View Full Version : Piston propeller check dilemma - which way?

Pugachev Cobra
27th Mar 2011, 14:24
Hello there once again,

I have been listening to contradictory information regarding how to check for a piston's compression in a pre-flight.

When doing your pre-flight inspection, you turn the propeller (magnetos off) with your hands as many times as the number of cylinders in the engine. I've always heard this was both for a little lubricating before starting the engine, and also to check for the piston compression. If it's not a little hard to turn the propeller after a small amount of movement, there's a problem with the compression of one (or more) of the pistons.

Now (and critics and feeback welcome about the above procedure), I heard two versions about how you do it.

First, I heard you always rotate the propeller in the OPPOSITE direction of the normal propeller movement (with engine running). This is for safety since the magnetos would not "springload" and generate a spark, and maybe the engine could start if there's some avgas in the combustion chamber. And I was ok with this, since you only hear the magneto "clicking" when you rotate it in the same movement as with the engine running.

Now recently I heard that turning the propeller in a contraty movement of the normal one is completely wrong, because the vacuum pump could damage if you do that.

Now, I'm confused. Both ways allow you to check for the pistons compression, and rotating it in the opposite direction helps you not to harm yourself even in the case the magnetos switch are off but there is the rare case of faulty wiring or bad contacts that are not really turning the mags off.

I want to know what you think about all this, because I also never read a POH/AFM that listed rotating a propeller manually to check anything, compression, or anything like that.

And if its really a good procedure in a pre-flight, how would you do it? Normal movement or opposite movement, and why?

Thank you in advance.

27th Mar 2011, 15:19
don't move the prop for just a normal preflight check...are you a mechanic ? if so, go to A and P school or mx tech school.

if you are just a pilot, why check compression.

when you do your run up, and takeoff if the plane isn't performing well, write it up.

and one day, the magneto will be hot and you will lose your head or hand

27th Mar 2011, 15:42
It doesn't matter if he's an A&P or not, he should be checking his plane prior to starting it.

The first thing to note here is that a radial engine (few and far between these days, but I've flown them) can experience something known as static lock, which can damage the engine if left undiscovered prior to start up. The idea of "lubricating" the engine prior to start is a bit of rubbish though, disregard that no matter who you hear it from.

As far as the direction you should turn a prop it will depend on the engine, so consult your mechanic for guidelines. One important thing to do when parking a propeller plane outside is to remember to turn your props to face a direction that doesnt allow rain to come down into the spinner and pool leading to corrosion.

Hope that helps.

27th Mar 2011, 17:23
Agree with sevenstrokeroll.

And the problem with radials is called "hydraulic lock". That's when oil pools in the lowest cylinder.

I've experienced that a few times. If the prop won't turn with the starter engaged and drawing amps then that might be the cause.

But this isn't what we're talking about.

We're talking about a new pilot being taught some interesting theoretical stuff, but in real life it serves no practical purpose. Like he says, that procedure is listed NO WHERE in his manuals. This is a TECHNIQUE that is taught as procedure....one of my personal pet peeves.

Some pilots (Capts and flight instructors) like to show how clever they are with no respect to operational necessity and thence make everything more complicated and then more UNSAFE.

But it's hard to argue when you're new and junior.

One thing I learned as a new flight engineer on the DC6 (where I learned about hydraulic lock) is that when you advance the power some Capts wanted the power applied faster and some slower...

You just can't win sometimes.

Shaggy Sheep Driver
27th Mar 2011, 18:08
Hydraulic lock is a potential killer. I used to fly a Yak52 (Vendeneyev 9 cylinder radial) and it had to be pulled through before start. And if it hadn't run for more than a couple of days, the drain plugs in the inlet manifolds had to be removed else the oil that collects there would be sucked into the engine on start, giving a 'lock'.

If you are lucky, that destroys the engine and you get your cheque book out. If you are unlucky, you are unaware of the 'lock' and it just bends a con rod. Thatbent con rod will fail at some time in the future. That's what kills people.

Our Chipmunk is always pulled through pre-flight, to check compressions (valves can stick in the Gipsy engine), to check for impulse mag click (if it doesn't click it will never start and the mag needs a clout with a hunk of wood to free the stuck impulse) and to check for hydraulicing.

28th Mar 2011, 01:56
Not recommended turning backwards on the larger TCM engines with the spring starter drive. The spring has been known to catch and scrape metal from the clutch or brake or damage to the spring itself.

