Hello,
I've never flown in complex aircraft, so a question for the guys who fly with constant speed props.
I suppose you also do a mag check at a certain RPM, but how can you see the drop, since I think the CSU would keep the RPM constant? Same for a carburetor ice check. Would you be able to detect ice forming through RPM decrease during flight and can you even do a carb heat check during preflight checks?
many thanks

Most CSU's will be fuel injected so I don't know too many aircraft with CSU's and carb heat.

Piper19

You do the mag check with the prop at the full 'increase' ( giving the finest pitch) setting, so it is in effect actnig as a fixed pitch prop.

I've only ever flown aircraft with fuel injection and CSUs, but you monitor the manfold pressure/fuel flow to detect trends affecting performance.

Most CSU's will be fuel injected so I don't know too many aircraft with CSU's and carb heat.

Yaks (M14 engine) are one example.

Actually you do the checks with the prop at full fine, but with an RPM well below the "full fine" RPM setting. So if full fine will give you 2700 RPM, normally, then you do the mag check & (if applicable, carb heat check) at 2000 or even less RPM. The prop can't go finer than full fine so as others have said, it effectively acts as a fixed pitch prop then.

Only when the power is increased and the RPMs exceed 2700 will the CSU unit kick in and make the blade pitch courses to maintain 2700 RPM.

Yaks (M14 engine) are one example.

Hence why I said 'Most' that means not all

using throttle lever in constant speed engines for run up will be just like running up a C150....

Supplementary question (I know the answer), how does a pilot lean for economy/power with a constant speed prop and no fancy engine monitoring system?

Fuel flow & EGT; I'm not considering that fancy engine management, but all of the admittedly small sample of CSU powered aircraft I've had have a combined manifold pressure / fuel flow gauge, and also EGT. (not multipoint etc).

Supplementary question (I know the answer), how does a pilot lean for economy/power with a constant speed prop and no fancy engine monitoring system?

Mark1234 is on to it....fuel flow and egt or if turbo, tit

Hence why I said 'Most' that means not all

Hence why I said "Yaks are one example". :rolleyes:

ha thanks everyone:ok:
now that I think about it, it's very obvious.

Piper,

A constant speed propeller must be in the "governing" range in order for the propeller governor to keep the RPM constant. Below that range, RPM is a function of throttle position, just like a fixed pitch airplane. When performing your engine runup at 1500 or 1700 RPM (the most common values for light airplanes when conducting mag checks and prop checks), your propeller isn't governing yet. It's acting as a fixed pitch propeller. Accordingly, when you ground a magneto (mag check), you'll see the appropriate RPM drop). More importantly, you're listening for the change and feeling for the vibration. This will tell you a lot about what's going with your engine.

In flight, a constant speed propeller works by changing the blade angle to increase or decrease the load on the propeller in order to help the propeller maintain a constant speed. If one is crusing with the throttle constant, and the propeller set for 2300 RPM, then imagine making some changes in flight without touching the controls. One pitches up, and decreases airspeed. As airspeed decreases, the propeller RPM on a fixed pitch airplane may also decrease somewhat...in a fixed pitch airplane your only choice would be to try to increase throttle. Not so in the constant speed propeller installation.

As you pitch up, the propeller governor allows the propeller blade angle to decrease ("fine pitch"), decreasing drag on the propeller, and allowing it to keep it's speed. In essence, you've just obtained a "climb prop."

Level the nose, and you begin to increase airspeed. As you do, if you kept the power constant on a fixed pitch propeller, the RPM would also increase. Not so with the constant speed installation. As your airspeed increases and your power and propeller settings stay constant, the propeller governor on the engine automatically increases blade angle for you...and the engine speed stays constant. You've not effectively got what in a fixed pitch propeller would be a "cruise prop."

Push the nose over and begin to dive, and the propeller governor now increases blade angle even more, increasing the drag on the propeller blades, to prevent them from increasing RPM. What's happening out there is the same thing that would happen in the runup pad if you pulled the propeller lever back...you're increasing blade angle. The governor does this in your dive to keep RPM constant.

The propeller governor does this all the time in flight, even in level cruise flight, to maintain a constant RPM. It does this any time the engine is in the "governing range.

The propeller's ability to increase blade angle or decrease blade angle are finite. That is, the propeller can only twist the blades so far, and then they hit mechanical stops that preven the twisting movement from going any farther. During low RPM operations, the blades sit on the high-rpm or "fine" stops, under spring tension (in some propellers). They're just like a fixed pitch propeller. Only when enough oil pressure is present to enter the propeller through a hollow crankshaft and actually move the blade position, do the propellers have the ability to change angle. When the propeller reaches this point, when it has enough oil pressure and is moving fast enough to allow the governor to do it's job, the propeller is referred to as being "in the governing range."

In order for the governor to be effective, it has to have somewhere to move the propeller. If you've commanded 1700 RPM and the propeller is already on the high RPM stops, then the propeller governor can't increase RPM any more...it's mechanically limited by the stops in the propeller assembly itself. It can decrease the RPM if you wish, but you have to make it do this during the runup by pulling the propeller control back and physically increasing blade angle. The propeller is said to be governing, but it can only govern in one direction...toward decrease-RPM...because it's physically prevented from increasing RPM by the mechanical stops in the propeller.

Because the propeller governor can't increase RPM at your 1700 RPM runup setting, you'll see the RPM drop as you check your magnetos...the propeller governor has no way of increasing the RPM to hide the RPM drop.

A good-neighbor way of flying with a constant speed propeller involves not allowing the propeller to wind-up during an approach to landing, either. You may have heard airplanes preparing to land where the propeller sound suddenly increases in the pattern, often abeam the numbers, as the pilot pushes the propeller lever all the way forward. This can be annoying to people on the ground, and is often unnecessary. What some will do (myself included) is leave the propeller set at a lower RPM setting until I'm far enough back on the throttle that pushing the propeller lever forward won't make any difference in sound or RPM. In other words, when my power is back far enough on approach (say, 15" of manifold pressure), I'm no longer in the propeller governing range, the propeller is already resting on the low-pitch/high-RPM stops, and there's no change in the sound or operation of the engine at that point if I move the propeller controls to full increase. This is the same condition in which the propeller governor and the propeller finds itself during a runup...at a low enough power setting, you've effectively got a fixed pitch propeller out there.

