Piston Engine Ops

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Hedley
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Piston Engine Ops

Post by Hedley » Wed Dec 01, 2010 9:27 pm

A few recent events have suggested to me that people need to review some piston engine operating considerations, especially now that winter is upon us.

1) I helped friend recently change a cylinder. Despite the low time, there was significant scuffing on the piston skirt

2) during an annual inspection of a different aircraft, I observed shiny, non-ferrous metal in the pressure screen (aluminum, from guess where)

Both of the above are signs of excessive RPM immediately after starting. Remember that our air-cooled engines have aluminum pistons and steel cylinder barrels (to which aluminum cylinder heads are screwed on).

If you get too much heat in the combustion chamber, too soon, the aluminum piston rapidly heats up and expands much faster than the cold steel cylinder. It's a function of the different materials (aluminum and steel) which can be used to make a cheap thermometer, like you'd find in your house thermostat.

So after starting your Lycoming/Continental/etc, idle it as the slowest possible RPM that it will run smoothly. This should distinctly be less than 1000 RPM. This is especially important during the winter - the colder the engine is, the more critical the piston-to-cylinder clearance is.

This initial idle (eg 800 RPM) I maintain for at least a minute or two, for the above considerations. After that, I like to normally idle at 1100 RPM with the mixture leaned for max RPM, to try to reduce plug fouling. I stay there until the Cylinder Head Temperature (CHT) is at around 200F.

Once I see 200F CHT, it's ok to rev the engine up to 1700 RPM and test the mags, cycle the prop, etc.

After runup complete, back to 1100 RPM and full lean mixture idle.

For takeoff, I like to see around 300F CHT. My redline is 400F CHT. If you're seeing CHTs over 400F you've got a problem (baffling, mixture, etc) and it needs to be fixed to avoid impacting engine life - even though Lycoming says 450F CHT is redline.

So you see, esp for winter ops, CHT is king. Notice that I haven't even mentioned oil temperature. That's because with multigrade oil, it's completely unimportant, in that you don't need to get it up into the green arc temp range (eg over 100F) before takeoff, like the old straight grade oil.

Now, you do need to see at least 150F oil temp in cruise, eventually. If you don't, you aren't burning off the water in the oil, and that's bad. You need to partially block off the oil cooler in this situation. Various manufacturers make "winterization kits" - I installed 2 today - which restrict the flow of air into the engine.

One thing you do need to worry about oil, is it's "pour point". Below this temperature, the oil will congeal - often in the oil cooler - and that can be really bad.

I am a big fan of aviation multigrade oils for winter piston ops. If you look at the spec for Aeroshell 15w50 you will discover that it has a pour point of -42C, which is fine with me, because I don't fly when it's colder than -40C.

However, straight grade Aeroshell 100Wplus - a marvellous summer oil - has a pour point of only -20C and is thus not suited for winter ops, even if you pre-heat the living crap out of it for every start, to avoid metal-on-metal from slow initial lubrication after cold starts, due to excessively thick oil. A symptom of this is excessive oil pressure after a cold start.

But I digress about the oil. What matters to your engine, esp in winter, is CHT. After start, lowest possible RPM that it will operate smoothly at - eg 800 RPM. No faster than 1000 RPM for a minute or two, depending upon ambient temperature and preheat. After that, normal idle 1100 RPM with mixture leaned for max RPM. No runup until the CHT comes off the peg - eg 200F. CHT should be approaching 300F for takeoff, and redline 400F.
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Tim
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Re: Piston Engine Ops

Post by Tim » Thu Dec 02, 2010 6:15 am

instructors make sure your students know this info!

one of my big pets peeves when i instructed was students immediately throttling up to 1000RPM after start (as per the checklist :roll: ) or worse having the throttle positioned so that it goes above 1000RPM as soon as the engine catches.

i found it pretty rare that students would know that 1000RPM is a max on a cold start, not a target. it was a frequent debrief point on supervisories.

when i was student we ate through 6 of 9 camshafts on our AEIO-360's in one winter due to high RPM starts. seeing the lobes that run 2 lifters being 1/4" smaller than the lobes than ran 1 lifter was a really good teaching tool - albeit very expensive for the school.

one big cause of this is people not knowing the proper procedure for starting/priming a cold engine. if an engine does not catch immediately DO NOT keep pushing the throttle up while cranking. when the engine DOES catch, what's the first thing it's gonna do with the throttle pushed way up?
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Hedley
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Re: Piston Engine Ops

Post by Hedley » Thu Dec 02, 2010 7:15 am

goes above 1000RPM as soon as the engine catches
I cringe whenever I see someone start a piston engine, and it immediately roars to 1500 rpm. Or even higher!