Turning forwards is OK and with the TCM engines and I do it regularly on the first flight out in the morning just to keep an eye on what is happening with the cylinders and the impulse couplings. I do a lot of back country operations with no engineering support other than what I can do myself so I like to look after my machine so I don't have to walk out of the hills. I only fly the one aircraft that I own and maintain myself, no one else flies the aircraft.

For a new pilot I don't think there is much point turning over the engine as it has taken years of fixing and flying to know what to listen and feel for so as a regular practice it is probably not worth the time if you operate only in civilization with engineers and workshop support.

The one rule I was taught both as a pilot and as an engineer was that the prop (engine) is always live whether the mags are on or off and to treat is as such - always!

28th Mar 2011, 03:16
it seems a bit pointless on a horizontally opposed engine, firstly hydro-lock from oil is not something they are known for and you will know as soon as you start whether the engine is running right or not.

I've been taught not to turn engines backwards as it is not good for vane pumps, I have a suspicion that its just an old wives tale but just as a precaution I don't turn engines backwards.

28th Mar 2011, 03:40
Some radials are too large to be pulled through by hand, so the procedure is slightly different:

1. If the bottom cylinder truly contains enough oil that has fallen, through the graces of Isaac Newton, into the bottom cylinder and the valves are stuck, the starter will not have enough torque to continue cranking the engine through the second complete revolution

2. So, the FCOM will usually instruct the pilot to count the roating blades (i.e. nine blades on a 3 blade prop, 12 blades on a four blade prop, to ensure three complete revolutions before introducing ignition and fuel by selecting ignition 'on' and mixture 'full rich'.

The result will usually be nirvana, if the pilot has cracked the throttle by just the right amount. A smooth light up, no backfire is the desired result, but that was in the days when a certain skill and a little sympathy for mechanical 'beings' was still a prerequisite.

28th Mar 2011, 07:59
you turn the propeller (magnetos off) with your hands as many times as the number of cylinders in the engine. := No matter how many cylinders your (ungeared) engine has, if it is a 4 stroke then you need two turns of the prop to have every cylinder turned through the compression stroke.

28th Mar 2011, 10:48
Hydraulic lock

After my experience with a set of oily 3350s the way I seem to remember it was if oil seeped into the bottom cylinders then the engine would lock up on the compression stroke, as no valves open, and that is where possible damage to con rods could occur.

To try and turn the prop by hand was almost impossible given the height of the outboard engines on a Connie, so you had to turn them on the starter prior to ignition selection, and should a hyd lock occur then the starter motor had a clutch which would slip so preventing damage to con rods.

A locked engine would require you to open the cowling remove the offending cylinder spark plug and drain the oil into a bucket. It was then a good idea to turn the engine again on the starter, with the plug out, to scavenge any remaining oil from that cylinder and also to ensure you had got the correct cylinder in the first place. Button everything back and start the engine.

A messy proceedure and to overcome it happening when we were ready to go we would about 30 minutes prior to start turn the engines through 12 blades [ cautious we were] to ensure there was no lock or if there was to give us time to go through the drain proceedure.

As to starting the engines, we would go 12 blades and I would do this with fully rich selected[ to prime each cylinder with fuel ], throttle one inch open and at 12 blades ignition ON and then watch through the flight deck door for 3 and 4 engines to see how the start was going. In the case of 1 and 2 engines listen to captains calls and watch their fuel flow meters which gave a kick when the engie lit up. Then fuel and mixture fiddling with to prevent the engine going to far above idle before the oil had warmed up.

Each of the engines had their own requirements and had to be nursed and coaxed to light up, but when you got a good start on all 4 it left you with a good feeling.

Jets are so easy, sometimes:rolleyes:

Air Tourer
29th Mar 2011, 08:07
Two blades for every cyl to do a compression stroke? But one blade gives at least a top dead centre with an exhause valve open?

Anyway, first up a chief pilot told me to wind a Jacobs backwards "because the exhause valve will open first". (IE. the excess oil won't run into the intake man.) That was all right until many pilots and engineers told me to always go forward. (Except on a tiger moth where we went both ways as I remember.) After many years of winding away an engineer told me never to hand wind "his" P&W 985 as it had a clutch and I had too much leverage. The aero club was usually "leave the damn thing alone".

The final amusement was my last engineer tells me that I could get a fuel hydro. lock on a horiz. opp. with my high tank and leaky injectors.
Along the way I was supposed to idle a 985 for a while before shut down so the scavanger pump would collect excess oil and dump it in the tank.

