ppl_06_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 1 : ELECT R ICAL SY S

	Ma l f u n c t i o n s In d i c a t e d o n t h e ‘ Lo a d m e t e r ’ .
	The loadmeter, the type of ammeter which has the zero on the left side of the dial, will
		The
	indicate alternator failure by the needle dropping to zero.
		loadmeter,
		the type of
		ammeter
		having the zero on the left side
		of the dial, indicates alternator
		failure by the needle dropping
		to zero.

Figure 11.24. Indication of Alternator Failure on the Loadmeter.

On the other hand, if the loadmeter reading remains excessively high, beyond the time period during which the battery would normally have recharged itself, this reading may indicate that the battery has an excessive charge-rate.

Figure 11.25. Indication of High Charge on a Loadmeter.

An excessive charge-rate will cause the battery to start losing some of the material from its plates. This will cause lasting and serious damage to the battery. The high charge will also cause the battery to get very hot, possibly so much so that the electrolyte will evaporate, exposing the plates to the air and once again causing them damage.

173

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 1 : ELECT R ICAL SY ST EMS

The needle staying central,

or just right of centre of

the gauge on the centre-zero ammeter is an indication that the battery is charged.

The pilot, however, should bear in mind that the high charge rate could perhaps be brought about by a faulty voltage regulator. If this is the case, all of the aircraft equipment will be at risk of becoming overheated and impaired, especially heat sensitive components like the radio and navigation equipment.

In d i c a t i o n s o n t h e Ce n t r e - Z e r o Am m e t e r .

The indication on the centre-zero ammeter that the battery is charged, and the system is functioning normally, is given by the needle staying central, or just right of centre of the gauge.

Figure 11.26. Constant Heavy Discharge on a ‘Centre Zero Ammeter’.

The centre-zero ammeter will indicate alternator failure by the needle showing a constant heavy discharge; that is, far into the left side of the scale.

If the needle stays just in the negative portion of the scale for any length of time, this may indicate that the alternator is incapable of supplying all the loads as well as re-charging the battery at the same time. This situation will require some of the electrical equipment to be switched off, otherwise the battery will eventually become completely discharged.

If, on a single engine

aircraft, the generator

fails, the electrical loads should be reduced to a minimum and a landing made as soon as is practicable.

Al t e r n a t o r F a i l u r e Dr i l l .

The following practices are not to be considered as being representative of emergency drills for any particular aircraft type. They are purely general recommendations which should be considered if an alternator should fail.

If the alternator warning light illuminates at normal engine speed, and either the loadmeter shows zero or the centre-zero ammeter indicates a heavy discharge, the following actions should be considered.

Initially, judiciously select off any electrical, radio and navigation services not vital to the safe operation of the aircraft.

174

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 1 : ELECT R ICAL SY S

Next, check the field circuit-breaker to see if it has tripped. If it looks normal, consider tripping and resetting it anyway. If your aircraft system has overvoltage protection and the field circuit-breaker has not tripped, these indications may be evidence that the over-voltage relay has tripped.

Switch off the alternator master switch, shown in Figure 11.22, and leave it off for about two seconds. Then switch it back on again. Now check the loadmeter or centre-zero ammeter and the alternator failure warning light. If the indications are that the alternator output has been restored, switch on the services singly in order of their importance to the safe operation of the aircraft. Do this relatively slowly and deliberately, allowing sufficient time between each service selection for the load to be taken up by the alternator. Should the alternator fail again, the drill should be repeated, but this time without selecting the faulty service.

If the pilot’s attempt at restoring the alternator is unsuccessful, it is advisable to reduce the electrical load as much as possible, and land as soon as is practicable. The battery will continue powering electrical equipment for a short while, but all unnecessary equipment should be shut down.

Remember that for some electrical equipment, the only way to switch off is to pull the circuit breaker. If the alternator has failed, you should turn off the alternator-half of the master switch because the field circuit of the inoperative alternator will still be drawing electric current and, if not isolated from the battery, will use up battery power unnecessarily.

Remember, too, that any radio transmissions you make will also use up battery power.

If, after carrying out the initial drill

for an alternator failure, the indications are that the alternator output has been

restored, restore the services singly, relatively slowly and deliberately, in order of

their importance to the safe operation of the aircraft, allowing time between each selection for the load to be taken up by the alternator.

Figure 11.27 A Rotating Field Alternator.

175

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 1 : ELECT R ICAL SY ST EMS Q U EST IO NS

R e p r e s e n t a t i v e P P L - t y p e q u e s t i o n s k n o w l e d g e o f t h e El e c t r i c a l Sy s t e m .

1.If the only alternator or generator fails during flight:

a.No action need to be taken

b.The master switch should be turned off and the flight aborted

c.The electrical loads should be reduced to a minimum and the flight aborted

d.An emergency landing should be carried out as soon as possible, because the engine ignition system gets its supply from the aircraft battery

2.Continuing the flight with a ‘flat’ battery:

a.Is not a concern, because the alternator will charge the battery during flight

b.Is not recommended, because the dilute battery acid may freeze during high altitude flight

c.Is not a concern because the battery is not essential

d.Is not recommended, as the alternator may not fully charge the battery during normal operations

3.If a fuse blows during flight then it:

a.Should be replaced, once only, by one of the same value

b.May not be replaced

c.Should not be replaced until after landing

d.May be replaced by fuses of greater and greater value until one is found that does not blow

