the printed technical guidance.

But, since this subject has been raised, I would like to see if there is any expert opinion in this forum that might substantiate or repudiate this recommended practice.

Obviously, either can be used. It is a matter of combining the two and the possible short/long term repercussions of doing so that interset me.

I will repost in Tribology forum.

 

[j79guy]

My field experience with aeroderivitive turbine engines, regarding mixing of various MIL spec oils has not been good. These oils are not governed by compatability requirements such as those in the automotive industry, where if a litre of oil has the "starburst" symbol, it is universally compatible with any other oil with the same symbol. Being directly involved with the teardown, overhaul and test running of aero engines, I occasionally see some nasty bearing sump contamination that has resulted from mixing "brand-X" with "brand-Y", usually with the customer's mind to reduce consumables costs. My opinion is to stick with what P&W recommends, and if you really want to know which oils perform the best thermally, well, thats another thread. (We've tried them all on the cell, and yes, some are clearly better than others.)
j79 guy