Most small aircraft are equipped with a 28 volt
direct current electrical system. The system is powered by an Alternator
which drives the electrical devices and stores energy in the battery.
The Master Switch (labelled MS) causes the electrical system
to connect the electrical buses and devices to the battery. The battery provides
the power to crank the starter. Once the engine is running, power is supplied by
the alternator and the battery is recharged.
Numerous circuit breakers feed off the Primary Electrical
Bus, and provide individual circuits to power the electrical devices. Although
the arrangement will vary from one make and model aircraft to another, the basic
principles are the same. By providing numerous circuit breakers and dividing the
electrical load into several different circuits, a malfunction in one system can
be turned off without adversely affecting the other circuits. The breakers will
be labelled as to their general use, and the amperage will be marked on the face
of the breaker push button. On older model aircraft, fuses are used instead of
Usually an alternator light is located on the instrument
panel to provide a means for the pilot to determine alternator is providing
power to the system. In addition, an ammeter on the instrument panel can
determine the general health of the electrical system. After the battery is used
for starting, a considerable “charge” should be shown, indicating that the
alternator is replenishing the power drained from the battery during engine
cranking. If the indicator shows zero while electronic equipment is ON, failure
of the alternator to charge the battery is indicated.
A second bus is provided to power the electronic and avionics
equipment. This bus is connected to the Primary Bus via the Avionics Switch.
This switch should not be turned on until the engine is started to prevent the
possibility of high voltage transient currents resulting from engine starting
from feeding into sensitive electronic equipment. The pilot should also turn
this switch OFF prior to engine shut-down for the same reason.
Prior to start-up the pilot should check the status of all
circuit breakers as a part of the pre-flight check. A “tripped” breaker will
project out farther from the control panel than does a properly functioning
breaker. Pushing the breaker in will reset it to it’s normal operating position.
If it pops out again, there is a malfunction in the circuit which it feeds, and
repair should be made prior to flight.
The pilot should turn on the master switch during the
walk-around pre-flight inspection to insure that the rotating beacon and strobe
lights (If present) are functioning. If all or part of the flight is to occur at
night, the navigation lights, instrument panel lights, taxi and landing lights
should also be checked for proper operation.
Generators and Alternators:
What's the Difference
Recently two people I work with
had an electrical problem in a light twin. Fortunately the electrical failure
happened in day VFR conditions and the aircraft had two pilots onboard. The
benefits of being day VFR and having two pilots on board cannot be over
emphasized. Although a single pilot could have safely handled the problem, being
able to share the workload with someone else makes any problem easier to handle.
With two pilots working the
problem and being in visual meteorological conditions, it was easy for one pilot
to fly the aircraft while the other pilot ran the appropriate pilot operating
handbook electrical checklist. They were able to return to their home airport
without incident. They were landing at an airport with a relatively short runway
where they wanted to use flaps. Once they had the runway made, they were able to
lower the electrically operated flaps using battery power without any problem.
They had left the gear down when they discovered the problem after takeoff from
a nearby airport to minimize the electrical drain on the battery. If the
electrical system had to fail, it chose the best possible time to fail. Some
pilots aren't so lucky.
NTSB and FAA
A cursory Internet review of the
National Transportation Safety Board's (NTSB) and Federal Aviation
Administration's (FAA) accident and incident data bank produced some interesting
reading. First, we want to acknowledge that accidents have occurred as a result
of electrical problems in flight. We want to emphasize that a serious electrical
problem under the worst circumstance can be a potential killer. One such bad
situation could be a total electrical failure in a complex, high performance
aircraft on a dark and stormy night in instrument meteorological weather
conditions over hostile terrain on an instrument flight plan with only one pilot
aboard. A pilot who has worked all day, and who is now fatigued trying to get
home. Now if you really wanted to make this a difficult situation, add in some
snow or freezing rain and the risk factor would go sky high. In such a
situation, what would you do? Fortunately, most electrical failures aren't this
Although we are discussing
general aviation aircraft, history has shown that modern air carrier aircraft
can crash under such conditions the same as your typical general aviation
aircraft can. We want to emphasize that these kinds of problems can be very
serious especially for the unprepared regardless of the type of equipment being
However, our non-scientific
look at a handful of general aviation electrical related problems that made the
NTSB or FAA incident or accident reports were more typical. In many cases the
damage to the aircraft was minor or none. The same was true of injury to pilot
types of problems
A review of some of the general
aviation reports seems to indicate that pilot error in responding to the
situation caused more of a problem than the electrical problem. Because many of
the reports had little or no damage reported, the narrative of the reports were
very brief without a lot of details. For example, one report about a Cessna 182
stated, "Electrical problem. Overran runway returning. Alternator field wire
loose. Struck rwy light." The airport conditions were day VFR. Although no
damage was reported, could the private pilot have handled the situation better?
We don't know. But the report begs the question of why did the pilot hit the
runway light in day, VFR conditions?
