Cowls closed or open on first engine start?
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Originally Posted by gassed budgie
Opening the cowl flaps on final will not 'shock cool' the engine. The engine is already operating at a relatively low power setting, with the aircraft at a relatively low airspeed. The CHT's hardly move if the cowl flaps come open on final. An engine is under its greatest thermodynamic stress at start up. This is where the most rapid change in CHT can be found.
Shock cooling I think, is a bit a of a furphy. I don't no how many times I've seen pilots constantly fiddling with the power settings on descent to avoid shock cooloing the engine. No more the 1" MP change per minute and it has to be at such and such a power setting 5 miles out, so on and so on.
Yet those same pilots in a Chieftan for example at top of climb, will pull the MP back to 31" from 38". A whole 7" all in one go!
In fact the power reduction at top of climb is where you will find the greatest change in CHT's, other than at start up. Not on descent.
Both of those temps are very hot. 232C equals 450F and 265C equals 500F. I like to keep the CHT's in the aircraft I fly at around 350F. Once they go above 385F I start to feel a bit uneasy and if they start to push 400F something needs to be done and quick. The one thing, the prime reason cylinders are junked is because of hot CHT's. I they're kept cool getting them to TBO should not be problem.
If you operate a Cessna 182 with the CHT's constantly at the top of the green arc, the cylinders will be garbage after a few hundred hours. They probably won't even get half way to the engines stated TBO. Many manufacturers over the years have suggested that it's OK to run the engine inside a particular set of parameters. Pilots see the operating procedures in the POH and follow the suggested power settings, only to find the engine lets them down way before it's TBO. To say it's OK to operate and engine as long as it stays in the green arc is not so simple.
Shock cooling I think, is a bit a of a furphy. I don't no how many times I've seen pilots constantly fiddling with the power settings on descent to avoid shock cooloing the engine. No more the 1" MP change per minute and it has to be at such and such a power setting 5 miles out, so on and so on.
Yet those same pilots in a Chieftan for example at top of climb, will pull the MP back to 31" from 38". A whole 7" all in one go!
In fact the power reduction at top of climb is where you will find the greatest change in CHT's, other than at start up. Not on descent.
Both of those temps are very hot. 232C equals 450F and 265C equals 500F. I like to keep the CHT's in the aircraft I fly at around 350F. Once they go above 385F I start to feel a bit uneasy and if they start to push 400F something needs to be done and quick. The one thing, the prime reason cylinders are junked is because of hot CHT's. I they're kept cool getting them to TBO should not be problem.
If you operate a Cessna 182 with the CHT's constantly at the top of the green arc, the cylinders will be garbage after a few hundred hours. They probably won't even get half way to the engines stated TBO. Many manufacturers over the years have suggested that it's OK to run the engine inside a particular set of parameters. Pilots see the operating procedures in the POH and follow the suggested power settings, only to find the engine lets them down way before it's TBO. To say it's OK to operate and engine as long as it stays in the green arc is not so simple.
Grandpa Aerotart
As far as shock cooling is concerned I can demonstrate in my Bo that it is a crock. I showed an experienced CFI once, using my EDM700, and he still couldn't get his head around it after 30+ years of believing in it and teaching it's existance.
Picture this...on descent at high IAS/cruise power setting and pull the MP back to a setting that is just above gear warning horn activation. What will the EGTs do and what will the CHTs do?
The EGTs increased and the rate at which the CHTs were cooling slowed. The increased EGT caused by changing the fuel/air ratio (richer mixture due less air/same fuel when you partially close the throttle) causes the cooling of the CHT to slowdown. You are puuting more heat into the cylinders. You can actually see it very clearly with modern digital engine monitors...the traditional EGT/CHT/Oil Temp triplex gauges show you nothing.
The temperature displayed on the EGT is merely the result of many instantaneous 'snap shots' as the exhaust valve opens and the temp probe is exposed to the exhaust gas being expelled from the combustion chamber. As an absolute value it is meaningless.
The temperature displayed on the CHT is much more important. It is, if you like, the residual heat left over from combustion that has not been able to be dissipated any other way...so it heat soaks the cylinders and engine block generally. Most of the heat of the combustion event is expelled out of the exhaust after finishing it's useful work in driving the piston down and turning the crankshaft.
The metal used to build the Cylinders and engine generally, but not the pistons, is an alloy that is strongest when it is cold. That strength decreases slowly until a certain temperature (around 400F) and then it decreases very quickly. Here is the above mentioned graph.
The common problem seen of engines losing compression and needing 'top overhauls' (new cylinders) is caused by running the engines at power settings, as decreed by manufacturers, that result in CHTs over 400F. This weakens the metal over time which leads to cracking. It also causes the cylinders to 'oval' or end up not round. The pistons made of much sterner stuff maintain their shape and this causes abnormal wear patterns which lead to loss of compression.
