You might need more than an evening!
There's a huge amount of good stuff in there.

My C152 POH clearly states the static RPM limits in the Normal Procedures section under the Takeoff Power check section, as does the C172 POH's. Unfortunately Piper is not as enlightened as it does not provide the values in their POH's. However you can do a lot better than a gut check by conciously noting the RPM as the throttle reaches the full forward position. This value should be consistant + - about 50 RPM regardless of the conditions for all subsequent takeoffs. if you are doing a check out on a new aircraft type the instructor "should" be able to tell what number to look for.

Yes you should always apply a gut check to every takeoff so if it doesn't seem right it is better to do alow speed abort than continuing with a sick airplane. However gut checks work best when they start with a comparision to hard data points and low hours PPL, in particular, do not have the reservior of experience to rely solely on "gut checks"

Big Pistons Forever

I entirely agree with your comments about the need to check static RPM at the start of each take off . But my experience in the UK is that very few PPL pilots actually perform this check,or if they do, then “Check Power” means no more than note the RPM is vaguely in the right area without any real understanding of what the correct figure should be and what tolerance is acceptable. As you said in your previous post.

In the UK at least it appears not be taught. Yet after all the mantra of pre take off checks of mags and carb heat function, the static RPM is the one instrument check that confirms whether or not the engine is delivering the correct power. Getting airborne with a malfunctioning engine when the evidence has been there right in front of you is inexcusable in my book. I wouldn’t mind betting it has been responsible for some of the take off accidents we have experienced in the UK.

Professional pilots understand the need to perform this check every take off and only continue if the correct power is achieved, so why not PPL holders too ?

There is a complication. Some aircraft types may be fitted with any of a range of propellors of different pitch, and this may affect the static RPM.. For example the Robin DR400 may be fitted with a fine pitch prop for glider towing giving a static RPM of 2500, yet the same aircraft with a “cruise” prop will produce only 2300, and others with a “compromise” prop will give 2400. So if I am going to fly a new aircraft, including one of a type with which I am familiar, I always ask about the static RPM. And how often do I get a sensible answer ?

I just checked the POH for the DR400-120 and in the take-off checklist it does specify a minimum of 2200 RPM.

According to the POH you can get this aircraft with three different types of props, and the minimum static RPMs are respectively 2200, 2250 and 2220.

Alex, just to extend slightly the discussion about leaning.....

flying from an airfield that is more than 5,000' above sea level adds another consideration! Density altitude.

The Wright Brothers found this out when the Flyer that had flown nicely from the Kitty Hawk Beach in North Carolina, was less than enthusiastic when they tried to take off on a hot summer day in Ohio! (Back to the drawing board!)

Morning!

Yep, looked it up and it clearly states a minimum static RPM :ok: *Adding-to-takeoff-checklist-NOW*

Have a nice weekend everybody!

Alex

....... although but a distant related thread - there was that other instance when an a/c newly returned from its annual C of A check experienced a form of 'rough running' - causing much white knuckle attempts to clear the boundary - When it was later discovered the
propeller had been fitted 'back to front !

While not the reason for running rough in this case, it could also be caused by running on ONE Magneto instead of BOTH. The a/c shakes like !!!! when full power is applied after a simulated forced landing and is only running on one mag. I only ever simulated FMMGT checks after that. It was a silly new PPL mistake. :O

Posted in the hope that others may learn from my mistake.

Personally I have to confess I'd expect that with 2 cylinders 'out' it'd run like a lame donkey! Would be suprised to achieve run up RPM - I'd expect it to be painfully obvious something was wrong on takeoff. Notwithstanding the diagnosis, it sounds more like a fouled plug.

There's no reason that the thing should shake with one mag off either, on the occasion of an engine developing a somewhat unpleasant vibration after meeting a rain shower, I finished up completing the flight on just the right mag because it was misfiring and shaking when the left was on - there's two for a reason.

Consider also that I'm pretty sure your static RPM will be affected by density altitude (but so will the power output of the engine, so I'm not too sure of the exact relationship..)

Hey Mark,

I can only say what I witnessed :) Mag check was ok (RPM drop on each of the mags). And yes, it was quite shaky, but it definitley went up to 1700RPM. It turned out that one cylinder had almost no compression and another was quite low on it.

However, I'm still on the status I got from a first quick check from our mechanic and haven't had any updates.

I'll keep you guys posted on the final diagnostic.

Cheers,
Alex

You would however clearly feel the vibration

I'm sorry but I certainly don't understand that. There are two mags in order to provide a full burn, not as a redundancy / choice exercise. Both mags should fire at the same time (possibly one may be slightly advanced?) Misfiring when both are on, not misfiring when only one is on does not compute very well?
Could this vibration have been caused by the rain shower / carb ice? switching off one mag so reducing the amount of "bang" & thereby reducing power & vibration?

