Engine starting

[sharpend]

There is a thread 'Why are aircraft engines so thirsty?'.

So I thought, 'why are aircraft engines so difficult to start?' OK, so they are not, on the whole, fitted with the most sophisticated electronic ignition systems; in fact most are fitted with twin magnetos. But at any airfield in the early morning, especially in winter, one hears the churn churn of an engine waiting to fire up.

Why is this? Surely twin mags + twin plugs should ensure that the engine starts like a car, ie instantly. After all, a car, even an older car with a carb and choke starts just about instantly (unless the battery is weak).

So why do we sit there with no much happening?

[Tupperware Pilot]

So I thought, 'why are aircraft engines so difficult to start?'

Are they????? Don't have any issues starting mine? C-85 hand start.....

[flybymike]

Aero engine mixture management is a black art. Wait till you see the fun that can be had trying to start a hot fuel injected turbocharged engine.

[Dave Gittins]

Because we stop an aeroplane engine by starving it of fuel with full leaning.

To get it going again, you need to drag the right mixture back into the cylinders.

Used to be just as hard to start my old MGB by pulling the choke out and cranking (anybody remember chokes on cars ?).

[Zulu Alpha]

Car starting these days is all done by computer. I do remember old cars that were very difficult to start.

Most aircraft engines don't have computer controlled injection and ignition and are therefore as difficult to start as old car engines.

I fitted electronic ignition to my last LAA Lycoming engined aircraft. It transformed the starting. I am going to fit it to my current aircraft this winter.

[Rod1]

I stop my aircraft engine by turning off the electronic ignition…

A Rotax aircraft engine has a choke and is exactly the same as starting a 1970’s car. Very easy if you know how and very hard if you have no clue.

Rod1

[Dave Gittins]

I have had very little exposure to Rotax's (only one in a Pipersport). The first time it stopped I thought it had seized.

[BackPacker]

Surely twin mags + twin plugs should ensure that the engine starts like a car, ie instantly.

Like others said, the issue is normally not the sparks. It's getting the right mixture to the cylinders that's tricky.

Sparks can be a problem too though. The magnetos are configured so that the spark happens about 20-25 degrees before Top Dead Center, which means that the flame front, at normal RPMs, reaches the piston shortly after TDC.

However at very low RPMs, such as when the engine is being turned by the starter motor, the flame front will reach the piston well before TDC and will cause the engine to stop, or even backfire. That's why aero engine magnetos are fitted with impulse couplings, which retard the spark by about 20-25 degrees at very low RPMs.

Impulse couplings are normally only fitted to one set of magnetos though, and it's only this set that is activated when you rotate the mag key to the "start" position. But in aircraft that have a separate starter button (not integrated in the Off/L/R/Both key mechanism), there might be a note in your POH that you should only start on the left (or right) magneto, not on both.

[Cobalt]

I would also add ageing engine / maintenance issues into the mix.

In autumn, several of our aircraft (C152) became progressively hard to start, and it became next to impossible in winter - heavy priming, throttle pumping, all the BAD things we had to do to get them going at all... and when really cold even that stopped working and we could not fly.

We leaned on maintenance, they replaced the primer nozzles, and hey presto, 2x primer pumps and they started every time for the rest of the winter. 1x would get it to fire, but would not always sustain it.


Also - is anyone actually following the POH procedure for priming a PA28? Read it an be surprised... I have yet to meet a pilot who uses it, everyone just primes 3 or 4 times and cranks. The POH says crank, if no start THEN prime ONCE and crank again. Works very well...

[A and C]

Slick have an SB that recommends a 500 hour check, the reason for this is to change the contact breaker points and check the internal timing in the mag.

The internal timing has a large effect on the strength of the spark, while on most occasions the Slick mag will run OK up to about 700 hours the efficiency however drops of to the point that the engine won't run at idle on one mag.

It follows that a weak spark will not start an engine very will especially when cold it may well be the mag internal timing.

