About PA28-161 checklist
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About PA28-161 checklist
I'm confused about PA28-161 checklist.
1. What does "throttle pump twice" in PA28-161 engine start checklist mean?
2. Why I have to set throttle to 1200 RPM before mixture idle cutoff in shutdown checklist?
1. What does "throttle pump twice" in PA28-161 engine start checklist mean?
2. Why I have to set throttle to 1200 RPM before mixture idle cutoff in shutdown checklist?
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If the engine does not have a separate primer, pumping the throttle will actuate the accelerator pump on the carburettor, and somewhat prime the engine. This is a non ideal way of doing it, and can be troublesome (carb fire). If the airplane has a separate primer, I would, instead, move the throttle smoothly once full stroke, just to assure it has full travel.
As for setting 1200 RPM before shutdown, they probably just want the engine running smoothly as it stops running, you could shut down at slow idle, and it would probably be fine too...
In truth, neither of those are actually "checklist" items, they have been merged into the checklist as operating instructions. The operating instructions are separate to a checklist, and should be understood by the pilot as a technique for correct operation, rather than "killer items".
As for setting 1200 RPM before shutdown, they probably just want the engine running smoothly as it stops running, you could shut down at slow idle, and it would probably be fine too...
In truth, neither of those are actually "checklist" items, they have been merged into the checklist as operating instructions. The operating instructions are separate to a checklist, and should be understood by the pilot as a technique for correct operation, rather than "killer items".
I own a PA-28-180 and have flown it over 1,000 hours. I always start a cold engine with no prime and pump the throttle while cranking. It usually takes one, sometime two pumps, to start it. Cranking time is seldom more than 2-3 seconds for an engine that may not have run for several weeks.
A couple of pumps while cranking introduces zero risk of of an induction system fire as all the fuel is drawn into the cylinders. Use of primer, or pumping before cranking, does introduce a significant risk of induction system fire if either is done to excess.
Pilots who do not use the "pump while cranking" technique will often advise against it. Not one of them can show that it was ever the cause of an induction fire. I don't think there is any doubt that induction fires have been caused by excessive primer use.
The ideal method of starting an engine is one that always works with the minimum cranking time.
A couple of pumps while cranking introduces zero risk of of an induction system fire as all the fuel is drawn into the cylinders. Use of primer, or pumping before cranking, does introduce a significant risk of induction system fire if either is done to excess.
Pilots who do not use the "pump while cranking" technique will often advise against it. Not one of them can show that it was ever the cause of an induction fire. I don't think there is any doubt that induction fires have been caused by excessive primer use.
The ideal method of starting an engine is one that always works with the minimum cranking time.
Last edited by EXDAC; 16th Jul 2023 at 02:33.
I beg to differ. Although pumping the throttle once you've started cranking and during cranking is a pretty good bet (since as you say the pots are sucking that all up), you'd need to prime a ridiculous amount to start an induction fire in a PA28. There are only 3 priming tubes and they're tiny like injection tubes. More importantly, they squirt so close to the 3 inlets that fuel would have a hard time finding it's way all the way back down the inlet pipes and, even then, it would have get through the "fuel make it happener" to start a fire underneath.
In the UK, because of our weather, we need much more priming for starting (but never more than 5 or 6 strokes), which is why virtually all our light aircraft have primers.
I've witnessed several induction fires over the years, all of them caused by pilots who use the accelerator pump before turning the starter. Some of them were instructors....... .
One backfire and the fuel that's naturally falling out of the carb is set off.
Throttle set at the 1200rpm position (and left in that position until combustion occurs), 2-5 primes and all is good.
In the UK, because of our weather, we need much more priming for starting (but never more than 5 or 6 strokes), which is why virtually all our light aircraft have primers.
I've witnessed several induction fires over the years, all of them caused by pilots who use the accelerator pump before turning the starter. Some of them were instructors....... .
One backfire and the fuel that's naturally falling out of the carb is set off.
Throttle set at the 1200rpm position (and left in that position until combustion occurs), 2-5 primes and all is good.