Now...if I approach an airport at cruise power with my propeller set at 2300 RPM, the governor is going to try to maintain 2300 RPM. Let's say I make a straight-in approach. I lower the nose and head for the runway, and the propeller governor is going to increase the blade angle to keep the RPM at 2300. As I reduce power and slow to my approach speed, the governor is going to try to decrease blade angle in order to maintain that same 2300 RPM.

At some point the propeller will be on the high-RPM mechanical stops, and any further reduction in throttle means a reduction in RPM, too. The propeller governor won't be able to decrease blade angle any further because it's reached it's mechanical limits in the propeller itself. Accordingly, as I drop my gear and flaps and set my approach manifold pressure (throttle), the RPM will be a lower value, just like it would be with a fixed pitch propeller. If I tried to move the propeller control at this point...it wouldn't make any difference, because the propeller governor is no longer governing. I'm out of the "governing range."

As I reach the threshold, a skunk runs onto the runway and I elect to go around. I push the power up, but don't touch my propeller control. It was set for 2300 RPM when I approached the airport, and I've left it just where it was (for the sake of illustration). I push my throttle in to go around, get a positive climb, retract my gear and flaps. Even though I'm carrying a lot of throttle and manifold pressure now, my RPM won't go above 2300...because the propeller governor starts increasing blade angle as soon as RPM reaches 2300...and thus prevents RPM from increasing past that point. The propeller is now well in the "governing range," and governing the RPM of the propeller...and of course, of the engine as well.

So much for what the governor does. In the runup area, the governor is out of the governing range and the propeller rests on it's internal mechanical stops...and is effectively a fixed pitch propeller. Accordingly, anything you do to reduce engine power is going to produce a drop in RPM. Ground one magneto (shut off the switch) and you've reduced engine power...and you'll see a decrease in RPM. Same for carburetor heat. Apply carb heat, you'll be enrichening the mixture, you'll see a decrease in RPM because you've caused a decrease in engine power.

To further illustrate (don't do this at home), if you were to perform a mag check in cruise flight, you might see a slight change engine operation, but the engine RPM would remain fairly constant. At best you might see a change in manifold pressure, but most likely not. If you had a bad mag, however, you could cause engine damage if you turned the mag back on at high power setting (which is why we normally do them before takeoff at lower power settings). The propeller governor wouldn't allow the engine RPM to drop when in the governing range, if the propellers weren't already at their maximum speed (and thus off the high-pitch stops).

A technique we often used when flying into rough, back-country mountain airstrips was to do our mag checks in the air. Doing them on the ground would be unwise in those cases, because of the damage to the propeller from rocks and chips...so we did our mag checks prior to landing. This can present a hazard to the airplane if done improperly, which is why I say don't do it at home. We didn't look for an RPM drop, but were feeling and listening for roughness in the engine.

In large radial engines, we would do this as well, usually to check for bad or fouled spark plugs. When we found roughness on one magneto, we'd leave it on and the other off, and land and taxi in on the one magneto. After shutdown, we'd use an "engine analyzer," which is a squirter bottle filled with water, to squirt water on each cylinder. The one that didn't sizzle when squirted with a little water would be the "cold cylinder," and the one with the fouled spark plug. This made isolating the problem much easier, and saved having to pull 28, 36, or 56 spark plugs to find the problem.

There's more to the runup than simply looking for the RPM drop, though any RPM drop still has to be within the values set by your manufacturer. One must listen to the engine, and feel the vibration, too. Just as important as seeing the RPM drop is seeing no drop...this tells you that your magneto isn't grounding out, and you don't have a way of electrically interrupting the spark to that magneto. The important thing there is that when you shut down the engine, that magneto is still "hot," because you can't ground it. Probably a broken "p lead." If the propeller is moved, the engine could fire up, because that mag is still "on," no matter what the switch position.

...and at what RPM are you carrying out the governor check?

Reduce 100RPM and watch it rise to stabilise again, etc.

Why ask if you already know the answer?

Your governor check is conducted at the same RPM that your mag check is done, in most cases. You can reduce the propeller speed through the governor, through the propeller lever...but the governor doensn't have the ability to increase RPM above your runup speed...because the propeller is already on the high pitch stops.

Reducing only 100 RPM doesn't tell you much during your propeller check. Part of the reason you retard the propeller lever during the runup isn't just to ensure you have control over the propeller. You want to fully cycle it, and should do so several times to get warm, fresh oil circulating through that prop. It's not just checking for an RPM decrease as you pull the propeller lever aft, it's exercising, lubricating, and warming the propeller. Additionally, some engines use a low RPM check on top of the normal runup...the engine is run to barometric and the propellers fully retarded to the low RPM setting. In this case the RPM should be noted, and should be at a specified value (typcially 1200 RPM, with manifold pressure set to barometric).

If the engine has a feathering propeller installed, this feature must also be checked.

SNS, thanks for that comprehensive and well explained reply. I may borrow a few snippets for my own teaching presentations.

Yes, very good explanation! I also did like that story of the engine analyzer. (I'm a mechanic myself, but only on jet a/c , and I'm definitly too young to have worked on big radials)

As I reach the threshold, a skunk runs onto the runway and I elect to go around.

Crap decision :} I only go around for Coyotes or larger, Skunks get squahed.

Excellent write up, and some interesting tales from experience there. It will help me explain VPP to my students from a practical point of view. Great :ok:

During a recent flight (PA-28) on a fly out with a guy that I have not flown with before, the following happened during the post-start up checks (not the power checks):

During the mag checks the engine slowly lost power and stopped. This happened a few times and several restarts were required. Eventually the pilot said this did not matter as the important mag chekcs were those during the power checks and proceded with the power checks. When the mag checks were done then there were no problems and the specified reduction in RPM was noted on each mag.

I did not have the chance to discuss this after the flight but was wondering: what is the normal practice if the engine stops during the after start mag checks. Are the power checks really the more important ones or should the flight have been abandoned in this case and the mags checked out?

Thanks for your inputs

If the engine slowly lost power, I would personally suspect carb icing instead of mag problems.

I've had this once when taxiing to the hold at the moment a rain shower (part of a front) started hitting the field. Indeed, about 15 degrees C, 100% humidity and imminent precipitation - the most likely conditions for carb ice. Applying carb heat solved the problem immediately, but I abandoned the flight anyway.

