Starting a 2-stroke (chainsaw) engine after running out of fuel

Starting a 2-stroke (chainsaw) engine** after running out of fuel

“Troubleshooting” your chainsaw

Everyone should and probably have heard that engines should never run out of fuel completely for many reasons. One of the reasons is that the last drops of fuel can bring in unwanted contaminants but other, more sensible reason is that air gets into the combustion circuit. (Even if you think about human organs, air bubble is an unwanted and can be fatal if it is present in the veins. Similarly, it can be "fatal" for engines too.) Imagine a running engine that just runs out of fuel and for a few seconds that engine will be running without a mixture. This is serious to the metal elements.

Now that I have mentioned some of the "don't-s", I can tell a solution on, how to resolve the problem if it happened. It can even take place after a long time of not being used, when you're trying to start your 2-stroke engine.
Main components under the cover (Note: not all parts are listed.)
Eventually the steps for a cold engine start are:
  1. Engage the chain brake when the chainsaw is started.
  2. Press the air purge repeatedly until fuel begins to fill the bulb. (The bulb need not be completely filled.)
  3. Pull the choke lever to full extent.
  4. Start throttling.
  5. Push the choke control to “half choke” as soon as the engine fires.
  6. Pull the starting chord until the engine starts.
What you should never do in case, it’s run out of fuel:
  • Never press the purge! By doing this, air will be pumped from the empty tank to the carburetor, that will be completely full of air and it will be difficult to get it to work.
  • Start pulling in the engine: This will move the pistons up and down without any lubrication and mixture. This can result in huge damage again!
Steps to resolve the problem:
  • Add fuel :)
  • Pull out the choke to full extent (fast idle),
  • Start pulling the starter handle 3-6x,
  • Press the air purge ~6x, 
  • Air purge: Press the air purge repeatedly until fuel begins to fill the bulb. The bulb need not be completely filled.
  • Start pulling the starter again.
  • Press air purge again to release air.
  • Do you see fuel in the purge?
    • Yes. Great, keep pulling in, the engine will start soon.
    • No. Try the following steps:
Try 1.:
  • Start pressing the purge.
  • Open the fuel tank, and while open, start pressing again. This will actually help the air to flow out, but not all. If there's too much of air, it'll not resolve the problem. But in some cases it may help. (Pay attention to have enough fuel while doing this and don’t end up sucking in air.)
Fuel lines
Try 2.:
  • Remove the cover which holds the purge button.
  • Look for the two fuel lines connected to the purge. (Picture above) One is coming from the carburetor, the other one is going to the fuel tank.
Fuel flow diagram
It may seem that the fuel flows to the purge (if you hold upside-down the engine and press a hundred times) but it'll never go beyond it, therefore this line will not be useful.
  • Try to remove the other line (the one coming from the carburetor). This will not flow, since it's the part, which is filed with air.
  • Now try to add manually fuel into this line. The best way is to use a syringe, because it can fit in the 2-3 mm inner diameter pipe.
  • Once you've added enough fuel (20-30 ml), you'll hear the air leaving the carburetor, and now you're ready to put back the pipe.
  • Start pressing the purge a few times.
  • (Now you should see fuel flowing through the purge.)
  • Assemble back the cover and do your normal starting procedure.
  • Success?

    • Yes.
    • No success? I'm afraid you need to contact a mechanic.

Engine characteristics:

Manufacturer: McCulloch
Type: CS 340

Cylinder displacement
Idle speed
Ignition system
Spark plug
Champion RCJ 7Y

Fuel tank capacity
Oil pump capacity at 8,500 rpm
Oil tank capacity

** About 2-stroke engines:

The Ideal Otto Cycle is used in all internal combustion engines. This is why it is important to know well the p-V (pressure-volume) diagram for Otto cycle. The one who is interested in the topic, is invited to read at:

Used reference: 
  • McCulloch - CS 340 Chainsaw User manual


Thesis topic evolution

Many of us, engineering students undergo the following dilemma when it comes to selecting the final thesis. In this blog you will read about how my topic has been evolved during the years, what ideas were circling my mind during the semesters, until the final version of the work was chosen. Because it's just not that simple to choose.

