Northwest727 From United States of America, joined Jul 2005, 491 posts, RR: 1 Posted (3 years 1 month 2 weeks 4 days 5 hours ago) and read 5102 times:
My instructor and I have hit a snag, and no one else has a clear answer. I can't find it anywhere in the PIM, nor can I find the answer online. Simply put, why does the priming system on the Piper Seminole inject fuel into cylinders 1, 2, and 4, but not 3? While the EGT probe is taken off of the exhaust of cylinder 3, most of the other instructors at my school seem to discount that that has anything to do with the lack of a primer nozzle in that cylinder.
Fly2HMO From , joined Dec 1969, posts, RR: Reply 1, posted (3 years 1 month 2 weeks 4 days 5 hours ago) and read 5078 times:
Oh man, I used to know exactly why, but can't remember for the life of me . I can say I did not find out until after I paid our MX department several visits just to look around the planes with all the hatches opened up and harrassed the mechanics with tons of questions. So your best bet is to ask your MX guys.
I've learned over the years as a student: never trust what an instructor tells you about aircraft systems unless he is an A&P with significant experience working on that specific plane.
KELPkid From United States of America, joined Nov 2005, 5934 posts, RR: 4 Reply 2, posted (3 years 1 month 2 weeks 4 days 1 hour ago) and read 4998 times:
Just a wild-ass guess here, but it seems that if you don't prime all four, then it would be impossible to overprime the engine to the point where at least one cylinder couldn't fire...
BTW, my very first flight instructor (in the Cessna 150) was dead-set against using the primer. He claimed that it was the #1 cause of ground fires in GA planes...he also showed me that in the 150, "pumping" the throttle while cranking the engine did the same thing (the accelerator pump squirted extra fuel into the cylinders...). He was also an A&P/IA.
I didn't start priming engines until I got my instrument, and all the instructors at the 141 school got on me for not following the cold start check list to the "T"
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
alaska737 From United States of America, joined Feb 2006, 1056 posts, RR: 6 Reply 4, posted (3 years 1 month 2 weeks 3 days 23 hours ago) and read 4966 times:
Quoting KELPkid (Reply 2): .he also showed me that in the 150, "pumping" the throttle while cranking the engine did the same thing (the accelerator pump squirted extra fuel into the cylinders...). He was also an A&P/IA.
One of my ground instructors told me to never do that. Each person has their own idea of whats right, I prime, but thats just me.
BTW, my very first flight instructor (in the Cessna 150) was dead-set against using the primer. He claimed that it was the #1 cause of ground fires in GA planes...he also showed me that in the 150, "pumping" the throttle while cranking the engine did the same thing (the accelerator pump squirted extra fuel into the cylinders...). He was also an A&P/IA.
Makes sense.
Quoting FighterPilot (Reply 3): To help from flooding the engine. It'll allow the third to still be "dry."
I'm almost certain that wasn't the main reason. Even if just one cylinder floods, your whole engine is, for all intents and purposes, flooded.
All I remember was that it was a stupid reason. I do remember thinking "well, duh!" after the MX guys told me why, but I still can't recall why
Quoting alaska737 (Reply 4):
One of my ground instructors told me to never do that.
It does more harm than good on fuel injected engines, depending on the situation. On carburated ones not so much. I never ever used the primer in my 65+ hours in the Seminole, not even when it was -5C out.
The only time you might need to use the primer/aux pump on a carburated engine would be if the plane had been sitting for a long time without flying and fuel completely evaporated from the carburetor's metering tank.
That's why the O-360 on the Seminoles are so easy to start. Whereas the IO-360s on the C172 could be a real pain at times, specially if you got vapor lock. But there's many tricks to fix that easily, but should not be attempted by noob pilots
TSS From United States of America, joined Dec 2006, 2870 posts, RR: 5 Reply 6, posted (3 years 1 month 2 weeks 3 days 22 hours ago) and read 4922 times:
Quoting Northwest727 (Thread starter): While the EGT probe is taken off of the exhaust of cylinder 3, most of the other instructors at my school seem to discount that that has anything to do with the lack of a primer nozzle in that cylinder.
If I recall correctly, an excessively rich air/fuel mixture will cause lower cylinder (and exhaust) temperatures and black smoke due to incomplete combustion. It's not hard to imagine that if Seminoles had a primer on all cylinders and a pilot was a little heavy-handed with the primer, all that soot from unburned fuel would gather on the EGT probe causing first a false low-temperature reading by accumulating on the business end of the probe then later a false high-temperature reading as the soot burned off... if the unburned fuel didn't foul the probe entirely.
But I'm just conjecturing here; I could be entirely wrong.
[Edited 2010-04-08 20:33:48]
Able to kill active threads stone dead with a single post!
alaska737 From United States of America, joined Feb 2006, 1056 posts, RR: 6 Reply 7, posted (3 years 1 month 2 weeks 3 days 18 hours ago) and read 4856 times:
Quoting Fly2HMO (Reply 5): It does more harm than good on fuel injected engines, depending on the situation. On carburated ones not so much. I never ever used the primer in my 65+ hours in the Seminole, not even when it was -5C out.
I try not to prime the Seminole, the only time I absolutely had to was a cold, wet morning at Sacramento Executive Field. Good old 77 just wouldn't start up. Then you have the Eagles (Cessna 150E) which are carburated and they will not start in the morning unless they are primed, Eagle One especially.
