That 2-Minute Dwell Time
There has been a lot of discussion on this subject here and wherever 206 pilots gather, so let me explain in detail how it all originated. John Eacott's post is excellent good but leaves out a few pertinent points.
The first JetRangers were fitted with 250-C18 engines, unfortunately their reliability was not nearly as good as it is today. So in the late 60' or early 70's, the manufacturer, DDA (Detroit Diesel Allison, a Division of General Motors) developed a whole range of modifications to address this reliability issue, DDA called this the 'Blue Ribbon Program'. Subsequently, engines that incorporated all of the modifications were known as 'Blue Ribbon Engines'. This term only applied to C18 engines, the modifications were standard on all subsequent engine models.
Briefly, the modifications relating to the 2-minute dwell time were to prevent coking and seizure of the labyrinth seals, not so much the bearings, and were as follows:
1. A check valve was fitted into the oil pressure tube supplying the No's 6 and 7 bearings in the turbine module. It can be seen on the R/H side of the engine attached to the horizontal firewall and just above the FCU. This check valve closes during the last few seconds following shut-down preventing oil getting into the bearing housing. The oil scavenge pumps continue to run and this minimizes the amount of oil in the bearing housing immediately following shut-down.
2. A small stainless steel sump was fitted to the bottom of the turbine casing to allow any residual oil, again from the No's 6 and 7 bearing, to flow down and collect in there after shut-down rather than remain in the bearing housing and turn to coke under the high residual temperature. This coking would cause the rotating and non-rotating blades of the labyrinth seals to lock preventing subsequent rotation.
3. A procedure was introduced that required pilot, after landing, to observe a 2-minute dwell time between rolling the throttle to flight-idle and shut-off. As John Eacott correctly points out, this results in a significant temperature reduction in turbine module as a whole and the bearings/seals in particular. Yes, there is a DDA-produced graph somewhere showing the temperature reduction and it's quite startling.
4. Another procedure was introduced that required the main rotor to be turned backwards for three revolutions immediately after shut-down (use the main rotor tie-down but be careful not to get it wrapped around the tail rotor blades - those who've done that will know what I mean). By turning the rotor backwards the freewheel unit is engaged and the N2 section of the engine is rotated, also backwards but a lot more than three revolutions. This rotation scrapes any potential coke from the labyrinth seals.
Frankly, I've always thought that there should have been a fifth item here. DDA should have specified which oils were more susceptible to coking, not by mil-spec but by brand name. For example, I believe that anyone who uses Aeroshell 500 oil in a 250 Series engines ought to be flogged. Use this oil and your asking for trouble, use Mobil Jet Oil II instead.
To clear up a couple of misconceptions the some seem to have regarding this issue:
A. Any carbon deposits in the oil will be found in the filter/s, not on the magnetic plugs.
B. Excessive carbon build-up on a labyrinth seal will not cause an in-flight failure, but it may very well prevent a subsequent start.
So now you can see that there are very sound reasons why ALL pilots should observe the 2-minute dwell time after landing and rotate the main rotor backwards three revolutions after it's stopped - that's TWO minutes and THREE revolutions - failure to do these simple tasks WILL eventually cost you a lot of money - now you know.