Do you recall what the conditions were at the time, and whether it was a carbureted or fuel injected engine?

Do you recall what the conditions were at the time, and whether it was a carbureted or fuel injected engine?

broken cloud at 3,000ft, no precipitation, temperature around 15degs, unsure about humidity

Carbureted engine

Idle set too low? Fuel problem, slow running jet issue?

Rod1

If your engine failed as a result of doing the mag checks, immediately after start, you've either got a nearly dead magneto with some internal timing issues, or low enough idle set that the engine can't sustain operation on the one magneto. Did it die on the same mag each time, or different mags? Both mags operating, or one? Did it actually die during the checks, before, or after?

Just as importantly, why are you checking the magnetos after engine-start?

Im with Guppy on this one, it is most likely that the aircraft is fitted with Slick Mags and the internal timing adjusment is not correct.

It is most likely that the reason for this is the contact breaker heel is worn, my guess is that this Mag has not had the RECOMENDED 500 hour check.

Not had the recommended mandatory recommended 500 hr check??;)


Just as importantly, why are you checking the magnetos after engine-start?

I do this to see if its worth taxying away to the hold for the normal checks and finding a dead or iffy mag up there. Pointless going all the way to the hold only to have come back if you find a dead or iffy mag you might have found at the start!:ok:

Just as importantly, why are you checking the magnetos after engine-start?

It's part of the checklist and as Malcolm rightly pointed out saves you the hassle later on....

It all depends who is the aircraft manufacture, some say it is mandatory some don't and of course Slick would !

My opinion is that it needs doing at around 500 hours and the vauge rules with some types allow me a little lee way to run to the next 50 hour check past 500 hours mag running time................... its all part of the EASA/LAMP farce!

Guppy. Thanks for that summary. I returned home yesterday after discussing exactly this issue with a flying friend. We both felt we had failed to explain the function of a CSU during mag checks and in the air to our satisfaction.
I have learnt much from your clear explanation. Extraordinarily apposite post after my discussion / questions of yesterday afternoon.
EB

If you want a GOOD write up on Manifold pressure and VP props look up the articles by AVweb's John Deakin :ok:

(do agree that SNS3Gs was good though)

Why would one do mag checks after startup?

Normally one does them during the power checks, at say 2000rpm.

There is an interesting school of thought which is to do them at high altitude cruise e.g. 10,000ft. Apparently this shows up all kinds of early-stage problems - because thin air is not such a good insulator. I used to design high voltage power supplies so can well believe this might be of value - if you have the balls to actually do this!

I remember during my instructor course that the FIC instructor carried out 'dead cut' checks after start. I have always carried out this procedure subsequently for the reasons stated above.

Being a pilot and not an engineer I wouldn't have continued with the flight if the engine cut during a mag check - and certainly not if I was curious enough to try it on several occasions!!! It would be in order to check for carburettor icing as part of the checks, but if the engine cut out after that it would be back for a cup of tea I think!

'Happier is the pilot who is on the ground wishing he was in the sky, than the pilot who is in the sky wishing he was on the ground' - comes to mind!

AJM :)

As an instructor I have experienced this on a number of occasions. Usually it's just a carbon build up on the mags which has always cleared following the power checks.

If after the power checks the engine were to cut again during another mag check (again at 1200RPM) I'd certainly abort the flight and taxi to the engineering hanger for closer inspection.

VFE.

Pray tell in just which part of the Mag is the carbon building up?

Oops - I meant the plugs! :}

VFE.

Won't help if you have pressurised mags.

I do this to see if its worth taxying away to the hold for the normal checks and finding a dead or iffy mag up there. Pointless going all the way to the hold only to have come back if you find a dead or iffy mag you might have found at the start!


Hopefully you're not taking the engine to runup RPM after engine start.

If you're checking mags after engine start, you're not accomplishing anything. The only thing you'll actually check is whether or not the engine can be cut off with the magneto grounding switch (mag switch).

A "dead mag?"

You're not going to find an "iffy mag." What's often taken as an "iffy mag" isn't the magneto at all, but a fouled spark plug, bad plug lead, or other problems. A mag check often isn't actually a mag check at all...but a check of other components. Chiefly the spark plug.

The spark plug will often fire with low cylinder pressures, but fail at higher power settings. Performing a "mag check" at low power settings right after start may in fact give you a false report; you may not see problems at all, which are really there. (This is the reason that when we test spark plugs in the shop, we do it in a chamber that's pressurized).

I remember during my instructor course that the FIC instructor carried out 'dead cut' checks after start. I have always carried out this procedure subsequently for the reasons stated above.


This is a problem today. Student does because instructor did.

Being a pilot and not an engineer I wouldn't have continued with the flight if the engine cut during a mag check - and certainly not if I was curious enough to try it on several occasions!!! It would be in order to check for carburettor icing as part of the checks, but if the engine cut out after that it would be back for a cup of tea I think!


Being a pilot and not an engineer I wouldn't have continued with the flight if the engine cut during a mag check - and certainly not if I was curious enough to try it on several occasions!!! It would be in order to check for carburettor icing as part of the checks, but if the engine cut out after that it would be back for a cup of tea I think!


What has carburetor ice to do with a mag check?

The carb heat check is another poorly understood check which most do blindly; check for an RPM drop, and call it good.

A rough engine during a mag check doesn't necessarily indicate a problem. A fouled spark plug, quite possibly due to an improper idle mixture or improper leaning during taxi. A power run while aggressively leaning to clear that fouled plug may settle the matter, and is an acceptable practice.

There is an interesting school of thought which is to do them at high altitude cruise e.g. 10,000ft. Apparently this shows up all kinds of early-stage problems - because thin air is not such a good insulator. I used to design high voltage power supplies so can well believe this might be of value - if you have the balls to actually do this!


"Balls" aren't required, and aren't necessarily warranted, either.

If you are flying high enough to have mag problems caused by low air pressure, then you won't need a mag check to find them. If you're getting mag problems at just 10,000', then you have bigger problems than the altitude...that's not a very high altitude to be having problems. Usually mag problems related to altitude don't show up until 18,000' or so.