[tmmorris]

Cobalt,

Interesting. That got me reading the POH for A and C's, which I rent. What you didn't mention (which is in ours, at least) is it says to operate the throttle accelerator pump 2-3 times first.

The Robin I used to fly had no primer and was started exactly as above. I'll have to try it. Though I've found the POH warm start procedure doesn't work...

Tim

[david viewing]

it says to operate the throttle accelerator pump 2-3 times first

I'd go easy on the throttle pumping.

Following a near-disastrous fire to our -161 (rescued only by a quick thinking pilot of another aircraft who discharged is own extinguisher into the cowling) my late partner and co-owner Trevor Shaw looked into what had happened. The renter pilot had pumped the throttle "5 or 6 times" in an effort to get the recalcitrant engine to start.

Trevor, who was a garage owner and race car mechanic, found that any more than 3 pumps had neat fuel running out of the air changeover box onto the inside of the cowling. On the other hand it took a great deal of primer pumping to get any fuel to emerge at all.

The real culprit was the recalcitrant engine which had got progressively harder to start. It turned out that the carb was badly worn and that there several other contributory factors. After Trevor finished with the engineers and they finished with the engine, it became the best starter I've ever known, starting first turn, every time, all the way through the rest of it's 2300 Hr career.

[Pegasus912]

Hi Rod1,

Very easy if you know how and very hard if you have no clue.

I think I fall into the latter category! Never had any problems starting the 912 from cold, full choke and no throttle. Same with hot, except no choke. It's in between that I tend to get into a muddle. Care to offer some advice?

[Ebbie 2003]

My PA28-181starts OK if the manual is followed, but I've seen some epic fails by others trying to get it started especislly when hot.

When hot no one seems to like the idea of the mixture being in the lean cut off position.

It seems to be a need thrre hands situation. Seems mixture in lean cut off, throttle half inch open as opposed to the usual quarter following a couple of throttle pumps, crank it and when it catches smartly xture to full rich - works every time from cold, same deal but full rich, quarter inch and three or four throttle pumps - can prime instead but the Essex primer lock is fiddly.

The engine is a Lyc 360-A4M - must admit this is in the Caribbean so it's never "UK cold"

[tmmorris]

Our PA28-181 POH states for cold start, throttle 1/4 open, not 1/4 inch. (Warm start says 1/2 inch.) Is that a misprint, or have others misread it?

[Ebbie 2003]

It's 1/4 INCH - not 1/4 OPEN

Amazed that it starts at all if it's being cranked with the throttle open that much, if it does it suggests that the throttle linkage has an issue with the 1/4 open being the fix - but that would not explain the 1/2inch on hot.

Maybe we have a solution to some starting problems - errors and misprints in rewrite POHs and chec lists?

[A and C]

Ebbie

It's my aircraft that Tim is talking about, there are no issues with the throttle linkage.

I favour 1/4 inch open for a cold start...... it fires first time for me.

[sharpend]

Any views on left mag (impulse) or both for starting a Lycoming IO 360 in a Bulldog? There seems to be several opposing thoughts re this.

[BEagle]

I favour 1/4 inch open for a cold start......

As I explained to a young lady student "¼ in means just that. You might have been told by some that an inch is perhaps rather different to its true size, but in this case boasting is irrelevant - ¼" means ¼"!".............

Bluntie, at the cranking speed of a 'dog, fatter sparks from using both magnetos is probably of more benefit than an impulse magneto alone. But what do the FRCs say?

[tmmorris]

Ebbie, and A and C,

Don't worry, I do in fact use 1/4 inch open. However, this is a verbatim quote from the POH (if I could work out how, I'd post a scan of it):

(Pages 41-42)
D. Starting Engine when Cold
...
4. Throttle Control - pump throttle full open and closed 2 to 3 times then set control 1/4 open.

I guess it's a misprint. The POH is typed, due to the age of the ac.

Tim