Monday, November 16, 2009

Engine Start-issues

The West Valley Flying Club has a monthly newsletter and my favorite contributor/CFI Dave Fry consistently has thoughtful and useful articles. With his permission he let me copy and paste it!

I Can’t Start the Plane, What do I do?
Dave Fry, Aviation Safety Councilor

-If you start it up
Kick on the starter give it all you got
The Rolling Stones

There are any number of ways in which airplane engines take on stubborn, mule-like characteristics when it comes to starting. Better yet, it is definitely a function of the type of engine, the type of plane, and sometimes even the particular airplane. In general, though, there are some basics that apply, and I’ll try to sort those out in the next several paragraphs.

Starting problems (in healthy engines) can be separated into those caused by cold engines and those caused by hot engines. In all cases, the things that make an engine start are the correct fuel/air mixture, and the spark. Oh, yes, and the engine turning.

So, taking them one at a time, and not in that order, let’s go to the engine turning first.

In larger airplanes, the Limitations section of the POH usually calls out specific starter and battery limitations. Even though these aren’t specified for most of our club planes, a good rule of thumb is to run the starter a maximum of 10 seconds with a 30 second rest period between attempts. After the third cycle like this, wait at least 10 minutes before trying again. It is possible to burn out a starter, and to drain the battery. The good news is that most starting problems aren’t related to the starter.

The steps to ensure the correct fuel/air mixture will differ depending upon whether the engine (and the weather) is hot or cold. Since we’re coming into winter, let’s take a look at the cold one first. All engines have some method of priming, and in cold weather they need more. Why? For two main reasons. First, the air is more dense, so more fuel is required, and second, when the air is cooler, the fuel doesn’t evaporate as fast.

Carbureted engines usually have primers that can prime even without turning on the master switch. In cold weather, a couple of extra strokes on the primer will help, but an even better way to get the engine to start is to prime very early in the process. Several of our instructors recommend priming the engine then doing the preflight. This gives plenty of time for the fuel to vaporize, and the starts are uniformly easy. However, it can mean messing with the propeller with fuel in the cylinders, something that makes me uncomfortable, if not queasy. Like most problems there is a way around this one. Do the propeller-related parts of preflight, then prime and do the rest of the preflight.

The early prime with a cold engine works just as well with a fuel injected engine, though a lot of the similarities begin to disappear at that point. In most fuel injected engines, priming is best done with the throttle open about 1/2 inch in the winter. The fuel pump is then turned on, and the mixture is advanced while watching the fuel flow meter. When the meter stops moving, pull the mixture to idle cut-off, and turn off the pump. Later, when starting the engine, the throttle should be open about 1/8th of an inch.

Hot engine starts are another beast entirely. And the problem comes in two types flooded start, and the vapor-locked start. How do you know which is which? A puddle of fuel under the engine, or a blue nose wheel faring may be a clue. Sometimes we can even smell the excess fuel. And usually, the flooded start with a warm engine is a result of not following the correct hot start procedure.

In a flooded start, the engine won’t start because the fuel/air mixture is too rich, so obviously, we want to increase the percentage of air while reducing the percentage of fuel. The best way to do this is to push the throttle all the way in, and keep the mixture at idle cut-off until the engine fires. Then things need to happen pretty fast – the throttle comes to idle (to keep the RPM from blasting right on up to 2000 or so) and the mixture needs to go rich (to keep the engine running). And it’s actually best to do it in that order, because it’s important to keep the RPM under control.

Now, about the vapor-locked start. The simplest, and often the most successful, way to start with a hot/vapor-locked engine is to start with the throttle open about 1/8th of an inch and the mixture at idle cut-off (no priming) until the engine fires, then bring the mixture rich. An alternative method is to open the throttle, keep the mixture at idle cut-off, and turn the master switch and fuel pump on for about 30 seconds. Then turn the fuel pump off, open the throttle about 1/8th of an inch, leaving the mixture at idle cut-off, and start the engine. It should fire quickly.

-She’s a mean, mean machine
Start it up.
The Rolling Stones