4.A 100 Amp/hour battery theoretically will supply 20 Amps for up to:

a.2 hours

b.3 hours

c.5 hours

d.30 minutes

5.Compared with the current flow through the starter motor, the current flow through the starter switch in the start position:

a.Is lower

b.Is higher

c.Is the same

d.Leads the voltage because of the inductive nature of the starter motor circuit

176

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 1 : ELECT R ICAL SY ST EMS Q U

6.Connecting two 12 Volt 40 Ampere hour capacity batteries in series will provide a battery with a total capacity of:

a.12 Volts and 80 Ampere hours

b.24 Volts and 40 Ampere hours

c.24 Volts and 20 Ampere hours

d.12 Volts and 40 Ampere hours

7.Below is a schematic diagram of a light aircraft DC electrical system using

acentre-zero reading ammeter. The most probable cause of the needle of the ammeter being in the centre-zero position would be that:

a.The battery is fully charged

b.The alternator has failed

c.The battery is flat

d.All electrical loads have been switched off

8.Below is a schematic diagram of a light aircraft DC electrical system. The most probable cause of the needle of the loadmeter being in the position shown would be that:

a.The battery is fully charged

b.The alternator has failed

c.The battery is flat

d.All electrical loads have been switched off

9.Secondary cells:

a.Cannot be recharged

b.Have an off-load voltage of 1.5 Volts when fully charged

c.Are used in hand-held equipment like radios and torches

d.Can be charged and discharged many times

177

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 1 : ELECT R ICAL SY ST EMS Q U EST IO NS

10.A double-pole electrical system:

a.Is not required in aircraft which are made of non-conductive materials

b.Requires one cable taking the current from the generator or alternator to the component and another to complete the circuit back to the of the generator or alternator

c.Has the return current flowing back through the metal of the airframe to complete the circuit

d.Is fail-safe, and so does not require a battery

Question	1	2	3	4	5	6	7	8	9	10
Answer

T h e a n s w e r s t o t h e s e q u e s t i o n s c a n b e f o

178

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 12

VACUUM SYSTEMS

179

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 2 : V ACU U M SY ST EMS

180

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 2 : V ACU U M SY

INTRODUCTION.

Most modern light aircraft use engine -driven suction pumps, like the one shown in Figure 12.1, to generate a vacuum, or, more accurately, a low pressure area which induces an airflow into the main gyro operated instruments. (There is usually at least one electricallydriven gyro instrument, usually the turn co-ordinator or turn indicator, in case of vacuum system failure.)

The airflow is directed over buckets formed in the rim of the rotor of the instrument gyros and this spins them round at very high speed.

The instruments which are usually powered by the vacuum system are the Attitude Indicator, or Artificial Horizon, and the Heading Indicator, often called the Direction Indicator (DI), shown in

Figure 12.2.

A vacuum

pump directs sufficient

airflow

onto buckets on the gyro rotor rim to drive it round.

Figure 12.1 An Engine Driven Vacuum Pump.

The

instruments usually

powered

by a vacuum system are the Direction Indicator and the Attitude Indicator.

Figure 12.2 The Direction Indicator and the

Attitude Indicator.

COMPONENTS.

The vacuum system consists of a means of generating a vacuum, a method of controlling the vacuum, a filter to clean the air being sucked through the instruments and the necessary pipework to join all the components together. The components forming a simplified vacuum system are shown in Figure 12.3.

Figure 12.3 Components of a Vacuum System.

181

Figure 12.4 A Venturi Tube and suction tube on an Auster Mk6.

Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 1 2 : V ACU U M SY ST EMS

			The vacuum system usually generates vacuum in one of the following ways:
		Because a
		dry vacuum	•	An engine driven vacuum pump
		system is
		vulnerable	•	A venturi tube placed in the airstream
to contamination by liquids,
a system lubrication
device is not used.			•	Connection to the inlet manifold of a normally aspirated engine

V e n t u r i T u b e s .

	A system	Older aircraft such as the Auster Mark 6, and the De Havilland Hornet Moth, may
		have a venturi tube fitted to the outside of the airframe to generate the vacuum, like
	powered by a
		the one shown in Figure 12.4.
	Venturi Tube
	will only work reliably when	The venturi tube works on the principle
	sufficient air is flowing through
	the venturi. This may not	that when it is placed in the airstream,
	happen for some time after the	its effect is to accelerate the air passing
	aircraft has become airborne.	through the tube.	When the air

accelerates its static pressure drops. At the narrowest point of the venturi the speed of the air is greatest and its static pressure lowest. Once through the narrowest point, the air decelerates and its static pressure rises once more. Placing the suction tube at the point of lowest pressure makes use of the partial vacuum thus generated.

The pilot should be aware that the system will work reliably only when

sufficient air is flowing through the venturi. This may not happen for some time after the aircraft has become airborne. A certain elapsed time is required to allow the partial vacuum to become effective in driving the gyros fast enough to give the correct indications on the instruments.

In l e t Ma n i f o l d Su c t i o n .

A few aircraft are fitted with an inlet manifold suction system similar to the one depicted in Figure 12.5.

This type of system works on the principle that whenever a normally aspirated piston engine is running, there is a depression created in the inlet manifold. This depression is greatest when the engine is just ticking over with the throttle almost fully closed, and gets less as the throttle is opened.

The partial vacuum thus created at low

to medium engine speeds is sufficient Figure 12.5 An Inlet Manifold Suction Fitting. to drive the instrument gyros.

182