Looking back at my 5th semester (3rd year on BSc studies), I knew that I would like to do 'something related to Aircraft Design'. Later in that year, I become more interested in landing gears and I imagined doing some design related to them.
Optimized trapezoidal strut
In the 6th semester, I was to choose an Intermediate Engineering Project (or a so-called pre-engineering thesis). This was the time when I first had to choose a topic wisely because it could have an effect on my final thesis. I was looking for a supervisor, who could give me a task about landing gears and this is how I made a project on a static analysis and a preliminary design of a UAV's landing gear.

In the following semester, I was about to continue this project and make a dynamic analysis as well and with more calculations involved, however I resigned from this topic. This was the time when I had an opportunity to work for an aircraft manufacturing company, doing some practical work for them. I was interested in structural design and I signed up for FEA (Finite Element Analysis) of an agricultural aircraft's control surface. Just before beginning the work in October 2015, I was offered an actual task about the vertical tail which has undergone some modifications and its calculations were required by the company.

Buckling calculation of stringers on the
vertical tail (Extract from thesis)
In December, the topic of my thesis was called: Structural analysis of PZL-106BT Aircraft’s Vertical Tail with a CAD/CAE Based Multidisciplinary Process. This is when the fun began with the introduction to the Multidisciplinary process. As I started working on the thesis, the work became more and more specific. I changed the main focus about the calculated case, and renamed the topic to: Multidisciplinary Structural Analysis of the Vertical Tailplane of PZL-106BT Aircraft.

This wasn't the end however, as I kept on working till May 2016, when the final topic became clear. The thesis is called: Finite Element Analysis of the Vertical Tailplane of PZL-106BT Aircraft with a CAD/CAE Based Multidisciplinary Process.
Flowchart of software compatibility for FEA (Extract from Thesis)

This would be all of my engineering thesis' evolution over the last two years. On the following timeline, this evolution is shown.
Others with similar studies might have similar workflow but for those who can't decide easily, I advise to try many things to find out about you real interests and to enjoy the work you have chosen. All the best for your future work.

More about the thesis will be presented in another article, or at special request.


Keep away from jet engine

"In thrust, we trust!"
Jet engines are powerful parts of aircraft that shall be always approached with care, based on their enormous thrust creating capability, which is the result of acceleration of gas (air) flowing through the propulsion system (engine, or engine + propeller). Actually, why jet engines are so dangerous?

Hazard warning decals on a CFM56 engine nacelle
As a civil traveler, turbofan engines are the ones you are most likely to come across at an airport (turboprops too, but let's leave them for another talk). If you look carefully, you may notice a red warning sticker on the side of the engine nacelles. If you've ever been wondering what does it mean, and never had the chance to observe it carefully, then here it is:
Speaking of turbofans engines, those are the ones with transonic velocity regime (Mach numbers from 0.75 to 0.9). Looking into the front core of the engine, the large fan can be seen, so the air entering the core first passes through the fan and is partially compressed by it. Most of the air, however, bypasses the core and goes directly to an exhaust nozzle. This is why these engines are also called as bypass engines. The remaining air inside, called the core stream, proceeds directly to its own exhaust nozzle, through a series of compressor blades (high- and low-pressure), to the combustion chamber and leavers the nozzle after the turbine stage.
Schematic illustration of major components on an engine (Trent XWB engine on Airbus A350-941, F-WWCF)
A key parameter for classifying the turbofan is its bypass ratio, defined as the ratio of the mass flow rate of the bypass stream to the mass flow rate of the core stream. Having very high bypass ratio involves the use of fans with very large diameters, even up to 3 meters. The bigger the diameter, the more efficient the engine will be. Those huge fans, rotating with large rotational speed can cause a huge suction in front of the engine. The core stream that is leaving the engine has temperature of 300-500°C, as they mix with the bypass air. Hot and strong stream of air leaving the engine can have very powerful blowing effects, as presented in the videos below. This is the reason why the one should avoid a running engine. (even a standing one can cause burning injuries if it's after operation)

Another major issue is the engine inlet and its risk of ingestion. The working principle of jet engines is based on simple physics. An operating engine will introduce low air pressure in the inlet. As a result of this low pressure, the air will move towards the engine core. As the air flows into the engine, the amount of air near the inlet will have higher velocity that the rest, further away from it. The suction is the strongest at the inlet. (Think about a similar example in a bathtub full of water. If you unplug the tub, the water will start flowing out. At the outlet, which could be our engine inlet, huge suction is present.) Due to this huge suction effect of an engine, it is dangerous for any ground personnel to stay in the vicinity of a jet engine.
CFM56 engine hazard area

The clearance distance for high-bypass engines, with 2-3 meters fan diameter, is usually about 10 m to the front and 50 m to the rare. The picture shows the hazard areas for CFM56 engine, which has fan diameter of only 1.55 meter. This area however can vary with different engine power settings.

