G'day,

I have always been taught to pump the throttle as a means of priming the engine prior to starting. I understand that the theory behind this has something to do with the accelerator pump.

My question is, how does the accelerator pump help prime the engine? It pumps fuel through the discharge nozzle. Because the discharge nozzle is downstream of the throttle butterfly, pumping the throttle with no airflow to catch the fuel would surely just result in the fuel 'dribbling' out of the nozzle. I have read a few forums stating that this can actually lead to a higher risk of fire on engine start.

Can someone please clarify why we pump the throttle prior to engine start and if it is actually a good practice?

Cheers,

Dan

When the throttle is opened suddenly, there is a rapid increase in the flow of mixture from the carburettor to the cylinders. However, since the fuel droplets are heavier than air, they are 'left behind' and the mixture becomes excessively lean often causing missing and spluttering of the engine just when you have an urgent need for increased power. The accelerator pump is operated by a cam on the throttle linkage and it squirts fuel under increased pressure just a fraction ahead of the demand. This helps to overcome the inertia of the fuel droplets and ensures that the mixture stays in the combustible range, ensuring a smooth increase in power.

You are correct in assuming that it was never designed to prime the engine. Pumping the throttle prior to start-up squirts fuel out of the discharge nozzle allowing it to evaporate in the vicinity of the venturi and the throttle butterfly. When the engine is cranked, that vapour-rich atmosphere is carried to the cylinders and it does increase the chances of a successful start. In that sense, it does perform the function of a primer (somewhat inefficiently). However, the engine hand (or pump) operated primer is far more effective for this purpose because it squirts fuel directly into the inlet port which is a far more efficient way to deliver it to the cylinders.

It should be remembered that many small engines do not have an accelerator pump, so pumping the throttle is a waste of time!

I have heard that excessive use of the throttle-pumping technique has caused engine fires at start-up, but I have never observed it personally. As far as I can see, an engine fire would also require a back-fire during the cranking period to provide a source of ignition. That would not be impossible with a very lean mixture during cranking.

Hi Aviatordan,

It's one of those things that's passed on by the good ol' "....beacause that's what my instructor told me to do!" method. I have seen LAME's start engines like this in the past and it was an old croppie who was my mentoring senior instructor who taught this method as a last resort.

You are 100% correct in that throttle priming without any airflow to deliever it to the cylinders is a fire risk. I have seen one go 'woof' and ignite the puddle of fuel sitting beneath the engine from incorrect priming. The student (and instructor) probably didn't really get fully explained about the whys and hows of using this technique and usually ends up with some mad pumping action without listening or smelling to what's going on up front.

At the end of the day, what does the flight manual say?

Is it a good practice? Depends on a number things. Will it flood the engine? What are my other options? Can I get help from a LAME or am I in the middle of nowhere and this could be my only chance starting?

I have taught this before to help start aircraft that are being difficult by using the the following and only after trying the method set out in the FM.

You've just been cranking the engine, so give the starter motor time to cool.
Brief yourself again on your actions for a fire on start up.
Crank the engine.
While cranking, pump the throttle once (and once only). Your movements should be very deliberate. In-Out-Set. This is to prevents you flooding the engine or having it roar to life with the throttle wide open. It is also delievering the fuel/air mixture to the cylinders in an atomised state. Engines like that!
Listen! You listening for little puffs of the engine starting fire. My experience has been that it will usually fires a couple of revolutions after you have the throttle set. That's why one pump then listen.
Have a smell for fuel. If you smell fuel, stop immediately.

If it hasn't started after one or two tries, see the LAME.

Remember! Starter motor limitations and when in any doubt, STOP.


This can be quite a hazardous undertaking so talk to your LAME. Be as informed as you can because your the one in the hot seat so to speak.

Hope this helps.

Cheers,

Mister W.

Hey Bob/Mister W,

Thanks for the replies.

It seems like, as Bob said, pumping the throttle (assuming there actually is an accelerator pump) does actually do something, in that it provides a fuel rich source of air that will be sucked in to the cylinders upon cranking. Albeit an inefficient method for priming, it is somewhat effective.

I am currently doing the flight instructors rating, which has made me extra vigilant with regards to faults that could damage the aircraft so I am questioning everything. It seems like pumping the throttle is alright as long as it is taught well, and it is stressed that you don't mess around after pumping it. The object is to keep the fuel atomized, and if you delay cranking after start, it may condense to liquid, resulting in an increased fire risk.

My last question on the topic...

Is there a reason people prefer to pump the throttle rather than using the perfectly good primer sitting basically right next to it? Does pumping the throttle provide some advantage over the primer? Or, inversely, does the primer have some disadvantage when compared to pumping the throttle, aside from the possibility of flooding the engine?

Thanks again guys,

Dan

Laziness or haste!

Good luck with the rating. Use to really enjoy teaching F.I.R. How are the long briefs coming along?

Cheers,
Mister W

Fair call. Well, I think I might be teaching primer over pumping the throttle!

Yeah, the long briefs are going really well, I think. I've had to design them all myself (I'm using powerpoint, with nice little animations and such!), and the biggest thing I struggled with was layout. I wasn't too sure what to put where and in what order, exact content that needed covering, etc... I ended up just doing the best I could, delivering what I had, and am now going through and amending them based on what I learnt in the flying sequences. I'll deliver the amended versions prior to taking the test so I know they are not only up to scratch for test day, but also so they're ready to learn a few people some stuff about flying once I'm an instructor!

The FIR has been amazing for my own personal flying technique, though. After quite a few solo hours, whilst I have always tried my hardest to fly accurately and efficiently, I had obviously picked up a few bad habits. But the FIR has been great for critiquing myself and really ensuring that I am 100% up to standard!