Northwest727 From United States of America, joined Jul 2005, 491 posts, RR: 1 Reply 8, posted (3 years 1 month 2 weeks 3 days 9 hours ago) and read 4728 times:
I heard yet another "possible reason," in that the manifold pressure gauge gets its readings via a probe in the #3 intake manifold. The holes in the intake manifold for the primer nozzles are apparently the same size as for the manifold pressure probe, so Piper simply cut costs and cut one hole in all the manifolds. Then they installed primer nozzles in 1, 2, and 4, while 3 got the manifold pressure probe. I am not sure of the accuracy of this explanation, just simple putting out yet another "reason."
Fly2HMO From , joined Dec 1969, posts, RR: Reply 9, posted (3 years 1 month 2 weeks 3 days 9 hours ago) and read 4718 times:
Quoting Northwest727 (Reply 8): I heard yet another "possible reason," in that the manifold pressure gauge gets its readings via a probe in the #3 intake manifold. The holes in the intake manifold for the primer nozzles are apparently the same size as for the manifold pressure probe, so Piper simply cut costs and cut one hole in all the manifolds. Then they installed primer nozzles in 1, 2, and 4, while 3 got the manifold pressure probe. I am not sure of the accuracy of this explanation, just simple putting out yet another "reason."
KELPkid From United States of America, joined Nov 2005, 5934 posts, RR: 4 Reply 10, posted (3 years 1 month 2 weeks 3 days 8 hours ago) and read 4668 times:
Quoting Northwest727 (Reply 8): I heard yet another "possible reason," in that the manifold pressure gauge gets its readings via a probe in the #3 intake manifold. The holes in the intake manifold for the primer nozzles are apparently the same size as for the manifold pressure probe, so Piper simply cut costs and cut one hole in all the manifolds. Then they installed primer nozzles in 1, 2, and 4, while 3 got the manifold pressure probe. I am not sure of the accuracy of this explanation, just simple putting out yet another "reason."
Ironic. If Piper *really* wanted to save money, they could have made the left engine and the right engine turn the same way Then, of course, you would have had a proper twin with a critical engine. As I recall, though, the Seminole I (of which there are very few examples) had the engines turning the same way. The Seminole II and beyond went to counter-rotating engines.
One would think that you would want a popular training twin to actually have a critical engine...just like you would want a good single engine trainer to be spinnable (but with benign spin recovery characteristics). I just might be off my rocker here in that thinking...
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
FlyHossD From United States of America, joined Nov 2009, 545 posts, RR: 1 Reply 11, posted (3 years 1 month 2 weeks 3 days 6 hours ago) and read 4635 times:
Quoting KELPkid (Reply 10): As I recall, though, the Seminole I (of which there are very few examples) had the engines turning the same way. The Seminole II and beyond went to counter-rotating engines.
Me thinks you're thinking of the Seneca I vs the Seneca II.
The Seminole has always had Lycoming 0-360s. The Seneca I also had Lycomings, but Piper changed to counter-rotating Continental TSIO-360s starting with the Seneca II (and III...).
My statements do not represent my former employer or my current employer and are my opinions only.
Fly2HMO From , joined Dec 1969, posts, RR: Reply 12, posted (3 years 1 month 2 weeks 3 days 6 hours ago) and read 4629 times:
Quoting FlyHossD (Reply 11): Me thinks you're thinking of the Seneca I vs the Seneca II.
Me too. I've never heard of such a thing as a Seminole I or II. It's been basically the same plane and same engine since its inception.
Quoting KELPkid (Reply 10): Ironic. If Piper *really* wanted to save money, they could have made the left engine and the right engine turn the same way
The solution they used, so I was told by my MX department, is that the engine blocks are basically "mirror" images of each other, except for a few components. So no need for gears or any other complex reversing system.
KELPkid From United States of America, joined Nov 2005, 5934 posts, RR: 4 Reply 13, posted (3 years 1 month 2 weeks 3 days 5 hours ago) and read 4609 times:
Quoting FlyHossD (Reply 11): Me thinks you're thinking of the Seneca I vs the Seneca II.
N353SK From United States of America, joined Jun 2006, 739 posts, RR: 0 Reply 14, posted (3 years 1 month 2 weeks 2 days 21 hours ago) and read 4503 times:
One other thing to note if you've been studying the POH - it's wrong regarding which cylinders are primed. On each engine the front two are primed and so is the rear outboard cylinder. Both rear inboard cylinders are not primed. Basically, each engine is a mirror image of the other one. Sorry about not remembering the cylinder numbers!
freeze3192 From United States of America, joined Oct 2006, 126 posts, RR: 0 Reply 15, posted (3 years 1 month 2 weeks 2 days 15 hours ago) and read 4452 times:
Yeah, anyone know how the cylinders are arranged in the Seminole? I couldn't find it for the life of me in the PIM.
"A passenger bets his life that his pilot is a worthy heir to an ancient tradition of excellence and professionalism."
N353SK From United States of America, joined Jun 2006, 739 posts, RR: 0 Reply 16, posted (3 years 1 month 2 weeks 2 days 4 hours ago) and read 4384 times:
Quoting freeze3192 (Reply 15): Yeah, anyone know how the cylinders are arranged in the Seminole? I couldn't find it for the life of me in the PIM.
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From United States of America, joined Jul 2005, 491 posts, 
. I can say I did not find out until after I paid our MX department several visits just to look around the planes with all the hatches opened up and harrassed the mechanics with tons of questions. So your best bet is to ask your MX guys.
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