The following incident is even
more common. The narrative said the air taxi "departed alternators off. Drained
batteries. Used manual gear. Not locked down. Folded landing."
Another report said, "Alternator
failed en route. Diverted. In confusion landed gear up." Again, minor damage was
done to the aircraft. The question is why did the pilot, a commercial pilot and
flight instructor, land gear up?
Another pilot while descending
from altitude did a "long cruise descent with the engines at a very low power
output. He said he was unaware that the aircraft had generators instead of
alternators, and that the engine speed he was using for the descent was below
the speed required to keep the battery charged." After landing and discharging
his passenger, the commercial pilot and flight instructor discovered the
aircraft's battery was too low to start the aircraft. The pilot set the brakes
and handpropped the twin's right engine. He then tried to use the operating
engine to produce enough electrical power to start the twin's left engine. When
that idea failed, the pilot got out of the aircraft and tried to handprop the
left engine. When the left engine started and went to a high power setting
before the pilot could get back into the aircraft, the twin went out of control
and started turning in circles eventually striking a fence and a tree with
substantial damage to the aircraft. The report listed a probable cause of the
incident as, "The pilot's failure to ensure the aircraft was secured prior to
attempting an engine start by handpropping."
or not to handprop
A good recommendation for anyone
attempting to start an engine by handpropping it is that a qualified, trained
pilot, knowledgeable in handpropping techniques, be in the pilot's seat to
safely operate and control the aircraft. Although people have handpropped
aircraft engines for decades, it is not without risk. Only trained people should
attempt to hand prop an aircraft because handpropping can be dangerous. A
rotating prop has the potential to inflict serious or deadly injuries to those
who make a mistake while handpropping an aircraft. Of course, the safest option
is to have the aircraft's battery replaced or charged and avoid the handpropping
completely for those aircraft with an electrical system.
aircraft system knowledge and stress
In another case there were
reasons to suspect a low voltage situation before the flight departed. There had
also been a previous electrical discrepancy reported. Then while preparing to
land at night, the electrical system failed and the aircraft hit trees during
the landing. Later it was discovered that a wire had broken.
A common thread in several
incidents was the failure of retractable landing gear aircraft to land with all
of their wheels down and locked. In some cases because of distraction or stress,
the pilot failed to extend the gear. In others, the manual gear extension
procedure was not done properly. Adding to the problem is the fact that in a
complete electrical failure, for those aircraft with landing gear indicator
lights, the lights probably will not be working. Without the lights, the pilot
may not realize the gear is not down or not down and locked properly. Adding to
the problem is the fact that most retractable gear aircraft have generally high
performance and therefore require more pilot attention to fly them.
typical general aviation
aircraft electrical systems
Since aircraft electrical
problems can occur at any time, we want to review the major differences
between aircraft electrical systems in your typical general aviation
For our readers with little
knowledge of aircraft electrical systems, we want to provide a very brief
discussion on your typical general aviation (GA) aircraft's electrical system.
First, modern piston-powered GA aircraft have two totally separate electrical
systems. One engine-driven, self-contained system provides the electrical power
for the ignition system needed to keep the engine running once it starts. This
system is based upon a self-contained magneto electrical generating system that
can keep the engine running whether or not the aircraft has any other type of
electrical system onboard. For those not familiar with a typical general
aviation piston-powered aircraft, you can compare such an engine's electrical
ignition system to that of a typical gasoline powered lawn mower. Although it
has a much simpler kind of magneto system, the lawn mower, once you start it by
pulling on its starting rope, will continue to run until it is out of gas or it
is shut off. The same concept is true of most small GA aircraft engines.
This is why older aircraft such
as the classic Piper Cub can fly without any other onboard electrical
system. To start a
Cub, just like a gas lawn mower, the J-3's engine must be rotated
fast enough to start running. Someone normally does this by rapidly turning the
propeller until the engine starts. Hence the term, "handpropping."
The fact that a piston-powered
aircraft can be started by rapidly turning its propeller when the magneto switch
is turned on and the fuel is on is why anyone working or standing around a
propeller is always warned to stay out of the propeller's arc when handling or
turning the propeller. The engine could inadvertently start and the rotating
propeller could injure or kill anyone within its rotational plane. Although the
magneto switch in the off-position is designed to prevent the engine from
starting by grounding the output of the magneto, a defective switch or a loose
magneto grounding wire could allow the engine to inadvertently start if the
propeller is turned rapidly enough and there is enough fuel for the engine to
Although the magneto system can
pose a potential safety problem for those turning the propeller, its biggest
advantage is that it provides an independent electrical system to keep the
aircraft running until the magneto system itself fails or the fuel is exhausted
or the engine stops running. To reduce the probability of a magneto failure,
modern piston engines have dual or two separate magneto systems firing two
separate spark plugs in each cylinder. Although both systems are normally used
together, in the case of a magneto failure, one system is adequate to fly the
aircraft to an airport where repairs can be made to the broken system.