The following collection of videos are intended to show how dangerous the blast, or exhaust of a running jet engine can be.
Starting with a van that is being destroyed by the jet blast:

Following with a famous car vs. Boeing 747 example in Top Gear TV series:

Lastly, humans can be blown away too, which occurs usually at St. Maarten:

Another topic involved with jet engines is: F.O.D.  (Foreign Object Debris/Detection/Damage), which now should be obvious why it is dangerous for the engines and rotating elements. Foreign object damage is a result of any foreign debris or object that is sucked by the inflow. The debris can come from any ground vehicle, wind, ground personnel or even from other aircraft. The importance of a clear ground and runways is therefore crucial at areas where airplanes are taxiing and flying. Not only it is a threat on the ground, as flying birds can be sucked too - known as 'bird strike' incidents. Coming back to the damage that they can cause are very varying. Engines are designed to withstand major damage that could be caused by small metal objects. Special tests have to be passed in order to give certificates to any engine. Usually a frozen chicken is shot to the engine, which has to keep all of its damaged parts inside the nacelles. They can not cause any further damage to the rest of the aircraft. The following video is presenting this testing.
The major issue here is however the functionality of the engine later on. The foreign objects could break off blades, those metal parts will cause further damage to other parts, which all will result in a reduction of thrust, or even an engine shut-down. This is very crucial when approaching the ground or during takeoff. Bird feather and bones are the most unwanted "by-products" of a bird strike, which can seriously damage the turbine blades by clogging its cooling holes.

I would like to finish with a funny but at the same time memorable example of a FOD case. It happened to a Delta Airline's plane, when during taxiing, another aircraft's jet blast blew the empty cargo container away, which was then sucked by the inlet of another taxiing airplane. All the topics of this article is covered on one photo.
Delta Air Lines cargo container in engine, Febuary 1999
Have you noticed too that the warning decals are missing from the side of the engine?


  • http://www.boeing.com/commercial/aeromagazine: Fred Zimmer - Preventing Engine Ingestion Injuries When Working Near Airplanes
  • http://www.cargolaw.com/1999nightmare.html
Cover picture: Engine Alliance GP7000 engine

Video's URL, in order of appearance:

  • https://www.youtube.com/watch?v=Q6AKVMtj5Kc
  • https://www.youtube.com/watch?v=ZJ9uWsvR1l0
  • https://www.youtube.com/watch?v=eV21f1MZ5iU
  • https://www.youtube.com/watch?v=_jfXX7qppbc


What is Aerospace Engineering really about?

Probably many of you (colleges) have came across the situation of telling someone about your studies like:
"-I study Aerospace Engineering..." 
"-Oh, Space Engineering? Cool!"
"-NO, It's not only about space..."

And this is the end of the conversation. (Ok, some people may be more interested and they want to know what is this about.)

"The primary focus of the aerospace engineer is the design, construction, testing, and evaluation of craft that move through the atmosphere or outer space. This broad focus includes vehicle as simple as sled, bicycle, or automobile and as complex as high performance fighter aircraft such as the F-22 Raptor, or spacecraft such as the Space Shuttle or SpaceShipOne.
Falcon-9 Rocket lands on droneship, SpaceX

Most aerospace engineers specialize in one of the four major disciplines that must be understood to design a successful vehicle or enable it to safely operate: aerodynamicsavionicsmaterials science, and propulsion."[1]

According to 'definition' it is;
"Aerospace engineers design aircraft, spacecraft, satellites and missiles. In addition, these engineers test prototypes to make sure that they function according to plans. These professionals also design components and subassemblies for these craft; those parts include engines, airframes, wings, landing gear, control systems and instruments. Additionally, engineers may perform or write the specifications for destructive and nondestructive testing for strength, functionality, reliability, and long-term durability of aircraft and parts."[2]

As I usually define it to people; it is Aircraft / Aeronautical Engineer + Astronautical engineer.

In practice, we are the so-called "rocket-engineers" or the "top engineers", which is true on one hand. We study everything from all other engineering fields, no matter what you are specializing in in the future.