More commonly, plug problems that don't show up at low power settings will be manifest at higher power settings, because of precisely the same issue. Weak plugs or a weak mag can't overcome the increased resistance in the air gap in the plug, and where it fires at lower power settings, it may fire intermittantly or fail to fire at all at higher power settings.

However, defective-dielectric (defective insulation) issues will need the thinner air to show up, unless as Tinstaafl says the problem is inside the mags and they are pressurised.

I wonder how many IFR tourers have pressurised mags? I don't think the TB21 has (ceiling 25k).

I would not expect dielectric issues to appear in the mags because they mostly rely on air gaps, whereas the ignition harness relies on plastic insulation and any cracks within that will end up relying on the air, and flight in thinner air will reveal them.

One should see changes in individual cylinder EGTs in cruise if that happened, as well as increased vibration, perhaps.

Wow!, what do I know!:p

Malcolm, don't worry, carry on as you are with a brief dead mag check after start up.

I even do this at shutdown so any problem can be addressed before the next planned outing.

Also, whenever I remember, I (very) briefly switch off both mags to test that they are both earthing correctly and I'm not leaving an aircraft with a potentially 'live' prop. (And before anyone jumps in, props should always be treated as 'live')

Thanks Studefather, I'm happy with my procedure and see no reason to reduce its content.
Each to his own, but I'm certainly not going to pressure test the plugs each preflight - thats just silly.:E

A power run while aggressively leaning to clear that fouled plug may settle the matter, and is an acceptable practice.
Thats a good way to cook up the engine, and pretty unlikely to clear a lump of lead stuck across the plug firing end!

Why would one do mag checks after startup?

Saves you getting all the way to the holding point/run-up area if you've got a duff one. Sure, you can't check the drop but you can check whether it works and you do your proper one during the power checks. Same reason I do "full and free" before I taxi as well as before departure. May as well find out then if there's a problem as later and have to come back.

FWIW the RAF has used the following for the last 30+ years (Bulldog and Grob. Can't remember what we did on the Chippy! Suspect also Firefly, though I've not flown that):

1. After start at 1200 RPM - live ck of both mags. Makes sure they're both creating the ergs to drive the plugs. Proves that you do actually have "two independent ignition systems, each capable of keeping the engine running".

2. Power cks - individual cks of both mags checking RPM drop within limits.

3. Before shutdown - dead cut ck at idle RPM. Makes sure yr not leaving a live mag when you walk way from the a/c.

Hope this helps.

DH

Thanks Thats a good way to cook up the engine, and pretty unlikely to clear a lump of lead stuck across the plug firing end!????
It is a pretty standard approach to clearing a fouled plug.

I don't know about you, but I do my runups at way less then 75% power, hence any mixture setting you like is authorised by the big two manufacturers.

There seems to be a fair bit of repetition between this and the other mag thread (CS Prop) - so I have merged them under a "Mag Checks - with / without CS Prop" heading.

Cheers

SD

Thanks Studefather, I'm happy with my procedure and see no reason to reduce its content.
Each to his own, but I'm certainly not going to pressure test the plugs each preflight - thats just silly.


Being happy with one's procedure makes more sense if it's sanctioned by the manufacturer, or has a basis in something other than "that's what I saw my instructor do." Where in the Continental or Lycoming Operator's Manual is counsel given to perform a check of the magneto immediately after engine start? It isn't. Then again, chances are that you haven't seen or read the engine manufacturers operating instructions. Most pilots don't bother. Most pilots also simply to what their instructor did...right or wrong.

Pressure testing...is a maintenance function. During each 100 hour inspection, we remove the spark plugs, check the gaps, and place the plug in a tester after cleaning them. In this tester we fire the plugs in a pressurized environment. A plug which fires at ambient temprature may produce a much weaker spark or no spark under pressure. We the plug fire through an angled mirror and verify that the spark or burn pattern is correct. This is done for each of the spark plugs as a final test before they are rotated and reinstalled in the airplane. It's not part of your preflight or after-start checks.

Also what's not part of the after-start checks is a mag check...because you're not really checking anything. You think you are...but you're not. Part of the reason, which does come back to the pressure under which the mag fires, is that you're not going to see most of the problems that can occur, happening at idle. The problems simply won't manifest themselves most of them time. The engine isn't operating at a high enough internal cylinder pressure, which will mask many of the problems. The engine is still not stable in it's mixture from the start process, especially if you used prime. This may create some uneven running or roughness which is absolutely no fault of the magneto...and simply needs proper engine operation to eliminate. In short, you're getting a false sense of security or understanding by doing a "mag check" at this early stage in the game, especially at idle power, or even a little above idle (say, 1200 RPM). You're hiding some potential problems, and manifesting some problems that don't really exist.

Thats a good way to cook up the engine, and pretty unlikely to clear a lump of lead stuck across the plug firing end!


No, it really isn't "a good way to cook up the engine."

Perhaps you can explain why you think that a cold spark plug will "cool up the engine." Do you understand that a cold spark plug, one which is completely bridged and thus shorted by carbon or lead fouling, or ice (more common than many realize) or oil...isn't going to heat up the engine or "cook" anything?

A common problem with engine operation is running the engine at too low a temperature. Aviation gasoline is high in tetraethyl lead content. Too low a temperature can result in lead deposits on the spark plugs and ultimately shorting of the plug, especially if it's a weak plug, or a rich mixture or both. Oil in the cylinder, either due to weakening oil control rings or valve guide play, may also serve to foul or weaken a spark plug. Operating at too low a power setting to burn this off or clean the plug can result in a rough engine. You may see it at the runup area. If the engine is missing on a spark plug due to fouling which has occured during taxi, often the most economical thing to do is increase power with both magnetos on and burn the fouling out of the cylinders. This is best done by leaning to bring the internal temperatures up and to help reduce any further fouling. This method of clearing the engine doesn't "cook" the engine. Often it can be used to good effect to clear the engine and make it ready for flight.

Prolonged operation at high power settings on the ground is always a bad idea for air cooled engines...but brief runs during the pre-takeoff phase are not a bad idea and can be very beneficial for the engine.

So far as mag checks and carburetor heat checks, ideally these are best done immediately before entering the runway, or if time permits, on the runway. Especially in the case of carburetor heat application. Too many pilots are in the poor habit of checking for an RPM drop, and calling the check good. This is wrong. One may even be causing ice to form by doing this. I've seen engines fail on the runway as a result or on takeoff; fairlure to properly perform the carb heat check.

