When it comes to the techniques for starting a hot engine, there are as many
opinions as there are pilots. Every airplane is different and comes with
different recommendations from the manufacturer. So instead of going through
all the possible hot start techniques, we'll explain a little bit about an
aircraft's fuel delivery system. The more you know about how the system works, the
more successful you'll be when it comes to hot starting. But first, what makes a
hot start hot? For air-cooled engines, it's usually
the period between 20 minutes to 2 hours after shutdown. During that window
of time, your engine and fuel system components haven't had adequate time to
cool down and allow for normal starting procedures. Now, on to the details -
airplane fuel delivery systems can be broken down into two main types -
carbureted and fuel-injected. Carbureted engines can occasionally be problematic
when it comes to hot starting, but fuel-injected engines are by far the
most difficult. The carburetor usually sits below the engine, so it's not as
likely to get hot enough to cause problems. In a typical fuel-injected
setup, however, the fuel lines are positioned right over the engine, above
the hot cylinder fins. Consider what happens after shutdown - the heat rising
up from the engine boils the fuel right out of the lines and turns it into vapor.
Some of this vapor then escapes into the intake manifold, which, by itself, is a
good thing for startup - vapor in the intake manifold means that it's
adequately primed. But, you still have vaporized fuel in the
lines. So at best, you might get the engine to fire up, only to have it die a
moment later. What you might need to do is purge the vapor from the fuel lines
without allowing excessive amounts of fuel into the intake ports. Let's take
one example of a hot start technique that deals with this problem. I'm
standing next to the A36 Bonanza with a Continental IO-550-B engine. The POH has
a specific procedure listed for hot starts, shown here. Note the first 3
steps in the Hot Start procedure - specifically steps 1 and 3. Mixture - idle cutoff, auxiliary fuel pump - high for 30 to 60 seconds, then off. After step 3, the procedure picks up as if it's a normal cold start.
So why run the fuel pump with the mixture off? Well, because this pressurizes the
system, and in pressurizing the system, you are circulating cool liquid fuel
through various components and purging the vapor from the fuel lines. And with
the mixture in idle cutoff, you aren't allowing any fuel flow to the
injectors. Again, this is just one example, and procedures will vary between
different types of aircraft. It's always best to start with the factory
recommendations in the POH first, if the manufacturer provides them. Let's take a
quick look at a Cessna 182T, with a Lycoming IO-540 engine. There's no
preliminary purging technique specifically listed, but a close look at
the wording in the POH will reveal that a similar vapor lock exists in the event
of a false start. Now that we understand what's going on in the fuel lines,
Cessna's procedure makes sense. The POH says, "Should the engine tend to die after
starting, turn on the auxiliary fuel pump temporarily and adjust the throttle
and/or mixture as necessary to keep the engine running." So essentially you're
purging the fuel lines of vapor after the engine has started, versus before. So
why does all of this matter? Well, let's go back and visit the pilot from the
opening scene. This entire time, he's been trying and trying to get his engine
started. He doesn't have a basic understanding of hot start technique, and
his POH is nowhere in sight. Now he's on his 6th attempt, and he's gotten pretty
frustrated. If he keeps going like this, in addition to wearing down his battery,
he's likely to burn out the starter. Airplane starters have limitations in
how well they can tolerate repeated starting attempts. These are called duty
cycle limits. Duty cycle limits will vary by manufacturer, but many starters won't
tolerate more than 10 seconds of engagement at a time. In addition, each
start attempt is meant to be followed by a specified cool down period. After a
certain number of start attempts, many manufacturers will specify at least a 30
minute cool down time, which is not insignificant.
The point is, it's important to fly well-maintained, reliable aircraft with
components that haven't been abused. Less stress on the aircraft equates to
less stress on us as the pilots. There may be times when the hot start
recommendation in the POH doesn't work - or the POH might not come with
recommendations at all. This situation, combined with repeated starting attempts,
may lead you to inadvertently flood your engine (meaning there's too much fuel in
the system that offsets the proper fuel-to-air ratio for engine firing). Luckily
there's a common procedure for a flooded start, and interestingly enough, it's a
process that pilots often think of as a hot start procedure. Because hot starts
can easily turn into flooded starts, you can see why the two sometimes get
confused. In the case of a flooded start, try this simple technique - mixture out,
throttle full forward, and start. Push the mixture in when it catches, and throttle
back rapidly. You might feel like you need three hands to succeed, but it's
worth a try. Here's what's happening under the hood - you've already got more
than enough fuel in the cylinders, so you want to keep the mixture out initially,
which doesn't allow any more fuel past the engine driven pump. By opening the
throttle, what you're doing is bringing the level of air intake up to the
correct fuel-to-air ratio, and allowing for combustion. If you've been flying for
any length of time, you know that hot starts are pretty much inevitable. To do
them correctly, all you need is an understanding of your aircraft's fuel
delivery system, a little patience, and a little practice.