Rough engine on run-up
 

I very much doubt that two cylinders "are out" because they are running lean.

It is much more likely that two cylinders have fowled plugs due to the wrong shutdown drill.

I see that the "dont lean below Bla bla bla" people have got to you, the Lycoming engine manual states that you can lean at any altitude when the power is set at or below 75%. leaning will save about 20% fuel burn.

As for question 4 with no EGT indicator you have the drill more or less correct
I have two C152's that we lean that way and get a fuel burn of IRO 23lts/hr.

Only to add there is so much conflicting advice about whether to lean or not.

Potentially there is concensus that it is worth leaning at higher levels.

At lower levels opinions seem more mixed.

Whatever view you hold the danger is leaning aggressively without the benefit of EGT and CHT probes on each of the cylinders. The consequence can be to leave one or more cylinders running hot (because you have no idea it/they are running hot). The end result will be cylinder damage or worse.

I can give you a good example where with my own aircraft one of the cylinders was running significantly hotter than the others when the engine was "fully" leaned and yet from sound and feel the engine showed no indication what so ever of a problem. The plug was beginning to fail.

Given the cost of an overhaul against the fuel saved by leaning I cant see the sense in "fully" leaning an engine unless you have some means of monitoring cylinders temps. I would run richer, take comfort in the extra cooling and pleasure that my engine will get much closer to TBO. If it were my own aircraft one of the first things I would do is add a temp gauge - they are relatively cheap, easy to install and worth their weight in gold.

Sadly of course when it comes to rental aircraft many pilots are less worried about the engine (which isnt theirs) or the next pilot that is going to be flying than their wallets, although at least when the aircraft is rented on wet tac time this is likely to be less of an issue.

If it was impossible to abort the take off I would have called a Pan x3 requesting return to the field. Your climb rate wouldn't be too great but if you were not loosing height probably enough power to stagger round the circuit. If you are loosing height and airspeed call a Mayday x3 and look for a suitable landing site ahead.....do not turn back to the field and risk a stall or spin:=

To address the "what if I had tried to take off" question somewhat..

I had an aeroplane with a 9 cylinder radial engine. I flew it across the US from Virginia to Wisconsin with never even a hint of an issue. When it went in for inspection shortly thereafter, it was found to have 1 cylinder virtually no compression and one cylinder with a low compression, so it was running on 7 and a half cylinders, basically. Showed no different indications and sounded no different.

But thats a 9 cylinder engine and I'm guessing your plane is a 4 cylinder?

Equally, our Cub came out of annual last year, we went to go flying in it, sounded slightly odd taxiing out, just a slight hint of being different but nothing definite. Tried to do a run up, but the RPM wouldn't go above about 1100, no matter where we put the throttle. Taxiied back, had a look under the cowl and found all the spark plugs on the left side had not been reconnected...duh! Taxiing was fine, with only low power required. Even with a Cub, it would have taken a v e r y long runway to get off the ground with the power the engine could produce.. :hmm:

Hi everybody!

Vizsla, I would never attempt a flight if something doesn't feel right :) My curiosity was more academic of the kind, would the engine deliver enough power for a somewhat normal take-off. At our field, my only option would have been the highway in case of an emergency right after takeoff. That or the town's main street ;)

@ fernytickles & cjboy: That's the creepy thing: although vibrating more than normal, the engine did rev up without hesitation.

I flew that airplane the weekend earlier on a long x-c at 8500ft, leaned it out but always reajusted when I changed altitude. It was then flown twice by students, one of them on his solo x-c.

And I too believe that this isn't due to a one-time misadjustment. If I'm pragmatic, thinking of what 152s are put through generally as student aircraft, they must be pretty sturdy and forgive lots of beginner errors.

Again, I'm still pretty "green behind the ears" myself :}, but eager to know what to look for in cases like this in the future.


Cheers,
Alex

Alex, the runup is generally done with about half the max RPMs. Somewhere in the 1700 to 2000 rpm range. When running on only half the cylinders but with full throttle, I would not be surprised to see the engine actually reach that RPM so you would not necessarily find anything wrong then.

At the start of the take-off run there should be a last check, and that's that the engine is actually achieving a given minimum "static" RPM when the throttle is fully opened. This is somewhere in the region of 2300-2400 RPM. Did you also do this check? It's a very rare engine that's able to reach its minimum static RPM with only half the cylinders operational.

BackPacker, I went through the normal pre-takeoff and only put it to 1700RPM. I didn't even try to fully open the throttle.

But then again, I only go full throttle on actual takeoff, and don't perform a last minimum static RPM check (never been told, not in checklist).