As pointed out above, even if you don't care about your pistons, especially if the engine has been allowed to sit for any length of time, the oil has drained down, and you are going metal-to-metal at 1500 RPM after start, before the oil pump has had a chance to circulate the oil throughout the engine. Horrible.

If you've ever operated a radial engine, you know that you have to sit and idle at minimum RPM. This is not optional. A friend of mine with a T-28 destroyed his engine by disregarding this. Sure enough, it started making metal and need a very premature overhaul.

Even if you don't care about how often the overhauls occur, are you happy flying behind an engine that's chewing itself to bits?
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into the blue
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Re: Piston Engine Ops

Post by into the blue » Thu Dec 02, 2010 1:01 pm

I always make sure that the throttle is set as precisely as possible for the RPM to not exceed 1000 after each engine start. 900 is the initial setting I'm looking for; it then usually rises slowly to 1000 by itself when the oil warms up and internal friction decreases. A frequent mistake that I notice is that pilots (with the best intentions in mind) push a vernier-type throttle 1/4-1/2 inch in as per POH instructions, but seem to forget that the engine has already previously been shut down at 1000 RPM and, effectively, the throttle is shoved a good inch when they engage the starter, giving a painful 1300-1500 on the tachometer. So, guys, if you shut down at 1000 RPM, then it's best to just leave the throttle untouched as a part of starting procedures (of course, that is if you were the last to fly that airplane).

Next I have a little question for the gurus here. The POH of one of the Cessna models I flew (C152) calls for the following action as a part of an engine start in temperatures below freezing (with or without preheat):
- Throttle -- ADJUST for 1200 to 1500 RPM for approximately one minute after which the RPM can be lowered to 1000 or less :shock: :!:
When I had to, I followed the instruction, but, being a little sceptical, I never went above 1200. What's the reasoning here?
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Big Pistons Forever
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Re: Piston Engine Ops

Post by Big Pistons Forever » Thu Dec 02, 2010 4:24 pm

An other issue with winter starting is to use enough prime. On cold days the engine needs an extra one, two or even three shots of prime. I have watched many pilots grind the poor starter for long periods on inadequately primed engines. I think this is often due to instructors passing on urban legends about how the engine will burst into flames if one milli litre too much prime is used.

However even if the correct amount of prime is used you can still have a problem on cold days. The throttle is correctly set but soon after the engine first fires it starts to falter and then die. The tendancy is to then to rev up the engine as a way to prevent it from stopping. A better way is to leave the primer part way out and set the throttle for 700 - 800 RPM. After the engine starts slowly feed in the prime and the extra gas will keep the engine ticking over untill it stabilizes.
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Re: Piston Engine Ops

Post by Hedley » Thu Dec 02, 2010 5:05 pm

One problem with hand primers is that they are rarely plumbed to all the cylinders.

Far more likely is that only one (perhaps two) cylinders receive gas when you pump the primer. This seems to me like a bizarre intentional handicap, for a marginal start when the battery is cold/weak. The other cylinders simply are not going to run during startup - you're hoping you can run on one or two cylinders at first.

It's far better to prime all the cylinders. You're far more likely to be successful.

Another intentional handicap: rarely are both magnetos equipped with an impulse coupling shim. 99% of the time, only one magneto has an impulse coupling, so only one spark plug will be firing.

Combined with a single-cylinder prime, you've got a lot of eggs in one basket. No wonder aircraft engines can be so difficult to start.
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Big Pistons Forever
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Re: Piston Engine Ops

Post by Big Pistons Forever » Thu Dec 02, 2010 5:20 pm

Hed

Excellent advice in your posts

With respect to the number of pimer lines , most of the common trainer types have 3 cylinder priming. You are right though if you are unlucky enough to have piming on only one cylinder (some of the PA 28 series are like this AFAIK, but then what do you expect from a cheap Piper POS) then good luck. Not mentioned but very important is to properly preheat the engine before starting in cold temps.
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Re: Piston Engine Ops

Post by KnownIce » Thu Dec 02, 2010 6:14 pm

into the blue wrote: but seem to forget that the engine has already previously been shut down at 1000 RPM and, effectively, the throttle is shoved a good inch when they engage the starter, giving a painful 1300-1500 on the tachometer. So, guys, if you shut down at 1000 RPM, then it's best to just leave the throttle untouched as a part of starting procedures (of course, that is if you were the last to fly that airplane).
Why would you shut down an engine from 1000 RPM?
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Re: Piston Engine Ops

Post by Hedley » Thu Dec 02, 2010 6:17 pm

Engine pre-heat is crucial for several different reasons:

1) reduce viscosity of oil, so that it flows sooner and reduces metal-on-metal at startup

2) increases tolerances between different metals

3) starter can turn engine over easier because of (1) and (2) above

4) improves evaporation of 100LL (mogas starts MUCH better in winter) for better initial combustion

What pisses me off is when the oil temp DECREASES after starting in the cold, after a pre-heat!
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Re: Piston Engine Ops

Post by Hedley » Thu Dec 02, 2010 6:45 pm

Another pre-heat tip:

Batteries lose cranking power in the cold - it's a fact of chemistry. Also, your battery may have lost some oomph if you haven't flown it recently.

