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Jabiru Ultralight Aircraft - Oops!

Jabiru EngineJabiru produce a range of Ultralight and Light Sports aircraft right here in Australia. They're popular with enthusiasts and flying schools alike because they are easy to handle and fairly robust - with one exception. The dummies have installed the pistons backwards! The offset pistons used were designed for clockwise rotation whereas the Jabiru rotates anticlockwise.

A quick lesson in 4 stroke engine operation

1. Induction Stroke

With the exhaust valve closed and the inlet valve open the piston is pulled down drawing air & fuel into the combustion chamber.

2. Compression Stroke

Both valves are closed and the piston is pushed up to compress the air and fuel to a fraction of its original size.

3. Ignition Stroke

The excitement starts on the third stroke when the sparkplug fires causing the fuel and air mixture to explode! The rapidly expanding gases push the piston back down again.

4. Exhaust Stroke

The fourth and final stroke completes the cycle by moving upwards pushing the exhaust gas out via the now open exhaust valve.

My late father, an Aeronautical Engineer with the Royal Australian Navy for 35 years, simplified this process by referring to it as "Suck Squeeze Bang Blow". That term pretty well sums up the 4 Stroke engine!

The amount of force exerted on pistons as they travel up and down is enormous, and like any "up and down" motion there has to be a “Top and Bottom”! By that I mean the piston must stop at the top before it can travel to the bottom and visa versa. Pistons with a 90 mm stroke at 3000 RPM travel at 9 meters (29 feet) per second and have to come to a dead-stop in an instant at the top and bottom of the stroke. This stopping and starting causes a hammering affect and creates stress on gudgeon pins, con-rods, big-ends, and more. The hammering is particularly horrific during the Ignition Stroke when the explosive force of the burning fuel literally "blows" the piston down.

Easing the strain

Traditionally pistons were connected to con-rods right in the middle but to ease the strain when the piston stops and comes back down they are offset in the direction of engine rotation, the connection between con-rod and piston is off centre. The crown of each piston is marked with an arrow indicating which direction the piston should be installed. By offsetting in this way you are giving the piston a bit of a headstart on its journey.

Engines with offset pistons run better! They're more durable, reliable and quieter but ONLY if you've put them in the right way.

Jabiru have it wrong

For some reason, as yet unexplained logically, Jabiru have chosen to put their pistons in reverse. They were made for General Motors motor vehicles, all of which run clockwise, which is correct based on the offset, but Jabiru engines rotate anticlockwise meaning they are all installed wrong. Any mechanical engineer will tell you what happens when you don't install your offset pistons correctly - you get problems, you shorten the life of your engine, it makes more noise, and they just become crappy in general.

One of the symptoms, and it is common to all Jabiru engines, is that when you shut-down there is a distinctive clunk-clunk before it stops completely. This clunk-clunk is caused by the engine trying to run itself in reverse as a result of the offset pistons.

By installing pistons in accordance with the Jabiru manual the offset is working against the flow and actually creating durability issues and potentially safety problems. Interestingly, a number of people have chosen to install their pistons in the opposite direction and claim their engines perform better and are significantly quieter.

Who knows about this

The Australian Civil Aviation Safety Authority (CASA) and the Australian Transport Safety Bureau (ATSB) were advised by various Jabiru owners and maintenance staff, but both organisations have chosen to do nothing. The engines were certified to fly and deemed appropriate for the aircraft type. I've been informed that engines are tested at idle and cruise RPM only! They did not test them through the full rev range, which was a mistake that should be rectified.

Jabiru have been made aware of this issue on multiple occasions and have cleverly devised a cover story “The pistons are installed in reverse due to the forces applied by the Prop”. What a load of cods-wallop. Every engineer I've spoken to about this issue has given me multiple reasons why these pistons should be turned around. My late father was very interested in discovering why Jabiru believe that an opposite offset could be beneficial. With 35 years of experience behind him working with various aircraft engines he could find no reason why Jabiru would pursue the opposite of what, from an engineering point of view, seems to be ridiculous.


The definition of a good pilot is “One whom has the same number of take-offs as they have landings” and most people when they go flying want to make certain they land in one piece. The secret to keeping yourself alive when flying light aircraft is more than just your own skills! Maintenance of your aircraft is vital to safe flying and going up in an aircraft that has such a design floor, in my mind, is madness.

How many accidents have occurred as a result of the offset issue. Just last week a good friend of mine was flying a Jabiru when a thru-bolt broke and he had to land with a faulty engine. It could have been much worse, the engine could have failed completely. These bolts are known to fail in Jabiru aircraft and in my mind this is a direct result of excessive stress placed on them by the ridiculous offset issue.


Jabiru are popular with flying schools because they offer easy shared access to controls via a v-shaped stick between the left and right seat. When an aircraft is used for training purposes it is considered "Certified" which means that under no circumstances can the aircraft be modified from manufacturer specifications. So even if they wanted to turn their pistons around to improve reliability and reduce maintenance costs they cannot by law. Private owners that rent their aircraft to flying schools are in the same boat. Owners that do not rent their aircraft are free to do whatever they like and some have already turned their pistons in the opposite direction with amazing results. Despite these results Jabiru continue to maintain that their manual is correct.

What should happen now

CASA should ground every Jabiru airplane! It's that simple. The ATSB needs to review any previous accident reports involving Jabiru, particularly when engine failure was considered a possible cause.

Jabiru should look carefully at their engines and if necessary swallow their pride and turn the pistons in right direction. They would probably find that maintenance costs would fall, performance would rise, and their customers would be very happy.

On a more personal note

I have spoken with a number of Jabiru owners whom tell me it's rare for an engine to go more than 250 hours before it requires serious and expensive repairs. I've seen evidence of excessive wear on cylinder walls and pistons. I've seen first hand the damage done to big-end shells as a result of excessive belting when the piston is trying to go in the opposite direction to engine rotation. Jabiru are unlikely to accept responsibility because they would then be liable for fixing the problem – that could be expensive!

Published 5 June 2013

Read a review of the Jabiru 160 on Aviator: Click Here.

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