So many things to learn! As they say, real training begins after you got your power ticket...

Thanks ;)

Alex

Alex,

How about these final checks that you can say to yourself, and therefore force yourself to glance quickly at the correct instrument, as you are starting to roll?

RPM - Good?

Temps and pressures - In the green?

Airspeed - Increasing?


RPM - if not what you are expecting then STOP.

T&Ps - if not greens then STOP.

Airspeed - if not increasing.....then you may have left the pitot cover on...and that won't help you at all = STOP.

An easy mantra that doesn't need a checklist but gives you that extra last chance to make sure that most things are working properly.

Did your instructor not tell you to do something like this?

G:ok:

For this check to have meaning you have to know what the alowable static RPM range is (this information will be in the POH). For example for the C 152 it is 2280 to 2380 RPM. So when you have got the throttle full in that is the number you should be seeing. Any lower or higher RPM is immediate grounds to reject the takeoff. Unfortunately my experience is that virtually none of the PPL's and even their instructors, I flown with know what the correct static RPM range is for their airplane and everybody just seems to use the "OK I have lots of RPM" method, which is pretty much useless as a check.
Note: This of course applies only to aircraft with fixed pitch props.

gijoe & Big Pistons Forever: Unfortunately you guys are right, static RPM check wasn't really taught. Until now, I relied on my guts-feeling on takeoff if something was not right. Although I force myself to go through an organized pattern to check things during takeoff, I still feel quite stupid as I have the impression that so many details still go unnoticed even though going through my checklists.

This is what I meant with "So many things to learn"....

Keep it up! :ok:

Alex

I understand the concept of leaning, never had to do it myself yet.

If leaned out at altitude and you want to start a decent say from 6000ft to 3000ft, what would you do?

Put mixture all the back to fully rich then re-lean at 3000ft?

liam548
 

Why have you never leaned the aircraft that you fly?
The engine should always be leaned in the cruise below 75% power otherwise you cant get the flight manual fuel flow.

How can you make the fuel part of your flight plan work if you don't know the numbers?

Liam,

That depends. If the aircraft is not equipped with an EGT indicator, I'll go full rich and lean out again. If it has en EGT (or even a G1000 :cool:) I'll readjust for the right temperature.

However, if I go to 3000ft, I'd just go full rich.

Question to the others:
Is this the right procedure? Or would you start the leaning process all over again everytime you have to adjust?

But I agree with A&C: How do you do your fuel planning then?

Cheers,
Alex

Ive not leaned because all my flights up to now have been short local (ish) ones and below 3000ft hence no need to.

I am interested in learning the correct technique hence my question above. (and your question number 4 Alex in your original post)

So back to full rich then re lean when settled in new cruise altitude?

Ask your instructor to cover a long brief on mixture control. Its a subject that needs to be covered properly(just my opinion!). Also have a read of the POH as well as the engine handling notes published by Lycoming. All will become clear.

Liam and Alex, have a look with the search under thread titles "lean", "peak", and "LOP". There's a book worth of good info written by people who know there stuff. Look up John Deakin's "Pelican's Perch" columns and read each of them several times.

You won't be following many of the LOP techniques people like Deakin suggest since you're in a carberated aircraft without all cylinder EGT/CHT, but the general knowledge to be gained there is pure gold. I used to be afraid of red knobs :}, but hours of reading has taught me that much of the damage blamed on "too lean" is rubbish. Its hard to know what to believe, instructors have told me things I know to be false, and there's so much that I don't know, but I've yet to hear anyone say that Deakin is mistaken about something!

The Lycoming info is, I feel, pitched in a less helpful way. The LOPers and Lycoming pretty much say the same thing, but Deakin's columns have much greater detail and less fear mongering. Rather than just "Rich is cooler so just do that OK?", there's a breakdown of what exactly is actually happening inside the engine.

liam548
 

Wrong answer........... please show me the part of the flight manual that says you don't lean below 3000ft.


The flight manual is not optional reading.

Actually, I just checked the PA-28-161 Cadet POH and the AFE C152 notes I have here (sorry, that's all I have available) and they do not specify a minimum static RPM that should be achieved in the early stages of the take-off roll.

The only place I found a reference about static RPM is in the Limitations section, and that only specifies a minimum and maximum static RPM with regards to propellor tolerance. And only for Sea Level/ISA conditions.

So although there is good reason to check the RPM straight after applying full power at the start of the take-off roll, I doubt whether all planes will have the minimum RPM number in their POH. Your gut feeling may be the best to go by.

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.