In the end I wound my prop from habit, forward, feeling for "something", and knowing it MAY kick. Kept me happy anyway.

29th Mar 2011, 09:17
People say "leave it alone", again, I disagree, I had the following happen to me.

Cold morning in the arctic, plane had been outside a few days so we pulled it inside to get the snow and ice off it. Before you know it we got a wet floor and a clean plane so we put it back outside and the mechanics pull in their "project" for the day. About an hour goes by before the weather at destination improves enough to get going, with the plane sitting outside the whole time. I make my way out to the plane and (out of habit) I turn the prop a bit as I walk past, but the darn thing won't move. "Strange" I think to myself, so I get a better grip and pull slightly harder to make sure it wasn't my imagination, "very strange", I think as it won't budge a millimeter. I head into the hangar and get a mechanic to take a look at it, after about a minute of investigating he tells me that there's ice all over the area between the prop and the engine. What had happened was ice had melted gotten into every crack up there and when we wheeled it back outside, it had refrozen and it was SOLID as could be (-40 outside at the time). Took a Herman Nelson a few minutes to get all the stuff out of there but the lead mechanic mentioned that if I had tried starting the engine like that it would have (in all probability) downed the plane for a few days at least.

29th Mar 2011, 19:09
Opposed engines in normal climates..'.pulling through' is NOT recommended by either Lycoming or TCM Continental, and....if either magneto is not properly grounded, there is a slim chance that the engine might attempt to start (especially if warm), so....I would avoid.
I own a TCM GTSIO-powered airplane, and I will not allow the propellor to be 'pulled through' by hand, as that is precisely what the installed electric starter is there for...:}

'Pulling throuth' by hand, must be a Brit thing...:rolleyes: where old outdated procedures die hard.:ugh:

19th Apr 2011, 13:13
where old outdated procedures die hard:ugh:
A somewhat ironical last post there Bob!
RIP 411A, I shall miss your forthright pot stirring and never wavering view on how aviation should be conducted.
The forums will be a less interesting place without your interesting views, anecdotes and advice for the young and inexperienced.

20th Apr 2011, 09:30
'Pulling throuth' by hand, must be a Brit thing...
Weren't Orville and Wilbur a couple of American pullers? :ooh: (and they weren't the first to fly. They were beaten by 9 months and no, it wasn't by a Pom/Limey/Brit)

20th Apr 2011, 11:38
(and they weren't the first to fly. They were beaten by 9 months and no, it wasn't by a Pom/Limey/Brit)

It was a kiwi!

Air Tourer
20th Apr 2011, 12:15
Well, sometimes starters don't work, and some planes don't have 'em.
I was never worried about winding a prop, and if you listen hard enough you may hear a "hiss" from that nasty crack in the cyl. head.

But I was always scared of people, me included, working around a running engine. Too many just walk into them. One mongrel, I had to leave running all day, during refuel, oil top up, and lunch and smoko.

If you told the boss the damned thing just won't start, he'd say, well just fly her back to base. :mad:

20th Apr 2011, 19:02
From over 50 years of having watched film clips from WWII of air crews walking through the props prior to starting the engines on USAAC 4 engine bombers, I had understood they did it due to hydraulic lock. I am talking about TV shows like Air Power back in the 1950's, stuff like that. Seems like every episode they had clips of the radials being turned over piror to start-up for the latest mission that was being documented in the show. I can't remember whether it was always the same clip spliced into each weeks episode, or even if they were different planes (like were the B-17 engines more susceptable to hydraulic lock than B-29 engines). Like I said, somebody who had been in the AAC during the war told me it had to be done. First time I flew in a radial engine equipped plane, probably a North Central Convair 340 wtih R2800s, they just fired up the engines and off we went. Progress, or television show film editors splicing in bits of good sharp film to make sure they had enough to fill a 30 minute episode?

20th Apr 2011, 19:26
I have spent 36 years around light aircraft as PPL, Instructor, examiner, NPPL,
and engineer. I have always pulled through the 4 cylinder horizontal opposed engines on first flight of the day. I wait for a quiet moment and listen for any odd noise and check compressions. On one occasion heard a hissing sound from under the cowling. It proved to be a cylinder head cracked two thirds of the way round and would have come off on the next flight. SO 36 years of checking has saved one forced landing!!