I would like to summarize the graduate course in a bit deeper way, focusing on the subjects and on the knowledge they can give us. (Here, I would like to note that many universities offering this course might have different syllabus, I am presenting what I have studied during my 3,5 years.) Many future or present students may find this collection useful for their studies.
just for fun

1st year:
This is the nightmare of all freshmen students. Your previous life changes from a normal human into a totally weird/nerd/unsocial one. You will be forced to study every day, no weekends guaranteed, daily attendance+studying time a day can be even up to 16 hours. If you are from a high-school that didn't teach you the basics properly, you'll suffer more.
  • You learn calculation tools like; Algebra, Calculus 1-2,
  • You spend endless hours of redrawing your Engineering drawings,
  • You start learning a programming language; this in most cases is C (the hardest one) to help with the crazy computations that you'll meet during your studies,
  • Mechanics 1-2, Materials, Physics, Mechanics of structures, Thermodynamics, Electronics are all part of this lovely year, just to make your life harder with those other subjects that you are already failing and struggling to pass. But these basic subjects are just to be an introduction for the upcoming years.
  • If all these wasn't enough, you learn additional, irrelevant subjects, just to have an even wider knowledge on all other fields of science; Philosophy, Environment, Economics, or a foreign language. (That last one is actually important.)

2nd year (3rd semester):
Let's assume, you have passed somehow that first year and still want to continue your Aerospace course. You hear it from others that "oh, second year will be easier". Definitely, but not on this major. It may feel slightly easier, because you are adapted to such a lifestyle that doesn't involve friends, you gave up all your hobbies and you are not freaking out if you have not a single weekend in half a year. You eat breakfast at 7, lunch at 19-21pm, dinner at midnight. The 2nd year is when the fun begins. This is when you actually see airplanes on the slides and are so excited about the new subjects.
Programming a CNC machine for milling
  • Calculus 3 is giving you more nightmares, but it is now making a lot of sense.
  • Engineering graphics is turning into Computer Aided Design (CAD),
  • Fluid mechanics and Basics of Control and Automation requires you to use all your Calculus skills, even the ones you have not yet studied. (Catch 22)
  • Machine Design begins, offering all nice and practical theory and calculations related to metal fatigue, calculations for those elements that you had to draw all the time on Graphics in the last year, you learn mainly about their design,
  • Manufacturing technology will teach you how things are made.
  • Mechanics of structures 2 seems like a piece of cake and you start to enjoy solid mechanics, as long as it's easier to understand than fluids.
  • Aeronautical systems 1. is you favorite subject this semester, which you enjoy following as an aviation addict, and you want to correct the teacher about ILS. But you may not knew the operation mechanism of the HSI.
  • Finally you have other introductory subject into Aerospace, Materials in Aerospace (now a more specific one as before),

still 2nd year (4th semester):
Now you start feeling something neutral. If you have passed everything so far, you start to meet your old friends or the new ones that also passed everything, if not, you are still determined and ready to retake some subject with the new 12 others. Here is when everything gets messed up. But it is also the turning point.
  • Aerodynamics, the long awaited subject, that is as cruel as Fluid Mechanics was, but you study the air more deeply, not those pumps, dams or Hagen–Poiseuille flow, for instance. Your favorite equation is Navier-Stokes, but you still have no idea how to solve it.
    Fluid Mechanics consultation with prof.
  • Astronautics, just in case to learn something at last about the space,
  • more Electronics, labs, and Electric circuits,
  • more Machine Design (now gears, clutches, and many more fun mechanism)
  • As for other laboratories are concerned, you will have them too. Machine Design, Mechanics of structures, Thermodynamics or even Aerodynamics labs. They are really amazing and you can't wait to attend the next class, apart from the fact that you have to prepare tonnes of reports and study for entry tests. This is where you either way have to work in a team, if so far you haven't gained this skill.
  • Mechanics of Flight, together with its lovely projects. Got to choose your aircraft, but pick wisely, you will spend your next one year working on it every week. Calculations of all kinds of graphs within Mechanics of aircraft. Pretty nice subject.
  • Propulsion systems, to know how piston engines and jet engines, etc. work,
  • Last but not least, Computer science continues, or i should rather call it Numerical methods. Yeah, those Runge-Kutta ODE-s and writing C programs to calculate Gauss-Elimination and the rest.
After successfully finishing the 2nd year, you came to a point where you can take a huge breath. You are able to survive whatever comes after, and you can start thinking of going for a beer or a date.

3rd year (5th semester):

This semester is a continuation of previous subject together with additional more.
  • Aeronautical systems 2.,
  • Aircraft Design 1. your favorite subject finally has come. Theory of everything on an aircraft, structures of fuselage, wing, empennage. Nice and time consuming projects where you can start the design of you own aircraft.
  • Aircraft Engine Design 1. if you like propulsion more, here you have it.
Did I mention propulsion?
Hinge system on a UAV rotorcraft
  • Chemistry of combustion, just to hate Chemistry even more.
  • Machine Design 3, to know a bit about metal contact, and elastohydrodynamic lubrication,
  • Mechanics of flight 2., now you completely hate that chosen aircraft, but still please calculate 5 more projects about its dynamical behaviors.
  • Rotorcraft Aeromechanics, 'cos you have to answer your friend's question about helicopters too, and you had Mechanics 2. too early, so in case not to forget all the knowledge,
  • Spacecraft design, so that now people can really say that you're a "rocket-scientist",
  • Aircraft Engines Maintenance, to get familiarized about GE, Rolls-Royce or PW engines.
  • Selected Applications of CAD/CAM/CAE Systems - to learn some CAD modelling using professional software like; Unigraphics NX, or Solidworks, or AutoCAD, etc.

(6th semester):
Now you can say, you've done something. Let's say you are over half of it. Keep working on!
Business Jet wing loads calculated with AVL
  • Aircraft Design 2, still airplanes, still projects, calculations, but it is getting more fun with Xfoil, AVL for aerodynamic computations, or even XFLR for airfoil design. Your programming skills are just about to pay-off.
  • Aircraft Maintenance, to get familiarized, how maintenance of Boeing or Airbus planes are done.
  • Finite element Method (FEM), is a lovely subject after those Mechanics of Structures classes. Now you learn the same as before but applied through FEM theory and FEAnalysis. Using a software like, ANSYS, you perform some calculations during labs. (Now the word lab is always about computer aided calculations or design.)
  • Machine Design continues but this is time for your own design. Design a shaft, control system or a gearbox, up to your requirements. Calculation from scratch, hand drawings, computer drawings, 3D models and calculations, documentations and huge printout drawings.
  • let's have a bit more Physics; Quantum mechanics and Quantum Physics,
  • Simulations of Aeronautical Systems, on which you perform simulation using MATLAB software, calculate dynamic motion equation, and present your ides. Flap, rudder, aileron or similar motion systems.
  • Structure and Assembly of airframes is also an integral part of aircraft manufacturing, learning about technologies used in production, design your own jig or tooling dock.

That'd be all? No!

  • Intermediate Project, or let's say, a pre-engineering thesis. You need to choose this on your own, get a supervisor and do something 'fun' to prepare you for your actual thesis. In some cases it is also a thesis but a bit less time consuming.

You've completed 3rd year, congratulations! You can proudly pass your printed A1 size drawings to your father, show your mid-thesis that you've made but you are still not an engineer with it.

4th year (7th semester):
Now that you have completed the most of it, you will have the same amount in one semester. Just kidding, you can sit back, relax and DO YOUR THESIS! Start immediately, because you will end up making it in the next semester. Don't forget about your still ongoing subject, such as:
Static Pressure plot of a flow in a T-connection, FLUENT
  • Computational Fluid Dynamics (CFD), the lovely Navier-Stokes equation is finally about to be solved, but unfortunately for compressible flow. Don't give up yet, FLUENT is only a user-friendly version for basic flows. Imagine what people with CFD doctor degree do, they code their own calculations for turbulent flow. Aw... too hard to even thing of it. So no reason to panic about your simple PDE and ODE problems. The only reason to panic is if you have forgotten Fluid Mechanics, Aerodynamics or Calculus 1-3, and Numerical Methods. So basically all the "best" subjects.
  • FEM 2. is just a continuation of the previous, more applications, and more colorful images in ANSYS Mechanical program,
  • Simulators, it can be flight or any other. No not flying, rather theory about the design principles, operations and computer network and algorithm used.
  • Vibrations and Aeroelasticity is another integral subject of any Aerospace Engineer, so here you get familiarized about vibration phenomena, the famous flutter, but all of it's types, computation of spring supported models and many more.
  • Aeronautical Regulations is needed in case you will once design your own aircraft, so to know how the certification process is.