The check should be done at runup power or higher, and should be done as close to takeoff as possible. A brief check just long enough to see if there's an RPM drop may do little more than serve the same purpose as partial carb heat application, and you may be moving the carb air temperature into the idea icing range...and either starting ice formation, or accelerating it.

The carb heat check should be done for fifteen seconds or so, long enough to ensure that any ice which may have formed has melted, long enough to verify that if ice had indeed formed during the taxi or low power operation, it's cleared, and to check to see if during that time any roughness developed as ice melted. Remember that idle icing isn't the same as open throttle icing. It develops in a different place on the carburetor, affects a different fuel jet, and will be manifest a little differrently than the open throttle icing you get in cruise. It may also not be immediately recognizable during the first application of carb heat, because it's not going to immediately melt, you will see the RPM drop and carb air temp rise (if you're equippped with a carb air temperature gauge)...but you have to give the ice time to melt.

If you simply do your runup and apply the carb heat just long enough to see an RPM drop, you then shut off carburetor heat and go on your merry way under the false sense of security that you've checked something...and you really haven't. Most notably, you haven't checked to see if you have carburetor ice right now...and your next opportunity to find out will be during the takeoff itself. Like I said, I've seen engines fail on takeoff for this very reason.

My preference is to enter the runway environment, get aligned with the runway and my line checks complete (Fuel, Flaps, Trim, Transponder, Air) as required...then bring up my throttles. Initially to the runup RPM, perform my mag check, and do a final carb heat check for at least fifteen seconds. Then with carb heat shut off and all engine indications stable, increase power to the takeoff power setting and initiate the takeoff roll. (This done on a smooth, hard surface runway only, of course). This way there has been no opportunity for ice to form in the intervening time I was wating to takeoff, and I'm certain of all operations secure immediately before doing the takeoff roll. Also, there's no power reduction from the time I brought the power up to runup RPM...increase power to do the checks, then increase it the rest of the way for takeoff.

This is even more important if I'm going to lean for takeoff. I often operate at locations where the density altitude can be as high as 10,000', which necessitates leaning for takeoff. One can't do this at idle RPM. One shouldn't really do this at runup RPM either...because leaning the engine is only valid for one altitude, one temperature, one airspeed, and one power setting. If any of those things change, then the engine must be re-leaned. If one is indeed going to lean for takeoff, then one must do it at the takeoff power setting...not at a lesser power setting. One must also know one's engine and understand why, or why not this is done.

As a side note, all manner of strange practices are introduced by instructors and others, but regrettably it's often because that's how their instructor taught them. Not because their practices are based on sound principle, and certainly not because that's how it's prescribed by the engine manufacturer. Few students have ever read or followed the engine manufacturers operating instructions, and few instructors have done so, either.

As an example of this, how many perform a post-flight runup as recommended by the engine manufacturers? This is done when you get back to your parking spot, and is part of the shut down procedure. You may not find it in your pilot operating handbook or aircraft flight manual...but there's an enormous amount of information that really ought to be understood about your aircraft that's not in that manual. The airplane maker didn't build the engine...the engine manufacturer did that...and the engine manufacturer also produces publications on operating the engine. One of the procedures that the engine manufacturer will tell you to do is perform a post flight runup immediately prior to shutdown...and this is primarily to clear the engine (and no, it won't "cook it"). The manufacturers will also direct you to perform an idle mixture check...which a fraction (if any) of the pilots out there do, or understand.

The truth is that when you're operating that engine, you don't know what it is that you don't know. Do not allow this lack of information to convince you that you know all you should about operating the engine, because you don't. Certainly do not permit yourself to be dragged into the trap of doing what you do simply because that's what your instructor did. Flight instructors tend to be some of the least experienced and least knowledgable of any out there...victims themselves of a heritage of inexperience where one inexperienced instructor teaches a pilot who then becomes an instructor and passes on the same wisdoms...each founded in inexperience, each passed on only because "that's how the last guy did it." This is no way to operate an aircraft engine or to learn to fly.

Saves you getting all the way to the holding point/run-up area if you've got a duff one. Sure, you can't check the drop but you can check whether it works and you do your proper one during the power checks. Same reason I do "full and free" before I taxi as well as before departure. May as well find out then if there's a problem as later and have to come back.


This is not the case, and your comparison is not an "apples to apples" one. The flight controls being free and correct will always remain free and correct, from taxi to takeoff, if indeed they're free and correct. There won't be any change in their free and correctness from taxi power to takeoff power, if the control lock is removed and they're indeed free and correct.

If you perform a check of the magnetos at lower power settings, especially right after takeoff, you'll be masking some problems which may exist, and you may see some problems which really don't exist...but think that they do exist.

A "duff" magneto? What are you really checking when you perform a mag check? The magnetos? Yes, partially...but it's seldom the magneto that's the problem. You're checking the entire ignition system, and most of the problems, especially the most common problems, won't be manifest at an idle power check. In other words, the most common things that will cause the roughness or excess drop in a mag check won't even be manifest until you have runup power or higher present on the engine. You'll miss them doing an idle "mag check" after engine start.

You may also see roughness that doesn't exist. You've just started the engine, it's not necessarily firing evenly. You may have a slightly fouled plug, you might have an excessively rich mixture in one or more cylinders. You may be still clearing out the engine after a primed start. The roughness isn't necessarily a bad magneto...which you won't notice at this point anyway...it will be due to other items normal to operation. The engine needs to stabilize, begin to warm up, the plugs need to clear, and the engine needs to be proper leaned. If you've go an improper idle mixture (a maintenance item: something you can't do anything about, but which you can check), it may run rough anyway, and may require additional leaning to compensate (somewhat...your cockpit control doesn't adjust idle mixture, and is only a rough approximation of fixing the problem). In the meantime, your mag check after start up is lying to you...because what you think you're seeing and hearing is primarily bad information. Real problems are disguised, and non-existent problems may appear to be present.

However, defective-dielectric (defective insulation) issues will need the thinner air to show up, unless as Tinstaafl says the problem is inside the mags and they are pressurised.

I wonder how many IFR tourers have pressurised mags? I don't think the TB21 has (ceiling 25k).