These are fundamentally air cooled engines. If you start the engine and merely sit at idle power eventually the engine will get very hot...hotter than at cruise power in flight...and by 'eventually' I don't mean several hours I mean 15-20 minutes. The heat is not being carried away efficiently by mass airflow in through the front of the cowl as in flight...only very innefficiently by propwash around the cowl and over the cowl flaps sucking air out of the cowl which is then replaced by a small airflow in through the front...the prop does not push air in through the front of the cowl.
As you descend at reasonably low power settings and high IAS they cool at a reasonable rate and as you fly the circuit at even lower power settings and still reasonably high IAS they cool more. The suggestion that opening the cowl flaps on final approach will cause a dramatic and quick drop in CHTs just cannot be supported by the physics. The cylinders are a significant chunk of metal...it takes time to effect a change in temperature...a relatively small increase in cooling airflow from opening the cowl flaps at 80-90kts just won't do that. As soon as you slow to taxi speed after landing the CHTS start to increase again. In a normally aspirated engine you merely serve to heat the engine up by sitting for 2 minutes 'cooling' the engine down after parking.
With turbo engines it is considered good practice to sit and allow the turbo impeller to cool for 2 minutes....but lets think about how a turbocharger works.
Exhaust gasses spin an impeller that essentially drives a pump that crams more air into the combustion chamber so that more fuel can be pumped in and produce more power at a similar fuel/air ratio.
At very low power settings on approach not much exhaust gas is driving the impeller so it is turning very slowly if at all. What is the difference between 12 inches on approach to 12 inches parked as far as the turbo impeller is concerned...zero.
So even in a turbo the real value of a cool down period of 2 minutes is, at best, marginal.
The old 1 in MP/ minute rule of thumb were not so much a engine cooling rule as a bastardised gearbox protection rule of thumb left over from the old geared engines. Pull off 1 in of MP and the aeroplane slows, repeat evey minute on approach and you get to a point where, below 25 inches MP, you could then do whatever you like, essentially, without putting undue negative stresses on the gear box. It was considered as a gentle way to treat the engine heat wise too and probably was....probably vastly more gentle than the engines needed from a cooling point of view. Of course then a pilot who perhaps didn't have all the facts goes to fly a Chieftain after flying something geared like the old Queenairs/C404s and introduces the same habits to that operation...perhaps as a Cheif Pilot or under a META...and before you know it all the pilots he trains start believing the 1in/minute rule is as 'essential' technique for engine cooling...and so on
This is how these 'legends' become institutionalised 'facts'.
There is an old rule of thumb which deals with loss of corporate knowledge as each new generation of pilots is trained by sucessive generations of training pilots after the initial batch get trained by the manufacturer...can't remember exactly but it may have been something like 'The 10% rule'.
As big engined piston twins have long passed from the hands of airlines with their training departments that 10% has increased exponentially.
Picture this...on descent at high IAS/cruise power setting and pull the MP back to a setting that is just above gear warning horn activation. What will the EGTs do and what will the CHTs do?
The EGTs increased and the rate at which the CHTs were cooling slowed. The increased EGT caused by changing the fuel/air ratio (richer mixture due less air/same fuel when you partially close the throttle) causes the cooling of the CHT to slowdown. You are puuting more heat into the cylinders. You can actually see it very clearly with modern digital engine monitors...the traditional EGT/CHT/Oil Temp triplex gauges show you nothing.
The temperature displayed on the EGT is merely the result of many instantaneous 'snap shots' as the exhaust valve opens and the temp probe is exposed to the exhaust gas being expelled from the combustion chamber. As an absolute value it is meaningless.
The temperature displayed on the CHT is much more important. It is, if you like, the residual heat left over from combustion that has not been able to be dissipated any other way...so it heat soaks the cylinders and engine block generally. Most of the heat of the combustion event is expelled out of the exhaust after finishing it's useful work in driving the piston down and turning the crankshaft.
The metal used to build the Cylinders and engine generally, but not the pistons, is an alloy that is strongest when it is cold. That strength decreases slowly until a certain temperature (around 400F) and then it decreases very quickly. Here is the above mentioned graph.
The common problem seen of engines losing compression and needing 'top overhauls' (new cylinders) is caused by running the engines at power settings, as decreed by manufacturers, that result in CHTs over 400F. This weakens the metal over time which leads to cracking. It also causes the cylinders to 'oval' or end up not round. The pistons made of much sterner stuff maintain their shape and this causes abnormal wear patterns which lead to loss of compression.