PZL-106 BT aircraft, CAD model of empannage
Now it's time for your final Thesis. It is not a descriptive 30 pages long, study about airplanes. No, it is engineering so do it as it supposed to be done. Design, create, calculate, model, analyse something that hasn't been done before! Something that can improve Aerospace technology in any way. This is your whole summary of learned few things. You choose your specialization based on this work. If you lack any knowledge, well, time to study on your own. When finishing it finally, you only need to worry about your final defense exam, where you just need to know all about 100 topics, related to all the subject that you've completed. (I hope you still remember what the layout of rockets are or what is the extensometry method in solid mechanics.)

This is all in a nut-shell about how to become an Aerospace Engineer graduate student!
just another day at your desk
Well, did I mention that all these years worth something if you do your Master studies too? As others often say; "this is not as hard a rocket science". I say, this is, so study it accordingly. Be a Master in it, if you want to make something out of it.

I would like to finish this summary with my favorite quotation that my cousin told be before university:
"When it gets really hard, remember that it will always be harder!"

[1] - Aerospace Engineering: From the Ground Up, Ben Senson, Jasen Ritter, pp.4.,2011
[2] - http://www.livescience.com/47702-aerospace-engineering.html


SpaceX SES-9 Mission Overview

26th February 2016 at 11:29pm UTC,

Space Launch Complex 40, Cape Canaveral, Florida, Earth

It is the second launch of the upgraded Falcon-9 rocket, Falcon-9’s first GTO mission, first attempt to try to land on a droneship and I will be writing about the special challenges of that landing. I will also be writing about the SES-9 Spacecraft and about the upgrades that were made to the pad in order to launch this more powerful rocket.

Let’s do a brief walk-through of what we’re looking at:
  • Firstly, the upgraded rocket itself and the Space Launching Complex.

  • Rocket first stage (bottom part): Carries the satellite up to the edge of space, about a 100km high, moving at about 2,5 kilometers/seconds (at the time of separation). (Usually it’s lower of that about 2 km/s for some lower Earth orbit.)

  • Interstage (second stage): Going to carry the satellite, speed it up, and carry it further towards to geostationary orbit, which is its final position that is going to.

  • Payload (top): The fairing, or the nose cone, which is made of a carbon composite structure that actually holds the satellite inside of it.
  • Just next to it, there is the transporter erector or the strong backThis is what actually carries the Falcon-9 out from the hangar to the launch pad, and then raises it up to its current position.

  • There are several lightning towers surrounding it. Florida is one of the lightning capitals of the world, and these safely direct that energy to the ground, instead of affecting some of the sensitive electronics there that are actually on the vehicle.

The launch will be a potential autonomous space-board droneship landing. After the first stage, the second stage separates, the first stage will be continuing on a ballistic trajectory towards the “Of Course I Still Love You” droneship.

About the SES-9 Satellite:
SES-9 is the 9th telecommunication satellite in the SES family, and the second SES satellite that SpaceX is putting into orbit. They will be delivering SES-9 to GTO (Geostationary Transfer Orbit) at about 38000km high (or even higher). After they drop off the spacecraft, it will use it's on board thrusters to adjust its position and to transfer from GTO into its final Geostationary Orbit.
Geostationary Orbit is so high up that it orbits the planet at about the same speed that the Earth rotates, and this makes the satellite appear like a star in the star. So it is going to be a fixed point in the sky, whenever you look up.

Let's compare this to the ISS (International Space Station) which is in low-Earth orbit, at about 300km, which is much closer to the Earth, therefore it means that the GTO orbits much faster, this is why you can see that star moving, when you look up at the sky, and you can actually see the Station transmitting across the sky. They will be dropping SES-9 over a 100 times as higher as the ISS is. To get that high, a lot of propellant is needed. They are not going to have a tonne of it left over in the first stage, they're flying back down to the droneship. In given the needs, use every single last drop of that remaining propellant to burn the Falcon-9 engines to slow it down and orient it onto the droneship, this landing attempt is going to be one of the most challenging one.
The launch window for the launch day is 96 minutes long, and the reason for that is because as the Earth rotates, the final orbit of the target that they're trying to drop the satellites up to, isn't a fixed distance from the launch-pad, which means that there is a certain window where you can get to the same destination easier than others. If they had to try the launch outside of that 96 minute window, they'd run the risks of not having enough propellant to get them to a desired orbit. Within that window, SpaceX can make as many attempts as necessary.
If they have to hold the countdown between the time of liquid oxygen loading and T-0, they'll scrub for the day, and will try again on another day. It takes probably more than the time they have in the window to offload the cold liquid oxygen and load a new batch on board.

