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  #1  
Old August 9th 05, 05:01 PM
Roger Fowler
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Default Towing

I recently rode with a towpilot who leaned the engine
aggresively after retarding throttle at the start of
the descent. While he is probably correct that it
slows the rate of cylinder cooling significantly, I
had never seen it done before. Is anyone else using
this procedure?



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  #3  
Old August 10th 05, 04:23 AM
COLIN LAMB
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One concern would be if the pilot has to do an emergency go around. If the
engine is leaned out enough, engine damage could be done unless the fuel in
enriched at or before full throttle application, or worse, the engine might
balk. Sometimes, during emergencies, things are forgotten - which is reason
to simplify things - especially if flaps must be retracted at the same time.

Colin


  #4  
Old August 10th 05, 06:07 AM
BTIZ
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On our warm up flight.. at about 3000ft AGL (6000MSL) we lean to max RPM and
note the position.
During tow, we lean to max RPM during the climb... after release we reset
the mixture to the position noted during the warm up flight and leave it
there for descent. Slow stage reductions in RPM during the descent.

I'll agree with another answer, once the power is below 1900RPM (2575
redline), I don't think the mixture will have too much effect. Except not to
be there on a go around.

BT

"Roger Fowler" wrote in message
...
I recently rode with a towpilot who leaned the engine
aggresively after retarding throttle at the start of
the descent. While he is probably correct that it
slows the rate of cylinder cooling significantly, I
had never seen it done before. Is anyone else using
this procedure?





  #5  
Old August 10th 05, 08:09 PM
[email protected]
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To All Concerned:

You know, it is still shocking to me that there are pilots with this
kind of "knowledge". I think that there is significant lack of
training, those pilots who are flying in this manner are accidents and
damages waiting to happen. That is also directly proportional to the
tow rates that we are paying. Imagine that our club would have to buy
or overhaul prematurely an engine. Who will pay for that? Are there any
clubs that are charitable organizations? Anyway, I am waisting my
breath ........

This is what the real experts are saying; these excerpts are from
Lycoming :

From time to time a field service report states that an engine has

damage. After further examination of the engine, this damage may be
classified as "induced damage." To clarify what is meant by this term,
induced engine damage is a failure or unsatisfactory condition which
results from operational or maintenance practices employed after the
engine is placed in service. Although there are a variety of conditions
which may fall into the induced damage category, this article will
discuss two particular types of failure and the circumstances which can
induce them.

Examination of an engine that is reported to have low compression, loss
of power, erratic operation, metal contamination, or even complete
engine stoppage may result in a determination that pistons are burned
or valves stretched. (Stretched valves are sometimes said to be
tuliped.) These two types of damage can be initiated in a number of
ways, but the chain of events is often the same; detonation is followed
by preignition and the engine damage has begun. To prevent burned
pistons and tuliped (or stretched) valves, action must be taken to
eliminate the possibility of detonation and preignition.

Detonation is a phenomena which can occur in any internal combustion
engine. The possibility of detonation cannot be completely eliminated.
By definition, detonation is a violent explosion. When used with
reference to a spark ignition internal combustion engine like the
Textron Lycoming aircraft piston engines, detonation indicates abnormal
combustion. Essentially, detonation is an uncontrolled explosion of the
unburned gases in the engine combustion chamber. Some engines are more
susceptible to detonation than others. For example, turbocharged
engines are more susceptible than similar non-turbocharged models and
engines with higher compression ratios are more likely to exhibit
detonation than engines with lower compression ratios.

Detonation may occur in an aircraft engine as a result of maintaining a
manifold pressure that is too high for the specific engine speed and
mixture setting being used. The engine power (i. e. speed and manifold
pressure) and mixture settings recommended in the Pilots' Operating
Handbook (POH) for a particular aircraft model have been determined by
a detonation survey. These surveys use special instrumentation to
detect and record detonation as it occurs. Based on these surveys, the
detonation limiting conditions are defined. Data from the surveys
indicate that detonation occurs in varying degrees; it is sometimes
possible to operate an engine for relatively long periods in the first
minor phase of detonation without inducing damage. Textron Lycoming
does not recommend or condone engine operation which even approaches
conditions which might cause detonation. The laboratory quality
equipment used for the detonation survey is not practical for use in an
aircraft engaged in normal flight operations. Without this equipment,
the pilot may not know that detonation is occurring, and it is
impossible to establish the fine line between the first phase of minor
detonation and the detonation magnitude which induces preignition
and/or engine damage. For this reason it is imperative that power and
mixture recommendations of the POH be carefully observed.

