Gremline Flight Safety Report # 1: Are noise reducing headsets necessary?
Gremline Flight Safety Report # 2: The use and abuse of batteries.

gremline.com

the online flight safety digest

Text Box: headsets & battery care

the gremline digest — noise reducing headsets / battery care

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
Winter is setting in to the northern hemisphere, the season when aircraft batteries are at most risk of degradation; but 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.

 

 

Search Now:
Amazon Logo

Google

 

Text and Photographs © 2007 Gremline & Hill House Publications, unless otherwise stated.

 

landing page   about gremline   copyright/conditions/contact   information exchange   glossary

uk emergency diversions   uk links, chirp & gasco   global & misc links   forum

the gremline cockpit — index of articles   the gremline bookshop   top of page