I would not expect dielectric issues to appear in the mags because they mostly rely on air gaps, whereas the ignition harness relies on plastic insulation and any cracks within that will end up relying on the air, and flight in thinner air will reveal them.


You're talking about several different problems. Altitude and reduced ambient air pressure isn't a problem for ignition harnesses. For aircraft where this may be an issue, the type of harness used determines suitability. That is, high-tension vs. low-tension systems.

Pressurized magnetos are another matter completely, and in these the issue is arcing internally between the mag distributor rotor, and the distributor cap of the magneto. Carbon tracking is also an issue within the mag distributor cap, particularly at higher altitudes. The phenolic or plastic in the cap can attract carbon particles which form lines that are electrically conductive, and can cause early or misfiring of the plugs, or even out of sequence firing. High altitude mag problems can lead to changes in internal E-gap firing which changes the internal timing, or can cause a weak spark due to changes in the way the opening of the points in the plug affect spark generation. An advance in firing can take place as the distributor cap approaches the plug lead in the magneto in extreme high altitude operation with an unpressurized mag, and the result can be a change in timing for the plug firing, by slight degrees. it also leads to burned points and burned contacts on both the distributor rotor, and cap.

A mag check won't isolate this or tell you what's going on, and serves little purpose.

Spark plug leads aren't simply lined with plastic. they're insulated in neoprene and nylon, and surrounded by wire braid to capture excess radio frequency noises and carry them to ground...prevents interference with the aircraft communications and navigations radios. An additional outer coating is present. Arcing through the spark plug leads isn't really an issue with high altitude flight, and the presence of pressurized magnetos makes no difference with respect to the ignition leads.

Where altitude may be an issue with ignition systems, they're designed around the kind of application for which they are intended. A low tension system, for example. Given that an airplane with a service ceiling of 25,000 has a standard high tension system with a normal set of ignition leads and harness, do not anticipate misfires due to "thin air" at altitude. The leads do not use insulative properties of air to protect them while conducting voltage to the spark plugs...and thin air will not prevent their proper operation.

Pressurized mags for the most part are troublesome, tend to get dirtier, and tend to have higher maintenance requirements.

My preference is to enter the runway environment, get aligned with the runway and my line checks complete (Fuel, Flaps, Trim, Transponder, Air) as required...then bring up my throttles. Initially to the runup RPM, perform my mag check, and do a final carb heat check for at least fifteen seconds. Then with carb heat shut off and all engine indications stable, increase power to the takeoff power setting and initiate the takeoff roll. (This done on a smooth, hard surface runway only, of course).

I'd really like that! But it would not be practical where I fly 90% of the time, and I imagine it is the same for many of us. Far to hectical at the airport for people to sit on the runway for 20 secs prior to takeoff.

So what's the second best way? If I've had a long wait at the holding position after the run up, I do a second run up and carb heat check (15 secs or so) when I can see it is about my turn to go. Newly fouled plugs etc I'd hope to catch at the RPM check immediately after setting takeoff power, which I do quickly to ensure it is a sufficiently static RPM check, but I imagine that check could be misleading in a strong headwind? Any other suggestions?

As an example of this, how many perform a post-flight runup as recommended by the engine manufacturers? This is done when you get back to your parking spot, and is part of the shut down procedure. You may not find it in your pilot operating handbook or aircraft flight manual...

Interesting you brought that up; I have thought of whether or not to do the post-flight runup... Yes, the engine manufacturer built the engine, but the airplane manufacturer integrated the engine with the airframe to create an airplane. If they leave out bits from the engine manufacturer's manuals, how am I to know if that's an omission, or because there is some reason why it's not a good idea on that particular aircraft? I take the view that the aircraft manufacturer will have included any truly important information from the engine manufacturer, so if I only do what the aircraft manufacturer says I will do well enough. If I add to the aircraft manufacturer's advice, I will probably do better much of the time, but I also risk unwittingly doing something downright dangerous, or at least quite expensive.

The aircraft manufacturer does not know as much about the engine as the engine manufacturer does. But I know even less, so I won't get creative with the procedures! :)

Interesting you brought that up; I have thought of whether or not to do the post-flight runup... Yes, the engine manufacturer built the engine, but the airplane manufacturer integrated the engine with the airframe to create an airplane. If they leave out bits from the engine manufacturer's manuals, how am I to know if that's an omission, or because there is some reason why it's not a good idea on that particular aircraft? I take the view that the aircraft manufacturer will have included any truly important information from the engine manufacturer, so if I only do what the aircraft manufacturer says I will do well enough. If I add to the aircraft manufacturer's advice, I will probably do better much of the time, but I also risk unwittingly doing something downright dangerous, or at least quite expensive.


Ignorance is bliss then, is that it?

Truly certain conditions exist beyond the construction of the powerplant in which special considerations must be given. These won't fall within the scope of the engine manufacturers generic operating instructions. An example is a propeller which has a particular harmonic operating range...this is a customer application which has occurred after the engine has been produced...and it's up to the airframe manufacturer (or entity which installed the propeller) to provide limitations on operation within, say, a certain RPM range.

General information, however, such as the propriety of performing an brief engine run prior to shut down, do NOT hurt the aircraft, and are both helpful and highly recommended.

Too many pilots falsely believe that everything they need to know is contained within the covers of the dumbed-down pilot operating handbook (which is designed for the lowest common denominator). In truth, you'll learn far more about the aircraft systems by delving into the maintenance manual...this ought to be required reading, but sadly, isn't. The pilot operating handbook isn't the only handbook provided by the manufacturer. Additionally, as only the airframe manufacturer provides a handbook for the finished product, wise pilots will seek out additional training material...which includes the engine manufacturers operating instructions, as well.

The mentality that the airframe manufacturer didn't provide it, so you don't need to know it, simply won't fly very far.

Your airframe manufacturer didn't provide your avionics, or make them, in many cases. Another company did that. In most cases, the operating instructions for your avionics, autopilot, and many aircraft systems come from some other source beyond the airframe manufacturer. The same is certainly true of your engine.

