From the Viscount 745 Pilot's Handbook dated May 5, 1960: “…(T)he following units have caused electrical fires in the past and may be used as a guide in troubleshooting.
1. Main Inverters
2. Nesa Inverters and transformers (transformers located forward of center carry on luggage rack)
4. Recirculating air fans
5. Temperature control actuating motor
6. Water pump
7. Cockpit fans
8. Voltage regulators
9. Hot cup (due to hostess holding circuit breaker “in” which had popped out).
10.Jettison system wiring (on test hop for jettison installation).”
From the Capital Airlines Viscount 745 Flight Manual dated January 15, 1961: “The following units have caused electrical source smoke in Viscount aircraft and may be used as a guide in locating defective units:
1. Main and nesa inverters
2.Nesa transformers (located forward of center carry on luggage rack).
3.Jettison system wiring.
4. Recirculating air fans.
5. Cabin temperature control actuating motor.
6. Flight recorder (located left side of electrical compartment).
7. Hot cup.
8. Radio equipment.”
The two lists of sources of fire are almost identical. Sonotone Nickel Cadmium batteries are not mentioned as a source of fire on Viscount aircraft in either list.
The reader may ask why a flight manual from Capital Airlines is given as a source of information since Flight 823 belonged to United Airlines. The answer is that Captain Oliver E. Sabatke began flying for Capital shortly after returning home from World War II and flew with them until Capital and United merged. Captain Sabatke had logged 1,700 hundred hours flying Vickers Viscounts. Source: Robert Serling's Loud and Clear.
It is reasonable to assume that Sabatke had familiarized himself with the various Viscount flight manuals over the years.
An entry in The Viscount Pilot's Handbook, dated September 16, 1960 has this to say about Sonotone batteries on board Viscount Aircraft: “After any start wherein ships batteries are used either alone or to assist an outside source, a few minutes of ground running under light electrical load will quickly replenish the batteries before takeoff. Heavy use of Sonotone batteries in this manner will do them no harm whatsoever, and may even do them some good, as it gives them a chance to 'flex their muscles'.”
Advisory Circular AC 00-33A, dated February 14, 1973 states: An increasing number of potentially hazardous incidents involving nickel-cadmium batteries, during flight and ground operations, have been reported. The failures are more prevalent where the batteries are charged directly from the DC (direct current) bus rather than by a separate battery charger. Although the nickel-cadmium battery is capable of delivering large amounts of current, the battery is inherently temperature sensitive and a majority of the reported incidents can be attributed to overheating…Basically, 'thermal runaway' is an uncontrollable rise in battery temperature that will ultimately destroy the battery. This condition can occur when a nickel-cadmium battery is operated at above normal temperatures and is subjected to high charging currents associated with constant voltage charging. As the temperature of the battery increases, the effective internal resistance decreases and higher current is drawn from the constant voltage charging source. The higher current increases the battery temperature which in turn results in even higher charging currents and temperatures.“
Slightly more than one year before the crash of United Flight 823, on April 8, 1963, the following appeared in the Viscount Pilot Program notebook: “The two Sonotone Nickel-Cadmium Batteries can be recharged rapidly from either ground power or the airplane generators without the overheating and 'boiling' associated with lead acid batteries.”
It appears the Bureau of Safety was on to something when it weighed the possibility that a loose Sonotone battery cable had somehow come into contact with a hydraulic line, while producing an arc that ignited hydraulic fluid. United Airlines attorneys referred to this as a “vague intimation” that took place during the course of the investigation.
Perhaps nothing more than a vague intimation could have taken place because the Bureau of Safety, as well as others who investigated the cause of the fire, were completely unaware of the danger posed by the aircraft's batteries.