Pawnee problems

"bds" wrote in message
...

"Andy" wrote in message
...
What do you mean by a power surge? To me a surge is a momentary
increase in something. Are you experiencing a momentary power
increase, or a momentary power reduction?

The PA28-180 I used to be a partner in experienced a few 200 rpm drops
at the most inconvenient times. It scared off 3 of the partners and
it took several years for me to find the problem. I became more
motivated after it happened to me during a high density altitude
takeoff.

It was caused by a P lead contact being shorter than spec. The P lead
contact pushes against the contact spring mounted on the mag
capacitor. The short contact didn't push the spring contact far
enough away from the rear mag cover and allowed it to sometimes arc to
the case. Dismantled the P lead contact and replaced the spacer tube
with a longer one from a spare P lead (same length as the other mag)
and the power loss problem has not been seen in the last 3 or more
years.

New mags, or even a complete engine rebuild, would not have fixed the
problem unless the P leads were also replaced. This wouldn't normally
be done since they are part of the aircraft wiring harness.

Really bad mag design since if the P lead comes unscrewed the mag is
shorted internally and behaves just as if the P lead is grounded.

Andy (GY)

Sorry - it is a sudden loss, not a sudden increase. Poor wording.

Thanks for the tip - we will take a look at this this weekend.

Bruce


To follow up on the P-Lead issue.. any short (break) in the old wire will
cause the problem as a bad P connector.
We traced one "mag" problem to the mag switch on the panel. Our PA-25-235
has two independent flip switches on the panel for the mags, one was bad.

We rebuilt one Mag 3 times in one day, it could not hold an internal timing.
We replaced the whole case.

BT

On Apr 29, 11:37*am, "Tim Mara" wrote:
we had a similar problem years ago with a Pawnee 150.....would usually run
up fine on the ground but every couple of flights just after take-off when
you were expecting it to get up to full RPM it would only get about 80% of
the normal power ....unfortunately this was about the time you'd be lifting
off with a glider on behind.....the mechanic replaced carburetor float,
changed plugs, adjusted magneto's .all resulted in the same behavior...then
finally we checked the exhaust and sure enough a baffle had broken off
inside and sometimes would lay harmlessly away from the muffler inlet or
exit and other times would just simply end up plugging the exhaust end of
the muffler.....what goes in has to come out or less can come in and
consequently when less comes in and out the motor doesn't work as
intended...you might also notice a higher than normal engine temperature as
an indication that this is the problem ...anyway, this is my bet!
tim

Please visit the Wings & Wheels website atwww.wingsandwheels.com

"bds" wrote in message

...





Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. *We are having intermittent problems with power surges
that can be significant and are sometimes even noticeable from the ground.
The problem will occur one day on one tow, and then sometimes disappear
for several weekends. *One tow pilot seems to think that the problem seems
to appear any time the airplane experiences noticeable negative Gs.

In the past year or so we have replaced or rebuilt the following:

Carburetor
Ignition harness
Fuel shut off valve
Exhaust system
Mags
Spark plugs

We have also cleaned the gascolator, checked the fuel tank for
obstructions, and checked the vent system.

The engine has about 1300 SMOH and early last season we did have a stuck
valve in one of the cylinders. *We switched from auto fuel to 100LL at the
beginning of last season and my recollection is that the problem might
have started or gotten worse shortly afterwards.

I am thinking that the engine is probably due for a top overhaul, but the
problem doesn't sound like sticky valves because some of the other tow
pilots say that the engine runs smoothly when the larger power losses
occur. I have never experienced one of the major losses myself, but I have
had a few of the 100 rpm drops which cleared up after about 15 seconds or
so. When it happened to me I did notice a very slight engine roughness
which does sound like it could be a stuck valve. *It also makes sense that
if it is sticky valves that the problem would get worse running on 100LL.

Any comments or suggestions on what to do next? *Two mechanics have looked
at it and have not been able to resolve the problem as yet, and we need
our Pawnee back on the flight line. *We are close to doing the top
overhaul as a next step.

Thanks,
Bruce

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Tim, that's a good guess.

Whacking the muffler gently with a rubber hammer can make a loose
baffle rattle enough to be heard. Airplane mufflers are made from
real thin metal and they do crack both inside and outside.

Bill Daniels

It sounds like a prop governor issue to me. A constant speed prop
will keep the RPM's constant until the engine power is reduced so
severely that the prop pitch has hit the limits. Surging RPM's on a
constant speed prop are usually the result of a bad prop governor or
fluctuating engine oil pressure (which the prop governor uses to
function). For example run the engine at 23" manifold pressure and
2400 RPM's. Reduce the manifold pressure to 20" and the RPM's will
remain constant. Continue reducing the engine power and eventually
when the engine is nearing idle the RPM's will start reducing (the
prop pitch has hit its limit of movement).