The argument over throttle or 'primer' for starting has been well rehearsed in these threads but is unlikely to come to an agreed conclusion. An earlier comment suggested that the primer if installed fed three cylinders, which may be the case, but it is not unusual for just one cylinder to be fed which is too often unsatisfactory. In over 40 years of operating light piston aircraft I have witnessed only one carburettor fire but many exhaust fires. These were all caused by the over priming of a hot engine with a hot exhaust. In these cases it mattered little what technique was used. Do not prime at all using either method if the engine is hot.
Light piston engines can run too rich during idle and whilst taxing for extended periods particularly in hot weather. Extensive taxing periods is common at some airfields to/from parking areas so keeping 1200rpm helps. Setting 1200rpm and then slowly moving the idle cut-off to shut down will leave the plugs in good shape for the next start. If the hot engine will not start suspect the plugs are wet or dirty, preventing a good spark, as your first consideration. Only prime a hot engine when proven as being necessary.
Light piston engines can run too rich during idle and whilst taxing for extended periods particularly in hot weather. Extensive taxing periods is common at some airfields to/from parking areas so keeping 1200rpm helps. Setting 1200rpm and then slowly moving the idle cut-off to shut down will leave the plugs in good shape for the next start. If the hot engine will not start suspect the plugs are wet or dirty, preventing a good spark, as your first consideration. Only prime a hot engine when proven as being necessary.
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I owned a C 150, which I had flown for 3500 hours, and accidentally set fire to it due to pumping the throttle during a cold start done wrong (after properly pump priming it)....
Definitely!
I have had two carb airbox fires resulting from pumping the throttle while starting, to try to "catch" an engine to run during a start on a cold day. Both required the use of a fire extinguisher even though I did keep cranking to pull the fire through the engine, Once the battery is discharged, and the fire persists, you have to get out and extinguish it. Ever notice the established warbird operators have a big wheeled fire extinguisher nearby for starts - they have learned the hard way....
The accelerator pump on the carburettor does squirt raw fuel into the induction airflow stream, which is it's intended function - while the engine is running already! The carburettor manufacturer did not include that function for starting - here's why:
Prior to accelerator pumps on carbs (and I've flown a few, too rapid application of throttle opened the air passage to the engine too much too fast, and the engine would lean and quit before it could draw enough fuel to increase it's power. The result could be (which I learned flying a Piper J2 on skis on the ice) the engine stops when you open the throttle [too quickly], and you have to stop in the remaining distance, rather than doing a touch and go. The you have to get out, and hand prop it on the ice, with no one to help you, and no way to tie it so it does not move, while you get back in. (now I carry rope and an ice screw for ice flying - but I diverge....). An accelerator pump eliminates the risk of the engine stopping when the pilot applies power too quickly. It does this by squirting a stream of liquid fuel into the carburettor throat. Liquid fuel does not burn well. Atomized fuel vapour burns very well.
During a start, particularly when cold, having the fuel needed to start with be a vapour makes the start much more certain (and fuel vapour does not fall backward by gravity into the airbox and pool there). So, have you noticed that engine pump primers seem much more stiff to push in than the throttle? That's because the engine primer is creating much greater pressure in the priming fuel, which is then being forced through a much smaller wedge orifice in the primer nozzle. That results in a noticeably finer mist of fuel vapour, rather than a stream of liquid fuel. If the accelerator pump on the carburettor were to be able to produce fuel vapour by high pressure, the flow would be inadquate to accelerate a running engine when the pilot jams the throttle, and the throttle would be really stiff to move!
So despite the fact that a carburettor accelerator pump will seem to prime an engine, it's a bad, and unsafe way to do it if a proper pump primer is installed. But - yes, I have flown planes which did not have a pump primer, so it was the only way to prime. Not good, but the only choice in that case.
If there is a pump primer, use it, rather than pumping the throttle.
Do move the throttle one full stroke immediately prior to start on old airplanes you don't know. Nothing whatever to do with priming, but rather some very old airplanes have a less than ideal linkage between the throttle knob and the carb butterfly - it can slip. The throttle arm on the carburettor shaft was a "pinch clamp" on the shaft, rather than the much more common radial spline connection. The result can be (and has happened to me) that if you have landed with some carb ice, and closed the throttle, the butterfly can jam on the ice, and the pinch clamp slips. The throttle control will move to idle, but the butterfly will not. The engine will produce enough power to [seem to] taxi in normally, and shut down. Then, while you stop for fuel, and the carb ice melts. You next start ('cause you did no think to check the throttle full travel before start) results in the engine going straight to 1800 RPM, at what you think should be the throttle control idle position. It has happened to me, and is very alarming, particularly on an icy apron with other planes around!