Preignition is a circumstance that causes destructive engine damage and
will be examined here briefly. Most Lycoming engines are designed for
ignition of the fuel/air mixture at 20 crankshaft angle degrees before
the piston reaches top dead center during the compression stroke. Some
engine models specify ignition at 18=F8, 23=F8, or 25=F8 before top dead
center. If ignition of the fuel/air mixture occurs before the scheduled
point in the operational sequence of events, preignition exists and the
compression stroke continues as the burning fuel/air mixture is trying
to expand. This subjects the combustion chamber and pistons to
temperatures and pressures far in excess of those experienced during
normal combustion. These excessive temperatures and pressures cause
damage to pistons and valves. In some cases both burned pistons and
stretched valves will be found in an engine which has been subjected to
preignition.

Considering the millions of hours flown each year in piston powered
aircraft, engine damage from detonation and preignition is quite rare.
The infrequency of the happening means little if your engine is the one
affected. Therefore it seems appropriate to look more closely at some
of the factors which lead to detonation and preignition.

The possibility of overboost is a characteristic of all supercharged
and turbocharged engines. Generally, overboost means the application of
manifold pressure which exceeds the limit specified by the
manufacturer. Early versions of the manually controlled turbocharger
allowed quite a few pilots to inadvertently induce damage by overboost.
With this system, the turbocharger wastegate was normally left full
open for takeoff; full throttle would produce 28 to 30 inches of
manifold pressure. After takeoff at full throttle, gradual closing of
the wastegate would slowly increase turbocharger speed and manifold
pressure to maintain climb power to cruise altitude or to the critical
altitude of the engine. The system worked fine until the wastegate was
inadvertently left in the closed position. If the pilot then applied
full throttle for takeoff or a go-round, it could produce 60 inches or
more of manifold pressure and failure of the engine.

More recent turbocharger installations usually include a pressure
relief valve and/or an automatic wastegate control which helps to avoid
the possibility of overboost. Even with these protective devices, it is
still possible to overboost by rapid throttle operation and/or
inattention to limiting manifold pressures at low engine speeds.

Automatic controllers may not be capable of preventing overboost if
full throttle operation is attempted before engine oil is warmed up
sufficiently. Textron Lycoming Service Instruction 369F addresses the
problem of overboost and recommends, depending on the severity and
duration of the overboost, a log book entry, engine inspection, or
complete engine overhaul including replacement of the crankshaft.

As stated earlier, ignition of the fuel/air mixture must take place at
precisely the right time. A spark plug which has been dropped, or
damaged in some other way, may induce preignition by causing a "hot
spot" in the combustion chamber which self-ignites the fuel/air
mixture. This could also occur from use of unapproved spark plugs.
Flight with defective magnetos or flight in excess of certified
aircraft limits may allow cross firing within the magneto, improperly
sequenced ignition of the fuel/air mixture, and engine damage. Proper
magneto to engine timing is also an important factor. The timing is
affected by wear and therefore should be checked and reset at specified
intervals. Regular, meticulous spark plug and magneto maintenance will
help to avoid preignition and possible engine damage from these
sources.

Although overboost and incorrect ignition timing are causes of induced
engine damage, this damage can often be attributed to fuel and the
fuel/air mixture. The first problem related to fuel is simply having
improper fuel in the aircraft tanks. A piston powered aircraft refueled
with jet fuel would have a fuel blend with greatly reduced octane
level. A piston engine should not be started when even small amounts of
jet fuel have been added to aviation gasoline because engine
contamination and detonation are likely; attempted flight under these
conditions will certainly result in destructive detonation and
preignition. The use of 80 octane aviation fuel in an engine certified
for 100 octane aviation fuel will produce similar results.

The lubricating oil may be a source of octane reducing fuel
contamination. Excessively worn piston rings may allow enough oil into
the combustion chamber to dilute the fuel/air mixture. The dilution
will reduce the octane rating of the fuel and can lead to detonation
and engine damage. While this scenario is not entirely typical of the
engine that uses large amounts of oil because of worn or broken piston
rings, it is possible for this situation to occur.

Even the use of 100 octane fuel in an engine in good mechanical
condition does not eliminate all the possibilities of induced engine
damage. Most engines operated at takeoff power or at a power setting in
the high cruise range need a relatively rich fuel/air mixture to help
cool the engine and reduce possibilities of detonation. Since lean
fuel/air mixtures and high power settings promote detonation, it is
recommended that Lycoming engines not be leaned at power settings which
produce more than 75% of rated engine power unless this operation is
approved in the POH. The pilot, by simply leaning the mixture
excessively at power settings above the cruise ranges, may be
responsible for inducing the detonation and preignition which leads to
tuliped valves and burned pistons.

