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

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 . 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..

[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

[Big Pistons Forever]

Originally Posted by 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.

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!

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]

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?

[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 ) 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]

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.

[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.