Not too long ago I watched a pilot attempt to hasten the warm-up by partially closing the cowl flaps.  I’d first heard of this technique more than forty years ago, and I must admit – back then it made some amount of sense to me.  Knowing what I do now, it makes a person wonder how many people believe in this Old Wives’ Tale, using it in a misguided attempt to speed up the operation.  Probably a fair amount of people, just from guessing.  I know that several people have mentioned it to me.  Allowing for the normal number of practical jokers just trying to get a rise out of me, I’d guess that a number of pilots really do use this technique.

The first time that I ever remember needing a really long time to warm up the engines occurred the first time it turned cold during my new-hire year with North Central Airlines.  There, in the wintertime, we preheated the engines at overnight stations such as Minot, Grand Forks, International Falls, Fort William/Port Arthur – well, you get the idea!  The airline pretty well had this preheating down to a science; it was done by a station agent who arrived at the airport in the middle of the night, several hours prior to the scheduled departure.  Canvas enclosures for the cowlings and wheel wells helped trap most of the heat generated by the Herman Nelson gasoline heaters.  Even after all of this effort, the airplane had to be started and then required a really l-o-n-g time with the engines running prior to returning to the gate for passenger boarding and a scheduled departure.  I’d remembered hearing somewhere in the distant past that the use of cowl flaps would speed up this heating process.  It was left for a captain to tell me why this was just another Old Wives’ Tale and useless in practicality.  He pointed out that obtaining an adequate cylinder head temperature was never any problem for us, rather the problem was obtaining an adequate oil temperature.  North Central’s power plant engineers would have probably looked askance at us if we’d baked the ignition harness while trying to accelerate the CHT warm-up for no good reason.  Wouldn’t have been considered smart, I’d guess! 

These same power plant engineers wanted the oil temperature to be at least 20^o and rising prior to exceeding 1200 RPM.  Then, after we’d advanced to this RPM we needed to see an indication of 40^o and rising to permit the RPM and power for the engine run-up.  By the term “and rising” I meant to convey the following meaning.  When you observe the gauge, you’ll see that the oil temperature usually fluctuates as it’s rising.  This is due to the fact that it’s passing through a mechanical system within the cooler that’ll show indications of opening and closing as the oil is warmed.  I.e., it might rise to 25^o but then the needle would suddenly fall back to 15^o, giving an average of 20^o.  Not good enough, the lowest observed temperature at the bottom of the fluctuation had to be 20^o, not be just an average.  The same applied equally to the requirement for 40^o.

I’ve discussed this with a test pilot/engineer who is familiar with the workings of a major aircraft manufacturer’s test facility.  He tells me that their testing disclosed that partially closing the cowl flaps on the ground resulted in temperature hot spots within the nacelle.  Without the use of some rather sophisticated test equipment these areas were impossible to detect – or locate.  But baking of the ignition harness and other problems are endemic.

Another thing that should be mentioned on this subject is the “venturi” effect the open cowl flaps create when the propeller airflow is driven past or over the cowling.  Some pilots may not realize the significance of this, however, remember that this airflow does create a low pressure area at the cowl flap position.  If you partially close them you’ll partially negate the benefits derived from this effect.    

I’m aware of one B-25 operator who routinely closes the cowl flaps just as soon as he’s shut down the engines after terminating.  Looks like the height of something here to me.  If one were looking for a guaranteed way of baking his ignition harness, I’d suspect that this would have to be at the very top of that list.  I asked another operator whom I respect highly about this, he said he closes them later - after things cool down.  This makes slightly more sense to me, his reasoning was “it keeps the birds out.”  But all the while I'm thinking, "Hmmmmmmm, must be pigeons or something, the gap I’ve seen on the closed cowl flaps would all let a pretty big bird get through."  To tell you the truth, I’ve never closed them at all in moderate climates.  Except for maintenance, they can stay open on the ground from now until eternity (or at least the next overhaul) as far as I’m concerned.  On the airline, we did close them during the winter on a turn around - after the CHT fell to 100^o or so - in an attempt to preserve some of the engine’s heat for the next flight.  But, in light of what I know now, I'm not entirely convinced that it wasn't another of those "Old Wives' Tales"

I've perused some military manuals about this; they universally fail to condone this procedure.

Last week I happened to notice a "booby-trap", if I've ever seen one.  Unintentionally –and done with good intentions – but still a booby-trap!  During a turn-around on the B-17, a couple of mechanics needed to gain access to an engine for some minor work.  About that time, a rain shower decided to come over.  A highly experienced mechanic closed the cowl flaps, "to keep the rain out".  When we came back out, I happened to notice it and asked why, the above answer was given.  Now, the problem – as I see it.  We've been experimenting with a "intermediate stop" checklist on the airplane, one that would cut down on those items that the crew must check each and every flight.  This would result in a check of only those items that need to be checked for each flight if the crew has not left.  Oh-oh!  A problem, right here, since no one expected the cowl flaps to be closed since we hadn't done so.   By the way, my thought when I asked why they had been closed and listened to the answer was, "Well, how about the Ford Tri-motor or any other airplane without cowl flaps?"

stuff yet to write:

more about military above

autrline

R. Sohn   © 1999

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