Anyone having engine trouble over chesterfield sun 23/8

[mark8647]

I was working in the garden today (sunday 23/08/09) when we heard a light aircraft passing overhead, as there had been all day.
We then heard the plane throttle back closely followed by two or three loud bangs. . .then silence.

We watched for about 20 to 30 seconds before it suddenly made a noise and turned as if it was turning back. (dont know where too)

Anyway i was just posting to say that i hope they got everything sorted as it did appear that everything was under control, Also what could cause the engine to appear to backfire during flight.

Thankyou
Mark8647

[worrab]

Backfires occur when unburnt fuel from the cylinder reaches the hot exhaust pipe and ignites. Reducing throttle rapidly (esp when running rich) is a common cause in cars, but can't say I've encountered it with aircraft.

[modelman]

Most prob an instructional flight for stalls and recovery.Close throttle,stick back,stall,stick forward and throttle open.Would give the appearance of what you saw.
Throttling back can cause a backfire ( I've heard it on EFATO) but I wouldn't know why for sure.(I'm sure worrab is correct)
MM

[theavionicsbloke]

Rapid retardation of the throttle on a hot Lycoming engine with Rich Mixture will very often cause a backfire. Usually two to three pops.

This is normal and very common.

The solution, well simplereally, gentle with the throttle guys.....
A gradual reduction, say over 1.5 to 2 seconds will ensure you don't righten the poor public, including yourselves..

[tomtom_91]

I think also I hured pops when doing stall testing on the C172 as I applyed the throttle if I moved it forward really fast it would make this noise could be wrong tho?

tom

[SoundBarrier]

Yeah very common, was part of our engine handling that we were shown to be easy on the engine as theavionicsbloke says. No need to pull the throttle back so quickly, not even for an EFATO. Look after the engine and she'll look after you!

[liam548]

ive had the engine backfiring in a PA28 when closing throttle to practice PFLs.

[Lightning6]

When practising PFLs and EFATOs, always throttle back slowly, otherwise you could end up in a real situation, the backfire could cause a head gasket failure, or worse.

That goes for throttling up also, treat your engine like a good woman, gently.

[IO540]

Always close the throttle slowly, over some tens of seconds if possible. This is to allow the engine to cool slowly, which greatly reduces the risk of cracked cylinders. Some evidence suggests this is an issue only if the CHT is above a certain value though, so a rapid closure during landing (following a prolonged low power period on final) is OK.

Backfires can damage the exhaust system. They must be avoided totally.

Instructors often close the throttle rapidly during training to simulate an engine failure. They are taking a risk... I would never do this with my own plane.

[JugglingSpence]

Backfires are caused when unburnt fuel/air mixture passes straight through the combustion chamber and into the exhaust. It is then ignited by the hot gases as they pass the exhaust valve and enter the exhaust manifold.

If you open the throttle then the carb will give a squirt from the accelerator jet to enrich the mixture as the RPM increases. If you open it too quickly then you may end up with more fuel in the cylinders than is necessary as the pump sprays the same amount over a shorter period, this passes straight through and gets burned on that cylinder's next exhaust stroke- set fire to fuel in an enclosed space and it will make a bang as the slug of exhaust gases hits the atmosphere.

The other extreme with opening the throttle too quickly is that if the accelerator pump doesn't squirt as strongly as it should then the engine will lose it's vacuum and therefore lose it's ability to draw fuel into the carb. This causes the bogging effect that you get on some older road engines.

Staying with the vacuum, if you close the throttle too quickly then the vacuum will shoot up to it's highest at ANY point of normal operation as the throttle butterflies close (don't forget when the engine is being pushed/ isn't under power then it acts as a pump- the airflow turns the prop which turns the engine which pumps fuel and air), this vacuum can't draw air so it will suck fuel through the carb's jets, even past the needles and into the combustion chamber (where there is still no air) and onto the exhaust. This vacuum is sometimes enough to draw oil past the piston rings, you might have seen this in your car when you get a puff of blue smoke from the exhaust after heavy engine braking. After a few seconds the vacuum will drop to a more normal level.

Backfires are a small explosion in your exhaust manifold so they can damage the manifolds themselves, the cylinder head or the exhaust valve so they are best avoided wherever possible.

Please don't take this as gospel, I'm not an aircraft engineer (although I do have a lot of mechanical experience in a different trade) but I learn a lot from this site and figured that this might be a chance to give something back and help you all understand backfires. Hope this helps...:ok: