Originally Posted by
FNFF
Any RB211-535C and E4 experts out there?
The RB211-535C and E4 have had a number of reported contaminated air events due to engine oil contaminating the bleed air supply. RR Service Bulletin RB.211-72-7651 of 1 Feb 1985 and others refer to the issue.
A 1999 article also states:"Oil related problems on the RB211-535E4 have multiple origins. First, high oil consumption can come from extended low power operation. "All the bearing compartments on the RB211 are sealed with air, so operating the engine at low power simply means that the available sealing air is less than it would be at high power or cruise settings. It has been shown that normal oil consumption can double when the engine is run at low power for an extended period of time, such as when taxiing or just sitting at the gate...."
Are there any engineers who could please explain a bit more about the relationship between increased oil leaks and the engine at idle and also the reported pilot reports of contaminated air events occurring in the decent again with the engines at idle?
Many thanks
You really have two questions there - why increased oil consumption at idle, and why Rolls.
First off, one of the big challenges of a 3 spool engine is bearing design and lubrication - it's simply far more difficult than with a more conventional 2 spool engine. Due to those complexities, oil going were it doesn't belong is more likely on a 3 spool, and Rolls has had more than their fair share of oil leak and bearing issues over the years on all of it's 3 spool engines.
Common design practice on large turbine engines is to use relatively simple oil seals on the bearings, and then use differential air pressure to help keep the oil from getting past those seals. They use air bled from various parts of the engine to provide those differential pressures - and at idle those pressure differentials are not large enough for this system to work as well as it does at power. That's also why fume events sometimes occur right after engine start - when the engine is shut down there is zero differential pressure so if an oil seal isn't up to snuff it can let oil into the flow path (it used to be common to see massive clouds of oil smoke out the tailpipe after engine start on the L1011).