When I work on an airplane, I will certainly have the manufacturers maintenance publications in front of me. I may have a Cessna maintenance manual, the illustrated parts catalog, and other data such as service bulletins and airworthiness directives in front of me. I also have before me, however, the maintenance publications for the various parts of the airplane. I may have a publication by Cleveland for the wheels and brakes. I'll have a Lycoming or Continental maintenance manual with it's own data in addition t anything the airframe manufacturer may have provided, and it will have it's own illustrated parts breakdown. I have to refer to all sources when performing maintenance, and I do...because I know that everything I have to know isn't found in one neat, tiny little dumbed-down guide.

The same goes for getting to know the airplane as a pilot. There's more to know about leaning the airplane than simply following a checklist which says "Mixture...LEAN." There's a LOT more to know. The engine manufacturer will provide a wealth of information on the topic. Perhaps you think that because you saw an instructor lean until the engine ran rough then enrichen a half inch, or two turns of the mixture setting, that you've learned enough. You might just take that kind of learning to a power loss one day by burning a hole in a piston...because while you can get away with it in a Cessna 150, you certainly can't in many other engines...especially at higher power settings.

When you have the gift of information that expands your understanding of the airplane beyond the bubble-gum little bit delivered in the AFM/POH, don't cast it away. It's provided by the manufacturers of the equipment you fly. It saves time, money, wear and tear, keeps equipment running better, makes for better efficiency, and makes you safer. It may even save your life.

If restrictions are in order that have been introduced by the manufacturer, then the manufacturer will provide these in the limitations section of your AFM/POH. You may be restricted from operating with a tailwind or in certain operating ranges, or from operating at a given power setting for longer than five seconds or five minutes. Certainly these limits must be observed, as they may stem from a propeller-engine combination or from the way in which the engine is cowled and cooled. Always respect these limitations. This in no way inhibits or suggests you shouldn't learn everything you can about the powerplant from the powerplant manufacturer. Obtain, read, learn, and use the powerplant manufacturers operating instructions...especially the ones written for you as a pilot.

The aircraft manufacturer does not know as much about the engine as the engine manufacturer does. But I know even less, so I won't get creative with the procedures!


Again, while you may think ignorance is bliss, you've no need for creativity by following the counsel of the engine manufacturer. Nor will it cause your engine (or you) harm.

Newly fouled plugs etc I'd hope to catch at the RPM check immediately after setting takeoff power, which I do quickly to ensure it is a sufficiently static RPM check, but I imagine that check could be misleading in a strong headwind? Any other suggestions?


The headwind won't hurt or mislead, but a tailwind might.

Remember that the drop in RPM (in a fixed pitch propeller installation) that's provided in your checklist and aircraft flight manual is given for your runup RPM. Don't apply the maximum drop and maximum difference numbers to a mag check at full power, because those numbers no longer hold true. They are correct values, and a limitation, when performed at your published runup RPM.

I take it you might be responding to a whole lot of other posts we have read here, rather than what I actually wrote... "Ignorance is bliss" is nonsense, as is blindly applying whatever you saw some instructor doing and thinking that's all you need to know. We're certainly in agreement there! :ok:

But I can see several reasons why it might be a bad idea to start using various procedures from different publications, just because it is written somewhere that says "manual" on the front. The POH is "dumbed down" for a reason! Using more advanced procedures requires more advanced understanding (quite the opposite to "ignorance is bliss!").

But as one gradually builds that understanding, I see it as a PPL holder's duty and responsibility to put it to use to improve one's personal SOPs. The regulators designed the system that way; we are our own operators, without an engineering and operations department to digest all available information and cook it down to safe and practical procedures. We have to do that ourselves. I think that is an interesting and fascinating aspect of flying! :)

Also, as you mention, the manufacturer's manual is not a regulated document, meaning, for instance, it might not be updated with new information or revised procedures, the way the POH is. Without an understanding and curiosity beyond what many PPL holders show, one risks using procedures that were deemed unnecessary or hazardous 30 years ago, using a 35 year old manual.

I have read the manufacturer's manuals for the engines I use though! :ok: Out of curiosity, and hoping to find something useful in them. I like the "Lycoming Flyer Key Reprints" publication as well, available from their site somewhere; learned a lot from those.

The headwind won't hurt or mislead, but a tailwind might.

I think it would though; a 30 kts headwind would mean a higher RPM at full throttle than zero wind, wouldn't it? So one might see the correct 2300-ish RPM, just inside the 2300 - 2400 RPM allowable range, although one does not actually have full power... For avoidance of any misunderstanding, I was refering to checking the RPM at full throttle and compare it to the allowable range in the POH; not doing a second mag check at full power.

The truth is that what you're seeing in the cockpit as RPM is very seldom what the RPM really is. A test of true RPM vs. the tach display in the cockpit very often reveals that the prop either isn't meeting the static RPM or is exceeding it. You might be surprised how much variation exists in cockpit displays. It's actually rare to find one that's accurate.

If you can bring your throttle up and see the engine making takeoff RPM and it's not stumbling, that's about as much as you're going to get for takeoff with a fixed pitch prop. The presence of a 30 knot wind isn't going to mislead you or show something that isn't there.

With a constant speed prop, of course, it's irrelevant.

sns3 guppy, you clearly know your stuff, so why the need to take a post out of context, twist it, beyond recognition and then pour scorn upon it?

I said
"Thats a good way to cook up the engine, and pretty unlikely to clear a lump of lead stuck across the plug firing end!"

You said


Im afraid you lost me there, You will need to explain to me about cooling up engines with cold plugs, because I know nothing about that - I never said anything about that in any way,shape or form.
The response was to [QUOTE] "A power run while aggressively leaning to clear that fouled plug may settle the matter, and is an acceptable practice."

I still am unsure how leaning will clear lead bridging a plug end, unless it is by melting it and/or shaking it loose. Oil, yes. Lead - no. And I remain convinced that if I have a dead mag after start - but dont actually bother to check there, there is little point going to the hold and finding out there when I could have found it earlier with a simple dead-cut check.

And I do a full & free before moving off to make sure my gut is clear of the column.:ok:

Im afraid you lost me there, You will need to explain to me about cooling up engines with cold plugs, because I know nothing about that - I never said anything about that in any way,shape or form.


This is a typo. Instead of "cool up the engine," it was intended to respond to the quote of "cook up the engine." It should have said "Perhaps you can explain why you think that a cold spark plug will "cook up the engine." Do you understand that a cold spark plug, one which is completely bridged and thus shorted by carbon or lead fouling, or ice (more common than many realize) or oil...isn't going to heat up the engine or "cook" anything?"