So, in addition to pre-heating your engine, to help get a good start, I like to put a little charger on the battery. This both tops up the volts, and actually physically warms the battery, both of which will give you a better start.

Remember that a cheap, constant-current charger CANNOT be left on a battery indefinitely - it will overcharge and damage it (dry cells out, etc). You must remove it after it has driven the voltage up, generally after a number of hours.

However, you can get more expensive constant-voltage chargers (aka tenders) which reduce the amps as the volts come up, and are safe to leave on permanently. If you have electrical power in an unheated hangar, I highly recommend one of these. Remember, if you don't fly your airplane for a while, the specific gravity of the acid will drop which can permit the fluid to freeze on a cold day. And when it freezes, it expands, cracks the battery, and acid runs out into your airplane. Not good.

If you're not going to put a smart charger on your battery, seriously consider removing it from your airplane if it's not going to be frequently flown in the winter. I know three people who wish they had done this (homebuilt, certified biplane and Piper).

Here's an incredibly cheap ($20) smart 12V charger on sale at Canadian Tire: http://tinyurl.com/3a327rv

PS It oughta be obvious that I worship at the Church of the Internal Combustion Engine, where our litany is the Four Strokes: Suck / Squeeze / Bang / Blow :wink:
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Re: Piston Engine Ops

Post by Hedley » Fri Dec 03, 2010 6:43 am

Why would you shut down an engine from 1000 RPM?
Read Lycoming Service Letter 185B:

http://www.lycoming.textron.com/support ... SL185B.pdf

which recommends techniques to avoid lead fouling your bottom plugs:
Prior to the engine shut-down, the engine speed should be maintained between 1000 and 1200 RPM
until the operating temperatures have stabilized. At this time the engine should be increased to
approximately 1800 RPM for 15 to 20 seconds, then reduced to 1000-1200 RPM and shut-down
immediately using the mixture control.
After reading Lyc SL185B, can you see why, above in the OP I recommended:
normal idle 1100 RPM with mixture leaned for max RPM
after the initial cold start minimum RPM operation?

A trick I learned from a long-dead three-decade CFI: if you have to start an engine without pre-heat (it happens) and you have the battery for it, start the engine and run it at one minute at minimum RPM, then shut it off. Let it sit, and the heat conduct throughout the engine to equalize temperature for clearances and to warm the oil. Then, start it again. This technique minimizes damage from a non pre-heated start.
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Re: Piston Engine Ops

Post by Hedley » Fri Dec 03, 2010 6:56 am

BPF: the above nicely leads into a discussion on hand-propping. Your turn.
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Re: Piston Engine Ops

Post by KnownIce » Fri Dec 03, 2010 8:35 am

Thanks for the link Hedley. I usually idled at 1000 (leaned) getting ready for shutdown then went throttle-idle immediately prior to mixture cutoff assuming it was easier on the engine to drop RPM gradually but I will change that practice.

The same bulletin however contradicts (I think) the startup process you outlined in your first post. ie.
2. The engine should be operated at engine speeds between 1000 and 1200 RPM after starting and during the initial warm-up period. Avoid prolonged closed throttle idle engine speed operation (when possible). At engine speeds from 1000 to 1200 RPM, the spark plug core temperatures are hot enough to activate the lead scavenging agents contained in the fuel which retards the formation of the lead salt deposits on the spark plugs and exhaust valve stems. Avoid rapid engine speed changes after start- up and use only the power settings required to taxi.
Do you think running at minimum smooth RPM generates enough heat on the spark plug core? I guess fouling is a lesser issue than scuffing the pistons during the initial warm-up, especially when you are minutes away from doing a run-up, but this isn't addressed in the bulletin.

If operating an aircraft without mixture control, or CHT gauges, would you do anything differently?

