F LY I N G O P E R AT I O N S
IGNITION CHECK
An understanding of how to check your ignition system can help you avoid
costly damage to engines and ancillaries.
BOB COOK
H
OW’S
YOUR
IGNITION
SYSTEM
knowledge? Good enough to correctly handle any irregularities or
emergencies that may arise?
First and foremost you should at all times
follow the recommended procedures contained in the engine and aircraft manufacturer’s handbook. Also, the advice and information in this article assumes the use of a rotary type ignition switch as opposed to
individual magneto switches, though for
most operations the difference has no effect.
The Magneto: A magneto is an engine accessory that generates pulses of high voltage
electrical energy which are channelled to the
spark plugs located in the engine cylinders.
Each spark is produced at precisely the right
moment to ensure proper ignition and burning of the fuel/air mixture present in the respective cylinder, thus generating usable
power. A magneto is completely independent
of the aircraft electrical system and therefore
requires no battery or aircraft electrical
power for operation.
The most popular unit is the rotating magnet magneto. As the name implies the system
uses a rotating magnet. The magnetic poles
pass in close proximity to a soft iron element
around which are wound two separate wire
coils, a primary and a secondary. The secondary has many more times the number of
turns of the primary, so the combination
forms a “step-up” voltage transformer. The
moving magnet produces a low voltage current in the primary coil but at this stage little
or nothing in the secondary. The primary
voltage produced is proportional to the speed
22 flig ht safet y aust r alia, march 1998
of rotation. The current flow in the primary coil generates a magnetic field which
tightly envelopes both primary and secondary coils.
By incorporating a means of cleanly interrupting the primary current flow – generally by the use of contact breaker points –
the magnetic field is made to collapse suddenly as the current flow stops. This produces a very high voltage pulse in the secondary coil at the instant of interruption. A
rotary distributor mechanism incorporated
in the magneto delivers successive sparks to
the cylinders in the correct firing order.
For safety reasons an aircraft engine is supplied with two entirely independent ignition
systems comprising a magneto, switch circuit, spark plugs and high voltage spark plug
leads. An added bonus is that two spark plugs
per cylinder gives better combustion and improved engine performance.
A minor penalty of duplication is that the
engineering task of setting and synchronising the ignition timing must be accomplished twice. Likewise, in operation the pilot
must conduct checks of two ignition systems
and ensure each is working within the limits
set out in the aircraft’s checklist.
Dead cut versus live magneto: Since engine
operation on one magneto will be almost
normal, how does the pilot know prior to
taxiing that both ignition systems are indeed
working? Certainly this will be discovered
during the pre take-off checks at the run-up
bay, but if there was a very simple and reliable check that could be done even before
leaving the parking area, wouldn’t it be worth
doing if only to save time and hassle?
Well, there is. It’s called a dead cut check.
Prior to taxi, the pilot moves the magneto
switch from “both” to “left” then to “right”,
and then back to ‘both’. There is no need to
select the ‘off ’ position in this check. If one
of the systems is dead the engine will cut out
when the respective magneto is selected. The
cut will be signified by a marked change in
Primary
coil core
Magnetic
field
N
S
Rotor
N
S
S
N
N
S
N
S
S
N
Magnetic fields of a rotating magneto which
produce low voltage AC current in the primary coil.
F LY
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P EHREAT
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S
engine note and a substantial RPM windDuring this check you should not pause
back.
unduly when running on only one magneOn the other hand, a similar check – called
to as the deactivated bank of spark plugs will
the live magneto check – may be conducted
start to become fouled by lead and carbon
when shutting down the engine. The reason
deposits resulting from inefficient combusfor performing the check though is different.
tion. Any such fouling will show up as an unYou will now be checking to ensure that neiacceptable RPM drop when the respective
ther of the magnetos is live when the ignition
magneto for those spark plugs is selected.
switch is selected ‘off ’. To conduct this check
Likewise for the same reason, returning the
the switch should be moved momentarily to
switch to ‘both’ between the single position
‘off ’ then back to ‘both’. The engine should
checks is essential.
cut out at the ‘off ’ selection. This check
Apart from confirming you still have the
should be performed with the engine at idle
reference RPM set (i.e. the throttle hasn’t
and by pausing only long enough in the ‘off ’
moved due to vibration for example), reposition to confirm engine cut either by a
turning to both allows time for the deactichange in engine note or the first inkling of
vated plugs to clear themselves of any posan RPM drop. If the engine continues to run
sible deposits which could otherwise yield an
in the ‘off ’ position then at least one of the
erroneous reading when re-selected.
magnetos is still live.