Let’s learn a bit about the satellite technology:
SES is one of the world’s largest satellites communication companies and with over 50 satellites in orbit. They are based at Luxemburg, in Europe. SES-9 is the latest satellite in their fleet and they have planned to launch 7 more satellites between now and 2017. SES-satellites offer all kinds of service, internet coverage, date services for mobile phones, airplane wifi, SES also broadcasts 7000 TV channels, more than any other media broadcasting in the world. They have satellites that they can mobilize in just 24 hours to help with disaster communications, with satellites that enable internet access in rural areas all around the world.
For today's launch, Falcon-9 is going to put SES-9 satellite at the highest possible orbit as it can, currently targeted for around 38000km or even higher. Then after it is separated from Falcon-9, SES will use its on board, liquid propellant thrusters to perform any major launch maneuvers or adjustments. A little it will use its electrical propulsion system to help finding its final position in GTO.

About the launching process:
SpaceX team monitors the temperature of liquid oxygen. It is important to keep the liquid oxygen cold enough under the rocket, especially in the second stage. When the team has the go/no go poll, they verify that everybody is ready to enter the launch out of sequence and begin propellant loading. Next step is a precision timing sequence between the ground computers and the Flacon-9 flight computers. They want to test as much as they can, as close to launch as possible, to make sure  that everything is working. So if that ends, the're moving the throttle valves on the engines. These set the power of the engines, so they are moving them to arrange the motion, to make sure if they are working. They're also setting the gimballing of the engines, making sure the actuators and the sensors are all connected and that the engines are able to steer the rocket throughout the flight.
In accordingly about T-10 minutes, the pre-valves will be opened and began chilling in the first stage engines. Getting the Merlin turbo pumps ready for the cold temperatures at ignition, just a couple of seconds before T0 (liftoff time).
When the Falcon-9 goes from ground power to internal power, it is possible to see the clamp arms, opening up on the second stage. The erector will then start to move always from the vehicle, that happens at T-4.5 minutes. Liquid oxygen loading is going to finish loading on the Falcon-9, between T-3 and T-2 minutes.

Upgrades made to the launch-pad system:

These upgrades allows the team to further improve the reliability at once with better processing time than ever before. One of the upgrades they have made are to the hold-down system, the hold clamps that physically hold the rocket to the ground while the engine comes up to full power. Right next to those hold-downs are the quick disconnects, these hold the electrical lines and the fuel plumbing that connects to the vehicle. They have made adjustments to those quick disconnects, changing the type of springs that they were using there so that they are not susceptible to a particular failure modes. 
The second big upgrade they have made is a much bigger increase of surge of liquid oxygen. As you know the rocket runs on liquid oxygen and RP-1 kerosene fuel and both of these fuels need to be as cold as possible before they are loaded into the rocket. The whole point of adding all of these capacity is to shorten the recycle time, which is the time it takes to completely empty and reload the rocket. Previously they just had one sphere that held the whole liquid oxygen (on Apollo testing). Now they have two much longer cylinders that keep that RP-1 at super cold temperature, and they have the ability to fill the first stage in 30 minutes.
They have also made upgrades to the transponder erector because the Falcon-9 is longer and heavier, and has a more powerful engine, they had to add blast place to the bottom, so that the transporter erector does not melt at the base.
They have also added more counter weights and more powerful lift cylinders, so they can have heavier payload on top of the Falcon-9 and still raise it to launch position without any problems. 

About the Mission Timeline:
On the spacecraft site, the SES spacecraft is gone on internal power. The range is good, the upper altitude winds are good.
At T-5 minutes accounting, everything seemed good.

At T-01:41, the SpaxeX team had to call for “hold-hold-hold” that stopped the count-down.

They were in the middle of propellant load, they were just loading up liquid oxygen on first and second stage. Falcon-9 was not in any significant concerns, static fires were revealed and lit down the engines and in additionally shut them down and then drained propellants.
The rocket then come back and the launch  was postponed for couple days later. 

Wish to see a successful launch in the near future.

Reference used:
  • SpaceX.com
  • SES-9 Full Webcast, narrated by: Lauren Lyons - Mission Integration Engineer, Michael Hammersley - Materials & Process Engineer, John Insprucker - Falcon 9 Product Director, Tom Praderio - Firmware Engineer