Thanks very much to all the contributors of this thread. As a recreational PPL this is very relevant to my flying.
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Re: Piston Engine Ops

Post by Big Pistons Forever » Fri Dec 03, 2010 9:37 am

Hedley wrote:BPF: the above nicely leads into a discussion on hand-propping. Your turn.
So it's a cold morning and you go to start. The prop turns about 10 deg, comes up against a compression and stops. What to do ?
Well you could hand prop, but it is worth first trying another approach. Assuming you have allready primed the engine turn off the mags and get and pull the prop through one turn for each shot of prime, then turn the prop untill it is just past a compression (that is the resistance to turning suddenly lessens). Get in and hit the starter again. The prop will have longer to turn before hitting resistance and will hopefully have enough momentum to get over the first compressions. The engine should have enough fuel vapour in the induction system as a result of pulling the prop through that it will fire on the first cylinder and then hopefully the engine will start. This technique has twice saved me from being stranded. BUT you need to be extremely carefull because if the mags are not grounding, the engine could start at any time throughout the procedure. Therefore make sure the aircraft is chocked, the throttle is not open, mixture is at ICO, and flip the prop a if you were hand propping.

I think most starting problems start with a weak battery. Aircraft batteries are designed for to be as light as possible for obvious reasons, however the downside is they are not very robust. The practical life of an aircraft battery is about 3 years, so if your battery is at that age you should consider replacing it in advance of the start of cold weather.
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Re: Piston Engine Ops

Post by Shiny Side Up » Fri Dec 03, 2010 12:03 pm

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Re: Piston Engine Ops

Post by into the blue » Fri Dec 03, 2010 12:12 pm

A question for Hedley:

I know it's possible, but, in reality, how likely it is to "hydraulic" a horizontally-opposed engine with liquid fuel after an accidental overprime (mostly in cold weather with no preheat when primed fuel evaporates poorly)? One technique I heard is to engage the starter briefly with the mixture at idle (float-type carb) and let the engine to turn through one revolution. If no resistance is seemed to be encountered, then mixture can be enriched while the engine is continued to be cranked. But is the risk appreciably high enough to go with this a bit odd sequence?
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Re: Piston Engine Ops

Post by Hedley » Fri Dec 03, 2010 3:48 pm

how likely it is to "hydraulic" a horizontally-opposed engine with liquid fuel after an accidental overprime
It absolutely can happen! Fuel is just as incompressible as oil - they're both liquids. I remember reading an accident report (Cessna 210) which had a connecting rod failure, and it was blamed on over-priming resulting in hydraulic lock which bent the connecting rod.

How I start our Maule (which has a Continental IO-360, and Continentals are known for their delicate starters!):

1) prime the engine until fuel runs out the bottom of the cowl. If fuel doesn't run out the bottom, it won't start (TCM GTSIO-520 in the C421B is exactly the same)
2) mags off, mixture lean.
3) pull the prop through 6 compression strokes
4) move the airplane back a couple feet to move the puddle of fuel ahead of the airplane
5) get in and start

That's worked for 39 years for me, so far. YMMV.
there's really no cause to start a non heated engine
Sounds good in theory, but sometimes reality intrudes. That long-dead CFI and I once went and picked up a 172 that had been abandoned by a student pilot during the winter at an airport with NO services. None. Calling it an airport is an exaggeration. It was just an empty runway, out in the middle of nowhere. No electrical power. No hangars. Nothing.

I suppose we could have just left the 172 there for the winter, until it warmed up in the spring?
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Re: Piston Engine Ops

Post by Shiny Side Up » Fri Dec 03, 2010 4:52 pm

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Re: Piston Engine Ops

Post by Hedley » Fri Dec 03, 2010 6:24 pm

I once tried the generator thing when we went to pick up the C421 when it was stranded, and it really didn't work very well. It had a lot of trouble trying to run both heaters, and didn't do a very good job of heating up the stone-cold GTSIO-520's outside, which simply won't start below zero C.

Next time I'm involved in a goat rope like that, I'll bring some ducting and a propane forced air heater and some way to generate a little 120VAC for the fan. Mind you, I'm sure a BBQ propane tank would be completely illegal to carry onboard. Sigh.
axecraft and firecraft
I'm not exactly the biggest fan of Cessna products, but making the 172 into a bonfire for a youtube video seems a little excessively cruel. Even worse than rick-rolling.
If operating an aircraft without mixture control, or CHT gauges, would you do anything differently?
Gosh, it's been a while since I flew without a mixture control - C120? Turned the engine off with the mags, IIRC. Felt weird. Anyways, with no mixture control, there's not much you can do except try to idle at 1100 RPM to try to stop the plugs from lead fouling and hope for the best.

With no CHT, you have to guess, using either time adjusted for ambient temp, or some other proxy source of info.
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