It is essential that you do not select the
A live magneto means a live propeller – a
switch to ‘off ’ during this check as damage
situation that has in the past had fatal results
may result if the switch is returned to a magfor people manipulating the propeller when
neto-operative position. However, if for any
the condition exists.
reason ‘off ’ is inAccordingly, this
advertently selecttype of unservice- “It is essential that you
ed, you should
ability should be redo not select the switch leave the switch at
ported immediately
that position, seto the off position durand a notice should
lect mixture to idle
be placed near the
ing this check as damage cut-off, and allow
propeller warning of
the engine to stop
may result if the switch completely. Otherthe danger, and the
unserviceability enwise, accumulated
is later returned to a
tered into the airfuel may
magneto-operative posi- unburnt
craft’s maintenance
ignite in the manrelease.
ifolds (backfire or
tion.”
Pre take-off igniexhaust
explotion checks: The
sion) and/or in the
checklist requires you to perform pre-take off
engine on the piston up-stroke causing kickignition checks. If the figures achieved in perback and possible mechanical damage.
forming these checks exceed published limits
The engine should then be re-started using
then the aircraft is unserviceable and must not
the specified hot engine start procedure.
be flown until the problem is rectified. In parThe aim of the run-up ignition check is
ticular, when snagging an aircraft for an ignitwofold. Firstly, examining the rpm drop protion problem you should note in the maintevides a check of each individual ignition sysnance release the precise position of the ignitem. If the drop exceeds the published limit
tion switch and the resulting symptoms for
the most likely cause is that one or more of
which the unserviceability was noted. This
the components for the affected system is opprovides considerable benefit to the mainteerating below par. Secondly, the RPM drop
nance engineer whose task it is to rectify the
differential is a measure of the closeness of
problem.
the timing of the two systems. A small differA typical check will call for you to set the
ential signifies that there is very little differRPM to say, 1800. You then move the ignition
ence in the timing between the two systems,
switch from ‘both’ to ‘left’, and note the RPM
which is good, while a differential large
drop. Typically the maximum allowable will
enough to exceed the limit indicates an unbe 125. You will then move the switch back
acceptable difference in timing.
to ‘both’ at which point the RPM should reOn rare occasions you may not get any
turn to 1800. You will then repeat the check
RPM drop at all when selecting a given magbut this time going from ‘both’ to ‘right’, notneto. This means that both magnetos are
ing the drop and returning to ‘both’. The
working but that the one not selected is ‘live’.
checklist will also stipulate the maximum alIf the fault lies purely in the switch, then the
lowable difference between the two single
problem maybe limited to the switch position
magneto RPM figures. A typical maximum
in question and both magnetos may still be
differential is 50.
grounded as per normal in the ‘off ’ position.
Schematic of typical ignition system
circuit diagram.
This can be checked at idle RPM. Operating
the aircraft when this condition exists is not
recommended since an unsafe condition may
exist, and because the differential RPM drop
run-up check cannot be performed. In any
case the aircraft should be placed unserviceable on shutdown and if appropriate, provided with a “live magneto” warning.
On completion of these checks and before
take-off it is important to confirm that the
magneto switch is returned to ‘both’. All of
the safety margins inherent in a duplicated
system are lost – not to mention the extra efficiency – if you inadvertently take-off and
fly using only one magneto.
Excessive RPM drop: An excessive RPM
drop during the engine run-up checks is
quite often the result of lead fouling of the
spark plugs. In most cases this can be burnt
off using the following procedure.
Set run-up RPM with the ignition switch
on ‘both’. Progressively lean the mixture until
either peak exhaust gas temperature (EGT) is
reached or the RPM just starts to decrease.
Leave the mixture at this setting for 30 seconds which provides enough time for the
higher combustion temperatures to burn off
any deposits. Return the mixture to full rich
and repeat the ignition switch check. If the
RPM drop is still excessive then the problem
is likely to be something other than lead fouling and the aircraft should not be flown until
the problem is rectified.
Incidentally, there is no chance of damaging the engine with high EGTs in performing
this procedure. At such a comparatively low
power setting (15-20 per cent power) there
is simply not enough fuel and air being
drawn into the combustion chamber to produce maximum EGT.
Airborne ignition problems: There are arflig ht safet y aust r alia, march 1998 23
AIRWORTHINESS
guments for and against selecting ignition
switch positions other than ‘both’ if ignition
problems are suspected when airborne.
Those against say that any magneto switch
selection away from ‘both’ must switch at
least one of the magnetos off. This could
have at least two undesirable results.
Firstly, because of subsequent switch malfunction, the deactivated magneto may remain off despite returning the switch to ‘both’.
This could make the problem worse when
there might have been a simpler solution.
Secondly, if the switch selection turns off
the only functioning magneto due to an already failed unit, serious engine damage may
result when the ‘both’ setting is re-selected.
