This has been a subject of discussion for a long time. And – many times – it’s been given a sort of “lick and a promise” consideration in the checklist or the operating procedures. In truth, probably not too much documented knowledge has really existed about it. Now, after discussing it at length with Larry New and Linc Dexter during the latest round of B-17 recurrent training, I’m developing second thoughts that we may have been giving it short shrift for all these years.
Ever since I can remember, we’ve generally followed the procedures in the military manuals on this. They generally seemed to have regarded 30 seconds at a prescribed RPM being adequate to achieve the desired oil scavenging results before shutdown. I.e. less residual oil remaining in the sump to help enable a “liquid lock” the next time the engine is started. This prescribed RPM varies from as low as 900 on aircraft such as the C-54 to as high as 1500 on the B-25. Personally, I think that 1500 is too much, it’d seem to me that it would rapidly heat up the engines – and for no
good reason. Almost universally, all the manuals indicate that the engines must be idled in order to cool them down to a reasonable CHT prior to shutdown. This only seems to make good sense to me. It also appears that the manuals seem to exhibit the same fault as they do in prescribing a proper RPM for the power check. I.e., the later the manual, the greater the probability that you’ll find a specific RPM recommendation for the scavenging process. I know that some manuals have shown 1500 RPM for scavenging, but again, this to me appears unnecessarily high.
Generally we’ve landed, shut down two engines on the four-engine equipment as we turned off the runway, then taxied in and then shutdown the last two after scavenging. The engines have been idling during all this taxiing and (if no one is too close behind where we park) we’ve just run the RPM up some in an effort to return a goodly portion of the engine lubricating oil to the tank. Anecdotal evidence indicates that a substantial amount of oil remains in the sump of the engine if this procedure isn’t utilized. Dave Clinton told me that his figures show that about 2½ to 3 gallons remain in the sump of an R-1820, even after
utilizing a proper period of scavenging time.
In those aircraft equipped with an oil quantity indicator – the T-28 is one – this gauge shows how much oil the tank contains. In this case, it’s possible to watch the gauge and see that oil is still being returned from the engine’s sump to the tank after at least a minute of advancing the RPM to a setting where we know that scavenging action is occurring.
We’ve always figured that if we shut down the applicable engines immediately after turning off the runway that they must’ve been pretty well scavenged, since they were turning at higher RPM’s on final just before landing. Well, maybe and maybe not, it appears that this may be a case where the perception perhaps hasn’t been really been too accurate. Probably the “proof-is-in-the-pudding”. We haven't generally had a history of hydraulic locks on the airplanes to point at so that’s a point in favor of doing it the way we’ve always done it. On the other hand, maybe we
could do a better job of scavenging if we religiously shut down immediately after running the engine up to a RPM that is known to scavenge the oil.
We've been experimenting with the DC-3 as to how long we can twiddle our thumbs while scavenging them for sixty seconds before cutting the mixtures. Seems to be an awfully l-o-n-g time waiting on the second hand on the clock, I’d describe it as “the longest minute in the world!
So from now on we’re utilizing the procedure of leaving all four engines on the B-17 running during the taxi-in. After parking, perform any necessary idle mixture checks or any other required items. Then, after making absolutely sure that the prop-blast will not damage or cause any problems behind the aircraft’s tail, advance all the RPM’s to between 1000 – 1200 RPM and scavenge for at least 60 seconds. After that period of time, place all the mixtures in the IDLE CUT-OFF position. In other words, complete all the items beforehand that call for any period of idle RPM, then scavenge and shut it off.
jd sez hot vs cold oil on the c-46
R. Sohn © 1999
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