Rough engine on run-up

[AlexUM]

Hi everybody,

So I'm a rookie-PPL getting my hours for a license conversion blabla...anyway: I was going to fly the club's 152 today. During the run-up, the engine was running quite rough and vibrated a lot. Doesn't feel like it should so I request a taxi back to the ramp. Turns out that two cylinders are out as it has been running too lean.

First I thought the whole thing was no big deal but now I have tons of questions.

1. What if I had attempted a take-off? Would I have made it into the air or into the woods at the end of the runway?

2. How do you notice in-flight that the engine is running too lean? One of our 152s doesn't have a EGT indicator. Is it only a power-loss? Does it run rough?

3. I have been taught to always lean out the engine on longer flights, more range, etc pp. But I'm getting more and more the impression that if I'm not going over 5000ft on a long x-c, I just shouldn't bother leaning.

4. How is the engine leaned properly anyway? Sure, turn the knob slowly until a slight RPM-drop, then turn it the other direction. But how is it really done?




Thanks in advance for your answers!

Alex

[A and C]

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.

[Fuji Abound]

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.

[AlexUM]

Hey guys,

First of all, thanks for your answers :ok:

From what I've heard after our mechanic made a first check, two cylinders have almost no compression. So it seems the valves also have suffered some damage. Can this be due to a single-time running too lean? Or must have somebody flown the aircraft consistently too lean?

This is a training aircraft flown by students every day, and this is the second time this happens in one year (!). This also makes me wonder if our CFIs should rethink their part of instruction regarding leaning.

@Fuji: You're right :uhoh:. There seems to be a common attitude of "If it's not mine, I don't care..." nowadays.

However, I'm still wondering what would have happened if I had attempted a take-off.


Cheers,
Alex

[Vizsla]

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:=

[fernytickles]

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:

[AlexUM]

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

[BackPacker]

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.

[AlexUM]

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

[gijoe]

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:

[Big Pistons Forever]

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.



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.

[AlexUM]

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

[liam548]

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?

[A and C]

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?

[AlexUM]

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

[liam548]

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?

[MIKECR]

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.

[Aerozepplin]

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.

[A and C]

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.

[BackPacker]

For this check to have meaning you have to know what the alowable static RPM range is (this information will be in the POH).

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.

[AlexUM]

Hey guys!

Aerozepplin, thanks for those hints! Guess I'll spend the night reading then.

A&C: You're right, lecture of the flight manual is not optional. However, being only taught the basics about leaning, I want to be on the safe side, as many other greenhorns, and prefer to fly a bit too rich before I know more about the subject. But, until now, my fuel numbers always worked out ;)

And to continue with those 3000ft: That's about 500ft above the pattern altitude of our surrounding fields and I will most probably have gone through my inbound-checklist already a couple of minutes ago when I started my descend.

If I go x-c I want to climb as high as possible. If there's somewhat minimum VMC, I just won't fly. 3000ft for me means patternwork and practicing maneuvers, so no leaning as I'll hit the throttle quite often.

So yes, you're right, there is no reason why not to lean at any altitude, but I guess it always depends on local factors, as so many other things :)

BackPacker: The guts it is then :ok:

Going for a beer ;) ...
Alex

[Aerozepplin]

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

[Big Pistons Forever]

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.

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"

[Tagron]

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 ?

[BackPacker]

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.

[mary meagher]

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!)

[AlexUM]

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

[pasir]

....... 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 !

[Biggles78]

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.

[Mark1234]

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..)

[AlexUM]

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

[Pull what]

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.

You would however clearly feel the vibration

[Crash one]

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.


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?

[Big Pistons Forever]

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.

[Big Pistons Forever]

.

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..)

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.

[AlexUM]

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

Pelican's Perch #46:<br>"But My Mechanic Says ..." (http://www.avweb.com/news/pelican/182135-1.html)

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

Cheers,
Alex

[mm_flynn]

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?
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).

[Piper.Classique]

You would however clearly feel the vibration
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.

There are two mags in order to provide a full burn, not as a redundancy / choice exercise
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.

[Crash one]

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.

[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

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.

[Mark1234]

AlexUM - wasn't calling you into question, just suprised..