This can occur if the engine controls are set
at a moderate to high power setting and the
no spark condition has existed for more than
a few seconds.
On the other hand there have been occasions where a rough running engine has been
caused by a considerable change in timing of
one magneto. This is particularly insidious
where the timing of the “problem” unit becomes excessively advanced thereby causing
pre-ignition which can be extremely damaging to the engine.
In this case selecting the problem magneto
‘off’ will immediately restore engine operation
to near normal. Certainly, engine operation
will be adequate to permit continued flight,
which should be to the nearest suitable airfield.
So, moving a magneto switch away from
the ‘both’ position when airborne should be
done only as a last resort. You should select
idle power between each change of setting
and then advance the throttle slowly to ascertain the result.
Pilot maintenance: Are you, as a pilot, permitted to replace spark plugs?
The short answer to this question is ‘yes’.
However there is much more to pilot maintenance than can be covered in this article.
You should contact your local CASA district
office (Ph: 131757) for full information.
Any spark plug maintenance attempted
must be performed strictly in accordance
with the conditions and limits specified in
Civil Aviation Regulations Schedule 8 and
carried out using approved maintenance
data. In addition it must be endorsed on the
maintenance release. Further, you are strongly advised not to attempt any pilot maintenance activities without first obtaining briefing, tuition, guidance or oversight from an
experienced maintenance engineer.
Bob Cook is a flying operations inspector and
Training Consultant for CASA.
24 flig ht safet y aust r alia, march 1998
SILENT EMERG
Vacuum pump malfunction can lead to loss of control.
Y
OU FLY IN INSTRUMENT WEATHER
conditions and make enough approaches to keep current, take your biennial flight review from a good instructor,
know the normal and emergency procedure
sections of your pilot's operating handbook,
and feel you are qualified to cope with any
emergency. Are you?
Maybe not.
US accident investigators have reported air
pump/system failure as a factor in an average of two accidents per year over the past
eight years. About one-half of the reported
cases involved other overriding factors such
as loss of control with a back-up electrical
gyro available, non-instrument rated pilots
flying in instrument weather conditions and
departing with pneumatic systems known to
be inoperative.
The most disturbing factor is the remaining half – an average of about one accident
per year – occurred to instrument rated pilots
who recognised the pneumatic system failure,
flew on partial panel in instrument weather
conditions for 30 to 45 minutes, and then lost
control during high task loads, such as during an instrument approach. Another common denominator was that all aircraft involved were high performance, retractable
gear, single-engine aircraft.
Lessons: The lessons are clear. Firstly, the loss
of a pneumatic system in actual instrument
conditions, without a back-up system is an
emergency that may become life-threatening
unless the aeroplane can be flown by partial
panel into visual weather conditions. This
may not be possible either due to weather
conditions or lack of pilot practice with partial panel flying.
An aeroplane with a single pneumatic system with no back-up system, or back-up instruments, should not be flown in any IFR
conditions that do not provide for quick access to VFR conditions. IFR flight "on top" of
cloud layers with good ceiling underneath
should create minimal problems with pneumatic system failure, but flying in actual IFR
with low ceiling and visibility underneath sets
the stage for serious difficulties.
Secondly, any aeroplane used regularly in
IFR weather should be equipped with either
a back-up power source, such as dual pneumatic systems, or back-up electrically powered gyroscopic instruments.
Although it is legal to fly single-engine aircraft without dual power sources for gyroscopic instruments, and the exposure rate to
accidents due to pneumatic system failure
while in actual instrument weather is low (1
accident for each 40-50,000 general aviation
instrument flight plans filed), prudence suggests that a back-up power source is good insurance against being forced to fly partial
panel in adverse weather without sufficient
practice.
Normal instrument flight relies in part on
three gyroscopic instruments, an attitude indicator (artificial t horizon), a heading indicator (directional gyro, or “DG”) and a turn
and slip indicator (“needle and ball” or “turn
AIRWORTHINESS
RGENCY
GENCY
and bank” or “turn co-ordinator”).
These gyroscopic instruments may be powered by pneumatic (vacuum or pressure) or
by aeroplane electrical systems. Which power
source is used for which instruments may
vary in the same make and model of aeroplane depending on use intended at time of
manufacture or modifications made after
manufacture.
The most common arrangement for single
engine aeroplanes without back-up instrumentation, or systems, is a single vacuum system which powers the directional and attitude
gyroscopic instruments. The other gyro instrument, the “turn and bank” or “turn co-ordinator” is usually electrically driven.
The gauge on the instrument panel may be
marked as either a “suction gauge”, a “vacuum
gauge” or a “pressure gauge” and indicates in
inches of mercury.