A few additional comments

1) There is no way an engine with two dead cylinders would make minimum static RPM although it would certainly make runup RPM. There may not necessarily be a lot of vibration depending on which combination of cylinders are dead, thus the importance of a power check a the start of the takeoff roll

2) While a quick check of the engine instruments to ensure they are in sensible possitions, is a very good idea at the start of the takeoff run the mantra "in the green" is often mindlessly applied to all flight regimes. This is a particular pet peeve of mind. When established in the cruise the engine guages should be checked regularly but not to assure that they are "in the green", rather to note the exact position of the needle on each guage and then ask the question " has there been any substantial change". If the answer is yes, then some thinking should be given as to why. Engines almost never just up and stop, they will almost always give some warning. For example impending catastrophic internal failures are almost always preceded by rising oil temperatures and declining oil pressures. By the time both have exceeded their red lines you had best have a field ready for the forced approach. If you have being paying attention you would have seen the situation devloping and started a diversion right away with the likely result of a uneventfull landing at the nearest airport.

2) Re the question of descent and leaning. This is what I do (for your typical trainer/light tourer with a fixed pitch prop and no multi probe EGT guage)
a) Slightly enrichen the mixture
b) Establish a nose down attitude which will give a 500 FPM descent rate
c) As the aircraft accelerates the RPM will start to rise so,
d) Reduce the throttle to maintain the cruise RPM I had before the descent
e) Trim
f) When I reach the desired attitude fine tune the RPM and relean

Prior to starting a descent, one of the worst things you can do is go to full rich, particularly on cold days and for long descents as this dumps a load of excess fuel into the cylinders which will cause the cylinder head temperature to dramarically drop, a great way to cause cylinder head cracking.

I had a similar case, although in this incident there wa an internal failure of one mag which greatly advanced the timing for the firing of the spark plugs of that mag. The engine all of a sudden started running very roughly. Swiching off the affected Mag returned normal engine operation and an uneventfull landing at the nearest airport (fortunately home base) was carried out.

With regards to the second point. The relationship between engine power and density altitude vs static RPM is not exact but close enough that for practical purposes the static RPM will not vary appreciably, unless you are at extremes of temperture and altitudes.

But my mechanic says....
 

I just had to chuckle as I find myself in exact the same situation....

Pelican's Perch #46:<br>"But My Mechanic Says ..."

First thing our mechanic said was "Somebody running it too lean again!".

Cheers,
Alex

Most likely there was either some level of water contamination in the the suspect mag causing intermittent/off-time firing or a slight breach of the insulation for one of the plugs (causing it to not properly fire). When both mags/plugs fire you get a full power stroke and when one messes up you get slightly less power. This difference causes the vibration. When you switch to one mag (either the dry mag or the one not powering the duff plug/wiring) you get slightly less power on all cylinders so no vibration.

In a fully instrumented engine you can see this weakness in EGT spread well before you can feel it and then have the issue sorted on the ground (as you can relative easily isolate the plug/plugs in question).

Not necessarily. We found one cracked cylinder and one with a broken piston ring on the following symptoms
Increased oil consumption. Not dramatically so, about 60 percent up on the previous 50 hours, and a slightly greasy fuselage behind the engine.
Slight white marking on the cracked cylinder from gas escape.
The cracked cylinder was well down on compression, the other down enough to make it worth changing at the same time. (both on the same side)
No vibration, static rpm 2350 (expected rpm for that engine/prop combination)
Lycomings (and Continentals) are pretty agricultural engines.

Well, not entirely. The general idea is to be able to continue flight to a suitable landing place if one plug is out, or of course one magneto. My car seems to manage an efficient burn just fine on one plug per pot, placed rather more centrally. Not choice, but redundancy, yes. I have done a fairish bit of flying on single ignition engines, but always in stuff that will either glide rather well, or alternatively is so slow that an average sized carpark would suffice for a forced landing. I like two magnetos, thank you.

Piper Classique,
I'll try to elaborate a bit. Two mags are there to (1) provide a full burn, (2) to produce some power if one fails "redundancy", (3) they work indepently of any power supply/generator/battery, (4)they "should" both work together & I was under the impression that they were, I may have been wrong there. If there was a serious mag malfunction then yes that would upset the thing, however the rain shower, in my limited experience of flying but long experience of driving/riding circa 1957, I consider more likely to produce carb ice than a flooded ignition system. My only experience of a swamped ignition was in a MK 8 Jaguar that blew the radiator & soaked everything. Within a few minutes things had evaporated/dried enough to restart. Coil/distributor system of course.
I may be wrong again but I believe that the magneto does not produce such a good spark as the coil system? Stuffing a wet rag into the plughole on a motorcycle magneto & kicking it over gave me a belt!! but not as serious as the latest, prob non survivable, electronic stuff. Which is why it was discontinued in cars.

Ok; I agree with you that there is more than one reason for having two mags, thought I would disagree as to the order of importance. I said nothing at any point about water, liquid or otherwise. That was mm.flynn

They are timed together but sometimes the timing can drift. I have timed a few mags..... "Some power" is a bit unkind. At least cruise power off only one out of two is an entirely reasonable expectation. Yes, I think we all know that a magneto does not need a battery. I have swung a lot of props in my time. As for coil ignition, we are talking about a typical Lyc/Continental here, rustic agricultural dark ages, so it's magnetos.