Crash one - think it's been covered pretty comprehensively, but yes, water was causing a missfire somewhere on the system for that on one mag, hence the roughness; i.e. it wasn't working *consistently* on that mag. Eliminating it from the equation leaves smooth running, at a (very) slightly reduced power. I'm not suggesting that's normal behaviour - in this case the dousing simply upset something that was probably failing anyway.

Yes, you do get better burn with both mags, but it's perfectly possible to design an engine that burns well with one (some cars, notably alfa romeo have run twin-plug installations). A two plug system running on one will most likely be worse than a one plug system running on one, but dropping one mag out does not make that much difference - IIRC about 200rpm from the cruise setting in this case.

[AlexUM]

AlexUM - wasn't calling you into question, just suprised..



Mark, Never thought you were :)

Update on the engine: Student on solo X-C. Went up to 8500ft and leaned out. On the way home forgot about the red knob (checklist? :=) and flew a couple of patterns practicing his short-fields before full stop for about 45min. Can this single-time misadjustment be the reason? We need two new pistons and the corresponding valves :uhoh:

Cheers,
Alex

[mm_flynn]

Mark, Never thought you were :)

Update on the engine: Student on solo X-C. Went up to 8500ft and leaned out. On the way home forgot about the red knob (checklist? :=) and flew a couple of patterns practicing his short-fields before full stop for about 45min. Can this single-time misadjustment be the reason? We need two new pistons and the corresponding valves :uhoh:

Cheers,
Alex

If the student leaned out to max power (something like peak EGT) at 8500 feet. I would have thought the engine would cough splutter and rattle at full power/sea level (it would be way lean of peak on at least some cylinders). The student couldn't help notice this.

HOWEVER, if the student 'sort of leaned' (at 8500 you need quite a bit of red knob back in most planes) they could have gotten to say 150-200 ROP (back to a reasonable performance level). In this case, when doing pattern work, they could have been at full power at peak EGT, deep deep in the Red Box and could have crisped the cylinders in a few minutes let alone 45 minutes.

The engine makes 100% power fairly rich of peak with a lot of partially burned fuel. Same RPM and Throttle setting, but with the mixture set to peak and you will pull much more than 100% out of the engine - and that is not a great idea for longevity

[Big Pistons Forever]

Another cause of burnt cylinders is prelonged climbs at low airspeeds on hot days. This will be exacerbated by engine baffles which are in poor condition, which seems to the case in many trainers. Since most trainers do not have a cylinder head temperature guage the first indication of engine overheating will be high oil temp, a lagging indicator, assuming the pilot is even monitoring the oil temp in the climb, a big assumption based on what I have seen with most PPL's....and their instructors.... By the time you see redline oil temps the cylinders will be well on their way to being cooked.

Another scenario which I think is relatively common arises from the fact that the student rarely leans the engine during their training. When they get their PPL and start going places they will lean the engine and then forget about it. Therefore if they decide to climb they wil probably just shove the throttle full in. To avoid this I make a big point in the after solo phase of the PPL that the student notes the mixture position (and indicates this to me by touching the mixture knob) before adjusting power for any flying outside the circuit. This to inculcate the habit of always linking throttle movements to the mixture setting, which will avoid the scenario mentioned above.

[gijoe]

Alex.

Going back to our posts about static RPM on 13th May I flew yesterday and rejected the takeoff when the RPM was much less than it should have been. Static should have been at least 2350 but was only 1900.

Fixed prop, shortish runway, no room for messing around on the climbout etc.

Rejected takeoff, the plugs pulled out and cleaned, back in and it was fine...after a very thorough run up with me and the engineer.

This is why you should do those final checks as you are rolling.

RPM
T&Ps
Airspeed

Have fun!

G:ok:

[Pull what]

This is why you should do those final checks as you are rolling.

RPM
T&Ps
AirspeedSo with a performance take off on a short strip you would do those checks while rolling?

[mcgoo]

Yes, why not?

[Jhieminga]

So with a performance take off on a short strip you would do those checks while rolling?
Line up on the end of the runway, brakes on, full power, note RPM, T&Ps, release brakes, note Airspeed, etc...

[Mark1234]

Pull what - It should be the work of a split second to glance inside and note these during the takeoff roll - any takeoff roll - do you not always check that the ASI is alive anyway?