The correct operating range (around 4.55.5 inches Hg) is given in the handbook for
each airplane. Some airplanes also have warning lights when the vacuum or pressure is out
of tolerance.
Pneumatic systems, like other mechanical
systems, can malfunction suddenly or slowly.
A slow decrease in gauge indication may indicate a dirty filter, dirty screens, sticking, regulator, worn out air pump or leak in the system. Zero pressure could indicate a sheared
pump drive, pump failure, a collapsed line, or
a malfunctioning gauge.
Any operation out of the normal range requires immediate attention by a mechanic.
A complete pneumatic loss is noticeable
immediately on the gauge or within minutes
by incorrect gyro readings. A slow deterioration may lead to sluggish or incorrect readings
which may trap a pilot who is not constantly
cross-checking all instruments, including the
vacuum or pressure gauge.
An additional factor involves an initial lack
of recognition of the cause of the conflicting
instrument indication which develops when
one instrument, usually the attitude indicator, malfunctions. Although possibly proficient in flying “partial panel”, many pilots are
not trained or skilled in deciding. to revert to
a “partial panel” scan unless an Instructor or
safety pilot has forced the scan by covering the
attitude indicator.
It is important for pilots to scan all Instruments whenever conflicting information develops and not attempt to make control inputs on the basis of the attitude indicator
alone. Once the all-important first step of
recognition of the need for partial panel scan
is accepted, it is also helpful to remove the
malfunctioning instrument from the scan;
usually by covering it with a disk or piece of
paper.
The possibility of pneumatic system or gyroscopic instrument failure is the reason every
instrument instructor drills students on partial panel flying without reference to gyroscopic heading and attitude instruments. It is
very rare that the failure itself results in a fatal
accident, but it can set the stage for one if the
pilot is not proficient in partial panel flying
and the failure occurs during instrument
flight conditions.
Knowledge: Every pilot should know the instrument power sources for each aeroplane
flown, and particularly know the consequences of loss of any source of power, air or
electrical, or loss of any instrument, and be
prepared to cope with the loss.
Aeroplanes can be flown safely with loss of
one or more gyroscopic instruments. Every
instrument rated pilot demonstrated the ability to do so prior to receiving the rating. The
problem is that many never practice the skill
and only a few have ever practised in turbulence as it seems an unlikely need in routine
operations.
Professional pilots who are required to take
semi-annual simulator training practice a lifetime of emergencies each training session although they rarely encounter emergencies in
daily operations.
Most general aviation pilots remain “current” by flying in the system and may rarely
face or practice emergency situations. For
most pilots, continued flight in IFR conditions with failed gyro heading and attitude instruments is a high work load situation that
could lead to a fatality.
If you are not instrument rated and inadvertently encounter instrument weather, the
180° turn is usually the best course of action.
If your pneumatic driven gyro instruments
fail, it is still possible to make a 180° turn by
using the turn and bank (or turn co-ordinator) magnetic compass and clock. Likewise
a descent through cIouds to VFR conditions
can be made using the turn indicating
instrument.
These procedures may be tailored to each
aeroplane type and model and should be
demonstrated by and practised with an instructor. It may be too late to learn them
when faced with actual need. Avoid conditions that risk, encountering instrument
weather.
If you are instrument rated: If you are instrument rated and gyro instruments fail or
mislead, do not be afraid to ask for help. ATC
personnel know where to find better weather and are able to give “no gyro” heading directions. The whole system – radar, weather
reports, communication, and personnel – is
instantly available to assist you.
Do not try to be a hero and continue on
bravely as if loss of pneumatic power was no
big deal. It can be a serious emergency unless
you have maintained high proficiency in partial panel flying.
Also, cover the dead or dying instruments.
Most partial panel practice is done with covered Instruments, but In real cases the artificial horizon will be sagging and giving erroneous information that your instincts want to
accept as correct. Auto pilots using these instruments as sensors must be turned off immediately.
Finally, if your aeroplane has no back-up
capability be cautious in the type of IFR you
fly. Solid IFR from take-off to touchdown can
be very difficult on partial panel.
Back-up: If your aeroplane does not have a
back-up, or stand-by system, and if you use
your aeroplane for IFR flight, consider a backup or stand-by pneumatic system. Several
manufacturers offer a variety of alternate systems that will supply vacuum or pressure if
the engine driven pump fails.
While the chances of pneumatic system or
pneumatic driven instrument failure while in
IFR conditions has been demonstrated to be
small, those same statistics also demonstrate
that the cost of a stand-by system is far less
than the too often fatal results of not having a
back-up.
Reproduced with permission from the US General
Aviation Manufacturers Association and the Federal
Aviation Administration's accident prevention
program.
flig ht safet y aust r alia, march 1998 25