These are fundamentally air cooled engines. If you start the engine and merely sit at idle power eventually the engine will get very hot...hotter than at cruise power in flight...and by 'eventually' I don't mean several hours I mean 15-20 minutes. The heat is not being carried away efficiently by mass airflow in through the front of the cowl as in flight...only very innefficiently by propwash around the cowl and over the cowl flaps sucking air out of the cowl which is then replaced by a small airflow in through the front...the prop does not push air in through the front of the cowl.
As you descend at reasonably low power settings and high IAS they cool at a reasonable rate and as you fly the circuit at even lower power settings and still reasonably high IAS they cool more. The suggestion that opening the cowl flaps on final approach will cause a dramatic and quick drop in CHTs just cannot be supported by the physics. The cylinders are a significant chunk of metal...it takes time to effect a change in temperature...a relatively small increase in cooling airflow from opening the cowl flaps at 80-90kts just won't do that. As soon as you slow to taxi speed after landing the CHTS start to increase again. In a normally aspirated engine you merely serve to heat the engine up by sitting for 2 minutes 'cooling' the engine down after parking.
With turbo engines it is considered good practice to sit and allow the turbo impeller to cool for 2 minutes....but lets think about how a turbocharger works.
Exhaust gasses spin an impeller that essentially drives a pump that crams more air into the combustion chamber so that more fuel can be pumped in and produce more power at a similar fuel/air ratio.
At very low power settings on approach not much exhaust gas is driving the impeller so it is turning very slowly if at all. What is the difference between 12 inches on approach to 12 inches parked as far as the turbo impeller is concerned...zero.
So even in a turbo the real value of a cool down period of 2 minutes is, at best, marginal.
The old 1 in MP/ minute rule of thumb were not so much a engine cooling rule as a bastardised gearbox protection rule of thumb left over from the old geared engines. Pull off 1 in of MP and the aeroplane slows, repeat evey minute on approach and you get to a point where, below 25 inches MP, you could then do whatever you like, essentially, without putting undue negative stresses on the gear box. It was considered as a gentle way to treat the engine heat wise too and probably was....probably vastly more gentle than the engines needed from a cooling point of view. Of course then a pilot who perhaps didn't have all the facts goes to fly a Chieftain after flying something geared like the old Queenairs/C404s and introduces the same habits to that operation...perhaps as a Cheif Pilot or under a META...and before you know it all the pilots he trains start believing the 1in/minute rule is as 'essential' technique for engine cooling...and so on
This is how these 'legends' become institutionalised 'facts'.
There is an old rule of thumb which deals with loss of corporate knowledge as each new generation of pilots is trained by sucessive generations of training pilots after the initial batch get trained by the manufacturer...can't remember exactly but it may have been something like 'The 10% rule'.
As big engined piston twins have long passed from the hands of airlines with their training departments that 10% has increased exponentially.
Last edited by Chimbu chuckles; 15th Aug 2006 at 11:50.
Nice post there Chuck.
Hehe. I just spotted that. I'll stand by with the can of petrol and box of matches just in case the fire goes out. Sore point indeed.
I wouldn't pi$$ on CASA if it was on fire.
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Was told by instructor to open cowl flaps on finals or major damage will occur on goround. Climbed up valley with cowl flaps closed @ full noise and after 10 min noticed cyl temp getting up there. Still in green though.
Grandpa Aerotart
qtn the top of the CHT green arc in most piston aircraft is 460F. Say you reached 450F before noticing...and just for arguments sake what if, like my aeroplane, your CHT probe was hooked up to one of the cooler running cylinders?
My OEM CHT gauge reads a full 45F cooler than my warmer cylinders as displayed by the EDM. It is entirely possible, even likely, that several of the cylinders on that engine were exposed to temps in the vicinity of 500F.
Nothing is likely to happen immediately from 10 minutes at 500F but the metal of those cylinders was weakened.
Suppose that same thing is visited on that engine once in a while by different pilots over the first half of the nominal TBO...that is exactly why, historically, VERY few IO520s and like engines every made TBO without being top overhaulled 600-800 hrs before TBO. Next time you glance through a Trader Plane looking at aircraft for sale note how many have had top overhaulls at around 1/2-2/3rds engine 'life'...almost all.
My OEM CHT gauge reads a full 45F cooler than my warmer cylinders as displayed by the EDM. It is entirely possible, even likely, that several of the cylinders on that engine were exposed to temps in the vicinity of 500F.
Nothing is likely to happen immediately from 10 minutes at 500F but the metal of those cylinders was weakened.
Suppose that same thing is visited on that engine once in a while by different pilots over the first half of the nominal TBO...that is exactly why, historically, VERY few IO520s and like engines every made TBO without being top overhaulled 600-800 hrs before TBO. Next time you glance through a Trader Plane looking at aircraft for sale note how many have had top overhaulls at around 1/2-2/3rds engine 'life'...almost all.