Unless there's a confusion and you're thinking that you check them rolling down the runway prior to starting the takeoff roll?

I'm personally pondering how you detect a loss of power with a CS prop - other than shaking, banging, or dramatic loss of power, likely to be quite insidious..

[palou89]

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

Pelican's Perch #46:<br>"But My Mechanic Says ..." (http://www.avweb.com/news/pelican/182135-1.html)

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

Cheers,
AlexI'd encourage you to keep reading those articles. After quite a good amount of reading, I'd definitively recommend leaning as long as you know exactly how much power (MCP %) you are producing at any time. Set power to 65% and lean to peak rpm, then enrichen 1 turn. Cant go wrong with that.

Leaning will do the following for you: Cleaner cylinders, less carbon deposits/build-up, cleaner sparkplugs and prevent valve sticking. Its not about fuel savings, its about running your engine properly and extending its operating life.

On the ground, lean until you can barely reach 1200rpm. At this power setting its absolutely impossible to damage the engine because of agressive leaning. At full rich the fuel/air ratio is WAY above what is required for such power setting so dont be affraid to lean on the ground. The overly rich mixture is designed to provide sufficient engine cooling at TO power on a low density altitude day. Leaving the mixture fully rich will result in incomplete combustion --> more carbon. Another advantage of using such an agresive leaning on the ground is that it will prevent you from taking off with a leaned out mixture, wich could lead to severe detonation if a low density altitude condition exists.

Unfortunatedly all of Deakins analysis are based on big bore, fuel injected engines with very reliable fuel injectors and engine monitoring instruments. At least it seems that for that kind of set up, he has managed to go beyond the recommended TBO with his engines following more or less the recomendations I've given above.

Remember, do not lean with a high power setting unless you have the appropriate monitoring gauges CHT/EGT for all cylinders and can efficiently monitor them.

I used to fly at Embry-Riddle Florida, where (as aproved by the POH) we would start leaning at 1000ft AGL to the top of the green arc on the F/F gauge (12GPH) and 1/4 of turn for each 1000ft there after on XCs. The 12GPH mark would result in about 1" of mixture out. Of course this was an approved procedure, so I definitively do not encourage you doing the same on a 152. I simply wanted to show that its not always "DO NOT TOUCH THE RED MISTERIOUS LEVER OR BAD THINGS WILL HAPPEN"

As a final advise, take a GOOD look at the CHT/EGT/SFC/POWER chats, you can learn something there for sure.

http://img709.imageshack.us/img709/7253/dibujojbj.th.jpg (http://img709.imageshack.us/i/dibujojbj.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

http://yfrog.com/jpdibujojbjj

[Big Pistons Forever]

Pull what -

I'm personally pondering how you detect a loss of power with a CS prop - other than shaking, banging, or dramatic loss of power, likely to be quite insidious..

For turbocharged engines it is easy, you should get full red line Manifold Pressure (MP) on every takeoff. Any less is grounds for an immediate reject.

For non turbocharged engines you should get about 1/2 inch less than field barometric MP on takeoff (field baro is what the MP guage reads when the engine is not running).

Obviously in both cases you would also check that you have full redline RPM as well.

On a related topic one area that does not IMO often get the attention it deserves is noting the fuel flow on takeoff in aircraft with fuel injected engines. All light aircraft engines depend on excess fuel flow to help cool the cylinders at full power. The fuel flow should be right at the redline maximum value (or even a little bit over) at full power. If it is even a little bit low it should adjusted by an engineer. Prolonged operation without suficent fuel flow will damage the cylinders.

[gijoe]

Like McGoo said, why not?

Soft field takeoff = don't stop at the end.

Shortish = probably enough time to start rolling and look at everything.

...it actually wasn't that short but practice etc. and I was only trying to get the point across...by putting it into context...hoping that Alex would remember it...

Anyway...

G:ok:

[Mark1234]

What effect would (say) a burnt valve, fouled plug etc., have on that manifold pressure? My gut feel says nothing.. the MP gauge is reading just that - MP. If the engine is spinning at the full RPM, the MP wouldn't be affected by the amount of power the engine is generating, it's still pumping the same volume of air, right?

Less sure for a turbo'd engine, but the logic should be the same.