On Apr 29, 9:44*pm, lanebush wrote:
It sounds like a prop governor issue to me. *A constant speed prop
will keep the RPM's constant until the engine power is reduced so
severely that the prop pitch has hit the limits. *Surging RPM's on a
constant speed prop are usually the result of a bad prop governor or
fluctuating engine oil pressure (which the prop governor uses to
function). *For example run the engine at 23" manifold pressure and
2400 RPM's. *Reduce the manifold pressure to 20" and the RPM's will
remain constant. *Continue reducing the engine power and eventually
when the engine is nearing idle the RPM's will start reducing (the
prop pitch has hit its limit of movement).


Pawnees have fixed pitch propellers. This could not be the problem.

On Apr 30, 6:20*am, Guy Byars wrote:
On Apr 29, 9:44*pm, lanebush wrote:

It sounds like a prop governor issue to me. *A constant speed prop
will keep the RPM's constant until the engine power is reduced so
severely that the prop pitch has hit the limits. *Surging RPM's on a
constant speed prop are usually the result of a bad prop governor or
fluctuating engine oil pressure (which the prop governor uses to
function). *For example run the engine at 23" manifold pressure and
2400 RPM's. *Reduce the manifold pressure to 20" and the RPM's will
remain constant. *Continue reducing the engine power and eventually
when the engine is nearing idle the RPM's will start reducing (the
prop pitch has hit its limit of movement).

Pawnees have fixed pitch propellers. *This could not be the problem.

Depends on the Pawnee. IIRC the 150 and 235 have a FP prop, and the
260 (like the one in our club) has a constant speed prop.

Pete

"vontresc" wrote ...
On Apr 30, 6:20 am, Guy Byars wrote:
On Apr 29, 9:44 pm, lanebush wrote:

It sounds like a prop governor issue to me. A constant speed prop
will keep the RPM's constant until the engine power is reduced so
severely that the prop pitch has hit the limits. Surging RPM's on a
constant speed prop are usually the result of a bad prop governor or
fluctuating engine oil pressure (which the prop governor uses to
function). For example run the engine at 23" manifold pressure and
2400 RPM's. Reduce the manifold pressure to 20" and the RPM's will
remain constant. Continue reducing the engine power and eventually
when the engine is nearing idle the RPM's will start reducing (the
prop pitch has hit its limit of movement).

Pawnees have fixed pitch propellers. This could not be the problem.

Depends on the Pawnee. IIRC the 150 and 235 have a FP prop, and the
260 (like the one in our club) has a constant speed prop.

Pete
*************

The one in question is fixed-pitch. I should have mentioned that.

Bruce

On Apr 29, 4:56*pm, "BT" wrote:
To follow up on the P-Lead issue.. any short (break) in the old wire will
cause the problem as a bad P connector.

That's confusing to me perhaps because I think short and break are
opposite conditions. Maybe you meant a break in the P lead insulation?

A short to ground in the P lead will disable the mag, an open circuit
P lead will leave the mag hot but will not make any difference to how
the engine runs. Similarly an intermittent open in the P lead will
have no effect on engine performance. An intermittent short in the P
lead could cause the problems described by the OP.

In the case of my PA28-180 an incorrectly fabricated P lead connector
allowed the P lead circuit to intermittently short inside the mag
case.

Andy

exactly... as you stated.. an intermittent short in the P-lead, in our case
it was in the switch on the panel.
caused intermittent engine rpm power loss.. and when happening within a
short span of elapsed time..
a sense of "engine roughness" and 150rpm drop/fluctuation

Most older Pawnee's have two separate switches on the panel, not a keyed
rotary switch.

"Andy" wrote in message
...
On Apr 29, 4:56 pm, "BT" wrote:
To follow up on the P-Lead issue.. any short (break) in the old wire will
cause the problem as a bad P connector.

That's confusing to me perhaps because I think short and break are
opposite conditions. Maybe you meant a break in the P lead insulation?

A short to ground in the P lead will disable the mag, an open circuit
P lead will leave the mag hot but will not make any difference to how
the engine runs. Similarly an intermittent open in the P lead will
have no effect on engine performance. An intermittent short in the P
lead could cause the problems described by the OP.

In the case of my PA28-180 an incorrectly fabricated P lead connector
allowed the P lead circuit to intermittently short inside the mag
case.

Andy

"vontresc" wrote in message
...
On Apr 30, 6:20 am, Guy Byars wrote:
On Apr 29, 9:44 pm, lanebush wrote:

It sounds like a prop governor issue to me. A constant speed prop
will keep the RPM's constant until the engine power is reduced so
severely that the prop pitch has hit the limits. Surging RPM's on a
constant speed prop are usually the result of a bad prop governor or
fluctuating engine oil pressure (which the prop governor uses to
function). For example run the engine at 23" manifold pressure and
2400 RPM's. Reduce the manifold pressure to 20" and the RPM's will
remain constant. Continue reducing the engine power and eventually
when the engine is nearing idle the RPM's will start reducing (the
prop pitch has hit its limit of movement).

Pawnees have fixed pitch propellers. This could not be the problem.

Depends on the Pawnee. IIRC the 150 and 235 have a FP prop, and the
260 (like the one in our club) has a constant speed prop.

Pete

Correct!!