The important thing to remember
is that a piston-powered aircraft engine does not need an alternator- or
generator-based electrical system or battery to fly. This is an important safety
point. As part of your preflight briefing to your passengers, you may want to
remind your non-aviator passengers that if they hear you say, "We have lost our
electrical system," the aircraft will continue to safely fly and not fall out of
the sky. This briefing is not required in those aircraft without an electrical
system onboard. Better yet, use your preflight check as a way to educate your
passengers about how your aircraft operates and important safety issues such as
We may have a problem
communicating and navigating. But there are safe operating FAA rules for that
eventuality too. If you are in VFR conditions, you stay in them. If you are in
IFR conditions, you follow the rules outlined in FAR §91.185. So read on.
Then why have an alternator or
generator and battery in an aircraft. There are many reasons. The most important
is pilots are like the drivers of the early automobiles. Most pilots don't want
to hand start (commonly called handpropping) their engine. It is potentially
dangerous, and it is nasty to do in the rain or snow. It is also nice, but not
required, to have two qualified people to do it. One trained person doing the
handpropping and one in the aircraft operating the controls (preferably another
pilot). So like automobiles, GA aircraft started being manufactured with
electrical starters in them.
This required not only a
starter, but some means of powering it. All of which lead to the need for some
type of battery to provide the necessary stored electrical power, a means of
keeping the battery fully charged, and a means of regulating the charging
process. Voila, the first aircraft electrical system based upon a battery, a
generator, and the all important electrical starter.
Once you had an electrical
system, it was easy to add all of the radios, navigational, and electrical
equipment we now have in modern aircraft.
But generators have a slight
problem. They like a minimum rotational speed to produce a specified amount of
electrical power. Too slow a speed and the output drops. If you want to make
sure the battery is being charged, you have to operate the engine faster. This
is normally not a problem in flight, but if you are number 25 waiting for
takeoff, it can become a problem on the ground. Or like the pilot listed in one
of the accident/incident reports who noted how his long, low-powered glide
caused him problems with his generator equipped aircraft. Generators are also
somewhat heavier than what has replaced most of them: The Alternator.
Enter the alternator; a
different way to make power. Again, like in cars, as electronics and technology
advanced, so did the way to produce power. Today, instead of a generator, cars
and new aircraft normally have alternators in them. The main benefit of the
alternator is that it can produce a specified amount of power at a much lower
rotational speed than a generator.
An alternator also operates
differently. It produces alternating current that is then rectified or converted
into direct current for use in most piston-powered GA aircraft. An alternator is
normally lighter in weight than a comparable generator. All of which provides
important advantages to the aircraft manufacturer and pilot. Better output at
lower revolutions per minute at a lesser weight not only improves efficiency,
but it also improves the useful load of the aircraft by a small amount.
maintenance and inflight decision making
So how do you know which one is
in your aircraft? The best way is to read the pilot's operating handbook.
Reading the handbook does several important things. First it allows you to
hanger fly with the best of pilots. You can also join any argument about the
type of electrical system in your aircraft. Plus when you have a problem you can
talk intelligently with your maintenance technician.
But the most important reason
for reading your operating manual or aircraft flight manual (AFM) is to learn
how to identify and possibly handle any electrical problem in flight.
Electrical problems need to be
handled correctly and promptly because they could cause an onboard electrical
fire, damage other electrical gear, or cause problems with other systems.
Another reason is once you
understand the electrical system in your aircraft or the aircraft you fly, you
can make important decisions about what you are going to do in case you have a
generator or alternator failure.
For example, by knowing and
understanding your electrical system, you may decide to continue your flight by
turning off non-critical electrical items such as your second radio and other
redundant electrical gear or start looking for the nearest airport to land.
Equally important is knowing
critical flight data such as what to do if you have electrically operated flaps
or gear. More than one pilot has put him- or herself in a "box" with no way out
by making the wrong decision during a "minor" incident or problem. Putting
electrically operated flaps down early and not having the electrical power to
raise them may mean having to fly with increased drag or minimal lift during a
go around or while having to divert to another airport. The same may be said of
electrically operated landing gear. Although in some aircraft the increased drag
produced by the lowered landing gear may be worth the drag penalty considering
the potential problems later of having to either manually lower them or
forgetting to lower them. Or if the pilot is in the clouds, the pilot may decide
that being able to talk and navigate is the most important use of any remaining
Because each flight is unique
and the needs of each pilot is unique, it is hard to say which electrical
devices should remain on and which devices should be turned off. This is why it
is important that each pilot review his or her aircraft's electrical system and
know and understand it to the point where the pilot can make the best decision
about the aircraft's electrical system before the loss of the generator (or is
it an alternator?) becomes critical to flight safety. Knowledge is power (pun
intended). And if your aircraft has electrically operated retractable landing
gear? Please remember that you still have to lower the gear before your next
landing, so you just may want to review your aircraft's emergency gear operating
procedure before your next takeoff.