I have learned a lot of hard lessons in airplanes. Sometimes this results in refining my understanding of how systems work, to try to operate them with the greatest possible safety, rather than just doing something the [apparently] easy way. As the saying goes, if you think doing it the more safe way takes too much time, try having an accident!
So i can skip the throttle pump action if the engine can be started directly?
I have had two carb airbox fires resulting from pumping the throttle while starting, to try to "catch" an engine to run during a start on a cold day. Both required the use of a fire extinguisher even though I did keep cranking to pull the fire through the engine, Once the battery is discharged, and the fire persists, you have to get out and extinguish it. Ever notice the established warbird operators have a big wheeled fire extinguisher nearby for starts - they have learned the hard way....
The accelerator pump on the carburettor does squirt raw fuel into the induction airflow stream, which is it's intended function - while the engine is running already! The carburettor manufacturer did not include that function for starting - here's why:
Prior to accelerator pumps on carbs (and I've flown a few, too rapid application of throttle opened the air passage to the engine too much too fast, and the engine would lean and quit before it could draw enough fuel to increase it's power. The result could be (which I learned flying a Piper J2 on skis on the ice) the engine stops when you open the throttle [too quickly], and you have to stop in the remaining distance, rather than doing a touch and go. The you have to get out, and hand prop it on the ice, with no one to help you, and no way to tie it so it does not move, while you get back in. (now I carry rope and an ice screw for ice flying - but I diverge....). An accelerator pump eliminates the risk of the engine stopping when the pilot applies power too quickly. It does this by squirting a stream of liquid fuel into the carburettor throat. Liquid fuel does not burn well. Atomized fuel vapour burns very well.
During a start, particularly when cold, having the fuel needed to start with be a vapour makes the start much more certain (and fuel vapour does not fall backward by gravity into the airbox and pool there). So, have you noticed that engine pump primers seem much more stiff to push in than the throttle? That's because the engine primer is creating much greater pressure in the priming fuel, which is then being forced through a much smaller wedge orifice in the primer nozzle. That results in a noticeably finer mist of fuel vapour, rather than a stream of liquid fuel. If the accelerator pump on the carburettor were to be able to produce fuel vapour by high pressure, the flow would be inadquate to accelerate a running engine when the pilot jams the throttle, and the throttle would be really stiff to move!
So despite the fact that a carburettor accelerator pump will seem to prime an engine, it's a bad, and unsafe way to do it if a proper pump primer is installed. But - yes, I have flown planes which did not have a pump primer, so it was the only way to prime. Not good, but the only choice in that case.
If there is a pump primer, use it, rather than pumping the throttle.
Do move the throttle one full stroke immediately prior to start on old airplanes you don't know. Nothing whatever to do with priming, but rather some very old airplanes have a less than ideal linkage between the throttle knob and the carb butterfly - it can slip. The throttle arm on the carburettor shaft was a "pinch clamp" on the shaft, rather than the much more common radial spline connection. The result can be (and has happened to me) that if you have landed with some carb ice, and closed the throttle, the butterfly can jam on the ice, and the pinch clamp slips. The throttle control will move to idle, but the butterfly will not. The engine will produce enough power to [seem to] taxi in normally, and shut down. Then, while you stop for fuel, and the carb ice melts. You next start ('cause you did no think to check the throttle full travel before start) results in the engine going straight to 1800 RPM, at what you think should be the throttle control idle position. It has happened to me, and is very alarming, particularly on an icy apron with other planes around!
I have learned a lot of hard lessons in airplanes. Sometimes this results in refining my understanding of how systems work, to try to operate them with the greatest possible safety, rather than just doing something the [apparently] easy way. As the saying goes, if you think doing it the more safe way takes too much time, try having an accident!
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Its to reduce/prevent spark plug fouling.
Aircraft and engine maintenance techniques benefit from frequent refinement in the form of service bulletins, revised maintenance manuals, and even AD's. It is much less common that flight manuals and checklists benefit from revision to present improved techniques.
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Then how about:
1. Annunciators Test in Run-up checklist
2. Electr. Equip. Off in Shutdown checklist?
How to test annunciators and what do i need to shutdown to meet "Electr. Rquip. Off"?