P.S. Ditto the fuel flow - both measure the inputs, and don't really give a clue whether there's power being produced, so long as there's enough oomph to get the prop into the governing range.

[gijoe]

I think that the subject of this original discussion would not have had a MP gauge - only a RPM.

Over to Alex.

[Mark1234]

Indeed, the subject of the original post appears to have been a fixed pitch prop, as such there is rarely a MP gauge, or really that much need for one. What is your point?

Static RPM is meaningful as a measure of engine output ONLY if you have a fixed pitch prop. I was pondering the manifestation of the same problem with a CS prop due to personal interest (being as I tend to fly behind CS props). Big Pistons Forever provided a suggestion, which to be honest, doesn't make sense (to me). As far as I can see, there is no way of objectively observing the power output when a wobbly prop is fitted. Perhaps someone knows better.

I don't think it's terribly off-topic for the thread.

[BackPacker]

I don't think it's terribly off-topic for the thread.

And even if it were (which I don't think it is): We're at page three already and the OP has been answered, so you're perfectly entitled to change the topic slightly... :O

[Big Pistons Forever]

Mark123

In large radial engine aircraft engine power (Torque) was directly measured and indicated on seperate dedicated (BMEP) guages. However there is no guage in a light aircraft which directly measures engine power. This is because for a non turbo/supercharged simple flat 4 or 6 cylinder light aircraft engine running normally, there is a very strong correlation between the induction manifold pressure and the power output. POH cruise charts show the percentage of power developed at a variety of RPM and MP combinations. Most will have a column for % of power and corresponding MP's at maximum RPM....or the takeoff condition. If at sea level your engine is only showing say 25 in at full RPM than it is only making about 75% of it s maximum horsepower....which IMO would be a good reason to abandon the takeoff.

Bottom line there is no way a normal smooth running engine with correctly calibrated engine guages could display full MP (close to field baro) and full RPM and not be making full power for those conditions.

[BackPacker]

If at sea level your engine is only showing say 25 in at full RPM than it is only making about 75% of it s maximum horsepower...

Well, that would be the case if there's a manifold restriction somehow, despite the full open throttle (dirty or blocked air filter, debris, kinked cable, throttle cable/linkage problem, whatever).

But suppose there's no problem in the inlet manifold whatsoever and the throttle is wide open. At max RPM the air is being sucked in normally so you have a slight pressure drop vs. ambient exactly like you would expect in a full throttle situation. But there's another problem. Say the engine is only running on one set of magnetos (or even less). Or there's a fuel flow restriction (e.g. dirty filter) which causes the engine to run too lean, but still (just) develop max RPM with the prop fully fine. Or something else internally to the engine. You wouldn't see that through an MP reduction or rise, would you?

[Big Pistons Forever]

In the case of a dead mag I can say from personal experience that the engine will not develop full MP because I aborted a takeoff in a twin last year after noticing in the "good engine check" the MP needles were split. It turned out the mag coil had died. As for a too lean mixture two things will happen

1) The engine fuel flow is so restricted that there is not enough to support full combustion at max power and the engine will start to run rough. or

2) There is enough fuel to support full combustion but not at the over rich condition necessary to allow proper cylinder cooling at high power.....in which case you will get abnormally high CHT's assuming you did not catch the problem during the runup.

Frankly guys if you are bound and determined to find some possible combinations of factors which could result in an indication of full MP and RPM and yet have the engine developing significantly reduced power I am sure you will probably be successfull.

However since from what I have observed over many years instructing most PPL's do not

1) Actually check the field baro indication prior to start up to see if the MP guage is reading a sensible number.

2) Do not know what they should be looking for in the runup particularly when it comes to testing the mixture control,carb heat,mags and cycling the prop

3) If they look at all at the engine guages at the start of the takeoff (which many do not) look but do not see.

So my recommendation is rather than worry about the .1 % probabilty problems...... concentrate on the basics so they get done properly on every single takeoff.

Finally if the aircraft is not accelerating normally it doesn't necessarily mean that the engine is not producing full power which points out the importance paying attention to what the aircraft is telling you and aborting (early) any takeoff which doen't feel right and for short fields having a physical go no go point allready established in your mind.

