The ball thrust bearings - some background
My comments are based on, or inferred from, information which I believe was developed at Stonehouse. It was stated that it was believed to have possible application to a contemplated aircraft designated the A3XX which could carry 450-500 people, among others. It appears to have originated with a technique used in fighter aircraft. Note is made of the fact the fighter and commercial aircraft differ substantially in the frequency of engine inspections, at least those involving tear-down of the engine.
I comment as follows:
The steadily rising oil temps-- from about 170-C to somewhere in the 190s, and at a comparatively leisurely pace (20-some seconds here before everything went pear-shaped), are compatible with a bearing running hot under very high thrust load, due to the inability to maintain an oil film under these conditions. This occurs even though the oil supply is completely normal (normal flow rate, and properly reaching the bearing(s), and being cooled in the normal fashion), in commercial A/C.
In the military precedent, the fighter A/C could totally lose oil flow to bearings due to violent turns, etc. A solution had be sought to this problem that would enable the bearing(s) to survive 30 seconds undamaged during oil starvation.
The inability to maintain an oil film can rapidly lead to spalled races and bearing seizure in plain metallic bearings, even those with several different types of surface hardening. Perhaps by accident or lucky chance, it was found that a coating called AP (advanced phosphate) tended to act as a solid lubricant (or perhaps an oil film would be adsorbed onto its surface?), and was not itself damaged in the process... at least not up to a point. (If there is such a point, it is not in the information I have, nor is a full explanation why the AP coating worked as it did.)
A model of a ball thrust bearing was made, roughly 1" x 1" in section including both races. This is thus about a quarter-scale model of what I tentatively believe to be the size bearing used in the OF32 #2 engine. The test bearing, with the AP coating on both races and the caged spherical balls, was test run for about 6 minutes with the appropriate high thrust loading. The inner and outer races rose to rather different temperatures, but by inspection of the graphs, 200-C is a fair estimate of the average of the two temperatures. This could be said to occur over the initial 40 seconds. The text said the AP coating tended to higher bearing temperatures, but that stabilization at about 200-C was typical. On reduction of the high thrust load back to more normal, the temperature fell without any bearing surface damage detectible.
Perhaps it can be said that there is a trade-off here in the weight of more oil cooling capacity. But maybe this type of AP coating is just going to run at about 200-C regardless (Edit: but only on failure to form the normal oil film). I don't want to get too far beyond my information; I have sought more recent information, but without success at the moment. I do know that such a bearing (or bearings, there are 5) was used in an advanced turbofan engine built by BMW Rolls-Royce AeroEngines.
At the moment, and I always stand to be corrected, I incline toward a bearing problem initiating a bearing overheat problem. Then obviously this will lead to hot lubricant in all the bearings of that engine, and they will all contribute to the overheating of the oil. Well, of course, the unrestricted certification oil temperature of the RR RB211-972 is up in the 190s (194-C? 197-C?).
The warning upon which there was time to take action was the rise and rate of rise of the oil temperature beyond 170-C. But this rise apparently is not communicated to the FD, just to maintenance. I cannot emphasize too strongly that we do not need any oil fire to have this happen. Yes, I've read the official report. I see a lot of clean metal in the area of the missing turbine casing-- a lot of that is clean as a whistle, including one large piece that had rotary symmetry sundered by a vicious, meandering tear. I also see a large, clean ball thrust bearing, with a buckled outer race.
I agree with barit1 that it is unlikely that a compressor stall was the start of this. But drag from a failing bearing may have disturbed the relation among the 3 compressors and 3 turbines leading to a stall. Then the forces involved in this stall may have made the 30-second delay, perhaps ordinarily enough to not incur disc disintegration while seeking stabilization of the engine, a wrong choice in the software. This is now changed in software.
A lot of questions arise out of this information. Did engine #2 have bearings with the AP coating? Or a later coating accomplishing the same purpose? Did vibration and impact from excessive spline wear damage this coating? There is certainly a heavy watch on spline wear. I leave more questions to later.
Last edited by Jetdriver; 29th Dec 2010 at 22:33.