1. Annunciators Test in Run-up checklist
2. Electr. Equip. Off in Shutdown checklist?
How to test annunciators and what do i need to shutdown to meet "Electr. Rquip. Off"?
Moderator
How to test annunciators and what do i need to shutdown to meet "Electr. Rquip. Off"?
The 1200 rpm rule is also appropriate to aircraft equipped with dynamos rather than alternators. Dynamos will typically not charge your battery when below 1200 rpm and during an extensive taxi the battery may become depleted. After landing check lists may also include reducing electrical loads before taxxing to park. Battery condition is always an issue with aeroplanes with large cylinders to turn plus the drag and inertia of the propeller to start. Maintaining peak charge of the battery as best you can is important. Even with an alternator the battery charge will unlikely be sufficient, indicated by the red warning light, when at idle rpm.
As above, a lot of the issues you're raising in this thread are things that should be discussed with an instructor, either during basic training or during a checkout on a, for you, new type. Could you tell us a bit more about the background to your questions perhaps? Are you learning a new type to fly on a PPL or is this a theoretical exercise?
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A masterly response from from Pilot DAR and excellent point about consulting your instructor! But ....
Instructors sometimes develop 'proprietary' starting techniques in response to poor starting engines. Vigourous throttle pumping can be one of these and inevitably students immitate them. How often do we see planes on the flightline cranking , and cranking, and cranking...? Sometimes it's the fault of the pilot, but more often it's poor maintenance. In UK at least instructors are often reluctant to 'go tech' on an aircraft that doesn't like to start and prefer to struggle on.
Now it's true that the 1930's magnetos that we are stuck with (in most certified aircraft, at least) are poor starting performers compared with electronic ignition and subject to frequent inspections and overhauls. They suffer from mechanical issues and are expensive to rectify, often having to be sent off for expert attention. Owners are often reluctant to incur the expense involved when "it wil go eventually". Until that day when it doesn't...
When a -161 that I was involved with went on fire with a renter pilot, we found that not only was starting excricable, but that throttle pumping had been taught. After recovering the aeroplane (saved by a nearby pilot with a fire extinguisher) we found that just 3 throttle pumps were sufficient to have liquid fuel running down the inside of the cowling. But the root cause was poor starting, allowed by casual maintenance.
Instructors sometimes develop 'proprietary' starting techniques in response to poor starting engines. Vigourous throttle pumping can be one of these and inevitably students immitate them. How often do we see planes on the flightline cranking , and cranking, and cranking...? Sometimes it's the fault of the pilot, but more often it's poor maintenance. In UK at least instructors are often reluctant to 'go tech' on an aircraft that doesn't like to start and prefer to struggle on.
Now it's true that the 1930's magnetos that we are stuck with (in most certified aircraft, at least) are poor starting performers compared with electronic ignition and subject to frequent inspections and overhauls. They suffer from mechanical issues and are expensive to rectify, often having to be sent off for expert attention. Owners are often reluctant to incur the expense involved when "it wil go eventually". Until that day when it doesn't...
When a -161 that I was involved with went on fire with a renter pilot, we found that not only was starting excricable, but that throttle pumping had been taught. After recovering the aeroplane (saved by a nearby pilot with a fire extinguisher) we found that just 3 throttle pumps were sufficient to have liquid fuel running down the inside of the cowling. But the root cause was poor starting, allowed by casual maintenance.
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But the root cause was poor starting, allowed by casual maintenance.
For any airplane with an MA-3 carburettor (C-150s, for example) the older (but better running) "two piece" venturi was vulnerable to a carb fire. You could have had a fire, and pulled it through the engine with the correct procedure to keep cranking, and still melt the inner portion of the venturi. If otherwise undected, the distorted inner venturi can change position in the carb throat, and then you definitely have an engine operating problem! The two piece venturis were AD'd to be changed to a single piece venturi decades back, but the new venturi worked poorly, so the AD was withdrawn. When I'd had a card fire in my C150 a number of years back, I inspected the carb, and had indeed distorted the inner venturi, and had to replace it.
So, avoid carb fires, and if there is been one, don't just go flying 'cause you got it extinguished, the plane needs an airbox off inspection...
