John due to the unfortunate limitations of human nature, combined with the lack of technology that blighted piston engined aeroplanes until recent times, what is in the POH is NOT always the best way of doing things.
POHs are written by airframe manufacturers not engine manufacturers.
The POH for my 1970 A36 Bonanza was written over 35 years ago and recommends techniques that are gauranteed to do damage...i.e operating 25-50 degrees rich of peak egt for best economy. It suggests operating lean of peak egt is very bad for the engine, and therefore not 'approved' when there is a graph in the TCM IO 550 manual that shows data that proves exactly the opposite. Conversely TCM standard injectors make operation LOP nearly impossible and the standard fit CHT/EGT gauges are bordering on useless. One would have to assume that TCM used 'special' injectors not available as standard to get the data that their graph displays.
In the last 10 years technology, in the form of balanced injectors and all cylinder CHT/EGT monitors, has provided the information and ability to operate piston engines in a far safer and better way but human nature is such that many people are extremely resistant to accepting change. I operate my IO550 LOP most of the time and have done so now for several years since it was overhaulled and new balanced injectors/all cylinder monitor was installed...nothing indicates that the engine is not loving it and that is shown at every annual/100 hrly when the engineers peer inside with their boroscopes and check the plugs.
The approved and widely accepted techiques are very likely the primary reason cylinders rarely ever make it to engine TBO...more typically being changed around the 500-700 hr mark because compression is down or they are cracked. The reasons for this are most likley to be because they were operated at temperatures that while 'approved' were too hot and weakened the metal. I have had veteran LAMEs tell me that LOP operation causes this because the engine runs 'hotter' lean of peak egt. This is a load of ****e.
The EDM 700 engine monitor in my aircraft gives me real time EGT/CHT temperature for every cylinder accurate to 1 degree. I can see that the CHT probe on the mandated 'must be installed' triplex oil/egt/cht gauge is not attached to the hottest cylinder and that it shows fully 50 degrees F cooler than my hottest cylinder. If I didn't have the all cylinder monitor I could be happily flying along with 4 of my 6 cylinders at temperatures well over 400F and not know it. The red line is 460F. I could look down and see 440F and blissfully believe all was well when in actual fact 4 of my cylinders were well in excess of 460F. Graphs exist, and are displayed in John Deakin's writings that show temperatures much over 400F permanently weaken the metal cylinders are made of....it doesn't take much of an intellect to accept that weakened metal leads to early failure from metal fatigue..and yet Beechcraft/CASA mandated practices will lead me to that exact point.
To the crux of this thread.
When you start thinking about it logically 'cool down' periods, typically 3 minutes, make no intellectual sense. An engine is a big lump of metal that is heated up from the internal combustion process. that big lump of metal then radiates that heat into the engine cowl heating up everything that is in the cowl. That heat is removed by airflow through the cowl. Cowl flaps increase/decrease flow depending on their position open or closed. In flight at high speed ram air flow increases cooling airflow dramatically so we close the cowl flaps, opening them when speed is low to maximise flow.
Descending from cruising altitude with relatively high airpseeds and progressively reducing power settings cools the engine down gently to the point that the last 3-5 minutes of flight the power settings are very low but with still significant ram airflow through the cowl.
Having landed we slowly taxi in with the cowl flaps hopefully open...the only airflow through the cowl will be 5-7 knots from taxi speed plus a little help from prop driven airflow around the cowl and passed the cowl flaps. The prop forces no air through the cowl...take a look at the first 20-30 cm of a prop blade...it is round. We then park and sit there for 3 minutes and the ONLY airflow through the cowl, unless parked facing a significant wind, is that caused by the prop blowing around the cowl and passed the cowl flaps producing low pressure over the cowl flap and a pressure differential between the upper cowl and lower cowl...sucking air out of the cowl. The engine, even idling at 700-800 rpm is still producing significant heat from the process of combustion and there is still heat radiating from the higher power settings involved in approach....that big lump of metal takes a while to heat up and cool down....but now there is little airflow to carry that heat away...so everything starts to get hotter...and the longer you sit there blindly following a procedure that makes little sense the hotter it will get....you'd never know it looking at a standard CHT because they are not accurate enough but you can see it on my EDM 700 as the CHTs begin to march upward.
The same applies to the engine oil...virtually no airflow passing through the oil cooler so the oil temp starts rising again.
Thinking about how the turbocharger actually works will lead to some logic being applied to that system as well. At descent/approach power settings the wastegate has been wide open bypassing the turbine for an extended period of time. By the time you land the turbine will have slowed about as much as it is likely to...sitting for 3-5 minutes is just heat soaking the turbine housing.
Last edited by Chimbu chuckles; 24th Sep 2005 at 04:33.