[Mark1234]

Bottom line there is no way a normal smooth running engine with correctly calibrated engine guages could display full MP (close to field baro) and full RPM and not be making full power for those conditions.

Frankly guys if you are bound and determined to find some possible combinations of factors which could result in an indication of full MP and RPM and yet have the engine developing significantly reduced power I am sure you will probably be successfull.

So my recommendation is rather than worry about the .1 % probabilty problems...... concentrate on the basics so they get done properly on every single takeoff.


That is rather the point - for a 'normal' smooth running engine there is a great correlation, absolutely. Nor am I setting out to find obtuse .1% probabilities to pick holes, or be pedantic. Maybe it's a burnt valve, or just some fouled plugs, but lets consider the dead mag....

In the case of a dead mag I can say from personal experience that the engine will not develop full MP because I aborted a takeoff in a twin last year after noticing in the "good engine check" the MP needles were split. It turned out the mag coil had died.

I have no knowledge or experience of turbocharged aero engines, but I guess your twin in this example must be - that is the only way I can conceive any feedback from combustion to the manifold pressure (altered combustion could affect the exhaust, which might affect the turbo, which might in turn feedback to inlet manifold pressure).

However, I cannot understand how it would make a difference for a normally aspirated engine - if it's turning full RPM, it sucks a set amount of air. The MP gauge has no clue whether the air is being used to oxidise fuel or not. If you could spin the prop at redline rpm on the starter with both mags off, it would show the same MP as it would running normally, and probably the same fuel flow - but CHT / EGT would be zero.

However, I guess in thinking it through I've answered my own question, the only thing that will tell me something is wrong is the seat of my pants.. EGT might give some clues - if it reacts fast enough (and yes, I know to look for a rise in EGT on one mag..)

[Big Pistons Forever]

Mark1234

My final comment.

When I am flying an aircraft with a CP prop and on application of full throttle the engine smoothly accelerates and I note that it is indicating close to field baro MP (or redline MP for a turbocharged engine), full redline RPM, full redline fuel flow (for fuel injected engines) and all other engine guages have normal indications I will be satisfied that it is almost certainly delivering full power and I will now concentate on controlling the aircraft.

You seem to be convinced that the above check does not assure you of full power on takeoff. I guess it is up to you to do what ever you feel necessary as the PIC and therefore I have nothing usefull to furhter contribute to this issue

[Pull what]

Pull what - It should be the work of a split second to glance inside and note these during the takeoff roll - any takeoff roll - do you not always check that the ASI is alive anyway? In my opinion the best task management shews that the best place to check Ts & Ps and Power is stationary against the brakes on a short field take off-its one less thing to do on the take off roll where you need to be more aware of airspeed, acceleration and surface. You are also more likely to be on a surface or area which does not have the same protections as a licensed airfield

It may only take you a split second to check but you obviously have not sat by enough students or low hour PPLs for long enough to know differently! Delaying the check, and again its something you see quite often, on a crucial take off can put the pilot into a dangerous go - no go, area. The best place to find out you have an engine problem is stationary at the beginning of take off, not when the hedge is coming up very quickly and you are trying to make a performance take off.

You also need to be sure where and how full power is indicated, for instance what setting are you going to accept as full power-to a pilot who has never seen or checked full power before and believe me there are many, this may not be as simple as it seems, especially on an unfamiliar type.

A slight deviation but it should be rembered that after selecting anything crucial on an aircraft a check should be made to see if you have the selection you have made-SELECT - CHECK, is missing from many pilots mental check lists.

It may all sound like splitting hairs but good task management is the hallmark of good airmanship - doing the right thing at the right time.

[Big Pistons Forever]

You also need to be sure where and how full power is indicated, for instance what setting are you going to accept as full power-to a pilot who has never seen or checked full power before and believe me there are many, this may not be as simple as it seems, especially on an unfamiliar type.
.

Full power should be checked on every takeoff not just the short field ones. However since normal takeoffs, unlike short field takeoffs, do not start with full power before brakes release than the check obviously has to be done on the roll a skill which should be taught at the beginning of the PPL. Your point about many pilots not knowing what engine indications constitute full power is unfortuantely very true and was why I made the point about the importance on knowing what the static RPM limits of the aircraft earlier in this thread.