You say you said nothing about a cold spark plug...but you did. You referred to a spark plug which is bridged, and therefore unable to produce a spark...which is referred to as a dead or cold spark plug (not to be confused with the technical reference to some types of spark plugs as cold or hot). You stated that a cold, or shorted spark plug will "cook up an engine." How do you propose that a spark plug which is not firing will "cook up an engine?"

And I remain convinced that if I have a dead mag after start - but dont actually bother to check there, there is little point going to the hold and finding out there when I could have found it earlier with a simple dead-cut check.


Have you ever experienced a "dead mag" after engine start?

When you do your "dead-cut" check after engine start, what is it that you believe you're actually checking?

I still am unsure how leaning will clear lead bridging a plug end, unless it is by melting it and/or shaking it loose.


Perhaps if you read the manufacturer literature, you would understand.

Bridging of a spark plug can take many forms. The entire electrode may be sealed off in the worst cases, but it may be something much more simple. You may very easily be able to clear it off, carbon or lead fouling, with a power run and by leaning, without having to needlessly return to the hangar for maintenance.

When I work on an airplane, I will certainly have the manufacturers maintenance .. and on .. .. and on

I have lost the will to live. :)

You stated that a cold, or shorted spark plug will "cook up an engine Where?

How do you propose that a spark plug which is not firing will "cook up an engine?"

It wont because it is not working. The other 11 in my 12 plug engine will when you overdo the leaning for a prolonged, often futile attempt at clearing the dead one, and not assertaining the original cause.

Have you ever experienced a "dead mag" after engine start?Yes.

When you do your "dead-cut" check after engine start, what is it that you believe you're actually checking? Compliance to MY satisfaction with N 76-07-12, the integrity of the P lead circuit and to annoy those who warrant it.

The truth is that what you're seeing in the cockpit as RPM is very seldom what the RPM really is. A test of true RPM vs. the tach display in the cockpit very often reveals that the prop either isn't meeting the static RPM or is exceeding it. You might be surprised how much variation exists in cockpit displays. It's actually rare to find one that's accurate.

I use one of these on each 50 hour and annual check, although only required to check calibrate at each annual really. I know exactly what my rpms are throughout the range. I also use it on my hovercraft whilst eel collecting. That way I know when it is full. :ok:

PropTach - Wireless Optical Tachometer (http://www.proptach.com).

You collect eels?

Where?


In post #42, where you stated "Thats a good way to cook up the engine, and pretty unlikely to clear a lump of lead stuck across the plug firing end!"

It wont because it is not working. The other 11 in my 12 plug engine will when you overdo the leaning for a prolonged, often futile attempt at clearing the dead one, and not assertaining the original cause.


You can't overdo leaning at runup power settings, or when operating at settings below 75%. You're not going to cook your engine by performing a power run to clear fouling.

Compliance to MY satisfaction with N 76-07-12, the integrity of the P lead circuit and to annoy those who warrant it.


Perhaps you mean Airworthiness Directive 76-07-12, which is not intended to determine a "dead mag," but to determine an ignition switch malfunction. This AD needs compliance each 100 hours. It's also to be performed using "regular ground runup procedures," not as an after-start check, and it's to be performed after the engine has been warmed. Specifically, the procedure is stated within the AD as follows:

Compliance required as indicated:

1. For switches subject to this AD, conduct the following checks within the
next 100 hours' time in service and each 100 hours thereafter to detect
possible switch malfunction:

(a) Observing regular ground run-up procedures, allow the engine to
reach operating temperatures and perform a normal magneto check.

(b) With the engine at normal idle, rotate the switch key or lever through
the "OFF" detent to the extreme limit of its travel in the "OFF"
direction.

(c) If the engine stops firing, this indicates an airworthy switch.

(d) If the engine continues to run with the switch in the extreme "OFF"
direction indicating a malfunctioning switch, prior to the next flight
accomplish Part III outlined in Bendix Service Bulletin No. 583, dated
April 1976, for Repair and Replacement or use an alternate method
approved by Chief, Engineering and Manufacturing Branch, Eastern
Region.


It should be noted that if you rotate the switch through to the off position and the engine quits firing, then the engine should be allowed to come to a rest before another start is attempted. The switch should not be turned back on while the engine is still rotating.

The AD, then, doesn't ask for, nor recommend, nor require a mag check after start. It specifically calls for a normal mag check, with the check having been done after the engine is warm, in conjunction with a normal runup.

If you wish to perform a dead-cut check (P lead integrity), there's absolutely no reason to do it before you go fly; it's a post-flight check to be done on shutdown, along with the post-flight runup.

The above airworthiness directive refers us to Bendix service bulletin 583, which includes similiar wording to the AD with respect to runup and operational checking of the switch assembly, as follows:

Procedure A - Check using engine reactions.

1. Observing the engine manufacturers ground run-up procedures allow
the engine to reach operating temperatures and perform a normal
magneto check.

2. With the engine at normal idle, rotate the switch key or lever through
the "OFF" detent to the extreme limit of its travel in the "OFF"
position direction.

3. If the engine continues to run with the switch manually held in the
"Past OFF" position, it is an indication that one magneto is still "Hot"
or ungrounded.

4. When the switch key or lever is released from the manually held
"Past OFF" position, it should automatically return to the normal
"OFF" position where the "Hot" magneto condition should no longer
exist and the engine should die.

5. Any switch exhibiting a "Hot" magneto condition when in the "Past
OFF" position should be repaired or replaced (Ref: Part III) at the
earliest possible opportunity.


Of note is the language in the bulletin...which doesn't say "using the airframe manufacturers ground runup procedures," but rather states "using the engine manufacturers ground runup procedures."

Sorry mate, but if you want to argue the toss about what has NOT been written by me then I will concede this argument as I also have lost the will to live.

No I dont collect eels.:ugh:

Sorry mate, but if you want to argue the toss about what has NOT been written by me then I will concede this argument as I also have lost the will to live.

No I dont collect eels.


I also use it on my hovercraft whilst eel collecting.


No sense in arguing. You seem to very quickly forget what you write, and then deny you've written it.

I believe the point has been made.

Sorry Guppy, pure mischief making on my part, hopeing to lighten the mood.

Seems to have backfired like an iffy mag!:ok: