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Accu
Accu's zijn er in vele soorten en maten. Je vindt ze in je auto. Vliegtuigen hebben ook vaak accu's waarbij de belangrijkste elementen bestaan uit Nickel en Cadmium. ( NiCd ) Daarom heten deze accu's ook wel Nicads. Deze accu's hebben een grote capaciteit wat van belang is voor de grote gevraagde inspanning bij het opstar
Het gevaar van Nicads bestaat uit de kans op thermal runaway. Het is al weer lang geleden dat ik iets over NiCad accus in vliegtuigen heb gehoord maar in de 80er jaren waren het de belangrijkste accus voor gebruik bij kleinere vliegtuigen,. Twin Otters hadden ze, en ook de Beechcraft B200 Super King Air. Bij de King Air was ik nooit zo gelukkig met de NiCads. Niet alleen werd er veel van gevraagd bij het starten van de motoren maar de accu zelf bevond zich in de vleugel. Een thermal runaway op die plaats kon het scenario worden voor een catastrofaal ongeluk. Om te laten zien wat dat betekende
I have not heard the phrase 'thermal runaway' for a long while, last heard on Be90/20 where i believe the battery was in the wing root (but please dont quote me) and that was the best part of 20 years ago. At that time, it was a master caution 'battery Charger' or 'battery charging' too long, or similar, that resulted in a diversion.
In aviator, or engineer terms, to the lay ops guy, what is 'thermal runaway' ?
Thermal Runaway
"Thermal runaway is a condition in which the current for a fully charged nickel-cadmium battery rises out of proportion to the impressed-voltage level. This condition is caused when heat from oxygen recombination, an inherent property of most rechargeable batteries, causes the battery's voltage to drop as it gets hot.
During thermal runaway, the battery can become dangerously hot, emit excessive amounts of toxic gas, and spew electrolyte. This condition can occur in either the battery workshop or the aircraft.
Low electrolyte levels, electrolyte contamination, no gas barrier, or a deteriorated gas barrier can cause thermal runaway.
When oxygen recombination occurs, heat is generated, causing the battery temperature to rise and the battery voltage to drop. This process causes the battery to draw a higher charge current. As the temperature of the battery increases, the battery voltage continues to decrease, and the current progressively becomes greater. As this process continues, electrolyte eventually reaches the boiling point. If allowed to continue, the electrolyte level may fall below the top of the plates, causing them to dry out. The cadmium plates may then ignite and burn like steel wool, melting the separator, causing the cell to short circuit, and igniting the hydrogen in the cell. A battery's stored energy can feed a short circuit, allowing it to burn through the entire battery.
In event of a thermal runaway, electrical power should be isolated, and no attempt should be made to handle or move the battery for at least 30 minutes. After this period or when the battery is cool enough, carefully remove the vent caps of the affected cells. Make sure you wear a facemask, gloves and protective apron when doing this work.
Thermal runaway is preventable, but it requires that maintainers rigidly follow servicing procedures at appropriately equipped battery-charging facilities. It is imperative only trained people service and maintain nickel-cadmium batteries. These preventive steps will work in most cases, but a situation occasionally is beyond the control of the battery maintainer. This usually happens when aircraft are required to operate in extremely high temperatures, which is less than ideal for premium battery operation."
It certainly makes one want to go the lead-acid option.
Thermal runaway is preventable, but it requires that maintainers rigidly follow servicing procedures at appropriately equipped battery-charging facilities. It is imperative only trained people service and maintain nickel-cadmium batteries. These preventive steps will work in most cases, but a situation occasionally is beyond the control of the battery maintainer. This usually happens when aircraft are required to operate in extremely high temperatures, which is less than ideal for premium battery operation. Not an expert here but I was able to see real benefits by the use of Ni Cads, first of all the pilot needs a little education on how to operate, our company engineer taught me not to go on board and check ATIS, get clearance etc., he also taught me that Beech's starting method deep cycled the batteries two times during each start sequence, we used a method (BE-20) of starting the first engine followed by an immediate start of the second engine followed by bringing the first GEN on line. We maintained our battery with a Christy charger and at midlife on the engines, there was no maintenance required, I think that says a lot.
I had a nicad battery blow up in flight over northern Kenya at night in one Mrs Windsor's HS Argosys just over 40 years ago.
We were going from Aden to Nairobi and had luckily just passed over the Ethiopian mountains when fumes became apparent. The battery box was in an electrical compartment downstairs in the freight bay/passenger compartment at the forward end of the port fuselage.
The F/E went to investigate and came back upstairs to get the asbestos gloves and some tools. He came back to report that the battery was overheating but that he had disconnected both terminals and therefore we should not have a problem until we got on the ground.
About 20 minutes later there was a muffled explosion when the battery blew apart. Fortunately, the damage was contained within the compartment and no one was hurt.
We did an emergency descent (which wasn't very far in an Argosy) and I had complete confidence in the fact that my navigator knew exactly where Mt Kenya was in the dark.
The interesting thing was that the thermal safety strip on the battery did not work as advertised which meant that there was nothing to stop the catalytic reaction once it got started. Needless to say, a lot of aircraft skin had to be replaced.
The funny bit (there is always a funny bit) was that 30 or so of the punters downstairs were going to Nairobi for a religious get together. They started community singing and Land of Hope and Glory could clearly be heard upstairs on the flightdeck!
That was a great help.
Hand-bagage
De verantwoordelijkheid voor alles aan boord van onze Twin-Otters lag bij de cockpitbemanning
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