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.