25th Apr 2011, 18:07
In my piston days I had never "pulled-through" my Lycoming/Continental horizontally opposed engines, nor have I ever heard or read in any Cessna/Piper operating manuals that this was a pilot's preflight requirement.

However, for whatever reasons, I have seen certified A&P mechanics do that during maintenance work.

25th Apr 2011, 20:01
If it is your own aircraft (mine had a 160hp lycoming) then I would pull through forwards. Only however for the first start of the day. It is very simple really, if there is some form of restriction in the movement that causes damage on start then the repair comes out of your pocket, or at least the hassle of insurance claims and time without an aircraft. Common sense always dictated that the prop was treated as live with the necessary precations therefore taken. Of course if it is your aircraft and you were the last to start it then you will have checked the mags on shut down so it is highly unlikely that they will have suddenly become live in between. If you are playing at this on subsequent starts then the engine may still be warm and the chance of it firing are higher.

Air Tourer
26th Apr 2011, 11:58
I'm sure I've seen one or two hot engines kick over well after shut-down, luckily with no one in the way.

27th Apr 2011, 06:06
A mechanic can pull an engine through and by feel know that it is normal or otherwise. A pilot usually does not have that experience or ability. I never pulled the P&W 1830s through because they had an inertial starter that did that for me. The P&W 985 I do pull through because the manufacturer says to do it, even if the engine has a separate starter (not included on the ignition switch). It always feels wrong to start those engines that have been modified with a Cessna-type start switch.

27th Apr 2011, 06:43
Many years ago I did my PPL with a very large well known aero club, never heard anything about turning over by hand. After a few check flights etc at other flying clubs, none there either. I would say in general it probally should be discoureged for obvious reasons. If you do an endorsment on a particular aircraft that requires it and its part of its published procedures, so be it.

27th Apr 2011, 07:15
I've been taught not to turn engines backwards as it is not good for vane pumps, I have a suspicion that its just an old wives tale but just as a precaution I don't turn engines backwards

Most vac pumps for sale today (every 215CC one I have seen) are either bidirectional, or are OK to run in the wrong direction for something like 50hrs.

28th Apr 2011, 18:28
How come "elf and safety" haven't banned propping yet?

cockney steve
28th Apr 2011, 23:23
My friend's Aeronca didn't have any starter other than the "armstrong"-indeed, there were no electrics other than the mags!
Communications were latterly by Icom handheld, to ground, or loud yells for cockpit conversation.

Hand-starting is great fun and takes me back to my model-aircraft diesels in the early 60's........starting them was a one-finger affair, but you could get some nasty cuts if they kicked-back. :\

30th Apr 2011, 16:49
for your pain, you got a good practical idea of what "hydraulicing" was.
Plus you could tell if an engine was rich or lean by the sound of the exhaust. Even more so on glo-plug engines.
Even worked on R2800 with open exhausts (DC-6), but NOT on Convair 440 because siamesed exhausts were fed into augmentor tubes which muffled everything. I was always much better at starting the DC-6 than the CV-440!

Piltdown Man
30th Apr 2011, 18:21
I'd read the Pilot's Operating Handbook and do what it says in there. If it doesn't say "Swing the prop.", don't. Unfortunately, there are hundreds of "old wives tales" in aviation and it takes a bit of time to sort out the little golden nuggets from the big, pointy ended brown jobbies. One of the best sorting methods is to listen for the noise of condemnation. Normally the bigger the noise, the more likely it is that the person is feeding you crap.


1st May 2011, 18:02
I learned to fly on Auster Aiglets and Autocrats, which had no starter. Furthermore the engines were inverted inline fours and although dry sump, oil tended to slowly drip into the cylinders over time. They were usually pulled through a couple of times, then primed, then "contact" and off you went.

What you did learn was that you had to regard the prop as live at all times, only open the throttle "one knob" when ignition on, and make sure you had chocks that were designed for the aircraft they were being used on.

If you flooded the engine, you had to make sure mags off, open throttle fully, pull through, then close throttle to one knob and "contact".
Forgetting to close throttle was a big problem and I heard of several incidents where Tiger Moths jumped chocks and in one case took off and slowly spiralled in.
The idle at "one knob" was also a liability if the chocks were metal, or a bit too big for the wheel in question. I have been chased at a walking pace from light chocks on wet grass and "one knob". The aircraft just pushed the chocks along.
To sum it up in the words of 411A, dont do it unless the book says so. And then be very very careful.