Are
Noise Reducing Headsets Needed?
Many GA pilots use noise-reducing
headsets in their single piston aircraft in order to cut down the
‘extraneous’ noises generated while they are flying. I wonder how
many pilots give serious thought to the actual need for
these headsets before they purchase them? Our article
Orbits in the Visual Circuit reviews
an accident where a contributory factor was a noise reducing headset that may
have caused the pilot to miss vital audible signs during communication with
ATC. And a noise reducing headset may also have contributed to the incident
referred to in our article
Automatic Pilot = Automatic Crash?
This was a fatal accident involving a Mooney M20J
when all on board were killed following an autopilot malfunction. The pilot
was wearing a noise reducing headset and may have missed aural clues from the
airframe and engine noises that all was not well as the aircraft climbed
through cloud at a steadily reducing airspeed.
Noise reducing headsets were first developed
for use in military jet aircraft where the levels and frequencies of noise in
the cockpit caused distraction, fatigue and physical damage to the hearing of
the crew. However, in a GA aircraft a properly designed and carefully fitted
‘normal’ headset will allow efficient communication without
blanking out all sounds from the surrounding cockpit, airframe and engine. I
believe these outside sounds are the source of sometimes vital clues to the
performance of the aircraft and its systems. By wearing noise reducing
headsets pilots isolate themselves from a valuable stream of information
about what is going on beyond the confines of their radio and intercom. I
don’t believe that noise reducing headsets are required in the majority
of GA aircraft.
Personally, I would rather invest in a protective helmet and a
complete seat harness than spend money on a noise reducing headset. Why do so
few GA pilots wear protective helmets in the air but would not think of
riding a motorcycle without one? Have a look at how many pilots die from head
injuries in an otherwise survivable accident.
The Use & Abuse of Aircraft
Batteries
Batteries require proper and
regular attention throughout the year. We offer a timely refresher on the
types and characteristics of aircraft batteries.
Batteries probably suffer more abuse and are given
less care and thought than any other component in the average GA aircraft.
How often does one hear an aircraft starter grinding away until the battery
is almost exhausted? The misguided pilot may then attach ‘jump
leads’ to the aircraft battery, start the engine and take off without
further thought about the state of the battery and the associated vital
components that keep the aircraft’s electrical systems working. An
airborne battery failure is always exciting and can quickly develop into a
major emergency. Batteries require regular checks and routine maintenance if
they are to provide reliable service. Modern batteries have become much more
reliable but should never be treated as ‘fit and forget’
components.
The majority of aircraft batteries are rechargeable types of
either Lead-Acid or Nickel-Cadmium construction.
Lead-Acid Batteries: Construction
A
Lead-Acid battery consists of a number of cells containing vertical
perforated plates with porous separators between each positive and negative
plate to prevent shorting. Each cell has a hard casing with a terminal on top
and a hole with a non-spill vent valve through which one can test the
electrolyte strength and top up with distilled water. The vent allows gas to
escape without loss of electrolyte. The cells are connected in series so
their potential is additive. The complete battery is fitted into a metal
battery box to give mechanical protection and electrical shielding.
Lead Acid Batteries: Ratings
The number
of cells connected in series determines the battery voltage. A 12 volt
battery has 6 cells in series and a 24 volt battery has 12 cells in series.
The current that can be supplied by the battery for a specified time rates
the capacity of the battery and is quoted in ampere-hours. A fully charged 50
ampere hour battery will provide 50 amperes for one hour, 25 amperes for two
hours or 10 amperes for five hours. The actual discharge rate can reduce
these figures because a heavy current demand heats the battery and decreases
its efficiency, and total output, prior to total discharge. Connecting
batteries in series increases their total voltage but not their ampere-hour
capacity. Connecting batteries in parallel increases the ampere-hour capacity
but not their voltage.
Lead-Acid Batteries: Life
Several
factors will reduce battery life. Over discharging causes sulphation, while
too rapid charging or discharging causes plate distortion and shedding from
the plates and permanent damage to the battery capacity. Shed material can
short the plates of individual cells, reducing both voltage and capacity. A
battery left in a low voltage or discharged condition will suffer permanent
damage so it is well worth removing your aircraft battery and keeping it
fully charged if the aircraft is not flown regularly. Lead-acid batteries
have a finite life and need to be replaced by calendar, no matter how
carefully they are maintained. A new battery is cheaper than a broken
aircraft.
Lead-Acid Batteries: Operation
Each cell
contains a positive plate and a negative plate immersed in an electrolyte of
sulphuric acid and water. Lead sulphate is deposited on both plates during
discharge while the water in the electrolyte increases and the acid content
decreases. This eventually retards the chemical reaction and the output
falls. When a battery is being recharged the lead sulphate is removed from
the plates and sulphuric acid is formed while the water content of the
electrolyte reduces, and the density of the electrolyte increases.
Lead-Acid Batteries: Testing
The
density of the electrolyte indicates the state of charge of a lead-acid
battery. A hydrometer measures the specific gravity of the electrolyte. The
hydrometer has an internal float with a printed scale from 1.100 to 1.300. In
a new, fully charged battery the electrolyte is 70% distilled water and 30%
sulphuric acid that is 1.300 times as heavy as water. A reading of between
1.300 and 1.275 indicates a high state of charge, down to 1.240 is medium and
below 1.240 is low.
The serviceability of aircraft batteries should be tested every three
months and the hydrometer readings should be corrected for temperature
according to the hydrometer temperature correction chart. Readings should be
taken BEFORE adding distilled water to top up a cell. The electrolyte will
burn skin and clothing as well as damage the aircraft structure so it is
important to immediately neutralise any spillage with clean water and
bicarbonate of soda.
Lead-Acid Batteries: Charging
Passing a
direct current in the opposite direction to the discharge current charges
batteries. The voltage of the charger must be greater than the open circuit
voltage of the battery. For example, the open circuit voltage of a fully
charged 12 volt battery is about 13.2 volts (6x2.2 volts) but 14 volts is
required to charge it. While in the aircraft, the battery is charged by the
generator. This is a constant voltage charge with a voltage regulator
controlling the generator output.
A lead-acid battery generates an explosive mixture of oxygen and
hydrogen while being charged. The vent caps must be loosened and left in
place while the battery is on the charging bench. It is dangerous to charge a
battery while it is still in the aircraft. Ensure that no sources of ignition
are in the vicinity of a battery while it is being charged.
Nickel-Cadmium Batteries: Construction
Ni-cad
batteries offer better reliability, lower maintenance cost, shorter charging
time, longer life and better engine starting capability than lead-acid
batteries. Ni-cad battery construction is similar to that of the lead-acid
battery, with positive and negative plates, separators and electrolyte in a
cell container. The specific gravity of the electrolyte remains between 1.300
and 1.240, without much change during discharge or charge so a normal
hydrometer cannot accurately detect the state of charge of a ni-cad
battery.
Ni-cad Batteries: Operation
As a
ni-cad battery is being charged the electrolyte level will rise slightly
until it reaches a maximum level at full charge. Distilled water should only
be added to a fully charged ni-cad battery. The process is reversed during
discharge with oxygen passing from the positive to the negative plates,
producing electrical energy. The plates absorb some electrolyte during
discharge, so the level will drop.
Lead-acid batteries and ni-cad batteries require separate
storage and charging facilities, as emissions from a lead-acid battery will
affect the ni-cad electrolyte. Keep acid away from ni-cads. Ni-cads use
extremely corrosive potassium hydroxide as an electrolyte and their servicing
demands the use of goggles, gloves and a protective apron. Any spillage,
however slight, must be rinsed thoroughly with clean water and a boric acid
solution.
Ni-Cad Batteries: Charging
It is
characteristic of ni-cad batteries that their voltage remains constant for
about 90% of their discharge cycle and then decays rapidly, unlike lead-acid
batteries that have a gradual voltage decay during discharge. A voltage test
will not indicate the state of charge until the battery is almost exhausted.
Recharging ni-cad batteries is not a ‘do-it-yourself’ operation
and should be left to specialists.
Thermal Runaway of Batteries
Thermal
runaways are not uncommon. Thermal runaway can occur in either type of
battery if their rated capacities are exceeded. Thermal runaway involves
violent gassing, boiling of the electrolyte, damage to the plates and even
melting of the battery case or an explosion. Placing excessive demands on a
battery during prolonged starting attempts can lead to thermal runaway and
destruction of the battery – and, possibly, the aircraft. Each year
there are several cases of thermal runaway in UK GA aircraft, leading to
severe damage and even total destruction. Don’t become another
statistic through ignorance or carelessness.
Text and Photographs © 2007 Gremline & Hill House
Publications, unless otherwise stated.
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