And finally, a small amount of dirt in the fuel system may be
responsible for clogging a fuel injector nozzle or nozzles. A partially
clogged fuel injection nozzle will reduce fuel flow to that cylinder
and will cause a lean fuel/air mixture. A nozzle which is partially
clogged in an aircraft that has a pressure operated fuel flow indicator
will cause that indicator to display a higher than normal fuel flow.
Leaning in an attempt to correct the high indicated fuel flow will
result in an even leaner mixture in the affected cylinder. Again it is
possible that a burned piston or tuliped valve will be the final
result.

Understanding and avoiding those factors which lead to induced engine
damage is certainly preferable to the discovery of tuliped valves or
burned pistons in your engine. This entire discussion is aimed at
promoting an understanding which will allow pilots and maintenance
personnel to direct their efforts to those elements which will reduce
the possibility of induced engine damage. Observing the refueling of
the aircraft and checking the fuel system for indications of
contamination are tasks expected of the pilot. Meticulous management of
power and fuel/air mixture as recommended by the POH is also a pilot
activity which will enhance the possibility of avoiding induced damage.

Maintenance personnel play an equally important role. Troubleshooting a
fuel injected engine for rough idle may lead to the cleaning or
changing of partially clogged fuel injector nozzles. Damage could
result if the engine were operated at takeoff or climb power with
reduced fuel flow to one or more cylinders. A close check of magneto
timing and magneto condition at regular inspection intervals will help
to insure the continued satisfactory operation of any engine.

There are some "after-the-damage" factors that maintenance personnel
should consider. Suppose that a power loss has been reported. A
compression check reveals low compression; a stretched or tuliped valve
may be found. This is an indication that the engine has experienced
detonation and preignition. A borescope examination should be conducted
to see if a piston has been burned. A burned piston often results in
damage to cylinder walls and piston skirts; it also may contaminate the
engine with metal particles. There is no healing process for this
damage. In some cases it is possible to repair the engine by removing
the metal contamination from the engine and oil system, including the
oil cooler, and by replacing all damaged parts, but often it is
necessary to replace the entire engine. If an engine is to be repaired,
it must be remembered that repairing the damage is not enough; the
cause of the malfunction which induced detonation and preignition must
also be found and corrected. Did a magneto malfunction produce ignition
outside the normal firing sequence? Were manufacturer-approved spark
plugs installed in the engine? Did a cracked spark plug induce
preignition? Was an approved fuel used, and if so, is there evidence of
fuel contamination? Whatever the malfunction, it must be corrected
along with the damage or the same problem could reoccur.

To conclude, induced damage in the form of tuliped valves and burned
pistons can usually be avoided by understanding the sequence of events
which lead to this form of engine damage. Careful attention to detail
is required of pilots and maintenance personnel. Compared to the
expense of repairing or replacing a damaged engine, it is worth the
time and effort necessary to avoid induced engine damage.

  #6  
Old August 10th 05, 08:34 PM
Andy
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What, if any, of this relates to the original posting. Could you
perhaps post only those parts.

Before I passed comments on anyone's letdown procedure I'd like to know
what airplane/engine was being used.

I've flown several tugs and jump planes and I operated them all the way
the owner, or chief tow pilot, asked me to do. There are certainly
differences of opinion but it's seems unwise to know better than the
person who will pay for the repairs. ( No I had a much better way than
you showed me, sorry about the cracked jug. Let me know when you want
me to fly next)


Andy

  #7  
Old August 11th 05, 04:25 AM
BTIZ
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Thanx Andy.. I was wondering what his problem was also..
we get TBO and better from our Pawnee O-540 with this operating technique.
Glider Tow is hard duty on an engine.

BT

"Andy" wrote in message
ups.com...
What, if any, of this relates to the original posting. Could you
perhaps post only those parts.

Before I passed comments on anyone's letdown procedure I'd like to know
what airplane/engine was being used.

I've flown several tugs and jump planes and I operated them all the way
the owner, or chief tow pilot, asked me to do. There are certainly
differences of opinion but it's seems unwise to know better than the
person who will pay for the repairs. ( No I had a much better way than
you showed me, sorry about the cracked jug. Let me know when you want
me to fly next)


Andy



 




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